High performance Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells are fabricated by selenization of the precursor films of Mo/Sn/Cu/ZnS/Sn/ZnS/Cu deposited by magnetron sputtering. The investigation of the solar cells with different Zn/Sn ratio in CZTSSe film discloses that the charge carrier concentration and depletion region width of the device is very sensitive to Zn/Sn ratio of CZTSSe layer. The CZTSSe film with Zn/Sn=1.05 has lower carrier density (5.0 x 10(15) cm(-3)), which is half of the cell with Zn/Sn=1.12, whereas the depletion region at the CdS/CZTSSe hetero-junction interface of the former (200-250 nm) is 100 nm longer than the latter. As a result, better collection of photo-generated charge carrier is found with the cell with longer W-d in the longer wavelength region above 800 nm. Therefore, the average power conversion efficiency is increased from 6.53% to 9.16% with enlarged depletion region width, and the best performance with 10.2% efficiency is achieved. (C) 2016 Elsevier B.V. All rights reserved.