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Tandem Solar Cell

Tandem Solar Cells

Tandem solar cells have emerged as a potentially revolutionary technology that is on its way to becoming the future of the solar industry. As the name suggests, tandem cells are made up of two layers of materials that convert sunlight into electricity, making the collected and converted energy significantly more efficient than traditional photovoltaics. This increased efficiency does not come without a price, however. By understanding the advantages and challenges associated with this budding technology, we can begin to appreciate the potential and limitations of tandem cell implementation.

What is a Tandem Solar Cell

Definition of Tandem Solar Cell

Tandem solar cells are commonly known as ‘double layer’ solar cells. They are made up of two solar cells of different configurations stacked on top of one another. These cells differ from single-layer solar cells and can potentially boost their efficiency beyond the theoretical limit for single-cell efficiency of ~30%. In this combination, one cell is designed to absorb certain wavelengths of light while the other absorbs different ones, resulting in an improved efficiency rating.

Tendem solar cell composition
https://www.pveducation.org/pvcdrom/tandem-cells

The technology is still in its infancy and faces various challenges, however. Several researchers are focusing on how to make them compatible with the current solar architecture. Which typically favors single-layer solar cells. Additionally, gaining a better understanding of how to optimize their performance is also a priority. Many of the components in a tandem solar cell configuration, such as thin films, need to be balanced in order to achieve ideal results. Finally, the fact that these cells are made up of two layers adds complexity when it comes to trying to find a suitable material combination.

Future of Tandem Solar Cell

Despite these challenges, it is possible that tandem solar cells will become a commodity product in the near future. The potential gains in efficiency are certainly a major incentive for many to continue their work and find the best possible solutions for the problems that the technology currently faces.

In terms of what kind of tandem solar cells we can expect, any combination of existing solar cells based on Si, GaAs, CIGS, Perovskites, etc. can be used. These types of solar cells come with their own benefits, so finding a way to combine them with tandem technology is a major step forward.

History

It was in the late 1950s when William Shockley and Hans-Joachim Queisser made a breakthrough discovery in the realms of solar energy and photovoltaics. They were the first to point out the existing limitations of solar panels with just one single layer and theoretically calculated what has now come to be known as the Shockley-Queisser limit. This theory states that such solar panels are unable to absorb solar light optimally, thereby limiting their capabilities.

In order to overcome this limitation, a novel and innovative design of the solar cell was proposed and developed. This new type of solar cell, now known as ‘Tandem Cell’, is based on the principle of stacking multiple layers of different semiconductor materials in order to allow for better and more efficient absorption of solar light.

Solar Spectrum

Solar Solar Spectrum
This picture is taken from metsolar

Orange: However, even in ideal conditions, some amount of energy from the photons is lost due to charge extraction losses. These losses occur due to imperfections in the material, such as impurities and defects, that prevent electrons from being fully extracted. This loss is commonly referred to as orange.

Blue: losses occur when photons of higher energy than the bandgap of the semiconductor material strike the cell. Since the cell cannot extract energy from these photons, the excess energy is converted to heat, which is lost and cannot be used.

Yellow: This process is known as the maximum available theoretical energy from a standard single junction cell, commonly referred to as “yellow”. Yellow is the maximum energy that can be extracted from a single junction cell under ideal conditions and is the highest attainable efficiency of solar cells.

Pink: Finally, pink losses occur when photons of energy lower than the bandgap of the semiconductor material strike the cell. Since these photons do not have enough energy to release electrons, the energy is lost and cannot be used.

Advantages of multiple layers

The main advantage of having multiple layers of different materials is that each of these layers has different energy bandgap characteristics which make them better capture certain wavelengths of solar light, thus harnessing more energy. This ultimately enhances the cell’s efficiency and is the technology at the heart of Tandem Cells.

Swiss Federal Laboratories for Materials Science and Technology

Rainer Klose from the Swiss Federal Laboratories for Materials Science and Technology explains how it works: “The idea is comparable to a double-blade razor: two work steps are more thorough than one. For solar cells, stacking two cells on top of each other, where the top cell is semi-transparent, efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity.”

The idea of stacking two or more cells has made a remarkable improvement in the efficiency of solar cells. In addition to helping capture more sunlight for conversion into electricity, it also works to extend the life of the cells by reducing the amount of heat generated. This reduces the amount of energy needed for cooling and thus, reduces energy costs.

Types of Tandem Solar Cells

The numerous kinds of tandem cells can mostly be classified depending on the materials employed; such as organic, inorganic, and hybrid variants. Furthermore, various connection types, like stacked, monolithic, or optical splitting, can be used to further differentiate the sub-cells. Here, we distinguish silicon-based tandem modules in order to demonstrate potential and efficacy.

Organic photovoltaics

Tandem cells made from organic materials offer the advantage of low-cost production and semi-transparency; however, this comes with a downside of low power conversion efficiency, typically under 10%. It is anticipated that tandem cells constructed using organic photovoltaics could reach an efficiency level of up to 15%; considerably lower than that achievable with other methods. In conclusion, tandem cells made with organic elements may be more economical, however, they lack the benefit of high efficiency.

These are cells that utilize polymers or other small molecules as the endogenous base material, as opposed to silicon or other inorganic materials. They offer a wide range of material properties, such as flexibility, lightweight, and low-temperature manufacturing processes. These cells are typically connected in the form of a stacked series of cells and are commonly used in a wide range of products, such as roller shades, backpacks, and textiles.

Inorganic photovoltaics

III-V group materials are commonly referred to as the “mother and father of technology”. Due to their single junction solar cells have led the way for conversion efficiencies since their initial emergence. Furthermore, tandem cells made from these materials have also seen steady improvements. With three-junction cells composed of GaInP/InGaAs/InGaAs achieving a world-record efficiency of 44.4% under 302 suns, and four-junction GaInP/GaAs; GaInAsP/GaInAs reaching 46.0% at 508 suns. Although these cells are slightly less effective than the lab-based world recorders. They are mostly used in space applications such as satellites and concentrator systems. Because of the high cost yet highest efficiencies they offer. To our knowledge, these are the only commercial tandem cells available.

Composition of Inorganic photovoltaics

Inorganic photovoltaics use a combination of silicon, germanium, and other semiconductor materials to create a more efficient and higher-powered cell. There are three main varieties of connection used in this type of tandem cell: stacked, monolithic, and optical splitting. Stacked cells are connected to form a multi-cell module, while monolithic cells are connected directly into a single unit. Optical splitting cells allow for maximum efficiency by using a beam splitter to separate the different subcells based on the reflective index.

Hybrid photovoltaic cells

Hybrid Tandem Cells constitute the third type of tandem cells and involve the solar industry perspective of Perovskite. Perovskite Tandem Cells offer great efficiency at a low cost, attributed to the cheap materials used. In addition to possessing strong optical absorption and long diffusion length, within the addition of being printed via roll-to-roll technology. Despite its promising qualities, the matter of perovskite stability is still in development and is thus a cause for concern raised in tandem cells using perovskite. As well as an array of other factors such as recombination losses, bandgap optimization, transparent conductive oxide reflections, and parasitic absorbance.

Hybrid photovoltaic cells: Revolutionize the solar industry

The hybrid type of tandem cells has the potential to revolutionize the solar industry, with its promise of partial transparency, low cost, and flexibility. Dye-sensitized solar cells (DSSCs) are one such example, where an interesting structure and promising future are seen. However, these cells are still in an early stage of commercialization, mainly due to the lack of efficiency.

Hybrid photovoltaic cells efficiency
credits go to Metsolo

Tandem cells with silicon

Tandem cells with silicon present an attractive option for the present, as opposed to the potential of DSSC technology in the future. Combining silicon with III-V group materials, CZTS, CIGS, perovskites, and polymers has been found to yield promising results. Consequently, this formal inquiry makes a strong case for further research into how best to take advantage of such opportunities.

 EfficiencyTandem cells with silicon
https://metsolar.eu/blog/what-are-tandem-cells-introduction-to-solar-technology-part-3/#!

Semi-transparent crystal

Molecular soccer balls, composed of PCBM (phenyl-C61-butyric acid methyl ester) molecules. Which contains 61 carbon atoms interconnected in the shape of a soccer ball. They served as a substrate to obtain a 14.2% efficient semi-transparent solar cell with 72% average transparency. The semi-transparent crystal was created by a vapor deposition and spin coating onto the soccer-ball-like PCBM layer, followed by a “lukewarm” temperature annealing. This perovskite-based cell is capable of absorbing visible blue and yellow light. Converting them into electricity whilst simultaneously allowing red light and infrared radiation to pass through. Through this process, a further solar cell can be incorporated underneath the semi-transparent perovskite crystal, allowing for a double success result.

Advantages of Tandem Solar Cell

The double-layered tandem cell structure offers improved use of the solar spectrum, due to the combination of materials with diverse bandgaps. Empa researchers specialize in the CIGS (copper indium gallium diselenide) technology, which is already in use for flexible solar cells. The two-layer cell utilizes both weak and intense radiation, increasing efficiency by reducing the heat and energy lost during conversion. This demonstrates how the versatility of the tandem cell structure offers a wider range of applications to better utilize the full spectrum of the sun.

The efficiency of Tandem Solar Cell

Over 30% efficiency can be achieved, suggested Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. Despite claiming the progress of achieving high efficiency, the efficiency in single-layer polycrystalline solar cells is only up to 25%. Which is higher than the figure that could be attained with tandem cells. Still, it requires many interdisciplinary efforts and combinatorial experiments to find the right semi-transparent high-performance cell. Which should be accompanied by the necessary base cell and electrical interconnection technologies.

Compatibility problem between layers

The key challenge to be addressed when introducing these devices is their compatibility between layers. The researchers are concentrating their efforts on examining the compatibility of the top and bottom solar cells in terms of the materials utilized. This entails the top cell needing to efficiently absorb a portion of the solar light so as to generate electricity whilst still remaining transparent enough to let the remainder of the light pass through and be absorbed by the bottom cell.

Moreover, due to the different voltages and currents that the two devices operate at, the need for significant development is required in order to extract the maximum power from both.

In order to produce a series of sandwich layers, the components must exhibit mechanical compatibility. This will mitigate against any fracturing from changes in temperature.

lack of a suitable technological solution

The issue, at hand, is the lack of a suitable technological solution for the manufacturing of a tandem device, specifically a routing card. To ensure efficient production and a properly functioning device, each step of the manufacturing process must be carefully sequenced. In order to not jeopardize the previously created structure. Additionally, scalability is paramount to creating a device suitable for mass production. Finally, the device’s durability should be taken into account, as there is no benefit in developing a tandem. In which one component fails while the other performs its duties for the duration of its lifespan.

Cost efficiency: Tandem Solar Cell

The introduction of tandems is an attempt to surpass the theoretical limit of single-cell efficiency (~30%). Naturally, this must be done while also remaining cost-efficient in comparison to established technologies, such as silicon-based solar cells. Consequently, cost efficiency is an important aspect of this endeavor.

How much efficiency is possible with Tandem Solar Cell?

Over 30% efficiency can be achieved, suggested Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. Despite claiming the progress of achieving high efficiency. The efficiency in single-layer polycrystalline solar cells is only up to 25%. Which is higher than the figure that could be attained with tandem cells. Still, it requires many interdisciplinary efforts and combinatorial experiments to find the right semi-transparent high-performance cell. Which should be accompanied by the necessary base cell and electrical interconnection technologies.

Conclusion

In conclusion, tandem solar cells are an exciting development in the solar industry with potentially revolutionary properties. Although more expensive, their increased efficiency can be beneficial in some circumstances. Having enlightened ourselves regarding the pros and cons of this technology, we will be better placed to understand how it can be implemented in various contexts. Moreover, by continually researching and developing new tandem cell technologies, we may yet usher in a renewable energy revolution.

FAQ

What is Tandem Solar Cell?

Tandem solar cells (also known as multijunction solar cells) are a type of solar cell design that has multiple layers of photovoltaic material, each with different band gaps. The two materials used in this type of design are usually a combination of crystalline silicon or Perovskite, and, depending on the application, a range of other materials such as organic materials and metal nitrides.

What is a tandem structure?

The structure is composed of two semiconductor absorbers stacked one above the other. The combination of the two absorbers has a higher optical absorption than either of them alone and allows the capture of a wider range of the solar spectrum than a single absorber. In a tandem design, the higher bandgap material (usually crystalline silicon or metal nitride) is placed on top and the lower bandgap material (usually metal nitride or perovskite) is placed at the bottom. This has the effect of capturing the higher energy light from the top absorber and leaving the lower energy light for the lower absorber.

How can the efficiency of the system be improved?

By using a tandem structure, the efficiency of the system can be significantly improved as the two absorbers (combined with a suitable transparent electrode) can cover the entire solar spectrum, resulting in higher voltage and higher current. The highest efficiency of these solar cells currently is around 28.3% with the use of a 4T perovskite/silicon tandem cell. This type of efficiency is achieved with the use of a semitransparent Perovskite top cell that makes use of an ultrathin gold film as a transparent electrode which also offers excellent conductivity and transmittance in the near-infrared region.

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