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

Abstract 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.