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[SANET-MG] quantum dots and solar cells
ISIS Press Release 19/01/06
Quantum Dots and Ultra-Efficient Solar Cells?
Exciting new possibilities in harvesting solar power over the next
decade. Dr. Mae-Wan Ho
A fully referenced version of this article is posted on ISIS members’
website. Details here
Limit on efficiency
The efficiency of solar cells is the electrical power it puts out as
percentage of the power in incident sunlight. One of the most
fundamental limitations on the efficiency of a solar cell is the ‘band
gap’ of the semi-conducting material used in conventional solar cells:
the energy required to boost an electron from the bound valence band
into the mobile conduction band. When an electron is knocked loose from
the valence band, it goes into the conduction band as a negative charge,
leaving behind a ‘hole’ of positive charge. Both electron and hole can
migrate through the semi-conducting material.
In a solar cell, negatively doped (n-type) material with extra electrons
in its otherwise empty conduction band forms a junction with positively
doped (p-type) material, with extra holes in the band otherwise filled
with valence electrons. When a photon with energy matching the band gap
strikes the semiconductor, it is absorbed by an electron, which jumps to
the conduction band, leaving a hole. Both electron and hole migrate in
the junction’s electric field, but in opposite directions. If the solar
cell is connected to an external circuit, an electric current is
generated. If the circuit is open, then an electrical potential or
voltage is built up across the electrodes.
Photons with less energy than the band gap slip right through without
being absorbed, while photons with energy higher than the band gap are
absorbed, but their excess energy is wasted, and dissipated as heat. The
maximum efficiency that a solar cell made from a single material can
theoretically achieve is about 30 percent. In practice, the best
achievable is about 25 percent.
It is possible to improve on the efficiency by stacking materials with
different band gaps together in multi-junction cells. Stacking dozens of
different layers together can increase efficiency theoretically to
greater than 70 percent. But this results in technical problems such as
strain damages to the crystal layers. The most efficient multi-junction
solar cell is one that has three layers: gallium indium
phosphide/gallium arsenide/germanium (GaInP/GaAs/Ge) made by the
National Center for Photovoltaics in the US, which achieved an
efficiency of 34 percent in 2001 .
Recently, entirely new possibilities for improving the efficiency of
photovoltaics have opened up.
Quantum dot possibilities
Quantum dots or nanoparticles are semi-conducting crystals of nanometre
(a billionth of a metre) dimensions. They have quantum optical
properties that are absent in the bulk material due to the confinement
of electron-hole pairs (called excitons) on the particle, in a region of
a few nanometres.
The first advantage of quantum dots is their tunable bandgap. It means
that the wavelength at which they will absorb or emit radiation can be
adjusted at will: the larger the size, the longer the wavelength of
light absorbed and emitted . The greater the bandgap of a solar cell
semiconductor, the more energetic the photons absorbed, and the greater
the output voltage. On the other hand, a lower bandgap results in the
capture of more photons including those in the red end of the solar
spectrum, resulting in a higher output of current but at a lower output
voltage. Thus, there is an optimum bandgap that corresponds to the
highest possible solar-electric energy conversion, and this can also be
achieved by using a mixture of quantum dots of different sizes for
harvesting the maximum proportion of the incident light.
Another advantage of quantum dots is that in contrast to traditional
semiconductor materials that are crystalline or rigid, quantum dots can
be molded into a variety of different form, in sheets or
three-dimensional arrays. They can easily be combined with organic
polymers, dyes, or made into porous films (“Organic solar power”, this
series). In the colloidal form suspended in solution, they can be
processed to create junctions on inexpensive substrates such as
plastics, glass or metal sheets.
When quantum dots are formed into an ordered three-dimensional array,
there will be strong electronic coupling between them so that excitons
will have a longer life, facilitating the collection and transport of
‘hot carriers’ to generate electricity at high voltage. In addition,
such an array makes it possible to generate multiple excitons from the
absorption of a single photon (see later).
Quantum dots are offering the possibilities for improving the efficiency
of solar cells in at least two respects, by extending the band gap of
solar cells for harvesting more of the light in the solar spectrum, and
by generating more charges from a single photon.
Extending the solar cell band gap into infrared
Infrared photovoltaic cells – which transform infrared light into
electricity - are attracting much attention, as nearly half of the
approximately 1000Wm3 of the intensity of sunlight is within the
invisible infrared region. So it is possible to use the visible half for
direct lighting while harvesting the invisible for generating
electricity . Photovoltaic cells that respond to infrared –
‘thermovoltaics’ - can even capture radiation from a fuel-fire emitter;
and co-generation of electricity and heat are said to be quiet,
reliable, clean and efficient. A 1 cm2 silicon cell in direct sunlight
will generate about 0.01W, but an efficient infrared photovoltaic cell
of equal size can produce theoretically 1W in a fuel-fired system.
One development that has made infrared photovoltaics attractive is the
availability of light-sensitive conjugated polymers - polymers with
alternating single and double carbon-carbon (sometimes carbon-nitrogen)
bonds. It was discovered in the 1970s that chemical doping of conjugated
polymers increased electronic conductivity several orders of magnitude.
Since then, electronically conducting materials based on conjugated
polymers have found many applications including sensors, light-emitting
diodes, and solar cells .
Conjugated polymers provide ease of processing, low cost, physical
flexibility and large area coverage. They now work reasonably well
within the visible spectrum.
In order to make conjugated polymers work in the infrared range,
researchers at the University of Toronto wrapped the polymers around
lead sulphide quantum dots tuned (by size) to respond to infrared .
The polymer poly(2-methoxy-5-(2’-ethylhexyloxy-p-phenylenevinylene)]
(MEH-PPV) on its own absorbs between ~400 and ~600 nm. Quantum dots of
lead sulphide (PbS) have absorption peaks that can be tuned from ~800 to
~2000 nm. Wrapping MEH-PPV around the quantum dots shifted the polymer’s
absorption into the infrared.
The researchers demonstrated a convincing, albeit very small
photovoltaic effect, giving a power-conversion efficiency of 0.001
percent. Professor Ted Sargent, the lead scientist, is optimistic
however, emphasizing that their device is simply a prototype of how to
capture infrared energy , and predicts commercial implementation
within 3-5 years.
Multiple excitons from one photon
Researchers led by Arthur Nozik at the National Renewable Energy
Laboratory Golden, Colorado in the United States really grabbed the
headline when they demonstrated that the absorption of a single photon
by their quantum dots yielded - not one exciton as usually the case -
but three of them .
The formation of multiple excitons per absorbed photon happens when the
energy of the photon absorbed is far greater than the semiconductor band
gap. This phenomenon does not readily occur in bulk semiconductors where
the excess energy simply dissipates away as heat before it can cause
other electron-hole pairs to form. But in semi-conducting quantum dots,
the rate of energy dissipation is significantly reduced, and the charge
carriers are confined within a minute volume, thereby increasing their
interactions and enhancing the probability for multiple excitons to form.
The researchers report a quantum yield of 300 percent for 2.9nm diameter
PbSe (lead selenide) quantum dots when the energy of the photon absorbed
is four times that of the band gap. But multiple excitons start to form
as soon as the photon energy reaches twice the band gap. Quantum dots
made of lead sulphide (PbS) also showed the same phenomenon.
The findings are further confirmation of Nozik’s theoretical prediction
in 2000 that quantum dots could increase the efficiency of solar cells
through multiple exciton generation. In 2004, researchers Richard
Shaller and Victor Klimov at Los Alamos National Laboratory New Mexico
were the first to demonstrate this phenomenon experimentally using
quantum dots made of lead selenide.
“We have shown that solar cells based on quantum dots theoretically
could convert more than 65 percent of the sun’s energy into electricity,
approximately doubling the efficiency of solar cells”, said Nozik .
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