Characterization of Intrinsic Amorphous Hydrogenated Silicon as a Thin-Film Photocathode Material. Efficient Photoreduction Processes in Aqueous Solution

Intrinsic amorphous hydrogenated silicon, a-Si:H, has been characterized as a thin-film (8000 Á) a-Si:H material. The intrinsic a-Si:H is deposited onto a 200-Å thick, heavily B-doped, layer of a-Si:H on stainless steel. The 200-Á p⁺-a-Si:H layer ensures a back contact to the valence band to give as large a field across the 8000-Á intrinsic layer as possible when the intrinsic layer is contacted on the other side by an electrolyte solution containing a redox couple. The a-Si:H photocathodes give good photovoltages in aqueous and nonaqueous media, up to 855 mV, depending on the E_1/2 of the redox couple in contact with the electrode. A plot of photovoltage vs. E1/2 shows a slope of only 0.42. A zero photovoltage is extrapolated to obtain for E_1/2 no more negative than +0.9 V vs. SCE; the photovoltage is constant for E_1/2 more negative than ~-0.8 V vs. SCE. The slope of less than 1.0 suggests a deleterious role for states situated between the valence and conduction bands of a-Si:H. The surface of a-Si:H can be derivatized with an N,N′-dialkyl-4,4′-bipyridinium reagent followed by deposition of Pd or Pt to effect H₂ evolution at an electrode potential up to ~700 mV more positive than on a conventional electrode. The durability and photovoltage of a-Si:H photocathodes are superior to those of single-crystal p-Si photocathodes, but the wavelength response, rectángularity of current-voltage curves, and the quantum yield for electron flow offset the advantages of the thin-film photocathode. The sustained energy conversion efficiency for 632.8-nm light to electricity or H₂ is about the same for a-Si:H and single-crystal p-Si under the same conditions. Importantly, constant output of electricity for 50 h at ~3.2 mA/cm² has been obtained from a-Si:H photocathode-based cells employing an aqueous Eu^3+/2+/KCl redox couple/electrolyte combination.