The impact of fluorination on both donor polymer and non-fullerene acceptor: The more fluorine, the merrier

Fluorination of the donor polymer or non-fullerene acceptor (NFA) in an organic photovoltaic device is an effective method to improve device efficiency. Although there have been many studies on donor polymer fluorination, blends containing both a fluorinated donor and fluorinated NFA have rarely been reported. In this study, we use two donor polymers (4 '-FT-HTAZ and 4 '-FT-FTAZ) and two NFAs (ITIC-Th and ITIC-Th1) with different amounts of fluorine (from 2F to 6F) to investigate how the degree of fluorination in a blend impacts device performance. We find that fluorinating the NFA leads to a higher short-circuit current density (J(sc)) and fill factor (FF), however, the open-circuit voltage (V-oc) is decreased due to a depressed lowest unoccupied molecular orbital (LUMO) level. Adding additional fluorine to the donor polymer does not have a large effect on the J(sc) or FF, but it does lead to an improved V-oc. By fluorinating the NFA and having more fluorine on the donor polymer, we obtain both a high J(sc) and V-oc simultaneously, leading to a power conversion efficiency over 10% in the case of 4 '-FT-FTAZ:ITIC-Th1, which has the most amount of fluorine (6F).