Small Molecule Thiol Self-Assembled Interlayer Enhances Carrier Extraction at CZTSSe/CdS Interface for High Efficiency Photovoltaics.

Efficient carrier extraction at the CZTSSe/CdS heterojunction is essential for high-performance CZTSSe solar cells. However, the uneven distribution of nucleation sites on polycrystalline CZTSSe often leads to nonuniform CdS nucleation and growth, resulting in defect-rich heterojunctions and enhanced interfacial recombination. Herein, we report a small molecule thiol self-assembled interlayer (thiol-SAI) strategy to regulate CdS nucleation and growth on CZTSSe through the directional interaction between thiol-containing molecules and the absorber surface. By comparing thiol molecules bearing hydroxyl, carboxyl, amino, and sulfonic acid terminal groups, sodium 2-mercaptoethanesulfonate (Mesna) is identified as the most effective interfacial modifier. The thiol group (-SH) anchors onto the CZTSSe surface to form a Mesna SAI with outward-oriented sulfonic acid functionalities. This interlayer significantly improves surface wettability and promotes uniform heterogeneous nucleation, enabling the growth of a compact, highly crystalline, and low-defect CdS buffer layer with enhanced grain-boundary coverage. Consequently, interfacial recombination is suppressed, and the heterojunction recombination activation energy increases from 59.27 to 69.50 meV. As a result, the power conversion efficiency is improved from 11.50% to 12.93%. This work demonstrates that thiol-SAI-assisted interface engineering provides an effective strategy for coordinating buffer-layer growth with heterojunction quality, offering a general molecular-level approach for optimizing charge extraction in solution-processed thin-film solar cells.