Origin of the Efficiency Roll-Off in Single-Layer Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence

The efficiency roll-off in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) is attributed to either singlet-triplet or triplet-triplet annihilation (TTA) as well as triplet-charge annihilation. The origin of the efficiency roll-off on a TADF OLED consisting of a host-less single-layer emitter is studied. Varying the charge-carrier concentration at constant exciton density or the exciton density at constant charge-carrier density with temperature unambiguously shows that the dominant contribution to the roll-off originates from TTA. Using an analytical model, also the TTA rate constant can be obtained. These results show that single-layer TADF OLEDs are suited not only to determine the roll-off mechanism, but also to provide its rate constant directly from OLED efficiency measurements.