The bulk photovoltaic effect in polar oxides for robust and efficient solar 
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energy harvesting
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Andrew M. Rappe
University of Pennsylvania

TCM Seminar Room 
Wed 26 June 11am

Solar energy is the most promising source of renewable, clean energy
to replace the current reliance on fossil fuels. Ferroelectric (FE)
materials have recently attracted increased attention as a candidate
class of materials for use in photovoltaic devices.  Their strong
inversion symmetry breaking due to spontaneous polarization allows for
excited carrier separation by the bulk of the material and voltages
higher than the band gap (Eg), which may allow efficiencies beyond the
Shockley-Queisser limit. Ferroelectric oxides are also robust and can
be fabricated using low cost methods such as sol-gel thin film
deposition and sputtering.  Recent work has shown how a decrease in
ferroelectric layer thickness and judicious engineering of domain
structures and FE-electrode interfaces can dramatically increase the
current harvested from FE absorber materials.  Further improvements
have been blocked by the wide band gaps (Eg =2.7-4 eV) of FE oxides,
which allow the use of only 8-20% of the solar spectrum and
drastically reduce the upper limit of photovoltaic efficiency.

In this talk, I will discuss new insight into the bulk photovoltaic
effect, and materials design to enhance the photovoltaic efficiency.
We calculate from first principles the current arising from the "shift
current" mechanism, and demonstrate that it quantitatively explains
the observed current.  Then, we analyze the electronic features that
lead to strong photovoltaic effects.  Finally, we present new oxides
that are strongly polar yet have band gaps in the visible range,
offering prospects for greatly enhanced bulk photovoltaic effects.