Molecular Engineering of Highly Efficient Small Molecule Nonfullerene Acceptor for Organic Solar Cells

A new molecularly engineered nonfullerene acceptor, 2,2′‐(5,5′‐(9,9‐didecyl‐9H‐fluorene‐2,7‐diyl)bis(benzo[c][1,2,5]thiadiazole‐7,4‐diyl)bis(methanylylidene))bis(3‐hexyl‐1,4‐oxothiazolidine‐5,2‐diylidene))dimalononitrile (BAF‐4CN), with fluorene as the core and arms of dicyano‐n‐hexylrhodanine terminated benzothiadiazole is synthesized and used as an electron acceptor in bulk heterojunction organic solar cells. BAF‐4CN shows a stronger and broader absorption with a high molar extinction coefficient of 7.8 × 10⁴m⁻¹ cm⁻¹ at the peak position (498 nm). In the thin film, the molecule shows a redshift around 17 nm. The photoluminescence experiments confirm the excellent electron accepting nature of BAF‐4CN with a Stern–Volmer coefficient (K_sv) of 1.1 × 10⁵m⁻¹. From the electrochemical studies, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of BAF‐4CN are estimated to be −5.71 and −3.55 eV, respectively, which is in good synchronization with low bandgap polymer donors. Using BAF‐4CN as an electron acceptor in a poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl) 2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5″′‐diyl)] based bulk‐heterojunction solar cell, a maximum power conversion efficiency of 8.4% with short‐circuit current values of 15.52 mA cm⁻², a fill factor of 70.7%, and external quantum efficiency of about 84% covering a broad range of wavelength is achieved.