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Organic semiconductor micro/nanocrystals (OSMCs) have attracted great attention due to their numerous advantages such us free grain boundaries, minimal defects and traps, molecular diversity, low cost, flexibility and solution processability. Due to all these characteristics, they are strong candidates for the next generation of electronic and optoelectronic devices. In this review, we present a comprehensive overview of these OSMCs, discussing molecular packing, the methods to control crystallization and their applications to the area of organic solid-state lasers. Special emphasis is given to OSMC lasers which self-assemble into geometrically defined optical resonators owing to their attractive prospects for tuning/control of light emission properties through geometrical resonator design. The most recent developments together with novel strategies for light emission tuning and effective light extraction are presented.

Thiophene‐based (Th‐based) derivatives have received wide attentions in organic optoelectronics due to their excellent and tuneable optoelectrical properties as well as chemical modification. However, the low photoluminescence quantum yield in solid state limits their application in solution‐processed light‐emitting optoelectronic devices, especially in organic lasers. Herein, a novel blue‐emitting steric Th‐based fluorophore (MC8‐Th) with excellent optical gain behavior for organic laser is reported. Interestingly, MC8‐Th neat film exhibits efficient exceptional deep‐blue amplified spontaneous emission (ASE) behavior with a remarkably low threshold of 6.0 µJ cm−2 and full‐width‐at‐half‐maximum value of 2.6 nm. Furthermore, random laser signals are also obtained with the lowest threshold of 4.1 µJ cm−2 when incorporating the compound into an inert polystyrene (PS) matrix. In addition, low threshold (16 µJ cm−2, fivefold lower than those of F8BT neat films) yellow–green ASE emission (560 nm) is achieved through Förster resonance energy transfer. Meanwhile, ultrafast transient absorption spectroscopy is deployed here to observe the single‐molecular excitonic behavior in solution and PS‐blend, intermolecular excited state in neat film, and efficient energy transfer in MC8‐Th:F8BT‐blend films. As far as known, MC8‐Th shows lower threshold ASE/lasing behavior for the Th‐based conjugated materials. [ABSTRACT FROM AUTHOR]