1D/2A ternary blend active layer enables as-cast polymer solar cells with higher efficiency, better thickness tolerance, and higher thermal stability

Recently ternary polymer solar cells (PSCs) have shown big potential to enhance the photovoltaic performance, however, the relevant systematic studies based on thick-film active layers are extremely scarce and have not been well elucidated. Herein, as-cast 1D/2A thick-film ternary active layers based on highly crystalline polymer PBODT as the donor and PC71BM plus non-fullerene ITIC as the acceptors were investigated, with full comparison to PBODT:PC71BM and PBODT:ITIC blends, the two binary systems. It was found that PBODT:ITIC binary PSCs showed very poor thickness tolerance, whose power conversion efficiencies (PCEs) dropped sharply from 6.45% for 100 nm thick active layer to 4.44% and 2.58% for 200 and 300 nm thick active layers, respectively. Relatively, PBODT:PC71BM binary PSCs could display much better thick-film performances, with a 5.14% PCE at 110nm and a maximum PCE of 7.55% at 300 nm. For the as-cast PBODT:PC71BM:ITIC (1:1.2:0.3 by weight) ternary PSCs, the PCE for 300 nm thick active layer could be further elevated to 8.42%. Interestingly, notable thickness tolerance was also achieved for the ternary PSCs, whose PCEs >= 7.22% could be maintained for thickness range from 110 to 520 nm. Based on AFM and TEM analyses,the PBODT:PC71BM:ITIC ternary blend film could show good surface and bulk-heterojunction morphologies. Relative to the two binary active layers, the ternary active layer could contribute higher short-circuit current due to improved photo-response and suppressed carrier recombination. Furthermore, the thermal stability of the as-cast ternary solar cells was quite better than those of binary solar cells. The simultaneous improvements of efficiency, thickness tolerance, and thermal stability via ternary blends in this work suggest that ternary polymer solar cells would be very promising in future.