Solar photovoltaic electricity

Solar photovoltaic electricityLin Wang, Yangyang Peng, Liang PanAbstract-This article simply introduces solar electricity. Solar electricity is one of the best options for sustainable future energy requirements of the world. In this paper , we review the technical progress made in the past several years in the area of photovoltaic technologies. It also simple describes the physical basis of photovoltaics.Keywords：Photovoltaics（PV）; Solar electricity; Solar cells; pn junction Ⅰ.INTRODUCTIONSolar electricity, also known as photovoltaics (PV), has shown since the 1970s that the human race can get a substantial portion of its electrical power without burning fossil fuels (coal, oil or natural gas) or creating nuclear fission reactions. Photovoltaics helps us avoid most of the threats associated with our present techniques of electricity production and also has many other benefits[1]. Photovoltaics has shown that it can generate electricity for the human race for a wide range of applications, scales, climates, and geographic locations. It allows homes and business a new level of guaranteed energy availability and security, and photovoltaics has been powering satellites orbiting the Earth or flying to Mars for over 30 years[2].Ⅱ.What is Photovoltaics?Photovoltaics is the technology that generates direct current (DC) electrical power measured in Watts (W) or kiloWatts (kW) from semiconductors when they are illuminated by photons[3]. As long as light is shining on the solar cell, it generates electrical power. When the light stops, the electricity stops. Solar cells never need recharging like a battery. Table.1 lists some of the advantages and disadvantages of photovoltaics.Tab.1 Advantages and disadvantages of photovoltaicsⅢ. HISTORY OF PHOTOVOLTAICSThe history of photovoltaics goes back to the nineteenth century. The first functional, intentionally made PV device was by Fritts[4]in 1883.He melted Se into a thin sheet on a metal substrate and pressed a Au-leaf film as the top contact. It was nearly 30cm2 in area. In the 1954s, researchers at Bell Labs in the USA academically discovered that pn junction diodes generated a voltage when the room lights were on. Within a year, they had produced a 6%efficient Si pn junction solar cell[5].In the 1980s, the industry began to mature, as emphasis on manufacturing and costs grew. Manufacturing facilities for producting PV modules from Si wafer pn junction solar cells were built in the USA , Japan and Europe.Ⅳ.THE PHYSICAL BASIS OF PV OPERATIONSSolar cells are made of materials called semiconductors, which have weakly boned electrons occupying a band of energy called the valence band. When energy exceeding a certain threshold, called the band gap energy, is applied to a valence electron, the bonds are broken and electron is somewhat “free” to move around in a new energy band called the conduction band where it can “conduct” electricity through the material. Thus the free electrons in the conduction band are separated from the valence band by the band gap (measured in units of electron volts or eV). This energy needed to free the electron can be supplied by photons, which are particlesof light[6]. Figure 2 shows the idealized relation between energy (vertical axis) and the spatial boundaries (horizontal axis). When the solar cell is exposed to sunlight, photons hit valence electrons, breaking the bonds and pumping them to the conduction band. There, a specially made selective contact that collects conduction-band electrons drivers such electrons to the external circuits. The electrons lose their energy by doing work in the external circuits such as pumping water, spinning a fan, powering a sewing machine motor, a light bulb, or a computer. They are restored to the solar cell by the return loop of the circuit via a second selective with. The movement of these electrons in the external circuit and contacts is called the electric current. The potential at which the electrons are delivered to the external world is slightly less than the threshold energy that excited the electrons; that is, the band gap. Thus, in a material with a 1 eV band gap, electrons excited by a 2 eV photon or by a 3 eV photon will both still have a potential of slightly less than 1 V. The electric power produced is the product of the current times the voltages; that is, power is the number of free electrons times their potential[7].Fig. 2Schematic of solar cell. Electrons are pumped by photos from the valence band to the conduction band. There they are extracted by a contact selective to the conduction band (ann-doped semiconductor) at a higher(free)energy and delivered to the outside world via wires, where they do some useful work, then are returned to the valence band at a lower (free) energy by a contact selective to the valence band (a p-type semiconductor)Sunlight is a spectrum of photons distributed over a range of energy.Photos whose energy is greater than the band gap energy can excite electrons from the valence to conduction band where they can exit the device and generate electrical power[7].Photos with energy less than the energy gapfail to excite free electrons.Instead, that energy travelsthrough the solar celland is absorbed at the rear as heat[8].Solar cells in direct sunlight can be somewhat(20-30℃) warmer than the ambient air temperature. Thus,PV cells can produce electricity without operating at high temperature and without mobile parts.At the heart of any solar cell is the pn junction.This pn junction results from the “doping”that produce conduction-band or valence-band selective contacts with one becoming the n-side(lots of negative charge), the other the p-side(lots of positive charge) [9].Ⅴ.THEDEVELOPMENT OF SOLAR CELLThe solar resource is huge although its energy density is rather low[10]. However, it is not so low as to lose any hope of massive utilization but it is not high enough to make it easy. Obviously, the proper strategy for recovering a dispersed resource is to do it with high efficiency at a low cost per area[11]. Table 2 give specific examples of where the research and manufacturing effort is concentrating to reduce costs and improve performance.Tab.2 Goals of current solar cell research and manufacturingⅥ.ConclusionsPhotovoltaics constitutes a new form of producing electric energy that is environmentally clean and very modular.PV electricity is highly appreciatedby the public.It is unique for many applications of high social value such as providing electricity to people who lack it in remote areas.Photovoltaics is poised to become a large global high-tech industry, manufacturing and selling modules in nearly every country.REFERENCES[1]Ofgem, Feed-in Tariff Update. Issue 7, Office for Gas and Electricity Markets, Editor, London, 2012.[2]Chaisitsak, S., Yamada, A., Konagai, M., 2001. In: The Proceedings of the Materials Research SocietySpring Meeting, San Francisco, p. 668.[3]Basore, P.A., 2004. 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