Charge Transport Properties of ZnO Nanorod Aggregate Photoelectrodes for DSCs

Dye-sensitized solar cells (DSCs) have been intensively studied as a promising alternative to the conventional Si-based solar cells due to the their good features such as low cost and high power conversion efficiency. 3 The main component of the DSCs is the photoanode, which is composed of a highly porous semiconducting oxide network on which a close-packed monolayer of dye molecules is formed through surface adsorption. The photons are captured by the dye molecules, and electron hole pairs are generated. Subsequently, the electrons are injected into the conduction band of the semiconducting oxide and diffuse through the oxide network and, finally, are collected to the anode charge collector, typically FTO glass substrate. At the same time, the oxidized dye molecules are regenerated by electron transfer from iodide ions in the redox electrolyte, and the tri-iodide ions diffuse through the liquid electrolyte to the counter electrode, where they are reduced back to the iodide ions to complete the whole process. In addition to the photon capturing by the dye molecules adsorbed on the oxide that is largely determined by the surface area of the oxide network, the charge transport properties in the oxide network play a crucial role in the power conversion efficiency of DSCs after the electron injection. There are two processes the electrons experience in the oxide network: charge transport and recombination. So, lots of effort has been focused on the determination of electron’s lifetime and diffusion coefficient that represent properties of the recombination and charge transport, respectively. 6 Also, these two factors could be used to calculate electron diffusion length, Ln, which is an average distance the electrons in the oxide network can diffuse before recombining, so it determines charge collection efficiency, one of the key factors for high performance of DSCs. For efficient charge collection, Ln would be 3 times longer than the thickness of the oxide film. The nanostructured oxide, which is used to provide huge surface area for high dye loading, is sintered to form the oxide network that acts as a path for charge transport. So, the charge transport behavior would be greatly affected by sintered structures of the nanomaterials. The nanostructured or porous materials are sintered to reduce surface area and finally surface energy, and, generally, the materials become spherical or are fused together to reduce their surface area. As the sintering is a process of time, temperature, and diffusion characteristics of materials, a well-sintered structure for improved charge transport and high performance of DSCs can be possible by controlling these parameters, especially the temperature that is the most powerful factor for the sintering. ZnO has been studied as a photoelectrode material of DSCs due to its promising characteristics such as easy synthesis process and high electron mobility, and recently lots of progress has been reported. 12 One is the layered structure composed of ZnO nanoparticle layer and ZnO scattering layer, followed by the prolonged dye loading time and the decreased distance between the two electrodes. With the comparable photocurrent to the TiO2-based DSCs, the efficiency over 6% was achieved. The other is ZnO aggregate structure, which allows scattering of incident light within the ZnO film, resulting in increased photon traveling distance and has hierarchical porous structure to minimize