Impact of chromium doping on physical, optical, electronic and photovoltaic properties of nanoparticle TiO₂ photoanodes in dye-sensitized solar cells

Herein, we report the impact of chromium doping on the physical properties, electronic band structure and photovoltaic characteristics of their corresponding devices based on dye-sensitized solar cells (DSCs). Different DSCs with various compositions of the photoanode electrodes are fabricated to study their optical, structural, conduction band edge, donor density, depth of trap states and photoelectrochemical properties using X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), field-emission scanning electron microscopy (FE-SEM), Mott-Schottky capacitance analysis, chemical capacitance and electrochemical impedance spectroscopy (EIS) measurements. For such studies, the Cr-doped TiO₂ films, with different Cr : Ti atomic ratios of 0.1-5% and a particle size in the range 15-30 nm, are synthesized by a facile hydrothermal process. We find that on introduction of Cr³⁺ into the TiO₂ lattice, the films exhibit small band gap, an upward shift in the conduction and valence band edges, and a low donor density. The results show that the Cr-doped TiO₂ electrodes have a lower recombination rate, a superior electron lifetime and a higher open circuit voltage than pristine TiO₂. However, these electrodes show a poor dye adsorption due to the surface defects of Ti³⁺ and the charge transport characteristics, resulting in a low short-circuit current. Furthermore, doping of TiO₂ with chromium may induce a high density of empty trap states with a deep position, leading to an entrapment of the charge carriers, a drop in electron conductivity and, therefore, an inferior photovoltaic performance of the solar cells.