Halogen-free Polymer Donors Based on 3,4-Dicyanothiophene for High-performance Polymer Solar Cells

Polymer solar cells(PSCs) consisting of a polymer donor and a small molecular acceptor is a promising photovoltaic technology,whose device performance is determined by both polymer donor and small molecular acceptor. Halogen atoms such as fluorine or chlorine atoms were usually introduced onto the polymer donors to downshift the highest occupied molecular orbital(HOMO) energy levels and improve the open-circuit voltage(VOC) of the PSCs. However, the introduction of the halogen atoms especially fluorine atoms greatly complicates the polymer synthesis. Herein, we report the use of a structural simple and easily synthesized building block, 3,4-dicyanothiophene(DCT), to construct a set of halogen-free polymer donors PBCNTx(x=25, 50, 75) via ternary random copolymerization. The introduction of DCT units not only simplified the synthesis, but also downshifted the HOMO energy levels of the polymers and improved the VOC of PSCs effectively.Encouragingly, the PBCNT75 afforded a power conversion efficiency up to 15.7% with a VOC of 0.83 V, which are among the top values for halogen-free polymer donors. This work shows that the introduction of DCT units is a simple yet effective strategy to construct halogen-free and low-cost polymer donors for high-performance PSCs.