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| Fig. 1: Best Research-Cell efficiencies for current photovoltaic technologies. [4] (Source: Wikimedia Commons. Courtesy of the DOE) |
As the XXI century is bringing significant changes to the way humanity thinks about the energy and climate there is a significant interest in developing new, green methods of generating electricity. [1] In 2019 the total production of renewable energy in the world achieved 2800 terawatt-hours, about 6-fold increase over last 10 years, as a result of several established technologies like wind turbines, hydroelectric plants, or photovoltaic cells. [2] As our demand for the renewable energy grows, the research on new, better methods of harvesting energy continues. One prominent example of new approaches to harvest sunlight and transform it into electricity is using perovskite solar cells. The method first proposed in 2009 by Kojima et al. [3] has experienced a tremendous development over the last few years. [4] The popularity of the idea grows from year to year, with citations reaching almost 2000 in the year 2018 alone. In this report we will discuss the reasons why perovskite solar cells are so appealing.
Perovskite solar cells have been first introduced in 2009 as a cost-effective way to make photovoltaic devices and started with efficiencies about 3.8%. [3] Over the last decade it was possible to increase the performance to about 25% for purely perovskite technology and 29.1% for silicone-perovskite tandem devices. [5-7] This rapid growth is outstanding compared to the other solar technologies, as can be seen in Fig. 1. [5] The development of perovskite technology in the past 10 years can be compared to about 50 years of monocrystalline silicone devices. The main advantages of the perovskite technology come from the method of production, their flexible chemical composition and atom arrangement in so-called crystal lattice. [4]
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| Fig. 2: Perovskite crystal structure, dark cyan ball represents cation A; white balls is lead, B; red balls represent halogen anions X. (Source: Wikimedia Commons) |
While perovskites are a broad category, when talking about the photovoltaic application we typically refer to the lead-halide perovskites. They share a common chemical formula - ABX3, where A is a cation, commonly CH3NH3, B stands for a divalent metal cation, typically lead, and X is a halogen anion. The crystal structure of lead- halide perovskite is shown in figure 2, where dark cyan ball represents cation - A; white balls are lead atoms - B; red balls represent halogen anions - X. The structure of lead-halide perovskites allows them to achieve semiconducting properties, necessary for light harvesting applications. In typical devices the perovskite film is sandwiched between p-type and n-type materials to form so-called p-i-n junction. In this geometry photons excite electrons and holes in perovskite and the built-in electric field is driving them towards the p-type and n-type electrodes, respectively. [7] While these characteristics allow perovskites to serve as a medium for photocurrent generation, they don't explain why such high photon conversion efficiencies and open-circuit voltages (VOC) are achievable. While some recent results suggest that coupling of photons and crystal lattice vibrations (phonons) can be responsible for these outstanding properties, they are a subject of ongoing debate and more theoretical work and material characterization is necessary for proper understanding. [8]
Perovskites, in opposition to the traditional crystalline semiconductors like silicon have ionic crystal structure which makes it easy to modify the composition of the crystal. [7] This property allows designing very specific materials which could target absorption of light in specific wavelength bands, for instance changing the content of I and Br in methylammonium lead halide (CH3NH3Pb(BrxI1-x)3-yCly, 0 ≤ x ≤ 1) and formamidinium lead halide (HC(NH2)2PbBr1-yIy, 0 ≤ y ≤ 1) can result in shifting of the absorption edge between about 550 nm and 800 nm. [9,10]
Fabrication of perovskite-based solar cells appears as a much easier process than traditional silicon crystal manufacturing. The latter one requires very specialized process in which material is melted at 1400°C and pulling it with the Czochralski method. When the crystallization process is finished another steps like cutting and polishing have to happen before the material is ready to work with, which makes the entire process expensive. At the same time perovskite technology allows fabrication of devices by drying a solution, spin coating, ink-jet printing, or precision spraying. [7,11] These methods allow reducing the cost of production and facilitate flexible photovoltaic cells at the cost of worse material quality, i.e. the crystal structure has more defects and the material is typically polycrystalline, as opposed to the Czochralski growth where pure, single crystals can be made. Despite this perovskite solar cells keep attracting the attention of the industry with plans for mass production as soon as 2021, for instance, to use on facades of buildings constructed by Skanska Group. [12,13]
In summary, we introduced the lead-halide perovskite solar cell technology and explained the underlying physics which not only allows formation of ion-based semiconductors but also facilitates easy bandgap tuning. The perovskite solar cells have a great potential in achieving high photon conversion efficiencies and maintaining the production cost low which allowed them to transition from pure R&D to startup stage very rapidly. At the same time university-level research is still ongoing and pushing the boundaries of this new technology at unprecedent scale.
© Hubert Stokowski. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The European Green Deal," European Commission, COM(2019) 640 Final, December 2020.
[2] "BP Statistical Review of World Energy 2020," British Petroleum, June 2020.
[3] A. Kojima et al., "Organometal Halide Perovskites as Visible- Light Sensitizers for Photovoltaic Cells," J. Am. Chem. Soc. 131, 6050 (2009).
[4] A. K. Jena, A. Kulkarni, and T. Miyasaka, "Halide Perovskite Photovoltaics: Background, Status, and Future Prospects," Chem. Rev. 119, 3036 (2019).
[5] M. A. Green et al., "Solar cell efficiency tables (version 56)," Prog. Photovolt. 28, 629 (2020).
[6] M. Jeong et al., "Stable Perovskite Solar Cells With Efficiency Exceeding 24.8% and 0.3-V Voltage Loss," Science 369, 1615 (2020).
[7] E. Köhnen et al., "Highly Efficient Monolithic Perovskite Silicon Tandem Solar Cells: Analyzing the Influence of Current Mismatch on Device Performance," Sustain. Energy Fuels 3, 1995 (2019).
[8] B. Guzelturk et al., "Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling," Adv. Mater. 30, 1704737 (2018).
[9] B. Suarez et al., "Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells," J. Phys. Chem. Lett. 5, 1628 (2014).
[10] G. E. Eperon et al., "Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells," Energy Environ. Sci. 7, 982 (2014).
[11] K. Amratisha et al., "Layer-By-Layer Spray Coating of a Stacked Perovskite Absorber For Perovskite Solar Cells With Better Performance and Stability Under a Humid Environment," Opt. Mater. Express 10, 1497 (2020).
[12] "Perovskites Take Steps to Industrialization," Nat. Energy 5, 1 (2020).
[13] A. Lydon, "Forget Silicon. This Material Could Be a Game-Changer For Solar Power," CNN Business, 14 Oct 20.
