On the effect of Al2O3 blocking layer on the performance of dye solar cells with cobalt based electrolytes

Dye solar cells with ${\text{Co}}^{\left(\text{III}\right)}/{\text{Co}}^{\left(\text{II}\right)}$ redox mediators have been prepared. To obtain higher conversion efficiencies, the recombination between photoinjected electrons and ${\text{Co}}^{\left(\text{III}\right)}$ species was minimized by deposition of a thin ${\text{Al}}_2{\text{O}}_3$ blocking layer over the mesoporous ${\text{TiO}}_2$ surface. Measurements of current-voltage characteristic curves, both under illumination and in dark conditions, together with electrochemical impedance spectroscopy demonstrate the great effectiveness of the addition of a blocking layer in cells containing cobalt based electrolyte, by substantially reducing the recombination current. The consequent power conversion efficiency increase is more than double, passing from 0.94% to 2.48% under $300\text{W}{\text{m}}^{-2}$ AM 1.5 illumination.