Abstract
The photoanode of dye-sensitized solar cell is composed of polycrystalline n-type oxide semiconductors, including nanostructured TiO2 or ZnO and transparent F-doped SnO2 electrode. The functions of nanostructured layers are to adsorb dye and to transport injected photo electrons. The analysis on electron transport in photoanode according to the energetic perspective illustrates that the interfacial Schottky barrier and grain boundary back-to-back Schottky barrier which restrict electron transport theoretically exist in the photoanodic materials. The former results from the difference of band structure between the nanostructured layer and the transparent electrode, and the latter is attributed to the nature of polycrystalline oxide semiconductor in the nanostructured layer. In order to promote the electron collection efficiency in photoanode, it is necessary to suppress the influence of the energetic barriers on electron transport.Introducing transparent Al-doped ZnO electrode into photoanode could eliminate the interfacial Schottky barrier. By using sol-gel method with the three-steps annealing procedure, it succeeded in preparing the Al-doped ZnO film with high transmittance and low resistivity. The origin of electric conduction of sol-gel derived AZO films is verified as the combining effect of the high temperature annealing to enhance crystal quality that provides higher mobility of electrons and the reduction annealing to release the localized electrons caused by oxygen absorption. According to the deductions from near infrared transmittance spectra based on Drude model, an effective method to improve electronic conduction is obtained. By modifying the preheating procedure, the grains of Al-doped ZnO films become smaller and more defected, resulting in better electrical conductivity. The results of impedance analysis demonstrated that the function of hydrogen annealing is to destroy the energetic barriers at grain boundaries and releases free electrons which are evidenced by the presence of parallel resistor-capacitor circuit, constant phase element and short Warburg element in equivalent circuits. It manifests that the energetic barrier which restrict electron transport is able to be destroyed.The existence of grain boundary barriers in the polycrystalline oxide semiconductors applied to photoanodic nanostructured layer (mesoporous TiO2, mesoporous ZnO, and nanowired ZnO) is verified by preparing samples with adequate structures and employing precise impedance analysis. The formation of the grain boundary barriers is dominated by material structural characteristics, such as density of lattice defects, crystalline orientation, and grain sizes. Three types of grain boundary barriers are observed in this study. The analysis for the samples with different sintering temperature showed that the characterization of grain boundary barrier is governed by the depth of depletion region determined by the grain size and defect density at grain boundary. Furthermore, it is also manifested that hydrogen annealing could destroy the grain boundary barriers which restrict injected photoelectron transport in the photoanodic nanostructured layer via impedance spectroscopy. The destruction of barriers seems to increase the short current of dye-sensitized solar cell.