Impact of cation substitution in all solution-processed Cu2(Cd, Zn)SnS4 superstrate solar cells

Abstract

Thin-film solar cells constituted of abundant elements are key to ensuring mass production, reducing energy costs, and meeting energy demands. Kesterite is a cost-effective light absorber but to date has shown low compatibility with a superstrate architecture, and thus studied mostly with costly substrates. In the present study, we demonstrated that a cation substitution of Zn by Cd could improve the absorber compatibility on a superstrate architecture. Moreover, we employed all solution-processing using a water and ethanol mixture as green solvents for thin-film deposition. An increase in the Cd amount in the absorber layer led to improved crystallinity and a crystal-size increment, band gap decrement, and a change in the crystal structure from kesterite to cernyite. The composition of the buffer layer was controlled by the diffusion of Zn into CdS, forming (Cd, Zn)S with a relative Cd–Zn ratio. Solar cells prepared with high Cd showed a thicker depletion width and a reduction in the defect carrier density, leading to a significant increment in the power conversion efficiency of up to 1.9%. Furthermore, the fabricated solar cells showed high stability upon continuous light illumination of over 80 h. Investigation of the electrical losses in Cu₂CdSnS₄ suggested an ample opportunity to improve solar cell performance in a superstrate type architecture.