热活化延迟荧光材料CzDBA掺杂器件的反常磁响应

Anomalous magnetic-field response of doped devices based on the thermally activated delayed fluorescent material of CzDBA

CzDBA是一种donor-acceptor-donor(D-A-D)型的热活化延迟荧光(TADF)材料,具有比普通D-A型的TADF材料更好的发光性能.为了探究基于CzDBA发光器件的微观过程,本文制备了发光层为CzDBA和4CzTPN-Ph的两种器件,并在室温下测量了它们的电致发光的磁效应(magneto-electroluminescence,MEL)和电导的磁效应(magneto-conductance,MC).实验发现,尽管它们都含有咔唑基团,但它们的MEL表现为两种不同的线型:前者表现为系间窜越过程占主导的正MEL,后者表现为反向系间窜越过程占主导的负MEL;且CzDBA器件的MC与其MEL具有相同的线型.为了深入研究CzDBA的这种反常磁响应,我们还制备了发光层为mCBP:x%CzDBA的掺杂器件.实验发现,与纯的CzDBA器件相比,掺杂器件表现出相似的MEL和MC线型,但却具有相反的温度依赖关系.分析可知,纯的CzDBA器件极化子对间ISC过程占主导作用,使其MEL始终为正值.但是,加入mCBP主体后,mCBP与CzDBA客体间存在较强的能量转移过程(ET),低温抑制掺杂器件的ET过程,导致反向系间窜越过程减弱,使其表现出不同于纯的CzDBA器件的反常温度效应.此外,主客体的能量转移过程与载流子陷阱作用的相互竞争,导致掺杂器件亮度-电流曲线和MEL幅值随掺杂浓度增大呈现不单调变化,这进一步证明了器件机制的正确性.显然,本工作有助于深入理解基于CzDBA发光器件微观过程的演化机制.

CzDBA is a donor-acceptor-donor(D-A-D)-type of thermally activated delayed fluorescence(TADF)material that has been recently reported in the literature;it has better luminous performance than ordinary D-A-type TADF materials.To explore the microscopic processes involved in Cz DBA-based light-emitting devices,we fabricated two kinds of devices with light-emitting layers of pure CzDBA and 4 Cz TPN-Ph films and measured their magneto-luminescence(MEL)and magnetic conductance(MC)at room temperature.Experimental results show that although both of them contain carbazole groups,their MEL traces present two different line shapes;the former(Cz DBA-based OLEDs)exhibits positive MEL curves dominated by intersystem crossing processes(ISC),whereas the latter depicts negative MEL traces governed by reverse ISC(RISC).In addition,Cz DBA-based OLEDs have the same line shapes of MEL and MC traces.To investigate this anomalous magnetic-field response in CzDBA further,we prepared a doped device with a light-emitting layer of m CBP:x%Cz DBA.Experimental results show that compared with devices with pure CzDBA as an emission layer,doped devices exhibit similar MEL and MC line types but have opposite temperature-dependent MEL traces.Detailed analyses related to MEL and MC traces show that the ISC process of polaron pairs dominates pure CzDBA devices,making their MEL values always positive.However,the doping of CzDBA into an m CBP host results in a strong exciton energy transfer(ET)process in the doped devices;this ET process is suppressed at low temperature,leading to a weakened RISC effect and an abnormal temperature effect different from that of pure CzDBA devices.In addition,the competition of the energy transfer process between host and guest and the trapping effect of charge carriers produce a nonmonotonic change in the MEL amplitude of the doped device by increasing the dopant concentration.This change further proves the correctness of the proposed microscopic mechanisms for these Cz DBA-based OLEDs.This work is beneficial to the further understanding of the evolution mechanisms of the microscopic processes in devices based on excellent CzDBA luminescent materials recently discovered in literature.