Uncovering solvent-engineering mechanisms in Y6:PM6 solar cells

Additives, like 1-chloronaphtalene (CN), are commonly used in Y6:PM6 solar cells as they lead to an increased power conversion efficiency. In this work, we investigate the influence of CN during spin coating of Y6:PM6 dissolved in chloroform via an in situ transmission setup. We show that, in the presence of CN, the film formation of Y6:PM6 can be divided into two parts: one related to the evaporation of chloroform and one related to the evaporation of CN. This is mostly related to Y6 being dissolved in CN. We find that even for low CN concentration, the film formation is not completed for several minutes after the spin coating process. Furthermore, the removal of CN is needed to achieve a smooth film surface. We demonstrate that this fast removal can be achieved by spin coating the electron transport layer PDINN from methanol. The methanol is acting as an anti-solvent for the CN, leading to its removal from the film. Using this approach, solar cells fabricated with a high CN concentration of 5% feature a comparable performance to ones with more common concentrations between 0.5% and 1%.