Modifying Oxide Single Crystal Surfaces to Study Photoinduced Electron Transfer

Single crystal oxide semiconductor electrodes provide an ideal experimental system for the elucidation of the models and parameters that control light absorption, charge separation, and transport at semiconductor/liquid interfaces.  Such fundamental investigations can prove very valuable in the understanding of dye sensitized nanocrystalline solar cells.  The single crystal experimental system is much less complex than its nanocrystalline counterpart and the study of well-defined crystallographic surfaces allows for more definitive conclusions to be made concerning the relation between sensitizer attachment and surface structure and the sensitizer’s performance.  I will report recent work on the sensitization of low index faces of both anatase and rutile forms of TiO₂ and ZnO single crystals with dyes, quantum dots and polymers.   We employ atomic force microscopy (AFM) to verify that atomically flat terraced oxide surfaces are prepared and to elucidate the structure of the sensitizers adsorbed on these surfaces.  Photocurrent spectroscopy and attenuated reflection spectroscopy are used to correlate the photocurrent response with the optical absorption of the sensitizing species and the relative electron injection efficiencies of for instance dye monomers and aggregates.  We have also recently begun to grow homoepitaxial layers on both anatase and rutile using atomic layer deposition (ALD).  These layers can have superior electronic properties compared to the substrate crystal and allows us to control the doping density in the near surface region.  The primary focus of this talk will be on CdSe and PbS quantum dots as sensitizers.  PbS quantum dots adsorbed on anatase (001) surfaces irradiated with light of >2.5 times their band gap have been shown to produce more than one injected electron per incident photon (1), a process known as carrier multiplication or multiple exciton generation (MEG).   Harnessing MEG may result in more efficient solar conversion devices.

1. Justin B. Sambur, Thomas Novet and B. A. Parkinson, “Multiple Exciton Collection in a Sensitized Photovoltaic System”, Science, 330, 63-66, (2010)