Interfacial energy levels and related properties of atomic-layer-deposited Al_2O_3 films on nanoporous TiO_2 electrodes of dye-sensitized solar cells

摘要

Low-temperature (__150℃), atomic-layer-deposited Al_2O_3 films on nanoporous TiO_2 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 Al_2O_3 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 Al_20_3 over-layers, while the thicker Al_2O_3 over-layers caused the values of valence band maximum and band gap to approach the values associated with pure Al_2O_3. A work function difference (ΔΦ_(A-T)) of 0.4 eV and a recombination barrier height (ε_(RB)) of 0.1 eV were associated with the highest PCE achieved by the first monolayer of the Al_2O_3 layer. Thicker Al_20_3 over-layers, however, caused significant reduction of PCE with negative ΔΦ_(T-A) and increased interfacial energy barrier height (~*ε_(IB)) between the N719 dyes and Ti0_2 electrodes. It was concluded that the PCE of the DSSCs may correlate with ΔΦ_(A-T), ε_(RB), and ~*ε_(IB) resulting from various thicknesses of the Al_20_3 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 TiO_2 solar cells.