XPS, TEM and NRA investigations of Zn(Se,OH)/Zn(OH) films on Cu(In,Ga)(S,Se) 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).