Photovoltage enhancement of M-series acceptor-based polymer solar cells and minimodules through the modulation of charge-transfer states

Side-chain fluorination can enhance the backbone organization and carrier mobility of non-fullerene acceptors (NFAs) but it often reduces their photovoltage due to the resulting deeper-lying lowest unoccupied molecular orbital (LUMO) levels. Herein, we present a strategy to regulate the LUMO levels of two NFAs, MC9F5 and MC7F5, by repositioning the highly electronegative -C₂F₅ moieties on the side chains. This approach mitigates the impact of fluorination on the energy levels, thereby improving the photovoltage and overall device performance. By incorporating 10,10,11,11,11-pentafluoro-2-(8,8,9,9,9-pentafluorononyl)undecyl side chains, the -C₂F₅ moieties are positioned away from the conjugated backbone of MC9F5, resulting in an elevated LUMO level compared with MC7F5, which features 8,8,9,9,9-pentafluoro-2-(6,6,7,7,7-pentafluoroheptyl)nonyl side chains. This modification reduces both the charge generation and the non-radiative energy losses in the MC9F5-based devices. The MC9F5-based small-area and minimodule devices achieve efficiencies of 18.02% and 15.66%, respectively, which are among the highest values reported for acceptor-donor-acceptor-type NFAs. This study highlights a valuable fluorination strategy for achieving high-performance NFAs.