Influence of sintering on the structural and electronic properties of TiO₂ nanoporous layers prepared via a non-sol–gel approach

In this work, a nonaqueous method is used to fabricate thin TiO₂ layers. In contrast to the common aqueous sol–gel approach, our method yields layers of anatase nanocrystallites already at low temperature. Raman spectroscopy, electron microscopy and charge extraction by linearly increasing voltage are employed to study the effect of sintering temperature on the structural and electronic properties of the nanocrystalline TiO₂ layer. Raising the sintering temperature from 120 to 600 °C is found to alter the chemical composition, the layer’s porosity and its surface but not the crystal phase. The room temperature mobility increases from 2 × 10⁻⁶ to 3 × 10⁻⁵ cm²/Vs when the sinter temperature is increased from 400 to 600 °C, which is explained by a better interparticle connectivity. Solar cells comprising such nanoporous TiO₂ layers and a soluble derivative of cyclohexylamino-poly(p-phenylene vinylene) were fabricated and studied with regard to their structural and photovoltaic properties. We found only weak polymer infiltration into the oxide layer for sintering temperatures up to 550 °C, while the polymer penetrated deeply into titania layers that were sintered at 600 °C. Best photovoltaic performance was reached with a nanoporous TiO₂ film sintered at 550 °C, which yielded a power conversion efficiency of 0.5 %. Noticeably, samples with the TiO₂ layer dried at 120 °C displayed short-circuit currents and open circuit voltages only about 15–20 % lower than for the most efficient devices, meaning that our nonaqueous route yields titania layers with reasonable transport properties even at low sintering temperatures.