Quantum dot solar cell | Wikipedia audio article

Quantum dot solar cell | Wikipedia audio article


A quantum dot solar cell (QDSC) is a solar
cell design that uses quantum dots as the absorbing photovoltaic material. It attempts to replace bulk materials such
as silicon, copper indium gallium selenide (CIGS) or CdTe. Quantum dots have bandgaps that are tunable
across a wide range of energy levels by changing their size. In bulk materials, the bandgap is fixed by
the choice of material(s). This property makes quantum dots attractive
for multi-junction solar cells, where a variety of materials are used to improve efficiency
by harvesting multiple portions of the solar spectrum. As of 2016, efficiency exceeds 10%.==Background=====Solar cell concepts===
In a conventional solar cell, light is absorbed by a semiconductor, producing an electron-hole
(e-h) pair; the pair may be bound and is referred to as an exciton. This pair is separated by an internal electrochemical
potential (present in p-n junctions or Schottky diodes) and the resulting flow of electrons
and holes creates electric current. The internal electrochemical potential is
created by doping one part of semiconductor interface with atoms that act as electron
donors (n-type doping) and another with electron acceptors (p-type doping) that results in
a p-n junction. Generation of an e-h pair requires that the
photons have energy exceeding the bandgap of the material. Effectively, photons with energies lower than
the bandgap do not get absorbed, while those that are higher can quickly (within about
10−13 s) thermalize to the band edges, reducing output. The former limitation reduces current, while
the thermalization reduces the voltage. As a result, semiconductor cells suffer a
trade-off between voltage and current (which can be in part alleviated by using multiple
junction implementations). The detailed balance calculation shows that
this efficiency can not exceed 33% if one uses a single material with an ideal bandgap
of 1.34 eV for a solar cell.The band gap (1.34 eV) of an ideal single-junction cell is close
to that of silicon (1.1 eV), one of the many reasons that silicon dominates the market. However, silicon’s efficiency is limited to
about 30% (Shockley–Queisser limit). It is possible to improve on a single-junction
cell by vertically stacking cells with different bandgaps – termed a “tandem” or “multi-junction”
approach. The same analysis shows that a two layer cell
should have one layer tuned to 1.64 eV and the other to 0.94 eV, providing a theoretical
performance of 44%. A three-layer cell should be tuned to 1.83,
1.16 and 0.71 eV, with an efficiency of 48%. An “infinity-layer” cell would have a theoretical
efficiency of 86%, with other thermodynamic loss mechanisms accounting for the rest.Traditional
(crystalline) silicon preparation methods do not lend themselves to this approach due
to lack of bandgap tunability. Thin-films of amorphous silicon, which due
to a relaxed requirement in crystal momentum preservation can achieve direct bandgaps and
intermixing of carbon, can tune the bandgap, but other issues have prevented these from
matching the performance of traditional cells. Most tandem-cell structures are based on higher
performance semiconductors, notably indium gallium arsenide (InGaAs). Three-layer InGaAs/GaAs/InGaP cells (bandgaps
0.94/1.42/1.89 eV) hold the efficiency record of 42.3% for experimental examples.However,
the QDSCs suffer from weak absorption and the contribution of the light absorption at
room temperature is marginal. This can be addressed by utilizing multibranched
Au nanostars.===Quantum dots===
Quantum dots are semiconducting particles that have been reduced below the size of the
Exciton Bohr radius and due to quantum mechanics considerations, the electron energies that
can exist within them become finite, much alike energies in an atom. Quantum dots have been referred to as “artificial
atoms”. These energy levels are tuneable by changing
their size, which in turn defines the bandgap. The dots can be grown over a range of sizes,
allowing them to express a variety of bandgaps without changing the underlying material or
construction techniques. In typical wet chemistry preparations, the
tuning is accomplished by varying the synthesis duration or temperature. The ability to tune the bandgap makes quantum
dots desirable for solar cells. For the sun’s photon distribution spectrum,
the Shockley-Queisser limit indicates that the maximum solar conversion efficiency occurs
in a material with a band gap of 1.34 eV. However, materials with lower band gaps will
be better suited to generate electricity from lower-energy photons (and vice versa). Single junction implementations using lead
sulfide (PbS) colloidal quantum dots (CQD) have bandgaps that can be tuned into the far
infrared, frequencies that are typically difficult to achieve with traditional solar cells. Half of the solar energy reaching the Earth
is in the infrared, most in the near infrared region. A quantum dot solar cell makes infrared energy
as accessible as any other.Moreover, CQD offer easy synthesis and preparation. While suspended in a colloidal liquid form
they can be easily handled throughout production, with a fumehood as the most complex equipment
needed. CQD are typically synthesized in small batches,
but can be mass-produced. The dots can be distributed on a substrate
by spin coating, either by hand or in an automated process. Large-scale production could use spray-on
or roll-printing systems, dramatically reducing module construction costs.==Production==
Early examples used costly molecular beam epitaxy processes. However, the lattice mismatch results in accumulation
of strain and thus generation of defects, restricting the number of stacked layers. Droplet epitaxy growth technique shows its
advantages on the fabrication of strain-free QDs. Alternatively, less expensive fabrication
methods were later developed. These use wet chemistry (for CQD) and subsequent
solution processing. Concentrated nanoparticle solutions are stabilized
by long hydrocarbon ligands that keep the nanocrystals suspended in solution. To create a solid, these solutions are cast
down and the long stabilizing ligands are replaced with short-chain crosslinkers. Chemically engineering the nanocrystal surface
can better passivate the nanocrystals and reduce detrimental trap states that would
curtail device performance by means of carrier recombination. This approach produces an efficiency of 7.0%.A
more recent study uses different ligands for different functions by tuning their relative
band alignment to improve the performance to 8.6%. The cells were solution-processed in air at
room-temperature and exhibited air-stability for more than 150 days without encapsulation. In 2014 the use of iodide as a ligand that
does not bond to oxygen was introduced. This maintains stable n- and p-type layers,
boosting the absorption efficiency, which produced power conversion efficiency up to
8%.==History==
The idea of using quantum dots as a path to high efficiency was first noted by Burnham
and Duggan in 1990. At the time, the science of quantum dots,
or “wells” as they were known, was in its infancy and early examples were just becoming
available.===DSSC efforts===
Another modern cell design is the dye-sensitized solar cell, or DSSC. DSSCs use a sponge-like layer of TiO2 as the
semiconductor valve as well as a mechanical support structure. During construction, the sponge is filled
with an organic dye, typically ruthenium-polypyridine, which injects electrons into the titanium
dioxide upon photoexcitation. This dye is relatively expensive, and ruthenium
is a rare metal.Using quantum dots as an alternative to molecular dyes was considered from the
earliest days of DSSC research. The ability to tune the bandgap allowed the
designer to select a wider variety of materials for other portions of the cell. Collaborating groups from the University of
Toronto and École Polytechnique Fédérale de Lausanne developed a design based on a
rear electrode directly in contact with a film of quantum dots, eliminating the electrolyte
and forming a depleted heterojunction. These cells reached 7.0% efficiency, better
than the best solid-state DSSC devices, but below those based on liquid electrolytes.===Multi-junction===
Traditionally, multi-junction solar cells are made with a collection of multiple semiconductor
materials. Because each material has a different band
gap, each material’s p-n junction will be optimized for a different incoming wavelength
of light. Using multiple materials enables the absorbance
of a broader range of wavelengths, which increases the cell’s electrical conversion efficiency. However, the use of multiple materials makes
multi-junction solar cells too expensive for many commercial uses. Because the band gap of quantum dots can be
tuned by adjusting the particle radius, multi-junction cells can be manufactured by incorporating
quantum dot semiconductors of different sizes (and therefore different band gaps). Using the same material lowers manufacturing
costs, and the enhanced absorption spectrum of quantum dots can be used to increase the
short-circuit current and overall cell efficiency. Cadmium telluride (CdTe) is used for cells
that absorb multiple frequencies. A colloidal suspension of these crystals is
spin-cast onto a substrate such as a thin glass slide, potted in a conductive polymer. These cells did not use quantum dots, but
shared features with them, such as spin-casting and the use of a thin film conductor. At low production scales quantum dots are
more expensive than mass-produced nanocrystals, but cadmium and telluride are rare and highly
toxic metals subject to price swings. The Sargent Group used lead sulfide as an
infrared-sensitive electron donor to produce then record-efficiency IR solar cells. Spin-casting may allow the construction of
“tandem” cells at greatly reduced cost. The original cells used a gold substrate as
an electrode, although nickel works just as well.===Hot-carrier capture===
Another way to improve efficiency is to capture the extra energy in the electron when emitted
from a single-bandgap material. In traditional materials like silicon, the
distance from the emission site to the electrode where they are harvested is too far to allow
this to occur; the electron will undergo many interactions with the crystal materials and
lattice, giving up this extra energy as heat. Amorphous thin-film silicon was tried as an
alternative, but the defects inherent to these materials overwhelmed their potential advantage. Modern thin-film cells remain generally less
efficient than traditional silicon. Nanostructured donors can be cast as uniform
films that avoid the problems with defects. These would be subject to other issues inherent
to quantum dots, notably resistivity issues and heat retention.===Multiple excitons===The Shockley-Queisser limit, which sets the
maximum efficiency of a single-layer photovoltaic cell to be 33.7%, assumes that only one electron-hole
pair (exciton) can be generated per incoming photon. Multiple exciton generation (MEG) is an exciton
relaxation pathway which allows two or more excitons to be generated per incoming high
energy photon. In traditional photovoltaics, this excess
energy is lost to the bulk material as lattice vibrations (electron-phonon coupling). MEG occurs when this excess energy is transferred
to excite additional electrons across the band gap, where they can contribute to the
short-circuit current density. Within quantum dots, quantum confinement increases
coulombic interactions which drives the MEG process. This phenomenon also decreases the rate of
electron-phonon coupling, which is the dominant method of exciton relaxation in bulk semiconductors. The phonon bottleneck slows the rate of hot
carrier cooling, which allows excitons to pursue other pathways of relaxation; this
allows MEG to dominate in quantum dot solar cells. The rate of MEG can be optimized by tailoring
quantum dot ligand chemistry, as well as by changing the quantum dot material and geometry. In 2004, Los Alamos National Laboratory reported
spectroscopic evidence that several excitons could be efficiently generated upon absorption
of a single, energetic photon in a quantum dot. Capturing them would catch more of the energy
in sunlight. In this approach, known as “carrier multiplication”
(CM) or “multiple exciton generation” (MEG), the quantum dot is tuned to release multiple
electron-hole pairs at a lower energy instead of one pair at high energy. This increases efficiency through increased
photocurrent. LANL’s dots were made from lead selenide. In 2010, the University of Wyoming demonstrated
similar performance using DCCS cells. Lead-sulfur (PbS) dots demonstrated two-electron
ejection when the incoming photons had about three times the bandgap energy.In 2005, NREL
demonstrated MEG in quantum dots, producing three electrons per photon and a theoretical
efficiency of 65%. In 2007, they achieved a similar result in
silicon.===Non-oxidizing===
In 2014 a University of Toronto group manufactured and demonstrated a type of CQD n-type cell
using PbS with special treatment so that it doesn’t bind with oxygen. The cell achieved 8% efficiency, just shy
of the current QD efficiency record. Such cells create the possibility of uncoated
“spray-on” cells. However, these air-stable n-type CQD were
actually fabricated in an oxygen-free environment. Also in 2014, another research group at MIT
demonstrated air-stable ZnO/PbS solar cells that were fabricated in air and achieved a
certified 8.55% record efficiency (9.2% in lab) because they absorbed light well, while
also transporting charge to collectors at the cell’s edge. These cells show unprecedented air-stability
for quantum dot solar cells that the performance remained unchanged for more than 150 days
of storage in air.==Market Introduction=====
Commercial Providers===Although quantum dot solar cells have yet
to be commercially viable on the mass scale, several small commercial providers have begun
marketing quantum dot photovoltaic products. Investors and financial analysts have identified
quantum dot photovoltaics as a key future technology for the solar industry. Quantum Materials Corp. (QMC) and subsidiary Solterra Renewable Technologies
are developing and manufacturing quantum dots and nanomaterials for use in solar energy
and lighting applications. With their patented continuous flow production
process for perovskite quantum dots, QMC hopes to lower the cost of quantum dot solar cell
production in addition to applying their nanomaterials to other emerging industries.QD Solar takes
advantage of the tunable band gap of quantum dots to create multi-junction solar cells. By combining efficient silicon solar cells
with infrared solar cells made from quantum dots, QD Solar aims to harvest more of the
solar spectrum. QD Solar’s inorganic quantum dots are processed
with high-throughput and cost-effective technologies and are more light- and air- stable than polymeric
nanomaterials.UbiQD is developing photovoltaic windows using quantum dots as fluorophores. They have designed a luminescent solar concentrator
(LSC) using near-infrared quantum dots which are cheaper and less toxic than traditional
alternatives. UbiQD hopes to provide semi-transparent windows
that convert passive buildings into energy generation units, while simultaneously reducing
the heat gain of the building.===Safety Concerns===
Many heavy-metal quantum dot (lead/calcium chalcogenides such as PbSe, CdSe) semiconductors
can be cytotoxic and must be encapsulated in a stable polymer shell to prevent exposure. Non-toxic quantum dot materials such as AgBiS2
nanocrystals have been explored due to their safety and abundance; exploration with solar
cells based with these materials have demonstrated comparable conversion efficiencies and short-circuit
current densities. UbiQD’s CuInSe2-X quantum dot material is
another example of a non-toxic semiconductor compound.==See also==Emerging photovoltaics
Nanocrystalline silicon Nanoparticle
Photoelectrochemical cell Polymer solar cell

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