The role of Cd and Ga in the Cu(In,Ga)S2/CdS heterojunction studied with X-ray spectroscopic methods

Photovoltaic cells with the structure Glass/Mo/Cu(In,Ga)S2/CdS/i-ZnO/n+-ZnO are currently among the most successful and promising thin-layer solar cells. In this system, the Cu(In,Ga)S2 (CIS) acts as the absorber, the CdS as the buffer layer and the ZnO as the window layer. The goal of this work is the investigation of the Cu(In,Ga)S2/CdS semiconductor heterojunction both as a component of the solar cell and as a separate material system. The characteristics of this junction were investigated both during junction formation through chemical bath deposition (CBD) and after the junction was completed. It is currently thought that the Cu(In,Ga)S2/CdS junction is responsible for several different properties of the solar cell: lattice matching and band offset optimization between the absorber and window layer and chemical passivation of the absorber surface by the CBD-CdS process on CIS which acts to reduce the surface defect density. The Cd may also pin the Fermi Level on the CIS surface or cause a type inversion of the absorber surface from p-type to n-type. In order to investigate the junction several new methods were used along side the conventional methods of X-ray, Ultraviolet and Inverse Photoelectron Spectroscopy. These were Near-UV Constant Final State Yield Spectroscopy for the measurement of the valence band offset at the interface between CIS and CdS and Near Edge X-ray Absorption Fine Structure to follow the development of the Cu(In,Ga)S2 conduction band edge with increasing Ga concentration. Additionally, the advantages and disadvantages of the established and new methods were compared and discussed. It was discovered that the deposition of CdS neither pins the Fermi Level on the CIS surface at a position important for the solar cell, nor does it dope the absorber surface, although the deposition does lead to the formation of a Cd-containing CIS surface layer (CIS:Cd). Because this surface layer is not soluble in HCl it cannot be CdS as this is readily dissolved by HCl. Also, because it is thought that Cd will replace the cations in the CIS lattice, most likely Cu, the resulting Cd-S bonds in CIS:Cd will be different from those in CdS. Further experiments could not exclude the possibility that Cd diffuses into the CIS. However, it was shown that Cu from the absorber diffuses into the buffer layer during junction formation, although the Cu does not reach the surface of a full ∝35 nm thick CdS layer. The valence band offset between Cu(In,Ga)S2 and CdS was independent of Ga concentration and had a value 1.35 eV±0.20 eV. However, the position of the conduction band did show a dependence on Ga and moved to lower binding energies with increasing Ga concentration. ∝8% Ga on the absorber surface opened the Cu(In,Ga)S2 band gap by ∝150 meV when compared to CuInS2. Although the opening of the band gap exacerbates the conduction band offset in the CIS/CdS junction, the inclusion of Ga increases the open circuit voltage of the solar cell by ∝100 mV. (orig.)