Anomalous magnetic-field response of doped devices based on the thermally activated delayed fluorescent material of CzDBA

CzDBA is a donor-acceptor-donor (D-A-D)-type of thermally activated delayed fluorescence (TADF) material that has been recently reported in the literature; it has better luminous performance than ordinary D-A-type TADF materials. To explore the microscopic processes involved in CzDBA-based light-emitting devices, we fabricated two kinds of devices with light-emitting layers of pure CzDBA and 4CzTPN-Ph films and measured their magneto-luminescence (MEL) and magnetic conductance (MC) at room temperature. Experimental results show that although both of them contain carbazole groups, their MEL traces present two different line shapes; the former (CzDBA-based OLEDs) exhibits positive MEL curves dominated by intersystem crossing processes (ISC), whereas the latter depicts negative MEL traces governed by reverse ISC (RISC). In addition, CzDBA-based OLEDs have the same line shapes of MEL and MC traces. To investigate this anomalous magnetic-field response in CzDBA further, we prepared a doped device with a light-emitting layer of mCBP: x %CzDBA. Experimental results show that compared with devices with pure CzDBA as an emission layer, doped devices exhibit similar MEL and MC line types but have opposite temperature-dependent MEL traces. Detailed analyses related to MEL and MC traces show that the ISC process of polaron pairs dominates pure CzDBA devices, making their MEL values always positive. However, the doping of CzDBA into an mCBP host results in a strong exciton energy transfer (ET) process in the doped devices; this ET process is suppressed at low temperature, leading to a weakened RISC effect and an abnormal temperature effect different from that of pure CzDBA devices. In addition, the competition of the energy transfer process between host and guest and the trapping effect of charge carriers produce a nonmonotonic change in the MEL amplitude of the doped device by increasing the dopant concentration. This change further proves the correctness of the proposed microscopic mechanisms for these CzDBA-based OLEDs. This work is beneficial to the further understanding of the evolution mechanisms of the microscopic processes in devices based on excellent CzDBA luminescent materials recently discovered in literature.