激基复合物有机发光二极管中系间窜越和反向系间窜越过程的非单调电流依赖关系

Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes

激基复合物有机发光二极管(exciplex-based organic light-emitting diodes,EB-OLEDs)中自旋对态(spin-pair states)的系间窜越(intersystem crossing,ISC)和反向系间窜越(reverse ISC,RISC)是重要的自旋混合过程.它们通常展示正常的电流依赖关系,即随电流的增大而减弱.本文利用磁电致发光(magneto-electrolumine-scence,MEL)作为指纹式探测工具,在具有不同电荷平衡的EB-OLEDs中观察到多种电流依赖的ISC和RISC过程.它们有趣的表现为:随着器件注入电流增大,非平衡器件中电流依赖的MEL曲线呈现从正常ISC(1-25μA)向反常ISC(25-200μA)过程的转换,而平衡器件中电流依赖的MEL曲线则展示从正常ISC(1-5μA)→反常RISC(10-50μA)→正常RISC(50-150μA)→反常ISC(200-300μA)过程的转换.通过拟合和解析MEL曲线,发现非平衡和平衡器件中的ISC和RISC过程随着电流增大都先增强后减弱.这些丰富而有趣的转换可归因于增大电流时自旋对态增加的数量与其减短的寿命之间的竞争.此外,平衡器件中的RISC过程比非平衡器件中的更强,从而导致平衡器件的外量子效率比非平衡器件的更高.显然,本文不但加深了对EB-OLEDs中电流依赖的ISC和RISC过程的理解,还为设计制作高效率EB-OLEDs提供了清晰的器件物理思路.

Intersystem crossing(ISC)and reverse ISC(RISC)between singlet and triplet polaron-pair and exciplex state are important spin-mixing processes in exciplex-based organic light-emitting diodes(EB-OLEDs).These two processes usually show normal current dependence which weakens with the increase of bias-current.This is because the bias-current increases by improving the device bias-voltage.When the bias-voltage rises,the electric field within the device is enhanced,which facilitates the electric-field-induced dissociation of polaron-pair and exciplex states and then reduces their lifetime.That is,less polaron-pair and exciplex states participate in the ISC process and RISC process,leading these two processes to weaken.Here,magneto-electroluminescence(MEL)is used as a fingerprint probing tool to observe various current-dependent ISC and RISC processes in EB-OLEDs with different charge balances via modifying the device hole-injection layer.Interestingly,current-dependent MEL traces of the unbalanced device display a conversion from normal ISC(1-25μA)process to abnormal ISC(25-200μA)process,whereas those of the balanced device show conversions from normal ISC(1-5μA)into abnormal RISC(10-50μA)and then into normal RISC(50-150μA)and finally into abnormal ISC(200-300μA)process.By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices,we find that the ISC process and RISC process in these two devices first increase then decrease as the bias-current increases.These non-monotonic current-dependent ISC process and RISC process are attributed to the competition between the increased number and the reduced lifetime of polaron-pair state and exciplex state during improving the bias-current.Furthermore,the RISC process in the balanced device is stronger than that in the unbalanced device.This is because the balanced carrier injection can facilitate the formation of triplet exciplex states and weaken the triplet-charge annihilation(TQA)process between triplet exciplex states and excessive charge carriers,which leads the number of triplet exciplex states to increase.That is to say,more triplet exciplex states can be converted into singlet exciplex states through the RISC process,causing the external quantum efficiency of the balanced device to be higher than that of the unbalanced device.Obviously,this work not only deepens the understandings of current-dependent ISC and RISC processes in EB-OLEDs,but also provides an insight into the device physics for designing and fabricating high-efficiency EB-OLEDs.