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New UK Research Could Squeeze More Power Out Of Solar Cells

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Physicists at the University of Warwick have published new
research in the Journal Science, on 19th April 2018 (via the Journal’s First
Release pages), that could literally squeeze more power out of solar cells by
physically deforming each of the crystals in the semiconductors used by
photovoltaic cells. The paper entitled the “Flexo-Photovoltaic Effect" was
written by Professor Marin Alexe, Ming-Min Yang, and Dong Jik Kim who are all
based in the University of Warwick’s Department of Physics.

The Warwick researchers looked at the physical constraints
on the current design of most commercial solar cells which place an absolute
limit on their efficiency. Most commercial solar cells are formed of two layers
creating at their boundary a junction between two kinds of semiconductors,
p-type with positive charge carriers (holes which can be filled by electrons)
and n-type with negative charge carriers (electrons). When light is absorbed,
the junction of the two semiconductors sustains an internal field splitting the
photo-excited carriers in opposite directions, generating a current and voltage
across the junction. Without such junctions the energy cannot be harvested and
the photo-exited carriers will simply quickly recombine eliminating any
electrical charge. That junction between the two semiconductors is fundamental
to getting power out of such a solar cell but it comes with an efficiency
limit. This Shockley-Queisser Limit means that of all the power contained in
sunlight falling on an ideal solar cell in ideal conditions only a maximum of
33.7% can ever be turned into electricity.

There is however another way that some materials can collect
charges produced by the photons of the sun or from elsewhere. The bulk
photovoltaic effect occurs in certain semiconductors and insulators where their
lack of perfect symmetry around their central point (their non-centrosymmetric
structure) allows generation of voltage that can be actually larger than the
band gap of that material (the band gap being the gap between the valence band
highest range of electron energies in which electrons are normally present at
absolute zero temperature and the conduction band where electricity can flow). Unfortunately,
the materials that are known to exhibit the anomalous photovoltaic effect have
very low power generation efficiencies, and are never used in practical
power-generation systems.

The Warwick team wondered if it was possible to take the
semiconductors that are effective in commercial solar cells and manipulate or
push them in some way so that they too could be forced into a
non-centrosymmetric structure and possibly therefore also benefit from the bulk
photovoltaic effect. For this paper they decided to try literally pushing such
semiconductors into shape using conductive tips from atomic force microscopy
devices to a “nano-indenter" which they then used to squeeze and deform
individual crystals of Strontium Titanate (SrTiO3), Titanium Dioxide (TiO2),
and Silicon. They found that all three could be deformed in this way to
also give them a non-centrosymmetric structure and that they were indeed then
able to give the bulk photovoltaic effect.

Professor Marin Alexe from the University of Warwick said:

“Extending the range of materials that can benefit from the
bulk photovoltaic effect has several advantages: it is not necessary to form
any kind of junction; any semiconductor with better light absorption can be
selected for solar cells, and finally, the ultimate thermodynamic limit of the
power conversion efficiency, so-called Shockley-Queisser Limit, can be
overcome. There are engineering challenges but it should be possible to create
solar cells where a field of simple glass based tips (a hundred million per
cm2) could be held in tension to sufficiently deform each semiconductor
crystal. If such future engineering could add even a single percentage point of efficiency it would be of immense
commercial value to solar cell manufacturers and power suppliers."

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