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Discovery Of the Photovoltaic (PV) Effect

Will Smith

The development of solar technology began in 1839 with the research of French physicist Alexander Edmond Becquerel (1820-1891). Becquerel discovered the "photovoltaic (PV) effect" while experimenting with a solid electrode in an electrolyte solution. At age 19, in his father's laboratory, silver chloride was placed in an acidic solution and illuminated while connected to platinum electrodes. During the experiment Becquerel recognized that a voltage had developed when light struck the electrode. The photovoltaic effect is sometimes called the Becquerel Effect.

Willoughby Smith (1828-1891), pictured at the left, was an English electrical engineer who discovered photo conductivity of the element selenium. This discovery eventually led to the invention of photoelectric cells. In 1848, he began working for the Gutta Percha Company in London where he developed iron and copper wires insulated for use as under water telegraph wires. In 1849, Smith supervised the manufacture and laying of 30 miles of underwater telegraph wire from Dover, England to Calais, France.

In 1873. Smith developed a method for continually testing an underwater cable as it was being laid. For his test circuit, he needed a semi-conducting material with a high resistance and selected selenium rods. Selenium seemed to do the job in the lab, but in actual practice the device was inconsistent. He discovered though a lab experiment that the conductivity of the selenium rods increased significantly when exposed to sunlight. Smith described the "Effect of Light On Selenium During The Passage Of An Electric Current" in an article published in the February 1873 issue of Nature.

William G Adams

Selenium produces an electrical current when it is exposed to sun light. Professor William Grylls Adams of Kings College in England (pictured at the left), and his student Richard Evans Day, proved that it was possible to convert solar energy directly into electricity without any moving parts or heat. During the late 1870s they subjected selenium to several experiments, and in one of these trials they lit a candle an inch away from the same bars of selenium Willoughby Smith had used.

The needle on their measuring device reacted immediately. Screening the selenium from light caused the needle to drop to zero instantaneously. These rapid responses ruled out the possibility that the heat of the candle flame had produced the current (known as thermal electricity). When heat was applied or withdrawn in thermo-electric experiments, the monitoring needle always rose or fell very "slowly". This was obviously not the case when some form of light was applied.

“Hence,” the experimenters concluded, “it was clear that a current could be started in the selenium by the action of the light alone.” They felt confident that they had discovered something completely new - that light caused “a flow of electricity” through a solid material. Adams and Day called the current produced by light “photoelectric.” However, modern scientists refer to this phenomenon as the "photovoltaic effect".  Top

The First Real Solar Module

Charles Fritts

The first "real" working solar module was built in 1883 by Charles Edgar Fritts, an American inventor, who constructed modules by coating a wide plate of copper with selenium and then topped it with an extremely thin semi-transparent layer of gold leaf. Fritts reported that the module produced a current "that is continuous, constant, and of considerable force". The current was responsive to not only sunlight, but to dim daylight and even candle light. Fritts suggested that "we may see the photoelectric plate competing with (coal fired) fossil fuel plants".

In materials like selenium, the some photons carry enough energy to knock poorly held electrons from their atomic orbits. When wires are attached to the selenium bars, the liberated electrons flow through them in the form of electricity. The resulting modules had an electrical conversion efficiency of only 1% due to the properties of selenium. In addition to the cost of selenium, the high cost of gold made these early solar modules not commercially feasible.

However, Fritts sent one of his solar panels to Werner von Siemens, whose technical reputation ranked up there with with Thomas Edison’s. The panels’ output of electricity when placed under light so impressed Siemens that the German scientist presented Fritts’s panel to the Royal Academy of Prussia. Siemens declared to the scientific world that the American’s modules “presented to us for the first time, the direct conversion of the energy of light into electrical energy.”  Top

Einstein Explains The Photovoltaic Effect

Charles Fritts

Albert Einstein (a 1905 photo at the left) believed that the science of the late 1800's failed to predict all the energy streaming from the sun. In a famous paper published in 1905, Einstein postulated that light had an attribute that had not yet been recognized. Einstein said light contains packets of energy which he called light quanta (now called photons). He suggested that the amount of power that light quanta carry varies according to the wavelength of light - the shorter the wavelength, the more power. The shortest wavelength, for example, contains photons that are about four times as powerful as those of the longest.

Einstein’s novel description of light combined with the discovery of the electron and the rash of research into its behavior, all happening at the end of the nineteenth century and early twentieth, provided photoelectricity with a scientific framework it had previously lacked. Theory could now explain the phenomenon in terms understandable to science. In semiconductor materials, photons can knock electrons from their atomic orbits and, if connected properly by a circuit, could generate enough electricity to do "work". In 1913 William Coblentz received the first US Patent (1077219) to convert sunlight into electricity.

However, the early pioneers in photoelectricity failed to achieve the goals they had set as the conversion efficiencies remained one percent or less for the first 50 years of the twentieth century. Maria Telkes (1900-1995), a Hungarian-American scientist at MIT who worked on several solar energy projects said, “Personally, I believe that photovoltaic cells will be the most efficient converters of solar energy if a great deal of research and development work succeeds in improving their characteristics.”  Top

Bell Labs - First Practical Solar Cell

Solar Dudes

In April, 1954 a slightly modified wafer of silicon, called a "solar cell", that converted sunlight directly into electrical energy was unveiled by Bell Telephone Laboratories in Murray Hill, NJ. The solar cell was an outgrowth of transistor research. It worked at an efficiency of just 6 percent, which was comparable to the performance of a typical gasoline engine at the time. The inventors were: Gerald L. Pearson, Daryl M. Chapin, and Calvin S. Fuller (shown at the left). The New York Times subsequently forecasted that solar cells would eventually lead to a source of "limitless energy of the sun."

The solar cell operates on the same principle that underlies the transistor. A junction is set up in silicon crystal. A junction means that the crystal is divided into two zones, one containing a slight impurity which produces an excess of movable electrons. The other zone also has a slight impurity which absorbs electrons and produces movable "holes" (spaces where electrons should be).

Across the junction between the two zones there is always a small voltage. A quantum of light (photon) falling on the junction will knock an electron from one of the crystal atoms, creating a loose electron and hole pair. Because of the constant voltage difference, the electron is pushed one way and the hole the other. If the zones are connected by an external circuit, a current of electricity will flow through the circuit.

The Bell scientists believed that the efficiency figure could be raised to 10 per cent through normal engineering techniques. An assembly of wafers covering a square yard could turn out 50 watts of power. Bell Labs expected it to be useful as a small power source for such applications as rural telephone systems.  Top

Solar Power Develops

Early Solar Panels

In the year 1956, the first solar modules were available commercially. The cost, however, was far from the reach of everyday people. At $300 for a one watt solar module, the expense was far beyond anyone's means. However, tiny solar cells began to be used in toys and radios. These novelties were the first items using solar cells that were available to consumers.

In the late 1950's and early 1960's satellites in the US and Soviet space programs were powered by solar panels where cost was not critical. Other considerations such as size, efficiency and durability were the major design concerns. By the late 1960's solar power was standard for powering almost all space satellites and efficiencies were up to about 14 percent.

In the early 1970s, Dr. Elliot Berman, Chairman and Founder of the Solar Power Corporation, designed a much cheaper solar module by using a less pure grade of silicon (rejects from semiconductors) and manufacturing techniques similar to photographic film production. This brought the price down from $100 per watt to around $20 per watt. His research was funded by Exxon Corporation whose off-shore oil rigs needed solar panels to power lights on the top of the rigs to warn oncoming ships. This development brought solar power down from outer space satellites to earth bound applications (see the Bell Labs employee to the left).

The period from the 1970's to the 1990's saw quite a change in the usage of solar cells. They began showing up in remote places to power homes. Australia used solar cells in their microwave towers to expand their telecommunication capabilities. Desert regions even saw solar power bring water where grid power was not an option.

Today we see solar cells in a wide variety of places. Many utilities are utilizing solar power during daylight hours. You may see solar powered cars. There is even a solar powered aircraft that has flown higher than almost any other aircraft. With the cost of solar cells well within everyone's budget, solar power has never looked so tempting.

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