Tailoring aggregation behavior of shamrock-shaped non-fullerene acceptors via an isomerisation strategy enables high-performance organic solar cells

The end-group halogenation strategy in banana-shaped Y-series non-fullerene acceptors (NFAs) plays a crucial role in the development of organic solar cells (OSCs). Compared to fluorinated end-groups, chlorinated end-groups offer advantages such as simpler synthesis, lower cost, and higher open-circuit voltage (V_OC). In this study, we replaced the benzothiadiazole (BT) unit in Y-series molecules with an acenaphtho[1,2-b]quinoxaline imide (AQI) structure and utilized IC-p2Cl and IC-o2Cl isomers as end-groups, respectively, to synthesize two shamrock-shaped NFAs, AQI16 and AQI17. Further investigations revealed that the substitution position of chlorine atoms plays a critical role in modulating the π–π stacking and crystallinity of the materials. Besides, end-group isomerization significantly influences the photophysical and photovoltaic properties of the materials. The D18:AQI16 combination achieved a power conversion efficiency (PCE) of 17.90%, which is obviously higher than that of the AQI17 based device (15.14%). This work highlights the impact of isomerization caused by halogen substitution positions on the photovoltaic properties of shamrock-shaped NFAs.