We report on the optimization of
the interfacial properties of
titania in mesoscopic CH3NH3PbI3 solar
cells. Modification of the mesoporous TiO2 film by TiCl4 treatment substantially reduced the surface traps, as is
evident from the sharpness of the absorption edge with a significant
reduction in Urbach energy (from 320 to 140 meV) determined from photothermal
deflection spectroscopy, and led to an order of magnitude enhancement
in the bulk electron mobility and corresponding decrease in the transport
activation energy (from 170 to 90 meV) within a device. After optimization
of the photoanode–perovskite interface using various sizes
of TiO2 nanoparticles, the best photovoltaic efficiency
of 16.3% was achieved with the mesoporous TiO2 composed
of 36 nm sized nanoparticles. The improvement in device performance
can be attributed to the enhanced charge collection efficiency that
is driven by improved charge transport in the mesoporous TiO2 layer. Also, the decreased recombination at the TiO2–perovskite
interface and better perovskite coverage play important roles.