Pentacene/TiO Anatase Hybrid Interface Study by Scanning Probe Microscopy and First Principles Calculations.

The understanding and control of the buried interface between functional materials in optoelectronic devices is key to improving device performance. We combined atomic resolution scanning probe microscopy with first-principles calculations to characterize the technologically relevant organic/inorganic interface structure between pentacene molecules and the TiO anatase (101) surface. A multipass atomic force microscopy imaging technique overcomes the technical challenge of imaging simultaneously the corrugated anatase substrate, molecular adsorbates, monolayers, and bilayers at the same level of detail. Submolecular resolution images revealed the orientation of the adsorbates with respect to the substrate and allowed direct insights into interface formation. Pentacene molecules were found to physisorb parallel to the anatase substrate in the first contact layer, passivating the surface and promoting bulk-like growth in further organic layers. While molecular electronic states were not significantly hybridized by the substrate, simulations predicted localized pathways for molecule-surface charge injection. The localized states were associated with the molecular lowest unoccupied molecular orbital inside the oxide conduction band, pointing to efficient transfer of photo-induced electron charge carriers across this interface in prospective photovoltaic devices. In uncovering the atomic arrangement and favorable electronic properties of the pentacene/anatase interface, our findings testify to the maturity and analytic power of our methodology in further studies of organic/inorganic interfaces.