One very attractive aspect of organolead-halide perovskite light absorbers relates to their low temperature processability, with the crystallization typically carried out below 150 °C. However, this is only advantageous if all the other layers in the device can be processed under similar conditions. Hence, alongside the efforts associated with optimizing the solution processing of these light harvesting materials, there has been a regain of interest in tackling processing issues related to the deposition of the compact TiO₂ (approx. 50 nm) blocking layer. Here, we demonstrate that a uniform crystalline TiO₂ layer can be produced by UV processing of a specially formulated aqueous suspension. This suspension contains crystalline TiO₂ nanoparticles (3-6 nm) derived from the hydrolysis of an aqueous TiCl₄ solution (80 °C, Patm) and dispersed using oxalic acid molecule adsorbents. Unlike traditional organo-titanate precursors which can only hydrolyse into crystalline TiO₂ anatase under severe heat treatment (typically 500 °C), our precursor is shown to convert into a uniform (50-100 nm) layer of crystalline anatase TiO₂ by application of a short UV treatment (3 mins, T monitored < 130 °C).

Two different types of UV radiation sources (UVA or UVB dominant) are investigated and compared to the effect of 30 mins heat treatments at 250 and 550 °C. The properties of the different TiO₂ layers are compared based on: 1) FEG-SEM imaging and cyclic voltammetry to assess the substrate coverage and impact of pin holes; 2) XPS analyses for surface chemistry data; 3) Raman spectroscopy for the evolution of the crystalline range; and 4) MAPIC cell performance. With these data we show that the UV-treatment plays a critical role in activating the surface of the TiO₂ nano-particles which exceptionally high surface area (200-300 m²/g) triggers a fast photocatalytic degradation of any residual organic components in the precursor while allowing the particles to flow into a non-conformal layer of crystalline TiO₂. MAPIC cells fabricated on our nano-TiO₂ blocking layer have registered up to 12% PCE (average 8% PCE) after heating at 550 °C for 30 mins  and up to 3% PCE (average 2% PCE) without any post deposition UV or heat treatment. While we are currently collecting cell performance data based on optimized UV-treated TiO₂ layers, the FEG-SEM, Raman and XPS analyses suggest that comparable performance to high-T processed samples (>10% PCE) may be expected.