Role of adsorption structures of Zn-porphyrin on TiO₂ in dye-sensitized solar cells studied by sum frequency generation vibrational spectroscopy and ultrafast spectroscopy

Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (η_rel) are affected by the molecular spacer connecting the porphyrin core to the TiO₂ surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO₂ surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO ₂ surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, η_rel, and the concentration of long-lived electrons in the conduction band of TiO₂ was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO₂ surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO₂ occurs "through- space" rather than "through the molecular spacer". It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties.