Real time adsorption and desorption kinetics of dye Z907 on a flat mimic of dye-sensitized solar cell TiO2 photoelectrodes
Journal article, 2014
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, which are injected into the TiO2 conduction band. For that purpose the dye molecules should adsorb on TiO2 with appropriate molecular orientation and close packing. We measured adsorption and desorption kinetics of dye Z907 on thin compact TiO2 films in real time using indirect nanoplasmonic sensing. From kinetic curves, we derived adsorption and desorption rate constants in a direct way, which has not been done for such systems previously. We then derived the equilibrium adsorption constant from both kinetics (by the ratio of the adsorption and desorption rate constants) and from a measured Langmuir isotherm obtained experimentally using the same method, the same sample, and the same experiment. The two values are in reasonably good agreement considering possible error sources; our approach thus constitutes an effective method of determining more reliable equilibrium constants for dye-TiO2 systems. Furthermore, by measuring a series of intermittent adsorption-desorption steps, we found successively less desorption at a given coverage after each rinsing step and conclude that there are different binding states and that reorganization of the dye molecules on the TiO2 surface occurs over rather long time scales. The rearrangement process seems to accelerate by intermittent rinsing and associated desorption of loosely bound molecules. The results suggest that the detailed conditions for the dye impregnation kinetics can be used to optimize the dye layer.