ALD Grown Aluminum Oxide Submonolayers in Dye-Sensitized Solar Cells: The Effect on Interfacial Electron Transfer and Performance

Effects of atomic layer deposited (ALD) aluminum oxide barriers on electron injection, recombination reactions, and efficiency of dye-sensitized solar cells were studied. The amorphous AlOx submonolayers prepared on nanocrystalline 2 mu m thick TiO2 anatase film were characterized by high-resolution transmission electron microscopy. Time-of-flight elastic recoil detection method was employed to study the growth of similar AlOx layers on planar anatase films. Density functional theory calculations of the first ALD cycle over a (101) anatase surface revealed atomic scale roughness of the deposited layer owing to unequal adsorption sites and lateral repulsions between adsorbed precursor molecules. Calculations also indicate that the holes in the first AlOx layer allow triiodide but not the ruthenium bipyridyl sensitizer to reach the TiO2 surface. After the first deposition cycle the dye binds to AlOx and is in average about 0.2 nm farther from the TiO2 surface than when binding to the bare TiO2 surface. Increase in average distance between the dye and TiO2 surface was considered as the main reason for reduced electron injection efficiency observed for all coated sensitized films. Electrochemical impedance spectroscopy indicated that also recombination reactions of the conduction band electrons with the electrolyte triiodide molecules were reduced. The exponential increase in charge transfer resistance at the dye-TiO2-electrolyte interface as a function of the number of ALD cycles indicated that AlOx barrier layers affect recombination mainly through the tunnel barrier mechanism. Decrease of both the short circuit current and efficiency of the solar cells prepared from above-mentioned films as the number of ALD cycles was increased suggests that the suppression of the recombination could not compensate for the barrier-induced losses of electron injection.