A direct correlation between structural and morphological defects of TiO2 thin films on FTO substrates and photovoltaic performance of planar perovskite solar cells

Titanium oxide (TiO2) precursor is often deposited on conductive fluorine doped tin oxide (FTO) to convert into crystalline anatase phase after a thermal annealing at 400–550 °C. A higher annealing temperature can increase the crystallinity of TiO2, but it may also induce morphological changes in both TiO2 and FTO. In this work, structural and morphological changes in thermally annealed FTO and TiO2/FTO are identified and semi-quantified by X-ray diffraction, scanning electron and atomic force microscopies. 400 °C annealed TiO2 shows the lowest crystallinity and the highest dislocation density, giving a poor photovoltaic performance of planar perovskite solar cells (p-PSCs). 550 °C annealed TiO2 exhibits the highest crystallinity and lowest dislocation density, but at such temperature the FTO films undergo an increase of surface roughness by more than 25%. As a result, 550 °C annealed TiO2 coatings on FTO substrates show the highest concentration of surface fracture and a larger exposed area of FTO, permitting a direct contact of upcoming perovskite with the FTO substrate. TiO2 thin films annealed at 450 or 500 °C show a compromising between the crystalline structure and defect density, giving a better energy conversion efficiency of 16.3% of p-PSCs prepared under ambient conditions.