Ternary Chalcogenide-Based Photoelectrochemical Cells VIII. Solution Composition Effects in n-CulnX₂(X = S, Se) Aqueous Polysulfide and Aqueous Polyiodide Cells

The photovoltaic characteristics of (photoelectro)chemically etched n-CuInX₂ crystals in basic polysulfide and in neutral polyiodide electrolytes were studied and compared. Because of the indium oxide-like nature of the n-CuInSe₂surface, dark current-voltage (I-V) measurements on In₂O₃ thin films were carried out, too. Photoelectrochemical etching improves the performance of chemically etched photoanodes, more so in polysulfide than in polyiodide. Surface analyses show that the former treatment leads to formation of elemental Se on the surface. Subsequent air oxidation yields a surface, which consists of Se° and In₂O₃. Solution effects of polysulfide and polyiodide solutions were studied. Polysulfide was modified primarily by changing its cation composition, and polyiodide, by addition of Cu⁺and/or H⁺. In polysulfide a strong positive cation effect is found on the photo I-V characteristics with Cs>K>Na>Li.A much weaker effect, with the same trend, is found in polyiodide, if dilute solutions are used. Measurements on In₂O₃ films show that their electrochemical behavior depends on the cation in the same way as found for CuInX₂. These effects are attributed mainly to differences in solution kinetics which stem from cationic activity trends in these solutions. CuInSe₂/polyiodide cells are superior, even to Cs-polysulfide ones, but no significant differences are seen with CuInS₂. This is explained by differences in the thermodynamic driving forces for electron transfer between these systems. Medium (4-5%) efficient CuInSe₂ electrodes show a positive temperature dependence of their photoresponse in polyiodide, similar to that in polysulfide. In the former electrolyte, furthermore, a memory effect is seen: after high temperature operation, some improvement persists for awhile, at room temperature. This might be due to trapping of Cu⁺in the Se top layer. It is concluded that, while the main difference between n-CuInSe₂ in polysulfide and polyiodide is a thermodynamic one, kinetic factors govern further optimization in both electrolytes.