Nanotube Film Enhancing the Longevity of Solar Cells

By Benedette Cuffari, M.Sc.Mar 22 2017

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Solar cells are photovoltaic cells that convert the sun’s energy into electricity through what is termed the “photoelectric effect.” This effect describes the ability of matter to emit electrons when light energy in the form of photons is supplied to them.

One of the most important elements in solar cells is the semiconductor, such as silicon, which acts 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. As a result of the imbalance present within the solar cells, an electric current is generated as these electrons are flowing in the same direction.¹

Silicon atoms are organized together in tightly packed structures. Through a process called doping, small quantities of other materials are introduced as impurities into the silicon structure, which will result in the formation of two types of silicon semiconductors. 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 activity of the electrons, therefore, 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, which can involve spin coating, dip coating, thermal evaporation, vacuum-induced crystallization³. 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).

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 occurs.

These high temperatures make the PSCs unsuitable for making flexible modules and monolithic tandem devices. Another disadvantage to metalorganic perovskites in solar cells is their inability to maintain a long life as compared to their silicon-based counterparts.

In order to address this longevity issue, researchers from several Swiss universities have developed novel perovskite solar cell technology comprised of random network nanotube films. Carbon nanotubes (CNT) are cylindrical materials comprised of carbon mesh, with diameters in the nanometer (nm) scale.

These CNTs have intriguing electronic, magnetic and mechanical properties while coming in a variety of structures of various lengths, thicknesses, and a number of layers. CNTs are usually classified into either single-walled (SWCNTs), which resembles a straw with a single layer of nanotube making the wall, or multi-walled nanotubes (MWCNT), which are comprised of layers of cylindrical nanotubes enclosed in one another of increasing diameter. The number of tubes making up the MWCNT could vary from two to as many as 100. The high strength, great flexibility, low weight, high conductivity of heat energy, and semiconducting properties of CNTs make it a great area of interest in the fields of nanotechnology, electronics, optics and material sciences.⁴

While a typical perovskite solar cell will exhibit a conductor layer that contains an organic material that is covered by a thin layer of gold responsible for diffusing light through the entire structure, the Swiss research team replaced this gold layer, as well as part of the organic material layer with their developed random network nanotube films. Nanotube films can either be black or transparent in nature, however, this study utilized thick black nanofilms in order to utilize the highest possible conductivity at the back of the solar cells, which is a location where light typically does not enter⁵.

Despite the use of black nanofilms in this study, researchers claim that thinner and more transparent nanofilms can also be useful, as these transparent structures could allow light to pass through. Upon comparing the traditional gold-coated perovskite solar cells with the newly developed SWCNT-coated devices in a 140h experiment, a dramatic and irreversible energy loss is observed in the solar cell with a gold electrode, whereas the SWCNT-coated devices demonstrated a small linear efficiency loss, while also exhibiting an extrapolated lifetime of 580 hours⁵.

While silicon solar cells are capable of being used for a total duration of 20-30 years, the energy required to manufacture such solar cells, which must also be composed of extremely pure quantities of silicon, requires researchers to look further towards alternatives to this method. As compared to the estimated 2-3 years required for the production of silicon cells, perovskite solar cells typically only require 2-3 months in order to produce the energy required for its manufacturing⁶.

Perovskite solar cells also exhibit an enhanced efficiency in converting the sun’s light energy into electrical energy as compared to its silicon counterparts. By strengthening the efficiency and lifetime associated with perovskite solar cell use, future applications of this tool into commercial buildings and even homes are limitless.

References and Further Reading

1. "How Do Solar Cells Work?" Physics.org. Web. https://www.iop.org/explore-physics
2. "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
3. Anaraki, Elham Halvani, Ahmad Kermanpur, Ludmilla Steier, Konrad Domanski, Taisuke Matsui, Wolfgang Tress, Michael Saliba, Antonio Abate, Michael Grätzel, 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.
4. "What Are Carbon Nanotubes?" Nanoscience. Web. https://www.nanoscience.com/
5. Kerttu Aitola et al, High Temperature-Stable Perovskite Solar Cell Based on Low-Cost Carbon Nanotube Hole Contact, Advanced Materials (2017).
6. "Nanotube Film May Resolve Longevity Problem of Challenger Solar Cells." Phys.org. N.p., 17 Mar. 2017. Web. 21 Mar. 2017. https://phys.org/news/2017-03-nanotube-longevity-problem-solar-cells.html

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