The term ‘photovoltaic’ is derived by combining the Greek word for light, photos, with volt, the name of the unit of electromotive force -the force that causes the motion of electrons (i.e. an electric current). The volt was named after the Italian physicist Count Alessandro Volta, the inventor of the battery. Photovoltaics thus describes the generation of electricity from light. The discovery of the photovoltaic effect is generally credited to the French physicist Edmond Becquerel who in 1839 published a paper (Becquerel, 1839) describing his experiments with a ‘wet cell’ battery, in the course of which he found that the battery voltage increased when its silver plates were exposed to sunlight.The first report of the PV effect in a solid substance appeared in 1877 when two Cambridge scientists, W G. Adams and R. E. Day, described in a paper to the Royal Society the variations they observed in the electrical properties of selenium when exposed to light (Adams and Day, 1877).
In 1883 Charles Edgar Fritts, a New York electrician, constructed a selenium solar cell that was in some respects similar to the silicon solar cells
of today . It consisted of a thin wafer of selenium covered with a grid of very thin gold wires and a protective sheet of glass. But his cell was ‘ very inefficient. The efficiency of a solar cell is defined as the percentage of the solar energy falling on its surface that is converted into electrical energy. Less than 1% of the solar energy falling on these early cells was converted to electricity. Nevertheless, selenium cells eventually came into widespread use in photographic exposure meters. The underlying reasons for the inefficiency of these early devices were
only to become apparent many years later, during the first half of the twentieth century, when physicists such as Planck and Einstein provided new insights into the nature of radiation and the fundamental properties of materials (see Section 3.3 below). It was not until the 1950s that the breakthrough occurred that set in motion the development of modern, high-efficiency solar cells. It took place at the Bell Telephone Laboratories (Bell Labs) in New Jersey, USA, where a number of scientists, including Darryl Chapin, Calvin Fuller and Gerald Pearson were researching the effects of light on semiconductors. These are non-metallic materials, such as germanium and silicon, whose electrical characteristics lie between those of conductors, which offer little resistance to the flow of electric current, and insulators, which block the flow of current almost completely.
Hence the term semiconductor. A few years before, in 1948, two other Bell Labs researchers, Bardeen and Brattain, had produced another revolutionary device using semiconductors – the transistor. Transistors are made from semiconductors (usually silicon) in extremely pure crystalline form, into which tiny quantities of carefully selected impurities, such as boron or phosphorus, have been deliberately diffused. This process, known as doping, dramatically alters the electrical behaviour of the semiconductor in a very useful manner that will be described in detail later. In 1953 the Chapin Fuller-Pearson team, building on earlier Bell Labs research on the PV effect in silicon (Ohl, 1941), produced ‘doped’ silicon slices that were much more efficient than earlier devices in producing electricity from light.
By the following year they had produced a paper on their work and had succeeded in increasing the conversion efficiency of their silicon solar cells to 6%. Bell Labs went on to demonstrate the practical uses of solar cells, for example in powering rural telephone amplifiers, but at that time they were too expensive to be an economic source of power in most applications. In 1958, however, solar cells were used to power a small radio transmitter in the second US space satellite, Vanguard I. Following this first successful demonstration, the use of PV as a power source for spacecraft has become almost universal
Rapid progress in increasing the efficiency and reducing the cost of PV cells has been made over the last few decades.
Their terrestrial uses are now widespread, particularly in providing power for telecommunications, lighting and other electrical appliances in remote locations where a more conventional electricity supply would be too costly. A single conventional PV cell produces only about 1.5 watts, so to obtain more power, groups of cells are normally connected together to form rectangular modules. To obtain even more power, modules are in turn mounted side by side and connected together to form arrays. A growing number of domestic, commercial and industrial buildings now have PV arrays providing a substantial proportion of their energy needs. And a number of large, megawatt-sized PV power stations connected to electricity grids are now in operation in the USA, Germany, Italy, Spain and Switzerland. The efficiency of the best single-junction silicon solar cells has now reached 24% in laboratory test conditions (see Box 3.1 and Box 3.3). The best silicon PV modules now available commercially have an efficiency of over 17%, and it is expected that in about 10 years’ time module efficiencies will have risen to over 20% (Appleyard, 2003). Over the decade to 2002, the total installed capacity of PV systems increased approximately tenfold, module costs dropped to below $4 per peak watt and overall system costs fell to around $7 per peak watt (see Figures 3.5 and 3.6). As we shall see, improvements in the cost-effectiveness of PV are likely to continue.
Reference: Godfrey Boyle Renewable Energy: Power for a Sustainable Future, Second Edition
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