Two donor–acceptor copolymers based on isomeric acceptor units, [7,7′‐bithieno[2′,3′:4,5]thieno[2,3‐d]thieno[3,2‐b]pyridine]‐5,5′(4H,4′H)‐dione (BTTP) and [2,2′‐bithieno[2′,3′:4,5]thieno[2,3‐d]thieno[3,2‐b]pyridine]‐5,5′(4H,4′H)‐dione (iBTTP), are developed to study the effect of isomeric structures on photovoltaic performance. Compared with PBDTBTTP, PBDTiBTTP possesses a smaller bandgap for good light harvesting and a better π–π stacking for higher hole mobility. PBDTiBTTP solar cells present balanced mobilities and good nanoscale phase separation, giving a power conversion efficiency (PCE) of 6.51%, with higher short‐circuit current (Jsc) and fill factor (FF). image