Band alignment engineering and performance prediction of Cu ₂ ZnSnS ₄ /Ag ₂ ZnSnS ₄ -based solar cells using first-principles calculations and SCAPS-1D device simulations

Cu₂ZnSnS₄ (CZTS) is recognized as a highly promising material for the absorption layer in solar cells, owing to its non-toxic nature and the abundance of its constituent elements. This study examines the use of CZTS and Ag₂ZnSnS₄ (AZTS), which is formed through the substitution of homologous elements, as absorption layer materials, while CdSe and ZnSe are utilized as buffer layers of solar cells. Utilizing density functional theory, the electronic properties of the investigated materials are computed, and surface models are constructed based on the semiconductor-exposed terminations to predict variations in electron affinity (<i>χ</i>). A comprehensive solar cell architecture incorporating multiple heterojunctions is also proposed, the one-dimensional solar cell capacitance simulator (SCAPS-1D) software is employed to optimize key parameters for each solar cell configuration, including the <i>χ</i> of the back surface field (BSF) layer and the buffer layer, the absorption layer thickness, as well as interface and bulk defect densities (IF <i>N</i>_t and bulk <i>N</i>_t), with the aim of maximizing efficiency. Finally, a novel device structure (ZnO/CdSe/AZTS/CZTSe/Mo) is introduced, wherein CZTSe functions as the BSF layer positioned between the absorption layer and the back contact electrode. We optimize the band alignment by modifying the <i>χ</i> parameter, enhance the photon absorption efficiency through variation of the absorption layer thickness, and systematically quantify the impacts of both IF <i>N</i>_t and bulk <i>N</i>_t on the performance of the solar cell. The findings reveal that optimal band alignment is achieved when the <i>χ</i> values of the BSF and buffer layers are 4.2 eV and 4.3 eV, the absorption layer thickness is 1300 nm, the bulk <i>N</i>_t and IF <i>N</i>_t are maintained at 10¹² cm^-3 and 10¹³ cm^-2, respectively. Under these conditions, the PCE reached 23.60%. This work provides valuable insights for the advancement of high-efficiency thin-film solar cells.