红荧烯掺入多种激基复合物器件的微观过程

Microscopic processes of Rubrene-doped devices with various exciplexes as hosts

红荧烯(5,6,11,12-tetraphenylnaphthacene,Rubrene)是一种典型的发射橙黄光的荧光客体材料,因具有丰富的微观过程而被广泛运用于主客体掺杂器件中,但将其掺杂在具有延迟荧光特性的激基复合物主体中的研究还较少.本文把2%的Rubrene掺杂到具有不同三重态激子能量的四种激基复合物主体里,以指纹式的有机电致发光磁响应(magneto-electroluminescence,MEL)曲线作为探测工具,研究了此类掺杂体系的能量传输和光发射机制.实验发现,当主体激基复合物的三重态激子(EX_(3))能量低于Rubrene客体的第二级三重态激子(T_(2,Rub))能量时,器件的MEL曲线表现为主体极化子对间的系间窜越(intersystem crossing,ISC)过程;否则,器件MEL低磁场部分的线型来源于Rubrene激子的高能级反向系间窜越(high-level reverse intersystem crossing,HL-RISC;T_(2,Rub)→S_(1,Rub))过程;MEL高磁场部分的线型在大电流密度下由Rubrene三重态激子间的聚合反应(T_(1,Rub)+T_(1,Rub)→S_(1,Rub)+S0,Rub)引起,在小电流密度下由单重态激子的分裂(S_(1,Rub)+S0,Rub→T_(1,Rub)+T_(1,Rub))过程决定.此外,温度和电流密度会通过调控Rubrene三重态激子(T_(1,Rub)和T_(2,Rub))的寿命和数量来影响上述微观过程的强弱程度.这是由于主体与客体的单、三重态激子的能级差会调控主客体间的能量转移,进而通过调节客体Rubrene分子上三重态激子的数量和寿命来影响三重态激子的利用率,最终影响器件的发光强度.本文工作既可加深基于Rubrene发光器件微观机制的认识,也可为增强其发光效率提供理论参考.

Recently,exciplex-based organic light-emitting diodes(OLEDs)have become hot topics in organic optoelectronics in terms of improving luminescent efficiency by controlling the energy transfer of excitons.This is because the formation of an exciplex can result in a small energy level difference,ΔEST between singlet and triplet exciplex states,and with the assistance of external thermal energy,nonradiative triplet exciplex states can transform into radiative singlet ones through the so-called reverse intersystem crossing(RISC)process.Moreover,this RISC process will enhance the luminescence of OLEDs based on host–guest systems through energy transfer from exciplexes to dopants.The energy-transfer capability of OLEDs will affect the number of polaron pairs and exciplexes in host–guest systems.Indeed,it is important to find an effective probing technique for simply studying these microscopic processes.Many literature reports have demonstrated that organic magneto-electroluminescence(MEL)traces could be used for exploring and understanding the formation of and interactions between polaron pairs,excitons,and/or exciplexes.This is because MELs exhibit sensitive fingerprint responses to intersystem crossing(ISC),high-level reverse intersystem crossing(HL-RISC),singlet exciton fission(SF),and triplet exciton fusion(TF).In this study,four different OLEDs with various exciplex hosts have been fabricated,and their MEL curves have been measured at different currents and temperatures.Four exciplexes with different triplet exciton energies were used as host materials,and Rubrene was used as a fluorescent dopant.To study the exciton energy transfer and luminescence mechanism in such doping systems,we fully analyzed the emission spectra of the host materials,absorption spectrum of the guest material,and triplet exciton energy of the host and guest materials.The experimental results show that when the combined energy of the exciplex’s triplet excitons(EX3)is lower than that of the second-order triplet excitons(T_(2,Rub))of the Rubrene dopant,the MEL curve is dominated by the B-mediated ISC from host polaron pairs(PP1→PP3).Otherwise,the MEL curves are composed of both low-field and high-field components.The low-field components of MEL are governed by the B-mediated HL-RISC(T_(2,Rub)→S_(1,Rub))process of the Rubrene molecules,while the high-field components of MEL are the result of the TF process between the triplet excitons of Rubrene(T_(1,Rub)+T_(1,Rub)→S_(1,Rub)+S0,Rub)at high current densities or by the SF process of singlet excitons(S_(1,Rub)+S0,Rub→T_(1,Rub)+T_(1,Rub))at low current densities.In addition,these microprocesses will be markedly affected by the operational temperature and bias-current of the devices,because the Rubrene triplet excitons have long lifetimes(T_(1,Rub) and T_(2,Rub))at low temperatures,and large quantities of Rubrene excitons are produced at high current densities.Moreover,the energy level difference between the single and triplet excitons of the host and guest will regulate the energy transfer from the former to the latter,i.e.,the combined energy from the singlet excitons of the host being higher than that of the guest is the requirement of efficient Förster energy transfer where the smaller is the triplet exciton energy difference between the host and guest,the stronger is the Dexter energy transfer.Consequently,the energy-transfer mechanism will affect the utilization rate of the triplet excitons by adjusting the quantity and longevity of the singlet or triplet excitons on the Rubrene molecules,which ultimately affects the luminous intensity of the device.This work is helpful for further understanding the microscopic mechanisms of Rubrene-based devices and provides a theoretical reference for enhancing their luminescent efficiencies.