Enhanced Optical Gain Through Efficient Polaron Pairs Recombination in F8_xBT_y

Abstract Organic semiconductors combine excellent optoelectronic properties with simple fabrication to obtain the desired features by tuning their chemical structures. Förster resonant energy transfer (FRET) is in designing blended gain media, but it does not enhance efficient optical gain in most cases. This is due to the competition between polaron pairs and stimulated emission (SE), leading to quenching of SE. Thus a challenge to design efficient optical gain systems via FRET. Here, a series of copolymers, F8_xBT_y, through uniformly inserting benzothiadiazole (BT) units into the 9,9‐dioctylfluorene (F8) chain, were synthesized. They consist of both charge transfer (CT) and FRET is synthesized. These copolymers can prevent the local F8 aggregation and lead to efficient polaron pair recombination into singlet excitons. The F8 excitons are thus spatially confined, increasing efficient SE. Efficient light amplification has low amplified spontaneous emission (ASE) thresholds (as low as 5.3 µJ cm⁻²) and significantly enhanced optical gain with gain coefficients up to 37 cm⁻¹. The distributed feedback (DFB) lasers has low lasing thresholds down to 5.4 nJ per pulse. The results suggest that these copolymers provide a organic gain media design strategy with efficient optical gain, introducing a new approach to developing laser materials for electrically pumped lasers.