The relationship between the composition and conversion efficiency of Cu2ZnSnSe4 (CZTSe)-based solar cells was investigated. CZTSe films were grown by thermal deposition using a coevaporation method. It was found that the composition of CZTSe and Na concentration [Na] have strong correlations to the conversion efficiency, and higher-efficiency samples were obtained with the composition ratios closer to Zn/Sn similar to 1.6, Cu/(Zn + Sn) similar to 0.8, and [Na] similar to 2%. The highest conversion efficiency (eta = 2.1%) was obtained in the composition region, and the composition was significantly nonstoichiometric. The CZTSe composition was automatically fixed on a tie line between Cu2ZnSnSe4 and ZnSe, and it was demonstrated that this composition can be expressed by one parameter. The structural and electrical properties of the high-and low-efficiency samples were also investigated. No significant differences in such properties were observed between the high- and low-efficiency samples by surface, x-ray diffraction, and Raman measurements. However, the low-efficiency sample showed a high carrier concentration. (C) 2013 Published by Elsevier B.V.