  Organic light emitting diodes (OLEDs) based on thermally activated delayed
fluorescence (TADF) show increased efficiencies due to efficient upconversion
of non-emissive triplet states to emissive singlets states via reverse
intersystem crossing (RISC). To assess the influence of the characteristic
efficiency-enhancing RISC process as well as possible efficiency-limiting
effects in operational OLEDs, we performed temperature-dependent measurements
of transient electroluminescence (trEL). With kinetic modeling, we quantify and
separate the impact of different temperature-dependent depopulation processes
and contributions to EL in the established donor:acceptor model system
m-MTDATA:3TPYMB. The underlying rate equations adapted for EL measurements on
TADF systems include radiative and non-radiative first- and second-order
effects. In this way, we are able to evaluate the non-radiative recombination
and annihilation processes with respect to their efficiency-limiting effects on
these OLEDs. On the one hand, we evaluate the depopulation of intermolecular
exciplex triplet states via non-radiative direct triplet decay, RISC and
triplet-triplet annihilation (TTA). On the other hand, we determine the
contribution to EL from the formation of singlet exciplex states via polarons,
RISC and TTA. Our results show that TTA accounts for a significant part to
triplet depopulation and contributes to EL while limiting the overall device
quantum efficiency.
