Simultaneously Enhancing Photoluminescence Quantum Efficiency and Optical Gain of Polyfluorene via Backbone Intercalation of 2,5‐Dimethyl‐1,4‐Phenylene

Abstract Simultaneous enhancement of photoluminescence quantum efficiency (PLQE) and optical gain in semiconducting polymer films is desirable for optically‐ or electrically‐pumped organic solid‐state lasers. In this work, a simple self‐dilution effect is achieved by introducing a small amount (≈10% by weight) of 2,5‐dimethyl‐1,4‐phenylene (DP) units in the backbone of poly(9,9‐dioctylfluorene) (PFO). The resulting copolymers, compared with PFO (PLQE 39%), exhibit a significantly increased PLQE (66%) while keeping similar absorption and photoluminescence profile, concomitant with an enhancement in optical gain properties. The radiative decay rate increases sharply along with a sustaining reduction in the non‐radiative decay rate in these copolymers, following similar principle of physical dilution of a luminescent compound in solution or in a polymer matrix. Among all the copolymers, the one containing 10% DP units exhibits the lowest amplified spontaneous emission/distributed feedback laser threshold (10.9 nJ/1.4 nJ, eightfold reduction), and the highest gain coefficient (54.4 cm⁻¹). The results demonstrate that a moderate DP/fluorene ratio can maximize the beneficial self‐dilution effects. These investigations shed light on new design strategies to achieve conjugated polymers with concurrent high PLQE and large optical gain properties.