XPS, TEM and NRA investigations of Zn(Se,OH)/Zn(OH)2 films on Cu(In,Ga)(S,Se)2 substrates for highly efficient solar cells

Structural and compositional properties of Zn(Se,OH)/Zn(OH)2 buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)2 (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)2 as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)2 grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)2 structure on CIGSS. Part of the Zn(OH)2 in both layers (i.e. the Zn(Se,OH) and the Zn(OH)2 layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)2) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)2/CIGSS devices exceed 14% (total area).