Role of the main adsorption modes in the interaction of the dye [COOH–TPP-Zn(ii)] on a periodic TiO2 slab exposing a rutile (110) surface in a dye-sentized solar cell

Abstract

The high solar-to-electric-power conversion efficiency reported for 5-(4-carboxylphenyl)-10,15,20-tetrakis (2,4,6-trimethylphenyl) porphyrinatozinc(II) (TPP-Zn(II)) prompted us to study at a molecular level the interaction of this dye on the rutile surface. The –COOH group was included in the complex to anchor the dye onto the semiconductor oxide. Three main modes of molecular adsorption of the anchoring group on the oxide surface were studied, and vibrational analysis was carried out to characterize it as either a minimum energy or a transition state structure. To investigate the geometrical and electronic structures of the different modes of COOH–TPP-Zn(II) adsorption on the periodic TiO2 slab with exposed rutile (110) surfaces, we employed time-dependent density functional theory to study the optical properties of the isolated molecule TPP-Zn(II) (which was used in the DSSC), followed by periodic DFT calculations in the completed system (COOH–TPP-Zn(II) on the periodic TiO2 slab). This procedure leads to a clear identification of the most stable position of the anchoring group, that binds strongly the dye on the surface and simultaneously facilitates the electron injection. On the other hand, frontier molecular orbital spatial distributions, and the energy diagram of the electronic density of states of the dye-surface system, suggest that the dye is capable of electron injection into TiO2, as has been shown from experiments. Our computational approach is able to provide considerable insight into the electronic structure of the bond system of TPP-Zn(II)–TiO2 and to get insight into the anchoring modes, which are very important for the coupling between the dye and the semiconductor surface. This leads to an effective photocurrent energy conversion in a DSSC device.