Abstract: A solar cell comprises a substrate, a first transition metal layer comprising an alkali element or an alkali compound located over the substrate, a second transition metal layer comprising gallium located over the first transition metal layer, at least one p-type semiconductor absorber layer including a copper indium selenide (CIS) based alloy material located over the second transition metal layer, an n-type semiconductor layer located over the p-type semiconductor absorber layer, and a top electrode located over the n-type semiconductor layer.