Molecular Engineering of 9‐Fluorenone‐Benzothiadiazole‐Based Donors: Synergistic Optimization of Photovoltaic Properties in Organic Solar Cells

9‐Fluorenone(FN) and benzothiadiazole units have become important building blocks for organic solar cells (OSCs) due to their excellent charge transport prperties, strong electron affinity, and planar molecular structures. However, the 9‐fluorenone‐benzothiadiazole ( FN‐BT ) system suffers from poor solubility and insufficient light absorption, limiting its photovoltaic performance. To overcome these challenges, three novel small‐molecule donors (SMDs) are developed: two asymmetric ( TPAFNBT and TPAFNDOBT ) and one symmetric ( TPAFN ) 2 DOBT , based on their molecular framework. Side‐chain modifications and π‐conjugation extension are investigated the effect on device performance. The symmetric donor ( TPAFN ) 2 DOBT shows particularly promising properties, with extended absorption and an optical bandgap of 1.93 eV. The introcuced alkyl side chains significantly improved material processability and film quality. Notably, ( TPAFN ) 2 DOBT ‐based devices achieve a high open‐circuit voltage ( V oc) of 1.09 V coupled with a power conversion efficiency (PCE) of 9.25%, representing the highest efficiency reported for solution‐processed FN‐BT photovoltaic systems. This work demonstrates that molecular engineering through strategic side‐chain design and π‐conjugation extension can effectively enhance the performance of SMDs. These findings provide valuble design principles for developing advanced active layer materials in organic photovoltaic.