Origin of the Additive-Induced V_OC Change in Non-Fullerene Organic Solar Cells

Additives are often used to adjust the morphology of the active layer to improve the performance of organic solar cells (OSCs). Here, taking typical high-efficiency non-fullerene systems as examples, the effect of the additive on the device performance in non-fullerene OSCs is systematically investigated. Surprisingly, an unpresented V_OC change is observed in the opposite direction of the two typical systems (PM6:Y6 and PTB7-Th: ITIC) appearing after the incorporation of the additive DIO, which can be affected by the morphological differences as indicated by the several morphological studies. The bewildering V_OC change caused by the additive in different material systems is supposed to originate from the different energy level variations as verified by the energy level studies. Molecular dynamic (MD) and density functional theory (DFT) calculations are also included to get an insight into the dynamic of the additive-induced morphological differences that are supposed to contribute to the energy level changes. Combining a series of morphological and energic studies as well as the theoretical calculations, the origin of unforeseeable V_OC changes caused by additives in non-fullerene OSCs is clarified, and provides in-depth insights into the effects of additives on device performance.