Cu2ZnSnS4‐based solar cells suffer from limited power conversion efficiency (PCE) and relative small grain size compared to selenium containing absorbers. Introduction of Na in Cu2ZnSnS4 absorbers either during the synthesis or after this step is used to improve device performances and to determine whether its effect is based on structural properties improvement (grain size enhancement, better crystallization) or on opto‐electronic properties improvement (defect passivation). In both cases, presence of Na in the absorber notably improves current and voltage of the solar cells, but the effect is more pronounced when Na is present during synthesis. Quantum efficiency analysis shows that these improvements can be related to longer minority carrier diffusion length and reduced absorber/buffer interface recombination. Introducing Na in the process mostly leads to preferential (112) orientation of the crystal which is clearly correlated with better device performances. Otherwise, the performance limitation due to small grain size is discarded by the joint use of Sb and Na, which has a significant impact on grain size but does not affect solar cells efficiency.