An effective defect passivation route has been demonstrated in the rapidly growing Cu2ZnSn(S,Se)(4) (CZTSSe) solar cell device system by using Cu2ZnSnS4:Na (CZTS:Na) nanocrystals precursors. CZTS:Na nanocrystals are obtained by sequentially preparing CZTS nanocrystals and surface decorating of Na species, while retaining the kesterite CZTS phase. The exclusive surface presence of amorphous Na species is proved by X-ray photoluminescence spectrum and transmission electron microscopy. With Na-free glasses as the substrate, CZTS:Na nanocrystal-based solar cell device shows 50% enhancement of device performance (similar to 6%) than that of unpassivated CZTS nanocrystal-based device (similar to 4%). The enhanced electrical performance is closely related to the increased carrier concentration and elongated minority carrier lifetime, induced by defect passivation. Solution incorporation of extrinsic additives into the nanocrystals and the corresponding film enables a facile, quantitative, and versatile approach to tune the defect property of materials for future optoelectronic applications.