Valence Band-Edge Engineering of Nickel Oxide Nanoparticles via Cobalt Doping for Application in p-Type Dye-Sensitized Solar Cells

We have systematically studied the effects of substitutional doping of p-type nanoparticulate NiO with cobalt ions. Thin films of pure and Co-doped NiO nanoparticles with nominal compositions CoxNi1-xOy (0 <= x <= 0.1) were fabricated using sol-gel method. X-ray photo-electron spectroscopy revealed a surface enrichment of divalent cobalt ions in the CoxNi1-xOy nanoparticles. Mott-Schottky analysis in aqueous solutions was used to determine the space charge capacitance values of the films against aqueous electrolytes, which yielded acceptor state densities (N-A) and apparent flat-band potentials (E-fb). Both N-A and E-fb values of the doped NiO were found to gradually increase with increasing amount of doping; thus the Fermi energy level of the charge carriers decreased with Co-doping. The photovoltage of p-DSCs constructed using the CoxNi1-xOy films increased with increasing amount of cobalt, as expected from the trend in the E-fb. Co-doping increased both carrier lifetimes within the p-DSCs and the carrier transport times within the nanoparticulate semiconductor network. The nominal composition of Co0.06Ni0.94Oy was found to be optimal for use in p-DSCs.