Magneto-conductance characteristics of trapped triplet–polaron and triplet–trapped polaron interactions in anthracene-based organic light emitting diodes

Abstract

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet-polaron interactions (TPI) and triplet-trapped polaron interactions (TPI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TPI and TPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TPI, TPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TPI and TPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TPI exhibited a negative sign, while that of TPI exhibited a positive one, with characteristic magnetic fields (B) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TPI was neglected above 200 K, while TPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet-polaron interactions.

Study of the microscopic mechanism of Ir(ppy)$_{3}$ regulating exciton splitting and luminescence process in Rubrene

“…我 们采用不同的比例混入 Rubrene 中作为器件的发光层, 制备了不同混入浓度的 Rubrene: x%Ir(ppy) 3 器 件, 并在 300∼20 K 温度范围内测量了器件在多种注入电流下的电致发光随外加磁场的变化 (magnetoelectroluminescence: MEL) 和器件发光强度随电流的变化情况. 实验发现, 在室温和小混入浓度时, 随 着 Ir(ppy) 3 混入浓度的增加, 其 MEL 线型与基于纯 Rubrene 发光层的参考器件的一致, 均为 STT 特征线型, 且 STT 过程和发光强度均随浓度增大而变强, 这与常规 Rubrene 掺杂器件 (如 mCP: y%Rubrene) 表现出的随浓度增大 STT 增强但发光强度减弱 [8] 的规律不同; 当混入 Ir(ppy) 3 分子 浓度超过某一值时, 器件 MEL 的线型仍为 STT 线型, 但是其幅值却随浓度的增加而降低, 同时其发 光则继续增强. 这些实验结果表明, 混入一定浓度的 Ir(ppy) 3 , 确实可以同时增强 Rubrene 器件的 STT 过程和发光强度.…”

Analysis of trap-assisted triplet exciton quenching by magneto- conductance in rubrene-based organic light-emitting diodes

“…(2) 束缚的T激子与自由极化子相互作用(trapped tripletfree polaron interaction, T t PI, T t +P→S 0 +P t ) [13,18,22] . 本 文将这两种过程统称为陷阱辅助的T激子淬灭作用.…”

“…例如, Chen等人 [24] 在Alq 3 中掺入3%的rubrene, 发现MC曲线为简单的系间窜 越(intersystem crossing, ISC)线型; Jia等人 [8] dioxythiophene)/poly(styrenesulfonate))/4,4′,4′-ris(N-3methylphenyl-N-phenylamino)riphenylamine(m-MTDA TA)/rubrene/4,7-diphenyl-1,10-phenanthroline(bphen)/ lithium fluoride(LiF)/Al. 在室温300 K和50~550 μA 的电流范围内, 就得到与Jia等人 [18] 报道类似的MC线 型, 即当|B|<6 mT时, MC 1 在50 μA表现为缓慢上升, 成的激子能被很好地束缚在rubrene层中. 图1(c)为室 温注入不同电流(100, 250, 500和1500 μA)时器件的 电致发光(electroluminescence, EL)光谱图.…”

“…从图中可 以看出, 发光的峰位约为565和610 nm, 这与已报道 的rubrene的峰位相一致 [4,9] , 且随着注入电流的增加, [27] , 此结果说明器件在室温下 就有陷阱存在, 并且随着温度的降低陷阱会增加. 流下对应的MC线型(图2(a), (e), (b)和(f))与Jia等人 [18] 采 用 器 件 结 构 ITO/CuPc/NPB/anthracene/BCP/LiF/Al [28] ; 也就是, 磁场抑制ISC作用将导致PP 1 的数 量增多, PP 1 解离形成的二次载流子增多, 从而导致 较小磁场范围内MC的增强. 但当B>>B hf 时, 外加磁 场对ISC过程的抑制作用达到饱和, 使得MC的幅值 不再随磁场的增加而改变.…”

“…因此, 受磁场影响的ISC 过程产生MC曲线的符号是正的, 具有Lorentzian线 型且其线宽只有几个mT [13,22] 态激子(trapped triplet exciton, T t ). 因此, S 1t , T t , P t , S 1 , [18,22] , 即T t +P→S 0 +P t , 使得流过器件的电流减小, 磁场抑制此过程将产生正的MC, 如图3(b)所示的第 (ⅱ)种情况; (4) P t 与自由T激子相互作用发生TP t I过 程 [13,17~21] , 即T+P t →S 0 +P, 通过去陷阱过程(detrapping process)产生二次载流子, 使得流过器件的电流增大, 由于超精细诱导的ISC过程对应的磁场效应往往 处于几个毫特斯拉的量级 [13,22] , T t PI与TP t I过程对应 的特征磁场通常也仅为十几毫特斯拉 [18] , 因此把它 们对磁场的效应称为低场效应(low field effect, LFE). TQA作用下的MC曲线的典型线宽约为100 mT [14,18] , 且为non-Loretzian线型, 将这种远大于超精细量级的 磁效应称为高场效应(high field effect, HFE).…”