The great aspect of solar cells is that they are producing non-polluting renewable energy by absorbing photons i.e. sunlight, and by extension converting the energy  of the photon to electrical energy for applications or conservation. 

One of the main challenges in the photovoltaic industry is the 33% efficiency limit for any traditional solar cell [1], which is where quantum dot solar cells (QDSCs) come into the spotlight as a solution to break the efficiency limit. 

The efficiency limit is due to excess energy lost to heat (thermalization loss), which could have been utilized [2]. The QDSC solves this issue with “quantum confinement” which will be elaborated further in the following.

What are quantum dot solar cells?

Quantum dot solar cells utilize crystalline nanoparticles known as quantum dots (QDs) as the absorbing photovoltaic material, and are investigated to replace common bulk materials such as silicon. QDs are typically in the size of 1-10 nanometers and are made from semiconductor materials or metals, e.g. cadmium selenide (CdSe) and lead sulfide (PbS). 

One of the biggest attractive attributes of QDSCs, is that they can cover the whole spectrum of wavelengths from sunlight. This is possible since the band gap i.e. the absorption spectrum is related to the size of QDs due to the quantum confinement effect. Therefore, the band gap can be tuned to match the solar radiation spectrum by adjusting the size of QDs, thereby providing great flexibility of light absorption and better efficiency in solar power generation [3]. 

Quantum dots made from lead sulfide are of most interest due to its great tunability [4]. In the case of bulk materials, the band gap is fixed in terms of what material is used, which makes thermalization loss inevitable.

Due to the very small size of QDs, it is also possible to print them into thin flexible sheets and make transparent solar cells more of a reality. These could possibly be incorporated into screens or windows as a form of electricity generation. The technology has also improved substantially in terms of efficiency, from 2.7% in 2010 to a reported 16.6% in 2020 [5].

Technical specifications of QDSCs

The QDSC is typically sandwich structured, with a photoanode, a counter electrode and an electrolyte. The photoanode consists of a transparent conducting electrode (TCE, typically indium tin oxide glass) and a metal oxide semiconductor (usually TiO2) coated with a layer of quantum dots [3]. 

The photoanode is where the sunlight is absorbed and as a result generates both positively charged electron vacancies (holes) and negatively charged electrons in the solar cell. The positive charges are transferred between the photoanode and the counter electrode with the use of an electrolyte. 

The purpose of the counter electrode is to transfer electrons from the external circuit (the electrical device) to the electrolyte, which results in catalyzing the redox reactions of the electrolyte [3]. A schematic of the QDSC is shown on Figure 1. 

Figure 1: A schematic of the quantum dot solar cell, which shows all the main components of a QDSC. The electrons transfer from the photoanode through the external circuit to the counter electrode. 

Outlook on Market

QDSCs are currently yet to be commercially viable in mass scale due to substantial challenges, such as reducing QD material cost and developing high-throughput deposition methods for mass production [6]. Although, several small companies have begun to produce QDSCs with the most notable ones being QD Solar, and Solterra Renewable Technologies, a subsidiary of the QD mass-manufacturer Quantum Materials Corp. 

Solterra gets a headstart in the immature market currently and has accordingly the proprietary technology to mass-produce QDSCs at hundreds of meters per minute using roll-to-roll printing technology. [7]

End note

QDSCs are currently seen as the next generation of photovoltaic devices, which has the most potential to break the thermodynamic efficiency limit of 33% with its excellent optoelectronic properties and the use of quantum confinement. QDSCs also have a maximum potential conversion efficiency of 66%, which is double the possible efficiency of traditional single junction solar cells [8], demonstrating that quantum dots could be the future of solar power.

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Sources

[1] Rühle, Sven “Tabulated values of the Shockley–Queisser limit for single junction solar cells”, published in ScienceDirect (2016).

[2]  Semalti, Pooja et. al.  “Advancements in Quantum Dot Solar Cells: Synthesis and Applications”, published in Sigma-Aldrich (2020). 

[3] Technical University of Denmark (DTU): Department of Chemistry. “Chemistry at the Nanoscale: Chapter 4”. (2020)

[4] Lutfullin, Marat et. al. (King Abdullah University of Science and Technology) . “Quantum Dots for Electronics and Energy Applications”, published in Sigma-Aldrich (2020)

[5] Hutchins, Mark. PV Magazine. “A quantum dot solar cell with 16.6% efficiency”. (2020)

[6] Jean, Joel. “Getting high with quantum dot solar cells”. Published in Nature (2020).

[7] Solterra. “Solterra: Business Objective”. (2020)

[8] NREL. “Quantum Dots Promise to Significantly Boost Solar Cell Efficiencies” (2013).

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Content

• Short facts
• Products and customers
• Comparison with other technologies
• New market opportunities
• Challenges
• Outlook

Short facts[i],[ii]:

• Status: Private
• CEO: Jason Hartlove
• Investors: 32
• Latest investment round: USD 20M
• No. of employees (approximately): 120
• Location: Silicon Valley

Products and customers

The main products that Nanosys produces are plastic films coated by quantum dots of green and red colors that are then used by other manufactures in displays[iii]. In 2019, Nanosys’ nanotechnology had been featured in more than 400 different products with more than 30,000 kg of quantum dot material having been shipped.

They are often integrated into LCD displays that use a blue LED backlight together with the quantum dots to produce RGB displays that are energy efficient and have extremely bright colors. The newest generations of quantum dot films are completely heavy metal free and are mainly made of Indium phosphide (InP)[iv].

Their main customer and biggest investor is Samsung. Other customers using Nanosys quantum dots in their products include Vizio, HP, Hisense, and TCL. They are collaborating with MIT, Lawrence Berkeley National Labs, Hebrew University, Philips-Lumileds, and more which is part of the reasons for their high achievements.

Comparison with other technologies

A technology that often gets compared with QLED (Quantum LED) displays is OLEDs (Organic LEDs). The two technologies are often competing with the best OLED TV displays being produced by LG.

While QLEDs are brighter and often more energy-efficient, OLEDs can produce better contrast and much thinner displays, which is something LG is particularly good at. Both technologies have multiple other pros and cons which can be a long discussion. However, what has of late become interesting is the possibility of combining the best of both worlds with Samsung investing in a new hybrid quantum dot-OLED technology that they hope to start mass-producing in 2021[v].

To complicate matters even further, a third promising technology, MicroLED is challenging both QLEDs and OLEDs by offering similar advantages of brightness, contrast, and energy efficiency. This technology is showcased in Samsung’s The Wall[vi]. Nanosys has, however, incorporated this technology into its roadmap and believes that quantum dots can play a part in improving microLED displays[vii].

What might be an outcome of these different technologies is a mixture of Quantum dot-OLEDs and microLEDs with quantum dots in the long run.

New Market opportunities

While the applications within display electronics seem to continuously grow, there are other opportunities awaiting quantum dots and Nanosys.

The CEO of Nanosys acknowledges solar cells as a future opportunity as quantum dots have much better theoretical efficiencies compared to current silicon-based ones. In addition, he mentions smart windows (incorporating solar cells in glass, in this case with quantum dots) as a research area and possible future application.

Challenges

Some main challenges that have already been addressed are the ‘scalability’ which Nanosys seems to have solved, now using 1250 L reactors compared to 5 L ones in 2013.  Others have been the price, which now has been brought down, making a QLED TV a much more mainstream product, and finally that cadmium and other heavy metals used for quantum dots are often toxic. However, this should no longer be an issue as Nanosys produce heavy metal-free ones already [iii].

While there are other companies working with quantum dots, Nanosys has close to 100 % market share[viii]. It therefore seems to be that current challenges mainly come from competing technologies, which could however in future scenarios be combined with quantum dots.

A good strategy that Nanosys seems to employ in its roadmap is trying to facilitate these combined technologies and being open to what OLEDs and microLEDs have to offer while still utilizing their expertise in quantum dots.

Outlook

With the prediction from MarketsandMarkets of an increase in the quantum dot market size from USD 3.5 billion in 2020 to USD 10.6 billion by 2025[ix] (annual growth: 24.6 %) and backing from large investors, Nanosys should be well off by continuously producing quantum dots at large scale. They need to keep collaborating with their partners in the industry and at universities to improve the application in displays. It is also going to be interesting to see how they might play a role in improving solar cells and create smart windows.

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[i] Nanosys overview, accesed at https://pitchbook.com/profiles/company/50754-52#competitors

[ii] Nanosys company profile, accesed at https://www.crunchbase.com/organization/nanosys?utm_source=linkedin&utm_medium=referral&utm_campaign=linkedin_companies&utm_content=profile_cta

[iii] Nanosys Products, accesed at https://www.nanosysinc.com/products

[iv] CEO keynote speech, 2019, accessed at: https://www.youtube.com/watch?v=74W4MRwe4d0

[v] Samsung OLED quantum dot hybrid could challenge LG for TV supremacy, Aug 2020, accessed at https://www.cnet.com/news/samsung-oled-quantum-dot-hybrid-could-challenge-lg-for-tv-supremacy/

[vi] Samsung’s The Wall, accesed at https://www.samsung.com/us/business/products/displays/direct-view-led/the-wall/

[vii] Nanosys Roadmap, accesed at https://www.nanosysinc.com/nanosys-roadmap

[viii] Nanosys Quantum Dots, CEO Jason Hartlove Interview, accesed at https://www.youtube.com/watch?v=6oERnxm4-ko

[ix] https://www.marketsandmarkets.com/Market-Reports/quantum-dots-qd-market-694.html?gclid=EAIaIQobChMIvZ2-qdf36wIVSubtCh1JDAj5EAAYASAAEgLfv_D_BwE

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Content

• What is a Quantum dot?
• How do you make them
• Applications
• Challenges to Quantum dots
• Outlook on Market 

What is a Quantum dot?[i],[ii]

Quantum dots are made from metals or semiconductor materials, usually with two-three different elements in a crystal structure. They are typically the size of 1-10 nanometers and normally need to be coated by another material that ensures they don’t degrade, in other words making them stable at a specific size.  An example is gold nanoparticles that are coated by a layer of a carboxylic acid. Another popular material choice is CdSe (Cadmium selenide), but quantum dots can be made from many other materials including heavy metal free such as silicon and carbon.

At very small sizes, particles behave similarly to atoms in the way that their energy bands are discrete, meaning the color of the light they emit or absorb is at a very precise wavelength. This property is what makes them excellent for applications such as displays.

How do you make them?

There are different chemical methods. The challenge is to find a method that gives a good yield while controlling the size of the particles. The method used mostly depends on the application. If the quantum dots need to be in a liquid, the best way would be a chemical bottom-up method where you create a large batch and add chemicals with a high temperature for specific amounts of time. This method is known as colloidal synthesis. [i]

If the quantum dots are instead needed as a powder, a method known as plasma synthesis can be used to produce them in the gas phase. [iii]

In addition, other methods to produce quantum dots include top-down fabrication using etching.

Applications

An application where quantum dots have highly succeeded is within displays. Companies such as Sony, Samsung, and LG have all made great products with quantum dots integrated into the display, branded as QLED displays. By doing that, they can produce a wider range of colors, giving a much more vivid experience.

Quantum dots for displays are produced by companies such as Nanosys Inc. with a range of options, some which are heavy-metal free.

Solar cells are a very promising market for quantum dots as they can be engineered to absorb a larger amount of light from the sun, with other materials having a narrower spectrum of possible wavelengths they can absorb[v]. Currently, silicon is widely used as it is the main component in modern-day electronics. This is the case even though there are fundamental limits to its efficiency with many other materials such as quantum dots providing a much higher theoretical efficiency. Now, however, there are no commercial quantum dot solar cells available, and the area needs more research and funding.

Other interesting applications are within LEDs, lasers[vi], and for sensors used for medical imaging[vii].

Challenges to quantum dots

One of the main challenges to the application of quantum dots is that they are often toxic and can interact with living cells. Hence, even though they possess excellent abilities relevant for medical applications, they are not likely to be applied in that field soon as regulations are high and very precautious. The toxicity is primarily due to heavy metals such as Cadmium often being part of the building materials.

Another challenge that is common for new materials is when they have to compete in industries that have specialized in working with other materials such as silicon. This is especially the case with solar cells as it is very hard to suddenly make the switch when production has already been optimized for something else. Thus, it requires more than scientific breakthroughs but a lot of investment and some patience.

As for other novel technologies, newly developed quantum dots often face the challenge of scalability. The synthesis at larger amounts takes some research to get right so it is an important aspect to keep in mind when looking at quantum dots from new materials.

Outlook on Market

Quantum dots seem to have come to stay within electronic displays, as big companies such as Samsung are investing highly in this technology[viii]. Furthermore, there is a well-established supply chain with companies such as Nanosys Inc. providing reliable and high-quality material.

The expectations for quantum dots are high, MarketsandMarkets predicts an increase in market size from USD 3.5 billion in 2020 to USD 10.6 billion by 2025[ix] (annual growth: 24.6 %) with the growth mainly from applications in displays. Meanwhile, the incentives to apply it in other markets such as solar cells are growing with large amounts of research being done[x].

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References

[i] Chemistry at the Nanoscale, DTU Chemistry, 2020, p. 16-23

[ii] What is a Quantum Dot, accessed at: https://www.nanosysinc.com/quantum-dot-basics

[iii] https://www.researchgate.net/publication/301290274_Plasma_Synthesis_of_Nanoparticles

[v] https://www.intechopen.com/books/solar-cells-new-approaches-and-reviews/quantum-dots-solar-cells

[vi] https://www.laserfocusworld.com/lasers-sources/article/14074945/colloidal-quantumdot-laser-diodes-could-be-coming-soon

[vii] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5546783/

[viii] https://nanophotonica.com/nanophotonica-announces-3-5m-investment-led-by-samsung-ventures/

[ix] https://www.marketsandmarkets.com/Market-Reports/quantum-dots-qd-market-694.html?gclid=EAIaIQobChMIvZ2-qdf36wIVSubtCh1JDAj5EAAYASAAEgLfv_D_BwE

[x] https://scitechdaily.com/solar-technology-breakthrough-world-record-quantum-dot-solar-cell-efficiency/

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