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Efficient heat transfer is beneficial to heat dissipation and the thermal durability of organic solar cell (OSCs). In this regard, heat transfer properties of organic semiconductors within OSCs should play important roles, but their thermal properties are rarely explored. Here, heat diffusion properties of Y‐series non‐fullerene acceptors processing different DA′D framework, named BZ4F‐5, BZ4F‐6, and BZ4F‐7 are probed; it is found that backbone rings extension from five‐ to six‐ and seven‐membered‐fused rings trigger longer phonon mean free path and higher thermal diffusivities (D) in their pristine solid films and bulk heterojunction blends. Particularly, the correlation between the thermal transport properties in Y‐series acceptors and their backbone geometry, molecule stacking, and thin‐film crystallinity is demonstrated. More importantly, both organic thin‐film transistors and OSCs confirm that thermal durability of organic semiconductor devices correlated with the thermal properties of their active layer. Although BZ5F‐6 and BZ4F‐7 based devices possess similar device performance at room temperature, superior heat dissipation in BZ4F‐7 molecule endows it with enhanced device lifetime. These results contribute to critical design criteria for future molecular optimization in photovoltaic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Recent studies have almost focused on finding active layer materials with extended π‐conjugation structures for high‐performance organic solar cells (OSCs). However, with the extension of conjugate length, the synthesis difficulty and cost of materials will increase. Achieving high efficiency while reducing material costs is a prerequisite for the commercialization of OSCs. Herein, two low‐cost A‐DA′D‐A‐type (where A and D represent an electron‐withdrawing unit and an electron‐donating unit, respectively) nonfullerene acceptors (Y25,Y26) are synthesized with pentacyclic fused backbone as the DA′D electron‐deficient core and 5,6‐difluoro‐3‐(dicyandiamethyl) indigo as the end groups. Compared with classical Y series acceptors with heptacyclic backbone, although Y25 and Y26 own the reduced conjugated length, they still show moderate performance (11.65% and 13.34%), and the cost of synthesis is significantly reduced. Therefore, we provide a new molecular design idea for commercially efficient nonfullerene OSCs acceptors. We also find that adding alkyl chains to the β site of thiophenes is beneficial to obtaining the reduced energetic disorder, dominant molecular stacking, and desirable morphology, which can facilitate charge carrier transport and prompt higher short‐circuit current density (Jsc) as well as fill factor. [ABSTRACT FROM AUTHOR]