Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light‐Emitting Diode with Multiple Reverse Intersystem Crossing Channels

Abstract With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light‐emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT‐SADF:PO‐T2T:CDBP is constructed with three RISC channels respectively on DBT‐SADF, DBT‐SADF:PO‐T2T, and CDBP:PO‐T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT‐SADF:PO‐T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.


DOI: 10.1002/advs.201801938
all exciplexes possess intermolecular charge-transfer (CT) transition and locate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on the D and A molecules, respectively. With these completely separated frontier molecular orbitals, exciplexes naturally have extremely small energy gaps (ΔE ST s) between their lowest singlet and triplet (S 1 and T 1 ) energy levels. [1,13,14] According to our previous work, by further controlling the T 1 energy levels of two constituting molecules higher than that of exciplex, exciplexes can exhibit significant thermally activated delayed fluorescence (TADF) behaviors, [12][13][14] which will promote the utilization of electrogenerated triplet excitons in the OLEDs. Besides, as the constituting D and A materials are generally capable of hole-and electron-transporting properties, respectively, exciplexes would possess bipolar transporting properties, and even reach a charge-transporting balance by adjusting the mixing ratio between D and A, which would be helpful to further simplify the structures and lower the driving voltages for OLEDs. [2,7,14,17] Therefore, exciplexes were widely investigated as the TADF emitters or hosts of OLEDs in recent years. [1][2][3][5][6][7][8][9][10][11][12][13][14][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] As the exciplexes can realize the balanced charge transport and suppress the triplet-triplet annihilation by upconvert triplet excitons to singlet excitons, the OLEDs using exciplexes as the hosts have realized remarkable progress till now. The traditional fluorescent emitters, phosphorescent emitters, and TADF emitters all realized superior performance in exciplex hosts than in conventional host materials, exhibiting lower driving voltages and improved efficiency roll-off in the devices. [17,26,29,[36][37][38][39] However, by using exciplexes as the TADF emitters in the OLEDs, only limited progress was realized. In 2012, Goushi et al. reported an OLED with a maximum external quantum efficiency (EQE) of 5.4% by using m-MTDATA:3TPYMB exciplex as the emitter, demonstrating the TADF characteristic of exciplexes for the first time. [1] In 2013, Hung et al. reported an interface exciplex-based OLED with a maximum EQE of 7.7%, suggesting the superiority of exciplex emitters on device design. [18] In 2014, Li et al. reported OLEDs based on mCP:HAP-3MF emitter with a maximum EQE of 11.3%. [21] In 2015, our With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light-emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP is constructed with three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.
It is generally understood that the exciton utilization of TADF emitter is decided by its reverse intersystem crossing efficiency of triplet excitons (Φ RISC ) and fluorescence quantum yield of singlet excitons (Φ f ). [1,[40][41][42][43][44][45] For exciplex TADF emitters, as their HOMO and LUMO are respectively located on different constituting molecules, their Φ f s are hard to be evidently improved due to such natural separation between HOMO and LUMO. As a result, further enhancing Φ RISC s of exciplex emitters should be the most possible way to improve their excitons utilization. As shown in Figure 1, conventional exciplexes only have one intrinsic reverse intersystem crossing (RISC) channel, which limits the upconversion of triplet excitons. In 2016, our group increased the RISC channels of the exciplexes by introducing a TADF emitter as one constituting molecule, and MAC:PO-T2T exciplex realized a high EQE of 17.8% in the OLED, [14] which is the highest efficiency among the reported OLEDs using exciplex as emitters till now. However, there is still conspicuous room for further improving the exciton utilization of exciplex emitters.

Results and Discussions
Molecular structures of the constituting materials are shown in Figure 2a. Common carrier transporting materials 13AB, CDBP, and PO-T2T were directly purchased from commercial sources, and DBT-SADF was newly designed and synthesized. Synthetic route and detailed synthesis of DBT-SADF are depicted in the Supporting Information. DBT-SADF was fully characterized and confirmed with 1 H, 13 C nuclear magnetic resonance spectroscopies and mass spectroscopy, and purified by sublimation before further characterizations. As shown in Figure   and phosphorescence of DBT-SADF doped in mCP (1,3-Di-9-carbazolybenzene) film were investigated at 77 K, and its ΔE ST is determined to be 0.030 eV from the difference between the peaks of the fluorescence and phosphorescence spectra, which is basically required for TADF emitters. To further demonstrate the TADF characteristic of DBT-SADF, the temperature-dependent transient photoluminescence (PL) decays of DBT-SADF doped in mCP film were shown in Figure S1b in the Supporting Information. From 100 to 300 K, the delayed parts are remarkably increased due to the enhanced RISC process with the increased temperature, which is the direct evidence for TADF behavior of DBT-SADF. [1,40] Moreover, DBT-SADF-based OLEDs were also fabricated by using mCP as the host material. The optimized device structure and the device performance are shown in the Supporting Information. The device exhibits a maximum EQE of 14.0%, which is far higher than the theoretical limit of 5% EQE for conventional fluorescent emitters, further demonstrating the TADF characteristic of DBT-SADF. To better understand our exciplex systems, we also investigated the fluorescence and phosphorescence of 13AB pure film at 77 K ( Figure S2b, Supporting Information), and its ΔE ST is determined to be 0.56 eV. The temperature-dependent transient photoluminescence decays of 13AB were also measured, and the trend of delayed parts is consistent with blank reference at 300 K, which is the direct evidence for non-TADF behavior of 13AB. Besides, we also investigated the carrier transport characteristics of 13AB and DBT-SADF, and both the 13AB and DBT-SADF are hole-dominated materials. (All the energy levels were newly measured in our laboratory and shown in Figure S3 in the Supporting Information.) Particularly, as the emission peak of DBT-SADF:PO-T2T is close to that of DBT-SADF neat film (516 and 514 nm), we further investigated PL spectra and transient fluorescence decays of DBT-SADF:mCP (1%, 10%, 20%, 5%, 70%, 100% doped) and DBT-SADF:POT2T (3:7, 4:6, 5:5, 6:4) films with varying concentration of DBT-SADF, and the results are shown in Figure S4 in the Supporting Information. Increasing the weight ratio of DBT-SADF from 1% to 100%, the PL spectra of DBT-SADF:mCP films are evidently changed with the peaks red-shifted from 490 to 514 nm and the full-widths at half-maximum enlarge from 72 to over 100 nm. These results suggest the emission of DBT-SADF:mCP should be from the intramolecular CT transition of DBT-SADF, which is sensitive with the environmental polarity increased with more D-A structure DBT-SADF molecules. [49] Reversely, all DBT-SADF:POT2T films exhibit similar PL spectra peaked around 516 nm, suggesting their emission should be from the exciplex between DBT-SADF and PO-T2T. Therefore, all three bicomponent mixed films can form exciplexes. Moreover, determined from the highest energy vibronic sub-band of their phosphorescence spectra at 77 K ( Figure S2, Supporting Information), the T 1 energy levels of 13AB, CDBP, DBT-SADF, and PO-T2T are 3.01, 3.02, 2.53, and 2.95 eV, respectively. These values are evidently higher than the energy values of their corresponding exciplexes, which can ensure the exciplexes harvest both singlet and triplet excitons. [12][13][14] In our previous report, we have demonstrated the TADF characteristic of CDBP:PO-T2T. [12] Thereby, we here measured the ΔE ST s and temperature-dependent transient PL decays for 13AB:PO-T2T and DBT-SADF:PO-T2T to prove their TADF behaviors. As shown in Figure S5 in the Supporting Information, 13AB:PO-T2T and DBT-SADF:PO-T2T respectively exhibit extremely small ΔE ST s of 0.047 and 0.032 eV from the differences between the peaks of their fluorescence and phosphorescence spectra at 77 K. And from 100 to 300 K, more significant decays in the microsecond range were observed with increased temperatures, confirming the existence of TADF characteristic. As shown in Figure 1, CDBP:PO-T2T and 13AB:PO-T2T are conventional bicomponent TADF exciplexes, and DBT-SADF:PO-T2T is the TADF-assistant bicomponent TADF exciplex.
Based on three bicomponent TADF exciplexes, we constructed two tricomponent exciplexes, 13AB:PO-T2T:CDBP and DBT-SADF:PO-T2T:CDBP, according to our strategy. As shown in Figure 2e,f, both the two tricomponent mixtures exhibit cumulative absorptions compared with the bicomponent exciplexes, suggesting still no formation of new ground-state transitions. And the PL spectra of 13AB:PO-T2T:CDBP and DBT-SADF:PO-T2T:CDBP are almost consistent with those of 13AB:PO-T2T and DBT-SADF:PO-T2T, respectively. These results do not only indicate that exciplex would be formed in the tricomponent mixtures as well, but also prove complete energy transfer from higher-energy CDBP:PO-T2T to lower-energy 13AB:PO-T2T or DBT-SADF:PO-T2T. We can realize the lower-energy exciplex emission with the assistance of the higher-energy exciplex in the tricomponent mixture. Therefore, as shown in Figure S6  With the increasing RISC channels, the Φ PL s of the exciplexes are evidently enhanced, and the highest Φ PL of 61.0% is realized with three RISC channels. These results indicate the multiple RISC channels in tricomponent exciplexes will indeed enhance the utilization of excitons. To better understand the superiority of multiple RISC channels in tricomponent exciplexes, we further measured the transient PL decays of four exciplex emitters at room temperature and calculated their key kinetic parameters. For blue and green fluorescent emitters, the internal conversion process of singlet excitons could be ignored compared with fluorescence decay and intersystem crossing process of singlet excitons, [41][42][43] thus the calculations were carried out via the following equations [41,42] where Φ F and Φ TADF are the quantum efficiencies of prompt and delayed fluorescence; Φ ISC is the efficiency of ISC process; k p , k F , k ISC, k TADF , and k RISC are rate constants of prompt fluorescence, fluorescence decay, ISC process from S 1 to T 1 state, delayed fluorescence decay, and RISC process, respectively. Φ F and Φ TADF of these exciplex emitters are estimated from the Φ PL with a relative ratio which was calculated from the transient PL results. [41] Consequently, we got all key kinetic parameters of four exciplex emitters and list them in Table 1. With naturally separated HOMO and LUMO distributions, four exciplexes exhibit relatively small k F s and relatively large k ISC s, resulting in all four Φ F s with small values. However, for the RISC process, k RISC s are evidently increased from 0.05 × 10 5 for 13AB:PO-T2T with only one RISC channel to 0.64 × 10 5 and 1.17 × 10 5 for 13AB:PO-T2T:CDBP and DBT-SADF:PO-T2T with two RISC channels, and finally to 14.2 × 10 5 for DBT-SADF:PO-T2T:CDBP with three RISC channels. As a result, Φ TADF s are also increased from 6.7% for 13AB:PO-T2T to 28.0% for 13AB:PO-T2T:CDBP and 33.0% for DBT-SADF:PO-T2T, and to 58.0% for DBT-SADF:PO-T2T:CDBP. Therefore, the evidently improved Φ PL of DBT-SADF:PO-T2T:CDBP is actually ascribed to the better upconversion of triplet excitons with three RISC channels and prove the feasibility of our strategy to improve the exciton utilization of exciplex emitters. We finally evaluated the electroluminescence (EL) performance of two tricomponent exciplexes and their contrastive bicomponent exciplexes by using them as the emitters. As the mixing ratios of exciplexes would change their charge balance and affect the device performance, [13,14] the carrier transporting properties of four exciplexes were carefully investigated. The current density-voltage characteristics of hole-only and electron-only devices for four exciplexes with different mixing ratios are shown in Figure S7  phenyl]-cyclohexane) and PO-T2T were used as holetransporting layer and electron-transporting layer, respectively; LiF was acted as electron injection layer; 13AB, DBT-SADF, and CDBP were used as electron-blocking layer; exciplexes with the optimal mixing weigh ratios were used as the emitting layer. As listed in Table 2, all four devices exhibit quite low turn-on voltages around 2.3 and 2.4 V, indicating the superiority of the exciplexes on carriers injection. As shown in Figure 3, both the two tricomponent exciplexes emit same-shape EL spectra with their corresponding bicomponent exciplexes, except 8 nm blue-shift. Such blue-shift is probably caused by the intermolecular interaction between 13AB and CDBP or DBT-SADF and CDBP, which slightly lowers the HOMO energy levels of 13AB and DBT-SADF. [47] With only one RISC channel, the maximum efficiencies of  Φ PL is the total photoluminescence fluorescence quantum efficiency value; b) The prompt fluorescence quantum efficiency; c) The delayed fluorescence quantum efficiency;   One additional RISC channel has already improved the device performance of exciplexes evidently, which is consistent with our previous report. [14] Particularly, with three RISC channels, DBT-SADF:PO-T2T:CDBP realizes quite remarkable performance with maximum CE/PE/EQE of 60.0 cd A −1 , 69.7 lm W −1 , and 20.5% in the device. From 13AB:PO-T2T to 13AB:PO-T2T:CDBP, DBT-SADF:PO-T2T to DBT-SADF:PO-T2T:CDBP, the maximum EQE values of the devices are evidently improved from 12.4% to 20.5%, suggesting multiple RISC channels can effectively improve the exciton utilization of exciplex emitters. Moreover, to the best of our knowledge, this is the first report with an EQE over 20% for the OLEDs based on exciplex emitters at room temperature (Figure 4; Table S2, Supporting Information), [1,2,[4][5][6][7][8][9][10][11][12][13][14][17][18][19][20][21]30,31,[46][47][48] thus our novel strategy of tricomponent exciplexes will provide a valuable approach for further development of exciplex emitters. To further confirm the proposed multiple reverse intersystem crossing channels in the tricomponent exciplex emitter system, a typical hole-transport material with (N,N′-Di(1naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) was chosen to replace the donor CDBP. As NPB has a higher HOMO energy level (−5.  Figure 5, DBT-SADF:PO-T2T:NPB exhibits similar but blue-shifted EL spectrum compared with NPB:PO-T2T (respectively peaked at 560 and 588 nm). This behavior is similar with the former two tricomponent exciplex emitters, and probably caused by the intermolecular interaction between NPB and DBT-SADF, which slightly lowers the HOMO energy level of NPB. [50] Moreover, the DBT-SADF:PO-T2T:NPB-based device exhibits the maximum efficiencies of 10.9 cd A −1 , 13.7 lm W −1 , and 4.1% for CE, PE, and EQE, respectively; while the maximum CE, PE, and EQE of NPB:PO-T2T-based device are respectively 2.0 cd A −1 , 1.84 lm W −1 , and 0.89%. The maximum efficiencies of the tricomponent device based on DBT-SADF:POT2T:NPB are nearly five times higher than that of the bicomponent device based on NPB:PO-T2T. These results further prove both the complete energy transfer from higher-energy exciplex to lower-energy exciplex in tricomponent exciplex system and the enhanced excitons utilization of multiple reverse intersystem crossing channels. Meanwhile, we also chose a classical electron-transporting host material bis [2-(diphenylphosphino) phenyl]ether oxide (DPEPO) with a wide gap material to replace the donor CDBP molecule, and constructed a tricomponent exciplex system DBT-SADF:PO-T2T:DPEPO. As DPEPO has a higher LUMO energy level and HOMO energy level than DBT-SADF, only exciplex DBT-SADF:PO-T2T can be formed in  Figure 5, with the different weight ratios from 6:3:1 to 5:3:2 and 4:3:3 and 2:3:5, all the devices exhibit nearly consistent EL spectra with the DBT-SADF:PO-T2T-based device. Such results prove that the EL of these devices should be emitted from exciplex DBT-SADF:PO-T2T, which is not sensitive to concentration of DBT-SADF. With a weight ratio of 6:3:1, the device based on DBT-SADF:PO-T2T:DPEPO exhibits the maximum efficiencies of 51.7 cd A −1 , 67.6 lm W −1 , and 17.3% for CE, PE, and EQE, respectively, which are also nearly consistent with the DBT-SADF:PO-T2T-based device, suggesting the positive impact of the higher-energy exciplex in tricomponent exciplex systems.

Conclusion
In conclusion, to further improve the exciton utilization, a novel tricomponent exciplex strategy was proposed to realize exciplex emitters with multiple RISC channels in this work. With a newly designed and synthesized single-molecule TADF emitter DBT-SADF, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP was developed with three RISC channels accordingly. And DBT-SADF:PO-T2T:CDBP successfully realizes much higher Φ PL (61.0%) and k RISC (14.2 × 10 5 ) than other contrasted exciplexes. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a low turn-on voltage of 2.4 V and high maximum efficiencies of 60.0 cd A −1 CE, 69.7 lm W −1 PE, and 20.5% EQE. To the best of our knowledge, this is the first report for exciplex emitter-based OLEDs with an over 20% EQE at room temperature. Such high performance demonstrates introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, and our novel strategy of tricomponent exciplexes will provide a valuable approach for further development of exciplex emitters.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.