To reduce the threshold current density and move closer toward the realization of future current-injection organic semiconductor lasers, the harvesting of triplet excitons is anticipated because 75% excitons are directly formed as triplet excited states under electrical excitation according to spin statics. However, the observation of light amplification in pure phosphorescent or thermally activated delayed fluorescence (TADF) materials has nor yet been reported even under optical excitation. Herein, light amplification was observed in the TADF emitter 9-([1,1′-biphenyl]-3-yl)-N,N,5,11-tetraphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-3-amine (DABNA-2). It is found that DABNA-2 possesses not only a large stimulated emission cross-section but also a favorable window for light amplification. These results indicate that TADF emitters can form a new category of laser dyes and are important candidates for the realization of an organic semiconductor laser.
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