Degradation Mechanisms in Organic Light-Emitting Diodes with

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Degradation Mechanisms in Organic Light-Emitting Diodes with Polyethylenimine as a Solution-Processed Electron Injection Layer Sebastian Stolz, Yingjie Zhang, Uli Lemmer, Gerardo Hernandez-Sosa, and Hany Aziz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15062 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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ACS Applied Materials & Interfaces

Degradation Mechanisms in Organic Light-Emitting Diodes with Polyethylenimine as a SolutionProcessed Electron Injection Layer Sebastian Stolz*,≠,‡, Yingjie Zhang√, Uli Lemmer≠,∏, Gerardo Hernandez-Sosa≠,‡, Hany Aziz√ ≠

Karlsruhe Institute of Technology, Light Technology Institute, Engesserstr. 13, 76131 Karlsruhe, Germany ‡

√

InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany

University of Waterloo, Department of Electrical and Computer Engineering & Waterloo

Institute for Nanotechnology, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada ∏

Karlsruhe Institute of Technology, Institute of Microstructure Technology, Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

KEYWORDS: OLEDs, lifetime, polyethylenimine, electron injection layers, solution-processing

ACS Paragon Plus Environment

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ABSTRACT

In this work, we investigate the performance and operational stability of solution-processed organic light-emitting diodes (OLEDs), which comprise Polyethylenimine (PEI) as an electron injection layer (EIL). We show that the primary degradation mechanism in these OLEDs depends on the cathode metal that is used in contact with the EIL. In case of Al, the deterioration in OLED performance during electrical driving is mainly caused by excitons which reach and subsequently degrade the emitter/PEI interface. In contrast, in case of Ag, device performance degradation occurs due to an additional mechanism; hole accumulation at the emitter/PEI interface and a consequent drop in the emitter quantum yield. As a result, the operational lifetime of OLEDs that use PEI as EIL can vary significantly with the cathode material, and at a current density of 20 mA cm-2, LT50 lifetimes of ~ 200 h and < 10 h are obtained for Al and Ag, respectively. Finally, we show that the first degradation mechanism can be significantly slowed down by using a mixture of PEI and ZnO nanoparticles as EIL. As a result, the operational lifetime of OLEDs with an Al cathode is increased to more than 1000 h, without adversely affecting device performance. This lifetime is significantly longer than that of a LiF/Al reference OLED.

INTRODUCTION In order to exploit new markets for large-area, organic optoelectronic devices, fabrication costs need to be reduced. High-throughput printing techniques are believed to have a significant potential in this regard and could eventually enable low cost fabrication of organic solar cells or light-emitting diodes (OLEDs). The preparation of such devices by printing techniques requires solution-processable electron injection layers (EILs).1,2 These materials reduce the work-function


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of common electrodes and provide a good energetic alignment between the Fermi level of the cathode and the LUMO of the organic semiconductor. Conventionally, either alkali salts like LiF or alkaline earth metals like calcium or barium have been used as EILs / cathodes in organic optoelectronic devices. While these materials have been shown to enable the fabrication of OLEDs with a high efficiency as well as a long lifetime, they cannot be easily solutionprocessed. Over the last few years, amine-based materials have been extensively investigated as EILs in OLEDs.3–8 While a a wide range of such polymers is available, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE) have received a particular attention.5–7,9 PEI and PEIE can be used in combination with a multitude of electrode materials in normal as well as inverted device architectures and both materials enable the fabrication of highly efficient OLEDs. In addition to their efficiency, the operational lifetime of OLEDs is equally important with respect to the application in future products. The OLED lifetime can be limited by a variety of degradation processes, which can either be related to phenomena within the bulk materials or to inter-layer interfaces within the device. Typical degradation processes of the bulk material of OLEDs have often been shown to be due to the instability of excited states of either host or emitter materials.10–20 As a result, non-radiative recombination centers are introduced in the organic semiconductors and hence device efficiency drops. Interfacial degradation mechanisms can involve both organic/organic as well as organic/electrode interfaces. For organic/organic interfaces, it has been shown that the interaction of accumulated charge carriers with excitons can lead to exciton quenching as well as an increase in the driving voltage of OLEDs.21–23 Organic/electrode interfaces can also degrade by excitons that reach the interface. As a result, charge injection into the semiconductor is lowered and device efficiency drops due to ensuing


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changes in electron/hole balance.24,25 While the different degradation processes are well understood in OLEDs with evaporated EILs, lifetime investigations have been mostly neglected with respect to solution-processable EILs. So far, there has not been any comprehensive study on the

effect

of

such

materials

on

the

operational

lifetime

of

devices.

In this work, we prepare solution-processed OLEDs that comprise PEI as EIL and either Al or Ag as cathode, and we study degradation mechanisms in these devices. We show that the emitter/PEI/metal contact is prone to degradation by excitons in general. We also show that when Ag is used as cathode, an additional degradation mechanism, caused by hole accumulation at the emitter/PEI interface, occurs. As a result of that, the quantum yield of the used emitter, a Poly(p-phenylene vinylene) (PPV) derivative, drops rapidly and hence also the efficiency of the OLEDs. LT50 lifetimes at a constant current density of 20 mA cm-² of < 10 h and ~ 200 h are obtained in case of OLEDs with Ag and Al, respectively. We also show that the lifetime of the OLEDs with an Al cathode can be further improved by using a mixture of PEI and ZnO nanoparticles as EIL. Due to the ZnO nanoparticles, the emitter/EIL/Al interface becomes less susceptible to excitons and as a result the operational lifetime improves by a factor of 5.

EXPERIMENTAL SECTION PEDOT:PSS (2.8 wt.% dispersion in H2O, low conductivity grade, acquired from Sigma Aldrich) was diluted with 2-propanol (vol. ratio of 1:5) and filtered with a 0.45 µm PTFE filter prior to use. PDY-132 (Super Yellow, light emitting polymer), acquired from Merck KGaA, was dissolved in Toluene with a concentration of 5 g L-1. PEI was dissolved in 1-Propanol at a concentration of 0.5 g L-1. Zinc oxide nanoparticle solution (N-10, supplied by Nanograde Ltd.) was filtered with a 0.45 µm PTFE filter prior to use.


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Substrates (glass covered by 180 nm of ITO, 10 Ohm sq-1 from Kintec) were subsequently cleaned in deionized water, acetone and isopropanol under sonication for 10 min, respectively, and treated by O2 plasma for 5 min. PEDOT:PSS was spin-cast and annealed (135°C for 20 min) under ambient conditions. Spin-coating parameters were ω = 5000 rpm, a = 1000 rpm s-1 and t = 60 s such that layers of 25 nm were obtained. PDY-132 was spin-cast and annealed (115°C for 30 min) in an inert atmosphere. Spin-coating parameters were ω = 2000 rpm, a = 1000 rpm s-1 and t = 60 s so that a layer of 65 nm was obtained. PEI was spin-cast and annealed (120°C for 10 min) in inert atmosphere. Spin-coating parameters were ω = 5000 rpm, a = 1000 rpm s-1 and t = 60 s. TPBi, MoO3, Al and Ag were thermally evaporated in a vacuum system with a base pressure of 1·10-6 mbar. Devices were kept in a nitrogen atmosphere during measurements. All electrical stress tests used a constant current density of 20 mA cm-2.


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RESULTS AND DISCUSSION

Figure 1. OLED stack (a), LIV-characteristics (b), current efficiency (c) and power efficacy (d) of prepared OLEDs. Performance of OLEDs with a PEI EIL. We fabricate solution-processed OLEDs, utilizing Supper Yellow (SY), a PPV derivative, as emitting layer and PEI as EIL. The full device stack, including layer thicknesses, is shown in Figure 1a. As we have shown in our previous work, PEI does not form homogeneous layers on top of SY and agglomerates are observed. As a result, the


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PEI thickness cannot be reliably measured on top of SY. In the literature, however, it has been reported that efficient devices require the thickness of PEI layers to be close to 10 nm.5–7,9,26 As top electrode, we use two different metals, Ag and Al, and compare between their effects on device efficiency and stability. Ag was chosen as it can be processed by various printing techniques and is therefore a popular electrode choice for fully solution-processed OLEDs. On the other hand, Al was investigated because of its shallower work-function in comparison to Ag (~ 4.3 eV for Ag while values as low as 3.4 eV have been reported for Al 6,27). As such, it should allow for more efficient electron injection in comparison to Ag. In Figure 1b, the luminance current density - voltage (LIV) characteristics of the prepared OLEDs are shown. As can be seen, OLEDs with both electrodes exhibit diode characteristics. However, the driving voltages are lower for Al than for Ag. Luminance values as well as current density values are larger in case of Al than in case of Ag at any given voltage. The turn-on voltage (VON) (here, VON is defined as the voltage which is required for a luminance of 1 cd m-²) of the OLED comprising an Al top electrode is ~ 2.2 V. This value corresponds to the highest peak in the SY emission spectrum (556 nm ≈ 2.23 eV), indicating an efficient charge carrier injection of holes and electrons.28 When Ag is used as top electrode, VON is marginally higher with a value of ~ 2.5 V. In Figure 1c, the current efficiency of these OLEDs is plotted vs. the luminance. While both electrode materials perform similarly, the use of an Al electrode leads to a slightly higher maximum current efficiency of ~ 10.5 cd A-1 compared to ~ 10 cd A-1 for the OLED comprising the Ag electrode. As a result of this slight difference in current efficiency in combination with the difference in driving voltage, the maximum luminous efficacy of the OLEDs differs considerably (Figure 1d). The OLED with an Al top electrode exhibits a significantly higher maximum


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Figure 2. Electron-only devices with either PEI/Al or PEI/Ag as cathode. The current density is more than one order of magnitude larger for Al than for Ag, indicating that electron injection is vastly superior for Al than for Ag.

luminous efficacy of ~ 10 lm W-1 compared to a value of ~ 6 lm W-1 in case of the OLED with an Ag top electrode. This difference in the performance of Al and Ag OLEDs can be understood by analyzing the current-density

voltage

(IV)

characteristics

of

electron-only

devices

consisting

of

ITO/ZnO/PEI/SY/PEI/metal (Figure 2). We use ITO/ZnO/PEI as bottom electrode as this material combination has been used as cathode in OLEDs due to its good electron injection capabilities.9 Therefore, under forward bias (i.e. when the ITO is at a more positive potential relative to the metal), only negligible hole-injection is expected from this electrode and the current in these devices consists predominantly of electrons that are injected from the top electrode. As a result, no light emission is observed.


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As Figure 2 shows, the IV-characteristics of these devices are strongly dependent on the metal which is used as top electrode. Over the whole voltage range, the absolute value of the current density, in this case electron current, is 1-2 orders of magnitude larger in case of Al than in case of Ag. This shows that the electron injection is vastly better for Al than for Ag, which is probably

a

direct

result

of

the

shallower

work-function

of

Al.

This result explains the lower driving voltage as well as the higher luminous efficacy observed in Figure 1 for Al in comparison to Ag. Interestingly, the current efficiency is observed to be very similar for both electrode materials and we will examine the reason for that in a subsequent section. Operational Stability of OLEDs with a PEI EIL. The operational lifetime of OLEDs with both electrode materials is studied by driving them at a constant current density of 20 mA cm-2 and recording both their luminance and driving voltage over time. In Figure 3a, the luminance, normalized to its initial value and the change in driving voltage (V-V0) is plotted vs. time. The initial luminance of the OLEDs was 1643 cd m-2 and 1893 cd m-2, while the initial driving voltage was 5.2 V and 3.9 V for Ag and Al, respectively. As can be seen, the LT50 lifetime of the two OLEDs differs by more than one order of magnitude. While the OLED with an Ag electrode exhibits a lifetime of ~ 8 h, the lifetime of the Al OLED is ~ 200 h. This significant difference in lifetime between Al and Ag was confirmed by testing multiple equivalent OLEDs. For each electrode material, at least four different devices were aged in this work and the average LT50 lifetimes were found to be 209±15 h and 6,9±1,3 h in case of Al and Ag, respectively. In the following, we will investigate which degradation mechanisms take place in both types of OLEDs, and we will explain what causes the significant difference in their lifetimes.


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Figure 3. Measurement of the operational lifetime at a constant current density of 20 mA cm-2 (a) and UV-aging (b) of both types of OLEDs. In panels (c) and (d), fresh and previously UVaged OLEDs are electrically driven and their degradation is compared to each other. Two possible explanations for the short lifetime of Ag OLEDs can be excluded. First, a comparison of the LIV characteristics of fresh OLEDs and OLEDs which were stored in a nitrogen atmosphere for three days after fabrication, indicate that the SY/PEI/Ag interface is not chemically unstable (Figure S1). Secondly, time resolved photoluminescence (PL) measurements show that the SY exciton lifetime is comparable in Al and Ag OLEDs (Figure S2). Furthermore, no change in the exciton lifetime is observed when the OLEDs are aged. Therefore, we can rule out the possibility of significant Ag diffusion into the SY layer during device operation.


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Primary Degradation Mechanism in Case of an Al Electrode. In an earlier work, we have shown that a common aging mechanism in OLEDs is caused by excitons that reach the electrode/organic interfaces in devices.25

Such excitons were found to cause cleavage of

interfacial bonds, and, as a result, the charge carrier injection from the electrode into the organic semiconductor is hampered. In order to test whether the OLEDs, prepared in this work, are susceptible to degradation by excitons, we test the effect of irradiating the OLEDs by external illumination with UV-light (365 nm, 5 mW cm-2). The UV-light is absorbed in the SY layer and as a result excitons are created. Therefore, the illumination by external UV-light allows us to study exciton-induced degradation without any impact from electrical current. Although the devices are not subjected to electrical bias during the light exposure, their LIV characteristics are measured and recorded at fixed time intervals during the test. Figure 3b presents the normalized luminance and V-V0 at 20 mA cm-2 versus the illumination time. As can be seen, both types of OLEDs are clearly affected by external UV illumination evident in the decrease in luminance and increase in driving voltage over time. Quite notably, the OLED with the Ag cathode is more strongly affected by the UV light than its counterpart with the Al cathode. For example, after 60 minutes of illumination, the Ag OLED exhibits a decrease in luminance of ~ 20 % and an increase in driving voltage by 0.35 V. In contrast, the Al OLED exhibits a luminance loss of only ~ 10 % over the same period of time. After 300 min of illumination, the luminance of this OLED is still at ~ 82 % of its original value and the driving voltage has increased by 0.55 V. It should be pointed out that, in these OLEDs, all layers are the same except for the top metal electrode. Since OLEDs with an Ag electrode are significantly more strongly affected by the UV illumination than OLEDs with an Al electrode, it is clear that the SY/PEI/Ag contact is susceptible to degradation by excitons. From figure 3b it can be seen that the Al OLED is also


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affected by the irradiation, although to a less extent. Such aging could be either due to the SY/PEI/Al or the ITO/PEDOT:PSS/SY contact being susceptible to degradation by excitons. In order to rule out that the ITO/PEDOT:PSS interface is involved in the observed degradation, we similarly test the stability of OLEDs comprising an alternative EIL (PFN) under external UV illumination. PFN is an amino-functionalized polyfluorene and it has been widely investigated as EIL in the last few years.3,29,30 In Figure S3, the respective data of the OLED using PFN/Ag as cathode is compared to the PEI/Ag OLED from Figure 3b. As can be seen, the PFN/Ag OLED is not affected by UV illumination indicating that the ITO/PEDOT:PSS interface is stable under external UV illumination. Therefore, it can be concluded that the SY/PEI/metal interface is degraded by external UV-light in case of both top electrode materials, Ag and Al. The fact that OLEDs with an Ag electrode are more strongly affected by the UV illumination than OLEDs with an Al electrode parallels the electroluminescence lifetime data for both types of OLEDs in figure 3a. Here, Ag devices are observed to be significantly less stable than the Al devices, suggesting that exciton-induced degradation of the PEI/metal contacts may be playing a role in the stability behavior of the devices under electrical bias. Following the UV stress, we subsequently drive the devices electrically at a constant current density of 20 mA cm-2, and study their stability behavior in order to investigate how it may have been impacted by the irradiation. In Figure 3c and Figure 3d, these measurements are plotted for Al and Ag, respectively. Data from fresh OLEDs, which had not been previously illuminated by external UV light, is also shown in the figures for comparison. The luminance of the UV-aged devices was normalized to the initial values before the UV-aging (i.e. the luminance curves start at 0.82 and 0.80 with the initial values equaling 1964 cd m-2 and 1689 cd m-2 for Al and Ag, respectively). Similarly, the change in driving voltage V-V0 refers to the initial voltage before the


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UV-aging procedure. Following the approach explained in our previous work, both the luminance and voltage curves of the UV-aged OLEDs were shifted in the x-direction so that the luminance curves of the UV-aged OLEDs intersect with the respective curves of the non-aged OLEDs at their starting point (i.e. the luminance curves intersect at a value of 0.82 and 0.80, for Al

and

Ag,

respectively).25

Experimental

details

are

explained

in

31

.

In case of the PEI/Al OLED in Figure 3c, the luminance curve of the UV-aged OLED follows that of the fresh OLED very closely. Since the fresh device is exclusively electrically aged, whereas the UV-aged one was first illuminated by external UV light and is afterwards electrically aged, their close similarity suggests that the effect of electrically aging the OLED can be replicated by illuminating it with UV light. This is also supported by the voltage curves of both OLEDs which again show very similar trends. It can, therefore, be concluded that the dominant aging mechanism in these OLEDs is caused by excitons, which degrade the SY/PEI/Al interface. The situation is different however in case of the lifetime data of the PEI/Ag OLEDs. This data is presented in Figure 3d. Here, starting from the intersection point of the luminance curves, the luminance of the UV-aged device drops faster than the one of the non-UV-aged device. Therefore, the aging behavior of this OLED cannot be entirely explained by the degradation of the SY/PEI/Ag interface by excitons. Instead, there needs to exist at least one additional major degradation mechanism in PEI/Ag OLEDs. This may also explain their significantly shorter lifetimes. Primary Degradation Mechanism in Case of an Ag Electrode. In order to get some insight into what additional aging mechanism could be relevant in case of the PEI/Ag OLEDs, we carry out delayed electroluminescence (EL) measurements. Delayed EL measurements can be used to


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assess differences in charge carrier accumulation among OLEDs. In this method, the OLED is driven using a pulsed forward bias with a pulse width of 0.5 ms. This pulse width is long enough to achieve steady state conditions in the OLED. During the time that the forward bias is applied to the OLED, an optical shutter blocks the resulting prompt EL from being detected by the photodetector. The optical shutter opens 0.3 ms after the forward bias has been turned off and the delayed EL can then be recorded by the photodetector. The delay time of 0.3 ms is many orders of magnitude longer than the exciton lifetime in fluorescent emitters such as SY. Therefore, all delayed EL can be expected to arise from excitons which are formed after the forward bias has been turned off. Such excitons are usually created by the late recombination of residual charges which accumulate mostly at device interfaces. To determine if the delayed EL indeed originates from delayed recombination of charges, as opposed to, for example, delayed emission from triplet-triplet annihilation, a reverse bias pulse is applied during the delayed EL signal collection. As a result of such a reverse bias, charge carriers are de-trapped or removed from interfaces and hence produce a sudden increase (a spike) in the delayed EL signal. A detailed description of the experimental setup can be found elsewhere.32–34


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Figure 4. Delayed EL measurements for reference OLEDs using a LiF/Al cathode with and without TPBi as HBL (a) and the respective measurements for PEI/Al and PEI/Ag OLEDs (b).


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As a reference data set, and to see how delayed EL characteristics can change depending on the extent of charge accumulation at interfaces, we first investigate a very simple OLED stack, consisting of ITO/PEDOT:PSS/SY/LiF/Al. This is basically a single-layer device, with the active layer (SY) being sandwiched between the anode (ITO/PEDOT:PSS) and the cathode (LiF/Al). The hole and electron mobilities in SY have been reported to be very similar.35 Furthermore, an efficient hole and electron injection can be expected in case of that device. Therefore, the recombination zone can be expected to be located close to the middle of the active layer. As can be seen in Figure 4a, the delayed EL signal of this device is very weak and applying a reverse bias of 5 V has a negligible effect on the signal, which may be due to a limited amount of accumulated charges. For comparison, we also fabricate and test an OLED with an organic/organic interface where charge accumulation can occur. For this purpose we use a device of the same structure as the first one but that further includes a 45 nm thick layer of TPBi in between SY and the LiF/Al cathode. TPBi is widely used as electron transport / hole blocking layer in OLEDs and its ionization energy has been reported as 6.2 eV compared to 5.45 eV for SY.36,37 Therefore, there is a large energy barrier for holes present at the SY/TPBi interface and thus holes can be expected to get accumulated at this interface. Furthermore, due to the addition of the TPBi layer which prolongs the electron transport path, the recombination zone in this device is expected to be located closer to the SY/TPBi interface. From looking at the delayed EL characteristics of this device it can be seen that they are significantly different from those of the first device. Without reverse bias, the signal is two to three times stronger relative to the first device, suggesting that there is a considerable amount of accumulated charge carriers present in the device, which recombine after the forward bias has been turned off. When a reverse bias is


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applied, these charge carriers are de-trapped and hence can recombine, resulting in a huge spike in the delayed EL signal. Demonstrating the differences in delayed EL characteristics in devices with versus without a significant amount of accumulated charges at an interface, we now examine and compare delayed EL in devices with PEI/Al and PEI/Ag contacts. The results are shown in figure 4b. A comparison of these signals to the ones in Figure 4a reveals multiple similarities. The delayed EL signal of the PEI/Al OLED is relatively weak in comparison to the respective signal of the PEI/Ag OLED, indicating that there is a difference in the amount of accumulated charge carriers in the OLEDs. However, in contrast to the LiF/Al OLED in Figure 4a, the application of a reverse bias leads to a spike in the delayed EL signals of both devices. At the same time, the height of the spike is considerably larger for Ag than for the Al. Therefore, the following conclusions can be made. First, in both OLEDs, holes get accumulated at the SY/PEI interface. And secondly, the number of accumulated holes is significantly larger in case of Ag than in case of Al. The accumulation of holes at the SY/PEI interface is probably the consequence of two properties of PEI. On the one hand, PEI is an insulator and hence it is expected to hinder (or block) the transport of holes from SY to the top electrode. On the other hand, it has been reported that tertiary amine groups, which are present in the backbone and side chains of PEI, act as hole traps.38 Such hole trapping would lead to the build-up of a positive space charge in the PEI layer which makes the transport of holes from SY to the top electrode even more difficult. The fact that less holes get accumulated at the SY/PEI interface in case of Al can be explained by the better electron injection capabilities of this electrode, as evident from the electron-only devices in Figure 2. As the electron current is considerably larger for Al than for Ag, the recombination


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zone is pushed further away from the SY/PEI interface and as a consequence, less holes are able to reach the SY/PEI interface. In order to get additional proof of the accumulation of holes at the SY/PEI interface, we also study hole-only devices with and without a PEI layer. The structure of these devices is ITO/MoO3/SY/(PEI)/MoO3/Al. Figure 5a shows the device structure as well as the IVcharacteristics of these devices. In the investigated voltage range, both devices exhibit unipolar charge transport without any light emission. However, the current density of the device with PEI is observed to be at least one order of magnitude lower over the whole voltage range, indicating that the PEI layer does indeed block holes. One can therefore indeed expect holes to accumulate at the SY/PEI interface over time, a phenomenon that can also be clearly seen from the time dependent measurements in Figure 5b. Here, both devices are driven with a constant current density of 5 mA cm-2 for 24 h and the change in their driving voltage to maintain this current is measured. In case of the device without PEI, the driving voltage is ~ 1.2 V and no change in voltage is observed over time. In contrast, the operational voltage of the device with PEI significantly increases over the course of the measurement. In the beginning, it is ~ 3.8 V and it increases to more than 7 V within the first hour of the measurement. Afterwards, it starts to saturate and a maximum value of ~ 8 V is observed after 24 h. Such voltage can be explained by the accumulation of holes at the SY/PEI interface. As a result, an electric potential builds up in the device and subsequent holes are repelled, increasing the external potential, which is necessary to sustain the current density.


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Figure 5. Hole only devices with and without PEI. IV-characteristics (a) and time-dependent


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measurements (b) confirm that PEI blocks holes. As a result, holes get accumulated in the SY film close to the SY/PEI interface. PL measurements of fresh and aged devices (c) show that these accumulated holes degrade SY as the quantum yield drops in case of the aged device with PEI. Here, the PL Intensity of the fresh devices was normalized to 1.

The hole blocking properties of PEI may also explain why similar current efficiencies for PEI/Ag and PEI/Al OLEDs are observed, despite the significant difference in electron currents in electron-only devices for the two electrodes (Figure 1c and Figure 2). As holes get accumulated at the SY/PEI interface, they are prevented from leaking through the OLED and hence the charge carrier balance is similar for Ag and Al. In order to check whether the holes, which get accumulated in the SY layer close to the SY/PEI interface, have a negative effect on SY, we carry out PL measurements on hole-only devices with and without the PEI (Figure 5c). For both types of devices, we compare the PL intensity of fresh and aged (j = 2.5 mA cm-2 for 24 h) devices by normalizing the PL spectrum of the fresh device to 1. As can be seen from figure 5c, in the devices without the PEI layer, the PL intensity of fresh and aged devices is the same. In contrast, when a PEI layer is introduced between SY and the top electrode, the PL intensity of the aged device is ~ 10 % lower than that of the fresh device. This suggests that the quantum yield of the SY decreases, which can be readily attributed to the accumulation of holes. While at first glance a 10 % decrease might be considered relatively modest, it is important to keep in mind that such accumulation would be primarily limited to the SY region close to the SY/PEI interface. Therefore, a decrease of the total SY PL signal by 10 % would correspond to a severe degradation of this part of the SY layer.

Furthermore, the delayed EL measurements in Figure 4 suggest that the exciton


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recombination zone is located in exactly that part of SY in case of PEI/Ag OLEDs. Therefore, the degradation of SY by accumulated holes should have a stronger effect on the OLED performance. One can therefore expect such hole accumulation to be the leading mechanism of EL aging in the PEI/Ag OLEDs and the main reason for their much shorter lifetime relative to the PEI/Al OLEDs. In order to confirm this conclusion we study the effect of increasing the number of (accumulated) holes on the lifetime of different types of devices. First, we investigate the aging of the reference OLEDs, which we studied in Figure 4a (Figure S4). As we pointed out previously, no accumulation of holes is observed for the device with a plain LiF/Al cathode whereas, in contrast, a significant accumulation of holes is observed when TPBi is introduced as a hole blocking layer in the OLED. As Figure S4 shows, the difference in lifetime between the OLED with and without a TPBi hole blocking layer is tremendous. Whereas the reference OLED, comprising a simple LiF/Al cathode, exhibits an LT50 lifetime of 300 h, the OLED which incorporates an additional TPBi layer has a lifetime of only ~ 10 h. This supports our hypothesis that the degradation of SY by accumulated holes is an important aging mechanism in these OLEDs. Secondly, we investigate the effect of varying the hole injection layer (HIL) in the OLEDs with PEI as EIL. In Figure S5, the IV-characteristics of hole-only devices with either PEDOT:PSS or MoO3 as HIL are presented. Over the whole voltage range, the current density of the MoO3 device is significantly larger than the one of the PEDOT:PSS device, suggesting that MoO3 is a more efficient HIL. Replacing PEDOT:PSS by MoO3 in OLEDs with PEI should therefore lead to an increase in the number of accumulated holes at the SY/PEI interface. In Figure S6, lifetime data of such OLEDs is shown. It is obvious that, regardless of the used


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electrode material (Al or Ag), using MoO3 as an HIL instead of PEDOT:PSS leads to a significantly shorter device lifetime. It has to be pointed out that, due to its acidity, PEDOT:PSS has been shown to negatively affect the ITO anode and hence the use of MoO3 as HIL usually leads to a better stability of organic optoelectronic devices.39,40 Therefore, we believe that the reduced stability of OLEDs with MoO3 is not related to the HIL/organic interface but instead confirms that the degradation of SY by accumulated holes is an important aging mechanism in these OLEDs. OLEDs with a PEI:ZnO Composite EIL. The significant accumulation of holes at the SY/PEI interface in Ag devices is, as explained previously, a direct consequence of the relatively poor electron injection capabilities of Ag in comparison to Al. Therefore, there is no straightforward way of improving the lifetime of these devices. One possibility would be to develop an emitting material which is not susceptible to degradation by holes. This however, is outside of the scope of this study and needs to be addressed in a future work. The situation is however different with respect to the degradation by excitons, which was found to be the primary aging mechanism for the PEI/Al OLEDs. It has been shown that the fluorescence of organic semiconductors can be quenched by metal nanoparticles due to energy transfer from the semiconductor to the nanoparticles.41–43 Instead of metal nanoparticles, we study if ZnO nanoparticles have a similar effect by mixing them with PEI and using that mixture as EIL in OLEDs. As ZnO is typically used as electron transport material and a mixture of ZnO and PEI has already been used as EIL in organic solar cells, we did not expect a significant negative effect of the nanoparticles on device performance.44 If the ZnO nanoparticles however quenched the fluorescence of the SY in the vicinity of the EIL, we should see an increase in operational lifetime of Al OLEDs.


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Figure 6. LIV characteristics (a) and current efficiency as well as luminous efficacy (b) of OLEDs, that use either plain PEI or a mixture of PEI and ZnO as EIL. In addition, a reference OLED with a plain ZnO EIL is shown.

In Figure 6a, the LIV- characteristics of an OLED which uses a PEI:ZnO composite EIL (wt. ratio of 1:7) are compared to those of reference devices, which comprise either a neat PEI or a neat ZnO layer as EIL. It can be seen that the OLEDs with PEI and PEI:ZnO perform very similarly. In contrast, the OLED with only ZnO exhibits a significantly higher driving voltage, indicating that electron injection from ZnO itself is not efficient. The current efficiencies as well


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as luminous efficacies of the OLEDs, which are presented in Figure 6b, support this conclusion. Here again, the OLEDs with either PEI or PEI:ZnO as EIL exhibit very similar values with a maximum current efficiency of ~ 10 cd A-1 and a maximum luminous efficacy of ~ 9.5 and ~ 8.5 lm W-1 for PEI and PEI:ZnO, respectively. The fact that the PEI OLED shows slightly higher efficiencies at lower luminance whereas the PEI:ZnO OLED exhibits slightly higher efficiencies at higher luminance could indicate that the charge carrier balance is slightly influenced by the addition of ZnO to the PEI. These differences are however small and both OLEDs exhibit a good performance. In contrast, the OLED with a neat ZnO EIL exhibits a significantly lower current efficiency as well as luminous efficacy with maximum values of only ~1.5 cd A-1 and 5x increase in lifetime. This substantial increase in lifetime by the use of a PEI:ZnO composit EIL was confirmed in two additional equivalent OLEDs which also exhibited LT50 lifetimes of well above 1000 h. It has to be pointed out that


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the OLED with the PEI:ZnO composite EIL even exhibits a significantly longer LT50 lifetime than the LiF/Al reference OLED presented in Figure S4. This result confirms that the degradation of the SY/PEI/Al interface by excitons is a major aging mechanism in these OLEDs. More importantly, the addition of ZnO to the PEI film proves to be a simple way of improving device lifetime. There is no need to change any processing parameters during device fabrication and the OLED performance is barely affected by the inclusion of ZnO to the EIL.

CONCLUSION In this article, we presented solution-processed OLEDs which comprised PEI as EIL and either Al or Ag as top electrode. We studied the influence of the electrode material on the performance as well as on the operational lifetime of the OLEDs. We showed that OLEDs with both electrode materials exhibit a good performance with current efficiencies of ~ 10 cd A-1. Operational lifetimes, however, differed significantly and LT50 lifetimes of < 10 h and ~ 200 h, were determined for Ag and Al, respectively. We showed that this difference in lifetime can be explained by different aging mechanism in the OLEDs. Whereas PEI/Al OLEDs mainly age due to the degradation of the Emitter/PEI/Al interface by excitons, PEI/Ag OLEDs additionally age by an accumulation of holes at the Emitter/PEI interface. As a result, we confirmed a drop in the emitter quantum yield and hence the OLEDs degrade rapidly. Finally, we were able to suppress the degradation of the Emitter/PEI/Al interface by excitons by using a mixture of PEI and ZnO nanoparticles as EIL. As a result, the operational lifetime of theses OLEDs could be raised to more than 1000 hours, which represents an increase by a factor of more than 5, without adversely affecting device performance. The lifetime of this device is significantly longer than that of a


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LiF/Al reference OLED, and therefore these findings underline the feasibility of efficient and stable OLEDs with solution-processed electron injection materials.

ASSOCIATED CONTENT Supporting Information. LIV characteristics and lifetime measurements of additional devices, PL exciton lifetime measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge the support of the German Federal Ministry for Education and Research (FKZ 13N13691) and from the Natural Sciences and Engineering Research Council of Canada (NSERC). S.S. acknowledges the Karlsruhe House of Young Scientists for supporting his research stay at the University of Waterloo. Nanograde Ltd. is acknowledged for providing ZnO nanoparticle suspension.


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GRAPHICAL TABLE OF CONTENTS


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Degradation Mechanisms in Organic Light-Emitting Diodes with

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