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. (C) 2015 Elsevier B.V. All rights reserved.