Improvement of the morphological and electrical characteristics of Al3+, Fe3+ and Bi3+-doped TiO2 compact thin films and their incorporation into hybrid solar cells

Compact titanium dioxide (TiO2) hole-blocking layers are commonly employed in organic-inorganic solar cells, however, their importance in terms of morphology and electrical conductivity is frequently overlooked in this novel type of solar energy converters. In this work, single TiO2 thin films were prepared by a sol-gel method, observing large pinhole densities and low electrical conductivities. As a means to solve the morphological issue, the deposition of a second TiO2 film was explored, which effectively reduced the surface irregularities obtained in single oxide films. The limited electrical conductivity of single and double layers was successfully increased by doping with the trivalent cations of aluminum, iron (III) and bismuth (III), observing an increase from 2.48 × 10−8 S/cm for an undoped TiO2 double layer to 51.41 × 10−8 S/cm for a Fe3+-doped TiO2 double layer. The incorporation of these hole blocking layers in hybrid solar cells led to further insights in the important role of trivalent doping cations in the transference and transport of electrons on the surface and in the bulk of the prepared TiO2 compact films.