The magnetic field effects of electroluminescence in thermally activated delayed fluorescence devices

In device with the activation of delayed fluorescence (TADF), triplet excitons can be activated by the environment and then converted into singlet excitons through reverse intersystem crossing (RISC), leading to delayed fluorescence and internal quantum efficiency of 100%. To further reveal the underlying mechanism of electroluminescence based on TADF devices, this paper selected three types of high efficient TADF materials, 4CzTPN-Ph, 2CzPN, and PIC-TRZ2 to fabricate organic light emitting devices. Our results show that, although the TADF devices have RISC process, their magneto-electroluminescence (MEL) curves exhibited similar line-shapes with those in the ordinary devices. This is due mainly to the facts that magnetic field can only affect the intersystem crossing (ISC) and Reverse ISC of polaron pairs rather than exciton in the these devices. At the same time, we found that the magnitudes of magnetic field effects in TADF devices increase with increasing the injection current, which is contrary to those from ordinary exciton-based devices. The relative change between the triplet polaron to triplet exciton rate k_t and the singlet Polaron to singlet excitons rate k_s is used to interpret these abnormal experimental results.