Solution processing route to Na incorporation in CZTSSe nanoparticle ink solar cells on foil substrate

Sodium is a key dopant in thin film photovoltaic cells with reported benefits including promotion of grain growth, passivation of grain boundaries and increased carrier concentration in chalcopyrite and kesterite based solar cells. Research-grade devices fabricated in substrate configuration often rely on diffusion of Na from a soda lime glass substrate into the photovoltaic absorber layer during high temperature processing. However, for samples on flexible substrates such as foils and plastics, this is not available and requires alternative approaches. In this work, we fabricate Earth-abundant Cu2ZnSn(S,Se)4 thin film solar cells from nanoparticle inks on flexible molybdenum substrates and demonstrate a simple, low-cost route to incorporating Na in solution thereby making it compatible with large area, high volume manufacturing. The technique is verified to improve the device efficiency relative to a reference flexible device built on molybdenum foil.