10.1117/2.1200812.1399
Quantum wells in concentrator
solar cells
N. J. Ekins-Daukes, I. M. Ballard, K. W. J. Barnham,
J. P. Connolly, and T. Tibbits
A novel, nanostructured photovoltaic material holds promise for largescale solar power generation.
The perennial challenge of solar energy conversion is to generate significant quantities of power at a reasonable cost. As part
of a CO2 reduction strategy, the International Energy Agency
recently set a goal of attaining 1TW of peak power capacity
from solar electricity by 2050.1 All photovoltaic technology sectors must grow rapidly to meet this goal. Yet solar panel manufacturing already uses more silicon than the entire microelectronics industry. In addition, though the technology can scale to
terawatt levels,2 expanding the manufacturing infrastructure is
very capital-intensive.
Concentrator photovoltaic systems may provide an alternative solution. By using mirrors or lenses, they focus sunlight onto
small, highly efficient solar cells. This shifts the manufacturing
burden from semiconductors to metal and glass, materials that
have more established manufacturing industries.3 Moreover, recent improvements in concentrator cell efficiencies suggest that
this approach may be cost-effective and rapidly scalable.
We recently demonstrated a single junction quantum well solar cell with an efficiency of 27.3% (see Figure 1), the highest value for any nanostructured solar cell to date. It is also
very close to the highest efficiency recorded for a single junction cell (27.8%).4 The device is made of a p-i-n structure with
a gallium arsenide phosphide (GaAsP) indium gallium arsenide
(InGaAs) multi-quantum well stack grown in the i-region. The
lower band-gap InGaAs layer is compressively strained, while
the GaAsP barrier layer is under tensile strain (see Figure 2).
Therefore, a judicious choice of composition and layer thickness
results in a stack of quantum wells where each GaAsP/InGaAs
bilayer exerts no net force on neighboring layers.5
By incorporating strained semiconductors into a solar cell
without introducing structural defects, we can adjust the absorption threshold for the solar cell. The single junction cell has an absorption edge at 1.33eV, which is fundamentally better matched
Figure 1. Efficiency for the GaAsP quantum well solar cell approaches
the gallium arsenide (GaAs) single junction record.
to the solar spectrum than a 1.42eV gallium arsenide cell. The
low defect density allows cells to become increasingly radiatively efficient at concentrator intensities, exhibiting photon recycling effects that further boost efficiency.6, 7
Adjusting the absorption threshold becomes particularly important when fabricating highly efficient, multi-junction solar
cells. Here the broad solar spectrum is absorbed using a seriesconnected stack of subcells with different bandgaps. While this
structure can lead to very high efficiencies, it requires careful
control of the absorption threshold of each subcell. The series
connection ensures that the lowest photocurrent in the stack will
limit the photocurrent in the entire cell.
By growing defect-free, strain-balanced stacks of material, we
can adjust the absorption threshold of the junctions without relaxing the semiconductor lattice or growing optically thin junctions. This strain-balanced approach makes double junction ef-
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10.1117/2.1200812.1399 Page 2/2
K. W. J. Barnham
Blackett Laboratory
Imperial College London
London, UK
Quantasol
Kingston-upon-Thames, UK
K. W. J. Barnham pioneered the quantum well solar cell at Imperial College London and founded the Quantasol company, where
he is now chief technology officer.
T. Tibbits
Quantasol
Kingston-upon-Thames, UK
T. Tibbits is the director of product engineering at the Quantasol
company.
Figure 2. The strain-balanced material combines thin layers of alternating tensile and compressively strained semiconductor. The resulting heterostructure is locally strained but exerts no net force on the
substrate.
ficiencies greater than 34% and triple junction efficiencies up to
42% under solar concentration feasible. Work is under way to
achieve these goals.
Author Information
References
1. International Energy Agency, Energy Technology Perspectives 2008: Scenarios
and Strategies to 2050, Organization for Economic Cooperation and Development,
Paris, 2008.
2. A. Feltrin and A. Freundlich, Material considerations for terawatt level deployment
of photovoltaics, Renew. Energ. 33, pp. 180–185, 2008.
3. R. Swanson, The promise of concentrators, Prog. Photovolt. 8, pp. 93–111, 2000.
4. M. Green et al., Solar cell efficiency tables, Prog. Photovolt. 16, pp. 435–440, 2008.
5. N. J. Ekins-Daukes et al., Strain-balanced criteria for multiple quantum well structures
and its signature in X-ray rocking curves, Cryst. Growth Des. 2, pp. 287–292, 2002.
6. K. Arthur and K. Barnham, Quantasol exploits quantum effects, Comp. Semiconduct., p. 21, January/February 2008.
7. D. Johnson et al., Observation of photon recycling in strain-balanced quantum well
solar cells, Appl. Phys. Lett. 90, p. 213505, 2007.
N. J. Ekins-Daukes, I. M. Ballard, and J. P. Connolly
Blackett Laboratory
Imperial College London
London, UK
N. J. Ekins-Daukes is a lecturer in the Department of Physics and
at the Grantham Institute for Climate Change at Imperial College London.
I. M. Ballard is a postdoctoral researcher in the Department of
Physics at Imperial College London.
J. P. Connolly is a postdoctoral researcher in the Department of
Physics at Imperial College London.
c 2008 SPIE