A Thieno[3,2‐c]Isoquinolin‐5(4H)‐One Building Block for Efficient Thick‐Film Solar Cells

An aromatic lactam acceptor unit, thieno[3,2‐c]isoquinolin‐5(4H)‐one (TIQ), is developed. Compared with its analogues, dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one (DTP) and phenanthridin‐6(5H)‐one (PN), TIQ shows its advantage in constructing donor–acceptor (D–A) copolymers for efficient solar cells. TIQ‐based D–A copolymer PTIQ4TFBT delivers a power conversion efficiency (PCE) of 10.16% in polymer:fullerene solar cells, while those based on DTP and PN copolymers, PDTP4TFBT and PPN4TFBT, afford PCEs around 8.5%. The higher performance of PTIQ4TFBT:PC 71 BM solar cells originates from enhanced short‐circuit current density ( J sc ) and fill factor (FF), because of favorable morphology, less bimolecular recombination, and balanced charge transport in the active layer. Moreover, the performance for PTIQ4TFBT:PC 71 BM solar cells is less sensitive to active layer thickness than PDTP4TFBT:PC 71 BM and PPN4TFBT:PC 71 BM solar cells. Over 8% PCEs can be obtained from PTIQ4TFBT:PC 71 BM solar cells when the active layer thickness is over 500 nm.