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With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existing in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results (Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 Adv. Funct. Mater. 5 765) have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S1,Rub ← T2,Rub) of high-lying triplet excitons (T2,Rub) in Rubrene when Rubrene with a typical value of several percent in content is doped into a host with high triplet exciton energy and there are no energy loss channels of triplet excitons from charge-carrier transporting layers either. Furthermore, this HL-RISC process can considerably increase the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e. the pure Rubrene film is used as an emissive layer) with no energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S1,Rub+S0,Rub → T1,Rub+T1,Rub) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP1 → PP3) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair is generated through the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. By comparing and analyzing these tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from B-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from B-mediated HL-RISC process of T2,Rub excitons. Note that such an abnormal and coincidental experimental phenomenon is the physical reason why the ISC process and HL-RISC process cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has not been found in the literature. Clearly, this work further deepeneds our understanding of some unique microscopic processes and physical phenomena in organic semiconductor “star” material of Rubrene (such as the energy resonance between 2T1 and S1 and the energy approach between T2 and S1).
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Keywords:
- Rubrene /
- magneto-electroluminescence /
- intersystem crossing /
- high-level reverse intersystem crossing
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图 1 (a) 器件A中每个功能层的能级图; (b) 室温下器件A在50 μA时的MEL曲线, 插图展示了器件A归一化的EL光谱; (c) 器件A的微观机制过程; (d) 采用三种过程(ISC, HL-RISC, SF)拟合器件A的MEL曲线
Figure 1. (a) Energy level alignments of each functional layer used in device A; (b) the room-temperature MEL curves of device A operated at a bias current of 50 μA, the inset shows the normalized EL spectrum of device A; (c) micro-mechanism diagram of device A; (d) fitted MEL curve of device A using three different processes (ISC, HL-RISC, and SF).
图 2 器件A在不同温度下电流依赖的MEL曲线 (a) 300 K; (b) 200 K; (c) 100 K; (d) 10 K. 图2(b)的插图是器件A在200 K时, 磁场为50 mT的MEL曲线
Figure 2. I-dependent MEL curves of device A obtained at various operational temperatures: (a) 300 K; (b) 200 K; (c) 100 K; (d) 10 K. The illustration in Fig. 2(b) shows the MEL curves of device A at 50 mT magnetic field and 200 K.
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[1] Lei Y L, Zhang Y, Liu R, Chen P, Song Q L 2009 Org. Electron. 10 889
Google Scholar
[2] Liu R, Zhang Y, Lei Y L, Chen P 2009 J. Appl. Phys. 105 093719
Google Scholar
[3] Chen P, Peng Q M, Bai J W, Zhang S T, Li F 2014 Adv. Opt. Mater. 2 142
Google Scholar
[4] 卢晨蕾, 贾伟尧, 白江文, 张巧明, 令勇洲, 刘洪, 熊祖洪 2015 中国科学: 技术科学 45 396
Google Scholar
Lu C L, Jia W Y, Bai J W, Zhang Q M, Ling Y Z, Liu H, Xiong Z H 2015 Sci. Sin.-Tech. 45 396
Google Scholar
[5] Liu Y, Wu X M, Zhao Z H, Gao J N, Zhan J, Rui H S, Lin X, Zhang N, Hua Y L, Yin S G 2017 Appl. Surf. Sci. 413 302
Google Scholar
[6] Qu F L, Jia W Y, Tang X T, Xu J, Zhao X, Ma C H, Ye S N 2020 J. Phys. Chem. C. 124 9451
Google Scholar
[7] Chen Q S, Jia W Y, Chen L X 2016 Sci. Rep. 6 25331
Google Scholar
[8] Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 Adv. Funct. Mater. 5 765
Google Scholar
[9] Wang Y, Ning Y R, Wu F G, Chen J, Chen X L, Xiong Z H 2022 Adv. Funct. Mater. 32 2202882
Google Scholar
[10] 张勇, 刘亚莉, 焦威, 陈林, 熊祖洪 2012 61 117106
Google Scholar
Zhang Y, Liu Y L, Jiao W, Chen L, Xiong Z H 2012 Acta Phys. Sin. 61 117106
Google Scholar
[11] Tang X T, Zhao X, Tu L Y, Ma C H, Wang Y, Ye S N 2021 J. Mater. Chem. C 9 2775
Google Scholar
[12] Tang X T, Pan R H, Zhao X 2020 J. Phys. Chem. Lett. 11 2804
Google Scholar
[13] Li J, Chen Z H, Zhang Q M, Xiong Z H, Zhang Y 2015 Org. Electron. 26 213
Google Scholar
[14] Xu Y W, Xu P, Hu D H, Ma Y G, 2021 Chem. Soc. Rev. 50 1030
Google Scholar
[15] Bai J W, Chen P, Lei Y L, Zhang Y, Zhang Q M 2014 Org. Electron. 15 169
Google Scholar
[16] Piland G B, Burdett J J, Kurunthu D, Bardeen C J 2013 J. Phys. Chem. C 117 1224
Google Scholar
[17] Chen Y B, Jia W Y, Xiang J, Yuan D, Chen Q S, Chen L X 2016 Org. Electron. 39 207
Google Scholar
[18] Zhang Y, Lei Y L, Zhang Q M, Xiong Z H 2014 Org. Electron. 15 577
Google Scholar
[19] Desai P, Shakya P, Kreouzis T, Gillin W P 2007 Phys. Rev. B 75 094423
Google Scholar
[20] Crooker S A, Liu F L, Kelley M R, Martinez N J, Nie W, Mohite A, Nayyar I H, Tretiak S, Smith D L, Ruden P P 2014 Appl. Phys. Lett. 105 153304
Google Scholar
[21] Kawata S, Pu Y J, Saito A, Kurashige Y K, Beppu T, Katagiri H S, Hada, M Kido J J 2016 Adv. Mater. 28 1585
Google Scholar
[22] Hu D H, Yao L, Yang B, Ma Y G 2015 Phil. Trans. R. Soc. A 373 20140318
Google Scholar
[23] Pan Y Y, Li W J, Zhang S T, Yao L, Gu C, Xu H, Yang B, Ma Y G 2014 Adv. Opt. Mater. 2 510
Google Scholar
[24] Kim H B, Kim J J 2020 Phys. Rev. Appl. 13 024006
Google Scholar
[25] Obolda A, Peng Q M, He C, Zhang T, Ren J J, Ma H W, Shuai Z G, Li F 2016 Adv. Mater. 28 4740
Google Scholar
[26] Tu L Y, Tang X T, Wang Y, Zhao X, Ma C H, Ye S N 2021 Phys. Rev. Appl. 16 064002
Google Scholar
[27] Wang F J, Bassler H, Vardeny Z V 2008 Phys. Rev. Lett. 101 236805
Google Scholar
[28] 陈秋松, 袁德, 贾伟尧, 陈历相, 邹越, 向杰, 陈颖冰, 张巧明, 熊祖洪 2015 64 177801
Google Scholar
Chen Q S, Yuan D, Jia W Y, Chen L X, Zou Y, Xiang J, Chen Y B, Zhang Q M, Xiong Z H 2015 Acta Phys. Sin. 64 177801
Google Scholar
[29] Peng Q M, Li W J, Zhang S T, Chen P, Li F, Ma Y G 2013 Adv. Opt. Mater. 1 362
Google Scholar
[30] Peng Q M, Li A M, Fan Y X, Chen P, Li F 2014 J. Phys. Chem. C 2 6264
Google Scholar
[31] Huh D H, Kim G W, Kim G H, Kulshreshtha C, Kwon J H 2013 Synth. Net. 180 79
Google Scholar
[32] 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
[33] Wu I W, Wang W H, Tseng W H, Chang J H, Wu C I 2012 Org. Electron. 13 13
Google Scholar
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Google Scholar
Ning Y R, Zhao X, Tang X T, Chen J, Wu F J, Jia W R, Chen X L, Xiong Z H 2022 Acta Phys. Sin. 71 087201
Google Scholar
[35] Goushi K, Yoshida K, Sayo K, Adachi C 2012 Nat. Photon. 6 253
Google Scholar
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