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 [1].
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
[2]. 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 [3]. 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 [4].
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 [5]. 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 [6], 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 [7].
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 [8].
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