Two different effects of Na on Cu2ZnSnS4 thin-film solar cells

•The Na content showed different values depending on the annealing process. The composition ratios calculated from the area ratio of Na in the absorber and the buffer layer are 7.6%, 4.9% (CZTS1), 4.9%, 3.1% (CZTS2), and 2.0%, 1.0% (CZTS3), respectively.•When an annealing process was applied, the voltage shortfall decreased, and the Ea of CZTS2 and CZTS3 become closer to Eg than the Ea of CTZT1.•Excess Na remains in the absorber layer or it acts as an impurity when it diffuses into the buffer and window layers during the follow-up processes.•Furthermore, because it traps minority carriers, the film characteristics can deteriorate.•By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased.

Among the many factors that affect the characteristics of Cu2ZnSnS4 thin-film solar cells, the effects of Na were examined. Because Na passivates the grain boundaries (GBs) in absorber layers, the GBs increase the minority carrier collection by providing a current pathway through which the minority carriers can reach the buffer and window layer and be collected. However, excess Na remains in the absorber layer and diffuses into the buffer and window layers, where it acts as an impurity and traps the minority carriers, thus decreasing the solar cell characteristics. By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased. By improving the solar cell characteristics, a power conversion efficiency of 6.43% was achieved.