Bandgap narrowing of iron oxide nanotubes upon doping with zinc and their spectral sensitivity used as photoelectrode
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Bandgap narrowing of iron oxide nanotubes upon doping with zinc and their spectral sensitivity used as photoelectrode Y. Kosugi1, T. Tomiyasu1 and S. Bandow2 1 Department of Materials Science and Engineering, and 2Department of Applied Chemistry, Meijo University, 1-501 Shiogamaguchi, Tenpaku, Nagoya, Aichi 468-8502, Japan ABSTRACT Iron oxide nanotubes (NTs) and zinc doped iron oxide NTs were prepared by sol-gel method. Obtained NTs have spinel structure represented by ZnGFe3-GO4. Iron oxide NTs indicated a spectral sensitivity maximum at the wavelength of 500-550 nm and a long tail toward red light. These features were explained by a typical bandgap of iron oxide NTs to be ~2.3 eV and bandgap narrowing for a small diameter NT. Organometal perovskite solar cell was constructed by using these NTs and evaluated by measuring the spectral sensitivity between the wavelength of 390 and 710 nm. For the present perovskite solar cell, drastic increase of the spectral sensitivity was obtained when we adjusted penetration depth of organometal perovskite into the porous layer of NTs. However, deep penetration worsened the spectral sensitivity. A similar trend was obtained for the cell using Zn doped iron oxide NTs, but the magnitude of incident power conversion efficiency (IPCE) became almost a half. This was explained by worsening the electrical conductivity, even the bandgap became narrower by Zn dope. INTRODUCTION Iron oxides are chemically stable and nontoxic materials, which widely exist around our living space. Because the elements of Fe and O are abundant on the Earth. This means that iron oxide materials are quite feasible for application with low cast and environment friendly. In addition, a part of Fe is easily substituted by other metal elements, forming many composites represented by MFe2O4, where M represents other metal elements, such as Mn, Co, Ni, Cu and Zn, etc., M4Fe12O22 (M: Ba, Mg, Zn etc.) and M3Fe5O12 (M: rare-earth elements), so on. These iron oxides exhibit a wide range physical properties covering with magnetism, ferroelectricity [1] and water splitting [2], etc. Regarding the conversion of sunlight to electric energy, TiO2 is widely used as photoanode for the dye sensitized solar cell. The bandgap of TiO2 is ~3.2 eV [3], so that the electron excitation from the valence band to conduction band needs the energy of ultraviolet light region. On the other hand, iron oxide has rather narrow indirect bandgap of ~2.2 eV [3], which is in the visible light region of sunlight, and hence it has ability to absorb a large part of the solar spectrum. However, the problem is that a short hole diffusion length in iron oxide. That is, the holes created near the interfacial region arrow to transport to the electrolyte, and most of the holes created would recombine with electrons in a short time. One approach to suppress such electron-hole recombination is to deposit Co-phosphate [4]. Another simple approach, we consider, is to make nanoscale tubular shaped iron oxide, which has internal and external surfaces and the elec
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