Quantitative Analysis of the Efficiency of OLEDs - ACS Publications

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Quantitative Analysis of the Efficiency of OLEDs Bomi Sim,† Chang-Ki Moon,‡ Kwon-Hyeon Kim,‡ and Jang-Joo Kim*,†,‡,§ †

WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea ‡ Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea § Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, South Korea S Supporting Information *

ABSTRACT: We present a comprehensive model for the quantitative analysis of factors influencing the efficiency of organic light-emitting diodes (OLEDs) as a function of the current density. The model takes into account the contribution made by the charge carrier imbalance, quenching processes, and optical design loss of the device arising from various optical effects including the cavity structure, location and profile of the excitons, effective radiative quantum efficiency, and out-coupling efficiency. Quantitative analysis of the efficiency can be performed with an optical simulation using material parameters and experimental measurements of the exciton profile in the emission layer and the lifetime of the exciton as a function of the current density. This method was applied to three phosphorescent OLEDs based on a single host, mixed host, and exciplex-forming cohost. The three factors (charge carrier imbalance, quenching processes, and optical design loss) were influential in different ways, depending on the device. The proposed model can potentially be used to optimize OLED configurations on the basis of an analysis of the underlying physical processes. KEYWORDS: EQE analysis, optical design loss, exciton profile, quenching factor, charge balance factor

1. INTRODUCTION Recently, organic light-emitting diodes (OLEDs) have become increasingly dominant in small-sized displays and are beginning to be used in even larger displays such as televisions. OLEDs are also utilized in lighting due to their ideal spectra and high color-rendering index. Even with the commercial success and in-depth understanding of the electrical and optical processes in OLEDs, a general quantitative model to describe the efficiency loss factors from the maximum achievable efficiencies for a given material in OLEDs is still lacking. The external quantum efficiency (EQE) of an OLED has been expressed by the following equation:1−4 EQE = ηb ·χS/T ·qeff ·ηout

emitting dipole orientation, refractive index of the organic layers including the birefringence, and exciton distribution in the devices are known.6−17 The maximum achievable EQE of OLEDs without any light extraction structure can be calculated for a given emitting dye using this model and is predicted to be about 45−50% if the emitting dipoles are oriented parallel to the substrate with a qPL of 1 and a refractive index of the organic layers of 1.7−1.8.4,17 The high efficiency of OLEDs (over 30%) has been demonstrated in recent years using phosphorescent and TADF dyes with horizontally oriented emitting dipoles.17−27 A very good match of the experimentally obtained EQEs with the predicted maximum achievable EQE using the device structures with little electrical loss has proved the validity of the classical dipole model. Therefore, it is now possible to optically optimize the structure of OLEDs using this model. In contrast, it is rather more difficult to obtain ηb and qeff quantitatively in real devices, not only due to the lack of available tools for the direct measurement of ηb but also due to the dependence of these parameters on the device structure (the Purcell effect) and current density. Many groups have studied extensively the variation in OLED efficiency with increasing current density or EQE roll-off at high brightness by considering exciton quenching factors, including singlet− singlet, field-induced quenching, singlet-polaron, triplet−triplet

(1)

where ηb is the charge balance factor; χS/T is the singlet−triplet factor (χS/T = 0.25 for fluorescent emitters; χS/T = 1 for phosphorescent and thermally activated delayed fluorescent (TADF) emitters); and qeff is the effective quantum yield, which depends on the photoluminescence (PL) quantum yield (PLQY) (qPL) in free space, the emitting dipole orientation factor (the horizontal portion of the emitting dipoles, Θ), and the Purcell effect5 in an optical microcavity structure. In a similar manner, the out-coupling efficiency of the emitted light, ηout, is influenced by Θ, the OLED structure, and the location of the emission zone in the device. Among the various factors, it turns out that a classical dipole model adequately predicts the optical characteristics of OLEDs, such as ηout and the electroluminescence (EL) spectrum, if material parameters such as the PL spectrum of the emitter, © XXXX American Chemical Society

Received: August 16, 2016 Accepted: November 3, 2016 Published: November 3, 2016 A

DOI: 10.1021/acsami.6b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) A general multilayer structure used for the modeling is represented. The structure has m layers between a substrate and an ambient. The each layer j (j = 1, 2, 3, ···, l, ···, m) has material parameters such as refractive indices and thickness, as indicated by nj and dj, respectively. The

(

EML of the device is located at layer l and the location of the emission zone adapts a δ function in the middle of the EML δ x =

dl 2

) for convenience

of the calculation. (b) Schematic device structures with the energy (eV) diagram of Device 1 (single host), Device 2 (mixed host), and Device 3 (exciplex-forming cohost). (c) The current density−voltage−luminescence (J−V−L) characteristics and (d) the EQEs of the three devices as functions of the current density.

annihilation (TTA), and triplet-polaron quenching (TPQ).28−38 Giebink et al. showed that the decrease in efficiency can be described by the charge balance factor and quenching factors using an electrically pumped transient PL method.34,38,39 Their analysis allows us to describe the variation of ηb and qeff as a function of the current density but does not provide absolute values of ηb and qeff. By combining the optical model with the description of the variation of EQE with current density, we propose a new expression for quantitative analysis of OLED efficiency, which describes loss factors of an OLED from the ideal device in which the ideal device is defined as the optically optimized device with no electrical loss and exciton quenching. This analysis allowed us to diagnose quantitatively the loss of efficiency arising from optical design loss, charge imbalance, and exciton quenching for the first time. A total of three highefficiency phosphorescent devices based on a single host,

nonexciplex-forming mixed host, and exciplex-forming cohost were used as examples to demonstrate the potential of this analysis. It turned out that the selected examples show different origins of EQE loss among the three main factors (optical design loss factor, charge imbalance factor, and quenching factor).

2. MODELING THE EQE A general multilayer structure used to model the EQE is schematically represented in Figure 1a. The structure has m layers between a substrate and an ambient. Each layer j (j = 1, 2, 3, ···, l, ···, m) has material parameters such as refractive indices and thickness, as indicated by nj and dj, respectively. The EML of the device is located at layer l, and the location of the emission zone adapts a δ function in the middle of the

(

EML, δ x = B

dl 2

), for convenience of the calculation. DOI: 10.1021/acsami.6b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces First, the ideal device for a given emitter in terms of its efficiency is defined as the optically optimized device with no electrical loss or quenching by electrical operation of the device. The maximum achievable EQE of the ideal OLED (EQE0max) using an emitter and the thickness of the EML and transporting layers can be calculated using a classical dipole model1−16 by the optical optimization with an ideal combination of the thickness of the consisting layers. With the given nj of the layers and emitter parameters (Θ, qPL, and emission spectrum) in the EML, EQE0max can be represented by following equations:

Q (J ) =

EQE(J ) = ηb(J ) ·Q (J ) ·O(J ) ·EQE 0max

O(J ) =

(2)

where Γ(d1, ···, dm) represents a combination of thickness of each consisting layer, and s(λ) is the normalized photon spectrum of the emitter in free space as a function of wavelength. In the above equation, qeff is expressed by

dl

⎡ ⎛1 ⎞ ⎤ IPL(t , J ) = C·exp⎢ −⎜ + KQ (J )⎟ ·t ⎥ ⎠ ⎦ ⎣ ⎝τ ηout(x)dx

(4)

F ·k r + k nr(J = 0) =

The exciton distribution in the device can be determined by experiments. A combination of thickness of consisting layers of the real device and the obtained exciton distribution are applied to the calculation of EQEreal max. EQE of the real device including electrical loss and quenching factors as a function of current density can be described by (6)

In the above equation, the electrical balance factor ηb(J) is represented by η b (J ) =

JCn − JAn J

=

JAp − JCp J

∫0

dEML

Iex(x) ·qeff (x) · (12)

1 τ

(13)

Here, C is the constant of proportionality between the optically generated exciton density and exciton profile at t = 0, Iex(x) is the exciton density, which can be obtained from the simulation of the optical electric field in the EML via the transfer-matrix method at an excitation wavelength as shown in Figure S1,43 and τ is the exciton lifetime measured from the electrically pumped transient PL. Similar to the EL intensity calculation, the microcavity effect on the transient PL decay is included in eq 12 and is represented by qeff(x)·ηout(x). The relationship between the radiative decay rate (kr), nonradiative decay rate (knr), Purcell factor (F), and lifetime of the emitter (τ) in the device structure is represented in eq 13. KQ(J) is the average quenching rate and is a function of the current density including the effect of TTA and TPQ. Because KQ(J) is the only fitting parameter in eq 12, it is obtained by fitting the equation to the measured PL decay curves as a function of the current density. The optical loss factor O(J) in eq 10 is coming from the optically not-optimized device reducing the outcoupling efficiency and the Purcell factor for a given emitter and can be calculated using the classical dipole model once the exciton distribution and the thickness of consisting layers are determined. The exciton distribution in the real device also depends on the current density. The value of g(x), depending

(5)

real EQE(J ) = ηb(J ) ·Q (J ) ·EQE max

(11)

where kEEA[N(J)], kEPQ,n[n(J)], kEPQ,p[p(J)], and kheat[heat] are the annihilation rate for exciton−exciton, exciton−negative polaron, exciton−positive polaron, and heat- related quenching, respectively. KQ(J) can be determined from the electrically pumped transient PL analysis. The optically excited population of excitons is exposed to all the possible quenching processes in the EMLs of the OLEDs. Taking the average quenching rate in the EML, the transient PL intensity is obtained with the optically generated exciton profile in the EML at time t = 0, Iex(x), as follows:

∫ ∫ s(λ)·g(x)·qeff (Θ, Γ, x , λ)·

g (x)dx = 1

(10)

+ k heat[heat] + ···

(3)

where g(x) is the normalized exciton distribution function in the EML with thickness of dl satisfying the condition

∫0

EQE 0max

KQ (J ) = kEEA[N (J )] + kEPQ, n[n(J )] + kEPQ, p[p(J )]

where F is the Purcell factor describing enhancement of the spontaneous emission rate in the structure.1−5,14,15 Here, F and ηout are functions of the emitting dipole orientation (Θ) of the emitter, refractive indices of the layers, and the device structure (Γ) of OLEDs. For a real device optimized experimentally to get the highest efficiency, for instance, by using the combinatorial method,40−42 the maximum achievable EQE (EQEreal max) is lower than EQE0max due to optical design loss arising from difference in the device structure and the distribution of the excitons in the EML from the ideal device. The EQE of the real the device is represented by below:

ηout(Θ, Γ, x , λ)dxdλ

EQE real max

In eq 8, KQ(J) is the quenching rate expressed by eq 11, representing all of the possible exciton-related quenching processes during the operation of the device, and is given by

F(Θ, Γ, λ) ·qPL

EQE real = χS/T max

(9)

where the optical design loss factor O(J) is defined by

∫

1 − qPL + F(Θ, Γ, λ) ·qPL

(8)

where τ is the natural decay time of excitons in the OLED, and KQ(J) is the quenching rate at a current density J. eq 6 can be rearranged as follows to explicitly express the optical design loss;

EQE 0max = χS/T ·max[ s(λ) ·qeff (Θ, Γ, λ) ·ηout(Θ, Γ, λ)dλ]

qeff =

1 [1 + τKQ (J )]

(7)

where J, JAn (JAp), and JCn (JCp) represent the current density of the device and the electron and hole current densities at the anode and cathode sides of the EML, respectively. The quenching factor Q(J) is described by34 C


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their fabrication processes are described in the Experimental section. 3.2. Distribution of Exciton Density: Sensing-Layer Method. The exciton distributions in the EMLs were probed directly by adding a sensing layer,63−65 which had a lowerenergy state than the phosphorescent emitter [Ir(ppy)2(acac)] (the maximum peak is at a wavelength of 524 nm) at different locations within the EML. Red sensitizer [Ir(mphmq)2(tmd)] with an emission peak at a wavelength of 604 nm was codeposited at 5 wt % at different positions separated by 5 nm in the EMLs with a width of 0.5 nm. The Forster energy transfer radius from Ir(ppy)2(acac) to the sensitizer was calculated to be 2.5 nm using the spectral overlap between the emission spectrum of Ir(ppy)2(acac) and the absorption spectrum of the sensitizer shown in Figure S2, which is sufficiently short to allow the use of Ir(mphmq)2(tmd) as a sensing material.66 The sensitizing layer did not alter the J−V characteristics of the devices as shown in Figure S3, indicating that the additional layers do not modify the distributions of excitons within the devices. The exciton profiles in the EMLs were extracted from the peak intensity of Ir(mphmq)2(tmd) resolved using the EL spectra, as shown in Figures S4 and S5, and calibrated by the position-dependent qeff and ηout at the peak wavelength (604 nm) of the sensitizer emission (Figure S6). Figure 2 shows the exciton profiles of the EMLs of Devices 1, 2, and 3 as functions of the current density measured by the sensing layer method. In Device 1, excitons were concentrated near the 1,3,5-Tris(N-phenylbenzimiazole-2-yl)benzene (TPBi) layer (electron transport layer [ETL]) side of the EML for all current levels. This can be explained by the hole mobility of the CBP layer (10−3 cm2 V−1 s−1), which is two orders higher than the electron mobility of the ETL.67,68 As the current density increases, the exciton distribution becomes wider because the Pool−Frankel constant (β) of the ETL is three times higher than that of the CBP. Thus, more electrons were transported toward the hole-transport layer (HTL) side of the EML at a higher current density. In Device 2, excitons were narrowly distributed at the HTL side of the EML for all current density levels, resulting from the low hole mobility of the mCP layer (HTL, 10−4 cm2 V−1 s−1)69 compared to the electron mobility of the TSPO1 layer (ETL). Moreover, the transport properties could be changed in the doped EML owing to the deep trap level (0.6 eV) of the emitter [Ir(ppy)2(acac)] in the system for holes. In Device 3, the exciton profile was slightly weighted toward the ETL side at a low current density (0.01 mA cm−2). As the current density increased up to 10 mA cm−2, excitons were evenly distributed in the EML. Compared with other systems, Device 3 had nearly even and wide exciton profiles for all current-density levels. 3.3. Quantitative Analysis of the EQEs in Different OLEDs. The measured transient PL decay curves at a high current density from the electrically pumped transient PL measurements were compared with curves without current flow (J = 0) along with the fitting curves in Figure 3 for the three devices. Although the three OLEDs had an identical green emitter [Ir(ppy)2(acac)], the radiative decay rates differed owing to the Purcell effect. Faster decays under the current flow than those without the current must result from the increased nonradiative decay rate at a high current density due to increased quenching processes. We can extract the natural decay rates (1/τ) from the decay curves at J = 0 and KQ from the decay curves at different J.

on the current density, can be determined by a modified sensing layer method corrected by considering the microcavity effect followed by spectrum separation. The loss from exciton quenching Q(J) in eq 9 can be evaluated from the electrically pumped PL transient method as a function of the driving current. Finally, ηb(J) can be determined from eq 9 using the experimentally determined EQE(J) and Q(J), and the theoretically calculated O(J) and EQE0max. eq 10 provides us with a few advantages over eq 1: (1) eq 10 explicitly show that the qeff and ηout cannot be calculated separately to get the maximum efficiency, but they must be combined to calculate maximum achievable EQEs using a given emitter with certain values of PLQY and Θ because both the qeff and ηout are functions of device structure and wavelength. In other words, EQE0max calculated by eq 2 is not the same as ηS/T max max × qmax eff × ηout or, more simply, ηS/T × qPL × ηout . (2) By 0 introducing EQEmax, which can be easily calculated using a classical dipole model, we can exactly calculate maximum achievable EQEs using an emitting dye without fabricating devices. If we assume that refractive indices of the HTL and ETLs have similar values of ∼1.8, we can use contour plot even without optical calculation.4 (3) In many cases, it is generally difficult to fabricate an OLED with EQE0max because it is difficult to get perfect electrical balance with optically optimized device structure. Therefore, the highest efficiency is obtained in most cases with optimized electrical balance under sacrifice of outcoupling efficiency or with an optically optimized device structure under sacrifice of electrical balance or compromise between them using the combinatorial method.40−42 By explicitly introducing the optical design loss O(J), which can also be calculated using the classical dipole model, we now can quantitatively analyze how much EQE losses come from charge imbalance and/or optical design loss. (4) In combination with the electrically pumped PL transient method, we can determine all of the loss factors, including quenching loss, as a function of current density.

3. RESULTS AND DISCUSSION 3.1. Device Structures. To verify the effectiveness of the theoretical model, the model was applied to three highly efficient OLEDs on the basis of previous literature. We selected one each from widely used three different host systems as examples, which are a single host17,18,44,45 (Device 1), a mixed host46−60 (Device 2), and a exciplex-forming cohost4,17,21,22,61 (Device 3) doped with Ir(ppy)2(acac) as the emitter. The device structures studied in this paper are shown in Figure 1b. 4,4′-Bis(carbazol-9-yl)biphenyl (CBP), N,N′-dicarbazolyl-3,5benzene (mCP) along with diphenyl-4-triphenylsilylphenylphosphine oxide (TSPO1), and 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) along with bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) are host materials in Device 1,19,44 Device 2,59 and Device 3,61 respectively. To improve hole-injection properties, we used MoO3 and ReO3 to simplify the fabrication on the basis of previous literature, although there are many reported strategies such as gradient HTL.60,62 One should note that the selected devices are not representing OLEDs with different host structures but taken as examples giving high efficiencies for the demonstration of the effectiveness of the model developed in this article. The current density−voltage−luminance (J−V−L) and external quantum efficiencies of the devices are displayed in panels c and d of Figure 1, respectively. Details of the device structures as well as D


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Figure 2. Exciton profiles as functions of the current density of (a) Device 1, (b) Device 2, and (c) Device 3 obtained using the sensinglayer method including the optical effect. (a) In Device 1, excitons are more concentrated at the ETL side of the EML for all the operating current levels. (b) In Device 2, excitons are narrowly distributed at the HTL side of the EML for all the current density levels. (c) Device 3 has nearly even and wide exciton profiles at all the operating current density levels.

Figure 3. Photoluminescence transient of the electrically driven OLEDs as a function of the current density and the fits according to eq 12 (dashed line). The unit of the quenching rate (KQ) is 106 s−1. The data show that the decays (exciton to photon conversion factor, Q < 1) of time-resolved PL at a high current density are faster compared to those without the current flow (KQ = 0 s−1, Q = 1) in (a) Device 1 (KQ = 0.177 × 106 s−1, Q = 0.84), (b) Device 2 (KQ= 0.07 × 106 s−1, Q = 0.93), and (c) Device 3 (KQ = 0.042 × 106 s−1, Q = 0.96) systems. The increased nonradiative decay rate at a high current density originates from increased exciton−exciton interactions and exciton−charge carrier quenching processes.

The maximum EQE values of Device 1, Device 2, and Device 3 were 24%, 27.3%, and 29.6%, respectively, as shown in Figure 1d. The efficiencies were lower than the EQE0max achievable with Ir(ppy)2acac were turned out to be 29.7%, 29.4%, and 31% for Devices 1−3, respectively, which were calculated using qPL = 0.95 for Devices 1 and 3 and 0.86 for Device 2, respectively, and Θ = 0.73 for the three devices. Device 3 gave the highest EQE0max among the three devices due to the birefringence of the ETL.13,15 One can note that the experimental EQE values of the devices are lower than the EQE0max values. Moreover, the efficiency roll-off properties differ among devices more than the efficiency. This raises questions regarding the origin of the lower EQEs and/or efficiency roll-off in terms of whether they originated from electrical loss, optical design loss, or exciton quenching in the devices. These questions can be answered using the method described in the previous sections. For our purposes, the EQEreal max values of the devices were calculated using the structures (see the Experimental section), g(x), and refractive indices of the composed organic layers. The quenching factors (Q(J)) were calculated by substituting τ and KQ(J) obtained using the electrically pumped transient PL

analysis into eq 12. Finally, the charge balance factors ηb(J) were determined using eq 6 by comparing these to the experimentally measured EQEs. Figure 4 shows the results of the analysis of the three OLEDs. The figures clearly manifest that the optical design loss, electrical loss, and quenching loss can be clearly quantified. Different devices show different loss characteristics to verify the usefulness of the approach. Device 1 shows optical design loss (around 5.7%) due to the low out-coupling efficiency, even though the device was well-designed electrically to have a good charge balance at a low current density. The comparison with the ideal device revealed that the optical design loss in Device 1 originates from the thinner HTL and the closer location of the emission zone to the anode than the optically optimized device. Device 2 represents optically optimized device structure, although it could not achieve unit charge balance at any of current density level. Device 3 (exciplex forming cohost) is also optically well designed with less quenching at high current E


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expectation demonstrate the importance of the analysis method, which cannot be identified without the detailed quantitative analysis. The different loss mechanisms also contribute to efficiency roll-offs with different degrees for different devices. First of all, the emission zone shifts in the devices do not contribute much to the roll-offs as revealed by the little change in the optical design losses in the devices. It must be general if OLEDs are optically designed to get reasonably high outcoupling efficiencies, where small changes in position of emission zone do not influence the efficiency so much. Second, the efficiency roll-off in the device with a large and sudden increase of efficiency roll-off with increasing current density (Device 2, for example) is mainly attributed to electrical loss. Third, in the devices with gradual degree of efficiency roll-off with less than 10% reduction in EQE, exciton quenching factors contribute significantly to efficiency roll-offs (∼5% in Device 1) along with charge imbalance if the emission zone is rather narrow at 10 000 cd/m2. If the emission zone is broadened up to 30 nm (Device 3), the contribution of exciton quenching is very small (less than 2%) even at 10 000 cd/m2. Fourthly, it is rather interesting that the exciton quenching is larger in Device 1 than in Device 2, even though the exciton zone in Device 1 is broader than that in Device 2. This can be explained by lower exciton-induced quenching owing to a lower exciton density in Device 2 at a high operating voltage (J > 10 mA cm−2) compared to that of Device 1 due to the poor charge balance in Device 2, as shown in Figure S9. It should be noted that the total exciton density in the EML is proportional to ηb × J.

4. CONCLUSIONS We presented a novel theoretical model for quantitative analysis of the electrical and optical effects on the efficiency of OLEDs. The model allows us to evaluate the contributions of the microcavity effect of the light emission process, exciton quenching processes, and charge-carrier imbalance on the efficiency of OLEDs by the unified equation. This can be done if the distribution of the exciton density in the EML and the exciton lifetimes are measured as functions of the current density, and information regarding the device structure, refractive indices of the composed layers, the PLQY, and the emission dipole orientation are known. The model was successfully applied to three highly efficient OLEDs based on different hosts: a single host (Device 1), a mixed host (Device 2), and an exciplex-forming cohost (Device 3). The analysis revealed that the three contributions (the optical effect, charge balance factor, and exciton-quenching factor) influence the efficiency in different ways for each of the different devices. For instance, the OLEDs based on the mixed hosts (Device 2) or exciplex hosts (Device 3) are considered to provide a better charge balance than single-host-based OLEDs (Device 1). However, the two examples in Figure 4b,c for the device show that this is not guaranteed on the basis of our specific examples. The Device 1 provides an excellent charge balance even with the low injection barriers from the EML to ETL or HTL in the device. Mobilities, the field dependence of mobilities, and charge trapping may play a role in the charge balance. This model has the potential to be utilized for the analysis of the physical processes of OLEDs. It can also be utilized for optimizing the OLED structure. Finally, one should note that the device structures were selected as examples based on previous reports, and their electrical and optical characteristics would not be generalized for the three different host systems.

Figure 4. Analysis of the efficiency according to eq 9 composed of the calculated optically optimized efficiency, optical design loss, exciton-tophoton conversion loss, electrical loss, and experimental EQEs of (a) Device 1, (b) Device 2, and (c) Device 3.

densities. However, the device has charge imbalance at low and high current densities. Surprisingly, electrical balance at low current density is excellent in this single host device. Up to now, large number of discussions on lower efficiency than commonly believed maximum outcoupling efficiency (for instance 26−30%) has been attributed to electrical loss without quantitative consideration of optical design loss factor. Furthermore, OLEDs based on the mixed hosts or exciplex hosts are considered to provide a better charge balance than single-hostbased OLEDs. However, the three examples in Figure 4 demonstrate that this is not guaranteed. The main losses in Device 2 and 3 arise from electrical loss, which is larger than that of Device 1, even though the total losses are small (2−3% and 1−4% EQE losses in Devices 2 and 3, respectively). The single-host EML of Device 1 provides an excellent charge balance even with the low injection barriers from the EML to ETL or HTL in the device. The single-host EML of Device 1 provides an excellent charge balance even with the low injection barriers from the EML to ETL or HTL in the device. Mobilities, field dependence of mobilities, and charge trapping may play roles in the charge balance. The higher efficiencies in Device 2 and 3 originate from the optical effect, even with higher electrical losses than that of Device 1. The contrast between Device 1 and Device 2 or 3 and the deviation from the F


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and Polymers with Chromophores. Philos. Trans. R. Soc., A 1997, 355, 801−814. (2) Nowy, S.; Krummacher, B. C.; Frischeisen, J.; Reinke, N. A.; Brütting, W. Light Extraction and Optical Loss Mechanisms in Organic Light-Emitting Diodes: Influence of the Emitter Quantum Efficiency. J. Appl. Phys. 2008, 104, 123109. (3) Furno, M.; Meerheim, R.; Hofmann, S.; Lüssem, B.; Leo, K. Efficiency and Rate of Spontaneous Emission in Organic Electroluminescent Devices. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 115205. (4) Kim, S.-Y.; Jeong, W.-I.; Mayr, C.; Park, Y.-S.; Kim, K.-H.; Lee, J.H.; Moon, C.-K.; Brütting, W.; Kim, J.-J. Organic Light-Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter. Adv. Funct. Mater. 2013, 23, 3896− 3900. (5) Purcell, E. M. Spontaneous Emission Probabilities at Radio Frequencies. Phys. Rev. 1946, 69, 681. (6) Chance, R. R.; Prock, A.; Silbey, R. Molecular Fluorescence and Energy Transfer Near Interfaces. Adv. Chem. Phys. 1978, 37, 1−65. (7) Neyts, K. A. Simulation of Light Emission from Thin-Film Microcavities. J. Opt. Soc. Am. A 1998, 15, 962−971. (8) Wasey, J. A. E.; Barnes, W. L. Efficiency of Spontaneous Emission from Planar Microcavities. J. Mod. Opt. 2000, 47, 725−741. (9) Smith, L. H.; Wasey, J. A. E.; Samuel, I. D. W.; Barnes, W. L. Light Out-Coupling Efficiencies of Organic Light-Emitting Diode Structures and the Effect of Photoluinescence Quantum Yield. Adv. Funct. Mater. 2005, 15, 1839. (10) Lin, C.-L.; Cho, T.-Y.; Chang, C.-H.; Wu, C.-C. Enhancing Light Outcoupling of Organic Light-Emitting Devices by Locating Emitters around the Second Antinode of the Reflective Metal Electrode. Appl. Phys. Lett. 2006, 88, 081114. (11) Krummacher, B. C.; Nowy, S.; Frischeisen, J.; Klein, M.; Brütting, W. Efficiency Analysis of Organic Light-Emitting Diodes Based on Optical Simulation. Org. Electron. 2009, 10, 478−485. (12) Kim, S. Y.; Kim, J. J. Outcoupling Efficiency of Organic Light Emitting Diodes and the Effect of ITO Thickness. Org. Electron. 2010, 11, 1010. (13) Wang, Z. B.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S.; Liu, Z. W.; Lu, Z. H. Unlocking the Full Potential of Organic Light-Emitting Diodes on Flexible Plastic. Nat. Photonics 2011, 5, 753−757. (14) Brütting, W.; Frischeisen, J.; Schmidt, T. D.; Scholz, B. J.; Mayr, C. Device Efficiency of Organic Light-Emitting Diodes: Progress by Improved Light Outcoupling. Phys. Status Solidi A 2013, 210, 44−65. (15) Moon, C.-K.; Kim, S.-Y.; Lee, J.-H.; Kim, J.-J. Luminescence from Oriented Emitting Dipoles in a Birefringent Medium. Opt. Express 2015, 23, A279−A291. (16) Callens, M. K.; Yokoyama, D.; Neyts, K. Anisotropic Materials in Oleds for High Outcoupling Efficiency. Opt. Express 2015, 23, 21128−21148. (17) Shin, H.; Lee, J.-H.; Moon, C.-K.; Huh, J.-S.; Sim, B.; Kim, J.-J. Sky-Blue Phosphorescent Oleds with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer. Adv. Mater. 2016, 28, 4920−4925. (18) Tanaka, D.; Sasabe, H.; Li, Y.-J.; Su, S.-J.; Takeda, T.; Kido, J. Ultra High Efficiency Green Organic Light-Emitting Devices. Jpn. J. Appl. Phys. 2007, 46, L10−L12. (19) Helander, M. G.; Wang, Z. B.; Qiu, J.; Greiner, M. T.; Puzzo, D. P.; Liu, Z. W.; Lu, Z. H. Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility. Science 2011, 332, 944−947. (20) Lee, C. W.; Lee, J. Y. Above 30% External Quantum Efficiency in Blue Phosphorescent Organic Light-Emitting Diodes using Pyrido[2,3-b]indole Derivatives as Host Materials. Adv. Mater. 2013, 25, 5450. (21) Kim, K.-H.; Lee, S.; Moon, C.-K.; Kim, S.-Y.; Park, Y.-S.; Lee, J.H.; Huh, J.; You, Y.; Kim, J.-J.; Woo Lee, J. Phosphorescent Dye-Based Supramolecules for High-Efficiency Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, 4769.

5. EXPERIMENTAL SECTION Device Fabrication and Characterization. A total of three different PhOLEDs were fabricated to quantitatively analyze the efficiency: indium tin oxide (ITO) (70 nm)/MoO3 (1 nm)/CBP (35 nm)/CBP−[Ir(ppy)2(acac)] (8 wt %, 15 nm)/TPBi (65 nm)/LiF (0.7 nm)/Al (100 nm) (Device 1 with single-host), ITO (70 nm)/ ReO3− mCP (15 nm)/mCP (55 nm)/ mCP−TSPO1−[Ir(ppy)2(acac)] (1:1 molar ratio and 8 wt %, 30 nm)/TSPO1 (45 nm)/Rb2CO3−TSPO1 (15 nm)/Al (100 nm) (Device 2 with mixed host), and ITO (70 nm)/ di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) (75 nm)/ 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) (10 nm)/TCTA− bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM)− bis (2-phenylpyridine)iridium(III)-acetylacetonate [Ir(ppy)2(acac)] (1:1 molar ratio and 8 wt %, 30 nm)/B3PYMPM (40 nm)/LiF (0.7 nm)/Al (100 nm) (Device 3 with exciplex-forming cohost). The precleaned glass substrates were patterned with 70 nm thick ITO using thermal evaporation under a base pressure of 10−7 Torr without breaking the vacuum. The devices were encapsulated in a nitrogenfilled glovebox prior to the measurements. The current density− voltage−luminance (J−V−L) characteristics and the EL spectra were measured using a programmable source meter (Keithley 2400) and a spectrophotometer (Photo Research Spectrascan PR650). The EQE of the devices were calculated from the J−V−L characteristics and the angular distributions of the EL spectra. Electrically Pumped Transient PL Method. Figure S7 shows the experimental setup of the electrically pumped transient PL method.34,38,39 The quasi-steady-state electrical signals were applied using a digital delay generator from Stanford Research Systems (DG645) with a pulse width of 100 μs and a repetition rate of 20 Hz; the electrical signal was combined with the optical signal excited by a N2 laser (337 nm, Usho Optical Systems Co.) at the middle of the voltage pulse. The optical signal was synchronized with the middle of each electrical pulse. The transient PL decays were analyzed using a streak camera (Hamamatsu, C10627).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10297. Optically excited exciton profiles (Figure S1), details on the modified sensing layer method (Figures S2−S6) and the experimental setup of the electrically pulsed transient PL (Figure S7), EL spectra as a function of current density (Figure S8), and relative exciton density (Figure S9) of the three devices. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea−Swiss Innovation Program (2015K1A3A1A14021006) and the Midcareer Researcher Program (2014R1A2A1A01002030) through a National Research Foundation (NRF) grant funded by the Ministry of Science, Information, and Communications Technology (ICT) and Future Planning (MSIP).

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