Abnormal Reverse Intersystem Crossing of Polaron-Pair States and Its Conversion to Intersystem Crossing via the Regulation of Intermolecular Electron-Hole Spacing Distance
Jing Xu, Xiantong Tang, Xi Zhao, Hongqiang Zhu, Fenlan Qu, Zuhong Xiong
Abstract
Reverse intersystem crossing (RISC) unexpectedly dominates the interconversion of polaron pairs in the traditional fluorescent material 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB), with a large energy gap (\ensuremath{\Delta}${E}_{\mathrm{ST}}$) between singlet and triplet excitons, by employing the magnetoelectroluminescence detection technique. Contrary to RISC among excitons with small \ensuremath{\Delta}${E}_{\mathrm{ST}}$ from conventional thermally activated delayed fluorescence, these RISC processes are increased at a high current density and low temperature, which is ascribed to an enhanced direct charge-trapping effect and prolonged lifetime of the triplet-polaron pair, respectively. More intriguingly, the conversion of RISC to ISC is observed by reducing the electron-hole distance of polaron pairs via increasing the concentration of DCJTB dopant. Furthermore, the host must have a high triplet exciton energy to prevent energy back-transfer of the triplet exciton from the guest to the host and ensure efficient occurrence of the RISC process. The RISC-dominated device has a markedly higher external quantum efficiency than that of the ISC-governed one. This work not only deepens the microscopic understanding of the spin-pair states of red-light-emitting DCJTB-doped devices, but also provides a useful method to utilize a conventional fluorescent dopant for designing high-efficiency organic light-emitting diodes.