n-CuInSe2 Photoelectrochemical Cells

The results of a wide research program, aimed at improving the conversion efficiency of n-CuInSe₂/liquid electrolyte solar cells, while maintaining the stability exhibited by this material in aqueous polysulfide, are described. Aqueous polyiodide is chosen as the electrolyte, on the basis of comparative studies with a number of aqueous and non-aqueous electrolytes. By making the polyiodide slightly acidic, both basic and acid electrolyte photo-oxidation can be avoided. Br₂/MeOH is shown to be a good etchant for n-CuInSe₂ and optimum conditions for its use are determined. Its chemical and physical effects studied. Chemically, a near-stoichiometric top layer is removed, leaving an O-rich surface. Physically, this treatment removes Fermi level pinning, possibly due to Se-related defect states. Subsequent air oxidation further improves cell performance and conditions for it are optimized. Chemically this treatment leads to formation of In-O bonds. Evidence is found for a low conductivity top layer, and for a ten-fold decrease in doping density, as compared to the etched sample, which is sufficient to explain the improved performance. Only the chemical etch has a significant effect on the charge collection efficiency, as measured by EBIC. From electrochemical decomposition, solid state chemical and surface composition studies we formulate stabilization strategies: adding Cu⁺ and In³⁺ to the solution and forming and additional indium oxide film on the electrode surface. In this way, stable, and 12% efficient cells are formed.