In CZTSSe solar cells,a simple sodium-incorporation post-treatment method toward solution-processed Cu2ZnSnS4 precursor films is presented in this work.An ultrathin NaCl film is deposited on Cu2ZnSnS4 precursor films by spin-coating NaCl solution.In subsequent selenization process,the introduction of NaCl is found to be benefacial for the formation of Cu2-xSe,which can further facilitate the element transportation,leading to dense and smooth CZTSSe films with large grains and less impurity Cu2Sn(S,Se)3 phase.SIMS depth profiles confirm the gradient distribution of the sodium element in Na-doped absorbers.Photoluminescence spectra show that the introduction of appropriate sodium into the absorber can inhibit the band tail states.As high as 11.18％ of power conversion efficiency (PCE) is achieved for the device treated with 5mg mL-1 NaCl solution,and an average efficiency of Na-doped devices is 10.71％,13％ higher than that of the control groups (9.45％).Besides,the depletion width and the charge recombination lifetime can also have regular variation with sodium treatment.This work offers an easy modification method for high-quality Na-doped CZTSSe films and high-performance devices,in the meantime,it can also help to further understand the effects of sodium in CZTSSe solar cells.