Effect of Cu/(Zn+Sn) ratio on the ZnSe position and performance of CZTSe solar cells

• CZTSe thin film was fabricated using vapor selenization.
• Micro-Raman depth profile of ZnSe phase along the film thickness.
• Solar cell performance was not correlated with the ZnSe existence.
• ZnSe at the absorber bottom promoted CuZn defect formation.

Cu2ZnSnSe4 photovoltaic absorber layers were prepared by the vapor selenization of stacked Cu/Sn/CuZn/Mo/glass precursor films with different Cu/(Zn + Sn) compositional ratios (CZT = 0.8, 0.85 and 0.9). The absorber was ZnSe-free at CZT = 0.9 but the films obtained from CZT = 0.85 or 0.8 contained a ZnSe phase mainly at the bottom or top of the absorber film, respectively. Although the solar cell fabricated from the CZT = 0.9 absorber showed poor-shunt characteristics (efficiency = 1.94%), that produced from CZT = 0.85 exhibited significantly distorted current–voltage characteristics (efficiency = 4.23%). The CZT = 0.8 ratio showed the highest efficiency of 5.8% and normal pn diode behavior. This suggests that the existence or location of the ZnSe phase within the absorber is not related directly to the solar cell performance. Instead, the ZnSe phase may alter the defect distribution near the space charge region. Several aspects of the device behavior depending on the CZT ratio are discussed with respect to the relative position of the ZnSe phase.

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