High-Performance Non-Fullerene Acceptor Derived from Diathiafulvalene Wings for Solution-Processed Organic Photovoltaics

A solution-processable small-molecule nonfullerene electron acceptor BAF-2HDT (7,7′-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(4-((4,5-bis(hexylthio)-1,3-dithiol-2 ylidene)methyl)benzo[c][1,2,5]thiadiazole) bearing hexadiathiafulvalene (HDT) wings as end groups has been synthesized for bulk heterojunction organic photovoltaics. The molecule shows broad absorption in the 300-600 nm range with a molar extinction coefficient (ε) of 9.32 × 10⁴ M^-1·cm^-1 exceeding to that of [6,6]-phenyl-C₇₁-butyric acid methyl ester of 2.8 × 10⁴ M^-1·cm^-1 at 461 nm. The HOMO and LUMO energy levels of the molecule are found to be -5.69 and -3.58 eV, respectively which is compatible with low band gap high-performance polymers such as 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)] (PffBT4T-2OD). Photoluminescence-quenching measurements confirm that the molecule BAF-2HDT has excellent electron-accepting capability. The organic solar cells made from BAF-2HDT blending with conjugated polymer donor PffBT4T-2OD exhibit a power conversion efficiency of 7.13% with high V_oc of 0.77 V, J_sc of 14.64 mA·cm^-2, and FF of 0.64. The design and development of such nonfullerene acceptors with high ε may be key to further development of high-performance and cost-effective solution-processed organic solar cells. (Graph Presented).