The regulatory role of the hole-transporting layer on the hot exciton evolution pathway in a coexistent system of exciplex and exciton

To investigate the influence of the hole transport layer (HTL) on the hot exciton channel in the systems with coexisting multiple excited states, this work selected 5,6,11,12-tetraphenyltetracene (rubrene) which exhibits hot exciton characteristics as the emitter, bilayer interfacial exciplex as the host, and materials with different triplet energies as HTLs to fabricate a series of rubrene-doped organic light-emitting diodes featuring coexisting exciplex and exciton states. For comparison, conventional exciton-type rubrene-doped devices were also prepared to establish a baseline for understanding the role of the HTL in modulating exciton dynamics. By measuring and analyzing the magneto-electroluminescence (MEL) curves under varying temperatures and currents, distinct MEL behaviors were observed depending on the different triplet energy levels of the host and HTL. Specifically, when the host material of the emissive layer possessed a high triplet energy but the HTL had a low triplet energy (E(T1,HTL)), the low-field (|B| 1→PP3) process from singlet to triplet polaron pair states (PP1 and PP3). This experimental result suggests that the primary spin-mixing mechanism is governed by the ISC process in conventional excitonic systems. In contrast, the low-field MEL of devices with coexisting multiple excited states (including exciplex and exciton) was primarily dominated by ISC, but the weak contribution from the high-level reverse intersystem crossing (hRISC) process is observed in the ultra-low field region (|B| 2 → S1) refers to the conversion from the high-lying triplet state (T2) to the lowest singlet state (S1), which is also called hot exciton channel. When both the triplet energies of the exciplex host (E(EX3,host)) and HTL were high, the low-field MEL of the rubrene-doped OLEDs with exciplex and exciton coexisting system was exclusively determined by hRISC process. Conversely, if E(T1,HTL) was low, the low-field MEL of the rubrene-doped devices with coexisting exciplex and exciton states was solely governed by the ISC process. Further analysis of the energy transfer and loss mechanisms between functional layers revealed that the exciton recombination zone was spatially distant from the HTL in the multiple-excited-state devices, making it difficult for E(T1,HTL) to directly influence the population of rubrene’s hot exciton T2 state. However, the energy transfer pathway between E(T1,HTL) and E(EX3,host) could not be neglected. Specifically, when E(T1,HTL) was low, the energy loss channel from the exciplex host to the HTL (E(EX3,host)→E(T1,HTL)) suppressed Dexter energy transfer between the host and guest, thereby hindering the formation of the T2 state and ultimately weakening or even completely inhibiting the hRISC process of hot excitons. This study enhances the fundamental understanding of the physical behavior of hot exciton evolution in systems with multiple excited states. It also offers practical guidelines for optimizing the performance of rubrene-based yellow-light OLEDs (an important illumination source) by strategically engineering the HTL’s triplet energy. In the future, we could explore alternative host-guest systems and different device architectures to further refine the control over exciton dynamics, potentially leading to even more efficient organic light-emitting devices.