Rear-Interface Engineering in Kesterite Solar Cells: Disentangling the Beneficial and Detrimental Effects of SnS

Abstract

Kesterite Cu₂ZnSnS₄ (CZTS) solar cells suffer from a persistent open-circuit voltage (V_OC) deficit, largely attributed to secondary phases and severe rear-interface recombination. In this work, a comprehensive SCAPS-1D numerical investigation is carried out to disentangle the dual role of the SnS secondary phase and to optimize rear-interface band engineering in CZTS solar cells. The influence of SnS thickness, acceptor concentration, defect density, and bandgap on device performance is systematically analyzed. Ultrathin SnS layers (~5 nm) are shown to induce a favorable spike-like conduction band offset (+0.15 eV), effectively suppressing electron recombination while preserving hole transport. In contrast, thicker SnS layers (>50 nm) introduce parasitic absorption and resistive losses, leading to significant efficiency degradation. Furthermore, a comparative study of various back surface field (BSF) materials demonstrates that Cu₂O provides the most advantageous band alignment, enabling strong rear-field formation and efficient suppression of Shockley–Read–Hall recombination. An optimized device efficiency exceeding 12% is achieved, offering clear guidelines for mitigating the V_OC deficit in kesterite photovoltaics.