Close-Spaced Sublimation of CdCl₂ Enables Uniform CdS Thin Films for Certified 8.08%-Efficient Sb₂S₃ Solar Cells

CdCl₂ treatment has emerged as a transformative protocol for augmenting the crystallinity and electrical performance of CdS thin films, representing a foundational advancement in the development of high-efficiency CdTe and Sb₂(S,Se)₃ solar cells. However, the conventional spin-coating method causes inhomogeneous deposition of CdCl₂ on the surface of CdS films, resulting in vastly different CdS(solid)-CdCl₂(solid) and CdS(solid)-CdCl₂(vapor) reaction modes during annealing. This process activates pinhole or crack formation and causes grain inhomogeneity due to the CdS(solid)-CdCl₂(solid) faster reaction rate, deleteriously impacting the device performance through compromised carrier extraction and enhanced interfacial recombination. This study introduces a close-spaced sublimation (CSS) approach for uniform CdCl₂ vapor treatment of CdS films. This methodology disrupts the inhomogeneous growth paradigm, enabling the conformal deposition of ultrathin CdS layers (∼20 nm) on FTO/TiO₂ substrates. The optimized CdS thin films demonstrated enhanced charge extraction efficiency and suppressed interfacial recombination-evidence of improved carrier transport kinetics. Under AM 1.5G illumination, the best Sb₂S₃ solar cell fabricated through this strategy delivered a record power conversion efficiency of 8.30% (certified 8.08%). The CSS technique, which enables controlled homogeneous crystallization, establishes a paradigm for realizing high-quality CdS films and is an essential breakthrough for high-efficiency Sb₂(S,Se)₃ and CdTe photovoltaic devices.