Aluminum oxides as alternative building blocks for efficient layer-by-layer blocking layers in dye-sensitized solar cells

All inorganic layer-by-layer (LbL) thin films composed by TiO₂ nanoparticles and [Al(OH)₄]⁻ anions (TiO ₂ /AlO _x) as well as Al₂O₃ and Nb₂O₅ nanoparticles (Al ₂ O ₃ /Nb ₂ O ₅) have been deposited to fluorine-doped tin-oxide coated glass (FTO) surfaces and applied as blocking layers in dye-sensitized solar cells (DSCs). Structural and morphological characterization of the LbL films by different techniques reveal that in TiO ₂ /AlO _x assembly, aluminate anions undergo condensation reactions on the TiO₂ surface leading to the formation of highly homogeneous films with unique optical properties. After 25 depositions transmittance losses below 10% in relation to the bare FTO substrate are observed. Electrochemical impedance spectroscopy shows that TiO ₂ /AlO _x layers impose an effective barrier for the charge recombination at FTO/electrolyte interface with an electron exchange time constant 50-fold higher than that for bare FTO. As a result, an improvement of 85% in the overall conversion efficiency of DSCs was observed with the employment of TiO₂/AlO_x blocking layers. Al ₂ O ₃ /Nb ₂ O ₅ LbL films can also work as blocking layers in DSCs but not as efficient, which is associated with the poor homogeneity of the film and its capacitive behavior. The production of cost-effective blocking layers with a low light scattering in the visible region is an important feature toward the application of DSC in other Building-integrated photovoltaic applications. (paper)