http://youtu.be/YWU89g7sj7s
Solar panels are a critical technology in our move towards net zero. Even though we’re seeing a decrease in silicon-based solar panel costs, we haven’t seen significant efficiency improvements … yet. But what if we could build panels using materials that aren’t supply-limited and with a more straightforward, lower carbon process? As well as achieving higher efficiencies at the same time? Perovskite solar panels have been promising that future for some time now, but where are they? And are they the future of solar panel technology? Let’s explore Perovskite solar panels and how they might energize our future.

Solar power is one of the most promising power sources to reduce carbon emissions. Electricity from photovoltaics into the grid jumped from 597 GWh in 2005 to about 545 TWh in 2018, and with many policies being rolled out to try and achieve net-zero in the next few decades, solar panel use continues to grow around the world.1

Crystalline silicon has been the go-to choice for decades, and although other materials like copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) have popped up, they only cover a small piece of the market — about 5%. The main reason for that small marketshare is because it’s hard to make them as efficiently and cheaply as traditional silicon-based solar panels.

But silicon isn’t perfect. It still has issues regarding cost as well as efficiency, which typically doesn’t go above 21% to 22% for the top selling panels. It’s also no secret that making solar panels is a dirty business due to the intense heat required to remove impurities from silicon, so researchers and companies have been looking for alternatives. One promising technology that provides simple manufacturing and highly efficient photovoltaics are perovskite solar cells. 2 3

Perovskites are a family of materials with a particular crystal structure discovered by a German scientist, Gustav Rose, when he was traveling to Russia in 1839. Any type of material that has the same crystal structure as calcium titanium oxide (CaTiO3) is considered a perovskite. 4 5

It wasn’t until the 1950s that research and development on oxide perovskites grew up, which included its use in fuel cells, glass-ceramics, superconducting devices, and more. But it was only in 1999 that perovskites started being applied to solar cells. Researchers from the National Institute of Advanced Industrial Science & Technology from Tokyo announced that they’ve manufactured an optical absorption layer for a solar cell using a rare-earth-based perovskite compound. After that, the new millennium came with extensive research on perovskite solar cells, and new fabrication methods and materials. 4

Perovskites are easy-to-synthetize materials, and are considered the future of solar cells since their distinctive structure has shown a great potential for high performance and low production costs. These solar cells have been improved considerably in a short time frame, with a boom in conversion efficiency — from reports of about 3% in 2006 to over 29% today — going over the maximum efficiency achieved in traditional mono- and poly-crystalline silicon cells. 6 7

In laboratories, perovskite cells are manufactured by spin-coating, spraying or “painting” them onto a substrate, which is a material that provides the surface for the chemicals to crystalize on. 5 8

These cells work much like a traditional solar panel, but for a quick recap…

The part of a solar panel that absorbs sunlight and converts it into electricity is a wafer made of a semiconductor material — usually silicon. A semiconductor is a material that usually doesn’t conduct electricity well, unless it’s under the right conditions. This is oversimplified, but the cell basically has two silicon layers. The top layer has a tiny amount of phosphorus, which has more electrons than silicon. This excess of electrons makes it more negatively charged, so it’s referred to as the N-Type layer. And the bottom layer has a tiny amount of boron, which has fewer electrons than silicon, to make it more positively charged and known as the P-Type layer.

When a photon of visible spectrum sunlight hits the panel and is absorbed, it knocks loose an electron while leaving a hole, which is positively charged. The free electron is attracted to the negatively charged top layer, while the hole moves down to the positively charged bottom layer. Wires connecting the top and bottom layers create a circuit for the electrons to reconnect with the holes … generating electric current. 9

So why does this matter when we’re talking about Perovskites?

Today, the mainstream solar technology – silicon – is reaching its practical efficiency limit when used alone. The physicists William Shockley and Hans-Joachim Queisser calculated the theoretical maximum efficiency of silicon single junction solar cells at around 30%.10 It’s known as the Shockley–Queisser limit. Although there are gains made by multi-junction cells that combine multiple layers and techniques together as I explained in a previous video. Another challenge is that silicon also has to be fairly thick and manufactured with very high heat.

But when it comes to Perovskite solar cells, they don’t require the heat and can be manufactured with much thinner layers. 11 12 They can also work with almost all visible wavelengths, resulting in a more efficient transport, recombination, and extraction of charges than silicon cells. 13

Perovskites can be tuned to absorb different colors in the solar spectrum. This bandgap flexibility opens up another useful application for these solar cells in high-performance tandem device configurations that achieve efficiencies above 30%. They can be combined with other materials, like silicon for example, to form hybrid structures … those multi-junction cells I mentioned earlier. Each junction, or group of layers, can be tuned to different wavelengths of light, increasing the over-all range of wavelengths the entire cell can absorb, which helps with the efficiency. 14

Perovskite can be produced utilizing simple solutions that don’t require expensive, complex equipment and facilities. Its thin-film structure uses 20-times less materials, and also doesn’t require materials that are uncommon or supply limited. Perovskites are only about half a micron thick while a silicon layer is roughly 200 microns. 15 16

A life-cycle analysis involving several PV technologies concluded that producing silicon cells or perovskite-on-silicon tandem cells results in both a higher carbon footprint and cost compared to multilayer perovskite cells. 17

Stanford scientists, for example, manufactured thin films of perovskite with a robotic device with two nozzles. This technique may be able to produce perovskite modules for $0.25 per square foot, while the cost of traditional solar panels range from $4 to $10 per square foot. 18 19

With better efficiencies, cheaper and easier manufacturing, why aren’t we seeing this take over the solar industry? The main hurdle for perovskites is whether they can last as long as silicon panels, which generally come with a 25-year warranty and last for much longer than that.

Perovskites are very sensitive to oxygen, moisture, and heat, requiring heavy encapsulation to protect the cells, which increases cost and weight. The most common electrode material in perovskite solar cells right now is gold, which obviously jacks up the price a little bit, and cheaper alternatives don’t last as long. At warm temperatures, the structures of perovskite cells shift, and although this change is reversible, it degrades the performance of the cell. 2 4 17

Although high efficiencies have been achieved, like Oxford PV’s perovskite-silicon cell, which reached a 29.52% conversion efficiency, most perovskite firms haven’t published their stability results. They all say that they follow a certification standard established for silicon solar panels, set by the International Electrotechnical Commission (IEC).

The IEC 61215 standard that the modules are subjected is composed of a series of accelerated tests to simulate their operation over years. In one of these tests, the modules are heated up to 85 °C for 1,000 hours at a humidity level of 85% … and even bombarded with hailstones.

After this heavy and hard series of tests, if a silicon panel still works, it’s assumed that they have a good chance of lasting 25 years. But, in the case of perovskites, even though they could pass these tests, there’s still doubts if in practical conditions they can last all those years due to their instabilities compared to silicon. We have decades of silicon use in the real world vs. nothing for perovskite … yet.

Microquanta’s perovskite modules, for example, got the IEC 61215 approval, but it turns out that the modules have their generation capacity reduced to 80% of their initial performance in 1–2 years on average, according to field trials in Hangzhou, China. 2

Another small but questionable problem with perovskite cells is their toxicity. Lead is used in the most common cell structures, and since it’s a toxic metal substance, it needs to be carefully controlled from its manufacturing to its recycling. 15

While these challenges still exist, a lot of research and development is being put into making perovskite solar cells a reality. Scientists and companies are working towards increasing efficiency and stability, as well as increasing lifespan and replacing toxic materials with safer ones.

Researchers from the School of Engineering at Brown University have recently published advancements to make perovskite solar cells more durable. First of all, the scientists found which was the weakest interface of perovskites. Then, they figured out how improve their resistance, which was done with a “molecular glue.” 16

Utilizing this “molecular glue” to glue the cells together, they increased adherence between the layers of the cells compared to traditional laboratory adhesives that would have destroyed the cell’s properties.20

Compared to the commercial perovskite cells used in this study, which could last about 700 hours, the technology developed by the researchers boosted the lifespan to 4,000 hours (that’s roughly equivalent to two years at five peak-sun-hours per day). That’s a massive improvement, but the researchers have identified other areas for improvement, so there’s still more to come.16

Today we have only estimates for perovskite solar cells, and not too much real world use data. Estimates show that perovskite solar panels could cost just 10 to 20 cents per Watt, but we still need to wait for this technology to be commercialized, and mature a little bit, to have a more precise grasp on its true cost and benefits. 21

And regarding toxicity due to lead, which is very low by the way, scientists at the Central University of Jharkhand, in India, have simulated a methylammonium tin iodide perovskite solar cell optimized with a hole transport layer made of copper oxide (Cu2O). That’s a whole bunch of random words … what does that even mean? Well, the structure they’ve created is a lead-free perovskite cell. I should have probably just … lead … with that.22

Their simulations showed that is has the potential to reach a power conversion efficiency of 27.43%. 22 And one of the researchers, Basudev Pradhan, pointed out that costs estimates are about 8-10 times cheaper than standard silicon-based solar panels.

Saule Technologies, one of the leaders in the perovskite solar cells market, has some really interesting products in the works. They have a perovskite photovoltaic glass that can be integrated with buildings. It’s a semi-transparent, perovskite solar cell printed onto flexible foils and overlayed with layers of glass, making it a window that generates electricity.23

Saule is also producing energy-harvesting sun-blinds that can block intense summer sunlight, but in the mornings and evenings allow sunlight to enter the building to provide natural light and passive heating. These blinds can be adjusted manually or automatically. 24

And the biggest news, in May 2021, Saule launched the world’s first industrial production line of perovskite solar panels in Poland. 25

Jinko Solar, another leader in the solar panel market, is also working on rolling out perovskite technology. In 2017, the company signed a non-exclusive deal with Australia-based Greatcell to explore options for commercializing Greatcell’s perovskite cell technology. At its financial statement for the first quarter of 2021 the Chinese manufacturer pointed out:

“We have also completed the construction of a high-efficiency laminated perovskite cell technology platform that is expected to reach a breakthrough cell conversion efficiency of over 30% within the year.” 26

So things look like they’re finally starting to heat up. The perovskite solar cell market is estimated to grow at 34.0% CAGR between 2020-2027, but factors such as instabilities and the use of toxic materials could slow down the growth of the market.

Even though companies such as Saule and Jinko Solar have been investing in this tech, perovskite still has issues to be addressed. But given that the material’s efficiency has increased from less than 4 percent to over 25 percent within a decade, it’s kind of easy to see why so many people are optimistic about perovskite’s future. Even though efficiency can be a great driver, it’ll never spur adoption on its own. For that it’s all going to come down the cost and value … even if it might not last as long as silicon. 27

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