采用瞬态电致发光研究磷光掺杂体系发光瞬时过冲的发射机理

掺杂型有机电致发光器件中载流子累积、载流子复合等物理过程的深入了解对提高器件效率和稳定性有重要作用。通过瞬态电致发光测量可以研究掺杂型有机电致发光器件内部载流子累积。对结构为:ITO/NPB(30 nm)/host:Ir(ppy)_3/BCP(10 nm)/Alq_3(20 nm)/LiF(0.7nm)/Al(100 nm)的器件分别研究主体材料以及客体掺杂浓度变化对有机掺杂型器件瞬态发光行为的影响。实验发现,当单脉冲驱动电压关闭后,只有TAZ:Ir(ppy)_3掺杂器件出现发光瞬时过冲现象,即发光强度衰减到一定时间时突然增强;且随着客体掺杂浓度的增加,瞬时过冲强度逐渐增强。通过分析TAZ:Ir(ppy)_3掺杂器件的瞬时过冲强度对主体材料与掺杂浓度的依赖关系,进一步发现,瞬时过冲效应强度主要受限于发光层内部积累的电子载流子;TAZ:Ir(ppy)_3发光层内电子容易被客体材料分子俘获并积累,电场突变时陷阱电子容易跳跃到主体材料上并与主体材料上积累的空穴形成激子,激子能量传递到客体材料上并复合发光继而出现发光强度的瞬时过冲现象。研究发光瞬时过冲行为可探究器件发光层内的载流子和激子的动态行为,有利于指导器件的设计,从而减少积累电荷的影响,提高器件的性能。

Study on the Overshoot Effect of Doped PhOLED with Transient Electroluminescence

The accumulation carriers and the trapped carriers are found in many organic light-emitting diodes (OLEDs) more or less,which can lead to a great loss of carriers and weaken the performance of devices .We have investigated a host-guest-system containing the green phosphorescent emitter tris2-phenylpyridinato-C2,N]iridium(Ⅲ) Ir(ppy)3] and one host material with transient electroluminescence (EL) .The charge recombination,accumulation and light emission mechanisms of the phosphorescent organic light-emitting diode s (PhOLEDs) with different host materials were analyzed .The structure was fabricated as ITO/NPB(30 nm)/host: Ir(ppy)3/BCP(10 nm)/Alq3(20 nm)/LiF(07 nm)/A l(100 nm),the hosts were CBP,PVK and TAZ respectively .These results showed the transient EL was strongly dependent on host materials .Compared to devices of host material CBP and PVK,only those with the host material TAZ as the emitting layer exhibited strong electroluminescence overshoots between 1 and 3 μs after turning off the voltage pulse at room temperatures .To further elucidate the generality of the overshoots,we monitored their dependence on the dopant concentration .The transient EL results in host-guest-system devices demonstrated a direct l ink between the strong overshoot effect and charge trapping in the emitting guest molecules .The excessive electrons in the guest sites could be a major factor inducing significant strong overshoot phenomenon in the TAZ: Ir(ppy)3 layer .W e attributed these overshoot effect to the electrons accumulated on Ir(ppy)3 sites and accumulated holes in the vicinity of the HBL/EML interface .As a result,w e obtained a better understanding of carriers' dynamics and recombination process of PhOLEDs after turning off the voltage pules .The new understanding of the charge carriers and exciton dynamics of PhOLEDs is instrumental in directing the efforts of developing stable and high-efficiency PhOLEDs .