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.