Adjustable refractive index of titania-alumina thin films prepared from soluble precursor powders

Triethanolamine was used as complexing agent to prepare soluble titania and alumina precursor powders from the respective metal alkoxides. Long-term stable coating solutions were obtained by dissolving varying ratios of these precursors in mixtures of ethanol and polyethylene glycol (PEG 200); the refractive index of the resulting films could be adjusted between 1.48 and 2.37. Thickness, refractive index and porosity were characterized as a function of annealing temperature. For an intermediate treatment temperature of 300 °C, partial liquefaction of the film material was observed. X-ray diffraction experiments reveal that the crystallization of TiO2 is effectively suppressed by the presence of alumina precursor material. The titania and alumina precursor powders were successfully used as index-matched binder material of SiO2 Stöber particles in the preparation of optical scattering layers. Such films may be used for light harvesting in thin film solar cells or the outcoupling of light from LEDs or OLEDs.