Suppressing the efficiency roll-off in thermally-activated-delayed-fluorescence—sensitized fluorescent OLEDs by triplet management under pulsed operation

Thermally-activated-delayed-fluorescence (TADF)---sensitized fluorescent organic light-emitting diodes (TSF OLEDs) can simultaneously exhibit high quantum efficiency and high color purity. However, the long-lived nature of excited states during a reverse intersystem-crossing (RISC) process may result in the accumulation and annihilation of triplets and consequently the efficiency roll-off in TSF OLEDs under high current density. In this study, we demonstrate that the pulsed operation of TSF OLEDs can suppress the efficiency roll-off by manipulating exciton dynamics to reduce the accumulation of triplets. Under pulsed operation, the on-cycle luminance of 5CzBN-based OLEDs is increased from 31 370 to 55 760 cd ${\mathrm{m}}^{\ensuremath{-}2}$ at an amplitude of 10 V, accompanied by an enhancement on on-cycle external quantum efficiency by approximately 100% at a high current density of 1 A ${\mathrm{cm}}^{\ensuremath{-}2}$. Exciton dynamics analysis on transient EL curves reveals that the triplet annihilation becomes predominated over the RISC process after the time delay of 1--2 \textmu{}s, offering a time window for pulse operation. The competition between triplet annihilation and RISC is further investigated by the magnetoelectroluminescence measurement, which confirms that the pulsed operation suppresses the triplet annihilation and thereby improves the conversion from triplets to singlets through RISC. These results show that the on-cycle electroluminescence performance of various TSF OLEDs can be optimized by tuning the pulsed operation parameters according to exciton dynamics, and provide an alternative way to address the issue of efficiency roll-off in next-generation OLEDs.