Fabrication and Characterization of Wide-Gap ZnCuInS₂ Solar Cells

We have successfully prepared ZnCuInS₂ (Zn_2xCu_1-xIn_1-xS₂, ZCIS) thin films by spray pyrolysis deposition (SPD). The bandgap of the ZCIS thin film was widely controlled from 1.4 to 3.4 eV by substituting Zn for Cu and In of CuInS₂ (CIS). The resistivity of the ZCIS film was controlled by adjusting deposition temperature and composition ratio. ZCIS solar cells with a structure of glass/indium tin oxide (ITO)/TiO₂/In₂S₃/ZCIS/Au were fabricated. The cell with a bandgap of 1.8 eV showed an efficiency of 4.4%. However, the average V_oc is much lower than what is theoretically possible for absorbers with the bandgap. The secondary ion mass spectroscopy (SIMS) depth profile showed that a narrow bandgap layer, corresponding to a low Zn/(Zn+Cu+In) ratio, was formed at the interface between the buffer and the absorber by interdiffusion. The low V_oc is attributed to the existence of the narrow bandgap layer.