Photoelectrochemical Cells Based on Amorphous Hydrogenated Silicon Thin Film Electrodes and the Behavior of Photoconductor Electrode Materials

The behavior of intrinsic a-Si:H photoanodes has been investigated. Thin films (∼3.9 µm) of intrinsic a-Si:H, on a stainless steel substrate coated with a very thin (∼200 Å) heavily n-type doped a-Si:H layer for ohmic contacting, behave as electrodes only under ≥E_g = 1.7 eV illumination, irrespective of the E^o′ of the redox couple investigated in the range −1.78 to +0.41 V vs. SCE in CH₃CN/0.1 M [n-Bu₄N]ClO₄. The nature of the contact, stainless steel/n-type a-Si:H/intrinsic a-Si:H, indicates that redox couples having E^o′ more positive than the bottom of the conduction band, E_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. Electrochemical measurements establish the E_CB position in EtOH/ or CH₃CN/0.1 M [n-Bu₄N]ClO₄ to be at ∼−0.7 ±0.1 V vs. SCE. For E^o′ more negative than −0.7 V vs. SCE, fast, one-electron, outer-sphere redox couples at ∼ 1 mM concentration respond to the illuminated a-Si:H as they do at Pt. For E^o′ more positive than −0.7 V vs. SCE, the oxidation of the reduced form of the redox couple can occur at a more negative position than at Pt. In cyclic voltammetry experiments, the extent to which the oxidation peak is more negative than at Pt is a measure of the photovoltage, E_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 (∼ 1 mM) concentrations of redox reagents used in cyclic voltammetry, and the largest E_v found is ∼0.4 V at ∼40 mW/cm² of 632.8-nm light. Derivatization of a-Si:H with (1,1′-ferrocenediyl)dichlorosilane suppresses photocorrosion, and E_v of ∼0.75 V has been obtained for this surface-confined ferrocene derivative, exceeding the E_v on single-crystal n-type Si photoanodes. Durable cells using a Pt cathode, the a-Si:H photoanode, and a EtOH/0.1 M [n-Bu₄N]ClO₄/10⁻³ M ferricenium/0.07 M ferrocene solution have been studied. The E_v seen is up to 0.75 V at 9.3 mW/cm² of 632.8-nm light. Constant output for ∼100 h has been observed with an optimum efficiency of ∼4.5% for conversion of 0.7 mW/cm² 632.8-nm light to electricity. This overall efficiency is slightly larger than that obtained for similar cells based on single-crystal n-type Si photoanodes.