2‐Ethylhexyl‐4,6‐Dibromo‐3‐Cyano‐3‐Thieno[3,4‐b]Thiophene Enables Low HOMO Energy Level Polymer Donor

Abstract The new halogen‐free donor polymer PCN6 is constructed using 2‐ethylhexyl‐4,6‐dibromo‐3‐cyano‐thieno[3,4‐b]thiophene as acceptor (A) block, and is compared in detail with the commercially available PTB7‐Th. It is found that PCN6 has a wider film absorption (300–700 nm) and lower highest occupied molecular orbital (HOMO) energy levels (−5.52 eV) than PTB7‐Th (−5.34 eV), suggesting a great advantage of the monocyano‐functionalized modification strategy in terms of molecular absorption and energy level tuning. The performance difference between PCN6:Y6‐ and PTB7‐Th:Y6‐based organic solar cells (OSCs) is compared by a series of studies including light intensity dependence, carrier mobility, AFM, TEM, and GIWAXS. The results show that PCN6:Y6‐based OSCs have stronger crystallinity, better charge transport, higher and more balanced carrier mobility, and less exciton complex loss. Therefore, the power conversion efficiency (PCE) of PCN6:Y6‐based OSCs reaches 11.34%, while the PCE of PTB7‐Th:Y6‐based OSCs is only 9.02%. These results suggest that 2‐ethylhexyl‐4,6‐dibromo‐3‐cyano‐thieno[3,4‐b]thiophene is an excellent A block for the construction of halogen‐free donor polymers with low HOMO energy levels, and also demonstrate that the introduction of cyano in the conjugated backbone of polymers is a good strategy to achieve high‐performance OSCs.