in this work using the combined density functional theory-finite element method (DFT-FEM) approach. First, a FEM simulation of a 4T 𝐶𝑠𝑆𝑛0.5𝐺𝑒0.5𝐼3∕𝐶𝑢2𝑍𝑛𝑆𝑛𝑆𝑒4 TSCs is performed. The total power conversion efficiency (𝑃 𝐶𝐸) for this structure is 13.44%. After results verification and losses identification, the top sub-cell’s electron transport layer (ETL) and hole transport layer (HTL) materials are modified to reduce losses and increase the structure’s total 𝑃 𝐶𝐸. The TSC’s total 𝑃𝐶𝐸 increases to 16.15% by changing these materials. Moreover, the addition of 100 nm of 𝑀𝑔𝐹2 as an anti-reflection (AR) layer with a reduction of reflection loss improves the total 𝑃𝐶𝐸 to 17.47%. Next, 𝑍𝑛(𝑂, 𝑆, 𝑂𝐻) material is used as a suitable replacement to eliminate the toxicity of the 𝐶𝑑𝑆 buffer layer in the kesterite SC. Next, various ratios of 𝑀𝑔 element are added to the ZnO window material in order to increase the 𝑃 𝐶𝐸 of the bottom sub-cell and subsequently the total 𝑃 𝐶𝐸. Using DFT to extract the opto-electronic parameters of the 𝑍𝑛1−𝑥𝑀𝑔𝑥𝑂 compound, this material is employed as a window material in the tandem structure. The composition of 𝑍𝑛0.8𝑀𝑔0.2𝑂 is introduced as the best material in order to achieve the highest 𝑃𝐶𝐸 of 17.63%. Finally, we suggest a highly stable and non-toxic 4T all-inorganic single-halide perovskite/kesterite TSC that is environmentally safe and has 31.18% greater total 𝑃𝐶𝐸 than the first TSC structure.
