Molecular Cluster‐Controlled Quasi‐Epitaxial CZTSSe/CdS Heterojunction Enables 12.3% Efficiency of Flexible Solar Cells

Abstract Flexible Cu ₂ ZnSn(S,Se) ₄ (CZTSSe) solar cells have garnered significant attention in photovoltaics. Interface defects in CZTSSe/CdS heterojunctions drive carrier recombination, leading to substantial open‐circuit voltage ( V _OC ) loss. Herein, a deposition strategy is proposed to achieve quasi‐epitaxial heterojunctions by controlling the CdS molecular clusters during the chemical bath deposition (CBD) process. The nucleation rate and size of clusters are regulated by manipulating the stirring process and a sealed NH ₃ atmosphere. Small‐sized CdS molecular clusters, densely adsorbed onto the CZTSSe surface at a controlled low rate, pair with dangling bonds to form the quasi‐epitaxial heterojunction structure. This effectively suppresses interface defects and mitigates tunnel‐enhanced recombination, resulting in an increased V _OC of 503 mV. The epitaxial growth of CdS thin films facilitates the formation of ultrathin buffer layers, thereby enhancing the short‐wavelength transmittance of the window layer, resulting in a 15% boost in short‐circuit current density ( J _SC ). Finally, the flexible CZTSSe solar cell achieves a power conversion efficiency ( PCE ) of 12.3% and demonstrates exceptional mechanical stability, retaining over 95% of its initial efficiency after thousands of bending cycles. The developed quasi‐epitaxial heterojunction strategy suppresses interfacial recombination, offering a promising route toward high‐efficiency flexible kesterite solar cells.