Passivation of Grain Boundaries and Defects in CZTSSe Solar Cells by In Situ Na Doping

Abstract

Alkali metal doping plays a great role in improving the efficiency of Cu2SnZn(S, Se)4 (CZTSSe) thin film solar cells. However, it is unclear how to realize the ideal distribution of alkali metal in CZTSSe films. Meanwhile, the mechanisms of alkali metal doping are still controversial. Herein, Na‐doped CZTSSe cells are fabricated by magnetron sputtering with Na‐containing Cu2SnZnS4 target which is in situ doping and annealing in selenization atmosphere. The incorporation of Na enhances the diffusion of K from the substrates to the absorbers. Na doping can increase the contact potential at the grain boundaries, which has beneficial effects on reducing the carrier recombination at the grain boundaries. Furthermore, Na incorporation modifies the conduction band offset at the CdS/CZTSSe interface from −0.07 to 0.03 eV. The defects and recombination in different regions are quantitatively extracted. It is demonstrated that the optimized grain boundary electrical properties and the heterojunction band alignment passivate the defects of the depletion region, the heterojunction interface, and the quasineutral region. Finally, a total‐area efficiency of 11.18% has been achieved in CZTSSe solar cell with an in situ Na doping concentration of 0.6 at%.