Thiourea-to-metal ratio modulates chemical-potential-driven precursor coordination and crystallization pathways in solution processed Cu₂ZnSnS₄ solar cells

Understanding and controlling precursor chemistry is essential for optimizing solution-processed Cu₂ZnSnS₄ (CZTS) solar cells. However, the microscopic mechanism linking thiourea concentration to coordination chemistry, sulfurization kinetics, and defect evolution remains poorly understood. Here, we systematically reveal how the thiourea-to-metal (TU/M) ratio determines the coupled processes of metal–ligand coordination, sulfur release, and grain growth during CZTS formation. A low TU/M ratio induces sulfur deficiency and the formation of secondary phases, whereas an excessively high TU/M drives surface-dominated crystallization accompanied by ZnS aggregation and a bilayer morphology. In contrast, an optimal TU/M (∼1.75) ensures balanced sulfur diffusion and a direct crystallization pathway form a short-range Cu-S-Sn-S-Zn coordination framework inferred from precursor chemistry and sulfurization behavior, facilitating the growth of compact grains with improved electronic properties and achieve an efficiency of 8.0% without an anti-reflection coating. Mechanistically, TU/M acts as a chemical potential regulator that governs the competition between direct crystallization and secondary-phase fusion pathways. This work clarifies the microscopic mechanism underlying TU/M-dependent crystallization and provides a general strategy for microstructure and morphology control in kesterite thin film photovoltaics.