Improved Chemically Deposited Zn(O,S) Buffers for Cu(In,Ga)(S,Se)₂ Solar Cells by Controlled Incorporation of Indium

Chemically deposited Zn(O,S) is one of the most promising materials replacing the commonly employed CdS buffer layer for Cu(In,Ga)(S,Se) ₂ solar cells. While yielding higher short-circuit currents, Zn(O,S) buffered cells commonly show initially low fill factors and open-circuit voltage. Different posttreatments (annealing, light-soaking) have to be employed to reach high efficiencies. In this paper, we introduce a method for controlled incorporation of indium into Zn(O,S) buffer layers up to [In]/([In]+[Zn]) = 0.5 within an alkaline process. Solar cells with such a new indium containing buffer layer show higher initial fill factor and open-circuit voltages, leading to efficiencies above 14% without extensive light-soaking. Photoelectron spectroscopy measurements are used to determine the composition of these zinc indium oxysulfide thin films and to extract the valence band alignment with the underlying Cu(In,Ga)(S,Se) ₂ substrate. Calculated conduction band offsets show a decrease of an initially high conduction band offset between Cu(In,Ga)(S,Se) ₂ and buffer layer upon indium incorporation, lowering the barrier for current transport and, thus, giving a reason for the improved solar cell behavior. We demonstrate a novel cadmium-free buffer layer material with wider bandgap than CdS, which is produced in a simple chemical bath process and yields efficiencies comparable with CdS buffered cells.