Quantitative analysis of the optical losses in CZTS thin-film semiconductors

For the first time, a quantitative approach was carried out to model the optical losses in several Cu₂ZnSnS₄ (CZTS) based thin-film heterojunction solar cells having different window and buffer layers. The modeling indicates that the heterojunction of glass/FTO/TiO₂/In ₂S₃/CZTS has a lower reflection and higher transmission coefficient than glass/FTO/In₂S₃/CZTS and conventional glass/FTO/CdS/CZTS structures. These optical losses at the interfaces (reflection) and inside the thickness (absorption) of stacked layers were calculated based on the known optical constants such as refractive index and extinction coefficient. While TiO₂ windowed device resulted in a promising high short-circuit current and energy conversion efficiency, the In₂S₃ windowed and conventional CdS windowed devices showed to be less efficient. We compared our theoretical analysis with the practical reports given for such structures in the literature. Further studies are required to reduce the reflection and absorption losses of CZTS structures in practice. So far, the overall performance parameters (e.g., J_sc and η) are much less than our theoretical estimations. The size of the TiO₂ nanoparticles on the layer is about 100 nm.