A backwall superstrate device structure that outperforms conventional substrate Cu(In,Ga)Se 2 devices for thin absorbers is described. The backwall structure of glass/ITO/MoO 3 /Cu(In,Ga)Se 2 /CdS/i-ZnO/Ag utilizes a MoO 3 transparent back contact to allow illumination of the device from the back. The device performance has been improved by modifying the Cu(In,Ga)Se 2 , including alloying with Ag to form (AgCu)(InGa)Se 2 absorber layers. In addition, sulfized back contacts including ITO-S and MoS 2 are compared. Interface properties are discussed based on the XPS analysis and thermodynamics of reactions.Index Terms-(AgCu)(InGa)Se 2 , backwall structure, solar cell, thin film, transparent back contact.