Molecular Orientation of -POH and -COOH Functionalized Dyes on TiO, AlO, ZrO, and ITO: A Comparative Study.

Modification of transparent metal oxide (MO) surfaces with organic monolayers is widely employed to tailor the properties of interfaces in organic electronic devices, and MO substrates modified with light-absorbing chromophores are a key component of dye-sensitized solar cells (DSSCs). The effects of an organic modifier on the performance of a MO-based device are frequently assessed by performing experiments on model monolayer|MO interfaces, where an "inert" MO (e.g., AlO) is used as a control for an "active" MO (e.g., TiO). An underlying assumption in these studies is that the structure of the MO-monolayer complex is similar between different metal oxides. The validity of this assumption was examined in the present study. Using UV-Vis attenuated total reflection spectroscopy, we measured the mean dipole tilt angle of 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid () adsorbed on indium tin oxide (ITO), TiO, ZrO, and AlO. When the surface roughness of the MO substrate and the surface coverage (𝛤) of the film were constant, the molecular orientation of was the same on these substrates. The study was extended to 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid () adsorbed on the same group of MO substrates. The mean tilt angle of and films on ITO was the same, which is likely due the intermolecular interactions resulting from the high and approximately equal 𝛤 of both films. Comparing films at the same 𝛤 on TiO and AlO having the same surface roughness, there was no difference in the mean tilt angle. MD simulations of and on TiO produced nearly identical tilt angle distributions, which supports the experimental findings. This study provides first experimental support for the assumption that the structure of the MO-modifer film is the same on an "active" substrate vs. a "inert" control substrate.