The compact TiO2 hole blocking layer, noted c-TiO2, was prepared by aerosol spray pyrolysis. The TiO2 nanoparticle solution employed for the preparation of the mesoporous layer, noted mp-TiO2, was prepared in advance and stirred for at least 12h. The TiO2 NR30-D paste was diluted in ethanol with a 1:7 w/w ratio. 45 μL of the solution was dropped on the c-TiO2 layer and spin coated at 2000 rpm for 15 s. The layer was then dried on a hotplate at 70 °C for 5-10 min and finally heated at 500 °C under an air flux for 30 min, cooled down to 200 °C and removed from the hotplate.