A Direct wafer 6 x 6 solar cell at the CubicPV facility in Bedford, MA on August 5, 2021.
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In 1839, German scientist Gustav Rose went prospecting in the Ural Mountains and discovered a dark, shiny mineral. He named the calcium titanate “perovskite” after the Russian mineralogist Lev Perovski. The mineral was one of many that Rose identified for science, but nearly two centuries later, materials that share perovskite’s crystal structure could transform sustainable energy and the race against climate change by greatly increasing the efficiency of commercial solar panels.
Solar panels accounted for nearly 5% of US energy production last year, nearly eleven times more than 10 years ago and enough to power about 25 million households. It is the fastest-growing source of new power, too, accounting for 50% of all new electricity generation to be added by 2022. But almost all solar modules used in electricity generation today are made of conventional silicon-based panels made in China, a technology that has changed little since silicon cells was discovered in the 1950s.
Other materials used, such as gallium arsenide, copper indium gallium selenide and cadmium telluride – the latter a key to America’s largest solar company First Solars growth — can be very expensive or toxic. Proponents of perovskite-based solar cells say they can outperform silicon in at least two ways and accelerate efforts in the race to fight climate change. Just this week, First Solar announced acquisition of the European perovskite technology player Evolar.
The silicon limits for solar cells
Photovoltaic cells convert photons in sunlight into electricity. But not all photons are created equal. They have different amounts of energy and correspond to different wavelengths in the solar spectrum. Cells made of perovskites, which refer to various materials with crystal structures similar to that of the mineral, have a higher absorption coefficient, meaning they can take a wider range of photon energies across the sunlight spectrum to deliver more energy. While standard commercial silicon cells have an efficiency of about 21%, laboratory perovskite cells have efficiencies of up to 25.7% for those based on perovskite alone, and as much as 31.25% for those that are combined with silicon in a so-called tandem cell. Meanwhile, even as silicon efficiency has increased, single-use cells face a theoretical maximum efficiency barrier of 29%, known as the Shockley-Queisser limit; their practical limit is as low as 24%.
In addition, perovskite cells can be more durable to produce than silicon. Intense heat and large amounts of energy are needed to remove impurities from silicon, and this produces a lot of carbon dioxide emissions. It also needs to be relatively thick to work. Perovskite cells are very thin – less than 1 micrometer – and can be painted or sprayed onto surfaces, making them relatively cheap to produce. A 2020 Stanford University analysis of an experimental production method estimated that perovskite modules could be manufactured for only 25 cents per square foot, compared to about $2.50 for the silicon equivalent.
“Industries will set up production lines in factories for the commercialization of their solar cells before 2025,” said Toin University of Yokohama engineering professor Tsutomu Miyasaka, who reported the creation of the first perovskite solar cell in 2009. “Not only for use in outdoor solar panels but also indoor IoT- power units, which will be a big market for perovskite solar cells because they can work even under low light.”
Supports the next generation of climate technology
Companies around the world are starting to commercialize perovskite panels. CubicPV, based in Massachusetts and Texas, has been developing tandem modules since 2019, and its backers include Bill Gates’ Breakthrough Energy Ventures. The company says its modules consist of a silicon bottom layer and a perovskite top layer and their efficiency will reach 30%. Their advantage, according to CEO Frank van Mierlo, is the company’s perovskite chemistry and its low-cost fabrication method for the silicon layer that makes the tandem method economical.
Last month, the Department of Energy announced that CubicPV will be the lead industry participant in a new Massachusetts Institute of Technology research center that will leverage automation and AI to optimize the production of tandem panels. Meanwhile, CubicPV is set to decide on the location of a new 10GW silicon wafer fab in the US, a move it says will accelerate tandem development.
“Tandem extracts more power from the sun, making each solar installation more powerful and accelerating the world’s ability to curb the worst effects of climate change,” says Van Mierlo. “We believe that in the next decade the entire industry will move to tandem.”
In Europe, Oxford PV also plans to start manufacturing tandem modules. A spinoff from Oxford University, it claims an efficiency of 28% for tandem and says it is developing a multilayer cell with 37% efficiency. The company is building a solar cell factory in Brandenburg, Germany, but it has been delayed by the coronavirus pandemic and supply chain hiccups. Still, the startup, which was founded in 2010 and is backed by the Norwegian energy company Equinor, Chinese wind power maker Goldwind and the European Investment Bank, are hopeful they can start deliveries this year pending regulatory certification. The technology would initially be priced higher than conventional silicon cells because tandems offer higher energy density but the company says the economics are favorable over the lifetime.
Many solar upstarts over the years have tried to break the market share of China and conventional silicon panels, such as the infamous now-bankrupt Solyndra, which used copper indium gallium selenide. First Solar’s cadmium telluride thin-film method survived a decade-long solar shake-up because of its balance of low cost relative to crystalline silicon and efficiency. But now tandem cells are seen as a key to the future of the solar industry as well.
“Perovskite is a disruptive material without disrupting the business model – the entrenched capability to manufacture based on silicon,” said Oxford PV CTO Chris Case. “Our product will be better at producing energy at a lower cost than any competing solar technology.”
The Brandenburg, Germany manufacturing facility for Oxford PV, an Oxford University spinoff, claims 28% efficiency for its tandem solar cells and says it is developing a multilayer cell with 37% efficiency.
Caelux, a spinoff from the California Institute of Technology, is also focusing on commercializing tandem cells. Backed by VC Vinod Khosla and Indian energy, telecom and retail conglomerate Reliance Industries, Caelux wants to work with existing silicon module companies by adding a layer of perovskite glass to conventional modules to increase efficiency by 30% or more.
Questions about performance outside the lab
Perovskites face challenges in terms of cost, durability and environmental impact before they can put a stop to the market. One of the best-performing versions is lead halide perovskites, but researchers are trying to formulate other compositions to avoid lead toxicity.
Martin Green, a solar cell researcher at the University of New South Wales in Australia, believes that silicon-based tandem cells will be the next big step forward in solar technology. But he warns that they are not known to work well enough outside the lab. Perovskite materials can degrade when exposed to moisture, a problem researchers have claimed some success with.
“The big question is whether perovskite/silicon tandem cells will ever have the stability required to be commercially viable,” said Green, who heads the Australian Center for Advanced Photovoltaics. “Although progress has been made since the first perovskite cells were reported, the only published field data for such tandem cells with competitive efficiency suggest that they would only survive a few months outdoors even when carefully encapsulated.”
In a recent field trial, tandem cells were tested for over a year in Saudi Arabia and was found to retain more than 80% of an initial 21.6% conversion efficiency. For its part, Oxford PV says its solar cells are designed to meet the expected lifespan of 25 to 30 years when assembled into standard solar modules. It says its demonstration tandem modules passed key industry-accelerated stress tests to predict solar module lifetimes.
Japan’s built-up perovskite experiment
In Japan, it is difficult to obtain large, flat lands that can host mega solar projects due to the mountainous terrain of the archipelago. That’s one reason companies are developing thin, versatile perovskite panels for use on walls and other parts of buildings. Earlier this year, Sekisui Chemical and NTT Data installed perovskite cells on the outside of buildings in Tokyo and Osaka to test their performance for a year. Electronics maker Panasonic, meanwhile, created an inkjet printer that can make thin-film perovskite cells of various sizes, shapes and opacities, meaning they can be used in ordinary glass installed on windows, walls, balconies and other surfaces.
“On-site power generation and consumption will be very beneficial to society,” said Yukihiro Kaneko, general manager of Panasonic’s Applied Materials Technology Center. “For Japan to achieve its decarbonization goal, you need to build 1,300 ball-field-sized megasolar projects every year. That’s why we think it’s best to build solar into windows and walls.”
On display at CES 2023, Panasonic’s 30cm square perovskite cell has an efficiency of 17.9%, the highest in the world, according to a ranking by the US National Renewable Energy Laboratory. The maker will get a boost from regulations such as a recently announced requirement that all new residential projects in Tokyo have solar panels starting in 2025. Panasonic says it aims to commercialize its perovskite cells in the next five years.
Perovskite cell inventor Miyasaka believes that perovskite-based power generation will account for more than half of the solar cell market by 2030, not by replacing silicon but by new applications such as building walls and windows.
“The rapid progress in power conversion efficiency was a surprising and truly unexpected result for me,” said Miyasaka. “In short, this will be a major contribution to realizing a self-sufficient sustainable society.”