Follow the voice of the storm
Elevate your thinking
Guide
Perovskites have the advantages of high conversion efficiency, sufficient material supply, low price and high recycling efficiency, but they also have disadvantages such as lead content and long-term stability.
This article is excerpted from Engineering, No. 8, 2021, journal of the Chinese Academy of Engineering
By Sean O'Neill
来源：Perovskite Pushes Solar Cells to Record Efficiency[J]. Engineering,2021,7(8):1037-1040.
Editor's Note
At present, the field of solar photovoltaic research is gradually beginning to use synthetic perovskites, and its potential application in solar cells has received widespread attention. Silicon/perovskite solar cells are made of a layer of silicon and a layer of synthetic perovskite thin film layer in series, the cell conversion efficiency is nearly 30%, far exceeding the conversion efficiency of the current industrial production of silicon solar modules (20% to 22%). In addition, perovskite can be made into a very thin film on a very light substrate, with good curvature or flexibility, and has broad prospects for development in electric vehicles, commercial buildings and other fields.
In the 8th issue of the Chinese Academy of Engineering in 2021, the 8th issue of the "Perovskite Solar Cell Efficiency Breaks the World Record", which reported the development and research status of perovskite solar cells, introduced its breakthroughs in battery conversion efficiency, and its future application prospects and directions. The article pointed out that the application of perovskite in solar photovoltaics is opportune, with high conversion efficiency, sufficient material supply, low price, high recycling efficiency advantages, but there are also unavoidable shortcomings, such as the highest conversion efficiency of perovskite still contains lead, long-term stability is insufficient, etc., to this end, further research is needed to promote the global popularity and widespread use of silicon perovskite solar cells.
In December 2020, the conversion efficiency of silicon/perovskite solar cells set a new world record, which made the development prospects of solar energy brighter. The cell is made of a layer of silicon and a thin film layer of synthetic perovskite in series, with an area of 1.12 cm2, and has passed independent test certification from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, USA, with a conversion efficiency of 29.52%. In short, tandem batteries convert nearly 30 percent of the simulated sunlight that shines on them into electricity.
The battery and perovskite technology was developed by Oxford PV, based in Oxford, UK. Currently, the company is commissioning a production line at its plant in Brandenburg an der Havel, Germany, to produce the world's first commercial perovskite/silicon cells with a side length of 156 mm and a conversion efficiency of about 26 percent (Figure 1). It is expected that the battery will be put into mass production in early 2022, when it will become the world's most efficient commercial solar cell, while the current industrial production of silicon solar modules is generally 20% to 22%.
Henry Snaith, co-founder and chief scientific officer of Oxford PV and professor of physics at the University of Oxford, said: "We started working on perovskites a decade ago in an attempt to find a material that costs less than machining batteries with silicon. This roughly includes the process of preparing perovskite batteries based on solution or sublimation. The material we are looking for should be crystallized below 2000 °C. We have a long-term goal, believing that one day we will develop a battery conversion efficiency of 10%, and, without exaggeration, the conversion efficiency of the first battery we made with perovskite is 6.1%, breaking all our previous laboratory records. While this may seem trivial today, at the time, its first reaction was: Wow! It's out of the box. ”
Figure 1 A series of silicon/perovskite solar cells manufactured at Oxford PV's plant in Brandenburg-sur-Havel, Germany. The first commercial batteries that the company will launch in 2022 will have a conversion efficiency of about 26%. Source: Oxford PV, licensed
The application of perovskites in solar photovoltaics (PV) is opportunistic, as decades of improvement have encountered major bottlenecks in continuing to improve the conversion efficiency of silicon cells; photovoltaic materials have a limit in converting solar energy into electricity. The level of this limit depends on their "band gap," which is the energy required for electrons to be released from the material, making them charge carriers needed to flow through the circuit. Crystalline silicon has a band gap of 1.1 eV, which means that photons from the sun with less than 1.1 eV energy cannot release electrons, and photons above 1.1 eV can still produce charge carriers, but some of the photon energy above 1.1 eV will be wasted in the form of thermal energy.
If the solar spectrum is considered, the theoretical conversion efficiency limit of ideal silicon is about 32%. However, since bell labs in the United States developed the first practical silicon solar cell in 1954, the maximum conversion efficiency achieved in the laboratory is about 27%.
Synthetic perovskites have the same crystal structure as natural minerals perovskites and perovskite oxides. In 2012, the field of solar photovoltaic research officially began to use synthetic perovskites, and its potential application in solar cells has received widespread attention. The synthetic perovskites we use today are usually organic-inorganic metal halide perovskites, where the metal is lead or tin. Joe Berry, chief scientist at the National Renewable Energy Laboratory (NREL) and head of the perovskite and hybrid solar cell team, said: "The metal halide system is very clever in handling photovoltaic tasks, which makes it very compelling. ”
The band gap of perovskite films covered on silicon cells can reach 1.7 eV to supplement the lower band gap of silicon. This allows more photons to be captured from more sunlight spectrum, more electrons are released, and more energy is generated. The theoretical conversion efficiency using the two materials in combination is 43%. Chris Case, Chief Technology Officer of Oxford PV, said: "Actual conversion efficiency is always unable to achieve theoretical conversion efficiency. Currently, the actual conversion efficiency is about to reach 30%, but we believe that with the existing knowledge collection alone, we can improve the commercial battery conversion efficiency to 33%. ”
When the efficiency of photovoltaics is greatly improved, from a financial and ecological point of view, solar energy is already a very attractive proposition for energy companies. Currently, in many countries around the world, utility-scale solar PV equipment is often cheaper than new coal-fired or gas-fired power generation equipment. In 2018, the Intergovernmental Panel on ClimateChange (IPCC) warned that to limit global warming to 1.5 °C, "rapid and far-reaching" changes in energy generation and other areas must be undertaken, as human-caused carbon dioxide emissions need to reach "net zero" around 2050. The International Renewable Energy Agency (IRENA) is an intergovernmental organization that supports countries in their quest for sustainable energy development, based in Abu Dhabi, United Arab Emirates. IRENA predicts a climate-resilient energy transition path based on the Global Renewable Energy Roadmap (REmapCase) developed by the Intergovernmental Panel on Climate Change. According to this path, by 2050, solar photovoltaics will become the largest source of electricity, with an installed capacity of 8.5 TW worldwide, and wind power will become the second largest source of electricity (Figure 2).
Figure 2 Projections of the transition to cleaner, renewable sources of energy need to reduce emissions fast enough to meet the IPCC's climate change target and enable the planet to achieve "net zero" carbon dioxide emissions by 2050. RE: Renewable Energy; CSP: Concentrated Solar. Source: IRENA, licensed
Solar photovoltaics are already accelerating. For example, in 2020, solar photovoltaic power generation accounted for 43% of the new installed capacity of new power generation in the United States, ranking first in power generation technology for two consecutive years. The U.S. solar industry is expected to have four times the capacity it has in the next 10 years. Part of the reason for this rapid growth is the sharp decline in the cost of solar PV technology over the past 10 years. According to NREL, the cost of installing utility-scale photovoltaic systems in the United States fell by 82% from 2010 to 2020 as the conversion efficiency of solar cells currently available has risen to about 20% and the associated hardware costs are also declining. Solar photovoltaic trends are similar around the world (Figure 3).
Figure 3 Over the past 10 years, the levelized energy cost (LCOE) of newly commissioned utility-scale solar PV projects in various countries has fallen significantly. LCOE is the minimum average price of electricity sold during the life cycle of a particular solar project to achieve financial break-even. Source: IRENA, licensed
Global solar PV generation rose to about 710 GW by the end of 2020 from 581 GW in 2019 (Figure 4). To further expand this power generation scale to the terawatt level, it is necessary to accelerate the production of solar photovoltaics, requiring sufficient materials. This is another advantage of perovskites, as the perovskite films required to produce solar cells are typically only 0.5 μm thick and the required materials are easily procured. Oxford PV noted that 35 kg of perovskite generates the same amount of electricity as 7 t silicon (typically used for 160 μm thick wafers) and said that one day perovskite can completely replace silicon.
The key challenge in scaling up other mature thin-film solar technologies is that they are based on cadmium telluride or copper indium gallium selenide. Given the toxicity of cadmium, and for these technologies, cadmium, tellurium and indium are rare metals that make it impossible to effectively scale solar photovoltaic power generation to the scale of terawatts.
Figure 4 Over the past 10 years, the total installed capacity of solar photovoltaics in the world has increased dramatically and shown signs of exponential growth. Source: IRENA, licensed
In contrast, the materials needed to make metal halide perovskites are well supplied and inexpensive. "They're great for some high-throughput, low-cost machining routes," says Berry. Although there are many ways to reduce production costs, the quality of the product will be reduced at the same time, and the way perovskites are processed will not affect the characteristics of the basic material. Perovskites also have some very unique advantages, and studies have confirmed that their recovery efficiency is very high. ”
However, perovskites are not without their drawbacks. Despite the current use of thin-film technology, the most converted perovskites still contain lead. A more pressing challenge for the widespread use of silicon/perovskite tandem solar cells is their long-term stability. The life of utility-scale photovoltaic panels needs to reach about 25 years. Although perovskite technology has developed rapidly since its first application in photovoltaic cells, its long-term stability has not been recognized.
Unlike silicon, perovskites are ionic materials that degrade more easily (especially when they are damp). Therefore, effective encapsulation of perovskite films is crucial. Oxford PV holds more perovskite solar PV-related patents than any other organization, and they are confident in its design process and perovskite encapsulation method. Snaith said: "In the past 10 years, in order to improve its stability, we have put a lot of effort into changing the composition of perovskites, materials, equipment structure. We didn't put much effort into improving its conversion efficiency; it took the most effort to achieve its stability. But now, we are very confident in the efficiency and stability of this technology. ”
Many industrial and scientific people have also begun to study and solve the stability problem of perovskites. In 2020, an international collaborative organization that included researchers such as Berry and Snow issued a consensus statement on the stability assessment and reporting of perovskite photovoltaics. "Over the past 10 years, we've been digging deeper into these perovskite materials to make predictions for the next 30 years," says Berry. This kind of predictive science is very technically demanding, but so far, our research at the level of basic materials has not achieved remarkable results. So the question becomes, 'What technical solution do you have?' Or'or'How much can you reduce?' ' and other such business issues. ”
Speeding up perovskite adoption in solar PV production will increase costs, and it's uncertain what impact this will have on the market. Oxford PV has not disclosed the approximate price of its commercial batteries. In large-scale power generation, LCOE is a key factor. Any potential increase in the initial price of this new technology will depend on the decline in LCOE due to the increase in conversion efficiency.
Oxford PV's manufacturing facility is currently being commissioned at a capacity of 100 MW per year, with the goal of expanding the plant to 10 GW per year by 2030, while the solar industry will increase its generation capacity by about 120 GW per year. This goal is not difficult to achieve for the solar industry. Other business organizations developing perovskite photovoltaic technology include The Japanese company Panasonic and Sekisui Chemical Company, the Chinese company Microquanta Semiconductor and Wonder Solar, South Korea's Frontier Energy Solution and Poland's Saule Technologies。
In the past 10 years, perovskite technology has developed rapidly. At present, it is uncertain how long it will take to commercialize perovskite technology, but Snow said it will take at least 10 years for the global adoption and widespread use of perovskite solar cells. In addition, he noted that the use of perovskites has other attractive possibilities. "Perovskite can be made into very thin films on very light substrates, so it has good curvature or flexibility. In the future, when the solar photovoltaic efficiency reaches 40%, it will be possible to cover electric vehicles with solar energy (because this will significantly improve the charging effect of electric vehicles). Similarly, if we can develop ultra-thin solar cells, we can consider using this cladding for commercial buildings. ”
In 2020, the US Manufacturing of Advanced Perovskites Consortium was formed with the goal of "regaining U.S. dominance in optoelectronics and photonic manufacturing." The organization is founded by NREL, the Washington Clean Energy Testbeds at the University of Washington in Seattle, the University of North Carolina at Chapel Hill, and the University of Toledo in Ohio of Toledo in Ohio) composition. The consortium includes six domestic commercial industry partners, one of which is First Solar, a utility-scale producer of thin-film solar PV modules based on cadmium telluride technology, based in Tempe, Arizona.
Note: The content of this article is slightly adjusted, if necessary, you can view the original text.
Adapted original text:
Sean O Neill.Perovskite Pushes Solar Cells to Record Efficiency[J]. Engineering,2021,7(8):1037-1040.