Unintentional doping in PM6:Y6-based solar cells from exposure to the ambient

Organic solar cells based on non-fullerene acceptors (NFAs) hold great potential for low-cost energy production; however, their large-scale manufacturability and long-term stability still pose challenges. In this work, we clarify how the doping concentration in PM6:Y6-based solar cells evolves as a function of time stored in the ambient. Our results show a rapid increase in the doping concentration, reaching 3 × 1016 cm−3 within the first few hours of ambient exposure. After 100 h of exposure to the ambient, the doping concentration saturates, reaching levels up to 1017 cm−3. A variation of the active layer thickness indicates higher doping concentrations in thinner samples. Previous work has shown that doping concentrations on the order of 1017 cm−3 can have a drastic effect on device performance—either positive or negative, depending on other device parameters. Our findings highlight the importance of properly characterizing unintentional doping in organic solar cell devices, not only in pristine devices, but also as a function of device aging.