Highly Efficient Cadmium‐Free Kesterite Solar Cell With the Lowest <i>V</i> _OC,def Enabled by Na‐Ion‐Assisted Defect Passivation

ABSTRACT Cadmium (Cd)‐free Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells are primarily constrained by a severe open‐circuit voltage deficit ( V OC,def ), arising from Cu Zn antisite defects and interfacial nonradiative recombination pathways that are difficult to suppress through bulk defect regulation alone. A major practical bottleneck is the lack of a controllable, scalable approach to regulate sodium (Na) in the defect‐rich near‐surface region. Here, we introduce a crystallization‐coupled Na treatment based on a solution‐processable NaCl–MOE surface treatment that enables controllable, surface‐confined Na incorporation. During selenization, the surface‐confined Na is activated and coupled to grain growth, leading to suppression of antisite‐related recombination pathways and a reduced interfacial defect density. This strategy suppresses band‐tail states and nonradiative recombination, driving a shift from interface‐dominated to bulk‐dominated recombination. As a result, Na‐treated devices exhibit reduced defect densities, moderated band bending, prolonged carrier lifetime, increased diffusion length, and overall improved carrier management. Consequently, the lowest open‐circuit voltage deficit ( V OC,def ) of 0.316 V and a power conversion efficiency (PCE) of 13.71% are achieved, placing this Cd‐free CZTSSe/ZTO device among the top‐performing Cd‐free kesterite solar cells reported so far. This work establishes surface‐initiated, crystallization‐assisted alkali regulation as an effective and scalable route to mitigate the V OC,def .