Characterization of Recombination Barrier Layers in Dye-Sensitized Solar Cells

 Research interest in solar energy due to climate and energy security concerns has resulted in many photovoltaic technologies.  One such example is the dye-sensitized solar cell (DSSC) which employs a monolayer of ruthenium-organic dye molecules adsorbed onto a TiO₂ substrate as the photon absorber. The original DSSC design in which an iodine electrolyte solution regenerates the oxidized dye has achieved efficiencies in excess of 12%.  More practical solid-state devices, however, have only achieved efficiencies of ~6% due to the higher rate of charge recombination between electrons in the TiO₂ and holes in the solid-state hole conductor, Spiro OMeTAD.  One way to slow this recombination route is to add an insulating layer between the TiO₂ and the dye layer.  The insulating layer can be either organic (e.g. a self-assembled monolayer) or inorganic (e.g. metal oxide) but charge injection considerations restrict layer thickness to approximately 1 nm.  Barrier layers under study in this work include amine-terminated phosphonic acids and metal oxides such as Al₂O₃ and ZrO₂.  In addition to the dielectric properties of such an insulating layer, the conformation of the dye layer at the bare or coated TiO₂ substrate will also affect device performance.  To determine the dye conformation, smooth TiO₂ substrates were grown via atomic layer deposition, and adsorbed dye layers were studied on these TiO₂ films—both bare and with barrier layers—by X-ray reflectivity (XRR) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy.  Using angle-resolved NEXAFS, we have elucidated the orientation of the recently-developed YE05 dye and the more common Z907 dye on bare TiO₂ and on barrier layers.  XRR provides insight into the dye and barrier layers' densities and thicknesses and confirms the deposition of a monolayer.  Solid-state DSSCs have also been fabricated and tested using conventional high-porosity TiO₂ substrates.  By coupling the structural characterization of the well-defined surfaces with real device performance, the effect of dye orientation and barrier layers on device performance in solid-state DSSCs can be better understood.