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

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.