POLYCRYSTALLINE N-CUINSE2 PHOTOELECTROCHEMICAL SOLAR-CELLS - ELECTROCHEMISTRY AND DEPOSITION OF COPPER AND INDIUM AT THE PHOTOANODE AND THE ACCOMPANYING EFFICIENCY ENHANCEMENT

The electrochemical behaviour of polycrystalline n-CuInSe2 (CIDS) with respect to removal of oxygen, anodic prepolarization, and deposition and stripping of copper and indium in aqueous KCl solution has been examined. Oxygen contributed to cathodic currents, while anodic prepolarization caused the appearance of a Cu2+/Cu couple, The reduction of the copper ion (Cu2+) to metallic copper at CIDS was found to be quasi-reversible and diffusion-controlled. Initially, deposited copper facilitated indium deposition at the CIDS electrode. Aged CIDS as well as CIDS deposited with copper underwent stripping (oxidation) with low currents at more negative potentials which could be prevented by codeposition of copper and indium. Further, deposition of copper, or copper and indium on CIDS was observed to improve the charge transfer at the CIDS/electrolyte interface. These results were applied for the fabrication of CIDS based photoelectrochemical solar cells employing polysulphide redox electrolyte. It was found that codeposition of copper and indium on CIDS electrode led to significant enhancement in the cell efficiency (rises to 11.5% from 3.4%) and slowed down the short-circuit current decay. A part (70-80%) of the decayed short-circuit current could be regained by renewing the CIDS electrode with codeposition treatment for the second time (redeposition). The possibility of applying deposition-redeposition treatment to regulate the output current was suggested. Previously unexplained cooperative effects of Cu+ and In3+ in polysulphide electrolyte on the cell efficiency were traced to the facilitated deposition of indium on the copper deposited CIDS electrode from the appropriate salt solutions.