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

Flexible Cu₂ZnSn(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 NH₃ 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.