Efficient tuning of the conversion from ISC to high-level RISC via adjusting the triplet energies of charge-transporting layers in rubrene-doped OLEDs

Abstract

Although the host material has been proved to be a crucial factor affecting the occurrence of the high-level reverse intersystem crossing (HL-RISC, T_2,rub → S_1,rub) process in rubrene-doped organic light-emitting diodes (OLEDs), the roles of the charge-transporting layers on this amazing process are still unclear. Very interestingly, herein, we found that the efficient conversion from intersystem crossing (ISC) to HL-RISC is observed by tuning the triplet energies of hole- and electron-transporting layers (HTL and ETL) in rubrene-doped devices. Specifically, when the triplet energies of ETL, rubrene guest, and HTL satisfy the criterion of E(T_1,ETL) < E(T_2,rub) < E(T_1,HTL), the conversion process can be observed under various current and temperature conditions, which can be reasonably explained by the weak energy-loss channel from T_2,rub to T_1,ETL states and the competition effects of excited states in the ETL and guest. Otherwise, ISC (HL-RISC) will dominate in devices due to the existence (non-existence) of energy-loss channels from T_2,rub to T_1,HTL states. Furthermore, very importantly, by inserting appropriate HTL or ETL materials with high triplet energies into the control devices with energy-loss channels, we found that the full confinement of E(T_2,rub) indeed facilitates the HL-RISC process. Our work deepens the cognition of the HL-RISC process and provides an effective method to achieve the HL-RISC process in organic optoelectronic devices.