(472b) 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.