Dye-Sensitized Core−Shell Nanocrystals:  Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide

Coating nanocrystalline SnO 2 electrodes for dye-sensitized solar cell applications with a thin layer of ZnO, TiO 2, ZrO 2, MgO, Al 2 O 3, Y 2 O 3, or other insulating oxides was found to improve dye adsorption and increase the sensitized photocurrent. The surface of the oxide coating is more basic than SnO 2, which renders dye attachment by its carboxyl groups more favorable. At the same time, the photovoltage and fill factor are strongly enhanced, resulting in much better energy conversion efficiencies. This change is ascribed to inhibition of electron back transfer from SnO 2 to the redox electrolyte (I 3 - ) by the insulating oxide. The optimum coating thickness is only a few angstroms, suggesting that electron transfer from the excited dye attached to the oxide surface to the underlying SnO 2 occurs by tunneling through the insulator layer. Different methods for coating SnO 2 nanocrystals with a thin layer of an insulating oxide were compared. The best results were obtained by adding a soluble metal salt to the SnO 2 colloid, which is converted to the oxide on firing of the electrode. Specific adsorption of the metal ions on the SnO 2 surface guarantees homogeneous coating at near-monolayer coverage. Although the energy conversion efficiency of such modified SnO 2 electrodes is still inferior to that of the usually employed TiO 2 electrodes, the larger band gap of SnO 2 offers the advantage of better long-term stability of the solar cell in the presence of UV light. Surface modification of nanocrystalline TiO 2 electrodes with very thin insulating oxide layers also improves the photovoltage but diminishes the current. Because of a blue shift in the photocurrent onset of insulator-coated TiO 2, the solar cell stability in UV light is again enhanced.