Effects of Na2S treatment and post-annealing on Sn-rich Cu2ZnSnS4-based thin film solar cells

In this study, chemical etching by Na2S and annealing under nitrogen atmosphere were combined to remove the secondary phases, which precipitated in the CZTS films produced by sulfurizing the metal-alloy precursors. Both treatments indicate positive outcome for removing SnS2 impurity crystals precipitated on the surface of the CZTS thin films. In addition, both treatments were found effective in dissolution of Sn-based secondary phases mainly from Sn-rich CZTS. By Na2S etching, spectral response at short wavelength of the thin film solar cell is enhanced, while that of the wavelength region of 500-700 nm is degraded. On the other hand, post-annealing improved the spectral response in this wavelength region, and subsequent etching did not degrade the spectral response. By applying an annealing treatment followed by Na2S etching, the spectral response was improved in both short and long wavelength regions and the photovoltaic conversion efficiency over 4% was realized. This treatment process was especially effective in the Sn-rich CZTS absorption layer, and hence open circuit voltage, short circuit current, and fill factor were greatly enhanced compared with the other treatments procedure.