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Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex state are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependence which weakens with the increase of bias-current. This is because the bias-current increases by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device is enhanced, which facilitates the electric-field-induced dissociation of polaron-pair and exciplex states and then reduces their lifetime. That is, less polaron-pair and exciplex states participate in the ISC process and RISC process, leading these two processes to weaken. Here, magneto-electroluminescence (MEL) is used as a fingerprint probing tool to observe various current-dependent ISC and RISC processes in EB-OLEDs with different charge balances via modifying the device hole-injection layer. Interestingly, current-dependent MEL traces of the unbalanced device display a conversion from normal ISC (1–25 μA) process to abnormal ISC (25–200 μA) process, whereas those of the balanced device show conversions from normal ISC (1–5 μA) into abnormal RISC (10–50 μA) and then into normal RISC (50–150 μA) and finally into abnormal ISC (200–300 μA) process. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC process and RISC process in these two devices first increase then decrease as the bias-current increases. These non-monotonic current-dependent ISC process and RISC process are attributed to the competition between the increased number and the reduced lifetime of polaron-pair state and exciplex state during improving the bias-current. Furthermore, the RISC process in the balanced device is stronger than that in the unbalanced device. This is because the balanced carrier injection can facilitate the formation of triplet exciplex states and weaken the triplet-charge annihilation (TQA) process between triplet exciplex states and excessive charge carriers, which leads the number of triplet exciplex states to increase. That is to say, more triplet exciplex states can be converted into singlet exciplex states through the RISC process, causing the external quantum efficiency of the balanced device to be higher than that of the unbalanced device. Obviously, this work not only deepens the understandings of current-dependent ISC and RISC processes in EB-OLEDs, but also provides an insight into the device physics for designing and fabricating high-efficiency EB-OLEDs.
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Keywords:
- exciplex /
- magneto-electroluminescence /
- intersystem crossing /
- reverse intersystem crossing
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图 1 器件的能级结构和光电特性 (a), (b) Dev. 1和Dev. 2的能级结构; (c) TAPC和PO-T2T的化学分子结构; (d) TAPC纯膜, PO-T2T纯膜和TAPC:PO-T2T共混薄膜的PL谱以及Dev. 1和Dev. 2的EL谱; (e) Dev. 1和Dev. 2的电流-电压特性曲线; (f) Dev. 1和Dev. 2的EQE-电流密度特性曲线
Figure 1. Energy-level structures and photoelectric properties of devices: (a), (b) Energy-level structures of Dev. 1 and Dev. 2; (c) chemical molecular structures of TAPC and PO-T2T; (d) PL spectra from pure TAPC and PO-T2T films and TAPC:PO-T2T co-deposited film and EL spectra from Dev. 1 and Dev. 2; (e) current-voltage characteristics of Dev. 1 and Dev. 2; (f) EQE-current density characteristics of Dev. 1 and Dev. 2.
图 2 (a) Dev. 1中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 1中ISC, RISC和TQA过程的强度因子
Figure 2. (a) Current-dependent MEL traces of Dev. 1 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 1 at different bias-currents.
图 3 (a) Dev. 2中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 2中ISC, RISC和TQA过程的强度因子
Figure 3. (a) Current-dependent MEL traces of Dev. 2 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 2 at different bias-currents.
图 4 不同注入电流下器件中PP态和EX态的形成和演变机制 (a) Dev. 1; (b) Dev. 2
Figure 4. Formation and evolution mechanisms of PP and EX states in devices at different bias-currents: (a) Dev. 1; (b) Dev. 2.
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[1] Yuan P S, Qiao X F, Yan D H, Ma D G 2018 J. Mater. Chem. C 6 5721
Google Scholar
[2] Zhang B H, Xie Z Y 2019 Front. Chem. 7 306
Google Scholar
[3] Yang H Y, Zheng C J, Zhang M, Zhao J W, Shi Y Z, Pu C P, Lin H, Tao S L, Zhang X H 2022 Sci. China Mater. 65 460
Google Scholar
[4] Niu L B, Zhang Y, Chen L J, Zhang Q M, Guan Y X 2020 Org. Electron. 87 105971
Google Scholar
[5] Jin P F, Zhou Z Y, Wang H, Hao J J, Chen R, Wang J Y, Zhang C 2022 J. Phys. Chem. Lett. 13 2516
Google Scholar
[6] 吴雨廷, 朱洪强, 魏福贤, 王辉耀, 陈敬, 宁亚茹, 吴凤娇, 陈晓莉, 熊祖洪 2022 物理学报 71 227201
Google Scholar
Wu Y T, Zhu H Q, Wei F X, Wang H Y, Chen J, Ning Y R, Wu F J, Chen X L, Xiong Z H 2022 Acta Phys. Sin. 71 227201
Google Scholar
[7] Yuan P S, Guo X M, Qiao X F, Yan D H, Ma D G 2019 Adv. Optical Mater. 7 1801648
Google Scholar
[8] Chen P, Peng Q M, Yao L, Gao N, Li F 2013 Appl. Phys. Lett. 102 063301
Google Scholar
[9] Xu J, Tang X T, Zhao X, Zhu H Q, Qu F L, Xiong Z H 2020 Phys. Rev. Appl. 14 024011
Google Scholar
[10] Zhao X, Tang X T, Zhu H Q, Ma C H, Wang Y, Ye S N, Tu L Y, Xiong Z H 2021 ACS Appl. Electron. Mater. 3 3034
Google Scholar
[11] Deng J Q, Jia W Y, Chen Y B, Liu D Y, Hu Y Q, Xiong Z H 2017 Sci. Rep. 7 44396
Google Scholar
[12] 宁亚茹, 赵茜, 汤仙童, 陈敬, 吴凤娇, 贾伟尧, 陈晓莉, 熊祖洪 2022 物理学报 71 087201
Google Scholar
Ning Y R, Zhao X, Tang X T, Chen J, Wu F J, Jia W Y, Chen X L, Xiong Z H 2022 Acta Phys. Sin. 71 087201
Google Scholar
[13] Zhao C G, Zhao F G, Wang K, Yu H M, Huang T Y, Wang R, Zhang C X, Hu B, Duan L 2020 Phys. Rev. Appl. 14 034059
Google Scholar
[14] Xu Y W, Liang X M, Zhou X H, Yuan P S, Zhou J D, Wang C, Li B B, Hu D H, Qiao X F, Jiang X F, Liu L L, Su S J, Ma D G, Ma Y G 2019 Adv. Mater. 31 1807388
Google Scholar
[15] Zhang T T, Holford D F, Gu H, Kreouzis T, Zhang S J, Gillin W P 2016 Appl. Phys. Lett. 108 023303
Google Scholar
[16] 赵茜, 汤仙童, 潘睿亨, 许静, 屈芬兰, 熊祖洪 2019 科学通报 64 2514
Google Scholar
Zhao X, Tang X T, Pan R H, Xu J, Qu F L, Xiong Z H 2019 Chin. Sci. Bull. 64 2514
Google Scholar
[17] Mermer Ö, Veeraraghavan G, Francis T L, Sheng Y, Nguyen T D, Wohlgenannt M, Köhler A, Suti-Al M K, Khan M S 2005 Phys. Rev. B 72 205202
Google Scholar
[18] Wang Y F, Tiras K S, Harmon N J, Wohlgenannt M, Flatté M E 2016 Phys. Rev. X 6 011011
Google Scholar
[19] Zhu M P, Yuan X T, Ni G 2019 Micromachines 10 546
Google Scholar
[20] 王辉耀, 宁亚茹, 吴凤娇, 赵茜, 陈敬, 朱洪强, 魏福贤, 吴雨廷, 熊祖洪 2022 物理学报 71 217201
Google Scholar
Wang H Y, Ning Y R, Wu F J, Zhao X, Chen J, Zhu H Q, Wei F X, Wu Y T, Xiong Z H 2022 Acta Phys. Sin. 71 217201
Google Scholar
[21] Crooker S A, Liu F L, Kelley M R, Martinez N J D, Nie W Y, Mohite A D, Nayyar I H, Tretiak S, Smith D L, Ruden P P 2014 Appl. Phys. Lett. 105 153304
Google Scholar
[22] Liu F L, Kelley M R, Crooker S A, Nie W Y, Mohite A D, Ruden P P, Smith D L 2014 Phys. Rev. B 90 235314
Google Scholar
[23] Janssen P, Cox M, Wouters S H W, Kemerink M, Wienk M M, Koopmans B 2013 Nat. Commun. 4 2286
Google Scholar
[24] Peng Q M, Li A W, Fan Y X, Chen P, Li F 2014 J. Mater. Chem. C 2 6264
Google Scholar
[25] Bagnich S A, Niedermeier U, Melzer C, Sarfert W, Seggem H V 2009 J. Appl. Phys. 106 113702
Google Scholar
[26] Inal S, Schubert M, Sellinger A, Neher D 2010 J. Phys. Chem. Lett. 1 982
Google Scholar
[27] Li B B, Gan L, Cai X Y, Li X L, Wang Z H, Gao K, Chen D C, Cao Y, Su S J 2018 Adv. Mater. Interfaces 5 1800025
Google Scholar
[28] Hung W Y, Chiang P Y, Lin S W, Tang W C, Chen Y T, Liu S H, Chou P T, Hung Y T, Wong K T 2016 ACS Appl. Mater. Interfaces 8 4811
Google Scholar
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