Simulation and study of Cd-free SnS-based solar cells with a ZnSe buffer layer using SCAPS-1D

Introduction:

Thin-film solar cells based on CdTe, CIGS, and perovskites have achieved high efficiencies, but their large-scale industrialization remains limited by toxicity issues related to cadmium or lead and by the scarcity of critical elements such as indium, gallium, and tellurium. In this context, tin monosulfide (SnS) has emerged as a promising absorber material because of the abundance and non-toxicity of its constituents, its direct band gap close to 1.3 eV, and its high absorption coefficient, which makes it suitable for thin-layer solar cells. However, the conventional SnS/CdS architecture still suffers from important limitations, including parasitic absorption in the blue–UV region by the CdS layer, unfavorable band alignment at the interface, enhanced recombination, and consequent degradation of the open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF).

Methods:

To address these limitations, this work proposes a detailed numerical study of cadmium-free SnS solar cells using SCAPS-1D. The study first considers replacing the CdS buffer layer with alternative Cd-free materials, particularly zinc selenide (ZnSe) and molybdenum disulfide (MoS₂), in order to optimize the conduction band offset at the interface with the absorber and reduce recombination losses. In parallel, the effect of introducing back surface field (BSF) or hole transport layers (HTL), such as NiO and Cu₂O, between the SnS absorber and the rear metal contact is analyzed, with the aim of blocking electrons, facilitating selective hole extraction, and lowering rear–interface recombination. Different combinations of buffer and BSF/HTL layers are systematically examined together with their physical and electrical parameters, including thickness, doping, band gap, and defect density.

Results:

The numerical analysis is intended to identify device architectures offering the best compromise between band alignment, recombination reduction, and simultaneous improvement of Voc, Jsc, and FF. The comparison focuses on Cd-free configurations based on ZnSe or MoS₂ buffer layers combined with NiO or Cu₂O as BSF/HTL layers, relative to the reference SnS/CdS structure.

Conclusions:

This study aims to identify a truly Cd-free SnS solar-cell structure with improved conversion efficiency compared with the conventional SnS/CdS configuration. It also seeks to provide practical guidance for the experimental implementation of environmentally friendly and efficient SnS-based solar cells.