Real Time Adsorption and Desorption Kinetics of Dye Z907 on a Flat Mimic of Dye-Sensitized Solar Cell TiO2 Photoelectrodes

A dye molecule monolayer formed on a TiO2 surface is a key component in Dye Sensitized Solar Cells. It is usually formed by adsorbing dye molecules from a solution. The dye layer should absorb as much solar light as possible and convert the light to photoelectrons, injected into the TiO2 conduction band. For that purpose the dye molecules should adsorb on TiO2 with the right molecular orientation, appropriate close packing to facilitate electron injection into TiO2 and to provide a barrier for charge recombination. The monolayer should also be stable towards desorption and bleaching over very long timescales. We measured adsorption and desorption kinetics in real time of dye Z907 on thin compact TiO2 films using Indirect Nanoplasmonic Sensing (INPS). From the results we derived the adsorption and desorption rate constants, and the equilibrium adsorption constant. The latter value is within error limits in good agreement with the one extracted from a measured Langmuir isotherm. By measuring a series of intermittent adsorption-desorption steps, we found successively less desorption at a given coverage after each sequential rinsing step, and conclude that there are different binding states and that re-organization of the dye molecules on the TiO2 surface occurs over rather long timescales. The rearrangement process seems to accelerate by intermittent rinsing and associated desorption of loosely bound molecules.