XXXIX. Techniques for Stabilization of n-Silicon Electrodes in Aqueous Solution Photoelectrochemical Cells

A number of different approaches have been used in the stabilization of small band gap n-type semiconductors against photocorrosion in photoelectrochemical (PEC) cells. Such stabilization is necessary in the design of practical cells for conversion of solar energy to electricity, and is of critical importance in photoelectrosynthetic systems where the photogenerated holes produce species (e.g. 02, C12) at quite positive potentials at the semiconductor surface, i) One approach involves the utilization of thin films of metals (I) or semiconductors (2) to protect the surface. For example, n-GaAs electrodes can be stabilized o in a solution of Fe(II) EDTA by a thin (~60A) film of gold (3). 2) Use of nonaqueous solvents (4) in PEC cells has been shown to aid in the stabilization of the semiconductor electrode by decreasing the extent of solvation of the oxidation products. A similar approach involves the use of concentrated electrolytes in aqueous solutions. Wrighton and co-workers (5) have recently demonstrated that n-MoSe 2 and n-MoS 2 electrodes are stable in concentrated LiCI solutions even in the presence of chlorine evolution. 3) The deposition of polymer layers on the semiconductor surface can also decrease photocorrosion. Noufi and co-workers (6) have described the stabilization of n-GaAs and n-Si by the electrodeposition of polypyrrole films on the surface. We demonstrate here that by combining these approaches even better stabilization can be accomplished and describe the photo-oxidation of Fe 2+ at n-Si electrodes covered by thin gold and thicker polypyrrole film and immersed in a concentrated electrolyte aqueous solution. Electrodes were fabricated from n-Si single crystals (0.4 to 0.6 ~ cm) donated by Texas Instruments. Ohmic contacts were made with In-Ga alloy. They were connected to a copper wire with silver epoxy cement. The crystals were mounted in a glass tube with all sides insulated with 5 min epoxy and covered by silicone rubber sealant leaving an area of 0.2-0.5 *Electrochemical Society Active Member.