First-Principle Characterization of the Adsorption Configurations of Cyanoacrylic Dyes on TiO2 Film for Dye-Sensitized Solar Cells.

The loading of sensitizers on a semiconductor is crucial for determining the light-harvesting efficiency of dye-sensitized solar cells (DSSCs). The interfacial properties of dyes adsorbed on a TiO2 film, such as adsorption configurations and adsorption energy, can influence the total amount of dye sensitizers that loads and the stability of a DSSC device. Therefore, it is important to characterize the adsorption properties of sensitizers on TiO2 films atomically and electronically to ensure rational structure-based dye design for high-performance DSSCs. Due to the complex properties of interfacial dyes, previous works on the identification of adsorption configurations of dyes on TiO2 have sometimes been controversial, in particular, the essential IR band assignments. In this study, we employed density functional theory to investigate the adsorption energies, geometries, and vibrational frequencies of various adsorption configurations of 2-cyano-3-(thiophen-2-yl)acrylic acid adsorbed on TiO2. We performed a comparative assignment of the calculated vibrational peaks of tridentate and bidentate configurations to the experimental FT-IR spectra simultaneously. Our work backs up the coexistence of tridentate and bidentate bridging configurations, first proposed by Meng and co-workers. Moreover, our comparative IR mode assignments provide clues for further studies of the interfacial properties of dyes adsorbed on TiO2. Study of the transformation mechanisms between tridentate and bidentate modes suggests that the bidentate bridging configuration is a kinetically trapped adsorption mode and the tridentate configuration is thermodynamically the most stable one. Finally, we investigated the photophysical properties of a D-π-A dye in tridentate and bidentate adsorption configurations.