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Solar cells describe a kind of photovoltaic cells that convert the sun’s energy into electricity using the photoelectric effect, which is the ability of matter to emit electrons when the energy in the form of light is supplied to them.
The key ingredients in solar cells are semiconductors, such as silicon, which act as a conductor in some situations and as an insulator in others. When energy from the sunlight is supplied as photons, the electrons in the silicon atoms are knocked off. When these electrons are flowing in the same direction due to the imbalance present in the solar cells, an electric current is generated1.
Silicon atoms are organized together in tightly packed structures. When small quantities of other materials are introduced as impurities into the silicon structure, a process known as doping, two types of silicon semiconductors are formed. These silicon semiconductors can either be negatively charged n-type, which have spare electrons or positively charge p-type, which are devoid of electrons, leaving holes in their place.
The side by side arrangement of these n-type and p-type semiconductors in the solar cells allow for the electrons to jump from the n-type to p-type in order to fill the holes. This electron activity allows the n-type semiconductors to attain a positive charge and p-type to attain a negative charge, allowing the electrons to flow in one direction1. The process of creating traditional silicon cells requires multiple steps, often involving temperatures higher than 1000 °C and in a vacuum, making this process expensive2.
Perovskite solar cells (PSCs), unlike traditional silicon-based solar cells, require much simpler processing techniques, such as spin coating, dip coating, thermal evaporation, vacuum-induced crystallization5. Spin coating, for example, involves the dissolving of perovskite material in a solvent, which is then coated and dried as a very thin layer onto a substrate.
These solar cells are made using perovskite structured compounds, which have a crystal structure of ABX3, where X is an anion which is a halogen atom such as iodine, bromine or chlorine, which binds two different sized organic cations, A, such as methylammonium (MA) and formamidinium (FA), and divalent metal ion, B such as lead (Pb) or tin (Sn).
The most commonly studied perovskite materials in solar cells are methyl ammonium lead trihalide (CH3NH3PbX3), formamidium lead trihallide (H2NCHNH2PbX3) and tin-based perovskite absorbers such as CH3NH3SnI33. Recently, transitional metal oxide perovskites, such as LaVO3/ SrTiO34 and atomic layer deposited SnO2 (ALD SnO2),5 were used to make solar PSCs.
While recent advances in the development of PSCs have found an impressive power conversion efficiency (PCE) of 22.1%, these processes still require high temperatures of 400 – 500 °C for the processing of mesoporous sintered titanium dioxide (TiO2) to produce the electron sensitive layer (ESL), where the extraction of electrons excited by solar energy occurs6. These high temperatures make the PSCs unsuitable for making flexible modules and monolithic tandem devices.
Researchers at the University of Toronto’s Department of Electrical and Computer Engineering have recently developed a planar PSC made at low temperatures of less than 150 °C using solution process6. Here, Dr. Hairen Tan’s research group developed PSCs using TiO2 coated nanoparticles that melt at 150 °C, which facilitates the solar ink to be printed on surfaces like glass and plastic using simple inkjet processes.
Furthermore, this research team addressed the problems of low performance and operational stability of low-temperature planar PSCs by passivating the interface between charge selective contact and perovskite absorbers. Through simple and effective interface passivation methods using chlorine capped colloidal TiO2 (TiO2-Cl) nanocrystal as the ESL,6 chlorine atoms at the interface suppressed the deep trap states leading to strong electronic coupling and chemical binding at the junction.
This resulted in low-temperature PSCs to have an initial PCE of about 20 %, which is outstanding operational stability, and 90 % retention of their initial performance after 500 hours of operation at their maximum power point (MPP) under constant 1-sun (AM1.5, or 1kW/m2) illumination6.
Tan and his group of researchers are hopeful that this new technology of low cost printable solar cells has the potential to convert virtually any surface into a power generator. By using techniques that are already established in the printing industry, this development and can lead to the introduction of a completely new class of solar panels7.
References and Further Reading
- "How Do Solar Cells Work?" Physics.org. Web. http://www.physics.org
- "Is Perovskite the Future of Solar Cells?" Engineering.com. Web. 20 Feb. 2017. http://www.engineering.com/Blogs/tabid/3207/ArticleID/6773/Is-Perovskite-the-Future-of-Solar-Cells.aspx.
- Eperon, Giles E., Samuel D. Stranks, Christopher Menelaou, Michael B. Johnston, Laura M. Herz, and Henry J. Snaith. "Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells." Energy & Environmental Science 7.3 (2014): 982. Web.
- Elias Assmann; Peter Blaha; Robert Laskowski; Karsten Held; Satoshi Okamoto & Giorgio Sangiovanni (2013). "Oxide Heterostructures for Efficient Solar Cells". Phys. Rev. Lett. 110: 078701.
- Anaraki, Elham Halvani, Ahmad Kermanpur, Ludmilla Steier, Konrad Domanski, Taisuke Matsui, Wolfgang Tress, Michael Saliba, Antonio Abate, Michael GrÃ¤tzel, Anders Hagfeldt, and Juan-Pablo Correa-Baena. "Highly Efficient and Stable Planar Perovskite Solar Cells by Solution-processed Tin Oxide." Energy Environ. Sci. 9.10 (2016): 3128-134. Web.
- Hairen Tan, Ankit Jain, Oleksandr Voznyy, Xinzheng Lan, F. Pelayo García de Arquer, James Z. Fan, Rafael Quintero-Bermudez, Mingjian Yuan, Bo Zhang, Yicheng Zhao, Fengjia Fan, Peicheng Li, Li Na Quan, Yongbiao Zhao, Zheng-Hong Lu, Zhenyu Yang, Sjoerd Hoogland, Edward H. Sargent. “Efficient and stable solution-processed planar perovskite solar cells via contact passivation.” Science, 2017. Web.
- "Printable Solar Cells Just Got a Little Closer." U of T Engineering News. 17 Feb. 2017. Web. http://news.engineering.utoronto.ca/printable-solar-cells-just-got-little-closer/?_ga=1.125028042.174570435.1487343043.