Effect of the TiO2 electron transport layer thickness on charge transfer processes in perovskite solar cells

Dense TiO2 thin films are widely used as electron transport layers in perovskite solar cells. TiO2 is a high band gap semiconductor with n-type conductivity showing excellent compatibility with metal-halide perovskites in terms of energy alignment providing efficient charge separation and less energy loss of electrons during electron transfer. In this work, TiO2 layers were deposited from TiO2 sol-gel solution by spin-coating methods. The optimization of TiO2 ETL thickness is a necessary procedure to reduce ETL resistance and minimize charge recombination. The thickness reduction leads to a decrease in resistance, however, also enhances charge recombination due to the formation of pin-holes and cracks. We found that TiO2 thickness reduction also causes the TiO2 defects density changes. By studying luminescence spectra of TiO2 with various thicknesses we found that the thickness reduction leads to the increase in the interstitial Ti+3 defects density, while the oxygen vacancies density decreases. Ti+3 species form deep levels trapping free electrons and as result increasing TiO2 resistance. TiO2 films with thicknesses in the range of 40–120 nm were deposited and used as ETL for PSCs with ITO/TiO2/CH3NH3I3PbClx/CuPc/MoOx/Ag structure. Our study revealed two competitive charge transport processes taking place in TiO2: electron transport through the TiO2 conduction band and electron trapping by deep trap levels formed by Ti+3 species. Therefore according to the IV and IS studies, the optimal TiO2 layer thickness at which there is a balance between the competitive charge transport processes is about 60 nm for our deposition condition.