Improvement of electron transport in DSSCs by using Nb-doped TiO2 electrodes

The performance of dye-sensitized solar cells (DSSCs) is closely related to efficiency of the electron transport within TiO₂-dye-electrolyte system. Electron transport can be improved by modification of the electronic structure of TiO₂ electrode by doping with niobium (Nb⁺⁵). For this purpose, the DSSCs based on undoped and Nb-doped TiO₂ layers were fabricated and their PV parameters and electrical properties were studied. The Nb-doped TiO₂ was prepared by a sol-gel method followed by a hydrothermal treatment with the Nb content in the range of 0.7 to 3.5 mol%. The transport properties of the TiO₂-dye-electrolyte junction were investigated using the electrical impedance spectroscopy. The electron lifetimes, estimated from Bode plots, were found to increase from 8 ms for DSSCs based on undoped TiO₂ up to 26 ms for the cells based on TiO₂ doped with 2.7 mol% of Nb. We have shown correlation between the parameters of the charge carrier transport and the main DSSC photovoltaic characteristics. The increase in the value of electron lifetime was shown to enhance the value of short circuit current (J_sc). When the Nb doping level was lower than 1.7 mol%, the electrical resistance at the TiO₂-electrolyte interface increased leading to rise of the value of the open circuit voltage (V_oc). Doping with the 1.7 mol% of Nb led to increase of both J_sc and V_oc, and significantly improved the device efficiency.