Photovoltaic perovskites increasingly combine superior efficiency with good durability

• Source: James Mitchell Crow, a freelance writer in Melbourne, Australia 
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• In August 2021, when the morning sun shines on the glass facade of Aliplast Aluminum Plant, the 40 square meter shutters will be rotated to appropriate positions to intercept the summer sun. In this place in Lublin, Poland, through a thin layer of solar energy material perovskite photovoltaic , the shutters began to collect the incoming sunlight.

  
  

  This perovskite solar cell is produced by Saule Technologies, which aims to commercialize perovskite solar cells. Aliplast's installation allows Saule to test the performance of perovskite solar cells in a real environment for one year.
  

  
  

  Some companies and academic research groups around the world are conducting field tests for several months to evaluate the durability and performance of photovoltaic (PV) perovskite. This is an important milestone in this field.
  

  
  

  Perovskite is a large class of crystal materials, and its general chemical formula is ABX3. It was not until 2009 that a Japanese research team found that some perovskite formulations had photovoltaic characteristics and could convert incident light into voltage 1.
  

  
  

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• The performance of the first perovskite photovoltaic is not high, and its 3.8% solar power conversion efficiency (PCE) is far lower than the 20% PCE of commercial silicon cells. However, the production of silicon solar cells requires special multi day high-temperature processing steps, which makes it time-consuming and expensive. Organic inorganic lead halide perovskite, on the other hand, can be made by mixing simple salt solutions. Other laboratories have tried to prepare perovskite photovoltaic, and the efficiency has soared.
  

  
  

  Physicist Sam Stranks recalled: "At the beginning, these devices worked very well, and their efficiency improved quickly." He began to study perovskite as a junior researcher at Oxford University in 2012. Now he is developing perovskite batteries in Cavendish Laboratory at Cambridge University, which is also part of the Department of Chemical Engineering and Biotechnology in the UK. More than ten years after the discovery of perovskite PCE, the record of perovskite PCE is now close to 26%, which is not far from the 27% of silicon solar cells with the best performance.
  

  
  

  However, challenges remain for practical applications. "There has been progress in efficiency in this area, but stability has lagged behind," Stranks said.
  

  
  

  Monica Lira Cantu, a photovoltaic researcher at the Catalan Institute of Nanoscience and Nanotechnology in Barcelona, Spain, said that the cheaper and easier to produce characteristics of these perovskite photovoltaic materials make them prone to failure, and they are soft materials with weak chemical bonds, which is positive for manufacturing because you can do it in low temperature solutions.

  
  

  
  

  But weak bonds imply a tendency to degenerate. The ions in the material tend to deviate from their positions in the crystal structure, leading to the accumulation of efficiency erosion defects. The roof silicon solar panel usually has a 25 year performance guarantee; Early perovskite will fail in a few days.
  

  
  

  
  

  Soaring efficiency

  Improving the durability of perovskite is a challenge. Jingjing Xue, a perovskite researcher at Zhejiang University in Hangzhou, China, said that perovskite is easily affected by external factors, such as water, oxygen, heat and light, as well as internal processes, including structural instability and ion migration. What's worse, these pathways will interact and make things more complicated. For example, exposure to oxygen will create vacancies in the lattice, thus accelerating ion migration.
  

  
  

  Lira Cantu said that early newspapers often advocated eye-catching PCE while ignoring stability. Now, the materials reported in the best papers combine high PCE and good stability, and devices are tested under increasingly strict accelerated aging conditions. Anita Ho Baillie, a solar cell researcher at the University of Sydney, Australia, said that the need to prevent perovskite photovoltaic devices from being affected by moisture has been established, but no one has considered the need to prevent gas escape. Ho Bellie's team used a technology called gas chromatography/mass spectrometry (GC/MS) to show that, at high temperatures, the organic part of the most common perovskite photovoltaic will decompose and volatilize in the form of gas2. "If we stop the gas release, it will stop degrading," Ho Baillie said. Because the device attenuation reaction is reversible, preventing the escape of gas can give perovskite time to self repair.
  

  
  

  "We can torture the device through a thermal cycle of 40 ° C to 85 ° C and 85% humidity in 1800 hours of accelerated testing," said Ho Baillie. These devices survived, and the team is now expanding its GC/MS analysis to perovskite decomposition under the influence of heat, humidity and light. Some recent advances come from the improvement of the active perovskite interface layer, which helps to guide charge transfer to the transport layer. "The perovskite active layer has made great progress. I don't mean we have completed it completely, but the active layer has made so much progress that the charge transfer layer needs to catch up." Ted Sargent from the University of Toronto in Canada said that he was combining chemistry, physics and engineering to improve the photovoltaic performance of perovskite.

  
  

  In a study on two-dimensional structure in March 2022, the study made electrons flow out from one side of the perovskite photovoltaic active layer. Sargent reported a device with 23.91% PCE, which worked for more than 1000 hours at 50% relative humidity without losing efficiency.
  

  
  

  In a related study also published in 2022, 4 the team led by Stefaan De Wolf of Thuwal KAUST, Saudi Arabia, realized 24.3% PCE in perovskite photovoltaic devices, and conducted 1000 hours of accelerated aging tests at 85 ° C and 85% relative humidity, maintaining an efficiency of more than 95%. "We have seen promising long-term stability tests at high temperatures and under pressure," Stranks said The focus now is to ensure that the performance of perovskite under indoor accelerated aging test is transformed into long life. Several companies are approaching commercialization, usually by superimposing perovskite on the silicon layer of "series" solar cells, and theoretically the PCE content may exceed 40%. Oxford PV is a photovoltaic start-up located in Oxford, UK and Brandenburg, Germany. Its German plant will be put into operation at the end of this year. Stranks is one of the co founders of Swift Solar, California, which plans to bring its products to the market within two years. Stranks said that the service life of the first devices will be shorter than that of roof solar panels. For example, by taking advantage of the low weight and thin film characteristics of perovskite photovoltaic, the company is targeting solar powered unmanned aerial vehicles and electric vehicles that can automatically charge. "The first roof product will be available in more than five years, but it is still in the process of development," Stranks said.
  

  
  

  In 2020, Xianna Optoelectronics, a start-up enterprise headquartered in Hangzhou, China, built a perovskite photovoltaic manufacturing plant. The company claims that its perovskite components have created multiple efficiency records and are part of a project of the Ministry of Science and Technology of China aimed at making perovskite life reach 10000 hours. "Perovskite is a research hotspot in China," Xue said. The old players in the silicon photovoltaic market led by Chinese enterprises are also very active.

  
  

  Stranks said that with the increase of the lifetime of perovskite photovoltaic, the research funds aimed at putting it into practical application are also increasing. "The United States is investing a lot of money, and the European Union is also investing heavily to establish a perovskite photovoltaic industry base."

  
  

  Sargent said that South Korea is another major investor. Investors who have not given priority to perovskite may soon be persuaded to invest.
  

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• 1. Kojima, A. et al. J. Am. Chem. Soc. 131, 6050–6051 (2009).

  2. Shi, L. et al. Science 368, eaba2412 (2020).
  

  3. Chen, H. et al. Nature Photon 16, 352–358 (2022).
  

  4. Azmi, R. et al. Science 376, 73–77 (2022).