ISIS Report 20/07/09
Quantum Well Solar Cells
Trapping solar energy in quantum wells increases gain and efficiency of
solar cells Brett Cherry
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A ‘quantum well’ is a potential well that confines particles
to two dimensions that are otherwise free to move in three dimensions. Both
electrons and holes can be confined in semiconductor quantum wells. The effect
is to increase the gain and efficiency of the solid state device such as lasers
in CD or DVD players, infrared imaging, and more recently, solar cells.
How quantum wells trap solar energy
A quantum well is basically a semiconductor with a small
energy gap (or band gap) sandwiched between two thicker layers of semiconductor(s)
with a large energy gap, such as gallium arsenide (GaAs). (see [1] Solar Power For The Masses for a
description of a solar cell). Quantum wells in solar cells confine electrons
and holes that normally move in three dimensions to two dimensions. The number
of electrons and holes confined is determined by the thickness of the semiconductor
used, usually ranging from 1-10 nanometres. Confining electrons within quantum
wells allows them to be easily converted to useful forms of energy, and it
is the thinness of the semiconductor material that allows this to happen.
Quantum well solar cells are built with multiple
nanoscale semiconductors layered on top of one another with a lateral conduction
layer between the substrate and n region to allow contact between each device
(Fig. 1). In solar cells, the quantum wells making up part of the thin i layer
in a p-i-n junction confines the electrons to two dimensions. This means electrons
and holes are quantized, having discrete levels of energy. Within the i layer
of the junction, the potential energy of an electron is less than the outside
layer so the flow of charge is confined to certain well-defined regions that
can be exploited in solar photovoltaics. Quantum wells are grown by molecular
beam epitaxy, where atoms of the materials are delivered to crystals using
a molecular beam or through chemical vapour deposition, using a flowing gas.
Figure 1. Quantum well solar cell. QW – Quantum Well. LCL
– lateral conduction layer [2]
A quantum solar journey in the making
Professor Keith Barnham of Imperial College London, who
invented the quantum well solar cell in 1989, was originally funded by the
Greenpeace Environmental Trust. Barnham is now Chief Scientific Officer and
Director of QuantaSol, an independent UK-based solar PV company that will
bring quantum well solar cells to the solar industry. The solar cells developed
by Barnham operate at high current. The i region consists of alternating
layers of indium gallium arsenide (InGaAs) and gallium arsenide phosphide
(GaAsp), while the p and n layers of the solar cell are made from gallium
arsenide GaAs. A “strain balance” technique is used to grow the different
layers, which matches the lattice structures of the different semiconducter
materials, preventing defects [3]. This method allows
more than 65 wells to be grown on top of one another without dislocation [2].
Figure 2. Strain-balanced quantum well solar cell [3]
The band gap of Barnham’s single junction quantum well solar cell
is also better matched to the solar spectrum at 1.33eV than a 1.42eV gallium
arsenide (GaAs) solar cell [4] (see [5] Very
High Efficiency Solar Cells, SiS 43). Quantum well solar cells avoid
efficiency losses that plague most solar cells because the quantum wells have
a lower band gap than the rest of the cell as illustrated in Fig. 2 where the
band gap (V) is higher than the absorption threshold (Ea) of the
quantum wells; this allows electrons to enter the wells after being hit by incoming
lower-energy photons, contributing extra current [6]. It also reduces dark
current, an electric current generated from the potential difference between
the terminals of the PV cell that flows in the opposite direction of the photocurrent,
decreasing net efficiency. A 1cm x 1cm multi-junction quantum well solar cell
can reach a high current of 7 Amps at 500x solar concentration [4]. “We can
never get rid of all the traps and defects and so on, but we can fill’em up
by operating at a higher current,” said Barnham.
Like conventional solar PV for micro-generation,
the cost per kWh for industrial scale solar PV depends largely on various
environmental factors such as the amount of solar insolation (the solar power
density incident on a surface of stated area and orientation, usually expressed
as Watts per square metre) and solar cell efficiency. Assuming a full life
cycle of 25 years for the concentrating photovoltaic (CPV) power plant, the
best estimate today would be near $0.20 US per kWh. QuantaSol expects to be
competitive with current CPV rates if not better. In comparison, current global
average cost per kWh for a 2 kWh residential system
is $0.37 US in a “sunny climate” and $0.81 US per kWh in a “cloudy climate”
[7]. This average does not include government rebates, such as feed-in tariffs
in countries such as Germany, so prices for individual countries will
vary. Projected costs for solar electricity are expected to continue to drop
in the future as manufacturing costs go down. China, for example, is planning
to reduce the cost of solar power generation to $0.146 US per kWh by 2012
[8]. With a combination of widespread solar systems
for micro-generation and industrial PV power plants, these lower energy prices
could be achieved at a fraction of the green house gases emitted today. Another
obstacle to overcome in making solar electricity more affordable for industrial
and small scale uses is storing solar energy [9]
(see Harvesting
Energy from Sunlight with Artificial Photosynthesis, SiS 43) when it is no longer available at night. In the
future, in order for solar electricity to be more sustainable and reliable,
new storage technologies must be developed, especially the use of solar light
to break down water into hydrogen and oxygen for
use as a fuel, as research in artificial photosynthesis has shown [10] [11]
(see Making Fuel
from Water and Splitting Water with Ease, SiS 43).
Photon Recycling
As losses are due to recombination, quantum wells create
a more efficient cell by allowing researchers to adjust the band gap in order
to minimise recombination. “If you adjust the band gap you can adjust how
much [energy] each cell produces,” said Barnham.
Once recombination is controlled in order to get
the most current possible, the next step is to find
ways of recovering losses of light absorbed by the cells through photon recycling
[3], as the bulk regions of the solar cell are transparent to radiation
from the quantum wells. Photon recycling can prevent incoming high-energy
photons from being wasted as heat.
To recycle photons, a reflector is used to reflect
light back to the cell so it can be reabsorbed to increase current. Reflectors
used include the Distributed Bragg Reflector (DBR, also used in fibre optics)
and the Luminescent Solar Concentrator (LSC) doped with quantum dots or nano-rods
to increase light absorption and remit it to the solar cells.
Barnham and his team discovered that internal
reflection within the cell itself is often the best form of photon recycling.
In order to achieve optimal reflection within the
solar cell in some cases an air gap is all that is needed [3]. Reflecting
back photon losses onto the solar cell results in an increase of 1.5 percent
efficiency for single junction solar cells and potentially more for multi-junction
solar cells.
New Solar World Record
Barnham and his team currently hold the record for highest
efficiency of a nano-structured solar cell at 30.6 percent, obtained from
a tandem-junction quantum well solar cell at a concentration of 54 suns. One
sun is about the amount of light that makes it to the Earth on a sunny day.
Recently, their gallium arsenide phosphide and indium gallium arsenide (GaAsp/InGaAs)
single junction solar cell has also broken the world record (28.2 percent)
for single junction cells at 28.3 percent with a concentration of 500+ suns.
Cost and Life Cycle
The life-cycle assessment (LCA) [12] (see Which Renewables? SiS
39) of solar PV cells provides an indicator
for how well solar cells perform in terms of energy payback and emissions
of greenhouse gases from ‘cradle to grave’. According to recent studies
on concentrating solar PV life cycles, the energy payback time (EPBT) is currently
0.7-1.3 years for one concentrating system examined, SolFocus [13].
This is slightly better than rooftop silicon PV solar cells that have an EPBT
of 1.1-2.7 years. Based on recent analyses, the amount of greenhouse
gases emitted by concentrating solar PV is not yet known, but for silicon
solar cells it is 30-55 g/kWh and CdTe (cadmium telluride or thin film PV)
is 21-25 g/kWh.
QuantaSol plans to make their quantum well solar
cells commercially available to solar concentrator manufacturers early next
year. Their goal is to develop quantum well solar cells specifically tailored
to the spectral conditions in relation to the placement of concentrators and
to optimise both for peak efficiency for utility-based solar PV. In time,
as the costs come down, quantum well solar cells with concentrators could
be used for micro-generation and for other conventional solar PV applications.
QuantaSol is currently researching how its quantum well solar cells will perform
under different environmental conditions to get the best possible cost per
kWh.
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There are 2 comments on this article so far. Add your comment
| Tom Blakeslee Comment left 20th July 2009 17:05:38
Such high solar concentration becomes very inefficient if clouds or haze are present. Here's a 42.8% cell with much less magnification:
http://www.udel.edu/PR/UDaily/2008/jul/solar072307.html | Brett Comment left 28th July 2009 12:12:30
Diffuse sun light due to cloud cover is certainly an important issue, this is why tracking is necessary. It is preferable to have direct sun in order to achieve high concentrations of sun light, but it depends on what type of concentrator you are using, along with environmental conditions (such as solar insolation) in order to accurately judge efficiency. The fact that single-junction solar cells are capable of reaching such high efficiencies is most important.
Research referred to at University of Delaware on very high effciency multi-junction solar cells is explained in this article: http://www.i-sis.org.uk/veryHighEfficiencySolarCells.php |
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