Investigation of buffer layers in copper indium gallium selenium sulfur solar cells

The purpose of the buffer layer (intrinsic wide bandgap semiconductor) in copper indium gallium selenium sulfur (CIGSS) solar cells has not been adequately explained in the literature; it is the goal of this dissertation to understand this purpose. New experiments completed in the course of this study have shown that materials from the top-conducting oxide layer, ZnO:Al, diffuse into the CIGSS absorber, decreasing lifetimes and degrading solar cell performance. Electronic characterization performed on ZnO:Al/CdS/CIGSS and ZnO:Al/CIGSS cells show that multi-step tunneling is a dominant loss mechanism in these structures without the buffer layer (called direct cells). The direct cells devices also had substantially less built-in voltage because of the formation of surface states or a high concentration of aluminum at the CIGSS surface. Some elementary calculations suggest that aluminum and zinc observed in the CIGSS absorber can shorten lifetimes and act as a recombination center. Based on these observations, a model of the buffer layer is discussed.