Improved Chemically Deposited Zn(O,S) Buffers for Cu(In,Ga)(S,Se) 2 Solar Cells by Controlled Incorporation of Indium
2016
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.
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