Decoupling of transport, charge storage, and interfacial charge transfer in the nanocrystalline TiO₂/electrolyte system by impedance methods

Processes in the dark of electron transport and recombination in several nanoporous titanium dioxide films have been studied as a function of the applied potential, using the electrochemical impedance technique. Contact and bulk characteristics have been identified, decoupled, and interpreted, applying a transmission line model that identifies the following elements: (i) the capacitance of the interface between the exposed surface of the substrate and the electrolyte, (ii) the electron transport resistance, (iii) the charge-transfer resistance distributed in the TiO₂/electrolyte interface, and (iv) a distributed capacitive element related to charging the porous matrix. The model provides a satisfactory description of the spectra in widely different conditions of conductivity of the TiO₂ phase. The electron conductivity has been determined as a function of applied potential and coincides for the different samples under study. Classical electrochemical frameworks of transport and interfacial charge transfer used in porous electrodes do not explain the obtained parameters completely. A more complete framework is suggested to explain the system on the basis of the following characteristic features: (i) existence of a large density of electron traps, giving rise both to a trap-limited mobility and to an exponentially increasing capacitance, and (ii) band-edge shifts under external polarization potential.