Asymmetric Alkylthio Branched Chain on Pentacyclic Small Molecular Acceptors Enables High Efficiency Organic Solar Cells

Solution‐processible organic solar cells (OSCs) have gained much attention as one of the most promising options for sustainable energy. With rapidly increasing efficiency of OSCs, developing small molecular acceptors (SMAs) with simple molecular structures are critical for reducing the cost of photovoltaics. Herein, three new SMAs (BZ4F‐C 2 SEH, BZ4F‐2C 2 SEH, BZ4F‐SEH) are designed by the incorporation of ethyl(2‐ethylhexyl)sulfane or (2‐ethylhexyl)‐sulfane at the β ‐position of thiophene. As a result, the BZ4F‐C 2 SEH‐based devices obtained an optimal PCE of 15.08% with a good balance between open‐circuit voltage ( V oc = 0.87 V) and short‐circuit current density ( J sc = 23.27 mA cm −2 ) by blending with low‐cost polymer donor PTQ10. Besides, the BZ4F‐C 2 SEH based devices treated by thermal annealing (TA) and solvent annealing (SVA) deliver a satisfactory power conversion efficiency (PCE) of 16.18% with a V oc of 0.88 V, and a J sc of 23.98 mA cm −2 . This work highlights that attaching asymmetric alkylthio side chain at the β ‐position of thiophene can be used as an effective molecular design strategy to trade off V oc and J sc , thus improving the photovoltaic performance of OSCs.