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Understanding Degradation of Organic Light-Emitting Diodes from Magnetic Field Effects

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Understanding Degradation of Organic Light-Emitting Diodes from Magnetic Field Effects ARTICLE https://doi.org/10.1038/s43246-020-0019-0 OPEN Understanding degradation of organic light- emitting diodes from magnetic field effects ✉ ✉ Masaki Tanaka 1, Ryo Nagata1, Hajime Nakanotani1,2 & Chihaya Adachi 1,2 1234567890():,; The impact of magnetic field effects on the electroluminescence of organic light-emitting diodes is commonly used to characterize exciton dynamics such as generation, annihilation, and performance degradation. However, interpreting these effects is challenging. Here, we show that magnetic field effects in organic light-emitting diodes can be understood in terms of the magnetic response of device characteristics derived from polaron-pair and triplet exciton quenching processes, such as triplet-polaron interactions and triplet-triplet annihi- lation. Device degradation shows a clear relationship with the amplitude of the magnetic field effects, enabling non-destructive measurement of the degradation. The results and proposed mechanism provide a better understanding of magnetic field effects on organic light-emitting diodes and device degradation phenomena. 1 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 2 International Institute ✉ for Carbon Neutral Energy Research (WPI-I²CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. email: [email protected]. ac.jp; [email protected] COMMUNICATIONS MATERIALS | (2020) 1:18 | https://doi.org/10.1038/s43246-020-0019-0 | www.nature.com/commsmat 1 ARTICLE COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-020-0019-0 n internal electroluminescence quantum efficiency of Results and discussion Anearly 100% can be achieved in organic light-emitting Assessment of magnetic field effects in various TADF-OLEDs. diodes (OLEDs) especially by utilization of phosphores- We focused on TADF emitter-based OLEDs (Fig. 1a). The cence and thermally activated delayed fluorescence (TADF) that complete device architecture is provided in Supplementary Fig. 1. involve intersystem crossing between the lowest excited singlet First, the MEL profiles of undegraded OLEDs were analyzed to fi and triplet states (S1 and T1). However, a signi cant improvement probe the origin of magnetic responses. Figure 1b shows typical in OLED stability is of crucial importance, particularly in blue MEL profiles of 4CzIPN-based OLED under constant-current fi OLEDs, so that they can be used in high-performance displays condition (MELJ). To assess the origin of the MEL pro les, we and light sources. To improve OLED lifetimes, a detailed performed a fitting analysis based on the Lorentzian and non- understanding of degradation processes is required. Several Lorentzian equations16–19 mechanisms have been proposed1–9. For example, Kondakov Magnetic field effect ¼ðLow À field effectÞþðhigh À field effectÞ et al. reported that chemical decomposition of the organic 1–4 2 2 materials is a critical degradation route that originates from ¼ ALB þ AHB 2 2 2 high-energy particles, such as highly excited triplet excitons (Tn) B þ B ðÞB þ B * – L H and polarons (P ), generated via triplet triplet annihilation ð Þ (TTA) or triplet–polaron annihilation (TPA)5–8 1 where AL and AH are the amplitudes and BL and BH are the þ ! þ ð Þ; characteristic magnetic fields for low-field and high-field effects, T T S0 Tn TTA respectively. The results are summarized in Fig. 1b and Table 1. The MEL profiles had two parts, indicating that there are two * different mechanisms. T þ P ! S þ P ðTPAÞ; 0 Because BL is a comparable value for the PP mechanism, i.e., ~5 mT16–19, the low-field effect originates from the PP mechan- “ ” fi where T, P, and S0 are a triplet exciton, a polaron, and the ground ism that increases bright singlet excitons. The magnetic eld * 1 3 state. Tn and P have high enough energy to decompose organic suppresses the intersystem crossing between PP and PP states. molecules and generate exciton quenchers and carrier traps. In contrast, BH was large (~100 mT). We compared it with the Recently, we clarified that TPA was identified as being responsible zero-field splitting values, i.e., D and E, of the excited triplet state for the degradation mechanism for TADF-OLEDs, and the gen- of 4CzIPN reported previously20,21, and good agreement eration of carrier traps, the change in carrier balance, and suc- suggested that the high-field effect results from the reaction of fi τ fi cessive exciton deactivation during device aging that signi cantly triplet excitons. The ds of TADF emitters affect the MEL pro les, 9 fi affect OLED lifetimes . It has been strongly suggested that the as shown in Fig. 1c. Although the signs of all the MELJ pro les dynamics of triplet excitons is largely responsible for device were positive, their shape and magnitude, especially in the high- fi τ degradation. However, no direct evidence has been presented, and eld region, depended on the ds. This behavior could be a detailed analysis of exciton dynamics via nondestructive mea- understood from the probability of triplet exciton reactions such τ 22–24 surements is complex. as TTA and TPI that should strongly depend on ds . In fact, To probe the dynamics of excited triplet states, external mag- devices based on TADF emitters exhibiting long τd, such as netic fields are used to lift the degeneracy. Magnetic field effects 2CzPN, PIC-TRZ, and 3CzTRZ, showed steep rolloffs in on the electroluminescence properties of OLEDs were first electroluminescence efficiency, as shown in Supplementary Fig. 2. reported in 2003 by Kalinowski et al.10, where the field modulated Magneto-photoluminescence measurements confirmed that 20 the ratio of the singlet/triplet exciton yield. This was the result of wt% 4CzIPN:mCBP and 2CzPN:mCBP films did not exhibit clear modulating the ratio of singlet and triplet polaron pairs (1PP and magnetic responses in high magnetic fields (Supplementary 3 PP). Numerous studies regarding the mechanism of magnetic Fig. 3). Thus, the contribution of a TTA event to the MELJ can field effects on OLEDs based on fluorescent11,12, phosphor- be negligible. Furthermore, no magneto-photoluminescence escent13, exciplex14, and TADF emitters15,16 were investigated to response suggests that PPs are not induced from excitons unveil the underlying dynamics of exciton generation, radiation, generated under photoexcitation, as introduced in phosphores- and annihilation processes. PP11,17, triplet–polaron interactions cent emitter and host systems25. 16,18 12,16 (TPI) , and TTA mechanisms were mainly used to Considering the effect of TPI on MELJ, Fig. 1d is a schematic of explain magnetic field effects. However, the interpretation of a TPI process in OLEDs under electrical excitation. A these effects has been unclear, and the relationship between triplet–polaron (trion) intermediate state is formed when a exciton dynamics and OLED degradation has been lacking. triplet exciton and a polaron collide, and has two possible spin Here, we demonstrate that magnetic-field-modulated electro- states: “doublet” and “quartet”18,19,26. Because the doublet-trion luminescence (magneto-electroluminescence, MEL) can be used reaction, i.e., TPA, is spin-allowed, the T1 that contributes to the to track triplet exciton dynamics in OLEDs during degradation. reverse intersystem crossing (RISC) in TADF-OLEDs can be MEL signals from TADF-OLEDs were divided into low-field and immediately quenched via energy transfer to the polaron, high-field effects that corresponded to PP and TPI mechanisms, generating a ground-state molecule and an excited (hot) polaron. respectively. We also find that the high-field effects exhibit a clear Because the energy of the hot polaron is enough high to dissociate fl τ dependence on the delayed uorescence lifetimes ( ds) of TADF chemical bonds, the TPA generates decomposed materials that emitters. Based on the assignment of the origin of the MEL sig- are exciton quenchers and/or carrier traps. In contrast, the nals, we analyze those signals of degraded OLEDs that exhibited quartet state reaction is spin-forbidden and the lifetime of a large amplitudes relative to those of pristine (undegraded) quartet-trion is longer than that of the doublet. Thus, there are OLEDs. Then, we confirm that the shapes of the MEL profiles two possible quartet–trion reactions: “carrier scattering” and changed due to an exciplex formation according to the unwanted “exciton dissociation.” In carrier scattering, a quartet–trion change in location of the carrier recombination zone. We thus separates into a triplet exciton and a polaron, and the net charge nondestructively reveal exciplex formation at the interface carrier mobility is decreased. In contrast, the dissociation process between emission and hole-blocking layers that results in a low increases the net carrier density because the triplet exciton electroluminescence quantum yield. dissociates into a hole and an electron via the quartet–trion 2 COMMUNICATIONS MATERIALS | (2020) 1:18 | https://doi.org/10.1038/s43246-020-0019-0 | www.nature.com/commsmat COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-020-0019-0 ARTICLE a N N N O N NC CN N NC CN N NN N N O N N N N N N N N O N N N N N N 4CzIPN ACRXTN PXZ-TRZ PIC-TRZ 3CzTRZ 2CzPN (2.8 s) (3.3 s) (4.1 s) (60 s) (75 s) (77 s) short Delayed emission lifetime long bc 0.18 1.6 Experimental result (4CzIPN) 4CzIPN 0.16 Fitting result 1.4 ACRXTN Low-field effect PXZ-TRZ −2 −2 0.14 High-field effect 1.2 PIC-TRZ 0.12 3CzTRZ 1.0 2CzPN 0.10 0.8 0.08 0.6 0.06 (%) @ 3 mA cm (%) @ 3 mA (%) @ 3 mA cm (%) @ 3 mA J J 0.4 0.04 MEL MEL 0.2 0.02 0.00 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Magnetic field (T) Magnetic field (T) d Fig.
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