基于激基复合物主体的高效TADF/磷光杂化白光有机发光二极管

Highly-efficient hybrid TADF/phosphorescent white organic light-emitting diodes based on an exciplex host

发光层中载流子的平衡以及拓宽的激子分布对于制备高性能白光有机发光二极管(WOLEDs)至关重要。采用蓝光热激活延迟荧光(TADF)分子DMAC-DPS、绿光磷光分子Ir(ppy)2(acac)和红光磷光分子RD071制备了基于激基复合物主体的TADF/磷光杂化WOLEDs。在发光层中引入TCTA:DPEPO激基复合物作为主体不仅平衡了电荷和空穴传输,拓宽了激子复合区,并构建蓝-绿-红发光层之间级联式激子能量传递,有效提升了激子利用率,降低了器件的效率滚降。通过调控发光层中载流子平衡及激子分布,白光器件的最大电流效率(CE)、功率效率(PE)和外量子效率(EQE)分别为37.1 cd·A^(−1)、36.4 lm·W^(−1)和17.5%,并且在1000 cd·m^(−2)亮度下依旧保持在26.6 cd·A^(−1)、18.2 lm·W^(−1)和12.3%,对应色坐标(CIE)和显色指数(CRI)分别为(0.451,0.428)和88。值得注意的是,在1000~5000 cd·m^(−2)亮度范围内,CIE变化仅为(0.006,0.004),表现出优异的色稳定性。同时,通过单极性主体和双极性主体的对比,阐明了双极性主体中载流子复合及激子能量传递机制。最终,通过器件传输层的优化进一步降低了器件的工作电压,提升了载流子平衡性,器件EQE及PE分别提升至19.3%和52.6 lm·W^(−1),并保持了高的显色指数(CRI=90)及良好的色稳定性。

Objective White organic lighting-emitting diodes(WOLEDs)have attracted significant interest in the fields of flexible flat panel displays and large-area solid-state lighting due to their merits of ultrathin,large-scale and lowcost.Phosphorescent OLEDs can achieve 100%exciton utilization.However,the lack of stable blue phosphorescent materials hinders the commercial application of all phosphorescent WOLEDs.Thermally activated delayed fluorescence(TADF)materials,which can harvest triplet excitons through efficient reverse intersystem crossing(RISC)and achieve nearly 100%internal quantum efficiency(IQE)are emerging as next generation emitters for OLEDs.Therefore,hybrid TADF/phosphorescent WOLEDs have become an alternative for preparing high efficiency and stable WOLEDs.Generally,in WOLEDs,unbalanced carrier transport in lightemitting layers(EMLs)usually leads to narrow exciton recombination regions,which reduces the efficiency and color stability at a high current density.Various methods,including inserting interlayers between EMLs have been proposed to improve color stability.However,the organic-organic barriers between the interlayers and EMLs enlarge the driving voltages and exacerbate exciton accumulation.Therefore,developing WOLEDs with balanced carrier transport and broadening the exciton recombination zones are the key to simultaneously achieving high efficiency and stable white emission.Methods High efficiency hybrid TADF/phosphorescent WOLEDs are prepared in this study.An exciplex system TCAT:DPEPO is chosen as the host to improve charge balance and optimize exciton distribution.Moreover,a cascaded exciton energy transfer route is constructed to improve exciton utilization efficiency.The working mechanism of devices is illustrated by examining host effects in EMLs.Moreover,the carrier balance is further enhanced by optimizing the transport layer.Results and Discussions The bipolar exciplex host(TCTA:DPEPO)and traditional host DPEPO are comparably investigated in blue TADF devices(Fig.1).By modulating the thicknesses of light-emitting layers,high-efficiency hybrid TADF/phosphorescent WOLEDS based on exciplex host have been achieved with excellent color stability and a high color rendering index(CRI)of 88(Fig.3).The comparison experiment shows that the outstanding performance of hybrid TADF/phosphorescent WOLEDs is attributed to the widened exciton recombination region and reasonable exciton utilization routes(Fig.4).In addition,by optimizing the electron transport layer,the power efficiency is further improved,achieving maximum values of 52.6 lm·W^(−1)and 19.3%for power efficiency and EQE,respectively(Fig.6).Conclusions High efficiency,color stable and low efficiency roll-off TADF/phosphorescent hybrid WOLEDs based on exciplex host are achieved.In the proposed WOLEDs,an exciplex host is utilized in EMLs to broad exciton recombination region and a cascaded exciton energy transfer route is constructed to improve exciton utilization.Hybrid WOLEDs exhibit excellent color stability and low efficiency roll-off.Maximum values of PE and EQE are 36.4 lm·W^(−1)and 17.5%(maintaining 18.2 lm·W^(−1)and 12.3%at 1000 cd·m^(−2)),respectively.With balanced white emission,the WOLED reaches a CIE of(0.451,0.428)and a high CRI of 88.By further optimizing the transport layer of WOLEDs,the EQE is further improved to 19.3%,and a maximum power efficiency of 52.6 lm·W^(−1)and a CRI of 90 are achieved.The design strategy proposed in this study provides a simple but feasible approach for high performance hybrid TADF/phosphorescent WOLEDs.