Degradation Mechanisms in Blue Phosphorescent Organic Light

Dec 13, 2016 - Isolating Degradation Mechanisms in Mixed Emissive Layer Organic Light-Emitting Devices. John S. Bangsund , Kyle W. Hershey , and Russe...
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Degradation Mechanisms in Blue Phosphorescent Organic LightEmitting Devices by Exciton−Polaron Interactions: Loss in Quantum Yield versus Loss in Charge Balance Yingjie Zhang* and Hany Aziz Department of Electrical and Computer Engineering & Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 S Supporting Information *

ABSTRACT: We study the relative importance of deterioration of material quantum yield and charge balance to the electroluminescence stability of PHOLEDs, with a special emphasis on blue devices. Investigations show that the quantum yields of both host and emitter in the emission layer degrade due to exciton−polaron interactions and that the deterioration in material quantum yield plays the primary role in device degradation under operation. On the other hand, the results show that the charge balance factor is also affected by exciton−polaron interactions but only plays a secondary role in determining device stability. Finally, we show that the degradation mechanisms in blue PHOLEDs are fundamentally the same as those in green PHOLEDs. The limited stability of the blue devices is a result of faster deterioration in the quantum yield of the emitter. KEYWORDS: degradation mechanism, blue PHOLEDs, exciton−polaron interactions, quantum yield, charge balance

1. INTRODUCTION Being able to exhibit 100% internal quantum efficiency,1,2 phosphorescent organic light-emitting devices (PHOLEDs) have attracted much interest in the field of displays. With advantages such as larger contrast, wider viewing angle, and better color quality, displays utilizing PHOLEDs are widely considered as the next generation of displays and now account for the second largest shipments in industry, after liquid crystal displays.3 One major issue that PHOLEDs still face is their limited operational stability. In particular, the stability of blue PHOLEDs is still not sufficient for commercial uses in displays. As a result, the current generation of mobile displays continue to use the less efficient fluorescent OLEDs as the blue-emitting units.4 Therefore, understanding the degradation mechanisms in PHOLEDs, particularly the blue ones, still remains an important issue to be addressed for the development of PHOLEDs. The degradation of OLED under operation is described as a gradual drop in device brightness, essentially a decrease in external quantum efficiency (EQE) over time. The EQE of an OLED is the product of four factors: the fraction of excitons that can decay radiatively, the quantum yield of the emitter, the charge balance factor, and the light outcoupling efficiency. Electroluminescence (EL) degradation of OLED can therefore be attributed to deterioration of one or more of these four factors. In general, both the fraction of excitons that can decay radiatively and the light outcoupling efficiency do not change with time during operation.5 Therefore, the remaining two factorsthe quantum yield of the emitter and the charge © 2016 American Chemical Society

balance factorcan change during device operation and have been identified as the pathways for OLED degradation. Changes in the quantum yield of the emitter, which are often a result of introduction of exciton quenchers or non-emissive recombination centers in the vicinity of the emission layer (EML), primarily arise from instability of organic materials used in OLEDs. Examples include exciton quenching by unstable cationic species,6,7 introduction of deep traps and non-emission recombination centers due to photochemical reactions of hole transport materials8−11 and fragmentation and/or dimerization of hole and electron blocking materials,12−14 and trap formation by exciton−polaron annihilation processes within the EML.15,16 The earlier discovery of out-diffusion of indium from ITO anode would also fall into this category.17,18 The simplest and perhaps most direct way for detecting changes in quantum yield is by measuring device photoluminescence (PL), which has been demonstrated before in the literature.6,19 On the other hand, changes in the charge balance factor are usually a result of deterioration in carrier transport across the organic layers or in charge injection at device interfaces20−22 and are inferred from changes in the current density−voltage (J−V) characteristics of OLEDs. Although the deterioration in material quantum yield and charge balance has been confirmed to contribute to device EL degradation over time, their relative importance to the stability Received: October 29, 2016 Accepted: December 13, 2016 Published: December 13, 2016 636

DOI: 10.1021/acsami.6b13823 ACS Appl. Mater. Interfaces 2017, 9, 636−643

Research Article

ACS Applied Materials & Interfaces

Figure 1. Structures of (a) OLEDs and (b) hole-only devices and (c) molecular structures of chemicals used in this study. current density of 20 mA/cm2 for 18 h; ⟨L only⟩: devices were exposed to external irradiation at 365 nm by an UV lamp with a power density of 2.3 mW/cm2 for 18 h; ⟨I + L⟩: devices were exposed to electric current flow and external irradiation described in ⟨I only⟩ and ⟨L only⟩ scenarios together, for 18 h.

of PHOLEDs, however, remains unclear. This question becomes particularly important to answer for blue PHOLEDs, whose stability still remain well below that of the other colors. In this work, we study what role the changes in material quantum yield and charge balance play in device EL degradation, with a special emphasis on blue PHOLEDs. Investigations show that quantum yield of both the host and the emitter in the EML degrade due to exciton−polaron interactions and that the deterioration in material quantum yield plays the primary role in device degradation under operation. The results show that charge balance is also affected by exciton−polaron interactions, but the phenomenon plays a secondary role in comparison. Finally, we show that the degradation mechanisms in blue PHOLEDs are fundamentally the same as those in green PHOLEDs. The limited stability of the blue devices is a result of faster deterioration in the quantum yield of the emitter.

3. RESULTS AND DISCUSSION We first study the role of material quantum yield deterioration in EL degradation of PHOLEDs. For this purpose, we fabricate blue and green PHOLEDs with the general structure ITO/ MoO3 (5 nm)/host (30 nm)/host: emitter (5%) (20 nm)/ TPBi (35 m)/LiF (1 nm)/Al (80 nm). For blue devices, the host and the emitter used are mCP and FIrpic, whereas for green ones, they are CBP and Ir(ppy)3. These material systems are chosen because they are most representative of their corresponding colors in the field and that devices incorporating these materials also show very good performances. The use of the green PHOLED serves as a comparison for the blue device and allows us to examine the difference in their degradation mechanisms. Figure 1a presents the device structure. Figure 1c illustrates the chemical structures of the materials. Both devices are subjected to electrical driving under a constant electrical current at 20 mA/cm2 until the luminance of the devices has degraded to 50% of its initial values (the degradation process takes ∼1 h for the blue device and ∼36 h for the green one; a similar lifetime has been reported previously in the literature,23 data not shown). To study the changes in material quantum yield, we study PL spectra for the devices collected before and after electrical aging. Figure 2 presents the spectra. The excitation wavelength is set to 330 nm (where both mCP and CBP absorbs strongly) for both devices. It can be clearly seen that for both blue (Figure 2a) and green (Figure 2b) PHOLEDs the PL intensities decrease after the devices have been electrically aged, which signifies decrease in material quantum yield. In the case of the blue device, the reduction in PL intensity is roughly 8%, whereas it is about 25% for the green device. Considering that the devices have been aged to 50% EL loss, it may seem at first glance that the degradation of material quantum yield in the blue device only accounts for a small portion of EL degradation and that about half of EL degradation of the green device comes from the reduction in

2. EXPERIMENTAL SECTION 2.1. Materials. The organic materials used in this work have the following chemical abbreviations: mCP = 1,3-bis(carbazol-9-yl)benzene; mCBP = 3,3′-di(9H-carbazol-9-yl)biphenyl; 26DCzPPy = 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine; CBP = 4,4′-bis(carbazol9-yl)biphenyl; TPBi = 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phenyl-1Hbenzimidazole); FIrpic = bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium; Ir(ppy)3 = tris(2-phenylpyridine)iridium. CBP and TPBi were purchased from Electronic Materials Index Co. mCP, mCBP, Ir(ppy)3, and FIrpic were purchased from Luminescence Technology Corp. 26DCzPPy was purchased from Shanghai Hanfeng Chemical Co. All materials have purity above 99% and were thus used without further purification. 2.2. Device Fabrication. Prior to device fabrication, the ITOcoated glass substrates were sonicated in acetone and isopropanol for 5 min each, in respective order. Devices were then fabricated in an Angstrom Engineering EvoVac system. All materials were thermally evaporated at a rate of 0.1−2 Å/s at a base pressure of 5 × 10−6 Torr. 2.3. Device Measurement. EL and PL spectra were collected using an OceanOptics spectrometer. External excitation for PL measurements were obtained from a 200 W Hg lamp equipped with a Horiba H-20 monochromator. All devices were kept and measured in a nitrogen atmosphere at all times. 2.4. Stress Schemes in Hole-Only Devices. ⟨I only⟩: devices were electrically driven under a forward bias to sustain a flow of 637

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Figure 2. PL spectra of (a) blue PHOLEDs doped with 5% FIrpic and (b) green PHOLEDs doped with 5% Ir(ppy)3 before and after they have been electrically stressed to 50% of their initial luminance.

material quantum yield, leaving the rest of the EL degradation to changes in charge balance. However, it is important to point out that in these devices, EL degradation is primarily interfacial;24 thus, only a thin layer of materials near the EML/electron transport layer (ETL) interface is degraded. Furthermore, because PL signal is collected from the entire organic stack, and not only from the interfacial layer, changes in PL reflect changes of material quantum yield in the entire EML on average and therefore cannot fully represent changes in quantum yield that occurs in the recombination zone of the device. In order to examine the relative importance of material quantum yield deterioration on device EL degradation, it is important to focus on only the recombination zone of the device. We therefore study devices with EMLs that are only 5 nm thick, since the width of the recombination zone of our devices is comparable to that.25 This way, degradation can be expected to occur across the entire EML and not just a fraction of it. The general device structure is as follows: ITO/MoO3 (5 nm)/host (45 nm)/host: emitter (5%) (5 nm)/TPBi (35 m)/ LiF (1 nm)/Al (80 nm). The host and emitter are mCP and FIrpic and CBP and Ir(ppy)3 for the blue and green PHOLED, respectively. Figure 3a presents the PL spectra of the blue PHOLED before and after the device has been electrically degraded by 60% of its initial luminance. As can be seen, the PL intensity decreases by roughly 40% after electrical aging. Similarly, when the green PHOLED is electrically degraded by 60%, its PL intensity also decreases by 40%, as shown in Figure 3b. The results indicate that the deterioration of the material quantum yield, which is presented by the decrease in device PL, accounts for roughly 2/3 of its EL degradation, for both blue and green PHOLEDs. The rest 1/3 of EL degradation in these devices therefore has to come from changes in charge balance. Knowing that the PL excitation at 330 nm is strongly absorbed by both hosts mCP and CBP and that cascade excitation from the host to the emitter occurs in both devices, the deterioration in material quantum yield could come from either the host, the

Figure 3. PL spectra of (a) the blue PHOLED under 330 nm excitation and the green PHOLED under (b) 330 nm and (c) 370 nm excitation, collected before and after the devices have been electrically aged to around 40% of their initial luminance.

emitter, or both. In order to identify the origin of quantum yield deterioration, PL spectra of the green PHOLED before and after electrical aging is also measured under 370 nm excitation. Since only the emitter Ir(ppy)3 is able to absorb significantly at this wavelength, the changes in PL intensity primarily reflect deterioration in the quantum yield of Ir(ppy)3. The results are presented in Figure 3c. As can be seen, the decrease in device PL intensity, and thus the deterioration of Ir(ppy)3 quantum yield alone is less than 30%, meaning that the quantum yield of the host CBP is also degraded during device operation. It is worth mentioning that in Figure 3a,b PL emission in the 400−450 nm range is still visible, which can be attributed to emission from the neat HTLs. This emission is significantly suppressed in the case of in Figure 3c, because in this case the excitation is almost exclusively absorbed by the emitter. Note that a similar experiment for the blue PHOLED cannot be performed due to the strong overlapping absorption spectra between mCP and FIrpic. Direct excitation on FIrpic without exciting the mCP cannot be achieved precisely. However, it is likely that the quantum yields of both mCP and FIrpic would degrade during device operation as well. Seeing that the quantum yields of both the host and the emitter degrade after electrical aging, we first investigate degradation of the host materials alone. For this purpose, we fabricate OLEDs with the structure ITO/MoO3 (5 nm)/host (50 nm)/TPBi (35 m)/LiF (1 nm)/Al (80 nm). The hosts are 638

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Figure 4. EL and normalized EL spectra collected before and after electrical driving at 20 mA/cm2 for various periods of time and PL and normalized PL spectra collected before and after 30 min of electrical stress for devices using mCP (a, b, c, and d), mCBP (e, f, g, and h), 26DCzPPy (i, j, k, and l), and CBP (m, n, o, and p).

more closely related to the deterioration of the emitter immediately arises. In order to examine the deterioration of the quantum yield of the emitter alone, we study devices with neat EMLs consisting of emitter materials only. The device structures are ITO/MoO3 (5 nm)/mCP (30 nm)/FIrpic (5 nm)/TPBi (35 m)/LiF (1 nm)/Al (80 nm) and ITO/MoO3 (5 nm)/CBP (30 nm)/ Ir(ppy)3 (5 nm)/TPBi (35 m)/LiF (1 nm)/Al (80 nm). Once again, the thickness of the EMLs is chosen to be 5 nm, comparable to that of the recombination zone. Figure 5a presents the EL spectra of the blue PHOLED before and after electrical aging at 20 mA/cm2 for various periods of time. It can be seen that the device degrades very quickly, with a halflifetime of under 1 min. Moreover, when these EL spectra are normalized, as can be seen in Figure 5b, a red-shift in the spectrum can be observed. We have previously identified this spectral shift to be the result of the emergence of a second band at longer wavelength, which may be attributed to aggregation of FIrpic.27 Figure 5c presents the PL spectra of the device before and after 10 min of electrical stress. As can be seen, the PL intensity of the emitter decreases by ∼73% after electrical aging, which is very close to the 70% decrease in device EL. Unlike the device in which FIrpic is doped in mCP in the EML, the deterioration of emitter quantum yield almost contributes wholly to the EL degradation of this device, without introducing significant changes in device charge balance. In addition to the drop in PL intensity, a red-shift in PL spectrum that is similar to the red-shift in device EL after electrical stress is also observed, as can be seen in Figure 5b. Since molecular

mCP, mCBP, 26DCzPPy, and CBP. In addition to mCP, mCBP and 26DCzPPy are also widely used in blue PHOLEDs. Figure 4a,b presents the EL and the normalized EL spectra of the mCP device before and after electrical aging at 20 mA/cm2 for various periods of time. As can be seen, the main mCP emission peak at 400 nm decreases over time. Accompanying the decrease is the appearance of a new band with peak at ∼500 nm, which we have previously attributed to emission from mCP aggregates.24 Figure 4c presents the PL spectra of this device before and after the electrical stress conditions. It can be seen that the PL intensity of the mCP device decreases by ∼25%. Once again, considering that the device degradation happens primarily at the mCP/TPBi interface, which leads to degradation of only a thin layer of mCP near the interface, the lower reduction in the PL intensity of the entire mCP film is expected. Furthermore, as can be seen in Figure 4d, when the PL spectra are normalized, an increase in the intensity of the aggregate peak, similar to that shown in the EL spectra, can be observed. When the same set of experiments are performed on the other three hosts, comparable decreases of host quantum yield and host aggregation after electrical stress are also observed, as shown in Figure 4e−p. It is important to point out that because the rates of quantum yield deterioration among all hosts are similar, the hosts used in blue and green PHOLEDs seem to have very similar stability. This result is very surprising since we have previously discovered that the degradation of the host in green and red PHOLEDs plays the primary role in determining device EL stability.26 The question whether the much faster degradation in blue PHOLED is perhaps somehow 639

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Figure 5. (a) Absolute and (b) normalized EL spectra collected before and after electrical driving at 20 mA/cm2 for various periods of time for a blue PHOLED with a neat FIrpic EML; (c) absolute and (d) normalized PL spectra collected before and after 10 min of electrical stress on the same device; and (e) absolute and (f) normalized PL spectra collected before and after a green PHOLED with a neat Ir(ppy)3 EML has been electrically stressed to 50% of its initial luminance.

aggregation in EMLs has been found to result in exciton quenching,24,26,28 the decrease in FIrpic PL after electrical aging may be attributed to the same effect. When the same set of experiments are performed on green PHOLEDs with Ir(ppy)3 as the emitter, after the EL of the device is degraded by 50%, the PL intensity of the emitter also shows similar degree of reduction in quantum yield, as shown in Figure 5e. However, different from the blue PHOLED, the normalized PL spectra in Figure 5f show that there is no significant shift in the spectrum before and after electrical stress. The results agree with our previous finding that aggregation in Ir(ppy)3 is slower than that in FIrpic.27 It is important to point out that it takes ∼10 h of electrical stress at 20 mA/cm2 for the Ir(ppy)3 device to degrade to 50% of initial luminance (as shown in Figure S1), whereas it only takes about 30 s for the FIrpic device to degrade by the same percentage. It is therefore not surprising that this large difference in emitter stability could lead to the difference in the EL stability of blue versus green PHOLEDs. Because the EL of these PHOLEDs come from cascade excitation from the host to the guest, when the quantum yields of the host and the emitter have similar stability, as in the case of blue devices, deterioration of both the host and the emitter contributes to the device EL degradation at a similar level. However, when the emitter is much more stable than the host, as in the case of

green devices, the host then plays the primary role in determining device EL stability, as shown in our previous work.26 In general, a decrease in quantum yield with electrical driving can be due to molecular degradation by one of the three factors: (i) charges (or polarons), (ii) excitons, or (iii) interactions between them. In order to understand the root cause of the deterioration in material quantum yield, we study a hole-only device. The benefit of using a hole-only device is that it allows decoupling of excitons and polarons and thus provides an opportunity to study if degradation in material quantum yield is primarily causes by either one identity. For example, the effects of having only excitons and only polarons can be induced by external UV irradiation or by applying an electrical current, respectively. Furthermore, by combining both UV and current stress conditions, it becomes possible to find any role of interactions between excitons and polarons in the degradation process. The details of the stress schemes can be found in the Experimental Section. Figure 1b illustrates the device structure. The host and the guest used are mCP and FIrpic, respectively. Figure 6a presents the changes in PL intensities of devices before and after being subjected to one of the three stress conditions for 18 h. The PL excitation wavelength is set to 330 nm. It can be clearly seen that the ⟨I only⟩ stress has minimal 640

DOI: 10.1021/acsami.6b13823 ACS Appl. Mater. Interfaces 2017, 9, 636−643

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Figure 6. (a) Absolute and (b) normalized PL spectra of hole-only devices before and after ⟨I only⟩, ⟨L only⟩, and ⟨I + L⟩ stress scenarios described in the Experimental Section.

Figure 7. Changes in J−V characteristics of hole-only devices consisting of (a) only mCP and (b) a mCP:FIrpic (20%) middle layer before and after ⟨I only⟩, ⟨L only⟩, and ⟨I + L⟩ stress scenarios described in the Experimental Section.

effects on the PL intensity, whereas the ⟨L only⟩ stress is able to degrade the PL intensity by ∼15%, suggesting that excitons alone, but not polarons, can degrade material quantum yield. More interestingly, when both excitons and polarons are present in the ⟨I + L⟩ stress scenario, the PL intensity drops by more than 25%. Clearly, the interactions between excitons and polarons, which are both present in PHOLEDs under operation, result in much faster deterioration of material quantum yield. Furthermore, the normalized PL spectra before and after the stress tests, as illustrated in Figure 6b, shows a similar spectral red-shift as in PHOLEDs after electrical aging. Seeing that the degradation of material quantum yield plays a primary role in determining device stability for both blue and green PHOLEDs (accounting for roughly 2/3 of device EL degradation) and that the degradation is mainly due to exciton−polaron interactions, it becomes interesting to see to what extent charge balance is altered in these devices during operation. To examine the changes in charge balance under exciton−polaron interactions, two hole-only devices with the following structures are fabricated: ITO/MoO3 (5 nm)/mCP (100 nm)/MoO3 (5 nm)/Al (80 nm) and ITO/MoO3 (5 nm)/mCP (40 nm)/mCP: FIrpic (20%) (20 nm)/mCP (40 m)/MoO3 (5 nm)/Al (80 nm). Figure 7 presents the J−V characteristics of these devices before and after being exposed to 18 h of ⟨I only⟩, ⟨L only⟩, and ⟨I + L⟩ stress scenarios (the stress conditions are the same as those used in Figure 6). As can be seen, the devices with and without FIrpic dopant under electrical bias have very similar J−V curves, suggesting that FIrpic is not a strong hole trap in mCP and does not affect hole transport in the device noticeably. Moreover, after both devices have been electrically stressed for 18 h, no significant change in the J−V curves can be observed, indicating that hole current alone does not change carrier transport in the devices appreciably. Interestingly, after the devices are exposed to external irradiation for 18 h, the carrier mobility in both devices decreases by a similar amount and that a small trap-filling regime29,30 in the J−V curves can be observed (indicated by the black circle in the figure). The difference of the two devices

comes after the devices are exposed to both hole current and external irradiation at the same time, i.e., the ⟨I + L⟩ stress scenario. The device with FIrpic doped in mCP sees a much larger drop in film conductivity as well as a much larger trapfilling regime in the J−V curve. The results are not surprising considering that FIrpic degrades faster than the host under exciton−polaron interactions. Given the spectral shifts and possibility of increased aggregation in the case of FIrpic, we can attribute the trapping effect to the smaller energy band gap of the aggregate. Furthermore, it can be believed that in blue PHOLEDs the hole transport layers should be stable because they are exposed mainly to hole current. On the other hand, within the EML, especially at the EML/ETL interface, both exciton and polaron densities are high. It can be expected that traps are quickly formed nearby to cause (1) quenching of excitons and thus deterioration of quantum yield as seen earlier and (2) an impedance to hole transport and thus the hole-toelectron ratio. It is worth noting that the changes in charge balance due to traps are interconnected with deterioration of material quantum yield. Since the change in charge balance accounts for roughly 1/3 of device EL degradation, it plays only a secondary role in determining device stability. It can be clearly seen that exciton−polaron interactions can lead to accelerated degradation of both material quantum yield and charge balance in PHOLEDs. In view of the above results, we can also see that the degradation mechanisms in blue and green devices are not dissimilar. In both cases, degradation is due to a deterioration in the quantum yields of the host and the emitter, with changes in charge balance playing a secondary role. With the hosts having very similar stability, however, the main difference is how fast the emitter degrades. In general, bond cleavage on emitter molecules has been found to play a major role in their degradation.5,31 As excitation energy in the case of blue emitters is necessarily higher than in the case of green emitters (due to the wider band gap), it is not surprising 641

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(6) Aziz, H.; Popovic, Z. D.; Hu, N.-X.; Hor, A.-M.; Xu, G. Degradation Mechanism of Small Molecule-Based Organic LightEmitting Devices. Science 1999, 283 (5409), 1900−1902. (7) Aziz, H.; Popovic, Z. D. Degradation Phenomena in SmallMolecule Organic Light-Emitting Devices. Chem. Mater. 2004, 16 (23), 4522−4532. (8) Kondakov, D. Y.; Sandifer, J. R.; Tang, C. W.; Young, R. H. Nonradiative Recombination Centers and Electrical Aging of Organic Light-Emitting Diodes: Direct Connection between Accumulation of Trapped Charge and Luminance Loss. J. Appl. Phys. 2003, 93 (2), 1108−1119. (9) Kondakov, D. Y. Direct Observation of Deep Electron Traps in Aged Organic Light Emitting Diodes. J. Appl. Phys. 2005, 97 (2), 024503. (10) Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F. Operational Degradation of Organic Light-Emitting Diodes: Mechanism and Identification of Chemical Products. J. Appl. Phys. 2007, 101 (2), 024512. (11) Kondakov, D. Y. Role of Chemical Reactions of Arylamine Hole Transport Materials in Operational Degradation of Organic LightEmitting Diodes. J. Appl. Phys. 2008, 104 (8), 084520. (12) Scholz, S.; Corten, C.; Walzer, K.; Kuckling, D.; Leo, K. Photochemical Reactions in Organic Semiconductor Thin Films. Org. Electron. 2007, 8 (6), 709−717. (13) Meerheim, R.; Scholz, S.; Olthof, S.; Schwartz, G.; Reineke, S.; Walzer, K.; Leo, K. Influence of Charge Balance and Exciton Distribution on Efficiency and Lifetime of Phosphorescent Organic Light-Emitting Devices. J. Appl. Phys. 2008, 104 (1), 014510. (14) Scholz, S.; Walzer, K.; Leo, K. Analysis of Complete Organic Semiconductor Devices by Laser Desorption/ionization Time-ofFlight Mass Spectrometry. Adv. Funct. Mater. 2008, 18 (17), 2541− 2547. (15) Giebink, N. C.; Andrade, B. W. D.; Weaver, M. S.; Mackenzie, P. B.; Brown, J. J.; Thompson, M. E.; Forrest, S. R.; D’Andrade, B. W. Intrinsic Luminance Loss in Phosphorescent Small-Molecule Organic Light Emitting Devices due to Bimolecular Annihilation Reactions. J. Appl. Phys. 2008, 103 (4), 044509. (16) Giebink, N. C.; D’Andrade, B. W.; Weaver, M. S.; Brown, J. J.; Forrest, S. R. Direct Evidence for Degradation of Polaron Excited States in Organic Light Emitting Diodes. J. Appl. Phys. 2009, 105 (12), 124514. (17) Vestweber, H.; Rieß, W. Highly Efficient and Stable Organic Light-Emitting Diodes. Synth. Met. 1997, 91 (1−3), 181−185. (18) Lee, S. T.; Gao, Z. Q.; Hung, L. S. Metal Diffusion from Electrodes in Organic Light-Emitting Diodes. Appl. Phys. Lett. 1999, 75 (10), 1404−1406. (19) Sandanayaka, A. S. D.; Matsushima, T.; Adachi, C. Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules. J. Phys. Chem. C 2015, 119 (42), 23845−23851. (20) Wang, Q.; Williams, G.; Aziz, H. Photo-Degradation of the Indium Tin Oxide (ITO)/organic Interface in Organic Optoelectronic Devices and a New Outlook on the Role of ITO Surface Treatments and Interfacial Layers in Improving Device Stability. Org. Electron. 2012, 13 (10), 2075−2082. (21) Zhang, Y.; Abdelmalek, M. M. A.; Wang, Q.; Aziz, H. Degradation Mechanism in Simplified Phosphorescent Organic Light-Emitting Devices Utilizing One Material for Hole Transport and Emitter Host. Appl. Phys. Lett. 2013, 103 (6), 063307. (22) Wang, Q.; Williams, G.; Tsui, T.; Aziz, H. Photochemical Deterioration of the Organic/metal Contacts in Organic Optoelectronic Devices. J. Appl. Phys. 2012, 112 (6), 064502. (23) Klubek, K. P.; Tang, C. W.; Rothberg, L. J. Investigation of Blue Phosphorescent Organic Light-Emitting Diode Host and Dopant Stability. Org. Electron. 2014, 15 (7), 1312−1316. (24) Wang, Q.; Sun, B.; Aziz, H. Exciton-Polaron-Induced Aggregation of Wide-Bandgap Materials and Its Implication on the Electroluminescence Stability of Phosphorescent Organic LightEmitting Devices. Adv. Funct. Mater. 2014, 24 (20), 2975−2985.

that such bond cleavage would occur more frequently, hence their generally faster degradation.32−34 Therefore, in order to improve the lifetime of blue PHOLEDs, more chemically stable molecules need to be developed. Moreover, since the exciton and polaron environment will always be present in the EML, an ideal blue emitter should perhaps have very short exciton lifetime to limit the amount of time excitons can interact with polarons. Finally, better designs of device structure that can reduce exciton35,36 and polaron37 densities near the EML/ETL interface should also be implemented to increase device lifetime.

4. CONCLUSION In conclusion, we have demonstrated that the quantum yields of both hosts and emitters in blue and green PHOLEDs can degrade when exposed to excitons and polarons. This degradation pathway plays the primary role in determining device EL stability. Furthermore, charge traps are found to be introduced due to exciton−polaron interactions. The resulting changes in charge balance is found to be of secondary importance in affecting device degradation. Finally, the degradation mechanisms in blue and green PHOLEDs are found to be very similar. The lower stability of blue PHOLEDs mainly comes from the faster deterioration of the quantum yield of the emitter.

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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.6b13823. Lifetime data of a green PHOLED with a neat Ir(ppy)3 EML (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Z.). ORCID

Yingjie Zhang: 0000-0002-6468-8583 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grant (RGPIN-2014-04940).

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REFERENCES

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DOI: 10.1021/acsami.6b13823 ACS Appl. Mater. Interfaces 2017, 9, 636−643