Jan 24, 2021
Review Paper: Using Quantum Dot Solar Cells to Improve Photovoltaics
Abstract
Solar energy provides us with free, abundant energy from the sun, supplying energy for one million homes in Canada. According to the National Academy of Engineering, the sun’s power output could supply 10,000 times more energy than the output of all commercial power plants combined. Solar’s potential is enormous and if we could get past its barriers, its efficiency could be significantly boosted and reliance on non-renewables like oil could slowly be shifted toward other resources. This paper covers the works of solar and its root cause issues, as well as how quantum dots could be utilized for boosting overall performance.
Outline of Paper
1. How Does Solar Energy Work?
2. The Problem with Solar
2.1. Band Gaps
2.2. Losses in Energy
3. Introduction to Nanotechnology
3.1. Different Dimensions
4. Quantum Dots and MEG
4.1. Quantum Dots
4.2. MEG
4.3. Materials and Toxicity
5. The Quantum Confinement Effect
5.1. “Artificial Atoms”
5.2. MEG via Quantum Confinement
5.3. EMA Model
6. Quantum Yields
7. Passivating Quantum Dots for Boosted Efficiency
7.1. Surface Defects
7.2. Dangling Bond Defects
7.3. Organic Passivation
7.4. Inorganic Passivation
7.5. PLQY
7.6. PL Lifetime
8. Thickness of Shells
8.1. Transmission Electron Microscopies
8.2. X-Ray Powder Diffractions
9. Engineering Band Gaps
9.1. Quantum Heterostructures
9.2. Strain
9.3. Alloying
9.4. Superlattices
10. Synthesis of Quantum Dots
10.1. Colloidal Synthesis
10.2. Plasma Synthesis
10.3. Electrochemical Assembly
11. Surpassing the Shockley-Queisser limit
11.1. Photo Upconversion
11.2. Multi-Junction Cells
11.3. Improving MEG
11.4. Thermal Upconversion
11.5. Light Concentration
12. Conclusion
1. How Does Solar Energy Work?
Solar panels are generally composed of layers of semiconductor-based photovoltaic cells, which absorb the sunlight’s rays during the day when the sun is shining.
Traditional solar cells are single-junction cells, meaning they only have one p-n junction. A p-n junction is like a divider between two semiconductor materials, one being the p-type and the other being the n-type. One side of a solar cell, the p-type, is doped with boron, and the other, the n-type, with phosphorus. This doping is what is responsible for the making of p-n junctions.
A solar cell essentially has two zones. The first zone is the p-zone having a low concentration of electrons, specifically one less valence electron compared to silicon, and the second is the n-zone having a high electron concentration, with one more valence electron than silicon. Because of this difference, a hole is left behind.
When the photons come into contact with a solar cell, an electron escapes the n-region, sprints through the circuit, and reaches the p-region. The travelling of the electrons from the n-zone to the p-zone creates electricity.
This electricity is direct current electricity, which must be converted to alternating current to power a home. DC can be changed into AC with the use of an inverter, a handy gadget whose job is to make this conversion. Depending on the household, this can also be done through microinverters, which are mini inverters that are connected to each panel instead of just one that works with the entire system.
Next, the AC electricity travels to the switchboard, which is responsible for sending electricity to household appliances. A switchboard is helpful because it warrants that additional electricity can be drawn from the grid when solar energy faces any shortages.
Households have bi-directional meters to measure the electricity flowing to and from a home (including if there is a surplus of power). From here, unused solar energy is returned to the electrical grid. This is what is called net-metering, and minimizes wastage of energy.
2. The Problem With Solar
Solar energy has hundreds of benefits, but the issue lies in how to harvest it effectively. The vast majority of commercially available solar cells are made from silicon. Despite costs having significantly decreased, they are still quite expensive which has always been a primary drawback with this energy source.
The other primary problem is that such cells only have an efficiency rate (efficiency rate meaning the amount of the sun’s energy converted to electrical energy) of 10–20%, which is rather low. Since the surface of solar cells reflect rays of sunlight (approximately 2–10%), then solar cells can lose up to 10% of acquired power due to direct optical losses. The remainder of the losses is primarily due to the intrinsic inefficiency of the electron-hole creation process.
2.1 Band Gaps
Solar cells can only harvest a specific amount of energy from a single solar photon. This is problematic, though, because they have the capability to do so much more. The energies of photons can range from 0.4 electron volts (infra-red photon) to 4 electron volts (ultraviolet photon). Why, then, do only some of these photons create electron-hole pairs? The answer lies in something called a band-gap. A material’s band gap is the energy needed to create an electron and a hole without any kinetic energy at a distance that makes the Coulombic attraction trivial — in other words, the range in which there can be no electron state. When two carriers come into contact, they create an exciton, a bound electron-hole pair.
Zero band-gaps mean there is no difference in energy between the valence electron band and the conduction electron band.
Colloidal quantum dots, which have solution based syntheses, have recently gained a lot of traction due to their optical and electronic properties, as well as their tunability. The quantum confinement effect allows energy gaps to be tuned with changes in the size of quantum dots.
2.2 Losses in Energy
Essentially, the band gap of a semiconductor dictates the amount of solar energy that can be converted to electrical power. Photons that have less energy than the band gap cannot be absorbed, photons with energy more than the band gap have excess energy released and wasted through emissions of vibrational energy (phonons). This is also called thermalization, which is a major contributor to losses within single-junction solar cells.
The available energy from current single-junction cells is only 33% per cell, another third being lost to thermalization, and another third being shared between non-absorbed photons and thermodynamic losses. This graph displays the losses on the solar spectrum.
3. Introduction to Nanotechnology
Nanotechnology is about studying and building structures and materials on the scale of atoms and molecules. A nanometre is equivalent to one billionth of a metre.
At these minuscule scales, quantum effects start to play a very significant role. A material’s entire behaviour can change, from its colour to its conductivity, hardness, strength, melting temperature, and reactivity, giving us the capability to control the properties of molecular structures and materials by changing their configuration and sizing.
3.1. Different Dimensions
When a solid displays a definite variation of optical and electronic properties with a variation of particle size below 100 nanometers, it is classified as a nanostructure. These can be broken down into two-dimensional nanostructures, which are made up of thin layers that have the thickness of at least one atomic layer, where two dimensions of the material are outside the nanoscale, like quantum wells and graphene.
There are also one-dimensional nanostructures, said to be the “building blocks” for nanoscale devices, where one dimension is outside the nanoscale, like nanotubes, and nanorods. Finally, there are zero-dimensional nanostructures where all the dimensions are at the nanoscale — like quantum dots! Zero-dimensional nanostructures have the sharpest density of states, meaning the number of states at energy levels that electrons can occupy.
Nanomaterials have to have at least one external dimension measuring 1–100 nanometers. Nanomaterials are each defined by their unique shapes and dimensions.
4. Quantum Dots and MEG
4.1. Quantum Dots
Nanotechnology solar cells are already in use, and although not yet quite as efficient, they are far cheaper. Advancements are quickly being made, and hopefully in the near future, with the help of quantum dots — otherwise called nanocrystals — solar cells will soon become more efficient than current solar cells.
Over 80% of today’s solar cells use silicon as their superconductor material, but with quantum dots, this might change. Quantum dots are tiny circular semiconductor particles that can carry electrons. They can emit light of various colours when struck with ultraviolet (UV) light. They are recognized for their astounding qualities at such minuscule sizes. One of these extraordinary characteristics is their ability to create multiple exciton generation, or MEG.
4.2. MEG
MEG is the process that occurs in quantum dots where the energy of an absorbed photon, generally two or more times the band gap, can generate one or more excitons, meaning the bound state of an electron and electron hole which are attracted to each other through the electrostatic Coulomb force, instead of being released as heat wastage.
In a faultless MEG absorber, one photon with energy equal to double the band gap would force more and more electrons to circulate through a photovoltaic device one by one. Although the MEG approach is not perfect as it cannot possibly retrieve all the heat energy waste, only one absorbing layer is required which makes it simpler to be constructed.
This is important for solar energy because the MEG effect has the potential to be able to create more electrons per photon from the sun’s rays. In fact, in 2006, the Los Alamos National Laboratory released a statement claiming this could harvest 7 electrons for one photon!
4.3. Materials and Toxicity
One primary worry with quantum dot solar cells is what they are made out of, which tends to be toxic elements such as cadmium and selenium or lead. However, studies are being introduced with the use of other materials like copper, indium, and zinc, and although they are not quite as efficient as current solar cells, developments are promising.
Somewhat worryingly is that studies performed regarding the metabolism and degradation of quantum dots have shown that quantum dots can accumulate in the kidney, spleen, and liver. In UV conditions, there is an increase in degradation.
5. The Quantum Confinement Effect
Quantum dots have the unique property of quantum confinement, modifying the density of state close to band-edges. Quantum confinement can be seen in a material when small enough, around 10 nanometers, where its electronic and optical properties change.
An exciton can be thought of like a hydrogen atom, where a hole represents the nucleus (with a smaller mass). The distance between the electron and hole is called the exciton Bohr radius. The confinement of an electron and hole is dependent on this Bohr radius.
5.1. “Artifical Atoms”
II-VI semiconductors are a group of compounds that display luminescent properties (due to their direct band gaps). Because of this they are sometimes used as a host for luminescent activators. When these materials are doped with certain chemicals, they will emit a certain colour. Quantum dots are a type of II-VI semiconductor particle.
Semiconductor nanocrystals such as quantum dots are often referred to as artificial atoms, and this is because of their separated energy states. Because of quantum confinement effects, quantum dots have atom-like electronic and optical structures and behaviours.
5.2. MEG via Quantum Confinement
Quantum confinement also enhances the multiple exciton generation because it plays a role in the Coulomb interaction, which the MEG is driven by. The quasimomentum is loosened, a vector that is associated with electrons in a crystal lattice and is responsible for transition rules that are a set of restrictions surrounding the ability of physical systems to change from a quantum state to another. This quasimomentum and energy have to be conserved in the MEG process.
5.3. EMA Model
In terms of predicting quantum confinement, there is a model used to do so called the Effective Mass Approximation Model. This is an efficient method for semiconductor nanostructures that are too large for their properties to be calculated using first-principles calculations.
This model is a spin-off from the particle in a box model, which is a prominent framework used within quantum mechanics. It outlines how a particle is free to move around in a deep well with impassable barriers that it cannot escape. This is an application of the Schrödinger equation, which yields insight into particle confinement through different dimensions, providing probability waves controlling the motion of small particles.
The EMA model uses the core principles of the particle in a box model, only now it is a 3-D box as opposed to a simple 1D quantum well.
6. Quantum Yields
The quantum yield, meaning the efficiency of converting absorbed light into emitted light, is also a barrier for quantum dots. However, this is not a hopeless matter. It has been found that the quantum yields of quantum dots can be increased by surface modifications, more specifically through passivating (discussed in the next section of this article).
The quantum yield, represented with symbol Φ, is directly correlated with the efficiency of solar panels, up to the value of unity, which is 100% (Φ100) and of course is very difficult to achieve. However, we are trying to get as close as possible to Φ100 to make the highest performing cell, and advancements are rapid.
7. Passivating Quantum Dots for Boosted Efficiency
7.1. Surface Defects
Quantum dots can have surface defects. This means that they act as “traps” which can control the ability of electrons and holes to rejoin, or recombine. These traps cause quantum dots to blink, while deteriorating the quantum yield. By putting a shell around the core, this flickering can be minimized, however shells can also be problematic because they change the optical properties of particles, so sizes are altered — which is very hard to regulate.
Scientists are still trying to understand why charges get trapped in this material, but they do know that the root of these traps is distribution. Elemental ratios must be perfectly distributed, and if this happens, electrons will behave as they are supposed to.
7.2. Dangling Bond Defects
This is where surface passivation comes in. Here, all leftover bonds of the quantum dot would be saturated so that no surface state is displayed. These dangling bond defects cause the charge transfer rate to be far too high, also affecting the band gap. When an organic or inorganic capping layer is deposited on the quantum dots, the surface can be modified, having a large impact on its efficiency. This is also called ligand passivation.
TiO2, or titanium dioxide, is used as an active material in a solar cell that is responsible for absorbing photons and converting them to electrical current. Passivation can change the position of the conduction band, as well as heightening the ability of the recombination of electrons and electron holes.
There are many different passivation agents, but the ones that have proved to be best for boosting solar cell performance have been those that contain thiol and amine groups (unlike acid groups which lower their efficiency). Passivation agents can have different effects — for example, when chemically reduced, bovine serum albumin can make quantum dots soluble in water.
Two types of passivation methods can be used: organic and inorganic. Each will be outlined in brief.
7.3. Organic Passivation
Organic molecules can be capping agents when held as a thin film on the surface of the quantum dots. With this method, quantum dots can be joined together in groups, which is called bio-conjugation. However, the majority of these molecules are oversized and deformed, and this is a key problem with organic capping. The organically capped quantum dots are also photo-unstable, there is weak bonding between the capping molecules and surface atoms when struck with a UV beam, and it is immensely difficult for there to be coinciding passivation of cationic and anionic surface sites.
7.4. Inorganic Passivation
Inorganic passivation is generally more effective in quantum dots with larger band gaps. A passivating shell of inorganic layers is grown on the quantum dots through epitaxy, where gas precursors are condensed to form a film on the substrate, or non-epitaxy, where a shapeless layer is grown on the core. This thin coating of the shell increases the quantum yield of quantum dots (though, keep in mind, it must be immensely thin to actually work; thickness effects are discussed in the next section).
In terms of being able to quantify the degree of trap passivation, this can be done through measuring the photoluminescence quantum yield (PLQY) and the photoluminescence lifetime (PL).
7.5. PLQY
The first method, measuring PLQY, is done with a photoluminescence spectrometer. Here a sphere is used to measure the scattered and emitted photons, allowing us to see the precise photoluminescence quantum yield which helps us understand and assign a number to the enhancement of dot brightness, also translated to the success of said passivation strategy.
7.6. PL Lifetime
Measuring the PL lifetime allows us to observe if the passivation of a quantum dot has increased the lifetime of a quantum dot and by how much. This can be done with the use of time-correlated single-photon counting, a statistical technique for being able to measure fluorescence lifetime, where a recurrent light source is utilized to detect each passing photon and their arrival time in respect to this light source. Being able to measure PL lifetime is important because it gives much information about the general properties of a semiconductor as well as any potential defects it may have.
8. Thickness of Shells
The luminescence properties of quantum dots are directly reliant on the thickness of their shells. The thinner the shell is, generally the better the surface passivation, the higher the quantum yield (optimizing the PLQY), and the brighter the quantum dot. Thick-shelled quantum dots, otherwise called giant quantum dots, reduce the quantum yield drastically and cause further blinking.
8.1. Transmission Electron Microscopies
An optimum shell thickness is required, which has been shown to be 1.9 nanometers, or 13 nanometers in quantum dot sizing. This is not as easy to accomplish as it sounds, though the shell thickness can be tuned and the thickness can be measured using transmission electron microscopy. This is where a beam of electrons is transmitted through an ultra-thin specimen to create an image on an imaging device, showing the interaction of the electrons. This can help us to approximate the thickness of a quantum dot shell.
8.2. X-Ray Powder Diffraction
The other primary method for measuring the thickness of a shell is with X-Ray powder diffraction, or XRD. This is used for observing the properties of crystalline materials and can provide us important information about them such as their atomic and molecular structures. In this approach, the crystalline structure makes a beam of incident X-rays disperse, or diffract, in different directions. From this, a crystallographic model is used, whose job is to examine the atomic and molecular structures of crystal forms by measuring the angles, intensities, and strength of the diffracted beams and come up with a 3-D picture of how the electrons look in the crystal.
This can be used to calculate differences from the particles sizes of the quantum dot shells, and draw conclusions as to what the thickness may be.
9. Engineering Band Gaps
Modulating activity in semiconductors is a huge challenge, and one of the most difficult parts of this is controlling the band gap.
It transpires however that band gaps of quantum dots can actually be engineered, changing their optoelectronic properties — which is a very powerful tool for semiconductor materials and devices. In fact, the vast majority of modern semiconductors now utilize band gap engineered configurations because of the unique capabilities it gives us to create custom-designed energies. This is generally done through heterostructures, strain, alloying, or superlattices. Each one will be discussed in brief.
9.1. Quantum Heterostructures
A heterostructure is a semiconductor structure whose chemical composition is directly correlated with its position. This is composed of at least one heterojunction, the interface between two regions in crystalline semiconductors having dissimilar band gaps (unlike homojunctions). Heterostructures can be used for more efficient solar cells by using thin film materials that are extremely absorptive. This would also prevent losses when an electron and hole recombine at the surface of the cell (which is responsible for a lot of energy loss!).
A quantum heterostructure occurs when a heterostructure is immersed in a semiconducting material so that the charge carriers are restricted from moving and are forced into quantum confinement. When multilayer heterostructures are taken they display optical confinement properties, improving light-emitting devices such as solar cells.
9.2. Strain
Due to their quantum confinement effects, semiconducting materials can be adjusted with strain-induced properties, allowing their shapes and sizes to have the ability to be tuned. This has been shown to significantly boost the hole mobility of transistors — not only this, but it enables precision over the emission wavelength of semiconductor and reduced threshold currents.
9.3. Alloying
The alloying effect on semiconductors has proven to be very effective for controlling band gap energy. This is where the core is alloyed, which is essentially the mixing of semiconductors with different band gaps. This has had extensive implications because when quantum dots are alloyed with several semiconductors they display mixed optoelectronic properties. They manage to reduce the level of surface defects without changing the electronic properties of their host material, which has always been a hole in engineering band gap concepts.
9.4. Superlattices
Lastly, there is the method of utilizing quantum dot superlattices (although there are certainly other methods; these are just four of the most prominent). These are grown by molecular beam epitaxy, which is a method commonly used for semiconductors to deposit single crystals on thin-films. Here, periodically arranged quantum dots form 3-dimensional structures of colloidal nanocrystals. By changing the distances between quantum dots, their dimensions can be altered.
Superlattices work like this: quantum dots each have a fixed, controlled size, sitting on a specific position on a large and thin MoS2 atomic grid, which is a 2-dimensional semiconductor otherwise referred to called molybdenum disulphide. The quantum dots must be placed distinct distances away from one another for this to work. A beam and Mo2D have a very interesting reaction when placed in contact with one another, where the properties of the semiconductors turn metallic. This causes electron-hole activity so that quantum dots essentially act like quantum wells, having only discrete energy values. When the position of quantum dots are changed, the band gap is changed, and that changes the wavelength of light emitted. This allows band gaps to be determined by design.
10. Synthesis of Quantum Dots
There are many different approaches for synthesizing quantum dots. Three of the most important will be summarized.
10.1. Colloidal Synthesis
Colloidal synthesis is likely the most commonly used method, as it allows for large batches of quantum dots to be made so is easier for widespread production and distribution, so it is effective for scalability. It is also the least toxic synthesis method (or one of). Produced from up to 100,000 atoms, quantum dots are generally on the scale of 2–10 nanometers from the colloidal synthesis method.
This method comprises nanocrystals being synthesized from solutions. A precursor is heated to such high temperatures that they decompose and become quantum dots. Two factors must be strictly controlled: 1.) temperature, as it must be high enough for the crystals to properly grow because annealing of the atoms is a key factor in the process, and 2.) concentration, because this is crucial to guiding the critical size of the nanocrystals — which is where nanocrystals do not have the capability to change in dimensions. Concentration needs to be maintained because if it decreases, the critical size becomes too big and the distribution defocuses. The higher the critical size, the smaller the particles, the better the growth of all the particles as a whole.
10.2. Plasma Synthesis
Plasma synthesis is a gas-phase approach. Because size, shape, and overall configuration of quantum dots can be controlled in nonthermal plasma, this is what is used as a synthesis method. With this, quantum dots are given as a product of powder and then modified to become nanocrystals — however, although this means it is slightly more difficult, it also means quantum dots can have solubility properties.
10.3. Electrochemical Assembly
Quantum dots can be synthesized with electrochemical processes, where templates of nanocrystals are produced from a reaction between a metal and electrolyte. These can then be etched onto a substrate. This is a common method for manufacturing semiconductor devices.
Electrochemical assembly for quantum dots has great potential for widespread production and distribution, as well as eventual lower cost.
11. Surpassing the Shockley–Queisser limit
The Shockley-Queisser limit, which can be calculated by measuring the electrical energy extracted from one photon, is the maximum efficiency of a single junction solar panel to be able to collect power from the cell. This allows the efficiency of around 30% with direct sunlight, which is not ideal. If we could find a way to overcome this, the photoconversion process could be enhanced, and efficiency boosted.
11.1. Photon Upconversion
One way to reduce this in single-junction cells is through photon upconversion, where a specific molecule or material that has the capability to absorb two or more photons below the bandgap and emit a photon above the band gap is used in a solar module, leading to the emission of light at shorter wavelengths.
11.2. Multi-Junction Cells
When using multi p-n junction solar cells, also called tandem cells, this limit can in fact be outperformed — if a multi-junction solar cell with an infinite number of layers were to be taken, there would be a limit of around 68% with sunlight. This is due to the fact that a material with just one band gap can not absorb sunlight below this band gap and cannot make use of sunlight significantly above the band gap. The good news is, tandem solar cells not only boost efficiency, but are also relatively cheap.
11.3. Improving MEG
There is also the utilizing of MEG, which allows extra carriers to be collected in a quantum dot solar cell. If techniques like altering the shapes and composition of materials were explored to see if MEG could be enhanced, for example, through quantum rods, efficiency of MEG could be boosted from its current ability.
11.4. Thermal Upconversion
Thermal upconversion alters traditional upconversion processes. In conventional solar cells, the upconvertor changes photons with an energy lower than the band gap to higher energy photons. These new transformed photons then proceed to be directed back and absorbed in the solar cell. What thermal photon upconversion does concerns controlling the optical density of states of the absorber, where characteristics can be tuned to provide better results, for example, changing a cell’s configuration so that one side is responsible for only absorbing low-energy photons, and the other is responsible for emitting high-energy photons.
11.5. Light Concentration
Another method is light concentration. Similar to organic sunlight with multi-junction cells, concentrated sunlight — usually created with lenses and mirrors to make rays more intense — can further boost the efficiency of a cell to around 87%. Essentially, the higher the concentration, the higher the efficiency. Despite this, too large amounts of concentrated sunlight will heat the cell which just ends up cancelling out efficiency rates.
There are several ways to achieve concentrated light for multi-junction cells, the most often used being parabolic troughs. These are U-shaped mirrors that can be more than 600 meters in length, and reflect sunlight toward metal absorber tubes. They heat a heat transfer fluid to act like sunlight that is reflected to the tube. These troughs move through the day, maximing the amount of solar energy that can be received. The largest barrier with concentrated sunlight is its cost, but this is decreasing.
12. Conclusion
The potential of quantum dot solar cells is immense and very exciting because they have the potential to overcome many of the problems that occur in our current silicon solar cells, where excess energy is released as heat wastage (which is why solar cells have such low efficiency). Quantum dots as the absorbing photovoltaic material may well solve the efficiency issue in solar cells.
Quantum dot solar cells would also solve the cost dilemma. They would be able to be made from inexpensive and abundant materials which would be relatively simple to obtain — unlike silicon, which requires substantial purification that is in no way easy. Another positive aspect is that these solar cells can be manufactured at room temperature, saving energy along the way, and can be implemented into a variety of flexible materials.
These photovoltaic devices are developing significantly, making their potential formidable.
What’s Next?
For visual learners, here’s a brief and simple overview I put together summarizing the basics of quantum dots in photonics!
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Sources
https://www.pnas.org/content/108/3/965#:~:text=Abstract,the%20toolbox%20for%20material%20design.
