Ag‐Promoted Na Retention for Synergistic Defect Passivation in Kesterite Solar Cells

The performance of kesterite solar cells is fundamentally limited by a large open‐circuit voltage deficit. Here, we demonstrate a precursor‐level Ag–Na codoping strategy via solution processing to overcome this limitation through synergistic regulation of lattice properties and bulk defects. A 10% Ag substitution triggers pronounced grain coarsening and reduces band‐tail disorder by suppressing Cu Zn antisite defects, evidenced by a modest but systematic reduction in Urbach energy. Depth profiling reveals a unique cooperative growth mechanism in which rapid Ag‐induced surface crystallization effectively traps Na within the film, suppressing severe Na loss during high‐temperature selenization and creating a concentrated Na reservoir near the back contact. Na subsequently diffuses upward to passivate bulk defects. Consequently, this Ag‐driven lattice framework enables a minimal 2% Na addition to significantly boost the majority carrier concentration, effectively reversing the conventional expectation that Ag alloying inevitably depletes carriers. Uniting these structural and electronic improvements at the optimal 10Ag–2Na composition, we achieve a champion power conversion efficiency of 13.1% alongside a severely reduced V oc deficit of 0.378 V. This synergistic codoping approach offers a robust and scalable pathway for advancing high‐efficiency earth‐abundant thin‐film photovoltaics.