Dye-sensitized solar cells: spectroscopic evaluation of dye loading on TiO2

Abstract

A new spectroscopic tool for the quantitative evaluation of dye-sensitizer uptake into TiO₂ semiconductor films for application in DSCs is presented, here applied to N719 dye and TiO₂-dye dipping times spanning from a few seconds up to 24 h. By the integration of the whole absorption spectrum in the Vis range or of the most significant low-energy peak, a conversion table could be built to be used as a reliable tool for evaluating the amount of adsorbed dye from the spectral response of the dye-coated films, independently of any spectral modification of the response with dipping time (indeed observed). In particular for N719 under the investigated conditions, we have demonstrated that the first hour is strategic for an effective uptake of the dye by the TiO₂ nanoparticles, with a saturation level reached within the first 5 h, suggesting that commonly used longer dipping times, typically 16–24 h, are useless or even undesirable when making DSC devices. In addition, we have found that the whole adsorption process, from few seconds up to 24 h dipping time, is very nicely described by a simple pseudo-first order kinetic model, in contrast with previous findings, with an observed rate constant of 3.1 × 10⁻⁴ s⁻¹.