Molecular Orientation of -PO 3 H 2 and -COOH Functionalized Dyes on TiO 2 , Al 2 O 3 , ZrO 2 , and ITO: A Comparative Study.

Modification of transparent metal oxide (MO x ) surfaces with organic monolayers is widely employed to tailor the properties of interfaces in organic electronic devices, and MO x 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 x -based device are frequently assessed by performing experiments on model monolayer|MO x interfaces, where an "inert" MO x (e.g., Al 2 O 3 ) is used as a control for an "active" MO x (e.g., TiO 2 ). An underlying assumption in these studies is that the structure of the MO x -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 ( A1P ) adsorbed on indium tin oxide (ITO), TiO 2 , ZrO 2 , and Al 2 O 3 . When the surface roughness of the MO x substrate and the surface coverage (𝛤) of the A1P film were constant, the molecular orientation of A1P was the same on these substrates. The study was extended to 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid ( A1C ) adsorbed on the same group of MO x substrates. The mean tilt angle of A1C and A1P 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 A1C films at the same 𝛤 on TiO 2 and Al 2 O 3 having the same surface roughness, there was no difference in the mean tilt angle. MD simulations of A1C and A1P on TiO 2 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 x -modifer film is the same on an "active" substrate vs. a "inert" control substrate.