Role of surface state on the electron flow in modified TiO 2 film incorporating carbon powder for a dye-sensitized solar cell

Abstract

An investigation of surface-related traps in nanostructured TiO 2 films modified by the incorporation of carbon powder was conducted by the potential-step chronoamperometric method. For the modification of the morphology and surface state of the nanoporous TiO 2 electrode, the incorporation of carbon into the white TiO 2 powder was accomplished. In the chronoamperometric data, all of the transients showed an initial fast phase (<1 s) followed by a slower phase which is related to the trap filling process. The trap-filling period of the carbon incorporated TiO 2 film becomes longer, as the applied negative potential increases, due to the widely distributed traps induced by the increased surface area. Furthermore, the film capacitance was derived as a function of the applied bias by integrating the current to time curves of the chronoamperometric data. The accumulated charge of the carbon incorporated TiO 2 film increases prominently in two regions. The dominant increase shown in the positive region (−0.7 to −0.9 V vs. Ag/AgCl at pH 13) of the flat band potential implies that the electron occupancy in the surface-related traps is increased. At a more negative potential (below −1.2 V vs. Ag/AgCl), electrons from the conduction band of the TiO 2 film substantially influence the total current, thereby inducing an exponential increase in the current. Therefore, it is found that most of the traps are located in the positive region of the flat band potential, since the Fermi level of the nanostructured TiO 2 film is positioned at −1.14 V vs. Ag/AgCl at pH 13. The trap sites in the sub-bandgap region of the TiO 2 film are important in the electron transport of photoinjected electrons from dye molecules and partially charge recombination with redox electrolyte in operating dye-sensitized solar cell. The influence of charge trap formed by increased surface states on the electron transport and electron transfer was investigated by photovoltage and photocurrent transient measurements.