Charge-Transfer Dynamics in OLEDs with Coexisting Electroplex and Exciton States

Abstract

The reverse intersystem-crossing (RISC) channels from triplet to singlet charge-transfer states (such as exciplex or electroplex states) frequently occur in donor:acceptor heterojunction organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency. Although exciplexes have been extensively investigated, there are few reports on the physical microscopic processes for electroplex-based devices. Herein, three kinds of donor:acceptor heterojunction OLEDs are fabricated: two coexisting electroplex and exciton devices and one pure exciplex-based device. Amazingly, via the fingerprint magnetoelectroluminescence measurement at room temperature, we observe an abnormal current-dependent RISC, which is enhanced with increasing bias current (I) in the coexisting electroplex and exciton but electroplex-dominated device; the conversion from ISC to RISC in the coexisting electroplex and exciton but exciton-dominated device; and only the normal I-dependent ISC, which weakens with increasing I in the pure exciplex-based device. This is because low-energy-triplet electroplexes are well confined by the high triplet energies of electron donors and acceptors, promoting the occurrence of the RISC process in electroplexes, and Dexter energy transfer from triplet excitons to triplet electroplexes enhances the RISC process with increasing I, causing the abnormal I-dependent RISC and the conversion from ISC to RISC. Moreover, as the operational temperature rises from 10 to 300 K, these two coexisting electroplex and exciton devices present the conversion from ISC to RISC at different temperatures, but the pure exciplex-based device shows only the normal temperature-dependent ISC, which is enhanced with increasing temperature. This is because the RISC channel is an endothermic process and temperature-dependent electroluminescence measurements show that the electroplex emission weakens as the temperature is reduced, causing the weakened RISC process and even the presence of the ISC process at low temperature. These experimental results demonstrate that the electroplex emission plays an important role in the occurrence of strong RISC processes. Our work deepens our microscopic understanding of the charge-transfer states in donor:acceptor heterojunction devices but also paves the way for utilizing electroplex states to design highly efficient OLEDs.