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‐C2SEH, BZ4F‐2C2SEH, 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‐C2SEH‐based devices obtained an optimal PCE of 15.08% with a good balance between open‐circuit voltage (Voc = 0.87 V) and short‐circuit current density (Jsc = 23.27 mA cm−2) by blending with low‐cost polymer donor PTQ10. Besides, the BZ4F‐C2SEH based devices treated by thermal annealing (TA) and solvent annealing (SVA) deliver a satisfactory power conversion efficiency (PCE) of 16.18% with a Voc of 0.88 V, and a Jsc 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 Voc and Jsc, thus improving the photovoltaic performance of OSCs. The trade‐off between Voc and Jsc of pentacyclic SMAs is realized by asymmetry strategy of sulfur‐containing branched side chains, and 15.08% PCE is obtained. Active layer morphologies are further optimized by solvent vapor annealing, and device efficiency is increased to 16.18%.