Thermal Activation Bottleneck in TADF OLEDs based on m-MTDATA:BPhen

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) can be highly efficient because of the conversion of non-radiative triplet to radiative singlet states by reverse intersystem crossing (RISC). Even highly efficient TADF OLEDs are limited by long excited state lifetimes though, which limit current densities and cause device degradation. When singlet-triplet energy gaps are comparable to thermal energies (~tens of millielectronvolts), RISC is fast and limited only by bottlenecks due to spin-selection rules. We have studied this phenomenon under device operating conditions using pulsed electrically detected magnetic resonance spectroscopy (pEDMR) in OLEDs based on the donor:acceptor combination m MTDATA:BPhen (4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine : 4,7 diphenyl-1,10-phenanthroline). These experiments showed magnetic resonance signatures of emissive exciplex states at the donor:acceptor interface, yet these signals did not reveal coherent spin propagation effects. Instead, the intensity of these signals scales linearly with the energy dose of the applied microwave pulses. This observation excludes the direct involvement of the resonantly prepared coherent spin states and indicates that the observed current response is due to magnetic resonant heating. This implies that the studied TADF blend is not limited by spin-forbidden RISC, but rather by the thermal activation step.