Photoelectrochemical cells based on amorphous hydrogenated silicon thin film electrodes and the behavior of photoconductor electrode materials

Abstract

The behavior of intrinsic a-Si:H photoanodes has been investigated. Thin films (approx.3.9 ..mu..m) of intrinsic a-Si:H, on a stainless steel substrate coated with a very thin (approx.200 angstrom) heavily n-type doped a-Si:H layer for ohmic contacting, behave as electrodes only under greater than or equal toE/sub g/ = 1.7 eV illumination, irrespective of the E/sup 0/' of the redox couple investigated in the range -1.78 to +0.41 V vs. SCE in CH/sub 3/CN/0.1 M (n-Bu/sub 4/N)ClO/sub 4/. The nature of the contact, stainless steel/n-type a-Si:H/intrinsic a-Si:H, indicates that redox couples having E/sup 0/' more positive than the bottom of the conduction band, E/sub CB/, can result in a field across the intrinsic a-Si:H that will drive photogenerated holes to the surface in contact with the redox couple while the electrons excited to the conduction band are driven into the bulk. In cyclic voltammetry experiments, the extent to which the oxidation peak is more negative than at Pt is a measure of the photovoltage, E/sub v/, that could be obtained at open circuit in the presence of equal concentrations of the oxidized and reduced forms of the couple. Surface photocorrosion of a-Si:H is a problem at low (approx.1 mM) concentrations ofmore » redox reagents used in cyclic voltammetry, and the largest E/sub v/ found is approx.0.4 V at approx.40 mW/cm/sup 2/ of 632.8-nm light. Derivatization of a-Si:H with (1,1'-Ferrocenediyl)dichlorosilane suppresses photocorrosion, and E/sub v/ of approx.0.75 V has been obtained for this surface-confined ferrocene derivative, exceeding the E/sub v/ on single-crystal n-type Si photoanodes. Durable cells using a Pt cathode, the a-Si:H photoanode, and EtOH/0.1 M (n-Bu/sub 4/N)ClO/sub 4//10/sup -3/ M ferricenium/0.07M ferrocene solution have been studied.« less