• Energy Research

In the search for a nontoxic replacement of the commonly employed CdS buffer layer for Cu(In,Ga)(S,Se) $_\mathrm{2}$ based solar cells, chemically deposited Zn(O,S) thin films are a most promising choice. In this paper, we address the usually slow deposition speed of Zn(O,S) in a newly developed ammonia-free chemical bath process, resulting in a deposition of 30 nm in 3 min with good homogeneity on 30 cm × 30 cm sized substrates. Solar cells with buffer layers prepared from this process match the efficiency of CdS reference cells. In a second step, we address the light-soaking post-treatment, still needed for maximum efficiencies. By addition of aluminum to the deposition process, the initial efficiencies can be increased slightly. With the addition of boron, the light-soaking post-treatment is rendered unnecessary, while maintaining high efficiencies above 15%, surpassing reference cells with CdS buffer.

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.