Degradation Mechanisms and Reactions in ... - ACS Publications

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Degradation Mechanisms and Reactions in Organic Light-Emitting Devices Sebastian Scholz,*,†,‡ Denis Kondakov,*,§ Björn Lüssem,†,∥ and Karl Leo*,† †

Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Strasse 1, 01069 Dresden, Germany Fraunhofer-Institut für Photonische Mikrosysteme, Maria-Reiche-Strasse 2, 01199 Dresden, Germany § DuPont Displays Inc., 4417 Lancaster Pike, Wilmington, Delaware 19805, United States ‡

5.3. Reorientation of Dipoles 5.4. Role of Charge Carriers 5.5. Charge Balance and the Position of the Recombination Zone 5.6. Electrochemical Reactions 5.7. Photochemical Reactions 6. Material-Specific Reaction Processes 6.1. Reactions of Metal Organic Complexes 6.1.1. Al-Organic Complexes 6.1.2. Ruthenium Complexes 6.1.3. Dissociative Reaction Mechanism of Iridium Complexes 6.2. Reactions of Aromatic Amines 6.2.1. α-NPD 6.2.2. TAPC 6.3. Reaction of Carbazole Derivatives 6.3.1. Reaction Pathway of CPB 6.3.2. TCTA 6.4. Phenanthroline Derivatives 6.5. Reactions of Hydrocarbons 7. Summary Author Information Corresponding Authors Present Address Notes Biographies References

CONTENTS 1. Introduction 1.1. Defining the Device Lifetime 1.2. Evolution of Device Lifetime over the Last Decades 1.3. Classification of Degradation Mechanisms 1.4. Materials Overview 2. State-of-the-Art Analytical Techniques for Thin Film Organic Semiconductors 2.1. Electrical Techniques and Methods 2.2. Optical Methods 2.3. Surface Analyzing Methods 2.4. Depth Profiling Techniques 2.5. Chemical Analysis Techniques 3. Abrupt Failure Modes and Their Visual Appearance 3.1. Catastrophic Failure 3.2. “Dark Spot” Degradation 4. External Processes of Degradation 4.1. Influence of the Process Parameters 4.2. Influence of External Water and Oxygen 4.3. Impurity-Induced Degradation 4.4. Influence of the Driving Scheme on a Lifetime Experiment 4.5. Voltage Increase 4.6. Temperature Effects 4.6.1. Morphology and the Glass Transition Temperature 4.6.2. Influence of Annealing 5. Internal Degradation Mechanisms 5.1. Diffusion and Drift 5.2. Accumulation of Nonradiative Recombination Centers

© XXXX American Chemical Society

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1. INTRODUCTION Since Tang and Van Slyke invented the first efficient organic light-emitting diode (OLED) based on small molecules in 1987, and Friend et al. in 1990, based on polymers,1,2 these devices have achieved tremendous progress. OLEDs are known for their high potential in display, signage, and lighting applications. Nevertheless, it is important to keep in mind that the development of organic semiconductors started more than 100 years ago with the discovery of the photoelectric effect in anthracene crystals in 1906 by Pochettino.3 A few years later, the photoelectric effect was found for other organic materials.4 The first experiments with electroluminescence on anthracene crystals were successfully carried out by Pope et al. (1963), as well by Helfrich and Schneider (1965).5,6 In 1982, the first thin film electroluminescence device, vacuum deposited and based on amorphous anthracene, was developed from Vincett et al.7 One

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DOI: 10.1021/cr400704v Chem. Rev. XXXX, XXX, XXX−XXX

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exponential decay (SED) function of the decreasing luminance,14−16 and (iv) additionally, the voltage increase during the aging process (direct current (DC) driven).15,17,18 Within the overview about the intrinsic degradation mechanisms, Aziz and Popovic discussed the influence of the morphological behavior of the used organic semiconductors, the “Alq3-instabiliy model”, the relevance of indium migration, as well as mobile ionic impurities and a model of immobile charge accumulation.19 Aziz and Popovic clearly pointed out that there is an obvious difference between the lifetime behavior of devices, which may occur due to intrinsic mechanisms, and the luminescence decay due to extrinsic effects. This concept was picked up by Xia et al., who assigned different degradation mechanisms either to intrinsic factors (the electro- or photochemical reactions, thermal behavior, or interfacial effects such as cathode delamination) or to extrinsic factors (impurities, insufficient encapsulation, and therefore the content of penetrating water and oxygen, fabrication environment like dust and vacuum pressure, and finally the substrate conditions). The latter are more or less conditions of device processing.20 Until now, a large number of such process-related failure mechanisms are well understood, and some of them, especially the “dark spot degradation”, can be avoided completely. Nonetheless, the visible appearances of mechanisms from these categories may overlap, and the analysis of the individual reactions may remain challenging. For this reason, recent work from the former Kodak research group uses a different view on the topic of device degradation.21 It focuses on studying intrinsic degradation mechanisms by different chemical and electrochemical analysis techniques like voltammetry techniques, electric field profiling, photoluminescence, as well as high performance liquid chromatography (HPLC).21,22 As a result, the former Kodak group, as well as the Leo group (Dresden) and other groups, discovered a wide range of photoand electrochemical reactions, which are detectable in current driven (aged) OLEDs and light-emitting electrochemical cells.22−28 Over the past decade, a wide range of different chemical reactions are found to have a major influence on OLED lifetime behavior. These chemical reactions occur mainly within an OLED stack and are induced by different factors like light, heat, electrons/holes, or excitons. So far there were only a few cases reported that link the lifetime of a device with the concentration of a discovered degradation product22,23 or even with the rate of a certain reaction.29 An understanding of the mechanisms of the reactions and their activation/inition as well as the influence and impact of the individual chemical structures will lead to further design rules for improved (and may be inert) molecules for OLED applications. In the following introduction section, we will discuss how OLED degradation can be described analytically, how it appears in physical parameters, and how it can be subdivided into different mechanisms. Additional, we will present an overview about the evolution of device lifetime over the last two decades. We will provide the reader with the presently available information about the cause and effect of external and internal mechanisms and reasons, which may appear on an organic device. In the following part, we will discuss the state-of-the-art analysis techniques, which are needed to microscopically understand various quenching mechanism, chemical reactions, or even the influence of morphological aspects. In the second part of the Review, we focus on the chemical reactions occurring in OLEDs during device aging. We will discuss different degradation mechanisms, based on the chemical structures of

year later in 1983, a similar device was made from polymer materials by Partridge.8 Partly due to their structures lacking organic heterojunctions, the efficiency and the lifetime of these devices were extremely low. Over the last decades, a huge increase in device lifetime and efficiency has been achieved,9,10 which allows organic electroluminescence to enter the display and lighting market.11 In 2013, several billion U.S. $ revenue was already achieved, mostly with small-molecule OLEDs for active-matrix display applications. According to IDTechEx, the OLED display market grew from 6 billion U.S. $ in 2012 to over 10 billion U.S. $ in 2013. The market is expected to grow to about 17.5 billion U.S. $ in 2015 and 25 billion U.S. $ in 2017.12 Nevertheless, one still has to face the challenge of insufficient device lifetimes for many applications. Typically, the higher is the luminance of a device and the higher is the energy of the emission, the shorter is the lifetime of an OLED. In Figure 1 is

Figure 1. Overview of relevant OLED brightness, needed for several applications. Current lifetime data are from the state-of-the-art red (red ■) and white (●) OLEDs from Meerheim et al. and Löser et al. respectively.9,13

displayed the required luminance of an OLED device for a certain application together with the state-of-the-art lifetime of red and white OLEDs. It is obvious that for display applications, a wide range of stable materials and OLED stacks has been already discovered and is available. Even though the issue of lifetime is still challenging, a wide range of OLED displays has been released over the past few years. For applications requiring very high brightness, like outdoor displays, HDR (high dynamic range) displays, billboards, or window integrated transparent devices, the current OLED technology still does not provide the required lifetimes. Over the last nearly 30 years of OLED research, much effort was undertaken to discover and overcome the failure and degradation processes causing the limited device lifetimes. Already in 1982, device lifetime was discussed for the first vacuum deposited device.7 Nearly 20 years later, an extended overview about the known degradation mechanisms was provided by Aziz and Popovic in 2004, who focused their review on the visible appearance of the OLED loss mechanisms.14 Certainly, these visible effects are (i) the immediate breakdown of the electroluminescent behavior (“catastrophic failure”), (ii) the growth of nonemissive areas, mainly known as “dark-spot degradation”, (iii) the (more or less) long-term degradation effect (“intrinsic degradation”), well described by the (stretched) B


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the molecules, and will look at different strategies to avoid these reactions. 1.1. Defining the Device Lifetime

The degradation of OLED usually implies some undesirable internal processes such as chemical reactions, morphological (phase changes, crystallization, and delamination processes), and other physical (e.g., charge accumulation) changes. These processes result in various changes in device properties, most notably in the color−luminance−current−voltage characteristic of OLEDs. Typically, the degradation manifests as a continuous loss of the device efficiency (primarily observed as the decrease of brightness at constant current or voltage). Therefore, the main parameter to evaluate the lifetime of an OLED is the behavior of the luminance over time (at a fixed or predefined physical environment, e.g., temperature, current density, voltage, and/or humidity), called the lifetime of the devices. Often, the lifetime (denoted as T50 or T1/2) is defined as the time the luminance drops to one-half of its initial device brightness at a constant current density.30 In some cases, different and more challenging lifetime metrics are preferred, for example, T70 and T97, which refer to the time the luminance drops to 70% and 97% of its initial value, respectively. The latter metric is of particular interest for display manufacturing because it corresponds to the threshold of our perception of differential brightness of adjacent display elements. Some groups reported the use of a constant voltage source to evaluate the lifetime of the devices. This typically causes a faster luminance drop, as compared to constant current-based aging tests. This is due to the simultaneous increase of device impedance over time, causing an additional decrease in current density. It is noteworthy that the OLED luminance is approximately proportional to current density over a substantial range.1,30,31 Considering the well-known empirical scaling law Ln0·T1/2 = C (eq 1),15,32 the higher is the applied current density, the lower is the lifetime of an OLED (Figure 1). It is even possible to extrapolate the lifetime of highly stable OLEDs within an adequate experimental time, known, for example, from Meerheim et al. as well as from Jarikov et al.9,33 Equation 1 defines the relationship between the initial luminance L0 (at time equal to 0) and the lifetime T(1/2) of a device, quantified with the constant C. The acceleration factor n is material and device specified and depends therefore on the device structure and the used organic materials. L0n ·T1/2 = C

Figure 2. Typical normalized L−t curve, showing the experimental data (full circles), together with the fit using eq 2 up to 1100 h (dotted line), and up to 2200 h (full line). Reprinted with permission from ref 32. Copyright 2005 AIP Publishing LLC.

whereas the use of the full (double) data set will fit the remaining measured data. Even here an underestimation of further data can be expected. An alternative to eq 2 is the stretched exponential decay (SED) function, eq 3.9,32,34 This functional form may be related to the widely accepted notion that the loss mechanisms of the degradation are related to the accumulation of defects created within the device.14 Such defects may act, depending on their energy levels, as luminescent quenchers, nonradiative recombination centers, and as deep charge traps.21,35 In one exemplary system, it has been measured that the areal density of fixed charge associated with such defects is ∼10−7 C/cm2 (or ∼6 × 1011 cm−2) at 50% luminance loss, which corresponds to ∼0.1% molecular density assuming that the defects are concentrated within several nanometers wide region.22 Using a mathematical model with several arbitrary parameters and assuming an otherwise unsubstantiated mechanism of electroluminescence loss, Giebink et al. have shown that a defect density of 1018 cm−3 (corresponding to nearly 0.1% of the molecular density) might lead to a loss of more than 50% of the device luminance.35 Similar to eq 2, this equation may also over- or underestimate the lifetime, when insufficient data are used for fitting.9 The SED involves two fitting parameters, τ as a decay constant and β as a stretching factor. Neither parameter has a clear physical meaning. Empirically, β varies for different materials setups and different stack designs but is nearly constant for different current densities of identical OLED stacks.

(1)

One of the main problems associated with extrapolating lifetime data of highly stable organic devices is the under- or overestimation of the actual lifetimes.9,32 In some cases, the degradation behavior of an OLED may be influenced by multiple independent degradation mechanisms. Thereby, the lifetime curve of such an OLED may show an initial rapid decay and a more moderate (long-term) luminance drop. One possible way to fit such a behavior is to combine different exponential decay functions, like it is shown from eq 2, where a, b, α, and β are fitting parameters.32 L(t ) L0

= a e − α t + b e − βt

L(t ) L0

⎡ ⎛ t ⎞β⎤ = exp⎢ −⎜ ⎟ ⎥ ⎣ ⎝τ⎠ ⎦

(3)

Aside from eqs 2 and 3, other functions, such as power function L0/L(t) = (t + a)b, can be used to fit luminance decay curves of OLEDs. Even though one of those equations might offer a better fit in some particular cases, all of these equations should be considered as purely empirical constructs, which are useful primarily for data fitting and extrapolation purposes rather than gaining insight into mechanisms responsible for OLED degradation. It is also possible to derive the analytical forms of decay functions from assuming that OLED degradation follows specific mechanism of degradation. In one study, the decay equations were derived on the basis of several different sets of assumptions/mechanisms and were shown to approximate experimental decays observed in the OLED degradation experiments.21 Unfortunately, these decays were also relatively

(2)

Nevertheless, even this equation might underestimate the real lifetime as well. For example, in Figure 2, a typical L−t curve (normalized to 100%) is shown. An extrapolation of the first half of the data set underestimates the real (measured) values, C


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nondistinctive and did not allow one to distinguish between different degradation mechanisms. Over the last years, the convention of how lifetime data are reported and compared has changed with the requirements for device application by industrial standards. Whereas in the beginning the device lifetime was often quoted at 100 cd/m2, a previously assumed display brightness, most lifetime values are published nowadays at luminance, which is 1 order of magnitude higher, 1000 cd/m2, a value appropriate for challenging display and simple lighting applications. It is widely accepted that the overall brightness of an (indoor) display ranges from 100 to more or less 300 cd/m2,36 which requires individual pixels on the device (depending on size, color, and device architecture) to emit at 200−600 cd/m2.14,15 Therefore, Howard and Orache from IBM (in 2001) and Chwang et al. from UDC predicted a need of more than 10 000 h at 140 cd/m2 for a display, in continuous operation and/or integrated, for example, in entertainment headsets.34,37 An additional implementation of filters, such as polarizers, leads to higher lifetime requirements for the devices. In the same study of IBM, it was noted that even simple displays designed for outdoor environment may require brightnesses as high as 3000 cd/m2. Besides the industry, as an example the Federal Ministry of Education and Research of Germany (German BMBF) is focusing on organic LEDs and founded a project (since 2005) with the aim to reach the 30 000 h lifetime goal for white OLEDs at 1000 cd/m2,38 which was already reached in 2008.39,40 In the last years, the requirements for device lifetime issues are more and more challenging. The industrial community now demands a device (display) lifetime of about 20 000 h at 500 cd/m2, where the lifetime is defined to reach 80% (T80%) or even 95% (T95%) of the initial brightness (drop of only 20% or even 5%).11 The simple reason is the sensitivity of the human eye, which recognizes even a minor (3−5%) brightness difference, meaning that burn-in effects will lead to annoying features, for example, on mobile displays. The question may arise whether the predicted lifetime data are realistic or not, especially in the light of the high requirements and the reported lifetime data, collected at very high driving conditions. Figure 3 displays a measured lifetime series from an OLED similar to the setup described in ref 9. When aging occurs at very high luminance and, respectively, at very high current densities, degradation will be accelerated beyond the scaling law eq 1 due to significant internal heating effects.41,42 This will lead to an overestimation of device lifetime by extrapolated high luminance values down to common values for display or lighting applications. To overcome this problem, some groups recalculate their lifetime data with empirical factors, obtained from hundreds of measured devices.33 Vice versa, low luminance data will not cover the uncertainties of Joule heating effects for high brightness applications. Facing the problem of Joule heating and discovering the fundamental issues of temperature enhanced degradation and the physics behind, the understanding of acceleration will lead to a more precise lifetime model. Acceleration tests similar to the so-called 80/80/80 model, where the lifetime will be measured consequently at 80 °C, 80% humidity, reaching 80% of its initial luminance, consider the increased degradation behavior and take them into account.

Figure 3. Lifetime measurement at different initial luminances displays the failure of the lifetime extrapolation due to the enhanced Joule heating of a red OLED,9 which leads to an accelerated device degradation at higher brightnesss; the figure displays the lifetime data at 80% (T80) as well as 50% (T50) of the individual initial luminance; lines serve as a guide for the eyes. Reprinted with permission from ref 43. Copyright 2014 Orgworld.

1.2. Evolution of Device Lifetime over the Last Decades

In Figure 4 is depicted the successive enhancement of the OLED lifetime separately for (a) fluorescent and (b) phosphorescent blue, green, and red OLEDs. Figure 4 shows the evolution of lifetime compared at the same initial luminance of 1000 cd/m2.1,7,9,13,20,33,34,39,44−118 In case of missing data of the initial luminance, we used the simple extrapolation of the available data with eq 1 and n = 1, even though it can lead to an overestimation of the lifetime data. Because of the fact that this extrapolation is only necessary for data mainly collected in the last century, this overestimation may be a tribute to the pioneering work of our colleagues. Besides this, we also notice that Burrows et al. report in 2000 about a phosphorescent 2,3,7,8,12,13,17,18-octaethylporphine platinum (PtOEP)-based device with an extrapolated long lifetime of about 107 h at 35 cd/m2.119 Because of the low luminance used, an extrapolation to 1000 cd/m2 with eq 1 and n = 1 would lead to a lifetime value of 350 000 h. Higher current driving conditions and other extrapolation methods may lead to other results.9 Figure 4 shows that the lifetime increases approximately by 1 order of magnitude every 5−10 years. Considering the history and considering the stack design of Tang and Van Slyke, the device stack was made out of two organic layers, an TAPC-hole transporting layer (HTL) and a Alq3-electron transporting layer (ELT) and emission layer (EML), the first remarkable increase in lifetime was obtained in the years around 1995, using an additional (fluorescent) doping material (rubrene or perylene) in a Alq3-matrix.49−51 Much work then was done to optimize the charge injection in (mostly) Alq3-based devices to avoid the known instability-Alq3-cation. In particular, the ITO work function was altered, the barrier height of hole transporting materials was varied, and the influence of different cathode materials was investigated. In 1998 the invention of the doping process for transport layers had a major impact of reducing the driving voltage of devices.120,121,294 In the same year, the doping of the emission layer with phosphorescent emitters was first reported by Baldo et al.122 At this time, phosphorescent OLEDs exhibit only poor lifetimes. Nevertheless, today the lifetimes of phosphorescent D


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Figure 4. Lifetime evolution of fluorescent (a) and phosphorescent (b) blue, green, and red OLEDs. The data points for the white OLEDs were not distinguished between stacked, hybrid, or separate phosphorescent/fluorescent devices.

and fluorescent devices are comparable, except for blue materials, where the lifetime for phosphorescent blue devices is still a challenge.20,98,100 An additional cohost material was introduced to a given matrix: doping emitter system at first in 2003 by Liu et al.75,89 and then 2007 from the former Kodak group33,89 to enhance the exciton transfer onto the emitter and to avoid high charge and/or exciton concentrations on the Alq3-matrix material. This approach led to the up to now highest lifetime for a red fluorescent OLED with 1 300 000 h at 1000 cd/m2.33

nisms and causes will be roughly divided into externally (on the right-hand side) and internally (left) caused mechanisms. The five main visible effects of device degradation are known (a) as “dark spot” degradation, where point defects growth (dark spots) and will accumulate over time to large non luminescent areas. A similar behavior is found at the edges of a device, where the active area may shrink over time. The main causes for this phenomena were found in external influences like particles, originating from production, or oxidized areas of the electrode. In section 3.2, we will have a closer look on this topic. Similar reasons may be responsible for the second effect of degradation, (b) the “catastrophic failure” or “abrupt device degradation”. Although the exact reasons are rarely known, one may consider that the main reason can be expected in external causes like delamination of the electrode or process related causes as well. This issue will be discussed in section 3.1. At this point, another phenomenon should be mentioned: bright spots, where a very high current density creates a local bright area and may lead to (b) or (a), depending on the destruction mechanism. The causes of it are defects or inhomogeneous layers like layer thickness variations or crystallization of the used materials. As mentioned above, investigations of device aging reveal that the lifetime curve frequently exhibits two different components that contribute to the luminescence decay, the short-term decay (c) and the long-term decay (d). For completeness, in Figure 5, a fifth observable effect (e) is included, which is an efficiencyenhancement effect. This phenomenon may be occasionally observed in the first minutes or hours of the device lifetime, followed by the decay of the device luminance. It should be stressed that the apparent separation of the decay curve into the long-term, short-term, and initial rise components might also be purely superficial and unrelated to the (frequently suggested) plurality of degradation mechanisms. Given the complexity of known degradation mechanisms, there is no reason why a single mechanism cannot yield a complex decay curve that does not conform to any simple mathematical expresssion.

1.3. Classification of Degradation Mechanisms

Our Review primarily focuses on the lifetime and degradation mechanisms of small molecule bottom emission OLEDs, normally processed on the anode material indium tin oxide (ITO). On the related subject of polymer devices, few review articles about the degradation of polymer-based devices mainly in the area of organic photovoltaic (OPV) have appeared.123,124 It is noteworthy that other OLED architectures like inverted structures or OLEDs with transparent top contacts often exhibit lower lifetimes than their conventional counterparts.77 This might be caused by the differences in charge carrier injection as well as the damage that may occur during device fabrication, especially by sputter deposition techniques for transparent top contacts.125−127 During the last two decades, a wide range of different mechanisms of OLED degradation were suggested. However, they result in only a small number of detectable and measurable effects. It is thus often difficult to clearly distinguish between the separate degradation mechanisms. We attempt to classify the mechanisms by first dividing them into effects caused either by intrinsic or by extrinsic factors.14,15,20,21 In the following, we will mention the different degradation mechanisms, and a detailed discussion of each mechanism or effect will follow in the next sections. On the top of Figure 5 are displyaed the appearances or visible behavior of the different degradation mechanisms. The four columns further below display the main causes (outer columns) of possible degradation mechanisms (inner columns). MechaE


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Figure 5. Schematic representation of known OLED degradation mechanisms and their essential constituents.

Beside the mentioned visible changes on OLEDs and their emission characteristics, there are some additional phenomena not included in the picture for brevity. During the device aging, shifts in current−voltage characteristic usually occur along with some shift in emission color, which may appear homogeneously over the emissive area or as a gradient across the emissive area. Reasons for the color shift can be attributed to microcavity effects due to emission zone shifting as well as accumulation of different emissive species or partial quenching of existing emitters. The following processes are usually considered in the context of intrinsic degradation mechanisms: exciton reactions, charge carrier reactions, migration of ionic species, charge accumulation, and changes in the electric field profile due to molecular reorientation.

The most commonly mentioned external causes of OLED degradation are arguably light,128,129 oxygen,130 water,130 and temperature.131,132 Some process parameters of the device production, like pressure133 (and therefore oxygen and water content), evaporation rate,134,135 temperature, and impurities, play an important role on device stability as well. The external influences are controllable due to improved production conditions, like an applied high vacuum in the range of 10−8 mbar, highly pure substrates and semiconducting materials, as well as the usage of the state-of-the-art encapsulation techniques.136 1.4. Materials Overview

For small molecule OLED applications as well as for organic solar cells, a wide range of thermally stable and (via physical vapor F


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Figure 6. Chemical structures and the most common abbreviations of the materials discussed in this Review.

deposition) sublimable organic compounds were developed. Beside these other organic compounds, polymers are used in a

wide range of organic semiconductor devices. The chemical structures of the materials, discussed in this Review, their G


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Figure 7. Chemical structures and the most common abbreviations of the materials discussed in this Review.

abbreviations, as well as their common names are shown in Figures 6 and 7.

The condensed aromatics and their substituted derivatives such as pentacene, perylene, and rubrene are widely known for H


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Table 1. Commonly Used Analysis Techniques To Investigate Organic Semiconductors and Their Devices and Estimates of Their Usefulness in OLED-Related Studiesa suitable for devices luminance−current−voltage characteristics charge modulation specroscopy voltammetry/impedance spectroscopy emission and absorbance spectroscopy Stark spectroscopy HPLC (MA)LDI-TOF-MS SIMS dyn. XPS AFM AES STM UPS XPS XRD electrochemical characterization of solid films IR/Raman EPR NMR thermally stimulated current spectroscopy ellipsometry a

surfaces

possible analysis of single layers

diffusion, migration

charge carriers, traps, and excitons

morphology

chemical reactions and products

+++

−

−

+

++

−

−

+++ +++ +++ +++ +++ +++ +++ +++ + ++ + − − + −

− − − − − − − + +++ +++ +++ +++ +++ + +

− − ++ − +++ +++ ++ ++ + +++ + ++ ++ +++ +++

− ++ − − + + ++ ++ − ++ − + + − −

+++ +++ +++ +++ − − − − − + − + + − ++

− − + + − − − − +++ − ++ − − +++ −

− − ++ − +++ +++ − + − ++ − + + − −

+ + + +

− − − −

++ ++ + −

+ + − −

+ ++ − ++

− − − −

+ ++ + −

−

−

+

+

+

−

+

(+++) very useful, (++) suitable only to a limited extent, (+) limited to special cases, (−) unsuitable.

their use in organic field effect transistors.137,138 Additionally, some of them are used in OLEDs as host materials or fluorescent emitters in blue emitting devices as well (TBADN and related materials as well as, e.g., different perylene derivatives).139 Some other blue emitting singlet emitters (usable as matrix materials, e.g., in phosphorescent emitter systems as well) are the spirobridged hydrocarbons like Spiro-DPVBi or Spiro-TAD. Because of the scope of this Article, to review OLED related degradation mechanism, the broad range of absorber materials (known from organic solar cells) is only represented by the C60 molecule, which may be used as an electron transporting material as well. Depending on the charge charier character and their HOMO and LUMO values, the different materials from the class of nitrogencontaining aromatics are used as electron transporting materials (e.g., CPB) or as hole transporting materials (as well as electron blocking materials). Some HTLs discussed in this Review are mMTDATA, α-NPB, TAPC, TPD, and TCTA. m-MTDATA was developed on the basis of the starburst concept, which was investigated together nearly parallel to the spiro-concept to create steric big molecules with appropriate charge transport character as well as a high TG. Another group of relative stable compounds are the metal phthalocyanines and porphyrins, which may act as HTL (CuPc) or emitter (PtOEP). The two materials CPB and TCTA may be also discussed as derivatives of the carbazoles, particularly because their reactivity during OLED aging is known to be related to this structural moiety. The group of electron transporting materials may be represented by the above-mentioned carbon derivative C60, metal chelates (like BAlq or Alq3), phenanthrolines (e.g., BPhen), the imidazole derivative TPBi, and many other N- and O-containing heterocycles. Also, a broad spectrum of emitting molecules is known. Efficient fluorescent emitters of various

chemical classes, aminostilbenes, aminoantracenes, coumarins, perylenes, chrysenes, quinacridones, etc., are widely used. Typical phosphorescent emitters are represented by the Ir and Pt organometallics, such as Ir(ppy)3 (green emitting), FIrpic (light blue), and Ir(MDQ)2(acac) (red), as we will see in the following.

2. STATE-OF-THE-ART ANALYTICAL TECHNIQUES FOR THIN FILM ORGANIC SEMICONDUCTORS In the following, we will discuss the different physical and chemical analytical techniques, which are essential to evaluate degradation and failure pathways within organic semiconductor devices. This part will give a broad overview of useful analytical methods for organic semiconductor-based single layers and more complex stacks present in practical devices. Within these structures, a wide range of physical and chemical characteristics have to be covered. To investigate the above-mentioned mechanisms and reasons for device degradation, a large number of topics, like diffusion/migration, chemical or morphological changes, charge accumulation, and many more, have to be considered in detail. During the last decades, a broad selection of different analytical techniques was used to shed light on these mechanisms. Some of the techniques are used more or less successfully for multiple areas of interest. The analytical tools and methods can be categorized in different ways. One is to group them according to their electrical, optical, chemical, or mechanical response. On the other hand, the different methods may be sensitive to different parts of a device: the surface, interfaces, or the bulk. A list of the main analytical techniques known to be effective in OLED studies is provided in Table 1, along with the individual strengths and limitations of each technique. I


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2.1. Electrical Techniques and Methods

breakthrough of some devices at the glass transition temperature (TG) could be investigated.132 Other optical methods are classical optical microscopy, taking micrographs, or even more complex photoluminescence microscopy or electroluminescence microscopy, or even together with a scanning near-field optical/ atomic-force microscope (SNOM-AFM) to investigate, for example, the growth of dark spot areas.164,165

The regular I−V (current−voltage) characteristic of OLEDs carries very substantial information about injection, transport, and recombination processes in a multilayer device structure. Although it is typically challenging to distinguish contributions of specific layers and interfaces, comparison of devices with systematically varied layer thicknesses can be used to effectively interpret shifts in I−V characteristic due to device aging.21 This approach, which was referred to as “electric field profiling”, essentially provides information about electric fields as well as mobile or fixed charge densities in operating devices. Alternatively, the electric fields in specific layers can be determined by Stark effect spectroscopy,4 and the mobile charge densities by charge modulation spectroscopy.140,141 Thermally stimulated current spectroscopy provides additional information about trap levels of mobile charges in OLEDs.142 Although these techniques are unquestionably powerful tools to study the inner workings of OLED devices, they have not been significantly used in OLED degradation research so far. Another technique to study trapping and transport in OLEDs is impedance spectroscopy, which provides certain information about charge carrier traps inside the layers and injection behavior at different interfaces.143−149 In this method, a DC voltage is superimposed with a small AC component, and a phase shift and amplitude of current are monitored during the measurement. With adequate equivalent circuit models, information about conduction and trapping in some layers and interfaces can be extracted. This technique is essential when it is required to evaluate devices at significant current densities. Similar information can be obtained in voltammetric measurements (also known as oscillographic method and DCM, displacement current measurement).150−153 Here, the applied voltage is ramped with a constant rate, and the resulting current can be monitored. At a certain scan rate, the capacitive component dominates the current, and therefore the observed transient current is proportional to the capacitance.154 In some cases, this capacitive current may undergo one or two voltage- and ramp-rate-dependent transitions between distinct layer-related capacitances.19,153,155,490 These transitions provide information about fixed charge and trap densities as well as injection barriers of the investigated devices. Unlike the impedance spectroscopy, the voltammetric technique is quite effective whenever a transient response (for example, an irreversible trapping) is of interest. On the other hand, this technique is normally limited to low current or the off-state of the OLEDs.

2.3. Surface Analyzing Methods

Investigating the surface of a device or a thin film can provide insight into different aspects of degradation. For instance, the atomic force microscopy (AFM) is commonly used to evaluate the roughness, roughness change, or even the degree of crystallization of a material (mainly single layers), and to interpret morphology-related lifetime aspects.165−168,239 Here, issues such as the domelike growth of the dark spots, respectively, bubbles in the electrode, are of great interest. Besides this, the AFM can be applied to investigate the effectiveness of various surface treatments and the deposition characteristics of organic materials.169 Less common is the usage of the scanning tunneling microscopy (STM) to evaluate the morphology as well as the molecular orientation with direct relation to an organic semiconductor degradation behavior.170 For a more detailed surface analysis, additional techniques can to be used, for example, X-ray (XPS) or ultraviolet (UPS) photoelectron spectroscopy, where chemical changes on the surface of the semiconductor can be observed.171 UPS172,173 and XPS174−179 are standard methods for surface and interface evaluation. Furthermore, it is possible to use these methods in a spatially resolved manner as scanning photoelectron microscopy (SPEM).180,181 For instance, with this method, it was possible to investigate chemical processes within “dark spots” or the delamination of the cathode. Gardonio et al. examined the delamination process caused by local overheating, and found an additional carbonation of the organic material.177 2.4. Depth Profiling Techniques

Various depth profiling techniques have proven quite useful in OLED investigations. Typically, the OLED materials are continuously removed by accelerated ions. With the dynamic XPS depth profiling technique, single elements can be individually detected by the chemical shift of their binding energy, resolved in the depth of the organic device.182,183 This method allows one to detect chemical changes on the bonding conditions (e.g., like oxidation of metals), a quantitative evaluation of materials in separate layers, and therefore to identify diffusion and migration mechanisms.184−186 Similar to the dynamic XPS, the secondary ion mass spectrometry (SIMS) depth profiling is used to characterize elemental distributions within the layer stack of semiconductor devices.187−192 Contrary to the dynamic XPS, the SIMS is less effective as a quantitative technique. The difference is due to the detection mechanisms (generation of the detectable particles) of both methods. While a defined cross section for individual atoms is fixed and the corresponding material-specific electrons can be detected in the XPS, the ionization of atoms and molecules in SIMS is highly material-, matrix-, and environmentally dependent.221 SIMS exhibits a high dynamic range (up to 1:107) and high sensitivity to specific elements, which was utilized for the investigation of Li diffusion.193,253 With the possibility to measure in positive and negative detection mode, different charged chemical species are detectable. As mentioned before for the XPS technique, SIMS

2.2. Optical Methods

With the help of the optical methods infrared (IR)156−160 and Raman27 spectroscopy, which can be used in transmission and reflection mode, specific changes in the molecular structure of the used materials are detectable. In a similar way, the “polarization-modulation reflection-absorption spectroscopy” (may be used in the UV range as well), also known as ellipsometry, can be used to determine optical characteristics of organic semiconductor layers.161,162 By modeling the experimental data, the optical and electrical characteristics (doping concentration) as well as the layer thicknesses of a given stack can be determined.161 Because of the complexity of the modeling and due to the use of IR-nontransparent metal electrodes, this method has not been adapted for complete devices thus far. A commonly used tool is imaging with an IR camera, measuring the temperature at certain driving conditions with the possibility of spatial resolution.163 With this method, the J


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can be even used as a surface analysis tool,194 and is believed to have a high lateral resolution.195 Another depth profiling method is Auger electron spectroscopy (AES).46,164,196 Other techniques like the Rutherford back scattering (RBS)197,198 or the radiotracer measurements199 are less commonly used for depth profiling in organic devices. A completely different form of depth profiling is scanning of the device cross section with some surface-specific methods such as the scanning electron microscopy (SEM) or the scanning ion microscopy (SIM), where the contrast of the image may provide information about diffusion or migration processes.196,200

retention peak of some materials, sometimes accompanied by a pronounced signal tailing, as is known for BPhen.23,208 Nevertheless, such conditions can be satisfied for a wide range of OLED materials like α-NPD or TAPC.23 The main advantages of the technique are the accuracy and reliability of quantification of both the loss of degrading materials as well as the accumulation of the detectable degradation products. Other chromatographic techniques may only be suitable for individual solutions. For instance, gas chromatography (GC) is limited to molecules with realtively low molecular weights. In contrast, the gel permeation chromatography (GPC) was successfully used to detect high-molecular weight products of α-NPD and TAPC containing OLEDs.23 Nonetheless, the overall wide range of available chemical analytical methods can be used for specific questions of device degradation. For example, one may use an online mass spectrometer, called a differential electrochemical mass spectrometer (DEMS), to evaluate the gas evolution during some processing or even during device operation.165 Using the nuclear magnetic resonance (NMR) technique (material can be collected on the outlet of the HPLC), the chemical structure of unknown organic materials can be evaluated, if enough material is available.23,209 Another useful method is the electron paramagnetic resonance (EPR), where the amount of unpaired electrons can be measured,210−212 even in a working device. This technique was successfully used by Pawlik and co-workers.209 Using EPR, it is possible to study structural changes in emitting molecules (like FIrpic or Ir(ppy)3) during UV excitation.213

2.5. Chemical Analysis Techniques

One of the main challenges of device investigations is an effective analysis of chemical composition of OLEDs. In the past few years, only a few techniques were used, which are suitable to provide chemical information. One of the most powerful methods for trace analysis is the matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS),201,202 which is mainly used in the areas of macro- and biochemistry, where long polymer chains as well as proteins are in the focus of investigation.203,204 MALDI was first used in 2006 by Soltzberg et al. to examine degradation of a Ru complex in a light-emitting electrochemical cell.27 For this experiment, the entire device had to be dissolved or mechanically removed from the substrate. Afterward, the material had to be mixed with a suitable matrix material and applied on a special conductive target, which is mounted in the MALDI setup. Because of the small quantities of the materials present in the device, the dissolving and mixing process is a significant disadvantage, as impurities can be introduced. Nevertheless, using this method to investigate the material of interest in the absence of external matrix (which is often called a laser ionization/desorption (LDI) process) provides a very effective analysis technique for organic layers205,206 or entire devices24−26 without contamination. Here, the investigation of devices with24 or without25 a capping electrode is possible. Because of the reaction and ionization behavior within the LDI, the analyzing conditions have to be chosen very carefully. For example, it is very important to avoid light-induced reactions within the layers.29,206 Using products of laser-induced photochemical reactions as a clue, one can get some insight about operationrelated chemical reactions as well as make device lifetime estimates.29,206 It is noteworthy that chemical degradation and analysis are implemented in one step in this method. Another commonly used tool is high performance liquid chromatography (HPLC) coupled with various detectors, for example, UV absorption or MS.22,28,207 Here, the dissolution/ extraction of the device components is an essential step. Arguably, the main challenge of this technique is limited sensitivity due to small absolute quantities of materials per unit of extractable area (typically, micrograms per cm2 of device area for minor device components and, potentially, orders of magnitude less for degradation products) in a typical HPLC experiment. Furthermore, some chemical products of device degradation are expected to be quite reactive and likely incompatible with solvent extraction and chromatography. Even the nominally stable device components such as metal chelates might be incompatible with chromatographic analysis because of the hydrolysis during extraction or during separation on the chromatographic column. Additionally, some challenges exist in the simultaneous investigation of some electron and hole transporting materials or even avoiding the broadening of the

3. ABRUPT FAILURE MODES AND THEIR VISUAL APPEARANCE After the overview of possible degradation mechanisms and the analytical techniques to study them, we will proceed to discuss the device aging phenomenon and the associated mechanisms in detail. A brief discussion of the two degradation processes, “catastrophic failure” and “dark spot growth”, will be provided in sections 3.1 and 3.2, respectively.The reason that we shortly cover the dark-spot problem, which has been largely solved using glass−glass encapsulation and chemical scavengers of oxygen and moisture (“getters”), is the current challenge of the thin film encapsulation for devices on flexible substrates and for top emitting stacks (see section 4.2).77,214,215 Other manifestations of device degradation are the short- and long-term decrease of the luminance as well as a possible color shift of the device. Such a color shift may or may not appear uniformly over the entire luminescent area. 3.1. Catastrophic Failure

Perhaps the most undesirable failure mode is the total breakdown of a device, which can be caused in most cases by the formation of electrical short circuits. The main reasons for the shorts are imperfections of the substrate surface such as particles,216 rough surfaces, and local heating effects during device operation.14,217,218 Such heating effects may cause morphological changes in the device structure as well. This may lead to contact formation between the electrodes,219,220 a destruction of the layer stack system by interdiffusion,221 or a delamination of the cathode material. Such morphological changes may disable the entire OLED. The delamination of electrode materials is mainly observed in combination with dark spot degradation and the formation of bubbles or domelike structures due to gas evolution as a direct result of material decomposition. This may occur due to electrochemical reactions, which may be thermally assisted. K


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Little work has been reported on the subject of the fine details of “catastrophic failure”. This is because the processing conditions are widely improved and this kind of a degradation pathway can be prevented in most cases. Nevertheless, investigations of polymeric OLEDs have shown fluctuations in voltage and luminance measurements right before a device failed. As mentioned above, these fluctuations are believed to be a direct result of bubble formation and corresponding dark spot growth, which will result in cathode delamination and a further device breakdown.222 Another mechanism proposed to lead to catastrophic failures is electromigration of the cathode metal during device operation resulting in the formation of thin metal filaments extending into organic layers or even reaching the opposite electrode.223,224 Some features of this mechanism make it difficult to distinguish it from other failure modes: strong dependence on current density and temperature, role of pinhole defects in initiation of filament growth, and self-healing behavior presumably caused by periodic formation and destruction of filaments. Although no new studies have been reported in the past decade, it is possible that electromigration is still an important contributor to both catastrophic failures and to leakage currents (e.g., as observed below turn-on voltage) even in state-of-the-art OLED devices.

spot growth are identified as moisture on the substrate surface as well as pinhole defects in the capping material, where oxygen and water can easily penetrate.41,130,216 The grow rate of such particle-induced black spots is particle size dependent and suggests that the larger are the particles (and respective pinholes), the larger is the grow rate of the nonemissive area.234 Kawaharada et al. distinguished between seven types of structures, which possibly initiate the dark spot growth: grain cluster and stain, protrusion, gap in cathode, broken cathode (occur mainly after relatively long-term operations), pinholes, and particles.200 Therefore, a clear difference in the behavior of a dark spot, generated by dust, particles or pinholes, and the dark area, which appears below generated bubbles, is distinguishable.130,229 The results suggest that substrate cleanness is highly important.235 Bubbles and dome-like structures are formed due to the mentioned oxidative processes, mainly with water,41,130,216 or due to electrochemical decomposition of the underlying organic compounds, which is known from polymer-based OLEDs as well.41,180,181,187 Because of such reactions, decomposition gases are generated that cause the observed bubbles.180,181 The usage of cathodes with a low ionization potential will even cause a faster “dark spot degradation” due to the higher affinity to oxygen.15,236 Besides these explanations for dark spot growth, a few other theories are worth noting: For instance, an interdiffusion of materials, made possible by internal Joule heating, was proposed, which should result in exciton quenching sites and parasitic current pathways. Such an interdiffusion may occur at the dark spot margin due to the high currents, which will lead to a further spot growth.164 A critical argument against this mechanism is provided by an investigation at elevated temperature, where a polymer-based OLED was heated to 450 °C (not IV-driven during the heating step), which resulted in no significant change in their IV-characteristics.237 Unfortunately, the investigated device consisted only of a single layer, which makes the conclusion about interdiffusion phenomena tenuous. In case of classic structures (HTL|Alq3 - devices), the crystallization of the HTL238 as well as the Alq3 could be considered as well.239−241 A very important contribution to this topic has been given by Lee et al., who used a multilayer stack, where the interdiffusion of the different layers was monitored by X-ray reflection at different elevated temperatures.242 This topic of temperature-dependent morphological changes will be further discussed in section 4.6.1. It is noteworthy that the main results on interdiffusion and crystallization of materials at elevated temperature are mainly based on experiments with single or multiple organic layers without capping electrodes.15,164,239 There are no reported results on regular OLEDs. Besides the above-mentioned interdiffusion of organic materials, another proposed mechanism of dark spot growth is the electro-migration of cathode metals into the device. Such a mechanism would lead to parasitic current pathways as well as to exciton quenching.200,223 Nevertheless, there are some indications that during the melting process and the decomposition of the organic materials, metallic In and Sn from the ITO will be formed.231 Such metal release may also result from electrochemical corrosion on ITO or the cathode, where the polymer PPV (poly(p-phenylenevinylene)) serves as a weak electrolyte.237 As a third mechanism and independent from oxidative reactions of the cathode material, oxygen and water might penetrate through existing pinholes and cause formation of charge carrier traps and nonradiative recombination centers

3.2. “Dark Spot” Degradation

An excellent overview on this topic was published by Turak, who mainly focused on the reasons for dark spot degradation in polymer-based OLEDs.225 “Dark spots” are generally defined as small, nonemissive regions of a (nominally) active area of OLEDs.14,16,136,226,227 Such regions may be already created during device production228 and will frequently grow during device aging and/or storage.229 Dark spot growth may occur during device storage under nitrogen15 and ambient conditions.45 Similar to dark spots, so-called “gray spots” have also been reported, which lose their brightness gradually, yet faster than the surrounding device area.227 The reason and the growth mechanism for dark spot degradation appear to be mostly known: It was found that usually the luminescent material within the dark spot area is not affected by any discernible degradation mechanism. This was demonstrated by photoluminescence measurements (PL) on degraded devices where the capping electrode was removed. The entire area, dark spots as well as unaffected parts, show the same PL brightness,226 leading to the conclusion that a delamination of the cathode material is responsible for the investigated dark spots.227,230,231 Cao et al. further demonstrated that, by removing the original cathode from an aged device and putting a fresh one on the device, the previously visible dark spot defects are eliminated.232 On the edges of such defects, which may also appear as bubbles or domelike structures, higher current densities and therefore higher brightness are visible.41,130,180,181,233 The resulting temperature increase at these edges can lead to a faster degradation and therefore to further delamination of the cathode.157,180,181,233 Because of the high temperature rise, the cathode material can degrade as well, resulting in a device short circuit.136,187,188 Infrared (IR) measurements suggest that some localized electrical shorts reach temperatures higher than 200 °C.41,42 The deformation of the cathode is afterward visible as bubbles, faulting, or ring like structures.230 Using temperature-dependent investigations, an activation energy for the dark spot growth of nearly 300 meV was found.130 In the case of a bubble formation, the growth of such structures is limited, leading to a bursting of bubbles at a critical dark spot area.130 The reasons for the dark L


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inside the OLED as well.243 As we will mention in section 4.2, such a penetration can be effectively prevented by using appropriate encapsulation techniques,136,244 like getter materials inside the glass lids.72 For flexible substrates, an efficient encapsulation is an absolute must.245,246 A fourth known, but relatively minor, discussion point is the influence of excitation (caused by carrier recombination or UV light) on the anode interface. It has been proposed that UV light degrades this interface, reducing conductivity of the device. If the UV-beam is restricted to a certain area, it will therefore appear dark or gray, depending on the degree of interface damage. This mechanism may be useful for patterninging large area devices as well to heal localized dark spots by illuminating special areas to separate them from the rest of the device.247−249

4. EXTERNAL PROCESSES OF DEGRADATION When classifying causes and mechanisms of degradation in the two major categories, we have to consider processes and conditions, which affect the OLED from the “outside” and “inside” of a device. External factors are usually easy to identify: Oxygen as well as water can limit the lifetime of OLEDs. Light- or temperature-induced changes are commonly observed. Some of these effects are well understood with respect to keeping them under control. However, aside from morphology changes induced by exceeding the glass transition temperature, which may lead to complete device breakdown, the effects of temperature on degradation rate are still poorly understood. Also, the impact of light on an OLED has not been thoroughly investigated. Because light might initiate chemical reactions, the topic of light-induced mechanisms will be discussed in section 5.

Figure 8. Dependence of lifetime on vacuum chamber base pressure and residue gas inside the process equipment. Lifetime drops with increased water content in the chamber. Figure created with data from Ikeda et al.133

atoms with the underlying organic, which may be the case in oxygen-containing organic layers.252 Nevertheless, attaching a mass spectrometer to a metal deposition chamber, one detects a decrease of the water content and a corresponding increase in hydrogen partial pressure. This is due to the reaction of metal with water by creating suitable (hydr)oxides and hydrogen.182,253,254 It is suggested that the water content has a stronger influence on the degradation reaction than the remaining oxygen255 in terms of quenching and in terms of an electrochemical reaction between water and (in the case of the publication of Ikeda et al.) Alq3. It has been suggested that the reaction products with water generate fluorescence quenchers and electron traps, which lead to an increase of the driving voltage.133,256 A possible doping effect of water and oxygen, as known from pentacene-based devices, has also to be taken into account. In the next section, we will discuss the direct influence of external oxygen and water on organic devices. Another critical process parameter is the deposition of metal oxide transparent electrodes, for example, for top emissive samples. One commonly used deposition method is sputtering. It is widely known that sputtering, which is mainly used for inverted and/or top emitting OLEDs, causes major damage due to the intense particle and UV irradiation.126,127 Even at low energy, irradiation damage can be observed.125 Damage from particles may be avoidable by using suitable protecting layers,126 but the protection from the intense UV irradiation is not achieved so far.127 Kwong et al. discussed an example where they compared a conventional OLED with a transparent OLED (sputtered cathodes), where the conventional OLED reveals a higher lifetime than the transparent counterpart.77 The UV irradiation might even cause photochemical reactions like fragmentation, dimerization, and other reactions within an OLED structure.127 As mentioned before, the UV radiation may damage the anode interface as well. These degradation processes are little understood and warrant further work.

4.1. Influence of the Process Parameters

Process parameters during device fabrication have an often underestimated effect on the device lifetime. In this part of the Review, we will briefly discuss the influence of the process pressure during the OLED fabrication. The cleanness of the substrate and therefore the content of imperfections on its surface have a major effect on pinhole density in a device structure. Other process parameters like the deposition rate have an influence on the device characteristics and the morphology of organic layers.250,251 Additionally, substrate heating (during deposition) may affect the device lifetime as well. Here, mainly a positive influence was observed. Currently, the mechanism of annealing improvement is not fully understood, but may be related to an in situ water and oxygen desorption during the heating step.235 Ikeda et al. showed that, together with the internal water and oxygen content, the process pressure itself does not change the initial device performance (current−voltage−luminance−characteristics).133 In contrast, the device lifetime drops with increased process pressure (see Figure 8). Ikeda et al. argued that at a given base pressure of 5 × 10−7 Torr and at a evaporation rate for Alq3 of 1 Å/s, the ratio of water molecules (2.38 × 1014 molecules per second and cm2 hitting the substrate surface) to Alq3 (1.7 × 1013 molecules per second and cm2) is nearly 10 to 1. Scholz et al. demonstrated that the remaining water and oxygen content within the process chamber has a major influence on the incorporated oxygen, particularly on the organic/metal interface, where the evaporated metal reacts with the remaining oxidative gases during the evaporation processes and oxidized components will therefore mainly locate on the organic/metal interface.182 Similarly, some authors assumed that the observed oxygen at the organic/metal interface originates from reactions of “hot” metal

4.2. Influence of External Water and Oxygen

A relatively well-known external degradation factor is the presence of water and oxygen. Both contaminants have been proposed to initiate various processes in OLEDs: (i) chemical reactions with electropositive metal cathode resulting in oxidation (and release of elemental hydrogen in case of water), (ii) hydrolysis of chelate molecules followed by various reactions M


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Figure 9. Overview about (a) the water vapor and (b) the oxygen permeability needed for different optoelectronic devices. Figure created with data from refs 265 and 266.

of hydrolytic products,257,258 (iii) oxidation of organic materials, including photo- and electrochemical reactions,259,260 (iv) charge-transfer doping resulting in formation of filled traps and/or free charge carriers, and (v) morphological changes such as crystallization induced by contaminants.158,261 Studying the effects of exposing various interfaces of multilayer devices to a separate vacuum chamber with controlled water vapor pressure led Yamamoto et al. to propose that exposing certain interfaces to water is particular, reducing device stability.262 It is interesting but not surprising that the HTL|EML interface, where the emission zone is typically located, showed the highest impact on device lifetime. However, even though the reported results are convincing with respect to identifying the interface that has the strongest influence on lifetime, it is not clear if the effect is due to water vapor or some other impurity, which is unintentionally present in the separate chamber used for water exposure. To understand the complexity of water and oxygen effects, it is necessary to discuss device encapsulation first. Despite the intensive work on flexible OLED displays263 during the last years, conventional encapsulation techniques are still not effective and show either high moisture permeation of the plastic substrate or are rigid as, for example, glass lids, known from Burrows et al., who showed first the urgent need for device sealing.136 To obtain reasonable lifetime values, water vapor permeability rates (WVTR) better than 5 × 10−6 g/m2/d and oxygen transmission rates (OTR) lower than 10−3 cm3atm/m2/d are an absolute need for OLEDs as well as other organic optoelectronic devices like organic solar cells (OSCs).77,215,264 Inorganic TFTs, for example, only need sealing conditions of about 10−3−10−1 g/m2/d WVTR, whereas organic TFTs are expected to be more sensitive.265 The transmission values required for OLEDs are several orders of magnitude lower than what is reached with conventional

sealing techniques, used, for example, for liquid crystal displays and food packages. For a comparison of some WVTR and OTR requirements, see Figure 9.265 It has already been proven that the permeation rates of water and oxygen correlate with the defect densities of the barrier materials,267 because the water vapor and oxygen typically enter the electrode materials at these defects at the edges of a cathode.130 It is therefore essential to minimize structural defects like pinholes, grain boundaries, or micro cracks.268 Up to now, it is quite difficult to achieve WVTRs less than 10−2 g/m2/d with inorganic barriers deposited near room temperature. For thin single inorganic sealing layers, the density of pinholes and defects is too high.77 To overcome these problems, it is possible to use thicker layers,266,267 but much more effective is the use of techniques that completely overgrow/cover existing defects,269 for example, by using multiple layers of materials with different properties.265,269−273 Schaer et al. presented several mechanisms for the penetration of water vapor and oxygen.130 Water vapor penetration was reported to be orders of magnitude more destructive than oxygen penetration. In this study, dark spot formation related to the presence of water was taking place only during device operation. This led the authors to propose that water, which penetrates into the device through defects such as pinholes, participates in electrochemical reactions at the organic/cathode interface. The electrochemical reactions are driven by electric current and, ultimately, result in evolution of hydrogen gas. The pressure increase from released hydrogen lifts the cathode and creates bubbles or domelike structures, resulting in a delamination of the metal layer. However, the authors do not present any argument to support the notion that hydrogen can accumulate and build up to any significant pressure rather than leave the device. Clearly, at least one water molecule needs to migrate into the “bubble” N


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4.3. Impurity-Induced Degradation

region per hydrogen molecule to be released. Considering that hydrogen molecules are far more diffusive than water molecules, no significant pressure buildup can be expected, and the domelike feature must have a different origin. Furthermore, the observation that the dark spot growth stops immediately after switching off the current suggests that the authors report results obtained with atypical OLED devices. From the classic study of dark spot growth,226 verified by many commercial OLED studies, it is well-known that the dark spot growth typically does not stop when the device is not driven and inadequately protected from water vapor and oxygen. Nevertheless, at high humidity levels, the cathodes oxidize. This was shown by Aziz et al. for Mg:Ag cathodes on Alq3, where the formation of Mg(OH)2 was proven by IR reflection spectra. A growth of elevated cathode bubbles is seen, which is believed to be a galvanic corrosion process only seen in the presence of Mg, and should therefore result from hydrogen production accompanied by the Mg(OH)2 formation.159 In addition to reactions of electropositive cathode metal, other device components can be associated with dark spot growth. Inspecting the device cross section, for example, by the 18O labeling technique of the penetrating water, one can see that the reactions of water will occur not only on the cathode and the cathode/organic interface, but even in the organic itself.254 Considering the oxygen-induced dark spots, one recognizes that the corresponding growth rates are orders of magnitude smaller than for the water-induced process. Assuming Arrhenius’ law, a thermal activation energy of 0.3 eV for the dark spot growth in the presence of oxygen was measured.130 Besides the dark spot formation, there are some reports that oxygen influences the device performance in additional ways. A form of “doping” by oxygen results in an increased conductivity,274 which may be reversible in some cases, for example, by purging the device with nitrogen.275−278 The enhanced conductivity275,279 and the quenching process of the device may be a result of the formation of a (weak) charge transfer state between the oxygen and the organic compound.182,274,276 As expected, exposing an OLED to ambient conditions (oxygen and water) causes a change in the device performance as well.275,279,280 Kaminorz et al. showed on a polymeric OLED that exposure to air may not cause a dramatic change in the I−V curve (a higher current is observed), but has a dramatic influence in the luminescence output.276 Light output was quenched by 2 orders of magnitude under ambient conditions as compared to devices stored under nitrogen. Experiments under oxygen exposure in addition to UV-light illumination show that the oxygen-containing environment will cause a photoinduced degradation reaction during the UV excitation, where similar experiments show that the exposure with air (without UV light) causes no change in a measured device PL response.276 It was found that excited OLED materials readily undergo photo-oxidation, which leads to excitonquenching products.223,281 A corresponding discussion of the photoinduced reaction between oxygen and different polymers is provided in section 5.7. Furthermore, investigations on Alq3 show that oxygen causes electron traps inside the OLED,282 which was shown before for exposed Alq3-based devices to ambient conditions.283 It is also known that, besides oxygen,233 water will produce fluorescence quenching as well.258 Both components, oxygen and water, are known to react specifically for different materials as well, which will be discussed for Alq3 (section 6.1.1.2), polymer chains (section 5.7), and a special Rucomplex (section 6.1.1.4) separately.

Impurities in organic materials are believed to have a major effect on device performance and lifetime.41,154,284 Besides some studies about powder, single layers, and devices, based on Alq3,133,285−288 only few publications describe systematic studies of the impact of impurities and moisture.287 For instance, Drechsel et al. showed that the concentration of impurities in organic materials has a significant influence on the performance of organic semiconductor devices like the efficiencies of OSCs and OLEDs.205 Tietze et al. show that doping efficiency is correlated with material purity of differently sublimed host materials.289 With the help of different sublimation techniques (for a review, see ref 290) or of optimized process parameters,288 highly pure materials can be obtained. Impurities may originate from the substrate as well. Liao et al. observed a strong desorption of impurities and moisture during annealing of glass|ITO-substrate at 200 °C.41 It is widely assumed that impurities may act as nonradiative recombination centers284 or will act as catalyzing agents for dissociation processes of metal organic emitters, already shown on Cu(110) surfaces.291 Using highly pure materials292 and ultrahigh vacuum conditions, the lifetime of OLEDs can be significantly increased.133 For instance, Xia et al. proved the importance of the use of highly pure materials by tripling the lifetime of a device by changing the dopant (emitter) purity from 99.8% to 100%, based on a HPLC-analysis.20 Additional to the intrinsic impurities of the organics, it is assumed that other materials such as dopants, metals from the electrodes, and possible dissociation products of organic materials will lead to an initial voltage rise accompanied by an efficiency drop.297,303,306,307 Such a voltage rise is partially reversible, according to the “mobile ion impurity model”. For example, Aziz et al. note that the reorientation of dipoles (see section 5.3) may have the same effects like the degradation effect proposed by the mobile ion impurity model.14,293,297 4.4. Influence of the Driving Scheme on a Lifetime Experiment

To discuss the lifetime behavior of OLEDs and the “mobile ion impurity model”, one has to consider the aging conditions of the devices. During the last two decades, the influence of the different driving schemes (alternating/pulsed current (AC) or direct current (DC)) was studied. Although some detailed studies about the reversibility of aging processes exist, we have to face a lack of systematic studies concerning these effects across a variety of devices, ranging from single layers to multiple layered p−i−n structures (p−i−n stands for p-doped−intrinsic−n-doped materials; for a recent review about doping, see Walzer et al.294) This large variety of investigated structures is likely the reason that contradicting statements exist for the use of different driving schemes. Whereas for small molecule OLEDs the positive influence of the AC driving scheme is known,30,295−300 polymerbased OLEDs do not show an improved lifetime when using the AC mode instead of a DC-driven scheme.301,302 One exception, however, was reported, the investigation of a coumarin 6 doped polymer (poly(N-vinylcarbazole))-based OLED with a Alq3|Al cathode.303,304 The improvement of the lifetime due to AC mode is an effect of the reversibility of device degradation, either spontaneous45 or induced by reverse bias.55,305−307 For small molecule OLEDs, the accumulation of charges at internal organic/organic interfaces is believed to be a major contributor to the degradation mechanism.17,18,308 Using an AC waveform may reduce such a mechanism due to the release of O


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accumulated charges induced by the reverse-bias component.30 Other reasons, like the electric-field induced migration of ions, might also play some role in device degradation.302 Concerning the use of AC, Van Slyke et al. were able to show that the reversibility of the degradation is independent of AC frequencies and AC waveform.30 In contrast to this, a detailed study was done by Tsujioka et al. on the waveform of AC and pulsed mode in comparison to DC-driven OLEDs.295 They found that OLEDs driven at the same luminance exhibit the longest lifetime behavior when driven at AC. The second longest lifetime is reported for a specific pulsed mode, and the OLEDs driven at DC exhibit the worst results. A detailed study of the reversibility of OLEDs was done by Langlois et al. and Shen et al.305−307 where they varied the forward/backward rate. By changing the ratio after 100 h of operation, the reversible behavior of the driving voltage was detectable. The authors were able to fit the voltage dependence of their devices in forward and backward voltage directions with an assumed electric field-dependent mobile ion migration model. The most remarkable observation of this report is the “healing process”, where the voltage increase was nearly completely reversible after applying a reverse bias to the aged device (Figure 10).

been reported so far about the influence of the AC mode in comparison to the DC mode on lifetime p−i−n structured OLEDs, and it is hard to link this behavior to (from today’s perspective) known mechanisms, such as charge accumulation effects or chemical reactions. It is also noteworthy that there is still a remarkably contrasting study by Iwakuma et al., where pulse-driven OLEDs exhibited a quarter of the lifetime of a DC-driven counterpart.74 4.5. Voltage Increase

Besides the luminance drop at constant current, a significant voltage increase (due to an electrical resistance) frequently accompanies the degradation behavior of OLEDs.306,307,309 Usually, voltage rise is attributed to an increased injection barrier at some interface and/or a decrease in conductivity of some transport layer, for example, because of accumulation of traps.23 Multiple interfaces and transport layers may contribute to this effect. A common interface, the ITO/organic, is well-known to contribute to the voltage rise. Experiments on polymer OLEDs illuminated by light revealed that the degradation of such a device is wavelength dependent.310 It was shown that while the polymer absorbs light differently at different wavelengths, the polymer itself does not degrade. As stated before, similar effects are seen for small molecule-based ITO/organic samples.249 It can be speculated that the oxidation reaction occurs at the ITO/organic interface between excited organic molecules and the oxygen-rich surface of ITO. With the help of X-ray photoelectron spectroscopy (XPS) (before and after the excitation of ITO with the relative intense UV light of an ultraviolet photoelectron spectroscopy (UPS) setup), a decrease of the ITO work function was observed. This phenomenon is only visible for chemically cleaned or oxygen plasma treated ITO surfaces, whereas for freshly sputtered ITO this effect is not present.311 Additionally, it was possible to avoid such ITO activation by the passivation of ITO surfaces by Au or pentacene.310 To improve the hole injection into the organic layer and to stabilize the organic/anode interface, properties of ITO can be modified,312,313 mainly by increasing its work function.339 For this purpose, oxygen plasma368 or UV ozone treatment is often used.314 Such surface treatments appear to have a stronger influence on single layer OLEDs, as known from polymer-based devices, than on multilayer stacks. The oxygen plasma treatment additionally leads to a lower leakage current and therefore enhances the efficiency of a device. Furthermore, such an O2treated device (ITO-surface) shows a significant improvement in lifetime performance.368 Using XPS-depth profiling, Kahn and co-workers showed that the oxygen treatment results in a homogeneous allocation and oxidative binding conditions for In, Sn, and O in the full ITO layer.368 Additionally, the use of hole injection layers is well-known to decrease the driving voltage. Sections 5.1 and 5.5 provide some discussion of this approach, where different materials are used to enhance the hole injection properties. For improving electron injection into the device, several materials were found to be suitable. In general, alkaline and earth alkaline metals as well as their salts are known to lower the energy barrier for electron injection into the organic effectively (e.g., LiF,315 Mg316). As an additional effect, the usage of such thin injection layers (e.g., Ba,232 CsF317) leads to an improved lifetime of the corresponding devices. Similarly, doped transport layers reduce injection barriers resulting in ohmic contacts to electrode. Furthermore, doped

Figure 10. Voltage change measurements (points) and calculated (lines) data set for a diode operating in forward (first 100 h) and in backward (90/10 cycle to collect light during the 10% in forward direction) directions. Nicely seen is the recovery of the voltage increase during the backward cycle. The data are fitted to an ion mobility model. Reprinted with permission from ref 306. Copyright 2000 Elsevier Limited.

In contrast, the simultaneously monitored luminance exhibits only a minor change (improved luminance) in the “reversible direction” right after switching, so a healing effect is not visible and the luminance decay continues.306,307 These results suggest that there is more than one mechanism involved in the degradation of the investigated devices. The ion mobility model itself cannot explain the incomplete recovery of the OLED performance, especially the behavior of the luminance. Furthermore, the study by Langlois and Shen did not disclose the “blue-green” device structure developed by Kodak. Little has P


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transport layers exhibit conductivities in the range of 10−3−10−5 S/cm, which corresponds to a voltage drop of only 0.001−0.1 V for a 100 nm thick layer at a current density of 100 mA/cm2.39 This results in a slightly lower power consumption of such a p− i−n device. OLEDs with doped transport layers also often show an improved lifetime.318,319 One additional reason for the voltage rise during the aging process of a device may be the oxidation of the cathode/organic interface as it is stated by Sato and Kanai.235 One way to overcome this was the use of calcium on top of a Mg:Al cathode as “sacrificial anode” creating an additional protective layer of a “getter” (scavenger of oxygen and water).320 Interestingly, calcium layer not in direct contact with underlying cathode (i.e., separated by a layer of organic dielectric) was shown to be less effective in suppressing dark spots growth. This observation led authors to propose that calcium acts as a positive element of electrochemical protection cell as opposed to a simple moisture barrier. Although beneficial effects of such bilayer cathode structure on device lifetime were not demonstrated, it is plausible that a reduction of dark spot growth will also result in less voltage rise and potentially to a longer device lifetime. Another interfacial contribution is the accumulation of traps at or near organic/organic interfaces (section 5.2), for example, between the HTL and the EML. A respective increase in immobile charge density due to filling those traps would typically result in a significant voltage rise (at constant current).

of the initial brightness or current density. Nevertheless, the effect of Joule heating is already known and scales linearly with the applied electrical power. Figure 11 shows the combination of

Figure 11. Temperature rise by increased power input into an OLED. Data from Zhou et al. as well as from Sato et al.15,42

the published data from two different groups. Both plots show the linear relationships between temperatures of the measured OLEDs and the respective applied input power.15,42 Assuming that the environment acts as effective heat sink, the temperature is indeed expected to scale linearly, but the slope will not be identical for different devices, considering the size, the stack design, the geometry, and the environment. This is noticeable in Figure 11, where two data sets exhibit slightly different slopes. It should be pointed out that the power densities used in these experiments are unusually high. Typically, OLEDs dissipate power far below 1 W/cm2 to produce 1000 cd/m2, which correspond to a very minor increase of internal temperatures relative to ambient. Joule heating is particularly problematic for larger area devices, where device edges contribute less significantly to cooling, which can lead to a faster degradation.90 To reduce Joule heating, the use of doped transport layers is effective,329 which lowers the voltage and, therefore, decreases the overall resistance of the device. Additional significant contributions to heating in practical devices such as displays and luminaires come from voltage drops on ITO|metal electric bus lines and TFT circuits.163 Using external heating techniques, one can control the reaction rate of the observed degradation.70,302,321,330 Applying the Arrhenius equation to the data of Ishii et al. (Figure 12 and the corresponding device structure: ITO|CuPc| TPTE|Alq3:MEQA|Alq3|LiF|Al) as well as to data published from Aziz et al. (ITO|α-NPD|α-NPD:Alq3:DMQ|Alq3|Mg:Ag), an activation energy for device degradation of 0.32 eV ± 0.01 eV (published from Aziz et al.: 0.27 eV) can be calculated (see Figure 13).70,321 Aziz et al. noticed that this activation energy appears similar in magnitude to the difference between the ionization potentials of the matrix Alq3 (5.6−5.7 eV)331,332 and the hole transporting material α-NPD (5.2−5.7 eV).331,333 Vamvounis et al. conducted similar experiments with three HTMs used in mixed layer with Alq3.334 They found substantial differences in both lifetimes and activation energies, with lower stability devices also showing lower activation energies. Interestingly, the lowest stability device (HTM = TAPC)

4.6. Temperature Effects

One of the major “external” factors affecting the device lifetime is the temperature. An increased temperature will not only result in a shorter device lifetime, it will even affect the initial IVL characteristics of a device. It is widely known that high temperatures will accelerate the device degradation,70,302,321,322 and will induce a device failure at a critical temperature.131,219,220,323,324 During investigation on Alq3, Cester et al. found a decrease of the electron mobility at higher temperature as well as at enhanced treating time.325 Nevertheless, it is widely accepted that the charge carrier mobility of Alq3 increases with increasing temperature.326 As discussed above, potential applications require a wide range of different environmental conditions and temperatures. Possible examples are a device in a car parked in the sun (above 60 °C) or a solar cell on a roof (above 50 °C).327 Therefore, organic devices should demonstrate a suitable temperature stability at least in the range of 70−90 °C.70 Arrhenius’ law dictates that thermally activated reactions occur faster at higher temperatures. Some chemical processes have activation energies in the order of 50 kJ/mol, which led to an empirical rule that the reaction rate doubles for every 10 K temperature rise. Other chemical reactions or other processes may have drastically different responses to temperature. Temperature increase in a device may occur because of an external heat source or internal Joule heating.15,42,328 The influence of local Joule heating was already pointed out in section 3.2, where structural defects cause darks spots, bubbles, or the local fusion of the metallic electrode. Here, we will discuss Joule heating due to the device resistance over the entire area of an OLED as well as the effect of intended external device heating. One focal point will be the discussion of the influence of the glass transition temperature (TG) on the device lifetime. Although several groups investigated the lifetime at increased temperature, no work was done so far to investigate the influence of Joule heating on the reaction rate separately from the influence Q


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change of the layer morphology as well as a phase transition of individual materials. As stated before, the temperature rise may occur from outside the device or due to the Joule heating, across the entire area,15,42,328 or even at localized defects.132,136,218 Most experiments that have been performed to investigate this subject are based on morphology-change measurements on single layer films and not on fully processed devices. The larger part of these experiments focuses on the crystallization behavior of the materials due to temperature rise.66,239,335 A minor part of the scientific contributions deals with the dewetting processes at anode interfaces. Some studies show a dewetting behavior directly after material deposition on ITO or other anode materials336 and therefore are believed to change the interface morphology,337 which can be prevented by using suitable buffer layers (e.g., MoO3)338 that enhance the adhesion of organic layers and therefore exhibit a better morphological behavior during annealing processes.339 It is reasonable to assume that morphological aspects can be neglected at moderate driving currents and only become more important at high current densities causing substantial Joule heating, or for higher environmental temperatures.14 4.6.1.1. Glass Transition Temperature. Tokito et al. examined the relationship between the TG of a particular HTL and its thermal durability, by measuring the critical temperature (TC),92,131,220,324 which is defined as the temperature where the device operation is drastically disturbed (see Figure 14a).219,340 This experiment was done on a simple bilayer structure, where the emitter layer Alq3 was used together with different HTLs (ITO|HTL (70 nm)|Alq3 (70 nm)|MgAg (180 nm)). In Figure 14 it is clearly visible that the TG of different used HTLs correlates linearly with the measurable TC of a device, suggesting that the failure mechanism is related to the phase transition of the HTL. Nevertheless, Tokito et al. could

Figure 12. Lifetime plot from Ishii et al. for different temperatures. The increased degradation due to the enhanced temperature is well visible. Reprinted with permission from ref 321. Copyright 2002 AIP Publishing LLC.

Figure 13. Arrhenius plot of temperature-dependent lifetime data from Aziz et al. and Ishii et al.;70,321 both devices show an equal activation energy of nearly 0.3 eV.

showed a nearly zero activation energy of 0.04 eV, which suggests that the rate-limiting step of degradation is an almost barrierless process controlled only by Arrhenius pre-exponential factor (approximately related to entropy of activation in transition state theory). In contrast, the most stable device (HTM = α-NPD) showed a substantial activation energy of 0.35 eV. Although the limited range of temperatures in these experiments does not allow for accurate comparison of pre-exponential factors, it is possible that all three types of the devices show similar values. Unfortunately, the overall range in activation energies (0.3 eV) is the same as the uncertainties in HOMO levels of the HTM (5.1− 5.4 eV, for α-NPD), which makes it impossible to confirm or rule out the hypothesis relating activation energy of degradation process and HOMO difference. Other authors reported that the growth mechanism of the dark spots also has an activation energy of approximately 0.3 eV. At this time, no explanation for the specific activation energy for certain degradation mechanisms is known. For comparison, the activation energy for a polymer-based OLED is found to be twice as large (0.67 ± 0.06 eV).302 It is thus not clear which degradation reaction is accelerated within the devices. 4.6.1. Morphology and the Glass Transition Temperature. Besides the accelerated internal reaction, some of the major effects that may occur at elevated temperature are the

Figure 14. Relationship between the glass transition temperature (TG) of hole transporting materials and their critical temperature (TC); displayed are the results from Tokito et al., where (a) TC of a device, composed with the HTL TPD, and (b) the dependence of TC from TG are visible. Reprinted with permission from ref 340. Copyright 1997 AIP Publishing LLC. R


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Figure 15. X-ray reflection data, stack design, and observed thicknesses of the temperature-dependent measurements, done by Lee et al.; well seen are the intermixing of Alq3 and α-NPD at an elevated temperature of 120 °C. Reprinted with permission from ref 242. Copyright 2007 Elsevier Limited.

demonstrate that stable devices, working up to temperatures of nearly 150 °C, are possible. Many other groups investigated high TG materials as well.131,219,324,341−343 Much work was done to find proper materials with high TG to increase the device durability at very high driving conditions. Some authors claim that the TG has a major influence on device stability.15,79 For this purpose, a wide range of investigations was done to obtain high TG hole transporting materials,48,324,344,345 as known from the starburst220,346 and spiro131,342,343 concept. To support the empirical work, a comprehensive study investigated the calculation of TG from the structural properties of the molecules of interest.347 It seems obvious that the device might fail at a certain temperature TC, which correlates with the lowest TG of the materials in the device.131,324 However, not all experiments support this simple theory. It has been shown that the breakdown of a device due to high temperature does not stringently depend on the lowest glass transition temperature of a device (e.g., Adachi et al. found that a wide range of materials do not show such a correlation between TG and device lifetime).48 It has been proven that even devices using low TG materials can reach high breakdown temperatures.78 Considering these investigations on high TG materials, one should keep clearly in mind that the device durability at high temperatures is not directly related to the lifetime at low temperature. 4.6.1.2. Crystallization. Crystallization of the individual materials has a major influence on layer integrity. It is known that unprotected single layer films tend to crystallize239,348−350 or to dewet from the anode substrate351 at elevated temperatures or at ambient storage conditions. Additionally, some authors proposed that such processes may have an influence on the dewetting process of the cathode layer as well.239 In fact, this suggestion is only based on crystallization observations, which are done on single layers without an applied capping material. Furthermore, some materials like α-NPD show an extremely stable behavior at ambient storage conditions even after 15 days.350 Spreitzer et al. stated that most organic semiconductors used in (small molecules) OLED applications exhibit relative low molecular weight and therefore a relatively low glass transition temperature (TG) leading to a thermodynamically unstable amorphous state of the layers. They also argued that the low TG

will lead to slow recrystallization effects when the OLED temperature comes close to the corresponding TG of the used stack materials.342 One possible way to enhance the lifetime of an OLED is indeed to use high TG materials. A significant study concerning the topic of recrystallization was done by Lee et al. where they investigated an organic stack (glass|ITO|CuPc|αNPD|Alq3|LiF) during applied annealing steps by X-ray reflection.242 The results of the experiments and the stack design are shown in Figure 15. They heated their sample in the range from 30 to 180 °C, where they only observed a thermal expansion of the α-NPD layer below 100 °C.216,323 At higher temperature, an intermixing of α-NPD and Alq3 is visible at an annealing temperature of 120 °C. The crystallization of both materials (Alq3 and α-NPD) was observable at a temperature of 180 °C. With this experiment, the authors could demonstrate that the crystallization observed at 80 °C for a single α-NPD layer on ITO coated glass does not match the behavior in a real device stack. The interdiffusion of organic layers at elevated temperatures and an assumed resulting dark spot growth was also investigated by other groups.15,164,239 In summary, the crystallization behavior observed in single layers has to be verified in real device structures. It is possible that single layer films behave differently from films capped with other layers. The free space adjacent to single layer film may be a critical element facilitating crystallization. 4.6.1.3. Mixed Layers. The findings of Lee et al. lead to the question of how the “bad” stability of a material can be improved.242 One possibility stated above is to use the material in a stacked design, which seems to stabilize the material itself. A second approach is to mix the material with other (suitable) materials. This approach was established as “bipolar transport and emitting layer” by Choong et al.; the group investigated an organic alloy of Alq3 and α-NPD at a variety of temperatures and found them to be more stable than bilayer heterojunction devices.330 This concept is also known as a “co-host emitter”.89 Another investigated mixed system is the doping of an HTL (like TPD or α-NPD) with the electron transporting material C60, done by Lee et al. and in parallel by Yuan et al.348,352 Yuan et al. argue that C60 forms stable charge transfer complexes with the aryl amines, which lead to a more “cross-linked” layer system.348,353,354 Such doped layers show a significantly higher S


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(section 5.5), as well as accumulation processes on interfaces (section 5.2) are the focus of this Review. In the last part of this section, chemical, electrochemical (section 5.6), and photochemical processes (section 5.7) will be discussed in more detail. The material-specific chemistries that are known for different materials will be subject of section 6. A broad variety of different reaction mechanisms of HTL-, ETL-, and emitter materials will be discussed.

thermal stability in the sense of suppressed dewetting processes at higher annealing temperatures for single organic layers on top of a ITO substrate.348 It has to be pointed out that the more important lifetime enhancing phenomenon for such systems should be addressed to the enhanced charge carrier delocalization over the emitter layer,16,37,63,68,355 as well as a shift of the hole density away from the Alq3-layer. 4.6.2. Influence of Annealing. Besides the influence of Joule heating and the external influence of the temperature, elevated temperature is an essential element of a distinct annealing process, which may be intentionally used during or after the deposition of the OLED materials. A wide variety of process sequences (deposition and annealing steps) may be used and provide a specific impact on the device performance. For example, devices processed at higher substrate temperatures have shown a better long-term stability as well as a better long-term storage behavior.356,357 Similar observations were made when the already processed device was post annealed. Because of the use of the annealing techniques, the lifetime was improved up to 5 times, depending on the material set and the annealing temperature. It was also shown that the lifetime improvement for a given annealing step is dependent on the composition of the materials, for example, the doping concentration.78 The nature of the lifetime improvement due to device annealing is not fully understood.78 One may speculate that oxygen and water are desorbed during an preannealing step. Additionally, it is widely known that high substrate temperatures during the material deposition may cause a change in the morphology of evaporated organic layers. This may have a positive influence on the device lifetime in particular cases.357 The morphology may affect the mobility of the charge carriers and the thickness of the recombination zone, as seen in an investigation on Alq3-layers, where the crystallinity was linked to the enhanced PL- and EL-efficiencies.358 Another important factor is presumably the higher purity of the grown films. Because of the heated substrates, as known from work where insufficient vacuum conditions are used (10−6−10−5 mbar), water and oxygen will be desorbed from the substrate as well as from the deposited material.322,359

5.1. Diffusion and Drift

As mentioned before, transport processes such as diffusion and drift are essential elements of OLED degradation. Electrically neutral entities like organic molecules, gases, and atoms from inand outside of a device as well as from the anode and cathode material can diffuse across some layers or through the entire device and can act as exciton quenchers or nonradiative recombination centers. Ions might also drift due to the applied or the built-in electric field. Several investigations show that heating will enhance metal diffusion into the organics as well.172,360 Migration of mobile ions inside an OLED was proposed in 1997 for elements originating from the ITO and Ca-electrode.361 Two years later, Lee et al. were able to support this model by using secondary ion mass spectrometry (SIMS) depth profiling and showed an increased Mg and In concentration within the organic part of an aged as compared to an unaged device.189 Nguyen et al. was able to measure the content of indium across a complete degraded OLED, consisting of an ITO|Alq3 (100 nm)| Al stack. 183 Indium atoms are known to quench the luminescence in the active area. This was shown by Lee et al. by intentional doping experiments on Alq3-emission layers in OLEDs.189 Aziz and Popovic studied the degradation mechanism based on the In migration as proposed by Lee et al. too, but found that the effect seems only marginal.14 Other investigations confirmed the In ion migration and also the minor importance of the effect.362 Therefore, the diffusion and migration of In ions have to be considered mainly for polymers with a lower pH (nearly 3),363 where ITO can be partially dissolved by etching processes,364 and In ions may be released. Because of this process in polymers, the In concentration in the organic layer can be enhanced as compared to small molecule-based OLEDs. This mechanisms of ITO etching in the presence of polymers were confirmed using XPS depth profiling as well as Rutherford back scattering (RBS) on ITO|PEDOT:PSS (poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)) and other ITO|polymer interfaces.197,221,365 Additional investigations on other interfaces like SnO2|polymer interfaces show that reduced ITO and SnO2 interfaces may convert into pure metal ions or oxides, which will diffuse through the entire organic.366 The content of In in the organic layers is found to be nearly 0.1 at. % (1:1000 In:C).365 Even for a test structure with the anhydride PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) on metallic In, such a mobile behavior of In is visible.367 Different experiments, like UV ozone treatment,189,368 suggest that some ITO-treating techniques will saturate open In valencies with oxygen, which immobilizes In before OLED processing.369−371 Another technique to passivate the ITO surface and to avoid In migration into the device is the use of thin passivation layers, for example, CuPc,30 copper oxide,372 organic small molecules like 4,4′,4″-tris(N-3-methyl-phenyl-N-phenyl-amino)-triphenylamine (MTDATA)47 or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN6),373 amorphous carbon,374 or plasma polymerized CHF3375 as well as Cl2-plasma treatment.376 A

5. INTERNAL DEGRADATION MECHANISMS In contrast to external degradation mechanisms, internal mechanisms are considerably more difficult to investigate. Whereas the process or environmental parameters can be intentionally varied to elucidate the effects of specific external factors, internal mechanisms are not as amenable to such an “active” method. Instead, a typical approach can be described in a single word as “observation”: apply any or all available analytical techniques to a degraded device hoping to pinpoint that crucial internal change that causes luminance efficiency loss and other undesirable effects of degradation. Unfortunately, there are often no a priori reasons to conclude that the detected changes are really deleterious for device performance rather than an unimportant “by-product” of device operation. Some of these analytical tools were briefly discussed in section 2, while this section is focused on the actual mechanisms. Physical and chemical mechanisms will be discussed separately. Afterward, we will summarize electrostatic mechanisms such as the influence of dipole reorientation (section 5.3), the influence of the charge carriers (section 5.4), as well as the influence of the excitons (included in section 5.7). To obtain a more detailed understanding of the processes in the devices, effects like charge balance, position of the recombination zone T


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Figure 16. Schematic evolution of the charge accumulation for the case of HTL degradation in a two-layer HTL|ETL system. Positive charges (⊕) as well as neutral hole traps (○) are created mainly in the HTL. Because of the accumulation of holes, the voltage at constant current rises, whereby electron injection will be enhanced, leading to a shift in the recombination zone within the HTL bulk region. Figure created with data from ref 23.

Parthasarathy et al. demonstrated that Li diffuses after processing through an underlying organic layer.193 Thus, a diffusion distance (ld) of Li in Alq3 (ld = 20 nm), CuPc, and BCP (2,9-dimethyl-4,7-diphenylphenanthroline) (each ld = 70 nm) could be determined by UPS, SIMS depth profiling measurements, and the interpretation of IV characteristics. Because of such high diffusion, the material is believed to create nonradiative recombination centers in the affected active layers.380 A similar result was found by D’Andrade et al. investigating the following stack: Si-substrate/intrinsic BPhen/Li with different Li thicknesses.76 This group could also show that Li diffuses into the BPhen underneath even during the processing, but does not penetrate the whole BPhen layer. Varying the Li layer thickness, that is, lowering the Li doping, reveals that the penetration of the material can be inhibited to a certain degree. This may be a result of the chelate-forming character by applying phenanthroline as ETL materials.182,381 If a concentration close to a “saturation” value is reached, the diffusion of such kind of materials can be limited. The use of a strong chelating agent, such as phenanthrolines, can be expected to prevent the diffusion of metals from the ETL as well as from the cathode. Unfortunately, demonstration of this behavior using a mixture of BAlq and BPhen was not successful,76 due to another degradation mechanism of BPhen, which is much more dominant than the quenching mechanisms expected from Li (see section 6.4). Coming back to the mechanistic behavior of the diffusion/ migration, Shen et al. suggested the concept of mobile ions based on fitting the increasing voltage of a device together with a corresponding healing process, which is applied afterward:306,307 Shen et al. proposed (section 4.4, Figure 10) that mobile ions may influence the aging of the current−voltage characteristics, but the influence on quenching mechanisms is marginal.295,306,307 Sato et al. mentioned that luminance loss and voltage increase during device aging may be independent from each other.15 Of course, it is obvious that the overall efficiency of a device strongly depends on the power consumption; therefore, the voltage increase has a significant influence on the device characteristics over time.

variety of different ITO treating methods are listed in the work of Turak.225 Treating the surface of ITO has a significant influence on the binding states of In. For example, it is possible to vary the density of hydroxyl groups on the ITO surface by varying the process as well as the process temperature. The relative density of hydroxyl groups correlates with the observed lifetime characteristics of the investigated devices, whereby the higher concentration of the hydroxyl groups corresponds to the lower lifetime of the device.377 For instance, due to the use of UV ozone plasma, the ITO workfunction is increased, carbon and carbon−hydrogencontaining impurities can be removed, and metal ions can be deactivated.369,371 As discussed before, thermal treating methods will help to desorb remaining oxygen and water from the ITO surface. However, if the temperature is too high, ITO will be partly decomposed. In case of an investigated (amorphous) ITO, the resistance will change, the morphology will change from the amorphous to a crystalline phase, the reflection index will increase, the fraction of oxygen in the ITO will decrease, and Sn ions will be activated for migration as well.378 Other metals like Ag and Mg are also known to migrate from the corresponding Mg:Ag alloys into the organic.184,188,190−192 It was discovered that the migration distance of such metals is matrix- and mobile-ion-dependent. For instance, Mg is mainly confined to a region of about 2−3 nm into the organic layer,189 a migration depth that is not likely to be important for OLED degradation. Beside the mentioned materials, Al, LiF/Al, Cs2CO3/Al cathodes are investigated as well and show a similar tendency to diffuse into the organic layers.149 Additionally, the evaporation sequence during the OLED processing appears to affect the penetration of metals and therefore the following diffusion of the materials into the device.379 This was already proven for metal-on-organic as well as for organic-on-metal stacks.172,182,253 A special case of it is the evaporation of a metal onto a thin LiF film. Heil et al. point out that Al will react with the LiF during the evaporation process to release Li, which will diffuse into the organic stack.253 Regardless of whether neutral Li atoms are actually formed in that process, the migration process must occur with Li cations rather than Li atoms because it is unrealistic to expect that atomic lithium would not be instantly oxidized by reacting with electron transport material that is readily reduced and, additionally, has propensity to bind to a metal ion. The migration of lithium cations is likely to be a diffusion process driven by concentration gradient. The ion drift may also play some role considering that the direction of built-in electric field favors the migration of positively charged ions toward anode. In case of an applied electric field, the cations will migrate toward the cathode.

5.2. Accumulation of Nonradiative Recombination Centers

In some OLED architectures, excessive charge can accumulate at HTL or ETL interfaces due to significantly different barriers for injection of holes and electrons.19,382 However, such charge accumulation exists even before the device has been operated for any significant time and, therefore, is neither a cause nor a consequence of device degradation. It has also been demonstrated that device operation may result in accumulation of different species (of degraded molecules), filled deep traps (fixed charges), which typically cause a loss of excitons by acting as nonradiative recombination centers and luminescence quenchU


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ers.14,19,59,209,306,307,383−387,394 The species responsible for charge accumulation can be products of chemical reactions of organic materials or simply migrating ions. Some groups state that mobile ions will cause such an accumulation at the heterojunction and within the recombination zone of the device.189,293,297,306,307 Therefore, the usage of an AC current was suggested to prevent or minimize the mobile ions accumulation effects.30 Using voltammetric measurements, it was demonstrated that the accumulation within the devices is mainly caused by fixed charges (deep traps),386 and the region of accumulation is mainly located at the interface, which serves in general as a charge recombination zone as well.19 Within this area, the accumulated charges were found to act as nonradiative recombination centers. In ref 23, the process of excessive accumulation is investigated for the case of a HTL reaction (Figure 16). Because of the dissociative reaction of the triarylamines α-NPD or TAPC, charged and neutral radical species (gap states) are created at and near the HTL|EML interface. Some of the neutral species may act as hole traps, which will carry the fixed positive charge after hole trapping. The charge accumulation and the respective change in electric field profile result in a gradual shift of the recombination zone deeper into the HTL, which leads to further reaction of the HTL bulk material. This phenomenon was observed for the less stable material TAPC, while α-NPD showed degradation confined to a narrow region (∼5 nm) near the interface. The details of the respective chemical reaction are discussed in section 6.2. As discussed below, charges as well as excitons play a central role in OLED degradation. The former one was proposed to cause degradation of Alq3 (cation instability model) and the latter to cause degradation of OLED materials of various chemical classes. By mixing of hole and electron transport materials in the emission zone, accumulation effects might be reduced due to a widening of the recombination zone.58,388 This approach might lower the local densities of charge and excitons as well.

moieties.293 Both dipole reorientation and reversible efficiency loss and recovery were observed. However, the focus of this study was on the class of elastomeric polymers, which could have a TG significantly below room temperature. Considering that the presence of cyano group on stilbene side chain should result in a large dipole moment, the electric-field-induced reorientation seems plausible, yet largely irrelevant with respect to typical OLED materials, which are amorphous solids with T G substantially higher than operating temperatures. In principle, the molecular reorientation in typical OLEDs should be readily detectable as change in internal electric fields, fixed interfacial charge densities, and dipole moments associated with specific layers and as functions of applied bias, temperature, and time. Even though many published studies measure and discuss these characteristics, no clear indications of molecular reorientation were reported so far. It is likely that the reorientation of molecular dipoles is only weakly (if significant at all) contributing to OLED degradation. Nonetheless, its contribution cannot be ruled out, particularly with respect to the initial part of the degradation curves. Further information about the dipole distribution can be obtained by modeling the outcoupled light, depending on its polarization.141,389−391 It is noteworthy that, because of its reversibility, the reorientation effect is not a degradation mechanism in the sense of a physical destruction of a molecule, layer, or a part of a device. 5.4. Role of Charge Carriers

Charge carriers are known to quench excitons (polaron quenching) when present in the vicinity of recombination zone at densities on the order of 1011−1012 cm−2.392−395 This behavior has a significant effect on device efficiency.396 Charges (either holes or electrons) may also interact with certain materials and can thereby cause chemical reactions. In 1999 Aziz et al. reported that Alq3 undergoes degradation when exposed to hole current. They were able to show a loss in photoluminescence by investigating Alq3-based holes-only devices. In subsequent studies, they confirmed the model of the instable Alq3-cation397 (in more detail in section 6.1.1.1) and, contrary to their original report, were able to demonstrate an electron-induced degradation of a Alq3/α-NPD structure in an electrons-only device as well. Considering a later study by the Kodak group, which demonstrated perfect stability of both photoluminescence and voltage in Alq3-based electrons-only device on a time scale, which is a magnitude longer (>1000 h), the instability of electrons-only devices reported in ref 397 may be related to a device component other than Alq3.154 There are many attempts to find a charge carrier-induced degradation in other OLED materials.154,398 For instance, Winter et al. studied the blue fluorescent emitter 2,2′,7,7′-tetrakis(2,2diphenylvinyl)spiro-9,9′-bifluorene (Spiro-DPVBi) at different charge carrier densities up to 100 mA/cm2 with unipolar devices.398 The measured PL decay over 1400 h does not support the theory of a carrier density depending on the degradation mechanism for the investigated material. The Kodak group observed lack of reactivity for a fluorescent hydrocarbons rubrene and (2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN)), various arylamines, carbazoles, and iridium complexes.154 These observations suggest that, although some metal chelates and other organic molecules may be susceptible to chemical reactions when ionized in the solid state, it is not a characteristic property of organic molecules used as OLED materials.

5.3. Reorientation of Dipoles

The reorientation of molecules possessing permanent dipole moments has been proposed to occur when amorphous solids are subjected to electric fields on the order of 1 MV/cm.303,304 This order of magnitude is comparable to electric fields created in typical OLEDs during operation. Such reorientation is expected to modify electric polarization of the respective layer, and potentially alters the electric field profile within the whole device. It is possible that the recombination efficiency and the extent of excited-state quenching by charge carriers are consequently changed as well. Additionally, if reorientation involves emitter molecules, the outcoupling efficiency is likely to be affected as well. All of these effects may contribute to the experimentally observed luminance decay, essentially comprising a distinct degradation mechanism. Because of the inherent reversibility of the reorientation process, such a degradation mechanism is expected to be reversible as well, and the initial luminance could be recovered. In fact, the experimental observations of partial recovery of luminance efficiency prompted the original idea that the reorientation of molecular dipoles may be a significant element of OLED degradation.303 Aside from indirect evidence in forms of spontaneous and reverse-bias-induced recovery of luminance efficiency, an optical technique has been reported to be able to prove a reorientation in polyurethane polymer bearing stilbene side chains as emitting V


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5.5. Charge Balance and the Position of the Recombination Zone

Often, improvement of charge injection improves lifetime. However, this is not always the case. It has been demonstrated with a large set of diverse experimental ETL materials that the device stability is substantially decreased when electron injection is improved relative to some optimum point.407 Nevertheless, to enhance the injection of electrons from the cathode into the organic, three different strategies are known: the choice of the cathode material,232,317 the use of an appropriate electron injection layer (EIL),232,317,408 and the use of doped transport layers.294 It is well-known that using metals with low ionization potentials (IP) enhance the electron injection into the organic. This can be demonstated by comparing alkaline or alkaline earth metals as cathode material or as HIL, where an improvement in device lifetime is visible.409 Of course this is only valid by the use of the appropriate encapsulation technique to avoid oxidation, which would occur preferable on low IP metals. Because of the common use of ITO as anode material for bottom emitting diodes, an optimization of the hole injection is mainly achieved by inserting an injection or buffer layer as well as the use of an appropriate doping of the HTL.410 A similar effect was discovered by using materials like CuPc,30 MTDATA,47 or plasma polymerized CHF3375 as mentioned above for ITO passivation. Adachi et al. investigated a wide range of HTLs (IP between 5.0 and 5.8 eV) in different ITO|HTL|Alq3|Mg:Ag stacks.48 Although a significant dependence of the device lifetime on the HTL IP was reported initially, the follow-up studies did not confirm that finding.30,411 There were many different approaches to enhance device. Several groups proposed that the charge balance in a α-NPD|Alq3 device can be improved by modifying the ITO by the use of different kinds of materials. Therefore, a lifetime enhancement (as compared to a conventional ITO anode) was observed with the materials like different oxides (e.g., silicon oxide,412 copper oxide,372 tungsten oxide,413 and molybdenum oxide339), conductors (e.g., carbon374), metals (such as Pt, Au, or Ni)414,415 and electron transporting materials (C60336,416−418 and Alq3419) as well. It should be mentioned that even due to different deposition process parameters, like the deposition rate or the temperature, the morphology and therefore the mobility of the materials can be varied, which influences the charge balance on the recombination zone as well.134,420 Such a behavior is rather pronounced when nominally identical stacks are processed as conventional and as inverted devices. Besides the damage of the underlying layers (by any kind of irradiation) during possible sputtering,127 inverted samples show usually significantly lower lifetimes.77 Another important point to consider is the location of the recombination zone.72 From our analytical work, we know that many phosphorescent emitter materials react with neighboring HBL materials under OLED operation, mainly when the emission zone is located at this interface.24,29,400,421 Other groups focused on the enhanced reactivity of HBLs (with quinoline ligands) by carrying holes on it (similar to the Alq3cation) and point out that shifting the emission zone away from the electron transporting HBL leads to an increase in device lifetime.49 Such a behavior is also supported by the finding that devices using matrix materials carrying only single carriers (e.g., holes), with and without a blocking layer, exhibit the same lifetime behavior.422 Nevertheless, an optimal charge distribution is likely to be a key to improve lifetime and efficiency. Now we should consider that excitons are likely to play a major role in device

An optimized charge balance is not only important for lifetime issues,25,399,400 but is also a very important field of investigation to reach high efficiencies as well as suitable color coordinates for display and lighting applications.10,401−404 The original use of the term “charge balance factor” in OLED referred to the percentage of carriers injected from electrodes that recombine rather than reach the opposite electrode and discharge nonradiatively there. In this definition, the charge balance factor is identical to recombination efficiency. It is a characteristic value, which varies from 100% for an ideal device to 0% for a nonemissive device, such as a unipolar (holes-only or electrons-only) device. It is noteworthy that a 100% charge balance factor in itself does not imply an ideal or even emissive device: it is easy to construct a device with excellent recombination efficiency that is still nonemissive (for example, by doping “emissive” layer with effective luminescence quencher such as copper phthalocyanine). Following the original concept of OLED as a heterojunction device, practical OLEDs are engineered to have an excellent recombination efficiency (charge balance factor ∼100%), utilizing injection barriers and/or carrier trapping to create an effective heterojunction and ensure carrier confinement to a designated recombination zone. Charge balance term is also frequently used in a much broader and nonquantiative manner, lumping together various mostly not understood or not quantified effects such as efficiency variation due to quenching of excited states by charge carriers. From this perspective, the optimum charge balance is achieved when electrons and holes create a maximum possible number of photons in the emission zone.25,396,405,406 It should be emphasized that an intuitive view of charge balance as a difference between densities of holes and electrons in recombination zone (or elsewhere in a device) is without any merit. There is no fundamental reason why an OLED with comparable densities of holes and electrons in recombination zone would be generally advantaged relative to an OLED that has density of one carrier greatly exceeding the density of the other one. It is a perfectly plausible situation when a modification of device leading to an increase in density of majority polaron relative to minority polaron would result in the improvement of device performance. Aside from an overly qualitative view of charge balance, there is no doubt that carrier densities have a major influence on device efficiency, lifetime, and other characteristics. Some materials (e.g., Alq3) appear to be unstable when in an ionized state. For such materials, it may be important to minimize density of the problematic carrier, for example, holes on Alq3.397 Generally speaking, by modifying charge injection at the electrodes, the carrier densities inside the device can be manipulated, which might reduce the concentration of ionized metastable molecules. Additionally, the nature of Langevin recombination dictates that the distribution of charge densities in the OLEDs determines recombination zone profile and location. Considering that excitons of some OLED materials appear more effective in initiating deleterious chemical reactions, it is obvious that controlling distributions of charge carriers can be used to suppress some degradation mechanisms. The improvement or even the inhibition of charge injection at certain interfaces may therefore be essential for a lifetime improvement in OLEDs, where charged molecules and subsequently formed excitons tend to react and form charged traps or nonradiative recombination centers. W


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degradation,247,423 and yet the discussed-above points of charge injection control do not overcome the problems of high exciton densities at a thin recombination zone, commonly located either on the HTL|EML or on the EML|ETL interface.25,72 It should be mentioned that the emission zone within an EL device may shift in dependence of the applied voltage.424 In some cases, this can be avoided by using doped transport layers in addition to the use of appropriate blocker layers by adjusting the emission zone.425 In 2000 Choong et al.330 and later 2002 Chwang et al. investigated the differences of (a) a conventional hetero device structure, (b) a graded mixed emission layer, and (c) a uniformly mixed layer (see for illustration Figure 17).37,68,426

in EMLs started around 1995 and displays an important development for the lifetime enhancement of organic devices. An important example of an doping system from the late 1990s is the use of Alq3 as matrix and the green fluorescent emitter DMQA as dopant.55 Besides the lifetime enhancement (reaching, e.g., 10 000 h at 1000 cd/m2), a significant improvement of device efficiency was achieved with this doping technique.55 Additional, another design is the use of multilayer structures (inaccurately referred to as quantum wells),438−440 where the approaches of charge and exciton blocking,441 exciton separation,442 centering of the emission zone,443,444 and broadening of the same are combined.445,446 5.6. Electrochemical Reactions

Besides the mentioned-above physical mechanisms, a broad variety of chemical reactions occur in an organic device. Such reactions may be caused by electrochemical processes near electrode such as corrosion, by charge carriers, or by the excited states created by recombination, energy transfer, or even light absorption. Below we will use the term “electrochemical reaction” to encompass chemical reactions initiated/caused by charges. Such electrochemical reactions can be classified according to the region of the device, where they occur. In principle, the three parts of the OLED can be considered: the anode, the cathode, and the organic part. In 1996 Aziz and Xu proposed for the ITO anode an electrochemical corrosion mechanism, where the used polymer PPV (poly(p-phenylenevinylene), spin-coated from precursor solution)447 serves as a weak electrolyte.237 Nguyen et al. were able to examine the ITO decomposition, where they could show dissolution of In and Sn and a further migration/diffusion into the organic layer from an ITO|Alq3 hetero stack during device degradation.183 Such a decomposition of ITO is sometimes accompanied by a volcano or dome-like structure forming at its surface, where In-rich regions are detectable.448 Another mechanism can be observed for cathode materials. The main reaction of this electrodes is the reaction with water and oxygen.41,130,165,216 Even at the interface between organic materials and the cathode, reactions may be visible. For instance, in encapsulated samples, a change in the cathode characteristics due to device aging can be visible. This was found for polymerbased OLEDs capped with a Ca cathode where the reflectivity of the cathode changes during the aging process.449 Other groups proposed an electrochemical decomposition of organic material in small molecule-based devices41 as well as in polymer based ones.187 In both cases, the observed bubble formation and the found gas evolution, detected by mass analysis, give a clear hint for electrochemical reactions on the organic|cathode interface. Electrochemical reactions within the organic part of the OLED are also possible. Here, the best known example is the instability of the Alq3 cation. Such a electrochemical process is also detectable in a standard electrochemical experiment, which shows the irreversibility of the Alq3 oxidation (see section 6.1.1.1).

Figure 17. Illustration of the structural behavior of HTL and ELT in different devices, (a) with a hetero structured stack, (b) with a graded mixed layer system, and (c) with a uniformly mixed EML; the bars below demonstrate the corresponding half-lifetimes of the systems. Data from ref 37.

Chwang et al. pointed out that the distribution of both charges should be spread over the entire EML by using a mixed EML (“bipolar transport and emitting layer”). Both concepts, graded mixed layers427−429 and uniformly mixed layers,430 display an improved lifetime behavior. Several factors were proposed to explain the improvement of the stability of mixed EMLs:16,37,63,68,355 (i) the accumulation at an interface can be prevented,431 (ii) the exciton density can be minimized at the same luminance, (iii) charges are carried by separate materials,80 (iv) charge transport and exciton transport can be separated, (v) the emission and recombination zone is shifted away from the blocking layers, and (vi) the emission zone will be broadened.355 The concept is also applied by using a bipolar transport and emitting layer together with emissive dopants,89 or it is used, supporting point (iv), by using two separate emissive dopants, where one sensitizes the other, being called the concept of the emissive-assisted dopants (EAD), initially proposeded by Hamada et al.432−434 This leads to a fast exciton transfer from a supposedly unstable (excited) material to the emitter and to some broadening of the emission zone as well. The concept of doping itself shows that EMLs prepared in this way exhibit a significantly longer lifetime as their single layer counterparts.49,55,435−437 The invention of the doping technique

5.7. Photochemical Reactions

Similarly to charge carriers, excitons are frequently suggested to play a major role in the device degradation. The first speculations about the role of the excitons in device degradation appeared as early as 1996.450 A decade later, based on the detection of identical chemical products of photo- and electrical degradation as well as the measurement of the action spectrum of the former, a compelling demonstration of major role of excitons was reported.22 Various OLED materials were demonstrated to be X


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susceptible to exciton-related degradation reactions.248,398,451,452 Another type of evidence of exciton involvement was reported by Winter et al.: the same shapes of degradation curves in photoand electrical degradations were observed.247,398 Minor differences were attributed to the dispersion of the excitons; in EL devices they are located mainly at the recombination zone and in PL devices across the entire absorption layer according to the Beer−Lambert law.247,468 In addition to chemistries observed in the small moleculebased devices, the keto defect in the polymeric devices will be discussed in some detail. We will not cover the wider field of potentially possible in-device photochemical reactions such as cis−trans isomerization, because there is no published work linking these reactions and OLED degradation.453,454 One has to consider that the influence of light can be distinguished by circumstances and the regions where the reactions occur: (i) at the interface between an electrode and the organic, (ii) intra- and intermolecular reactions inside the organic (bulk), between molecules from the same type, and (iii) reactions of the organic with reaction partners like oxygen or neighbored molecules from another class of materials. Experiments on polymer-based devices were carried out to study lightinduced reactions at electrode|organic interfaces (case (i)) in more detail.179 The anode material ITO is the most thoroughly investigated electrode. Spectroscopic investigations exhibit a dependence of the device degradation on the absorption behavior of the organic compounds. It was found that the polymer itself (at different absorption wavelengths) does not degrade.310,455 This nonreactivity of the organic materials was proven by PL intensity measurements, where no changes in the PL luminance between aged and unaged samples are visible, whereas the EL properties are significantly altered. On the basis of these investigations, it was assumed that these light-caused reactions occur at the corresponding ITO−organic interface. Thus, the following mechanism was proposed: Light is absorbed by the polymer, and afterward the excitation is transferred to the ITO and the last oxygen from the ITO is activated and released. Additionally, XPS investigations of ITO sample before and after UV light irradiation revealed some lowering of the work function. This behavior was only found on chemically cleaned or oxygen plasma treated ITO.307,311 Sputtered surfaces instead remain unaltered. Using Au or pentacene buffer layers between the ITO|organic interface will also prevent this activation of the ITO oxygen.310 By the way, pentacene is also known to be reactive via a photochemical reaction step. Choi et al. point out that illuminating pentacene (at a short wavelength of 254 nm) led to a reaction with the oxygen of an underlying SiO2 substrate.456 One should mention that this observation may contribute to a cyclization reaction after Coppo and Yeates.457 As expected, other pentacene derivatives are known to undergo the 4 + 4 cycloaddition457,458 or will oxidize in the presence of oxygen or even will be excited for further reactions.459 Such photo oxidation effects are well-known for condensed materials like rubrene and naphthacene as well.259 As expected, photoinduced reactions with oxygen were already discovered for illumination experiments with UV460,461 as well as for the visible part of the light.128,129,462 Many OLED materials, for example, α-NPD and TPD, are known to degrade (changes in their absorption and PL emission characteristics) after UV irradiation (365 nm, 150 W Hg arc lamp) of single layers in air.350 Colditz et al. observed the degradation behavior of a polymerbased OLED at a constant current and were able to show the wavelength-dependent luminance loss and voltage drop due to

light illumination.128 The luminance loss was attributed to an absorption-reaction mechanism, where the used blue emitting polymer acted as the absorbing material. PL observations of the degraded device show no significant decrease in PL efficiency of the sample, whereas the EL examinations exhibit the illuminated spots as areas with lower efficiencies. One conclusion from these findings is that the UV light-induced damages may occur at one organic|electrode interface. In a second experiment, where samples with and without cathodes were illuminated with green light, it was revealed that the observed degradation phenomenon only occurs with the samples possessing the organic|cathode interface. The effect of voltage increase due to UV light illumination can be used for structuring and patterning of area-emitting OLEDs as well as for healing steps to prevent further dark spot growth by separating the dark spots from the unaged device area by surrounding the dark spot with high resistance areas.249,463−465 In this context, a reversible effect was found, where healing processes were observed for UV-treated466 or even visible lighttreated regions.467 The nature of this effect is not understood. According to investigations with PL and EL measurements, the active material itself was not affected, which leads to the conclusion that the interface has be changed in a way where a reversible mechanism can take place.467 The second process (ii) of how light may interact with an organic semiconducting device is a reaction of and within the bulk material. In that case, some authors showed that light only interacts with the bulk material. For instance, Alq3-based devices illuminated by UV-light show a decrease in the PL intensity of the sample during light exposure. Besides this, corresponding single carrier devices were investigated as well. For instance, a decrease in the conductivity of a Alq3|α-NPD-based holes-only device was observable, whereas the corresponding Alq3-based electronsonly device remains unchanged after the UV exposure.468 Other materials, for example, Ir(ppy)3 doped into 4,4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl (mCBP), also show such a behavior, where some device degradation appears to be induced by a combination of light and electric current.35,385 However, the nature of the degradation processes is unclear in these cases. Although it has been proposed that the mCBP anion itself becomes more unstable in the excited state, many alternative interpretations of the experimental observations can be invoked just as well. For example, by analogy with a hypothesis put forward by Aziz, an interfacial reaction involving separate anionradical and exciton could explain the observations in the mCBP/ Ir(ppy)3/BAlq system. Such a mechanism has an advantage of being able to explain a surprising observation of no change in Ir(ppy)3 emission transient. This observation is essentially inconsistent with proposed degradation of bulk layer of mCBP/ Ir(ppy)3 unless an unprecedented and implausible assumption is made that the mCBP degradation products are incapable of quenching luminescence and are acting exclusively as carrier traps. In practice, degradation processes in emissive layers of various OLEDs always show signatures of accumulating both luminescence quenchers and nonradiative recombination centers. Alternatively, sequential chemical mechanisms, wherein products of current-induced degradation undergo subsequent light-initiated reactions, can be invoked to explain observations reported by Giebink et al. as well as those by Aziz et al.35,469 Of course, it is also possible to draw a sequential mechanism with an opposite order of action, in which products of light-induced degradation are reacting further with charge carriers that are Y


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energy diagram suggests that the photoinduced oxidation of polymer materials may occur even at a wavelength located in the emitting range of the device, which means that self-absorption may contribute to the degradation mechanism as well.462 The highly reactive singlet oxygen is well-known to undergo 1,2-cycloaddition to vinyl double bonds resulting in the formation of various oxidized species, which are described in ref 157 on the example of an alkoxy-substituted poly phenylenevinylene (PPV) derivate.281,473 This behavior was confirmed by IR measurements. The observed photoinduced 1,2-cycloaddition on the vinyl part of the molecule implies a chain cutting on the engaged double bond, a creation of radical chain ends, as well as different possible carbonyl-containing species. One example of a similar photooxidation of a small molecule semiconductor material is the reaction of 2,2′,7,7′-tetrakis(2,2diphenylvinyl)spiro-9,9′-bifluorene (Spiro-DPVBi) with oxygen under UV light.516 The mass spectra of the UV-irradiated samples revealed an oxidative addition reaction on one of the four double bonds on the molecule (see Figure 19). As a result,

present in abundance due to passing current. Either way, synergetic effects of light and current can be readily explained without invoking a direct chemical reaction of excited state and ion-radical or excitation of an ion-radical. Not only is the probability of such processes involving two transient species relatively low, but the ion-radicals are also typically excellent quenchers of the excited states due to the presence of low-lying excited states and, consequently, provide a very effective channel for nondestructive deactivation of electronic excitation (internal conversion). Up to now, there are several photochemical reactions known, which may occur inside an OLED due to light exposure (case (iii)). One process is the formation of the so-called keto defect, which was observed in polymer-based OLEDs. The keto defect was primarily observed in vinyl group-containing polymers157,470 as well as in polymers containing fluorene units.471,472 The reaction was named after the corresponding main reaction product (ketone) and may occur via two different reaction pathway in the presence of light (excitons) and oxygen. List et al. proposed a photoinduced as well as an electro-induced reaction of the polyfluorenes they investigated.471 They proposed that the excitation of a fluorene unit would enable the molecule to dissociate an alkyl side chain from position 9, followed by reaction with the surrounding molecular oxygen (see Figure 18). In a subsequent step, the ketone is formed.

Figure 18. Proposed reaction mechanism for the keto defect on fluorene units during device aging after List et al. Figure created with data from ref 471.

IR measurements revealed the accumulation of the carbonyl unit (>CO), identified by the increasing absorption at the characteristic wavenumber of 1721 cm−1 during the photoinduced device aging process. Corresponding PL measurements suggested the efficient energy transfer from the polyfluorene main chain to the keto defect site. The strongly trapping character of the keto defect was also evident, which appeared, in some cases, along with an enhanced light emission from the new generated (red-shifted) energy level.471,472 Examination of smallmolecule devices led Thangaraju et al. to propose that the observed quenching may occur due to formed carbonyl units, which were identified in Alq3 thin films after some minutes of white light exposure.160 Similar reactions may involve vinyl double bonds via a 1,2cycloaddition of singlet oxygen.157 The organic is excited by incident light, or electrically, and a singlet oxygen is formed due to an energy transfer originated from the excited organic. It was found that the singlet state of the polymer has a too low lifetime to contribute to the energy transfer on to the oxygen, and instead the triplet state of the organic was found to be involved in this transfer reaction. The lowest singlet state of oxygen is 0.98 eV above its ground (triplet) state. Cumpston et al. compared the energy levels for an exemplary chosen material poly[2,5-bis(cholestanoxy)-1,4-phenylenevinylene] (BCHA-PPV) and for molecular oxygen.157 The

Figure 19. Proposed reaction pathway of the light-induced oxidation on Spiro-DPVBi with oxygen. The (last four) reaction products are found in MS analysis.

the two corresponding products are detectable. At this point, it is not verified whether oxygen or water is the responsible oxidative agent as well as this reaction may occur in EL-driven OLEDs. Another well-known molecule for photoinduced oxidation behavior is the C60 buckminsterfullerene and its derivatives. C60 reacts with pure oxygen even without light exposure.474 A comparative investigation with UV and visible light shows that both irradiation sources lead to polymerization and oxidation of C60.475 Such reactions were observed mainly in C60 powder, single crystals, single amorphous layers, or OPV devices, where the oxidation of the fullerene leads to a reduction of the conductivity of the corresponding film.476−478 Depending on the exposure time, the temperature, and the intensity of light, the amount of reaction products varies,475,479 and CO, CO2, and carbonyl-like reaction products could be detected.479 Even photopolymerization may occur by a 2 + 2 cycloaddition between two parallel double bonds from two C60 molecules.480 In case of O2 exposure and under light illumination, the effect of Z


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Figure 20. (a) Direct dissociation of an AB-species after excitation; (b) predissociation after Nakashima and Yoshihara; as well as (c) hot-molecule mechanism. Figure adapted with permission from ref 481. Copyright 1989 American Chemical Society.

charge carriers in an OLED device, the double excitation would require interaction of two excited states, one of which transfers energy to the other. Although processes of this kind have been demonstrated to occur in OLEDs (triplet−triplet annihilation being a fairly common and well-documentd example in both fluorescent and phosphorescent devices), they are unlikely to play a major role in OLED degradation because of relatively low power densities and, respectively, low probability of double excitation involving short-lived excited states with sufficient energies to cleave σ-bonds. Taking for example a typical fluorescent blue OLED, the annihilation of two singlet excited states of emitter molecules might result in an excited state with sufficient energy to dissociate any single carbon−carbon bond present in those molecules. However, considering nanosecond lifetime of a typical fluorophore and fast deactivation of a potential upper excited state by internal conversion, the bond dissociation appears rather unlikely. Still, it cannot be rigorously ruled out, especially for the systems where long-lived, high energy species are generated, for example, blue phosphorescent devices. It is also noteworthy that the acceleration factor discussed in section 1.1 may provide some insight as to the likelihood of the double excitation mechanism. If the dissociation occurs as a bimolecular reaction of two excited states, it may be expected to follow a second-order rate law with respect to excited-state concentration. Considering that the excited-state concentration is approximately proportional to device luminance and current density in a wide range, we would expect that the acceleration factor eq 1 would be approximately 2. In reality, typical acceleration factors are substantially less than 2 (albeit often higher than 1).

conductivity reduction may influence the performance of C60 containing OLEDs (minor use as ETL or EIL) as well. Many other chemical reactions are known to occur in operating OLEDs without the involvement of oxygen or water. The specific material-related reactions will be discussed in more detail below. The subject of photochemical reactions in OLEDs is certainly much broader than just the processes induced by externally (or internally) generated light. It includes most of the known chemical mechanisms of OLED degradation because of the likely primary role of excited states in these processes. Although the details of these material-specific mechanisms will be discussed in the next section, we will provide an overview of what appears to be the most commonly encountered photochemical process associated with OLED degradation, homolytic bond dissociation. Nakashima and Yoshihara discussed three models of excitedstate bond dissociation reactions, wherein the first case is the well-known dissociation of a molecule after its excitation into a repulsive potential (R, see Figure 20a).481 The second dissociative process may occur along another route (Figure 20b): After an excitation into a stable state and provided that the repulsive potential R crossing is thermally accessible, the transition to R (with subsequent dissociation) may occur radiationlessly. The third mechanism (Figure 20c) is called the “hot-molecule mechanism”, whereby an excited state undergoes a radiationless internal conversion to form a highly vibrationally excited ground-state S0**. After this initial step, a dissociation may take place, forming two fragments.482−485 It is noteworthy that the quantum yield of dissociation can be expected to approach unity in the first model. This would translate into extreme instability in the OLED device and, therefore, is unlikely to be relevant for the molecules used in the OLED device. Instability of the emissive OLED materials when exposed to light during preparation and handling even in inert atmosphere can be expected as well. The known examples of carbon−carbon and carbon−nitrogen bond dissociation in OLEDs involve σ-bonds, which mean that the respective repulsive potential requires σ−σ* excitation with energy considerably larger than the HOMO−LUMO bandgap, and therefore cannot occur under normal electric excitation. From that perspective, it is unlikely that the homolytic dissociation in OLED occurs in this manner. Similar arguments make the second model unlikely as well, leaving dissociation of vibrationally excited ground states as the most likely scenario. Additional possibilities include electronic excitations of already vibrationally or electronically excited states. Considering that the excited states are formed primarily by recombination of

6. MATERIAL-SPECIFIC REACTION PROCESSES In recent years, several intrinsic chemical reactions have been identified in OLEDs. Before that, only some speculations about in-device chemical reactions existed, such as the proposed hydrolysis/oxidation/polymerization of Alq3 due to a reaction with water.257 Besides the additionally found decomposition of organic materials below the dark spot areas and below formed bubbles, no direct observations of chemical changes had been reported. Nevertheless, due to the more recent results, it is now a widely accepted notion that chemical reactions are one of the main reasons for device degradation, occurring intrinsically due to device operation at or near the recombination zone. There is ample evidence that the corresponding reaction products act as efficient exciton quenchers or charge traps, resulting in nonradiative recombination centers.33 In the following sections, AA


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Figure 21. Proposed water-caused dissociative reaction of Alq3 after Papadimitrakopoulos et al. The formation of 8-Hq is only observed at elevated temperatures of 180 °C. Figure adapted with permission from ref 492. Copyright 1998 American Chemical Society.

quenchers occurs mainly on the HTL|Alq3 interface rather than in the Alq3 bulk material.383,384 This is quite understandable considering expectations of the higher hole density at the interface than in the bulk of the electron transporting Alq3. Cyclovoltammetric measurements on Alq3 solutions at high scan rates support the finding that holes induce an instability at Alq3 molecules (irreversible oxidative sweep) whereas electrons do not.257,487 Such hole-induced reactions may be preventable by the use of an appropriate HBL.488 In contrast with several reports showing the absence of electrons-induced degradation of Alq3,14,16,59,489 Luo et al. presented some indications of degradation in a relatively high electron current condition in an Alq3 layer in an electron only device.397 The degradation process was indicated and monitored by a PL signal loss over time. It was suggested that the previous investigations might have missed such behavior due to a relatively low electron density. However, it is unclear whether electron densities of such magnitude are ever present in emissive, bipolar OLEDs or whether the degrading component is Alq3. As mentioned in section 5.5, it may be advantageous to avoid high charge carrier densities within the emitting region. One of the main approaches is the use of mixed emissive layers that include materials to reduce effective barriers to inject carriers into emissive layer. For instance, Brown et al. could show that an additional mixing of the matrix material Alq3 with a more hole transporting material (such as rubrene) enhances the lifetime of a DCJTB doped OLED.490 Similarly, very high lifetimes (1 300 000 h at 1000 cd/m2) were reached by doping Alq3 with 70% DNP and with 1% of the emitter molecule DCJTB.33 6.1.1.2. Reaction of Alq3 with Oxygen and Water. In 2001 Liao et al. exposed Alq3 layers to different gases to investigate the influence on the UPS and XPS response.491 It was possible to show that oxygen reacts with Alq3, while water does not. In the same experiment, even additional doping effects of both gases were observed, where water causes a lowering in the LUMO level whereas oxygen causes a shift to higher values. In 1996 Papadimitrakopoulos et al. proposed a water-induced dissociative degradation mechanism for Alq3 (see Figure 21) and an oxygen-induced polymerization step for the created 8-hydroxyquinoline (8-Hq) based on results obtained at elevated temperatures of 180 °C.257,492−494 One has to mention that the investigations of the dissociative mechanism were done in the presence of water at elevated temperatures (180 °C) and the polymerization was performed with pure 8-Hq in solution. The

we will provide an overview of the chemical reactions currently reported to occur in common OLED structures. We will consider both metal organic and hydrocarbon compounds, which are commonly used as functional materials (HBL, EBL, host, and dopant materials) in the region near or in the recombination zone of an OLED. 6.1. Reactions of Metal Organic Complexes

Metal organic chelates are well-known to be useful for a broad range of applications in OLEDs, such as electron transporting and hole blocking materials, as well as emissive layer hosts and dopants. Some of these materials and their degradation mechanisms will be discussed in the following sections. 6.1.1. Al-Organic Complexes. Al-organic complexes (e.g., Alq3 and BAlq) are well-known to be suitable for use as electron transporting materials, hole blockers, as well as host materials for the emission layer. Although both materials have similar chemical structures, clear differences in their degradation behavior were found.9,25,67,206 In some cases, OLEDs with BAlq as host or hole blocking material were found to be considerably more stable than OLEDs with other electron transporting materials.69,72,486 6.1.1.1. The “Cationic Instability Model” of Alq3. Alq3 is one of the oldest and most investigated OLED materials. Because of the early use of the material in the first efficient OLED and the broad adaptability as ETL, emitter, and host material, many material properties have been investigated.286 One of the reported degradation processes of this material is driven by formation of electronically oxidized molecule, which is found to be unstable in solid films as well as in solutions during electrochemical oxidation. There are also some indications of different chemical processes within Alq3-based OLEDs, which do not support directly the “cationic instability model”.26,257 Although the instability of cationic species of OLED materials is mainly reported for Alq3, similar chemical reactivity may be expected for a wide range of metal chelates as well. In contrast, investigations do not show evidence of such a behavior for various other classes of organic molecules, such as fused aromatic hydrocarbons, arylamines, and carbazoles.154,398 With photoluminescence experiments, it was possible to identify the hole current as one of the key aspects for the degradation. If holes are injected into an Alq3 layer, cation radicals are formed and undergo some unknown chemical reactions. Because of the finding that the PL signal decrease is lower than the corresponding EL decrease during lifetime measurements, it is reasonably assumed that the creation of the AB


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Figure 22. Proposed reaction pathway of the dissociative reaction of Alq3 in aged OLEDs. Reaction (a) displays the dissociation of Alq3, where (b) shows possible further reactions between the Alq2+ fragment and the HBL BPhen, which was also detected in an investigated aged OLED. Figure created with data from ref 26.

Alq3, and a complex ([Alq2BPhen]+ at 647 Da) of the 315 Da fragment and the HBL BPhen. As depicted in Figure 22a, excited Alq3 molecules appear to dissociate, which is likely to be a reversible reaction. On the basis of thermodynamic comparisons to the alternative homolytic and heterolytic reactions, it is likely that the dissociation occurs as a heterolytic process resulting in quinolate anion (q−) and Alq2+. As mentioned above, further reactions of the fragments may occur. In our experiments, we were able to detect another complex in form of the [Alq2BPhen]+ ion. Because similar products are detected in phosphorescent OLEDs as well, the existence of such [Alq2BPhen]+ complexes within aged OLEDs at the Alq3|BPhen interface is most likely. This conclusion is based on OLEDs containing Ir-based triplet emitters where BPhen is used as HBL as well.25 These observations are conceptually aligned with a direct correlation of the device lifetime and the complexation strength of the individual complexes.29,421 During the LDI experiment, a possible further complexation of the observed Alq2+ with BPhen may occur, so an alternative reaction pathway can be considered (Figure 22b), where Alq2+ interacts with other surrounding molecules to be stabilized as a charge transfer complex, which may react in a further step with BPhen. Additionally, the investigation revealed that the fragmented Alq3 does not react with another Alq3 molecule, meaning that no Alq3-dimer formation takes place. In another LDI-TOF-MS experiment, we observed Al2q5+ dimer formation due to laser light illumination within a single layer experiment, which could not be proven for the electrochemical degradation of Alq3 inside an OLED.26,206 Unfortunately, in these studies it was not possible to link the observed dissociation of Alq3 to a specific mechanism, for example, an exciton-, hole-, or electron-induced one. 6.1.1.4. Reactivity of BAlq. Low cost and versatility of Alq3 as electron-transport material and emissive layer host resulted in its

reaction conditions itself do not resemble OLED operation conditions at all. The authors introduced via spin-coating the 8-Hq-derived polymeric material into a conventional OLED structure and observed that the electroluminescence is suppressed relative to a reference sample without the polymer. Both samples showed a similar I−V characteristic. A quenching behavior of various 8-Hq derived-products, which may also include carboxyl containing molecules, was inferred. Liao et al. showed evidence of the proposed water-induced dissociation step of Alq3. They doped a water/oxygen treated (5 min on air) Alq3 bulk layer with Li and found a well-pronounced healing procedure, which fully recovers the water/oxygen caused quenching reaction/process.495 They proposed that Li reacts with the evolved 8-Hq as well as with the introduced water and oxygen. At this point, a direct evidence of the formation of Liq in situ from free 8-Hq is not available. Overall, the reactivity of Alq3 with oxygen and water has been reasonably well established, but it is likely to be irrelevant from the perspective of OLED degradation in operating conditions where extremely low concentrations of oxygen and water molecules are present after vacuum evaporation. 6.1.1.3. Dissociation and Further Reactions of Alq3. We investigated aged OLEDs with the highly sensitive analyzing technique “laser desorption ionization time-of-flight mass spectrometry” (LDI-TOF-MS), where we could show the dissociative processes involving Alq3 under conventional OLED driving conditions.26 These investigations were performed under oxygen- and water-free conditions. An unaged OLED was compared to an aged one, and newly appearing signals were classified as products resulting from chemical degradation reactions within the emission layer as well as with adjoining blocker layers. Two main signals/products were identified as directly related to the degradation of the OLED: the signal at 315 Da, which corresponds to a dissociated fragment of AC


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Figure 23. Proposed dissociation of BAlq after the excitation of the material. The mechanism is investigated with the LDI-TOF-MS technique. Figure created with data from ref 206.

Figure 24. Chemical structure and reaction pathway of the dimerized Ru-complex [(bpy)2(H2O)RuORu(OH2)(bpy)2]4+, proposed by Kalyuzhny et al. Figure adapted with permission from ref 497. Copyright 2003 American Chemical Society.

ubiquitous presence in OLED research and development, especially in early devices and academic research. However, its incompatibility as a host with phosphorescent emitters and even blue fluorescent emitters led to a search for a higher bandgap replacements. Although most practical solutions ended up with materials of completely different chemical classes, some higher bandgap analogues of Alq3 were developed as well. Arguably, the most notable material of this kind is known as BAlq. The main structural feature of this molecule, setting it apart from Alq3 and responsible for the increased bandgap, is the 2-methyl substituent on the quinolate ligands. However, the presence of this substituent also results in too much steric congestion in a hypothetical complex bearing three identical ligands, making it synthetically inaccessible. Because of that, BAlq molecule is designed with only two bidentate methylquinolate ligands and one monodentate phenolate ligand. Generally, bidentate ligands bind metal much stronger than monodentate ligands. BAlq does not appear to be an exception, and, because of the presence of a weaker bound phenolate ligand, it is more susceptible to hydrolysis and ligand loss and exchange during purification and handling. Nonetheless, it provided a significant boost in the stability of green phosphorescent devices when used instead of phenanthroline derivative as adjacent-to-emissive layer.67 It also provided a significant boost in efficiency when used instead of Alq3 in a similar role in green phosphorescent devices. The increased efficiency was originally attributed to exciton and hole blocking ability of the layer between the emissive and electron transporting layers, which received a common name “hole blocking layer” (HBL). However, it is noteworthy that blocking hole current at the cathode side of a typical emissive layer cannot be expected to increase the efficiency significantly because very few holes reach that interface, and recombination of those that do reach it produces little light because of the unfavorable position in an optical cavity. Therefore, preventing these holes from injecting into ETL is mostly moot from the efficiency perspective. Similarly, exciton blocking does not appear to be important because the exciton diffusion lengths in typical phosphorescent devices are shorter than the distance between

the recombination zone and ETL layer boundary. In fact, green phosphorescent devices with excellent efficiencies have been constructed using anthracene derivatives in place of BAlq. Considering very low triplet energy and shallow HOMO of anthracene, the layer commonly referred to as HBL has a different function in the device. In case of BAlq in green phosphorescent devices, the most likely function is to improve electron injection between ETL such as Alq3 and an EML host with a shallow LUMO level such as CBP. As mentioned above, BAlq shows improved performance in comparison to the electron transporting materials TPBi, CBP, BCP, and BPhen.25,69,72,486 Another advantage is the higher glass transition temperature of BAlq as compared to those of BPhen and BCP.486 Other results suggest that BAlq does not react with phosphorescent Ir-based emitters (see section 6.1.3), whereby other ETLs like Alq3, BPhen, and BCP do.29 Nevertheless, LDITOF-MS investigations on Alq3 and BAlq single layers show that both Al-based materials fragment in a similar way during laser irradiation.206 Alq3 fragments into Alq2+ and q− in contrast to BAlq (512 Da), which loses the p-phenylphenolate ligand (169 Da), and an analogue positive charged fragment [Al(Me-q)2]+ (343 Da) is formed. These investigations led to the conclusion that BAlq may fragment in an aging OLED according to an dissociative pathway, displayed in Figure 23, where the p-phenylphenolate will dissociate from the molecule. Because of this preferred reaction pathway, the dissociation of a quinolate ligand may be suppressed. It might be possible that the dissociation of pphenyl-phenolate is reversible, which has not been verified yet. Investigations of highly stable red phosphorescent OLEDs, utilizing BAlq as matrix material, exhibit an additional reaction step, where the [Al(Me-q)2]+ fragment coordinates with an BAlq2 molecule by forming a charged [BAlq2+Al(Me-q)2]+ molecule.496 This kind of reaction was found for devices driven above 150 mA/cm2, corresponding to a luminance above 8000 cd/m2. 6.1.2. Ruthenium Complexes. In 2006 Soltzberg et al. published a degradation analysis of a single layer light-emitting electrochemical cell (LEC), where a ruthenium-based complex AD


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was spin coated on ITO and afterward covered with Au contact.27 This investigation was the first proof in the literature of an internal chemical reaction associated with organic electroluminescence. The active material of the investigated LEC is based on the ionic metal complex [Ru(bpy)3]2+ (Figure 24). For this compound, it is assumed that it reacts with water by creating fluorescence quenching reaction products.497 With the help of matrix assisted (MA) LDI-TOF-MS measurements, where the aged LEC was scratched from the substrate and mixed with a suitable matrix, a dimer-like reaction product was found. This product exhibits a oxygen bridge between both core Ru atoms ([(bpy)2(H2O)RuORu(OH2)(bpy)2]4+) as well as two coordinative bonded water molecules. Comparing the results with synthesized oxygen bridged species, the results could be confirmed. Nevertheless, the origin and role of water within the reaction pathway could not be evaluated in detail. 6.1.3. Dissociative Reaction Mechanism of Iridium Complexes. As a representative of the d-shell group of the periodic table, iridium forms a wide range of different organometallic complexes, which provide a variety of different materials with emissive character over the entire range of the visible spectra.498,499 In general, using heavy metals leads to an enhanced spin−orbit interaction and therefore to a more efficient triplet emission.500,501 The excitation of such molecules can lead to the formation of the metal-to-ligand-charge-transition states 1MLCT or 3MLCT, meaning that the ligands may be viewed as partially reduced while the positive charge remains on the central atom. Up to now, the exact reaction mechanism(s) leading to the different observed dissociation products are not finally discovered. See Figure 25 for examples of possible reactions, which are valid for different known materials.25,29,421,496,502,503 Here, the different binding atoms at the ligands are labeled A and B, as well as X and Y, in case of a further complexation, where X and Y may also be N, O, or other atoms with free electron pairs. M indicates the metal atom Ir, or another metal of common metal organic compounds. The dissociation reactions of such compounds may occur after the electronic or thermal excitation of the molecules (see (a) in Figure 25). It is likely that the overall dissociation does not take place in one single reaction step. Instead, the reaction may occur in a stepwise manner, where the first step is the “opening” of one side of a ligand. Following the exciton-induced dissociation of metal organic compounds, further reaction steps occur, which may form more complex molecules or ions within the recombination zone of the OLED. Such complex formation may be the dimerization (b) of the molecule itself by bridging the two core metals by a ligand. Another case is the reaction of the excited Ir-based molecule with another ligand, such as BPhen or BCP, forming the corresponding complex (c). Additionally, one may think about hydrolyses steps (d), which may occur as well. In the following, we will review several known reactions between Ir-based charged fragments and adjacent molecules. 6.1.3.1. Complex Formation between Ir-Based Fragments and HBL Materials. One recent finding in OLED degradation is the dissociative reaction of Ir-based emitters, like Ir(MDQ)2(acac), followed by a subsequent complexation step with other materials. Similar reactions are found to occur for the related complexes: Ir(ppy)3, Ir(piq)3, FIrpic, and facial-tris(1phenylpyrazole)Ir(III) (fac-Ir(ppz)3).400,502,504,505 It may be speculated that the dissociation of the material occurs as a first step due to an excitation (see also Figure 25). This excitation may occur by exciton creation due to recombination in OLED

Figure 25. Scheme of the dissociative reaction mechanism of metalbased phosphorescent emitters and possible following reactions. Binding atoms (e.g., N, O) at the ligands (bold lines) are labeled as A and B, as well as X and Y, in case of a further complexation partner. M indicates a metal atom such as Ir. Different numbers are for a better distinction.

operation25 or due to applied external light.29 Next, the resultant fragments react with adjacent molecules. In case of Ir(MDQ)2(acac), the presence of the acac ligand, which may act as an effective leaving group as an anion, suggests that the dissociation is likely to occur heterolytically, yielding [Ir(MDQ)2]+ cation. Figure 26 illustrates a subsequent reaction of [Ir(MDQ)2]+ with the HBL BPhen, which is well-known to complex with various metal ions to form chelates.506−508 As indicated by the complexation behavior with various metal ions, complexes with BPhen are quite stable. The sequence of reactions, like that described, may result in an overall irreversibility of the dissociation of Ir-based emitters by forming charged complexes or complexes with band gap characteristics different from those of the former emitter molecule. Although complexes of this type have not been isolated and studied, it is possible that they will be nonemissive and act as luminescence quenchers or nonradiative recombination centers. Analogous complex formation was not found exclusively between different Ir-based emitters and the BPhen, but also for the singlet emitter Alq3. It is indeed possible to show that similar reactions occur with other HBL materials as well.25,400,421 In ref 29, different observed complexes with several HBL materials were suggested. Besides the reaction with BPhen, the complexAE


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Figure 26. Complexation of [Ir(MDQ)2]+ with BPhen as a direct result of the dissociated Ir(MDQ)2(acac) emitter molecule in the direct neighborhood of a BPhen HBL molecule.

light-induced reactions appear to have a similar chemical mechanism as compared to exciton-induced reactions generated by the hole−electron recombination in an OLED.247,423 To confirm this, we conducted an experiment by comparing UVlaser light treated samples and EL-driven (aged) OLEDs with the same EML|HBL sequences.29 Five different blocking materials (TPBi, BCP, BPhen, Alq3, and BAlq) were investigated. In this experiment, a close correlation between the laser-induced complex formation of a certain emitter-fragment-blocker complex and the lifetime of a corresponding OLED could be demonstrated. Thus, the formation of complexes between the Ircontaining fragment of Ir(MDQ)2(acac) and the HBLs was caused by the laser light (excitation wavelength was 337 nm) of the LDI experiment. It was possible to show that the strength of the complex building mechanism can be expressed by the signal ratio, which varied with the used HBL. In the second part of the experiment, the corresponding OLEDs were used, and the electroluminescence decays over time were investigated. Plotting both results against each other in one graph, the intensity ratio between the complex against the fragment signals and the lifetime, the correlation is fairly obvious in the double logarithmic scale (see Figure 27). Because of the specific reaction mechanism with the HBL TPBi, the data of the corresponding complexes ([Ir(MDQ)1(TPBi-H+)]+ and [Ir(MDQ)2TBPi]+) differ from

ation with BCP, Alq3, and TPBi is known. As stated in section 6.1.1.4, the HBL BAlq does not show any reaction with other materials, and Ir-based emitter fragments make no difference. This can be explained by the mentioned dissociative reaction of the phenylphenolate, which has only one binding side and is therefore not able to bridge between Al and Ir like it is assumed for Alq3. One remarkable reaction is the apparent complexation of the Ir-based fragment with the HBL TPBi.29 From mass spectra, it is known that the ion [Ir(MDQ)2(TPBi)]+, formed from the first emitter-based fragment, exists as a detectable ion. Another signal was observed with a substantially higher intensity, which was assigned to the reaction product [Ir(MDQ)1(TPBi-H+)]+. In the course of investigating other emitter materials in combination with the HBL TPBi, similar products were also detected. For example, with FIrpic as emitter, the corresponding reaction product with TPBi in aged OLEDs was identified as a monofluorinated species [Ir(F1ppy)(TPBi-H+)]+. This suggests that, after a complexation reaction between the HBL TPBI with the first fragment of the emitter [Ir(F2ppy)]+, a proton abstraction from the TPBi occurs, which itself is assisted by a reaction with a fluorine atom.504 The reaction products between the Ir-based emitter fragments and the different HBLs were identified by investigating aged OLEDs with LDI-TOF-MS. Because of the excitation of the material by UV light (337 nm), one might argue that these reactions are at least partly caused by the laser excitation. To address this potential issue, a direct comparison of aged and unaged devices was performed. Investigations further revealed that such reactions can be suppressed by using low intensity laser light.24,25 Indeed it is quite obvious that fragmentation even at the emitter molecules occurs, which creates charged molecules or even other species of nonradiative recombination centers. One can therefore argue that the final reaction between the HBL and the fragment will not occur in the OLED, but is rather due to the LDI conditions in the LDI-TOF-MS experiment. This is indeed a possibility, which does not affect the conclusion about the dissociation of the emitter itself and has to be verified with other analytical techniques. Nevertheless, other methods like XPS measurements support the finding of a strong interaction between the emitter and the HBL by measuring a charge transfer at the interface between both materials.174 As discussed in the next section, a correlation between the lifetime of a certain emitter|HBL-interface containing OLED and the measured value for a reaction constant for both materials (emitter fragment and HBL-material) provides additional support of the proposed in situ complexation reaction. 6.1.3.2. Correlation between the Ability of Complex Formation and the Device Lifetime. As mentioned above,

Figure 27. Correlation between the complexation constant, expressed by the ratios between the complex and the Ir-based fragment during a LDI-experiment, and the lifetime of the corresponding OLED. Well visible is the dependence of the [Ir(MDQ)2(HBL)]+ complexes within the double logarithmic plot. Note that the complex type [Ir(MDQ)1(TPBi-H+)]+ does not fit in this behavior. Figure created with data from ref 29. AF


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Figure 28. Proposed decomposition reaction of FIrpic. Figure created with data from ref 504.

tional isomerization of Ir-based emitters due to device aging as well as due to thermal treatment.28,454,510 At this time, the contribution of the isomers to the degradation mechanisms is still unknown. One can argue that isomers are still able to emit albeit with different efficiency and color, which might cause a change in efficiency and color of the device. Although minor changes in emission color are often observed during device degradation, no definitive link to isomerization has been demonstrated so far. In case of the work of Baranoff et al., a change in the PL-emission of the treated material was not detectable whereas the absorption behavior was apparently affected.511 Another reaction found in aged OLEDs is the defluorination of the emitter molecules. Fluorinated compounds, like those who are known from the blue emitters (e.g., FIrpic or Fir6), tend to be able to eliminate fluorine atoms due to OLED processing (during thermal material evaporation) as well as due to OLED aging.248,451,510 Up to now, the mechanism of the observed defluorination as well as the quenching mechanisms of the reacted species are still unknown.

the expected correlation behavior in contrast to the other stable ions ([Ir(MDQ)2(HBL)]+). This experiment demonstrates a correlation between the observed lifetime behavior of certain OLEDs and the proposed degradation reaction between the red phosphorescent emitter Ir(MDQ)2(acac) and different HBLs. As mentioned in ref 21, the synthesis and the intentional doping of such identified complexes are still outstanding. 6.1.3.3. Curious Behavior of the Picoline Ligand − CO2 Abstraction. Another interesting reaction associated with device degradation was found for picoline ligand-containing emitters. Besides the fragmentation and the complexation, a decomposition reaction of the picoline ligand was observed in case of FIrpic. The tentative reaction mechanism is depicted in Figure 28, where the elimination of the neutral and thermodynamic stable CO2 is shown. On the basis of LDI-TOF-MS measurements on FIrpic-single layers, where a MS signal corresponding to a loss of CO2 fragment from FIrpic was detected, the reaction pathway was initially proposed in 2008400 and could be confirmed in 2011 by the clear detection of the degradation product in degraded OLEDs.504 Because of the formation of thermodynamic stable CO2, which is sufficiently small to diffuse through organic layers, the degradation of FIrpic is likely to be irreversible. Depending on the used host material for Firpic, the lifetime of an OLED can be enhanced.509 6.1.3.4. Other Known Chemical Reactions on Ir-Based Emitter Molecules. Some groups have reported the conforma-

6.2. Reactions of Aromatic Amines

The chemical reactions of various aromatic amines were extensively studied in degrading OLED devices.21,23 It was concluded that the degradation appears to be initiated by a minor, yet significant fraction of recombination events yielding respectively small, but non-negligible population of excited states of arylamines. Because the singlet excited-state energy and carbon−nitrogen bond strength are comparable for typical AG


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Figure 29. Proposed reaction products (a−e, g) found for the degradation of α-NPD within a HPLC/MS experiment on an aged sample; the proposed reaction (below) is based on the assumption of a (prior occurring) homolytical bond cleavage of α-NPD into (c) and (d), and it is based on the detected reaction product (g) − a mass, which was found in an aged sample as well. Figure created with data from refs 21 and 23.

found to be useful as host and EBL in phosphorescent OLED devices.9 In good agreement with solution electrochemical reversibility, the studies of hole-only and electrons-only devices revealed that neither holes nor electrons alone appear to initiate chemical reactions of α-NPD.23 On the other hand, investigating a regular bipolar device led to the conclusion that α-NPD degrades in the singlet excited state, which is formed primarily via recombination of holes and electrons or even by absorption of light. This was confirmed by using samples degraded with UV light. Using sublayers of electronically identical and chemically distinguishable arylamines, it was demonstrated that the reaction of α-NPD occurs in the immediate vicinity of the α-NPD|EML interface, and the degraded products are confined within 2−10 nm of that interface. The reaction products of α-NPD were detected with HPLC/ UV and HPLC/MS methods, thus allowing the measurements of quantities of formed degradation products and losses of initially present materials as well as assignment of masses. The main reaction products of α-NPD were identified as fragments of the original molecule as well as some adducts of α-NPD and its fragments. In Figure 29 are depicted most of the identified reaction products of α-NPD. 23 Additionally, a product

OLED hole transport materials, homolytic bond dissociations are likely to take place. The initially formed σ-radicals are highly reactive and capable of unselective addition to almost any aromatic systems to form stabilized delocalized π-radicals. Other radical reactions, such as hydrogen atom abstractions, radical recombinations, additions, and disproportionations, were proposed to involve both carbon- and nitrogen-centered radicals, forming a very complex mixture of degradation products, ranging from fragments of the initial arylamines to polymeric materials that were observed in HPLC and GPC analysis. We will now discuss reactions of such specific aromatic amines, which are frequently used as hole transporting, electron blocking, and matrix materials. 6.2.1. α-NPD. Together with Alq3, α-NPD is one of the archetypal OLED materials. α-NPD was found to be completely reversible in the oxidation and reduction cycle in cyclic voltammetry measurements. Electrochemical or chemical oxidation can be used to produce solutions of α-NPD in cation-radical form, which are stable for days. Even the double oxidation to produce [α-NPD]2+ cation was found to be similarly reversible. This material shows a moderate glass transition temperature of ∼95 °C, which is sufficient for most of practical purposes.30 In addition to common use in HTL, α-NPD was AH


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Figure 30. Proposed reaction products of TAPC via different possible homolytic cleavage reactions; products of (i)−(iii) are found in the HPLC/MS analysis of aged samples. Figure created with data from refs 21 and 23.

originating from a α-NPD molecule and a phenyl fragment was detected in heavily aged OLEDs.496 On the basis of the molecular structures of products generated in electrically and photoexcited devices, it was concluded that the degradation of α-NPD and structurally related aromatic amines was caused by their relatively weak carbon−nitrogen bond, which was calculated to be similar in energy to the excited singlet state of α-NPD. As mentioned above, the initially generated radicals may further react with other molecules to form additional products, molecules with higher molecular weights than the initially present materials. The radicals may recombine and disproportionate with other radicals, and undergo a wide variety of reactions as ions (charged closed shell species) after trapping hole or electron. In Figure 29 are depicted some examples of such reactions. The generated radicals are expected to be long-lived because their extended π-system stabilizes the unpaired electron, preventing it from attacking neighboring molecules in a solid matrix, where molecular diffusion is virtually impossible. Such radicals are also expected to act as deep traps, nonradiative recombination centers and fluorescence quenchers. Additionally, the presence of such radicals will lead to an irreversible hole trapping in HTLs and manifest as accumulation of positive fixed charge.23 6.2.2. TAPC. TAPC as a common hole transporting material is found to degrade in a qualitatively similar way to α-NPD within the operating OLEDs.23 This molecule exhibits several differences in the molecular structure, which, at least in some device types, results in greatly accelerated loss of luminance efficiency. Although this effect has been reported very early in the development of HTMs in OLEDs, the nature of such instability was not understood at that time.30,44,47,411

Theoretical analyses predict that, due to one of the sp3-carbons in the cyclohexyl ring being attached to aromatic rings, the TAPC structure has relatively weak exocyclic carbon(sp3)−carbon(sp2) bonds with the dissociation energies calculated to be almost exactly the same as carbon−nitrogen bonds in the same structure (nitrogen−carbon bond (75−76 kcal/mol)).23 Additionally, there are endocyclic carbon(sp3)−carbon(sp3) bonds with calculated energies as low as 64 kcal/mol. Although the lower dissociation energy of these bonds may in itself suggest a faster rate of chemical degradation, it is possible that, by virtue of being part of the cyclohexyl ring, the dissociation of these bonds is more reversible than the dissociation of exocyclic carbon− carbon and carbon−nitrogen bonds, and therefore less damaging in an OLED device. Chemical analysis of the degradation products confirmed that not only the relatively weak carbon− nitrogen bonds undergo dissociations, but also the carbon(sp3)− carbon(sp3) and carbon(sp2)−carbon(sp3) cleave to yield a wide range of degradation products. Some of the proposed reaction products and pathways are depicted in Figure 30. Similar to the α-NPD case, the initial step of the degradation is proposed to be the homolytic dissociation of the excited molecule. It is noteworthy that, as compared to α-NPD, TAPC not only has additional weak bonds, but also substantially higher energy of the first excited singlet state (∼10 kcal/mol above the respective state of α-NPD), which is the most plausible reason for higher relative instability of various OLED devices utilizing TAPC as an HTM immediately adjacent to the emissive layer. Evidently, the higher reactivity of excited singlets of TAPC is considerably more important than their expectedly lower concentration (the excited singlet of TAPC is expected to be less likely formed during recombination and more likely to be quenched by adjacent emissive layer materials). AI


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Figure 31. Proposed reaction pathways of CBP. (a) The dissociation of the CBP after the excitation of the material, and (b,c) further reactions of the generated radicals. Part (d) shows the mass balance of the CBP degradation. Figure created with data from refs 22 and 209.

commonly used transport materials TCTA and CBP, both of which are frequently used as emission layer components as well. From previous investigations, carbazole radical cations are known to tend to dimerize by forming 3,3′-biscarbazole in solution,20,512 which is additionally proven by cyclic voltammetry measurements, where irreversible oxidation at position 3 of the carbazole unit is detectable.513 6.3.1. Reaction Pathway of CPB. As was already mentioned, the CBP was one of the first materials found to undergo chemical transformations caused by OLED device operation.22,209 For this analysis, an OLED with the main components CBP as emission layer matrix, Ir(ppy)3 as the emitter, and α-NPD as the HTM was chosen. Because the degradation of α-NPD is relatively slow and confined to a narrow region, the bulk of the thick HTL remained approximately unchanged relative to CBP and Ir(ppy)3, which provided a

Interestingly, it was pointed out that the chemical degradation of TAPC does not appear to remain confined to the immediate vicinity of the emissive layer and, in contrast to α-NPD devices, gradually spreads into the bulk of the HTL. However, this difference may be simply due to a much faster degradation of the TAPC-containing devices, rather than some material-related qualitative change of degradation mechanism. In other words, the apparent expansion of degradation into HTL bulk may be related to a much larger extent of degradation, which cannot be achieved with α-NPD-based devices because of practical constraints on the duration of experiments.23 6.3. Reaction of Carbazole Derivatives

Carbazole derivatives are a frequently used class of OLED materials, chemically related to aromatic amines. Their degradation behavior received considerable attention since 2006. In the following, we will have a closer look on the AJ


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Figure 32. Possible reaction pathway of the degradation of excited TCTA; the signal of 4 was found in aged devices. Combining the LDI results, the chemical homolytical pathway 1 can be proposed, where no hints for the homolytic cleavage are observable after pathway 2.

Similar to the α-NPD and TAPC mechanisms discussed above, the initial step of the CBP degradation is proposed to be the homolytical cleavage of the carbon−nitrogen between the carbazole and the biphenyl moiety of an excited CBP molecule. The authors obtained a theoretically calculated dissociation energy for the C−N bond of 84 kcal/mol, which is comparable to the first excited singlet state energy of the material.514 The resulting radicals (the carbazolyl and the 4-(N-carbazolyl)biphenyl radical are expected to react further, as depicted in Figure 31. Electron paramagnetic resonance (EPR) studies show the radical formation within an UV-aged CBP single layer.209 It is widely expected that the formed free radicals may undergo various further reactions, which also will lead to a subsequent degradation of the active region and an accumulation of degradation products within.23 The resultant long-living πradical species, even in their charged form as well, may act as nonradiative recombination centers and luminescence quenchers. Quantitative analysis of degrading OLEDs made it possible to determine the mass balance of the CBP conversion during the OLED aging process (see Figure 31d).209 For example, for a given number of 1000 molecules of CBP initially present in the OLED, 209 were consumed after 4000 h of operation at 40 mA/ cm2, approximately 72 molecules of BPC were formed, as well as approximately 8 molecules of carbazole, 9 molecules of 3-CCBP, and an estimated 10 molecules of other detected, but

convenient reference to quantify changes in the emission layer. In the course of device operation, both CBP and Ir(ppy)3 showed a substantial loss in relative quantities, indicative of chemical transformation into some degradation products. Several chemical products, structurally related to CBP, were also detected and tentatively identified (in one case identification was performed via isolation and comparison of 1H NMR spectrum to an authentic sample). In Figure 31 the reaction products and the proposed reaction pathways are exemplified. Using quantitative chemical analysis, it was also demonstrated that the accumulation of one of the reaction products of CBP (the identified BPC) is directly related to the total charge passing through the device.209 Additional investigations on single carrier devices reveal that the unipolar currents (either electrons or holes) do not result in any detectable changes in chemical composition of the devices. These findings strongly suggested the pivotal role of the electronically excited molecules in the investigated OLED devices. The excitation may occur either by the recombination of holes and electrons to forming an excited state or by photoexcitation processes, which could also be demonstrated with UV-degraded samples. The additional confirmation of the role of excited states in degradation of CBP was obtained by demonstrating that the introduction of luminescence quenchers reduces the rate of chemical transformation of photoexcited samples.209 AK


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Figure 33. Proposed reaction scheme of the dimerization reaction of the two phenanthroline derivatives BPhen and BCP. Figure created with data from ref 206.

the blocking layers.443,444 The corresponding device was aged at 15.3 mA/cm2 at an initial luminance of 5900 cd/m2 until 6.8% of this brightness was reached. The comparison between aged and unaged samples revealed the chemical reaction products (4) as indicated in Figure 32.515 In principle, in addition to homolytic dissociation depicted in Figure 32, two heterolytic reaction pathways could be envisioned as well. However, they are less likely considering the nonpolar nature of the medium and large energy penalty associated with charge separation. Because the potential reaction products related to 2b and 3b were not detected in aged OLEDs and in UV light exposed films, the pathway 2 in Figure 32 is unlikely to play a major role in the degradation mechanism. This conclusion is also consistent with the computational estimates of higher dissociation energy for the carbazole−phenyl bond as compared to the amine−phenyl bond. Nevertheless, it should not be dismissed completely due to some detection of carbazole ions using the negative detection mode in LDI-TOF-MS measurements on unaged OLED

unidentified products. Additionally, it was concluded that more than one-half of the bulk of the degradation products remained undetectable with the chromatographic method due to their irreversible retention on the column or due to being insoluble polymers. 6.3.2. TCTA. The triaryl-amine- and carbazole-containing material TCTA is structurally similar to the molecules discussed above. It is not surprising that this material also undergoes chemical transformations in operating OLEDs. Considering the versatility of TCTA, its degradation pathways are of some practical and theoretical interest. Investigations of TCTAcontaining OLED structures or films excited with UV light revealed a dissociative reaction involving a central amine moiety, which suggests the initial dissociative step to occur via electronically excited molecules.24 With the LDI-TOF-MS technique, it was possible to investigate the reactive behavior of the material within an degraded OLED, where the double emission layer structure consisting of a TCTA:Ir(ppy)3|TPBi:Ir(ppy)3 bilayer was used to separate the recombination zone from AL


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structures.516 A further reaction ((b) in Figure 32) of the initially formed radicals with TCTA molecules results in a complex product mixture. As depicted in Figure 32, multiple positional isomers may be expected. As discussed in previous sections, accumulation of degradation products, neutral radicals as well as their charged forms, is expected and leads to nonradiative recombination and exciton quenching manifesting as monotonic loss of luminance efficiency.

In Figure 33b, a tentative further reaction pathway of the phenanthroline dimers is also depicted. Unintentional Na-doped BPhen layers show the presence of the neutral dimer (2), indicating a further reaction of the charged dimers 1 and therefore 1a as well.206 Similar to 1/1a, the 2/2a structures are not supported by analytical methods capable of structure elucidation, such as 1H NMR. Additional to the found dimers of the phenanthroline derivatives, BPhen shows the ability to form trimers during laser excitation experiments.206 Nevertheless, this further polymerization could not be verified to occur in operating OLEDs so far. As it is known from phenyl amines, an extended π-system shrinks the band gap of the corresponding dimer/trimer in contrast to its monomeric species. This may lead to an additional (nonradiative) exciton decay directly on the EML|HBL interface.

6.4. Phenanthroline Derivatives

Because of the appropriate LUMO level and the well-known ability to bind metal ions, BPhen and BCP are suitable as electron transport materials capable of making effective injecting contacts with the commonly used cathodes, such as CsF/Al, Li/Al, Mg:Ag. Their deep HOMO levels are also believed to be useful in some cases to block holes.443,444,517 These materials can be doped with alkaline metals, forming n-type semiconductors capable of efficient injection of electrons into common emission layer materials.76,294,443,444,518 Comparing BPhen, used in ETL/ HBL, with Alq3, it is typically observed that that BPhen-based devices show substantially reduced lifetimes.33,519 Similarly, the phenanthroline derivative bathocuproine (BCP) is also known to be relatively unstable.64 Previously, some experiments were done to understand the lower stabilities of the phenanthroline-based OLEDs. For instance, for a mixture of BPhen and BAlq, it was found that the lifetime of the device increased with higher content of BAlq, which was interpreted in terms of the higher TG of BAlq.76 As discussed in section 6.1.3, the phenanthroline derivatives appear to be involved in degradation chemistry of OLEDs by acting as ligands to coordinatively unsaturated metal complexes that are believed to form during device operation. Aside from this type of reaction, which does not alter the chemical structure of phenanthroline moiety, the phenanthroline derivatives also show a remarkable propensity to dimerize.24,206 Such dimerization was detected in degraded OLEDs as well as during laser excitation. Because of the observation of the positive ions in LDI-TOFMS experiments and the known electrochemical instability of cation-radical of azines, it may be speculated that the initial reaction step is the formation of the respective cation-radical after electronic excitation or oxidation. In Figure 33, few plausible mechanisms are depicted for the two phenanthroline derivatives, BPhen and BCP. Our investigations via LDI-TOF-MS show clear evidence for this dimerization of the phenanthroline derivatives by forming two different species. Examples of tentative chemical structures of such products are shown as 1/1a and 2/2a in Figure 33. Reaction pathway 1 is essentially a nucleophilic addition reaction involving attack of a lone pair of one of the nitrogens of phenanthroline moiety on cation-radical form of the respective phenanthroline derivative. Alternatively, the process can be viewed as electrophilic addition of cation-radical to a lone pairbearing nitrogen of a neutral phenanthroline. The resulting product is detectable in aged OLEDs as well after the UV-treating within LDI-TOF-MS measurements. Although the exact reacting position of the cation-radical involved in the reaction is uncertain, we consider some alternatives to be less likely considering steric congestion (pathway 2 in Figure 33) and the absence of the corresponding reaction product (3a), which would be visible if pathway 3 were valid.24,25 Interestingly, the steric congestion associated with the substitution at position 2 (or 6) of the phenanthroline moiety appears to correlate both with the lower amount of the detected dimer and with the higher lifetime of the corresponding OLED.29,206

6.5. Reactions of Hydrocarbons

Various chemical reactions discussed in the previous sections involve reactive moieties such as weakly bonded groups (or ligands) or heteroatom-bearing substituents with strong nucleophilic or electrophilic properties. A multitude of dissociation reactions of the former and addition reactions of the latter appear to be allowed for electronically excited molecules or respective ion-radicals. Although the inverse statement that a molecule without such reactive moieties would be immune to in-device chemistry is not necessarily true from the perspective of formal logic, it may seem as an intuitive and practically useful guiding principle. The fully aromatic hydrocarbons present an excellent subject to test validity and generality of this notion: they typically lack bonds with less than 5 eV dissociation energies and strongly electro- or nucleophilic centers. Incidentally, such hydrocarbons, for example, anthracene and tetracene derivatives, have proven to be important as emissive layer hosts as well as appropriate emitter materials in fluorescent OLEDs due to high oscillator strengths and high quantum yields of fluorescence as well as low disorder in amorphous films, resulting in excellent electron- and holetransport properties and the absence of significant densities of trapped carriers. Even though the operational stability of the current generation of OLEDs is likely to be limited by chemical reactions of other device components, the potential inability of fully aromatic hydrocarbons to initiate in-device chemical reactions would be clearly crucial to achieve maximum device stability. In principle, one-by-one elimination of the device components capable of detrimental chemical reactions can be expected to eventually yield OLEDs with arbitrarily long lifetimes. From such perspective, examination of in-device reactivity of hydrocarbons is important even if their reactivity does not currently play a decisive role in determining device stability. Because in-device chemistry associated with OLED operation is typically dominated by more reactive components of the adjacent layers, such as arylamines in HTL and heteroaromatic materials in ETL, detecting products of chemical transformations of hydrocarbons does not prove their inherent reactivity in OLEDs. The brute force experimental approach such as elimination of all of the OLED materials with known reactivity from the device structure is difficult to implement because it essentially precludes carrier confinement and formation of an efficient heterojunction. Hence, it is not surprising that the initial detection of inherent reactivity of hydrocarbons in OLED-like conditions was reported in a model system, which employed photogenerated excited states in thin films.423 The films were AM


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Figure 34. Tentative mechanism of degradation of fully aromatic hydrocarbons exemplified by intermolecular cyclization of an anthracene derivative ADN.

structure of the identified product (b) (in Figure 34) corresponds to a cyclization of the hydrocarbon, formally releasing two hydrogen atoms per cyclized molecule. This type of chemical transformation resembles oxidative coupling, which is common for oxidized arylamines23 and carbazoles23,209 in solution. The photodegradation of hydrocarbons reaction likely follows a different mechanism: Instead of electrophilic addition of cation-radical, the initiation step, carbon−carbon bond formation, appears to involve neutral excited molecules and result in the formation of a metastable intermediate biradical (a). Although the mechanism depicted in Figure 34 is just an example of subsequent chemical reactions, it is likely that, on average, multiple quenchers/traps are formed as a result of a single cyclization. Furthermore, the degradation products such as (c), (d), (d), (f), and (g), resulting from formal addition of hydrogen atom to starting hydrocarbon, are not expected to be observable in typical methods of chemical analysis of OLEDs. They would either regenerate starting hydrocarbon during extraction or produce the same [M+1] signal as the starting hydrocarbon, known from LDI-TOF-MS experiments. Overall, the degradation mechanism specific for hydrocarbons appears to produce the higher yields of quenchers and traps per initiating chemical reaction, yet the lower yields of the chemically detectable species as compared to dissociative mechanisms of arylamines and carbazoles. The same degradation product of Figure 34 was found in low concentration (∼0.02%) after extended operation of OLEDs,423 which is consistent with the excited state of the anthracene initiating the same chemical degradation mechanism irrespective of whether the excited state is generated by absorption of a photon or by the recombination charges. Furthermore, the detection of high molecular weight products in photodegraded

produced by the same vapor process and protected by the same metal layer and encapsulation as regular OLED devices. In addition to being a supposedly impermeable oxygen and water barrier, the metal layer, in combination with the ITO layer on the substrate, allowed measurements of electric field- or lightinduced currents. The fully aromatic derivatives of anthracene and tetracene were chosen for these experiments because of the widespread use of these material classes in fluorescent OLEDs. Photoexcitation was performed with light filtered appropriately to exclude populating higher excited states, which are likely to be more reactive, yet inaccessible during OLED operation. The degradative processes in photoexcited films were clearly detectable by several physical effects: loss of photoluminescence intensity, decrease in conductivity, accumulations of populated deep traps, and increase in concentration of free electron spins.423 The last two effects allowed quantitative evaluation of concentrations of the degradation products detectable with these techniques, leading to a conclusion that the quantum yields of degradation of excited hydrocarbons are order-of-magnitude comparable with yields of degradation in operating OLEDs. Interestingly, the chemical detection of the degradation products has proved to be much more challenging relative to the similar experiments with arylamines and carbazoles:423 no significant losses of the starting hydrocarbons were observed for any practically accessible experimental conditions. This discrepancy between the quantitative results of physical measurements and chemical analysis suggests that the hydrocarbons may undergo fundamental different mechanisms of degradation as compared to arylamines and carbazoles. The detection and identification of a degradation product found in photodegraded film of an anthracene derivative (9,10bis(2-naphthyl)anthracene (ADN)) shed some light on this distinctive characteristic of degradation of hydrocarbons. The AN


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solid films suggests that the intermolecular coupling reactions also occur to a significant degree.

incident light and heating are to be considered as significant contributors to device instability, particularly when the highest ambient temperature is comparable to TG of some material in OLED. There are also some empirical guidelines for chemical design of OLED material, which are mostly based on anecdotal evidence connecting device instability to some certain features of molecular structure. Although these guidelines are rarely stated publicly even in some abbreviated form and are likely to vary considerably between research groups, it is probably fair to say that they typically result from a large body of empirical selection and optimization experiments within distinct industrial and academic organizations. Considering the highly competitive nature of OLED material market and apparent continuation of large-scale development efforts (as judged from the proliferation of patent publications), such extensive empirical optimization has so far proven nonideal as a way to eliminate OLED stability concerns, particularly for demanding applications. The authors hope that the subject of this Review, summarizing the knowledge of mechanisms associated with OLED degradation, would be instrumental in developing improved OLED materials and, consequently, longer-lived devices.

7. SUMMARY In this Review, we classified known degradation effects to give a broad overview about possible occurring mechanisms and reactions in operating OLEDs. Although we choose to focus on small-molecule-based OLEDs here, some chemical reactions of polymeric compounds are discussed as well in some cases. The focus on small-molecule devices can be justified by the fact that, today and probably in the near future, commercial applications of OLED are dominated by this material class. As discussed, during the past two decades, OLEDs have seen a dramatic improvement in device lifetimes. The initially often observed phenomenon of localized areas where the devices do not emit (“dark spots”) can be avoided today almost completely. Nonetheless, OLEDs still invariably age by a slow decrease of brightness, which is primarily due to reduced current efficiency. While for red and green devices excellent lifetimes of more than one million h at 1000 cd/m2 have been achieved, blue phosphorescent devices still lack the stability needed for widespread applications. Although many different degradation mechanisms have similar effects and appearances, they can be classified on the basis of external and internal causes of degradation. Although all known degradation mechanisms are discussed in some detail, this Review focuses on the chemical degradation mechanisms of different materials in OLEDs. In the past 7−12 years, the knowledge and understanding of different pathways of chemical reactions in operating OLEDs has improved dramatically. For some materials, actual chemical reactions have been identified and, in some cases, even quantitatively linked to efficiency losses in degrading devices. Nevertheless, predictions of device stability based on material properties and overall device architecture are still out of reach. This is partly due to the fact that the known chemical reactions can be thought of as a tip of an iceberg. In particular, the nature and reactions of reactive intermediate species (ionic and radical) remain highly speculative even though such species are likely to be directly responsible for degradation in performance characteristics. These deficiencies in our knowledge of in-device chemistries are primarily due to the lack of suitable available analytical tools for such thin and delicate devices: for example, a small percentage of reacted molecules located in only a minor fraction (recombination zone) of a nominal EML may lead to a decrease of light output by an order of magnitude. Considering that in every case of OLED degradation, where chemical products were quantified, there are many individual products formed in low yields, detecting and identifying such small amounts of reactive species is a great challenge for any available analytical method. This Review also gives a broad overview of the most common analytical tools suitable to investigate degradation phenomena in OLEDs. A rough classification of these tools according to their significance and field of application is also provided. So far, two analytical techniques, the HPLC-UV-MS and the LDI-TOF-MS, were the most widely used for trace analysis as well as for the identification and quantification of the degradation products. It is well-known that there are some empirical design rules to improve the lifetime of an OLED significantly. The usage of highly purified materials and substrates, high vacuum conditions in the evaporation tool, and an appropriate device encapsulation generally results in better device stability. External factors such as

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address ∥ Department of Physics, Kent State University, Kent, Ohio 44242, United States.

Notes

The authors declare no competing financial interest. Biographies

Sebastian Scholz received the Diplomchemiker degree from the University of Leipzig (Germany) in 2004. The topic of his diploma thesis was the epitaxial growth of III−V semiconductors. In 2008, he received a Ph.D. degree from the Technische Universität Dresden under the supervision of Karl Leo. His thesis focused on the characterization of chemical reactions occurring during OLED aging. After a postdoc period with Karl Leo’s group, he worked in 2010−2011 at the Fraunhofer Institute for Ceramic Technologies and Systems (Dresden) on topics like carbon nanotubes, silicon-based solar cells, and CVD prepared nanomaterials for hard coatings. Since 2011 he has worked at the Fraunhofer Institute for Photonic Microsystems (Dresden), focusing on the topic of atomic layer deposition for MEMS applications. He wrote ∼20 publications on organic semiconductor materials and AO


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devices, of which 15 publications are directly linked to degradation mechanisms in OLEDs.

Karl Leo obtained the Diplomphysiker degree from the University of Freiburg in 1985, working with Adolf Goetzberger at the FraunhoferInstitut für Solare Energiesysteme. In 1988, he obtained the Ph.D. degree from the University of Stuttgart for a Ph.D. thesis performed at the Max-Planck-Institut für Festkörperforschung in Stuttgart under the supervision of Hans Queisser. From 1989 to 1991, he was postdoc at AT&T Bell Laboratories in Holmdel, NJ. From 1991 to 1993, he was with the Rheinisch-Westfälische Technische Hochschule (RWTH) in Aachen, Germany. Since 1993, he is full professor of optoelectronics at the Technische Universität Dresden, and until 2013, he was also working at the Fraunhofer-Institution for Organics, Materials and Electronic Devices COMEDD. Currently, he is visiting professor at the King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi-Arabia. His main interests are novel semiconductor systems like semiconducting organic thin films, with special emphasis to understand basics device principles and the optical response. His work was recognized by the following awards: Otto-Hahn-Medaille (1989), Bennigsen-Förder-Preis (1991), Leibniz-Award (2002), award of the Berlin-Brandenburg Academy (2002), Manfred-von-Ardenne-Preis (2006), Zukunftspreis of the German president (2011), Rudolf-JäckelPrize (2012), and a Dr. techn. h.c. of the University of Southern Denmark (2013). He is the cofounder of several companies, including Novaled AG and Heliatek GmbH.

Denis Kondakov finished his doctorate from St. Petersburg University, Russia, working in the area of free radical chemistry of organometallics in 1991. After doing postdoctoral research in the field of early transition metal complexes-mediated synthesis with T. Takahashi at IMS, Okazaki, Japan and E. Negishi at Purdue University, IN, he joined Eastman Kodak to work on reaction mechanisms and photochemistry of azomethine photographic dyes. Since 2001, his work has focused on device physics, photochemistry, and material design of small molecule-based OLED devices. In 2011, he joined DuPont to work on printed OLED technologies.

REFERENCES (1) Tang, C. W.; Van Slyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913−915. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-emitting diodes based on conjugated polymers. Nature 1990, 347, 539−541. (3) Pochettino, A. Photo-electric properties of anthracene. Lincei Rendic. 1906, 15, 171−179. (4) Stark, J.; Steubing, W. Fluoreszenz und lichtelektrische Empfindlichkeit organischer Substanzen. Phys. Z. 1908, 9, 481−495. (5) Pope, M.; Kallmann, H.; Magnante, P. Electroluminescence in Organic Crystals. J. Chem. Phys. 1963, 38, 2042−2043. (6) Helfrich, W.; Schneider, G. Recombination Radiation in Anthracene Crystals. Phys. Rev. Lett. 1965, 14, 229−232. (7) Vincett, P. S.; Barlow, W. A.; Hann, R. A.; Roberts, C. G. Electrical conduction and low voltage blue electroluminescence in vacuumdeposited organic films. Thin Solid Films 1982, 94, 171−183. (8) Partridge, R. H. Electroluminescence from polyvinylcarbazole films: 1. Carbazole cations. Polymer 1983, 24, 733−738. (9) Meerheim, R.; Walzer, K.; Pfeiffer, M.; Leo, K. Ultrastable and efficient red organic light emitting diodes with doped transport layers. Appl. Phys. Lett. 2006, 89, 061111−061113. (10) Reineke, S.; Lindner, F.; Schwarz, G.; Lüssem, B.; Leo, K. White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459, 234−238.

Björn Lüssem studied electrical engineering at the Rheinisch-Westfälische Technische Hochschule (RWTH) in Aachen and the University of Bath and obtained his degree as Diplomingenieur in 2003. He prepared his Ph.D. thesis at the Research Center in Jülich, Germany, in the field of molecular electronics. His thesis concentrates on scanning tunneling microscopy of pure and mixed self-assembled monolayers and has been awarded the VDE-Promotionspreis and the GüntherLeibfried-Preis. After staying at the Materials Science Laboratory of Sony in Stuttgart from 2006 to 2008, he joined Prof. Leo’s group at the Technische Universität Dresden where he headed the Organic Light Emitting Diodes and the New Devices group. Recently, he started as Assistant Professor at Kent State University in OH. His main interests are new semiconducting devices based on organic materials and their differences from or similarities to inorganic semiconductors. AP


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