Zn(O,S) Buffer Layer for in Situ Hydrothermal Sb₂S₃ Planar Solar Cells

Hydrothermal deposition is emerging as a highly potential route for antimony-based solar cells, in which the Sb₂(S,Se)₃ is typically in situ grown on a common toxic CdS buffer layer. The narrow band gap of CdS causes a considerable absorption in the short-wavelength region and then lowers the current density of the device. Herein, TiO₂ is first evaluated as an alternative Cd-free buffer layer for hydrothermally derived Sb₂S₃ solar cells. But it suffers from a severely inhomogeneous Sb₂S₃ coverage, which is effectively eliminated by inserting a Zn(O,S) layer. The surface atom of sulfur in Zn(O,S) uniquely provides a chemical bridge to enable the quasi-epitaxial deposition of Sb₂S₃ thin film, confirming by both morphology and binding energy analysis using DFT. Then the results of the first-principles calculations also show that Zn(O,S)/Sb₂S₃ has a more stable structure than TiO₂/Sb₂S₃. The resultant perfect Zn(O,S)/Sb₂S₃ junction, with a suitable band alignment and excellent interface contact, delivers a remarkably enhanced J_SC and V_OC for Sb₂S₃ solar cells. The device efficiency with the TiO₂/Zn(O,S) buffer layer boosts from 0.54% to 3.70% compared with the counterpart of TiO₂, which has a champion efficiency of Cd-free Sb₂S₃ solar cells with a structure of ITO/TiO₂/Zn(O,S)/Sb₂S₃/Carbon/Ag by in situ hydrothermal deposition. This work provides a guideline for the hydrothermal deposition of antimony-based films upon a nontoxic buffer layer.