Proposal for New High-Performing Flexible Transparent Perovskite Solar Cells

Making higher-performing synthesized transparent bendable Perovskite solar cells using graphene conducting electrodes, placed between fluorine-doped tin oxide Willow glass substrates, with an interior graphene quantum dot layer for enhanced efficiency and a Spiro-IA layer to prevent recombination, coated with moth-eye AR structures

*Note, September 5th, 2021* I am now building my solar cell out in a lab and this design has been modified!

Abstract

Our fossil fuel consumption is destroying Earth’s biodiversity, habitats, quality of life, and, eventually, will play a role in demolishing humankind. Solar energy has never really been looked at as a full, or even partial, replacement to these non-renewable resources, but it could be in the future. The sun’s energy is abundant, and yet, solar cells’ efficiency rates mostly fall in the range of 10–20%. Although efficiencies around 30% are now appearing, this is still not exceptional. This proposal outlines a new solar cell design. Not only would these panels be equally as efficient, if not more, than conventional solar cells at converting sunlight to electrical power, but they would also be transparent to the human eye, and flexible. This paper will outline each component of this idea, which a miniature 3D model has been printed of!

Miniature 3D model of proposed solar cell

Outline of Proposal

1. Background — Transparent Solar Cells

1.1. Obstacles

1.2. How Transparency is Determined

1.3. Implications of Transparent Solar Cells

2. Taking Advantage of Thin-Film Photovoltaics

2.1. Why is This a Good Idea?

2.2. Using a Perovskite Structure

3. Graphene Conducting Electrode

3.1. The Problem With Indium Tin Oxide

3.2. Benefits of Graphene for Conducting Electrode

3.3. Synthesized Graphene Through APCVD

4. Achieving Flexibility with Willow Glass Substrates

4.1. Implications of Flexible Solar Cells

4.2. Using Fluorine-Doped Tin Oxide Willow Glass Substrates

5. Using Layer of Graphene Quantum Dots

5.1. The Problem With Conventional Quantum Dots

5.2. Why GQDs?

5.3. Hydrothermal Synthesis Method

6. Using Dopant Free Spiro-IA for Hole Transport Material

6.1. Effects of Surface Recombination

6.2. The Problem With Spiro-OMeTAD

6.3. The Role of Spiro-IA

7. Using Anti-Reflection Coatings

7.1. Avoiding Reflections

7.2. Conventional AR Coatings

7.3. Moth-Eye Structures

8. Conclusion

1. Background — Transparent Solar Cells

1.1. Obstacles

Transparent solar cells are tricky because they have to permit the transmission of the visible wavelength of absorbed light while still actively absorbing photons to provide a decent efficiency rate of light absorption to electricity produced. Therefore, if 100% transparent solar cells were to be made, then 100% of photons would be passed straight through and the process of generating electricity could not even begin. Furthermore, if a 30% transparency were taken, 70% photons could be absorbed. If a 90% transparency were taken, 10% photons could be absorbed (which would not allow a very efficient solar cell). Finding materials that allow the absorption of light while still being able to work with a transparent cell is difficult. These materials that have high transmissions of course also need to be fabricated in a constructive way, and in one which is cost-effective. The substrate used to protect the solar cells also must be taken into consideration, such as plastic or glass.

1.2. How Transparency is Determined

The arrangement of a material’s electrons within atoms is directly correlated with its transparency. In a material where little light can pass through, an electron’s energy gap is equal to the energy of a photon so that a photon is absorbed and the electron is moved to a higher energy level. Therefore, in transparent materials, the electron’s energy gap must be higher than the photons, preventing electrons from using the photons’ energy and allowing light to pass through. This, of course, means solar cells can not be 100% transparent — instead, they will be translucent. However, to humans, even if there were a 50% transparency in solar cells, this would look see-through to us. This is because our eyes are logarithmic detectors. Transparent solar cells have to be measured by the efficiency of their light conversion rate.

1.3. Implications of Transparent Solar Cells

A large problem with solar cells is that they have a limited amount of places where they can be used. This restricts the potential for solar cells to be distributed widespread. What if solar cells could be easily set up on your window? Transparent solar cells could achieve this. They could power large buildings, and create self-charging, solar-powered cars, airplanes, boats, and trains, as well as electronic devices, like our laptops, cell phones, e-readers, and etc. Additionally, millions of other people would use solar panels for their roofs, because a key reason why many do not desire these is due to the fact that they find them bulky, or “ugly”. Essentially, every glass surface could be a solar cell. This could have phenomenal impacts.

2. Taking Advantage of Thin-Film Photovoltaics

2.1. Why is This a Good Idea?

Thin-film photovoltaic modules are a vastly growing area within the solar sector because they can produce consistent power at a low cost, not only on sunny days, but also on cloudy and dark ones where there are low sun angles. These versatile cells are incredibly light but also durable because of their narrow design, and can be easily installed. Such panels are made by depositing thin layers of photovoltaic material onto a substrate.

Thin-film photovoltaics are becoming increasingly popular because they can be used on a large scale for inexpensive prices, being ideal for solar farms, as well as for schools and corporations. However, they can still be used for residential sectors, as they provide a cost-effective method for particularly aesthetic benefits and are flexible. As advancements accelerate, this will become more open of an option.

2.2. Using a Perovskite Structure

A Perovskite solar cell is one which has a Perovskite-structured compound. Perovskites are a scientific breakthrough and have been quickly gaining traction over the past few years. Their efficiency has faced rapid progress, from 3% efficiency in 2006 to over 25% in 2020. They offer themselves as a promising alternative to conventional solar cells and are a significant area of research and testing. Perovskites can be manufactured as thin-film products, are flexible, and allow for 20 times less required materials (which are all abundant and accessible). This results in low cost production paired with high efficiency, which is the ideal combination, since no high-cost machinery or facilities are needed. Although the majority of thin-film photovoltaics require complex engineering processes, Perovskites are defect-tolerant and are easy to produce.

The current primary challenge with Perovskites is that they contain lead, which is toxic, therefore research is being done to find replacements that are more environmentally compatible. However, these solar cells highly outperform many other energy sources in terms of environmental implications, therefore, although this is an issue, it is not a deal breaker in any way (and advancements are rapid in this field).

Figure 2: 3D Printed Perovskite Layer

3. Graphene Conducting Electrode

3.1. The Problem With Indium Tin Oxide

For transparency to be enabled without lowering the performance of solar panels, synthesized transparent materials must be used. Finding transparent conducting materials is a primary challenge with solar cells.

Indium tin oxide is most often used as the transparent electrode material in solar cells. However, its applications are limited by high costs, poor flexibility, severe toxicity levels, and high temperature of deposition. Indium tin oxide is also very chemically unstable and has a light transmission that is not ideal.

3.2. Benefits of Graphene for Conductive Electrode

Graphene, a one-atom thick carbon allotrope, has atypical but remarkable electrical and mechanical properties. Graphene has been tested and proven to act as a flexible, chemically stable, electrically conductive, and overall more effective alternative to indium tin oxide with all factors taken into account. When graphene is used, resistance is minimized, light transmission characteristics of the electrode improved, and reliability boosted. Additionally, graphene is optically transparent due to its thinness, hence containing the main upside of indium tin oxide. This graphene would have to be coated on glass layers, which will be discussed in the next topic of the proposal.

Figure 3: 3D Printed Graphene Layers

3.3. Synthesized Graphene Through APCVD

Graphene has to be deposited over a large surface area of glass to be a conductive electrode. There are many methods to achieve this, but the most promising has been chemical vapour deposition, or CVD. CVD graphene is known to have low resistance, high transmittance, and outstanding physical properties altogether.

CVD is a vacuum deposition production method generally used for thin films, with the aim to create high quality solid materials. This method works best with metallic substrates because of the surface’s catalytic nature. This is commonly done under high temperatures where a hydrocarbon gas flows onto a transition metal, like nickel, copper, platinum, etc. Once the graphene has been produced, the underlying metal can be removed, and the graphene deposited onto glass. However, graphene can also be grown directly on a glass substrate using a simple atmospheric pressure CVD method called catalyst-free atmospheric CVD (APCVD). This is very promising as a uniform and large-area graphene can result from this, and deposition rates are fairly high!

4. Achieving Flexibility with Willow Glass Substrates

4.1. Implications of Flexible Solar Cells

Flexibility is defined as the ability of a material to be bent without mechanical failure. Flexible solar cells could have many broad reaching implications — for example, they could be used for smart textiles, clothing containing modern integrated technologies, eg, clothes which can change colour depending on body heat. Do to the cells’ ulta-thinness, they have the capability to revolutionize wearable tech. This could also result in bendable electronic devices!

4.2. Using Fluorine-Doped Tin Oxide Willow Glass Substrates

Having a flexible substrate is essentially the building block of having a flexible photovoltaic device. Willow glass substrates would be a perfect fit. This is a flexible glass produced by Corning, the company who furnishes the front of iPhones and some Samsung devices with Gorilla Glass. Willow Glass is a flexible and transparent borosilicate glass purposed to be a substrate, having a thickness of only 0.1 millimetres. It is heat resistant, allows for high temperature processing, and can be rolled up for roll-to-roll processes, which makes for easy and quick production and low costs. Fluorine-doped tin oxide (FTO) Willow glass has already been proven to increase the power conversion efficiency of solar cells. FTO coated glass is conductive and beginning to be used for transparent electrodes for thin film photovoltaics.

Figure 4: 3D Printed Willow Glass Layers (with graphene-coat on top)

5. Using Layer of Graphene Quantum Dots

5.1. The Problem With Conventional Quantum Dots

Quantum dot solar cells have been a very exciting finding within photovoltaics. Quantum dots are small circular semiconductor particles that can carry electrons. When struck with UV light, they emit their own light of different colours. When used as the absorbing material, they boost the efficiency of solar cells, namely due to the fact that they can create multiple exciton generation, or MEG, which occurs when the energy of an absorbed photon, generally two or more times the band gap, generates one or more excitons instead of being released as heat wastage. The only difficulty with quantum dots is that their materials are very toxic, generally being composed of cadmium, selenium, and lead. Quantum dots can also accumulate in the kidney, spleen, and liver, and, in UV conditions, there is an increase in degradation.

5.2. Why GQDs?

Graphene quantum dots, or GQDs, are a mixture of both graphene and quantum dots. Unlike graphene on its own, GQDs have a zero band-gap, making them a very good conductor, which also plays a role in their rare optoelectronic and electric properties. They are physically and chemically stable, being non-toxic, soluble, and photoluminescent. They have a large surface to mass ratio and better surface grafting than QDs, as well as much easier to handle.

Perovskite solar cells already have high performance, and therefore, locating an effective additive is key. GQDs can do this, supplying tunable band gaps, chemical stability, quantum confinement, and large surface area.

Because GQDs harvest excess photon energy for electricity, paired with the fact that this is a tandem solar cell, the Shockley Quessier limit can be surpassed, which is a restricting factor for single-junction solar cells.

Figure 5: 3D Printed GQD Layer

5.3. Hydrothermal Synthesis Method

Synthesis methods for GQDs are divided into top-down and bottom-up approaches. Top-down approaches are best for mass production and the operations are simple, utilizing abundant and cheap materials, therefore, this will be the best proposition if GQDs will be used in widespread solar cells. A commonly used technique is hydrothermal treatment.

This is a one-pot, cheap, novel, and simple treatment method for GQDs. Surface-passivated polyethylene glycol graphene quantum dots can be produced by a hydrothermal reaction with the use of graphene oxide sheets and polyethylene glycol as starting materials. This has been tested and proven to be very effective for solar cells, displaying higher photocurrent generation capability, exceptional luminescence properties, and a photoluminescent quantum yield double that of pure GQDs.

6. Using Dopant Free Spiro-IA for Hole Transport Material

6.1. Effects of Surface Recombination

Surface recombination occurs when an electron and hole come together between non-current generating electrons and holes. This has a negative impact on the efficiency of solar cells because maximum efficiency can only be achieved if all the electrons and holes generated by photons move through the circuit and produce current for electricity. If 50% were to recombine before doing this, then 50% current would be reduced, and hence efficiency.

6.2. The Problem With Spiro-OMeTAD

Using Spiro-OMeTAD as the hole transport material, or HTM, can minimize surface recombination and therefore boost the efficiency of solar cells. Many solar cells today use Spiro-OMeTAD because of its implications in protecting Perovskites against degradation. Perovskite solar cells are also sometimes looked down upon due to their poor stability, which can be improved with the addition of this layer. However, Spiro-OMeTAD is very expensive, having a commercial price 10 times more than gold and platinum. Therefore, new hole transport layers are an active field of research, one successful finding being Spiro-IA.

6.3. The Role of Spiro-IA

Spiro-IA is a fluorene-based dopant-free hole-transporting material that has high yield, and is more soluble, stable, conductive, and simple to produce than conventional Spiro-OMeTAD. Spiro-IA is also inexpensive, as its unit cost is only 1/9 of the price of Spiro-OMeTAD. Lastly, it further heightens the ability of the performance of Perovskites, because of its UV absorption, making the efficiency fairly high (and higher than dopant free Spiron-OMeTAD).

Figure 6: 3D Printed Spiro-IA Layer

7. Using Anti-Reflection Coatings

7.1. Avoiding Reflection

Reflection in solar cells has to be mitigated as much as possible, because this decreases the amount of light that can be absorbed. Looking at silicon solar cells, for example, these have a reflection of over 30%! This reflection can be avoided by using anti-reflection coatings, which are generally made of silicon nitride or titanium oxide that are fabricated to a certain thickness. This allows them to produce “destructive interference”, making the wave reflected from the top surface of the AR coating become out of phase with the semiconductor surface’s reflected wave. The two waves interfere, with each other and the outcome is a reduced reflected energy. AR coatings can be used on the glass surfaces of solar panels, which improves light transmittance and efficiency.

7.2. Conventional AR Coatings

The most common ways to producing AR coatings are 1.) multilayer coating and 2.) graded index coating. Multilayer coating uses alternating layers of different materials of various indexes, allowing low reflectivities to be achieved at single wavelengths, whereas graded index uses varying indices of refraction deposited on glass or silicon substrates. However, there are barriers to each of these approaches. Since such coatings are synthesized from vapor deposition methods, there are large size surface areas generated that are hard to control. Additionally, multilayer coating has low durability, thermal deformation, and absorption and scattering losses.

7.3. Moth-Eye Structures

Moths’ eyes have hexagonal arrays of nanostructured hemispherical bumps, smaller than wavelengths of visible light. This means the air-lens interface is effectively graded making it so that when light hits their eyes, the majority of it is transmitted through the outer surface (almost zero reflectivity). This is helpful to moths because it means they can see well in limited light conditions.

These moth-eye structures can be roll printed in plastic and stuck directly onto the glass surface. Moth-eye AR structures are arrays of the same kinds of bumps as seen in moth eyes. This is a more economically viable and effective alternative to conventional AR coatings, allowing them the capability to outperform their counterparts by 20% while costing much less.

Figure 7: 3D Printed Moth-Eye AR Layers

8. Conclusion

If tested, this solar cell could have broad-reaching implications. Each layer of the transparent and flexible solar cell is cost-effective and efficient. If this proved to function at a higher efficiency compared to conventional solar cells, we could have solar windows on our houses and cars, bendable electronics, smart textiles, and more.

Check out the presentation I gave on this idea here!

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Acknowledgement; I would like to thank Kamal Balaa for 3D printing these designs for me!

Sources

https://www.researchgate.net/publication/298714122_Building_integrated_photovoltaics_BIPV_Costs_benefits_risks_barriers_and_improvement_strategy

https://www.energy.gov/eere/solar/perovskite-solar-cells#:~:text=Perovskite%20solar%20cells%20have%20shown,to%20over%2025%25%20in%202020.

https://www.researchgate.net/publication/283678572_Transparent_electrodes_for_organic_optoelectronic_devices_A_review

https://www.researchgate.net/publication/259427107_Graphene_growth_on_metal_surfaces

https://phys.org/news/2014-01-perovskite-solar-cells-cheaper-materials.html#:~:text=Although%20the%20perovskite%20material%20itself,that%20of%20gold%20and%20platinum.

https://www.researchgate.net/publication/271553522_Graded_index_Sol-Gel_antireflection_coatings

Innovator at TKS ~ Sustainable energy through nanotechnology