Physics

Photovoltaic Cells

Photovoltaic Cells

Photovoltaic cells are a type of semiconductor diode which generates direct current from visible light. This article will briefly explore the origination of photovoltaics, how photovoltaic cells work, disadvantages and advantages, and their application in the real word.

Whilst the hydrogen atoms in the Sun fuse to form helium, gamma photons and neutrinos are also formed. Thousands of year later, these small energy packets reach the surface of the Sun and then discharged into space. The sunlight that we receive is made up of these photons, or quanta of radiant solar energy. Based upon the measurements from NASA’s  satellite missions’, approximately 1,360 watts per square meter of solar energy reaches the  Earth’s atmosphere. Therefore on an average, every square meter of Earth’s surface receives 340 watts of solar energy (1).Despite this light and heat energy being useful, it could not be used directly in our everyday lives. There was a need for an invention that utilised the Sun, an abundant energy resource, to make electricity. This invention was the photovoltaic cell which converts light energy directly into electricity at an atomic level.

In the year 1839, a French physicist named Edmund Becquerel discovered that when exposing some materials to light, they released small quantities of electric current. Based on the quantum idea, Einstein explained the nature of light and how atoms absorb and release light in discrete packets (2). Despite Einstein’s work on the photoelectric effect in 1905, there was no practical way of collecting solar energy and directly converting it into electricity until 1954. Calvin Fuller, Gerald Pearson and Daryl Chaplin, who worked in Bell Laboratories, were merely just trying to fix issues with the Bell telephone system and were asked to investigate substitute power sources for the telephones since the batteries they used were inefficient in warm temperatures (3). While doing that, by chance they discovered that silicon could be used as a semiconductor. As a result, they contrived the first photovoltaic cell which was able to directly convert light photon, which it absorbed from the sun, into electricity (4). Despite their photovoltaic cell only having an efficiency rate of 6%, it was the birth to the photovoltaics industry.

Semiconductors are substances which can only allow electricity to flow through them under some specific conditions. Silicon is  a very useful semiconductor and is used in  photovoltaic cells (5), as well as in other products such as transistors in microchips. Silicon’s semi-conductive properties enable it to be modified by doping. Doping is when impurities are added into the crystalline lattice to adjust its electrical properties, forming an extrinsic semiconductor.

A photovoltaic cell consists of two different layers of silicon. The bottom layer of silicon is a positive-type silicon since impurities called acceptors are added into the crystalline lattice. The deficiency of free electrons means it is incapable of fully bonding with the rest of the crystal. The P-type silicon has the same number of protons and electrons but it has a positive character which attracts electrons. Often this P-type semiconductor layer is created by heating polysilicon to melting temperature and then adding small amounts of boron. The top layer of layer is a negative-type silicon since impurities called donors are added into the crystalline lattice. The excess of free electrons means it forms bonds with the vicinal atoms. It has the same number of protons and electrons, but more electrons are free to move within the layer. Typically this N-type semiconductor layer is created by diffusing small amounts of phosphorus into silicon.

When the N-type silicon and the layer of P-type silicon are in contact, the free electrons from the N-type layer move into the positively charged holes in the P-type layer.(6) As a result they form negative ions with the holes and leave behind positive ions in the N-type layer. The opposite charges generates an internal electric field which prevents electrons from flowing between the two sides. When light photons hit the photovoltaic cell’s surface, electrons are stimulated and are released from their atoms. The electric field results in the flow of current since the free electrons are attracted to the n-type silicon’s positive charge and repelled by the p-type silicon’s negative charge. The electrons are converted into usable power since the photovoltaic cell consists of a metallic grid, which acts as an electrode which collect the electrons from the silicon and transfers them to the external load. The circuit is completed with the back contact layer.

Photovoltaic cells have many advantages and most importantly, they create renewable energy since the sun provides an abundance of energy. They also provide clean energy since there are no greenhouse gas emissions or any contribution to pollution. It is silent and hence does not contribute to noise pollution. Photovoltaic cells are low maintenance, so once they are installed they do not require much attention. They can also be adjusted or enlarged according to energy requirements. Investing in photovoltaic cells is expensive since the manufacturing and installation costs are high. Also they require further equipment such as inverters to convert direct current to alternating current. Furthermore, they often include high insurance costs due to their fragile nature. The production process of photovoltaic cells uses toxic chemicals, like cadmium and arsenic, which are harmful to the environment. Also energy from photovoltaic cells is also too reliant on the weather which is uncontrollable and unreliable. It is a concern that solar farms will take up land that could be used for farming and food production, but small scale photovoltaic systems can utilise empty space on existing buildings’ rooftops. It can be argued that the efficiency levels of photovoltaic cells are relatively low in comparison to the efficiency of other energy sources.

 

 

Despite the disadvantages, photovoltaic cells have increasing applications in today’s world. Most commonly photovoltaic systems are seen on rooftops of buildings. They are also used in solar-powered wristwatches and calculators and smartphones, which does not require charging, road and railroad signs. Developing countries (8), such as India, have almost inexhaustible amounts of sunlight but lack strong electrical infrastructure. They use solar power to power water pumps and phone boxes. It is also benefits developing countries since they can use solar power to refrigerate vaccines and blood. Photovoltaic power is used for most satellites which orbit the earth because they does not require high maintenance so can operate by themselves. The photovoltaics industry is still growing and new technology is being introduced such as concentrator photovoltaics which uses lenses and curved mirrors in order to focus sunlight through reflection and refraction onto solar cells. Also photovoltaic thermal hybrid solar collectors maximise the energy created since it converts both light and thermal energy from the sun into electrical energy so there is less waste. Floatovoltaics are like solar panels but are kept in the sea so there is no need to sacrifice land. Photovoltaic Systems are being used increasingly to power transport such as cars, boats and airplanes.

 

Reference List

1. Lindsey, R. (2009) Climate and earth’s energy budget: Feature articles. Available at: http://earthobservatory.nasa.gov/Features/EnergyBalance/page2.php

2. O’Neill, M. (2015) OCR AS/A level physics a student book 1 Activebook. United Kingdom: Pearson Education.

3. Perlin, J. (2002) From space to earth: The story of solar electricity. Cambridge, MA: Harvard University Press.

4. Solar energy conversion and Photoenergy system: Thermal systems and Desalination plants: V. 1 (2010) Isle Of Man: EOLSS Publishers Co.

5. Sen, B. (2008) Super-powered earth: Energy from the rays of the sun. India: The Energy and Resources Institute, TERI.

6. Society, A.C. (2017) How a solar cell works – American chemical society. Available at: https://www.acs.org/content/acs/en/education/resources/highschool/chemmatters/past-issues/archive-2013-2014/how-a-solar-cell-works.html?cq_ck=1396892718960

7. (2008) How do Photovoltaics Work? Available at: https://science.nasa.gov/science-news/science-at-nasa/2002/solarcells

8. Foroudastan, S.D. and Dees, O. (2006) Solar Power and Sustainability in Developing Countries. Available at: http://www.udc.edu/docs/cere/Solar%20Power%20and%20Sustainability%20in%20Developing%20Countries.pdf

About the Author

Methuna Kailanathan, 17, is studying Physics, Maths, Further Maths and Economics at St Dominic’s Sixth Form College in the UK. She aspires to delve deeper into the world of Physics at university. Attending placements and summer schools/taster courses at leading universities such as Imperial, UCL and Oxford has provided a strong insight into physics beyond the curriculum.  

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