A Thin In₂ S₃ Interfacial Layer for Reducing Defects and Roughness of Cu₂ ZnSn(S,Se)₄ Thin-Film Solar Cells

Cu₂ ZnSn(S,Se)₄ (CZTSSe) has generated considerable research interest owing to its composition of abundant elements and excellent light-absorption properties. However, CZTSSe thin-film solar cells suffer from a considerable deficit in the open-circuit voltage (V_OC ), which is mainly due to the severe interfacial recombination induced by the rough surface of CZTSSe and numerous physical defects. In this study, to improve the morphology and reduce the interfacial recombination, an In₂ S₃ passivation layer was introduced between the CZTSSe and CdS layers via a chemical bath deposition process, and the effects of the In₂ S₃ layer on the device performance were systematically examined by performing various electrodynamic analyses. The CZTSSe solar cells with thin In₂ S₃ layers exhibited impressive increases in V_OC and conversion efficiency (from 7.33 to 9.24 %), due to the suppression of physical defects and the refined surface morphology resulting from filling the voids and pinholes. In addition, the nanoscale roughness of the In₂ S₃ /CZTSSe surface increased the number of nucleation sites for the CdS nuclei, which may reduce the activation energy of the heterogeneous nucleation. The presence of In₂ S₃ layer resulted in uniform growth of CdS without macroscopic CdS agglomerates (i. e., reduced roughness of full devices), which improved the quality of the interface. These findings confirmed that the reduction of physical defects and the improved deposition of the CdS layer enabled by the added In₂ S₃ passivation layer improved the device performance.