Solar trends: Perovskite Solar Cell technology
A new trend in solar, Perovskite Solar Cell. Perovskite is a crystalline material that possesses interesting properties which are proving to be promising for solar PV cells and future market opportunities.
Technically known as metal-halide perovskites, perovskites are crystalline materials with interesting properties for solar PV cells that have promising market opportunities.
Development on this technology has evolved rapidly, as just a few years ago the efficiency of perovskites wouldn’t exceed 5%, while nowadays the promising record of 30% could be exceeded by 2020, according to Oxford PV.
This means that in just a few years, perovskites have reached efficiency levels that silicon cells took half a century to reach!
What is the Perovskite Cell?
The perovskite material was first discovered in the Ural Mountains which is a large mountain range located in Occidental Russia and so the mineral was named after the founder of the Russian Geographical Society, Lev Perovski.
This mineral is based on a compound of calcium, titanium and oxygen (CaTiO3) with a cubic structure as the one shown on the image.
How are Perovskite Cells made?
The construction process of perovskite cells consists in placing three layers that are intercalated between metal contacts.
The first layer is of a transparent contact on a glass substrate made of a compound known as FTO, then, on the top of this layer, an element called titanium dioxide is added. This is an n-type semiconductor used to conduct electrons (and therefore current).
This layer creates the so-called anode of the perovskite cell.
Then the perovskite layer is added, which acts as the semiconductor material that transforms photons into electrons.
Finally, the ultimate layer is made out of a whole transport material (like Spiro-OMeTAD) that acts as the cathode of the cell.
Once the layers are mixed up and after making some chemical reactions, the cell is heated to high-temperature values and finally, a gold contact is added to make conduction of electrons even better.
The whole manufacturing process, known as solution processing, is actually a well-known process which is used to make newspapers as well.
Check out the video explanation of what to expect from Perovskite solar technology in the future:
What is the big deal with Perovskites? Do they make the D]difference?
There is huge academic and industry interest in this technology because of the advantages that can be achieved by using modules with perovskite cells.
One of those advantages is the high open-circuit voltage that perovskite cells can generate when high illumination is applied; this translates into lower losses during the conversion of light into electricity.
Moreover, lower losses are also related to radiative recombination of electrons which allows the possibility of pushing the perovskite cells towards the Shockley-Queisser limit (maximum theoretical value of silicon cells).
As mentioned before, perovskite efficiency values have very promising targets. As of today, 25% efficiency can be achieved on lab tests, values that drive the attention of the academic and industrial sectors, as more efficiency means more conversion of light into DC power with the same module size.
Another important advantage is related to the lower cost of materials and manufacturing processes. This translates into a full-scale price reduction of solar cells that can be as low as USD $0.25 per watt according to Richard Caldwell from Great Cell Solar.
As it can be seen in the solar balance of system costs made using NREL data, the module cost per watt was close to $0.64/Watt by 2016. Therefore, a reduction of nearly 60% in costs of modules could be achieved by using the perovskite technology.
If you put those numbers on a big scale level, it is a huge amount of money that could be saved, making PV technology even more attractive for large-scale installations.
And if you think of it for residential scale level, the reduction in price per module is still very attractive.
Perovskite cells are also flexible in their conception.
The composition involves mixing elements into an ABX3 crystal structure which can be changed on several configurations to give multiple options to study a variety of electrical and optical properties.
This flexibility also allows them to capture a broader light spectrum.
Typical silicon cells can only capture photons on the visible wavelength range, while perovskite cells could capture photons from other spectrum ranges like an infrared or ultraviolet light.
Moreover, perovskite cells are intended to be constructed for thin-film PV modules, which makes them physically malleable, lightweight and with the possibility to even become semi-transparent.
Furthermore, the malleable ABX3 composition allows them to take any colour from the light spectrum. These features also make them highly attractive for Building Integrated Photovoltaic installations, and also easier and cheaper to transport.
Likewise, the perovskites composition can also be printed as photovoltaic electronics which makes them even cheaper to produce than other thin-film technologies.
Finally, an ongoing innovation that has received a lot of the media’s attention is the possibility of the perovskite to be applied as a liquid solution, or in other words, spraying the perovskite as a liquid layer on a substrate material which would allow the solar cells to be printed at a higher volume.
Among other particular benefits beyond the PV industry, they have also been considered good for the manufacturing of lasers, photodetectors, and light – emitting diodes due to their inherent properties like ambipolarity, high charge carrier-mobilities and high diffusion lengths.
Limitations of the technology
There are ongoing challenges for perovskite cells.
One of the most negative for the renewable way of life is the toxicity associated with lead quantities in high-performance perovskites.
This is a big hold-up from environmental and some photovoltaic communities. In spite of the low amount of lead content in a solar panel (just a few milligrams), the potential quantity of metal associated with utility or commercial scale introduction of the perovskite should be treated with caution.
Moreover, under extended exposure to humidity, elevated temperatures or oxygen, perovskites tend to degrade into harmful compounds that carry heavy metals which could leach into the environment.
This environmental disadvantage also presents another limitation that can influence the policy aspect regarding this technology.
In order to present a solution to this issue, the proposal is to substitute the lead constitution by Stannum (Sn).
However, Stannum perovskite cells degrade rapidly, are extremely unstable in the air and can still cause environmental and health effects, making it unsuitable for commercial purposes..
Some other propositions include copper, germanium and nickel.
On the other hand, the most pressing matter for perovskite researchers is proving that the technology can be manufactured on a large scale and that perovskite cells can last at least 25 years in outdoor environments without using any expensive encapsulation methodology. That is the biggest barrier to commercialisation.
Typical silicon cells can last up to 25-30 years, but perovskite technologies are quite sensitive to moisture, making them quite fragile for outdoor environments in countries with high relative humidity in the long term.
That is why sealing needs to be carefully done in the manufacturing of these cells in order to avoid moisture contact. However, as the technology is still new (less than 6 years) there is no practical way to find out how many years a perovskite would last.
Stability is also a typical concern when speaking of perovskite cells.
Again, the problem in this sense is generally attributed to the presence of moisture in the cell, which causes a rapid decomposition of the semi-conductive material. Deep research is required on the topic as the elevated humidity not only influences the perovskite layer itself, but also affects the surrounding layers (anode and cathode).
Modifications in the geometry and materials are needed to address this issue.
Another pressing matter is related to the current-voltage curves of perovskite cells which exhibit hysteresis shapes that change the parameters of performance under different scenarios.
The reason for this phenomenon is highly debated and there is no conclusion about it, making it an interesting field of deep research.
Some of the estimated explanations are related to the grain size and boundaries, and surface imperfections of the perovskite and ion migration within the crystal structure of the perovskites.
Despite these limitations, market interest is widespread due to the promising advantages and governments pushing interest to make the technology work in a commercial environment.
Therefore, today many companies are involved in the research and manufacturing of this material.
Actually, one of the principal characters in the industry is an Australian company, formerly known as Dyesol, winner of $6 million from the Australian Renewable Energy Agency (ARENA), with the purpose of commercialising very high-efficiency perovskite cells in the Australian market.
Today, the name of the company has changed to Greatcell Solar and they have definitely become the greatest market leader of this third generation PV technology in Australia.
One of their leading projects here in Australia is the Major Area Demonstration (MAD) prototype, which will aim to take perovskite cell manufacturing to a new level by building a world-class prototype facility that will increase the potential for manufacturing high-quality large-area perovskite devices.
On the other hand, Oxford PV is perhaps the most important company involved in the perovskite industry worldwide with more than 8 years in the business.
The company was first funded in 2010 by Professor Henry Snaith and Kevin Arthur from the University of Oxford. Since then, the company has focused on the research development of perovskites, on silicon tandem cells, technology that presents very interesting projections. They have already .proved that 25.2% of efficiency can be achieved (goal reached on June 2018) and they are working together with other development partners to bring this technology from the research lab to the industrial sector.
Currently, their product line focuses on developing commercial size 156×156 mm perovskite-silicon tandem solar cells.
The Americans are not left behind and Solar-Tectic leads the way in that market as the only US company that holds exclusive patents on an innovative manufacturing process. This uses a highly textured MgO (111) thin-film on a glass substrate that successfully grows silicon films on which textured perovskite thin-films can be placed to mix them with high efficiency tandem cells.
Among other market leaders, Microquanta Semiconductor and Solliance also take part in the industry scenery.
The first one is a Chinese company that was established in 2015 and holds its headquarters in Hangzhou, China.
Microquanta owns a huge and highly advanced lab that is dedicated to merely producing perovskite cells of 6×6 cm², although their reported efficiency values only reach 16%.
On the other hand, Solliance is a Dutch company established in 2010 which focuses on research studies to develop upscale perovskite PV modules.
Solliance has also developed interesting methods of increasing the resistance of perovskite cells to water by using Atomic Layer Deposition (ALD).
This is a technology that when applied to the inner layers of the cell, means that the barrier layer needed to make the modules moisture-resistant need not be as strong.
We have provided you with an overview of the market and current status of perovskite cells, major breakthroughs, key leaders, barriers and opportunities of this very promising technology.
The development of perovskite cells is still an evolving process. The potential benefits of the technology are that the perovskite cells could increase the efficiency of solar panels, reduce the cost of solar modules and bring new trends and opportunities to the growing solar market. For example, perovskite solar cells can adopt any colour and they can be manufactured as easily as newspapers.
All of this means more profits and an expansion of the market.
Although important breakthroughs have been made in the introduction of perovskites to the commercial environment, there are still important challenges ahead that research institutes and companies must face to take this technology to the roof of Australian households.
Toxicity, lack of durability and instability in the presence of humidity, are significant barriers that must be eliminated before the technology can be considered as commercially viable.
Nevertheless, we must remember that perovskites made an incredible progress in less than 5 years, the progress that took 50 years for silicon cells to achieve.
Therefore, it is probably just a matter of time before these problems can be solved and we start looking at perovskite cells on the rooftops of Australian households.
Photo credit: Metsolar, NREL, ResearchGate, Depositphotos