We used titanium(IV) isopropoxide (Ti-(OiPr)4) in isopropanol with room temperature processing to achieve a PCE of 16.3% with MAPbI3 perovskite solar cell in the attached publication. Commonly, titanium diisopropoxide bis(acetylacetonate) in butanol is used but it requires high temperature (450°C) annealing.
Article Room Temperature Processing of TiOx Electron Transporting La...
You cannot use a precursor assisted (titanium isopropoxide) or sol-gel (commercial paste with binders) method to uniformly deposit TiO2 which requires high temperature firing, then this would also burn your plastic substrate.
So disperse commercial TiO2 particles in ethanol ( evaporates @ ~ 80 deg C).
Concentration should be varied according to the requirement. Then use any thin film technique depending on the required thickness.