Effect of Replacing Thiophene by Selenophene on the Photovoltaic Performance of Wide Bandgap Copolymer Donors

Two polymers J75 and J76 with selenophene instead of thiophene on the conjugated side chain of benzodithiophene (BDT) unit or π bridges of polymer J71 were designed and synthesized, for investigating the effect of selenophene substitution on the photovoltaic performance of the conjugated polymer donors in comparison with J71. The selenophene π bridges in J76 can narrow optical band gap and red-shift absorption of the polymer film by ca. 25 nm, but the highest occupied molecular orbital (HOMO) energy level (E_HOMO) of J76 is up-shifted slightly by 0.04 eV. Two typical electron acceptors of fullerene derivative PC₇₁BM and the nonfullerene acceptor m-ITIC were used to investigate photovoltaic performance of the polymer donors. For the PC₇₁BM-based polymer solar cells (PSCs), J76 with selenophene π bridges shows the best power-conversion efficiency (PCE) of 8.40% in comparison with the J71-based device (PCE = 6.79%), benefitted from the red-shifted absorption, larger coherence length, purer average domains, and proper domain size of J76 donor in the blend film with PC₇₁BM. For the nonfullerene PSCs with m-ITIC as acceptor, the J76-based PSCs showed a slightly lower PCE of 11.04% in comparison with the J71-based device (PCE = 11.73%), which could be due to the up-shifted HOMO level and slightly larger domain size of J76 in the blend film with m-ITIC. The results indicate that the selenophene π bridges of the polymers red-shifted absorption and increased PCE of the fullerene-based PSCs; nevertheless, it has weak influence on the photovoltaic performance of the nonfullerene PSCs.