Close‐Spaced Sublimation of CdCl ₂ Enables Uniform CdS Thin Films for Certified 8.08%‐Efficient Sb ₂ S ₃ Solar Cells

ABSTRACT CdCl 2 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 Sb 2 (S,Se) 3 solar cells. However, the conventional spin‐coating method causes inhomogeneous deposition of CdCl 2 on the surface of CdS films, resulting in vastly different CdS(solid)–CdCl 2 (solid) and CdS(solid)–CdCl 2 (vapor) reaction modes during annealing. This process activates pinhole or crack formation and causes grain inhomogeneity due to the CdS(solid)–CdCl 2 (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 CdCl 2 vapor treatment of CdS films. This methodology disrupts the inhomogeneous growth paradigm, enabling the conformal deposition of ultrathin CdS layers (∼20 nm) on FTO/TiO 2 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 Sb 2 S 3 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 Sb 2 (S,Se) 3 and CdTe photovoltaic devices.