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

Article abstract of DOI:10.1021/ja00351a015

Intrinsic amorphous hydrogenated silicon, a-Si:H, has been characterized as a thin-film (8000 Angstroem) a-Si:H material.The intrinsic a-Si:H is deposited onto a 200-Angstroem thick, heavily B-doped, layer of a-Si:H on stainless steel.The 200-Angstroem p⁺-a-Si:H layer ensures a back contact to the valence band to give as large a field across the 8000-Angstroem 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.E_1/2 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 ca. -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 ca. 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, rectangularity 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 ca. 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.