Abnormal current dependence of high-level reverse intersystem crossing induced by Dexter energy transfer from hole-transporting layer

Abstract

High-level reverse inter-system crossing (HL-RISC, T_2,rub → S_1,rub) in rubrene molecules has been recently found as a potential evolution channel of excitons for realizing high-efficiency organic light-emitting diodes (OLEDs), and this channel usually presents a normal bias-current dependence. Here, an abnormal current (I) dependence of the HL-RISC process, which enhances with increasing I, is observed in rubrene-doped devices with low dopant concentration (C) of 1% and always exists from room temperature down to 20 K, but disappears as C increases up to 3%. By analyzing the energy level structures and I-dependent electroluminescent spectra of devices with different C, the extra Dexter energy transfer (DET) process from hole-transporting layer (HTL) to rubrene emitter enables the population of T_2,rub states to increase with I, leading to the abnormal current dependence of HL-RISC. Additionally, this unusual phenomenon is independent of the changes of the host material or the electron-transporting layer but is closely associated with the HTL of the control devices (with C = 1%). This is because the energy level barrier at the HTL/host interface plays an important role for the occurrence of the extra DET process and the increase of T_2,rub states. Undoubtedly, this work not only deepens the comprehensive understanding of the amazing HL-RISC channel in rubrene-based devices, but also provides ideas for designing reduced efficiency roll-off OLEDs at high bias currents.