Kinetic and thermal study of dye adsorption on TiO2 and sequential dye adsorption model for dye sensitized solar cells (DSSC)

Abstract

As the site to generate electrons, dye and its attachment process on TiO2 in dye-sensitized solar cells is critical for high cell performance, yet it is not well studied. The aim of this project is to understand the dye adsorption mechanism on TiO2 and investigate the relationship between the dye loading conditions (namely dye soaking time and temperature) to cell photovoltaic performance.

For the kinetic study, the correlation of cell efficiency and UV-visible spectroscopy has showed that maximum efficiency can be achieved after 4hours of dye adsorption instead of 24 hours. The period after the achievement of maximum dye absorption and before maximum cell efficiency suggested the existence of a period in which aggregated dye molecules have to ‘self-assemble’ themselves to form strong hydrophobic monolayer on TiO2 before reaching the maximum cell efficiency.

The thermal study showed the increase of 10% of photocurrent density and cell efficiency at the optimum temperature of 30°C. It is suggested that with the aid of heat, the equilibrium is pushed to higher dye adsorption rate according to the Arrhenius equation and collision theory. The ‘Arrhenius’ relationship between temperature and cell performance has been discovered and it can be divided into two regions which are limited by temperature and dye molecular orientation respectively.

The proposed dye adsorption model is a sequential 3-step process: initial, aggregated and self-assembled steps which progressed from weak physisorption/ hydrogen bonding to strong covalent and then collective hydrophobic interaction for ‘self-assembly’ monolayer of dye molecules on TiO2.

More studies have to be done on validating the effects of the types of dye (with different functional groups) and dye adsorption conditions (e.g. pH, dye concentration, etc) to validate the dye adsorption model.