Optically uniform thin films of mesoporous TiO2 for perovskite solar cell applications

Mesoporous titanium oxide (mp-TiO2) thin films are effective electron transport layers in hybrid perovskite solar cells. In this work the mp-TiO2 are obtained via spin coating by mixing titanium isopropoxide and poly (vinylpyrrolidone) (PVP). Optically uniform anatase TiO2 thin films are formed after annealing at temperatures ≥500 °C. The surface morphology shows porous structure with pore sizes of ≅15–30 nm. The volume porosity of a mp-TiO2 thin film is estimated for the first time by using the Volume Average Theory with the experimentally measured effective refractive index of the film, and confirmed by a statistical and 3D interactive method for void volume determination in the cross sectional image of the same film. It is found that this volume porosity varies slightly, from 40.3% to 43.6%, when the spin speed is between 1000 and 2500 rpm. However, the volume porosity is significantly reduced to 33% for a very slow spin speed like 500 rpm. Using these mp-TiO2 thin films as electron transport layers in perovskite solar cells, it is confirmed that the power conversion efficiency (PCE) of the cells is a function of both the mp-TiO2 layer thickness and volume porosity, giving the maximum PCE in a cell sample that combines a larger thickness and higher volume porosity.