EXAFS, ab Initio Molecular Dynamics, and NICIS Spectroscopy Studies on an Organic Dye Model at the Dye-Sensitized Solar Cell Photoelectrode Interface

The organization of dye molecules in the dye layer adsorbed on the semiconductor substrate in dye-sensitized solar cells has been studied using a combination of theoretical methods and experimental techniques. The model system is based on the simple D−π–A dye L0, which has been chemically modified by substituting the acceptor group CN with Br (L0Br) to offer better X-ray contrast. Experimental EXAFS data based on the Br K-edge backscattering show no obvious difference between dye-sensitized titania powder and titania film samples, thus allowing model systems to be based on powder slurries. Ab initio molecular dynamic (aiMD) calculations have been performed to extract less biased information from the experimental EXASF data. Using the aiMD calculation as input, the EXAFS structural models can be generated a priori that match the experimental data. Our study shows that the L0Br dye adsorbs in the trans-L0Br configuration and that adsorption involves both a proximity to other L0Br dye molecules and the titanium atoms in the TiO₂ substrate. These results indicate direct coordination of the dye molecules to the TiO₂ surface in contrast to previous results on metal–organic dyes. The molecular coverage of L0Br on mesoporous TiO₂ was also estimated using NICIS spectroscopy. The NICISS results emphasized that the L0Br dye on nanoporous titania mainly forms monolayers with a small contribution of multilayer coverage.