The development of organic semiconductor lasers (OSLs) is of significant research interest as it offers a versatile and cost-effective solution for a variety of laser applications including spectroscopy, biomedical imaging, and optical communications. A laser comprises a gain medium and an optical resonator, and can be excited optically or electrically. Many organic materials, including small molecules, oligomer, dendrimers, and polymers have been studied as gain media for amplified spontaneous emission (ASE) by optically pumping to the excited state to establish a population inversion. Organic laser diodes are challenging to realise due to the inherent drawbacks of organic semiconductors, such as a low exciton utilization ratio and long-lived/broad absorption from triplets and polarons. Electrically injected charge carriers generally recombine to form emissive singlet and non-emissive triplet excitons in a ratio of 1:3 according to spin-statistics, which limits the branching ratio to 25% for organic injection lasers based on fluorescent emitters. The long-lived triplet excitons lead to excitation energy losses through triplet excited-state absorption (ESA) and exciton annihilation, resulting in self-termination and poor stability of the laser devices. To overcome the problems associated with triplet accumulation, different strategies have been proposed, such as introducing triplet scavenger materials within the organic gain layer to quench the long-lived triplet excitons, using organic materials whose triplet absorption is spectrally separated from the stimulated emission (SE), or using thermally activated delayed fluorescence (TADF) materials as triplet harvesters that can convert triplet excitons into singlet excitons via reverse intersystem crossing (RISC) (Figure ). This latter strategy provides a route to circumvent the triplet absorption and annihilation in OSLs.