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

摘要

High-level reverse inter-system crossing (HL-RISC, T-2,T-rub -> S-1,S-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 increasingI, 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 asCincreases up to 3%. By analyzing the energy level structures andI-dependent electroluminescent spectra of devices with differentC, 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 withI, 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 (withC= 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.