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

ABSTRACT 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.