Effects of Fabrication Atmosphere on Bulk and Back Interface Defects of CdTe Solar Cells with CdS and MgZnO Buffers

The performance of Cadmium telluride (CdTe) solar cell devices is limited by the properties of front and back interfaces as well as the absorber layer, which can be influenced by the fabrication atmosphere. In this work, we report a detailed study of electrical properties of ZnMgO (ZMO)/CdTe and cadmium sulfide (CdS)/CdTe solar cells with the cadmium chloride (CdCl2) treatment performed in different atmospheres using temperature-dependent impedance spectroscopy and capacitance – voltage measurements. An equivalent circuit model consisting of two serial combinations of the front and back junctions is employed to fit the complex impedance spectra of devices measured in dark. Fitted data from equivalent circuit provides the value of each element, from which the bulk conductivity, back contact barrier height, as well as spatial inhomogeneities within the cell are extracted. Impedance spectroscopy analysis shows that there are negative and positive effects of back surface treatment in oxygen free ambient on device performance. For oxygen-free treatment, an obvious increase in the bulk conductivity is observed, suggesting an increased copper doping in the device. Additionally, ZMO devices show less junction inhomogeneity. All these improvements lead to better device performance of ZMO/CdTe solar cells.