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.