Mediation of energy transfer on magnetic field effects in Px-CNP-based OLEDs

Abstract

Thermally activated delayed fluorescence (TADF) material, Px-CNP, has a low singlet–triplet states energy gap (ΔE_ST = 0.04 eV). Thus, Px-CNP can realize the reverse intersystem crossing (RISC) process at room temperature, and the process usually has linear voltage dependence. In this paper, we prepared a series of mCP:Px-CNP doped devices and measured the magneto-electroluminescence (MEL) effect of the devices. It was found that the MEL of the devices exhibited a RISC linear pattern at room temperature. Surprisingly, as the voltage increases, the intensity of RISC shows a nonlinear dependence of increasing and then decreasing. After replacing the high-energy triplet material mCP with the low-energy triplet material Alq₃, the MEL curve of the device changes to an intersystem crossing (ISC) line, but the intensity of ISC also shows a nonlinear dependence on voltage (decreasing and then increasing) as the voltage increases. Analyzing the microscopic mechanism of the doping layer and the energy transfer process, it can be seen that the different binding ability of the Px-CNP triplet charge transfer (³CT) excitons by the host materials with different triplet energy levels, leads to different energy transfer channels and energy loss of ³CT in each device, which results in different RISC and ISC linear patterns in the MEL of each device. Through the Dexter energy transfer (DET) and direct charge trapping (DCT) processes, voltage modulates the concentration and lifetime of ³CT and triplet state excitons (T₁) in the devices, leading to a non-linear voltage dependence of RISC and ISC in the devices. This work not only reveals the regulation of RISC and ISC in Px-CNP-based devices by energy transfer, but also provides a feasible idea for the efficient implementation of the RISC process in TADF devices and the controllable application of ³CT.