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

Abstract

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 ${\text{{[}In{]}/({[}In{]}+{[}Zn{]})}=0.5}$ 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.