Ternary thick active layer for efficient organic solar cells

Ternary organic solar cells (OSCs) hold great promise in enabling the roll-to-roll printing of environmentally friendly, mechanically flexible, and cost-effective photovoltaic devices. Nevertheless, many ternary OSCs display the best power conversion efficiency (PCE) with a thin active layer at the thickness of about 100 nm, which can be hardly translated in to the roll-to-roll processing with high reproducibility. In this paper, the ternary OSCs with a high PCE and a thick active layer were reported, which was obtained by incorporating a dye small molecule named as 2'-(5,5'-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))-bis(3-ethyl-2-thioxothizolidin-4-one) (FTR)) into a PTB7-Th:PC71BM binary system. Specifically, the addition of FTR into the PTB7-Th:PC71BM binary system was found to improve the hole mobility of the active layer, which resulted in faster charge transport, more efficient charge separation, and higher PCEs even in the presence of a thick active layer. The single-junction PTB7-Th:FTR:PC71BM ternary OSCs with the active layer thickness of 160 nm presented an outstanding PCE of 9.4%, which was much higher than that 7.5% of the PTB7-Th:PC71BM binary OSCs with the active layer thickness of 160 nm. Notably, the PTB7-Th:FTR:PC71BM ternary OSCs devices exhibited excellent thickness insensitivity. In other words, the PTB7-Th:FTR:PC71BM ternary OSCs device with a thick active layer (200 nm) could still demonstrate a high PCE of over 8.2%, which was well compatible to the requirement for future roll-to-roll printing.