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 planets 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?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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).

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.

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.

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!).

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.

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.

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.

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.

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.

What’s Next?

For visual learners, here’s a brief and simple overview I put together summarizing the basics of quantum dots in photonics!

Innovator at TKS ~ Sustainable energy through nanotechnology

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