(512c) Doped TiO2 Based Core-Shell Structures for High Efficiency Hybrid Solar Cells

Hybrid solar cells, with an inorganic/organic interface for charge separation, have been extensively investigated in
the past decade in order to replace the expensive Si based technology with an inexpensive alternative. Typically,
these devices incorporate a mesoporous TiO₂ film which is decorated with dye molecules and filled with a hole
transport polymer, for example P3HT, to conduct the electrons and holes, respectively. Recently, we have shown
that  the  efficiency  of  nanowire  based  hybrid  solar  cells  can  be  increased  from  ~1.8  %  to  2.5  %  through  the
formation of a Sn-doped TiO₂|TiO₂ core-shell device created via a hydrothermal growth and subsequent TiCl₄
treatment.  However,  this  surface  treatment  presents  difficulties  in  creating  a  crystalline  conformal  coating,
limiting the control over the extent of coating and the crystallinity, directly affecting the charge injection from the
polymer  into  the  TiO2  array.  In  this  work,  we  directly  deposit  a  controllable  TiO₂  film  through  atomic  layer
deposition to conformally coat the nanowire arrays with various thicknesses. By changing the thickness and TiO₂
crystallinity, we are able to engineer the energy levels at the TiO₂-dye-P3HT interface due to the magnitude and
position  of  the  Fermi  levels  of  the  core  and  shell  material,  influencing  the  rate  of  charge  injection  and
recombination.  Furthermore,  the  crystallinity  of  the  shell  layer  directly  affects  the  amount  of  dye  that  can  be
absorbed on the surface of the nanostructures with a reduction in light absorption by roughly 30% from anatase to
rutile TiO₂. Finally, a detailed mechanism will be proposed for the device performances based on the energy level
alignment between the pinned Fermi-level TiO₂ structure and the HOMO of the P3HT resulting in a shifting open
circuit  voltage  based  on  the  crystal  phases.  Additionally,  the  core-shell  structures  are  characterized  with
photovoltage decay and impedance spectroscopy measurements to study the charge transport and recombination
across these various interfaces.