Interfacial energy levels and related properties of atomic-layer-deposited Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells

Low-temperature (∼150 0C), atomic-layer-deposited Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells (DSSCs) were investigated using electron spectroscopy. The power conversion efficiency (PCE) of the DSSCs was increased from 5.7% to 6.5%, an improvement of 14%, with one monolayer of Al2O3 with a thickness of ∼0.2 nm. The formation of Ti-O-Al(OH)2 and interfacial dipole layers exhibited a strong influence on the work function of the Al2O3 over-layers, while the thicker Al2O3 over-layers caused the values of valence band maximum and band gap to approach the values associated with pure Al2O3. A work function difference (ΔΦA-T) of 0.4 eV and a recombination barrier height (εRB) of 0.1 eV were associated with the highest PCE achieved by the first monolayer of the Al2O3 layer. Thicker Al2O3 over-layers, however, caused significant reduction of PCE with negative ΔΦT-A and increased interfacial energy barrier height (*εIB) between the N719 dyes and TiO2 electrodes. It was concluded that the PCE of the DSSCs may correlate with ΔΦA-T, εRB, and *εIB resulting from various thicknesses of the Al2O3 over-layers and that interfacial reactions, such as the formation of Ti-O-Al(OH)2 and dipole layers, play an important role in determining the interfacial energy levels required to achieve optimal performance of dye-sensitized TiO2 solar cells.