Effects of the Cu/(Ga+In) ratio on the bulk and interface properties of Cu(InGa)(SSe)2 solar cells

In this study, we systematically investigated the performance loss factors in Cu(InGa)(SSe)2 (CIGSSe) solar cells with various Cu/(Ga+In) (CGI) ratios, which were fabricated with sputtering and sequential selenization/sulfurization processes. The effects of the CGI ratio on the CIGSSe solar cells were determined by measuring the current-voltage (IV) curves and junction capacitance of the solar cells, as well as by performing X-ray analysis techniques on the cells. An increase in the defect density and decrease in the free carrier density were observed in samples with high Cu concentrations (CGI ratio >0.89), which resulted in a drastic decrease in the open-circuit voltage (VOC) and fill factor (FF). The temperature-dependent IV (IVT) and X-ray diffraction (XRD) results of the bulk characterization corresponded well with the capacitance measurements for all CGI ratios. The low crystal quality and short minority-carrier diffusion length at high CGI ratios resulted in a significant bulk recombination rate. A qualitative analysis of the interface characteristics was performed with IVT measurements, and the results showed that the recombination activation energy in the samples with high CGI ratios was lower than the bandgap (Eg), decreasing the VOC of these devices. For the samples with high CGI ratios, their inferior bulk and interface characteristics caused recombination to occur at the interface as well as in the bulk. However, the interface recombination rate was negligible for the samples with low CGI ratios (CGI ratio <0.89). In addition, a CIGSSe solar module was fabricated on a mass production assembly line, with the size of the CIGSSe solar module the same as that of commercially available crystalline-Si-based solar modules. The record efficiency of a solar module with a total area of is 16.0%.