Enhanced reverse inter-system crossing process of charge-transfer stated induced by carrier balance in exciplex-type OLEDs

Wang Hui-Yao; Wei Fu-Xian; Wu Yu-Ting; Peng Teng; Liu Jun-Hong; Wang Bo; Xiong Zu-Hong

doi:10.7498/aps.72.20230949

Abstract

The reverse inter-system crossing (RISC, CT³ → CT¹) process in charge transfer (CT¹ and CT³) states is an effective approach to improving the energy utilization rate of excited states, and precise control and full use of the RISC process have important scientific significance and application prospect for fabricating and realizing the efficient exciplex-type organic light-emitting diodes (OLEDs). The conventional exciplex-type OLEDs based on m-MTDATA: Bphen have received extensive attention among researchers owing to the fact that the energy difference between CT¹ and CT³ around zero promotes the efficient occurrence of RISC process. But up to now, only transient photoluminescence can infer the existence of RISC process in experiment, which is quite unfavorable for the comprehensive understanding and application of this process to design high-performance OLEDs. Fortunately, in this paper, a series of balanced and unbalanced exciplex-based devices are prepared by changing the donor-acceptor blending ratio in the emitting layer (x% m-MTDATA:y% Bphen; x%, y% is the weight percent) and the carrier density flowing through the device. The RISC process of CT states is directly observed via analyzing fingerprint magneto-conductance (MC) traces of the balanced device at room temperature, and the balanced device has higher electroluminescence (EL) efficiency than the unbalanced device. Specifically, the low-field MC curves of unbalanced device only show an inter-system crossing (ISC) line shape, whereas those from the balanced exciplex device present an RISC line shape at low bias-current and the conversion into an ISC line shape with the further increase of bias current. The line shape transition from RISC to ISC is attributed to the triplet-charge annihilation (TQA) process caused by excessive charge carries under high bias current. Combining the physical microscopic mechanism of device, the above-mentioned MC curves of various exciplex devices can be explained as follows: under the same bias current, extra holes or electrons are generated in the emitter layer of unbalanced devices due to the mismatch of donor-acceptor molecular concentrations. These superfluous holes or electrons will react with the CT³ state, which aggravates the TQA process in the device and weakens the RISC process in which the CT³ state participates. That is to say, there are strong TQA process and weak RISC process in unbalanced exciplex device. Contrarily, the strong RISC process and weak TQA process in the balanced exciplex device are beneficial to the occurrence of delayed fluorescence, resulting in its EL efficiency higher than that of the unbalanced device. This work not only deepens the physical understanding of the influence of donor-acceptor blending ratio on the carrier balance in exciplex devices, but also paves the way for designing highly efficient OLED by fully employing the RISC process of balanced device.

Keywords:

Authors and contacts

School of Physical Science and Technology, Chongqing Key Laboratory of Micro & Nano Structure Optoelectronics, Southwest University, Chongqing 400715, China

Corresponding author: Xiong Zu-Hong, zhxiong@swu.edu.cn

Funds: Project supported by the National Natural Science Foundation of China (Grant No. 11874305).

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References

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Cited By

图 1 (a) 激基复合物器件的能级结构; (b) m-MTDATA纯膜、Bphen纯膜和m-MTDATA:Bphen混合膜的归一化PL谱, 插图展示了m-MTDATA和Bphen的化学分子结构; (c) 器件A—D的归一化电致EL谱; (d) 器件A—D的EL强度随偏置电流的变化曲线

Figure 1. (a) Energy level structure diagram of exciplex device; (b) normalized PL spectra of pure films of m-MTDATA, Bphen and composite film of m-MTDATA:Bphen, the inset shows the chemical molecular structure of m-MTDATA and Bphen; (c) normalized EL spectra of devices A–D; (d) the EL intensity as function of bias current for devices A–D.

DownLoad: Full-Size Img PowerPoint

图 2 (a)—(d) 室温下器件A—D中电流依赖的MC曲线; (e) 器件A—D中MC_L幅值随偏置电流的变化曲线; (f) 不同偏置电流下MC_H幅值随给体/受体共混比例的变化曲线

Figure 2. (a)–(d) The current-dependent MC curves of devices A–D at room temperature; (e) the MC_L values as a function of bias current in devices A–D; (f) the MC_H values as a function of donor-acceptor blending ratio at various bias currents.

DownLoad: Full-Size Img PowerPoint

图 3 (a)—(d) 室温下器件A—D中电流依赖的MEL曲线; (e), (f) 器件A—D中MEL_L和MEL_H幅值随偏置电流的变化曲线

Figure 3. (a)–(d) The current-dependent MEL curves of devices A–D at room temperature; (e), (f) the MEL_L and MEL_H values as a function of bias current in devices A–D.

DownLoad: Full-Size Img PowerPoint

图 4 (a)—(d) 室温下器件A—D中电流依赖的η_M曲线

Figure 4. (a)–(d) The current-dependent η_M curves of devices A–D at room temperature.

DownLoad: Full-Size Img PowerPoint

图 5 平衡器件与非平衡激基复合物器件的微观机理图

Figure 5. Microscopic mechanisms in balanced and unbalanced exciplex devices.

DownLoad: Full-Size Img PowerPoint

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[2]	Park Y S, Lee S H, Kim K H, Kim S Y, Lee J H, Kim J J 2013 Adv. Funct. Mater. 23 4914 Google Scholar
[3]	Kim K H, Yoo S J, Kim J J 2016 ACS Chem. Mater. 28 1936 Google Scholar
[4]	Goushi K, Yoshida K, Sato K, Adachi C 2012 Nat. Photon. 6 253 Google Scholar
[5]	Zhang T Y, Chu B, Zhao B, Jin F M, Yan X W, Gao Y, Wu, H R, Li W L, Su Z S 2014 ACS Appl. Mater Interfaces 6 11907 Google Scholar
[6]	Chen P, Peng Q M, Bai J W, Zhang S T, Li F 2014 Adv. Opt. Mater. 2 142 Google Scholar
[7]	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
[8]	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
[9]	Chen Q S, Jia W Y, Chen L X, Yuan D, Zou Y, Xiong Z H 2016 Sci. Rep. 6 25331 Google Scholar
[10]	张勇, 刘亚莉, 焦威, 陈林, 熊祖洪 2012 物理学报 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]	Lei Y L, Zhang Y, Liu R, Chen P, Song Q L 2009 Org. Electron. 10 889 Google Scholar
[12]	Liu R, Zhang Y, Lei Y L, Chen P 2009 J. Appl. Phys. 105 093719 Google Scholar
[13]	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
[14]	Xu J, Tang X T, Zhao X, Zhu H Q, Qu F L, Xiong Z H 2020 Phys. Rev. Appl. 14 024011 Google Scholar
[15]	Tang X T, Hu Y Q, Jia W Y, Pan R H, Deng J Q, He Z H, Xiong Z H 2018 ACS Appl. Mater. Interfaces 10 1948 Google Scholar
[16]	管胜婕, 周林箭, 沈成梅, 张勇 2020物理学报 69 167101 Google Scholar Guan S J, Zhou L J, Shen C M, Zhang Y 2020 Acta Phys. Sin. 69 167101 Google Scholar
[17]	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. 33034 Google Scholar
[18]	Zhang Y, Liu R, Lei Y L, Xiong Z H 2009 Appl. Phys. Lett. 94 083307 Google Scholar
[19]	Chen P, Song Q L, Choy W C H, Ding B F, Liu Y L, Xiong Z H 2011 Appl. Phys. Lett. 99 143305 Google Scholar
[20]	Wang Y F, Sahin-Tiras K, Harmon H J, Wohlgenannt M, Flatté M E 2016 Phys. Rev. X 6 011011 Google Scholar
[21]	Sheng Y, Nguyen T D, Veeraraghavan G, Mermer Ö, Wohlgenannt M, Qiu S, Scherf U 2006 Phys. Rev. B 74 045213 Google Scholar
[22]	Wang D, Li W L, Chu B, Su Z S, Bi D F, Zhang D Y, Zhu J Z, Yan F, Chen Y R, Tsuboi T 2008 Appl. Phys. Lett. 92 053304 Google Scholar
[23]	Naka S, Okada H, Onnagawa H, Tsutsui T 2000 Appl. Phys. Lett. 76 197−199.Google Scholar
[24]	Tierce N T, Chen C H, Chiu T L, Lin C F, Bardeen C J, Lee J H, 2018 Phys. Chem. Chem. Phys. 20 27449 Google Scholar
[25]	Zhu L, Xu K, Wang Y, Chen J, Ma D 2015 Front. Optoelectron. 8 439 Google Scholar
[26]	Wu F J, Zhao X, Zhu H Q, Tang X T, Ning Y R, Chen J, Chen L X, Xiong Z H 2022 ACS Photonics 9 2713 Google Scholar
[27]	宁亚茹, 赵茜, 汤仙童, 陈敬, 吴凤娇, 贾伟尧, 陈晓莉 2022 物理学报 71 087201 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
[28]	Attar H A A, Monkman A P 2016 Adv. Mater. 28 8014 Google Scholar
[29]	Huang Q Y, Zhao S L, Wang P 2018 J. Lumin. 201 38 Google Scholar
[30]	Nakanotano H, Furukawa T, Morimoto K 2016 Sci. Adv. 2 e1501470 Google Scholar
[31]	张巧明, 陈平, 雷衍连, 刘荣, 张勇, 宋群梁, 黄承志, 熊祖洪 2010 中国科学: 物理学力学天文学 40 1507 Google Scholar Zhang Q M, Chen P, Lei Y L, Liu R, Zhang Y, Song Q L, Huang C Z, Xiong Z H 2010 Sci-Phys. Mech. Astron. 40 1507 Google Scholar
[32]	Desai P, Shakya P, Kreouzis T, Gillin W P 2007 Phys. Rev. B 75 094423 Google Scholar
[33]	Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 Adv. Funct. Mater. 30 2005765 Google Scholar
[34]	Wang Y, Ning Y R, Wu F G, Chen J, Chen X L, Xiong Z H 2022 Adv. Funct. Mater. 32 2022882 Google Scholar

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