Comprehensive coverage

The shining successor of silicon

The perovskite material, which has recently made headlines, could eventually be used to make solar cells that are cheaper and more efficient than those made using conventional silicon technology

The article was published with the approval of Scientific American Israel and the Ort Israel network

Perovskite crystal. Photo: Shutterstock
Perovskite crystal. Photo: Shutterstock

Crystalline silicon has dominated the solar cell market for decades, but prototype cells made from another crystalline material, perovskite, are rapidly approaching the same level of efficiency.

Perovskite may be cheaper than silicon because it can be produced at much lower temperatures. The perovskite cells can be rolled in flexible and colored layers, which leads to a wider variety of uses and products than those made possible by the rigid cells of silicon.

But the challenges are still great. Ways must be developed to reliably seal the cells against water, to prevent the disintegration of the perovskite cells within a few hours.

Lead, which is found in cells in small amounts, should be completely sealed inside the cells, for safety. Also the dimensions of the cells must increase. Today, the most efficient cells are no bigger than a fingernail.

As evening fell, research student Michael Lee sat in a dim bar in Japan and quickly scribbled on a piece of cardboard, used

The current record, 20.1%, achieved by the Korea Research Institute of Chemical Technology in November 2014, marked a fivefold increase in efficiency over three years. For comparison, after decades of developing silicon solar cells, the increase in the efficiency of the cells at the forefront of research has stopped at around 25%, a goal that perovskite researchers like us already see on the horizon. We are also expecting a commercial breakthrough, perhaps through industrial companies established following the research, such as Oxford Photovoltaics, which one of us (Snaith) co-founded.

Perovskite cells are of interest for several reasons. The ingredients are abundant, researchers can easily create thin crystalline layers from them, at a low cost and at low temperatures, in contrast to silicon surfaces with a similar structure, which are more expensive to produce and are made at high temperatures. Rolls of thin and flexible perovskite layers, in contrast to the thick and rigid silicon "slices", will one day be able to flow out of special printers and it will be possible to produce lightweight, designable and perhaps even colorful solar sheets and coatings from them.

However, perovskite cell developers have several high hurdles to overcome before they can threaten silicon's reign. Their prototypes are no bigger than a human fingernail. To compete with silicon panels, researchers must find ways to produce larger perovskite layers. They also have to greatly improve the safety and long-term stability of the cells - a challenge as difficult as climbing up the mountain.

 To win the efficiency race

Today, the most efficient silicon cells reach 25.6 percent conversion. Why can't solar cells convert all the light energy of the sun into electricity? And why may perovskite cells break the silicon record?

The answers to the questions are related to the mobile and excitable electrons found in the material. When a solar cell is in the dark, each electron in the material remains bound to its atom, and there is no electric current. However, when sunlight hits the cell, it can release some electrons. These "excited" and high-energy electrons wander drunkenly through the crystalline lattice of the solar cell until they exit the end of the cell - then they are launched by an electrode as a useful electric current - or encounter a barrier or trap and lose their energy as wasted heat.

Image of gold electrodes adorning red solar cells made of perovskite. Credit: Plenam Petkov
Gold electrodes adorn red solar cells made of perovskite, built by researchers at the Massachusetts Institute of Technology. The size of the cells is like a postage stamp but they are much thinner.
Credit: Photographs by Plenam Petkov

The higher the quality of the crystal, the fewer defects it has that could divert the electrons from their path. To remove defects in the structure of silicon cells, they are usually heated to a temperature of 900 degrees Celsius. Perovskite cells have far fewer defects, even though they are processed at much lower temperatures, around 100 degrees Celsius. As a result, the electrons that the light excites in perovskite cells manage to get out of them no less than from silicon cells and their chances of losing energy when encountering obstacles are low. Since the electric power of a cell is a product of the flow of electrons leaving it (the electric current) and the energy that these electrons carry (the voltage), the efficiency of perovskite cells can successfully compete with silicon cells, with a much lower production effort.

However, there is a limit to the ability to convert light energy into an electric current using cells made of semiconductors such as silicon and perovskite. The reason for this is due to one of the properties of semiconductors: the forbidden energy gap, or the minimum amount of energy needed to release electrons. Sunlight includes many wavelengths, but only some of them have enough energy to overcome the forbidden gap. Light at other wavelengths will simply pass through the material without doing anything.

Different semiconductors have different forbidden gaps and this requires compromises in principle: the smaller the forbidden gap, the cell can absorb light in a wider range of wavelengths and use it to excite more electrons, but the energy of each excited electron will be lower. Because electrical power depends on both the number of electrons and their energy, even an ideal bandgap semiconductor can convert only 33% of sunlight into electricity.

Silicon has a fixed gap size that is not ideal, but it dominates the solar industry because the efficient ways to manufacture this technology are well understood. However, when producing perovskite cells, it is possible to change the size of the gap through changes in the mixture of ingredients. This increases the chances of exceeding the efficiency of silicon cells. It is also possible to place layers of different perovskite materials with different forbidden gaps on top of each other. It appears that bilayer perovskite cells may break the 33% barrier. There are predictions that they will be able to use even 46% of the sun's energy for work.

 Old stuff learns new tricks

Mineralogists have known natural forms of perovskite in the earth's crust since the 19th century [perovskite is the name of the mineral calcium titanate whose formula is CaTiO₃]. Perovskite crystals graced the cover of Scientific American in 1988, when scientists thought they could make superconductors at high temperatures (and some research is still being done in this area to this day.) Also, for the past 20 years, engineers have been making electronic devices based on artificial perovskite , but they missed its possible use in solar cells.

Finally, in 2009, a group of researchers at Twynedd University used a perovskite containing lead halide, an artificial material first prepared in 1978, and produced a solar cell from it. The researchers dissolved some selected substances and then swirled the solution and dried it on a thin plate of glass. The drying left behind, on top of the plate, a nanometer-thick layer of perovskite crystals, very similar to salt crystals that appear at the edges of puddles of seawater drying in the sun. This layer produced electrons when the sun shone on it, but not very efficiently. The researchers added thin layers of material on both sides of the nanoscale perovskite crystals that helped them transfer the electrons to an external electrical circuit and produced useful power.

The efficiency of the first tiny crystals was only 3.8% and their stability is very low. They fell apart within a few hours. Lee changed the composition of the perovskite, replaced a layer that caused problems in the cell and raised the efficiency beyond 10%. Another group of researchers, led by Michael Gratzel from the Swiss Federal Institute of Technology in Lausanne and Nam-Gyu Park from Sungkyunkwan University in Korea, reached a similar achievement.

The last step beyond 20% was made through some clever inventions. Since it is difficult to produce a crystal layer without defects, the researchers in Sang Il Seok's research group from the Korea Research Institute of Chemical Technology devised a multi-step process that made the crystal layer produced from the swirled solution more ordered. Seok gradually improved the process and achieved in 2014, one after the other, three efficiency peaks, from 16.2% to 20.1%.

Other scientists have made the layering of additional materials simpler; The newest perovskite cells look more like silicon cells: a simple stack of flat layers. It is this layered structure of the silicon that has made it possible to lower the cost of mass production of silicon cells. Recently, perovskite researchers tried to heat the solution and the glass plate on which the substance sinks. The result is crystals several orders of magnitude larger than those used in the initial cells. This is an encouraging sign of continued improvement in the formation processes.

Scientists also create new features. A change in the quantitative ratios between the substances can create cells with a subtle yellow tint or a crimson blush. Deposition of perovskite on glass in the form of islands, instead of in a uniform layer, makes it possible to create opaque or transparent layers or intermediate levels of transparency. All these possibilities give architects a refreshing choice of materials compared to the rigid and opaque silicon cells that are black-blue in color. This diversity could help them design ceiling skylights, windows and facades of houses that would contain colored perovskite solar cells. Imagine a skyscraper with colored windows, which protects the interior from the hot sunlight by turning it into an electric current, reducing the cost of cooling and also providing electricity.

 The road to commercialization is long

Perovskite products face a long journey until they realize this vision. Although researchers from Korea and Australia recently demonstrated printing perovskite cells that are 10 by 10 centimeters in size, large enough to be used in competitive commercial products, the most efficient cells are still in the small prototype stage. In the up-scaling processes of the devices, the laboratories and start-up companies involved in this must meet three prerequisites in order to reach the market: make sure that the cells will remain stable enough to generate electricity for decades, design a product that consumers will not be afraid to install in their homes or other buildings, and answer the skeptics who warn that the efficiency data of excessive perovskite cells.

The Achilles heel of perovskite solar cells is probably their instability. Perovskite cells may deteriorate quickly due to their sensitivity to moisture, so they must be sealed. We produced such cells in a chemically inert atmosphere and sealed them with epoxy resin. Then, we exposed them to continuous light and they worked stably for more than 1,000 hours. Researchers at Huazhong University of Science and Technology in China, working in collaboration with Gratzel, reached 1,000 hours of operation without epoxy sealing. They recently published a paper on a field trial deployment of solar panels in Saudi Arabia to show that the cells they designed can operate under realistic conditions. At the recent Materials Research Society conference in San Francisco, we revealed results from Oxford Photovoltaics that showed perovskite cells can produce stable electrical power for more than 2,000 hours in full sunlight.

However, the duration of the warranty on the integrity of solar panels accepted in the industry is 25 years. That means about 54,000 hours of bright, continuous sunlight. Finding a seal against moisture, which will work over such a period, in a wide temperature range, is really essential. Silicone panel manufacturers solved the problem by sealing the cells between two layers of glass. This is a perfect solution in large facilities standing on the ground. But since it is possible to produce perovskite cells in light and flexible thin layers, sealing methods must be found that will allow for wider applications, such as coating walls or windows that can generate electricity.

Illustration of the operation of silicon and perovskite cells in the conversion of sunlight to electricity. Credit: Jean Christensen
Credit: Jean Christensen

Fortunately, companies trying to commercialize other flexible solar materials, such as a semiconductor made of copper, indium, and gallium selenide, have made progress in this direction. The sealing technologies work well, but these companies struggle to compete in this market segment against the silicon industry, as their cells are less efficient and more expensive. Perovskite cells, which are supposed to be more efficient and cheaper, may be able to take advantage of advances in sealing methods.

Sealing the cells from getting moisture in is no more important than preventing the cell components from leaking out. The cells contain a tiny amount of toxic lead, so the market will demand firm proof that generating electricity from perovskite is safe. Researchers can draw inspiration from an alternative solar material, the only material other than silicon with commercial success: cadmium tellurium (CdTe).

The manufacturer of the panels made of this material, First Solar, deployed them around the world and met the safety standards, even though the panels contain cadmium, an element much more toxic than lead. First Solar has convinced consumer communities that their panels are well sealed so that cadmium will not leak from them even in a desert fire that will reach a temperature of 1,000 degrees Celsius. However, these plates are placed on a glass substrate and are therefore not flexible and light as perovskite cells are supposed to be. However, the perovskite manufacturers can learn from First Solar's success in sealing and rigorously testing the products.

An encouraging lead-related development recently emerged from the Massachusetts Institute of Technology (MIT): Angela Belcher and her colleagues have shown that car batteries containing lead and acid can be safely recycled. The recycled lead from the batteries could be used to produce perovskite cells. This research gives the cells an environmental advantage. Belcher estimates that the lead from one car battery could be used, approximately, to produce 700 square meters of perovskite cells. If the efficiency of the cells is 20%, this area will provide enough electricity for the consumption of 30 houses in a hot and sunny climate, such as that in desert areas.

Another solution could be avoiding the use of lead. Both our group and another group of researchers at Northwestern University have published preliminary results regarding cells containing tin instead of lead. However, this impairs the efficiency and stability of the cells, because over time the tin tends to damage the crystalline structure of the perovskite and delay the release of electrons from the cells. A lot of development will therefore be required before tin will enable long-term performance like lead.

Apart from the issues discussed here, the researchers have to answer a smaller and stranger issue. Critics claim that the measured efficiency values ​​of the perovskite cells may be higher than their true value due to hysteresis: a jitter in the measurement that may result from the movement of charged molecules migrating from one side of the cell to the other and apparently causes a stronger measured current. But this ion migration happens for a short time and scientists are looking for ways to stop it. In the meantime there is a simple solution: wait until the migration of the ions stops and measure the efficiency for a longer time. In most cases, the results of measuring efficiency with such a method are similar to the rapid initial measurements, but the researchers may be tempted to report only the highest measurements. We are now working together with researchers from all over the world to standardize the measurement process so that our results will stand up to more stringent tests.

Finally, to be commercially successful, perovskite pioneers need to provide a compelling economic plan to attract investors to move to full production. Admittedly, the materials needed to create perovskite cells are abundant, the production of the thin layers is done at low temperatures and the price of the equipment to produce them in rolls is low, but companies producing perovskite solar cells should be careful not to fall into the trap of competing directly with silicon. There is no place to offer cheaper prices than silicon, because the main cost of a facility is not related to the price of the boards but to what is known as the "balance of the system", a concept that includes the cost of the facility's materials, labor, licenses and supervision and other expenses related to the establishment of the system. The average price of a residential solar installation in the US in 2014 was $3.48 per watt of electrical power. However, the cost of the solar panel itself was only 72 cents per watt. Even if the prices of the perovskite panels drop to the ground, i.e. to 10 or 20 cents per watt as the researchers predict, the improvement will reduce the final price of building the facility only by a small amount.

However, perovskite companies can build on these small discounts and design products that are more efficient than silicon cells. A high-efficiency perovskite solar panel will reduce the installation cost per watt because it will save on the necessary ground or roof space and therefore also reduce labor and equipment. An even more creative example of changing the rules of the game is to sell perovskite products for uses that silicon cannot compete with, such as layers that will be integrated into building materials in walls, roofs and windows.

 The integrated solution

Now is the opportunity for the perovskite to reach the market not as a competitor to silicon but as its ally. Perovskite products will be able to "ride", literally, on top of the silicon plate, thus achieving entry into the 50 billion dollar market.

The combination can occur by adding a perovskite layer directly above the silicon layer and creating a two-layer solar cell (tandem cell). Perovskite excels at converting sunlight into the higher-energy wavelengths, such as blue and ultraviolet, which silicon fails to capture, thus allowing for a higher electrical voltage to be produced. Researchers at Stanford University and MIT recently stacked a perovskite cell on top of an opaque silicon cell and increased the efficiency from 11%, the original value of silicon, to 17%. They also built a two-layer cell by placing a layer of perovskite on top of a non-sealed layer of silicon, thus creating a single structure. The combination reached an efficiency of only 14%, but there is no doubt that this value will increase with the introduction of improvements in production. Based on these two experiments, the researchers sketched a scenario according to which if they combine the most efficient silicon and perovskite components using the most sophisticated engineering methods, it will be possible to reach an efficiency of more than 30%, without a noticeable change in either of the two technologies.

If two-layer solar panels reach 30% efficiency, the impact on the balance of the system will be enormous. Two-thirds of the number of panels is enough to reach the same power of panels that are 20% efficient. This means a significant reduction in the area of ​​the roofs or the area of ​​the ground, in the amount of construction materials and in the amount of work and equipment. The company Snaith co-founded, Oxford Photovoltaics, collaborates with silicon manufacturers to increase the efficiency of silicon cells using perovskite coatings. The company plans to produce a prototype of a two-layer cell in 2015. In the future, the company intends to incorporate cheap solar coatings into roofing and glazing materials, which could fundamentally change the cost structure of buildings powered by solar energy.

 running back

The rapid development of solar perovskite cells has stimulated scientists and engineers to produce more primary products, which may one day reach the market. We recently produced, in collaboration with our colleagues at the University of Cambridge, light emitting diodes (LED) and lasers using a compound of metal halide in a perovskite. These devices emit light (instead of absorbing it) very efficiently in a process called luminescence.

This turn is not surprising at all. When you operate the most efficient solar cell in the world, gallium arsenide (GaAs), in the opposite direction it works as an LED. LED devices and lasers, which can be produced with a printer, could lead to fascinating uses, from large-scale lighting to medical imaging.

The research towards such innovative products is still at an early stage, of course. But we believe it will become more acceptable. Perovskite-based materials make scientists feel like kids in a candy store. We found a material whose properties allow us to fulfill every item on our wish list: high efficiency, low cost, light weight, flexibility and good looks. To fully realize the potential inherent in perovskite, academia, industry and governments must cooperate, in a global and coordinated manner, and move beyond the silicon era. But if you think about the result, cheap and clean energy and the next generation of electronic devices, perovskite seems to be a safe bet.

5 תגובות

  1. Good news. Fortunately, the treasury and the electricity company are currently saving public money.
    There is no problem with installing a few solar cells that you only want to push electricity to the electricity company in the wrong amount, at the wrong times and at an exorbitant rate.

  2. Israel needs to invest more in nano technology (there are surpluses in the budget, they will not go to health. They will go to security). With the help of nano technology, a traditional industry can become innovative (as was written about combining materials in silicon panels). Since "KMG" owns an old atomic reactor (Dimona) Dimona should be turned into the Weizmann Institute of Nanotechnology. We need a new reactor, and I believe that it should join the Treaty on the Prohibition of Nuclear Weapons, and thus get rid of old and unsafe weapons, build a new reactor, and renew atomic research. If we want to renew the arsenal of weapons, then we should have a reactor New, and a new generation weapon.

  3. Good news. Unfortunately, the Treasury and the electric company
    Harassing Israeli citizens and preventing the deployment of solar cells
    (Just like the controlled charging stations for cars were required
    hybrids) and therefore for the citizens of Israel it is science fiction...

Leave a Reply

Email will not be published. Required fields are marked *

This site uses Akismat to prevent spam messages. Click here to learn how your response data is processed.