Formation of Blocking Contacts at p-Type Porphyrin/ITO or Electrolyte Junctions

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Abstract
Energetics of ITO/p-type porphyrin/electrolyte interfaces are discussed on the basis of photoelectrochemical and photovoltaic measurements. When sensitized by a layer of 5,10,15,20-tetraphenylporphyrinatomagnesium(II)(MgTPP), the indium-tin oxide (ITO) electrode acts as only a photocathode in an aqueous electrolyte solution. However, the ITO electrode modified with a film of ZnTPP, CuTPP or H2TPP having oxidation potential more positive than that of MgTPP acts as not only a photocathode but also a photoanode, depending on the electrode potential and the direction of irradiation on the ITO/porphyrin electrode. The cathodic component in the photocurrents is attributed to the downward band-bending in a space charge layer at the porphyrin/electrolyte (blocking) contact and the anodic component to that at the ITO/porphyrin (blocking) contact. The upward band-bending, on the contrary, is found inefficient in producing charge carriers, because the accumulation layer is conducting (ohmic) for majority carriers (holes). The porphyrin/electrolyte junction shows certain similarities to an ITO or metal/porphyrin (semiconductor) junction. However, it should be noted that a Helmholtz double layer exists at the porphyrin/electrolyte interface. The results are rationalized from the energy levels of the porphyrin films, ITO and the redox couple solutions with due regard to the orientation of water dipoles at the porphyrin surface.