Solar cells have attracted attention as environmentally friendly power-generation devices. Silicon-based semiconductors utilizing p-n junctions are widely known as such solar cells. However, the manufacture of silicon-based solar cells requires high vacuum and high temperatures, making it difficult to reduce costs, preventing practical use of silicon-based solar cells.
With an expectation for the development of lower-cost solar cells, Graetzel et al. reported dye-sensitized solar cells wherein titanium dioxide or the like that is modified with a dye is used as an active electrode (see Patent Literature 1). Dye-sensitized solar cells have attracted attention as solar cells that can be readily manufactured at low cost.
However, further improvement in the performance of dye-sensitized solar cells is presently required, for example, in terms of the electron conduction of titanium oxide used as an active electrode.
Titanium oxide nanoparticles are generally known to exhibit high performance as an active electrode. The use of nanoparticles is intended to provide a large area for a dye that is adsorbed on the titanium oxide, thereby efficiently absorbing incident light. However, the formation of a film of spherical nanoparticles involves the following trade-offs: the presence of an interface between the particles precludes efficient transfer of charge-separated electrons; and the narrow gap between neighboring particles makes the transfer of ions in the electrolytic solution near the nanoparticles difficult, thereby precluding the accompanied transfer of electrons.
Thus, there has been a need for titanium oxide that enables efficient transfer of both electrons and ions as an active electrode for dye-sensitized solar cells.
In view of this problem, cases where titanium oxide in the form of nanowires is used as active electrodes have been reported (see Non-Patent Literatures 1 and 2). However, because these nanowires are only composed of titanium oxide having a flat surface, dyes cannot be deposited thereon in an amount sufficient to absorb incident light; thus, there has been a need for an increased current density.
Patent Literature 2 investigates an active electrode wherein a titanium oxide coating is formed on carbon tubes whose lengthwise direction is arranged substantially perpendicular to the film-formation surface of a substrate. This is done in order to improve the conductivity of a current flowing in the lamination direction of films, thereby providing increased electron conductivity in the titanium oxide and efficient transfer of electrons from the titanium oxide to the electrode. However, the active electrode of Patent Literature 2 has the problem of an increased leakage current, possibly due to the difficulty in forming a uniform titanium oxide coating.
Patent Literature 3 teaches mixing an active material, i.e., oxide particles, with carbon nanotubes. However, if the carbon nanotubes are not sufficiently coated with the active material oxide, the leakage current will increase, resulting in the problem of reduced power-generation efficiency.