One-dimensional (1-D) structured TiO2
nanorods show improved electrical and optical properties in the photoelectrodes of dye-sensitized solar cells (DSSCs)
]. They can provide straight moving paths for electrons and reduce the e−
]. Further, they scatter sunlight so that the incident light stays longer in the cell
]. As these properties enhance the solar energy conversion efficiency, much research into the effects of the 1-D structured TiO2
on the photoelectrode have been conducted
In principle, photoexcited electrons from dye molecules move on a TiO2
nanocrystal undergoing a series of trapping and de-trapping events during diffusion. The 1-D nanorods, which are densely packed TiO2
nanoparticles, could act as a single crystal and be involved in rapid electron transport, thereby reducing the chances for electron recombination. Furthermore, the TiO2
film with random packing of 1-D rods helps the electrolyte to penetrate into the photoelectrode because of the porosity
]. The enhanced interpenetration of electrolyte leads to the dye regeneration by redox process of the electrolyte and enhances the energy conversion efficiency with improved photocurrent.
Few grain boundaries in the TiO2
nanorods induce fast electron transport and decrease the electron recombination due to the reduced number of trapping sites in the interfaces
]. In order to reduce grain boundaries in the nanorods, the crystal size should be increased. TiO2
crystal structure (anatase and rutile) and size can be controlled by sintering temperature. The anatase phase has been reported to be developed at temperatures below 800°C, and above the temperatures, it transforms to the more stable rutile phase
]. Also, the TiO2
nanorods sintered at a high temperature have high crystallinity, meaning reduced grain boundaries and decreased trap sites. Electrons moving through the rutile structure undergo less stress because of the reduced number of trap sites on the grain boundaries
]. In addition, the transported electrons can easily migrate from the rutile to anatase phase
]. As the conduction band of the pure anatase phase is typically 0.2 eV more negative than that of the rutile phase, photoexcited electrons injected into the rutile phase migrate to the conduction band of the anatase phase, before passing through the external circuit. The resulting synergistic effects between the anatase and rutile phases lead to energetic electron flows and enhanced photocurrents
However, even though the rutile 1-D nanorods provide the electrons with a better moving path and improve electrolyte penetration, a large number of rutile phases simultaneously can become a barrier for electron transport
]. The increased amount of rutile phase increases the probability of the moving electrons facing a higher energy level, which increases the internal resistance.
In this study, in order to make photoelectrodes with the 1-D rutile nanorods, the electrospun TiO2 nanofibers were sintered at various temperatures. The photoelectrodes considerably improved the DSSC energy conversion efficiency, depending on the amount of TiO2 nanorods. The intensity-modulated photocurrent spectroscopy, intensity-modulated photovoltage spectroscopy, charge-transfer resistance, and I-V characteristics of the DSSCs were investigated in order to study the effects of the rutile TiO2 nanorods on the cell performance. The purpose of this study is to investigate the effects of the crystal size and amount of the rutile TiO2 nanorods on the electron transport in the photoelectrodes of dye-sensitized solar cells.