Nanocrystal solar cell
Encyclopedia
Quantum dot solar cells are an emerging field in solar cell
research that uses quantum dot
s as the photovoltaic material, as opposed to better-known 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 the quantum dot size. This is in contrast to bulk materials, where the bandgap is fixed by the choice of material composition. This property makes quantum dots attractive for multi-junction solar cells, where a variety of different energy levels are used to extract more power from the solar spectrum.
The potential performance of the quantum dot approach has led to widespread research in the field. Early examples used costly molecular beam epitaxy
processes, but alternative inexpensive fabrication methods have been developed. These attempts rely on quantum dot synthesis using wet chemistry (colloidal quantum dots – CQDs) and subsequent solution processibility of quantum dots. CQD solar cells currently hold the performance record for quantum dot solar cells. Their efficiency of 5.1% is yet low compared to that of commercial bulk silicon cells (about 17%), but it has a potential for improvement.
producing an electron-hole (e-h) pair. This pair is separated by an internal electric field and the resulting flow of electrons and holes creates electric current. The internal electric field is created by doping
one part of semiconductor with atoms which act as electron donors (n-type doping) and another with electron acceptors (p-type doping) that results in a p-n junction
. Generation of e-h pair requires that the photons of light have energy exceeding the bandgap of the material. Whereas photons with lower energies produce negligible amount of e-h pairs, higher energy photons are relatively inefficient: they produce an energetic e-h pair which quickly (within about 10−13 s) loses its energy through collisions with the lattice ("thermalizes"). As a result, most photon energy is lost into heat that lowers the conversion efficiency of light into electricity. The detailed balance calculation shows that this efficiency can not exceed 31% if one uses a single material for a solar cell.
Numerical analysis shows that the 31% efficiency is achieved when the solar cell material has a bandgap of 1.13 eV, corresponding to light in the near infrared. This band gap nearly matches that of silicon (1.1 eV), one of the many reasons this material dominates solar cell production. It is possible to greatly improve on a single-junction cell by stacking extremely thin cells with different bandgaps on top of each other – the "tandem cell" or "multi-junction" approach. The same basic analysis shows that a two layer cell should have one layer tuned to 1.64 eV and the other at 0.94 eV, with 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 loss mechanisms accounting for the rest.
Traditional silicon preparation methods do not lend themselves to this approach. There has been some progress using thin-films of amorphous silicon
, but other issues have prevented these from matching the performance of traditional cells. Most tandem-cell structures are based on higher performance semiconductors, notably gallium arsenide (GaAs). Three-layer InGaAs/GaAs/InGaP cells (bandgaps 1.89/1.42/0.94 eV) hold the efficiency record of 42.3% for experimental examples.
considerations, the electron energies that can exist within them are limited. These energy levels, defined by the size of quantum dots, in turn define the bandgaps. The dots can be grown to any needed size, allowing them to be tuned across a wide variety of bandgaps without changing the underlying material or construction techniques. In typical preparations, the tuning is accomplished by varying the duration or temperature of synthesis.
The ability to tune the bandgap is what makes them desirable for solar cell use. In this respect they are similar to the existing expensive GaAs tandem cells, and in theory have efficiencies on the same order. But CQDs can improve this further. In particular, lead sulfide
(PbS) CQDs have bandgaps that can be tuned into the far infrared, energy levels that are normally unseen to traditional materials. Half of all the solar energy reaching the Earth is in the infrared, most of it in the near infrared region. With a quantum dot solar cell, IR-sensitive materials are just as easy to use as any other, opening the possibility of capturing much more energy cost-effectively.
Moreover, CQDs are far easier to make than GaAs materials, and in some cases even simpler than traditional silicon. When suspended in a colloidal liquid form they can be easily handled throughout production, with the most complex equipment needed being a fume hood while the solvents outgas. The entire production process takes place at room temperature or on a hotplate, dramatically reducing handling issues and energy input. Although the base semiconductor material might require a complex preparation before being made into dots, even then the material does not have to be produced in large blocks, significantly reducing operational costs. Although current production is limited and the materials are relatively expensive, the price should be significantly reduced in mass production.
The dots can be distributed on a substrate through spin coating, either by hand or in an easily automated process. In large-scale production this technique could be replaced by spray-on or roll-printing systems, which dramatically reduces module construction costs.
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 provides the electrons. This dye is relatively expensive, and ruthenium
is a rare metal. Another drawback of the design is that it requires direct contact between the dye molecules suspended in the film and the rear electrode to return electrons to the dye. In most designs, this is handled by a liquid electrolyte between the two, making the design susceptible to leakage and freezing. Finally, in order for the energy levels to work out, the front electrode has to be transparent. Such electrode is usually made of indium tin oxide
(ITO), which is fragile and contains expensive indium metal.
Quantum dots as an alternative to the molecular dyes was considered from the earliest days of DSSC research. The ability to tune the bandgap means the designer can select a wider variety materials for other portions of the cell. The collaborating groups from the University of Toronto
and École Polytechnique Fédérale de Lausanne
developed a new design based on a rear electrode directly in contact with a film of quantum dots, eliminating the electrolyte and forming a depleted heterojunction
. To date these cells have reached reached 5.1% efficiency, comparable to the best solid-state DSSC devices, but still below those based on liquid electrolytes.
(CdTe). A colloidal suspension of these crystals is spin-cast onto a suitable substrate, often a thin glass slide, potted in a conductive polymer. These did not use quantum dots, but had a number of features in common with them. In particular, the method of casting a thin layer of crystals would work just as well as with quantum dots, and the use of a thin film conductor would both be applicable with few changes. In low scale production quantum dots are more expensive to form than mass producing of nanocrystals, but the crystals are based on rare metals that are already subject to major price swings, whereas a wide variety of materials can be used to make suitable dots.
Experiments using a variety of CQDs materials with spin-casting techniques started at the Sargent Group in the mid-2000s. In one noteworthy experiment, the group used lead selenide as an infrared
-sensitive electron donor to produce the highest-efficiency IR solar cells ever built. The true advantage of this technique, however, is that it yields the prospect of combining the quantum dots inherent tunability with a simple manufacturing process to allow the construction of "tandem" cells of greatly reduced cost. The cells use a rear layer of gold
as an electrode, but recent experiments have shown that nickel
works just as well. This would greatly reduce the cost of the system in large-scale production.
In the case of quantum dots, or other nanostructured donors, it is possible to cast cells as uniform films that avoid the problems with defects. These would still be subject to other issues inherent to quantum dots in general, notably resistivity issues and heat retention. It appears there has been little active development along these lines.
in Golden, Colorado
reported a spectroscopic evidence that several excitons could be efficiently generated upon absorption of a single, energetic photon in a quantum dot. This opens up the possibility of a different approach to the same problems that tandem cells attempt to solve, capturing more of the energy in highly energetic visible photons in sunlight. In this approach, known as "multiple exciton generation" (MEG), the quantum dot is tuned to release multiple electrons at a lower energy instead of one high-energy electron. This increases the cell efficiency. The dots in NRELs example where made from lead sulfide
.
In 2010, a team at the University of Wyoming
demonstrated similar performance using cells based on the DCCS design. In their examples, PbS quantum dots demonstrated two-electron ejection when the incoming photons had about three times the base bandgap energy.
NREL maintains an active research effort developing silicon-based quantum dots. In order to become cost effective, any new solar cell design will have to compete with the existing silicon industry. Silicon is plentiful and inexpensive in bulk form, it is only the processing that makes it expensive. If quantum dots with suitable properties can be made from silicon, they can compete on a cost basis. In 2007, NREL demonstrated that MEG occurs in silicon quantum dots as well as in the PbS dots.
as well as low cost, clean power generation
and an efficiency of 65%.
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
research that uses quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
s as the photovoltaic material, as opposed to better-known bulk materials such as silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, copper indium gallium selenide
Copper indium gallium selenide
Copper indium gallium selenide is a I-III-VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide and copper gallium selenide...
(CIGS) or CdTe. Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing the quantum dot size. This is in contrast to bulk materials, where the bandgap is fixed by the choice of material composition. This property makes quantum dots attractive for multi-junction solar cells, where a variety of different energy levels are used to extract more power from the solar spectrum.
The potential performance of the quantum dot approach has led to widespread research in the field. Early examples used costly molecular beam epitaxy
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
processes, but alternative inexpensive fabrication methods have been developed. These attempts rely on quantum dot synthesis using wet chemistry (colloidal quantum dots – CQDs) and subsequent solution processibility of quantum dots. CQD solar cells currently hold the performance record for quantum dot solar cells. Their efficiency of 5.1% is yet low compared to that of commercial bulk silicon cells (about 17%), but it has a potential for improvement.
Basic solar cell concepts
In a conventional solar cell, light is absorbed by a semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
producing an electron-hole (e-h) pair. This pair is separated by an internal electric field and the resulting flow of electrons and holes creates electric current. The internal electric field is created by doping
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
one part of semiconductor with atoms which act as electron donors (n-type doping) and another with electron acceptors (p-type doping) that results in a p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
. Generation of e-h pair requires that the photons of light have energy exceeding the bandgap of the material. Whereas photons with lower energies produce negligible amount of e-h pairs, higher energy photons are relatively inefficient: they produce an energetic e-h pair which quickly (within about 10−13 s) loses its energy through collisions with the lattice ("thermalizes"). As a result, most photon energy is lost into heat that lowers the conversion efficiency of light into electricity. The detailed balance calculation shows that this efficiency can not exceed 31% if one uses a single material for a solar cell.
Numerical analysis shows that the 31% efficiency is achieved when the solar cell material has a bandgap of 1.13 eV, corresponding to light in the near infrared. This band gap nearly matches that of silicon (1.1 eV), one of the many reasons this material dominates solar cell production. It is possible to greatly improve on a single-junction cell by stacking extremely thin cells with different bandgaps on top of each other – the "tandem cell" or "multi-junction" approach. The same basic analysis shows that a two layer cell should have one layer tuned to 1.64 eV and the other at 0.94 eV, with 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 loss mechanisms accounting for the rest.
Traditional silicon preparation methods do not lend themselves to this approach. There has been some progress using thin-films of amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
, but other issues have prevented these from matching the performance of traditional cells. Most tandem-cell structures are based on higher performance semiconductors, notably gallium arsenide (GaAs). Three-layer InGaAs/GaAs/InGaP cells (bandgaps 1.89/1.42/0.94 eV) hold the efficiency record of 42.3% for experimental examples.
Quantum dots
Quantum dots are particles of semiconductor material with the size so small that, due to quantum mechanicsQuantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
considerations, the electron energies that can exist within them are limited. These energy levels, defined by the size of quantum dots, in turn define the bandgaps. The dots can be grown to any needed size, allowing them to be tuned across a wide variety of bandgaps without changing the underlying material or construction techniques. In typical preparations, the tuning is accomplished by varying the duration or temperature of synthesis.
The ability to tune the bandgap is what makes them desirable for solar cell use. In this respect they are similar to the existing expensive GaAs tandem cells, and in theory have efficiencies on the same order. But CQDs can improve this further. In particular, lead sulfide
Lead(II) sulfide
Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....
(PbS) CQDs have bandgaps that can be tuned into the far infrared, energy levels that are normally unseen to traditional materials. Half of all the solar energy reaching the Earth is in the infrared, most of it in the near infrared region. With a quantum dot solar cell, IR-sensitive materials are just as easy to use as any other, opening the possibility of capturing much more energy cost-effectively.
Moreover, CQDs are far easier to make than GaAs materials, and in some cases even simpler than traditional silicon. When suspended in a colloidal liquid form they can be easily handled throughout production, with the most complex equipment needed being a fume hood while the solvents outgas. The entire production process takes place at room temperature or on a hotplate, dramatically reducing handling issues and energy input. Although the base semiconductor material might require a complex preparation before being made into dots, even then the material does not have to be produced in large blocks, significantly reducing operational costs. Although current production is limited and the materials are relatively expensive, the price should be significantly reduced in mass production.
The dots can be distributed on a substrate through spin coating, either by hand or in an easily automated process. In large-scale production this technique could be replaced by spray-on or roll-printing systems, which dramatically reduces module construction costs.
Early concepts
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 production 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 TiO2Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
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 provides the electrons. This dye is relatively expensive, and ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
is a rare metal. Another drawback of the design is that it requires direct contact between the dye molecules suspended in the film and the rear electrode to return electrons to the dye. In most designs, this is handled by a liquid electrolyte between the two, making the design susceptible to leakage and freezing. Finally, in order for the energy levels to work out, the front electrode has to be transparent. Such electrode is usually made of indium tin oxide
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
(ITO), which is fragile and contains expensive indium metal.
Quantum dots as an alternative to the molecular dyes was considered from the earliest days of DSSC research. The ability to tune the bandgap means the designer can select a wider variety materials for other portions of the cell. The collaborating groups from the University of Toronto
University of Toronto
The University of Toronto is a public research university in Toronto, Ontario, Canada, situated on the grounds that surround Queen's Park. It was founded by royal charter in 1827 as King's College, the first institution of higher learning in Upper Canada...
and École Polytechnique Fédérale de Lausanne
École polytechnique fédérale de Lausanne
The École polytechnique fédérale de Lausanne is one of the two Swiss Federal Institutes of Technology and is located in Lausanne, Switzerland.The school was founded by the Swiss Federal Government with the stated mission to:...
developed a new design based on a rear electrode directly in contact with a film of quantum dots, eliminating the electrolyte and forming a depleted heterojunction
Heterojunction
A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
. To date these cells have reached reached 5.1% efficiency, comparable to the best solid-state DSSC devices, but still below those based on liquid electrolytes.
Multi-junction efforts
During this period, other teams were working with nanocrystals of other semiconductors, notably cadmium tellurideCadmium telluride
Cadmium telluride is a crystalline compound formed from cadmium and tellurium. It is used as an infrared optical window and a solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell...
(CdTe). A colloidal suspension of these crystals is spin-cast onto a suitable substrate, often a thin glass slide, potted in a conductive polymer. These did not use quantum dots, but had a number of features in common with them. In particular, the method of casting a thin layer of crystals would work just as well as with quantum dots, and the use of a thin film conductor would both be applicable with few changes. In low scale production quantum dots are more expensive to form than mass producing of nanocrystals, but the crystals are based on rare metals that are already subject to major price swings, whereas a wide variety of materials can be used to make suitable dots.
Experiments using a variety of CQDs materials with spin-casting techniques started at the Sargent Group in the mid-2000s. In one noteworthy experiment, the group used lead selenide as an infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
-sensitive electron donor to produce the highest-efficiency IR solar cells ever built. The true advantage of this technique, however, is that it yields the prospect of combining the quantum dots inherent tunability with a simple manufacturing process to allow the construction of "tandem" cells of greatly reduced cost. The cells use a rear layer of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
as an electrode, but recent experiments have shown that nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
works just as well. This would greatly reduce the cost of the system in large-scale production.
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. There was great hope in the 1980s that thin films of silicon or other materials would avoid this, and capture some of this extra energy. These films are amorphous, and in practice the defects that are inherent to these materials overwhelmed this advantage. Modern thin-film cells are generally less efficient than traditional silicon.In the case of quantum dots, or other nanostructured donors, it is possible to cast cells as uniform films that avoid the problems with defects. These would still be subject to other issues inherent to quantum dots in general, notably resistivity issues and heat retention. It appears there has been little active development along these lines.
Multiple exciton generation
In 2005, the National Renewable Energy LaboratoryNational Renewable Energy Laboratory
The National Renewable Energy Laboratory , located in Golden, Colorado, is the United States' primary laboratory for renewable energy and energy efficiency research and development. The National Renewable Energy Laboratory is a government-owned, contractor-operated facility; it is funded through...
in Golden, Colorado
Golden, Colorado
The City of Golden is a home rule municipality that is the county seat of Jefferson County, Colorado, United States. Golden lies along Clear Creek at the edge of the foothills of the Front Range of the Rocky Mountains. Founded during the Pike's Peak Gold Rush on 16 June 1859, the mining camp was...
reported a spectroscopic evidence that several excitons could be efficiently generated upon absorption of a single, energetic photon in a quantum dot. This opens up the possibility of a different approach to the same problems that tandem cells attempt to solve, capturing more of the energy in highly energetic visible photons in sunlight. In this approach, known as "multiple exciton generation" (MEG), the quantum dot is tuned to release multiple electrons at a lower energy instead of one high-energy electron. This increases the cell efficiency. The dots in NRELs example where made from lead sulfide
Lead(II) sulfide
Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....
.
In 2010, a team at the University of Wyoming
University of Wyoming
The University of Wyoming is a land-grant university located in Laramie, Wyoming, situated on Wyoming's high Laramie Plains, at an elevation of 7,200 feet , between the Laramie and Snowy Range mountains. It is known as UW to people close to the university...
demonstrated similar performance using cells based on the DCCS design. In their examples, PbS quantum dots demonstrated two-electron ejection when the incoming photons had about three times the base bandgap energy.
NREL maintains an active research effort developing silicon-based quantum dots. In order to become cost effective, any new solar cell design will have to compete with the existing silicon industry. Silicon is plentiful and inexpensive in bulk form, it is only the processing that makes it expensive. If quantum dots with suitable properties can be made from silicon, they can compete on a cost basis. In 2007, NREL demonstrated that MEG occurs in silicon quantum dots as well as in the PbS dots.
Other issues
Although research is still at a pre-commercialization stage, in the future quantum dot based photovoltaics may offer advantages such as mechanical flexibility (as in quantum dot-polymer composite photovoltaics)as well as low cost, clean power generation
and an efficiency of 65%.
Other third generation solar cells
- Photoelectrochemical cellPhotoelectrochemical cellPhotoelectrochemical cells or PECs are solar cells which generate electrical energy from light, including visible light. Some photoelectrochemical cells simply produce electrical energy, while others produce hydrogen in a process similar to the electrolysis of water.-Photogeneration cell:In this...
- Polymer solar cellPolymer solar cellPolymer solar cells are a type of flexible solar cell. They can come in many forms including: organic solar cell , or organic chemistry photovoltaic cell that produce electricity from sunlight using polymers. There are also other types of more stable thin-film semiconductors that can be deposited...
External links
- Science News Online, Quantum-Dots Leap: Tapping tiny crystals' inexplicable light-harvesting talent, June 3, 2006.
- InformationWeekInformationWeekInformationWeek is a weekly print magazine, an online site with corresponding face-to-face and virtual events, and research. It is headquartered in San Francisco, California and was first published in 1979 by CMP Media, later called CMP Technology. On February 29, 2008, CMP Technology was...
, Nanocrystal Discovery Has Solar Cell Potential, January 6, 2006. - Berkeley Lab, Berkeley Lab Air-stable Inorganic Nanocrystal Solar Cells Processed from Solution, 2005.
- ScienceDaily, Sunny Future For Nanocrystal Solar Cells, October 23, 2005.