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| The Influence of Deep Trap on the Efficiency Decrease in PhOLEDs Based on Double Dopants Strategy |
| WANG Hao1, 2, ZHAO Su-ling1, 2*, XU Zheng1, 2, SONG Dan-dan1, 2, QIAO Bo1, 2, WANG Peng1, 2, ZHENG Wei-ye1, 2, WEI Peng1, 2 |
1. Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University), Ministry of Education, Beijing 100044, China
2. Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China |
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Abstract In this paper, in order to study the effect of trapped carrierson the efficiency decrease in PhOLEDs based on double dopants strategy, threegroupsof devices, in whichthehostwas 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) andtheguestswere tris(2-phenylpyridine) iridium(Ⅲ) (Ir(ppy)3)，tris(1-phenylisoquinolinato-C2, N)iridium(Ⅲ)(Ir(piq)3)andpoly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1, 2-b:4,5-b′]dithiophene-2, 6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]] (PTB7), with the emitting layer of CBP∶Ir(ppy)3, CBP∶Ir(ppy)3∶Ir(piq)3 or CBP∶Ir(ppy)3: PTB7 were prepared by spin-coating method utilizing the double dopants strategy to realize high performance PhOLEDs. Transient photoluminescence measurement was used to test the lifetime of the films with different doping ratios. Asthe concentration of Ir(piq)3 increased, the lifetime of green emission became shorter which indicated that internal energy transfer between the dopants existedwhen the doping ratioofthe Ir(piq)3 was at a low value. When the concentration of Ir(piq)3 increased to 100∶3 and 100∶5, the luminescence of the dopants became independent, in which the internal energy transfer could be negleted. The devices with PTB7 or Ir(piq)3 performed significantly lower power efficiency compared with Ir(ppy)3 only devices, in which the PTB7 and Ir(piq)3 had become the traps which would influence the perfomance of the devices. Transcient electroluminanscence was investigated to penetrate how the trapped charges work in the double doped devices. When the device was driven by a pulse power, a spike occured when the reverse bias turned on. This was because that the trapped charges were released and then recombined under a high reverse bias. The results showed that the device with more Ir(ppy)3 showed weaker spike which indicated that charges trapped by the Ir(piq)3 reduced, and as the concetration of the Ir(ppy)3 increased, the sipke became weaker. This was mainly because that the Ir(ppy)3 could transmit charges, which would reduce the trapped charges in Ir(piq)3. Through the transicient electrolu minescence mesurement, we also found that the spike would decay faster when suppllied a higher reverse bia which was caused by the deep trapped charges releasing and then excerbate the triplet-polaron quenching (TPQ) effect. Hence, the material with deep energy level would trap a large number of charges and then aggravate the interaction between the triplet exciton and polaron, causing the efficiency decrease and roll-off in the PhOLEDs based on double dopants strategy.
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Received: 2018-01-24
Accepted: 2018-06-07
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Corresponding Authors:
ZHAO Su-ling
E-mail: slzhao@bjtu.edu.cn
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