Kesterite Cu2ZnSnS4 (CZTS) thin film solar cells are fabricated based on sputtering and post sulfurization process on stainless steel substrates. NaF layers with different thickness are used as external sodium source to address the lack of Na issue in Na-free steel substrates and 10 nm of NaF layer is proved to be the optimal thickness. Sodium incorporation into CZTS induces the metal element redistribution, facilitates CZTS film recrystallization and promotes the phase evolution. Appropriate amount of sodium is evidenced to yield an enhancement in power conversion efficiency from 3.07% to 4.10% whilst further increase in sodium tends to deteriorate the device efficiency. Device performance deterioration is correlated to the rather small grain size accompanied with a large amount of grain boundaries and unfavorable SnS2 secondary phase for CZTS samples without sodium, while pinholes and lower carrier density in absorber for CZTS samples with over amount of sodium. (C) 2016 Elsevier B.V. All rights reserved.