  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.
