We investigated the impact of various excitonic and photonic losses on the lasing threshold and slope efficiency of organic semiconductor lasers (OSLs) under optical and electrical excitations. The rate equations are solved numerically using the Euler method for an OSL and an organic semiconductor laser diode, including 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) as a gain medium. The results showed that the loss mechanisms that affect the exciton and photon densities cause an increase in the laser threshold and a decrease in the slope efficiency. Further, we demonstrated that by using a thermally activated delayed fluorescence (TADF) emitter as a gain medium, the triplet excitons could be harvested by increasing the reverse intersystem crossing rate (k RISC ), resulting in an appreciable decrease of the laser threshold and an increase of the slope efficiency. Accordingly, the TADF emitters with a fast k RISC are expected to significantly reduce the current density required for electrical excitation.
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