An Overview of Progress

French physicist Edmond Becquerel first described the photovoltaic effect in 1839, but it remained a curiosity of science for the next three-quarters of a century. Becquerel found that certain materials would produce small amounts of electric current when exposed to light. The effect was first studied in solids, such as selenium, by Heinrich Hertz in the 1870s. Soon afterward, selenium photovoltaic (PV) cells were converting light to electricity at 1 % to 2% efficiency (The conversion efficiency of a PV cell is the proportion of sunlight energy that a cell converts to electrical energy.) Selenium was quickly adopted in the emerging field of photography for use in light-measuring devices.

Major steps toward commercializing PV were taken in the 1940s and early 1950s when the Czochralski process for producing highly pure crystalline silicon was developed. In 1954, scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic (or solar) cell, which had an efficiency of 4%.

Although a few attempts were made in the 1950s to use silicon cells in commercial products, it was the new space program that gave the technology its first major application. In 1958, the U.S. Vanguard space satellite carried a small array of PV cells to power its radio. The cells worked so well that PV technology has

been part of the space program ever since. Today, solar cells power virtually all satellites, including those used for communications, defense, and scientific research. The US space shuttle fleet uses PV arrays to generate much of its electrical power.

The computer industry, especially transistor semiconductor technology, also contributed to the development of PV cells. Transistors and PV cells are made from similar materials and operate on the basis of similar physical mechanisms. Advances in transistor research have provided a steady flow of new information about PV cell technology. Today, however, this technology transfer process often works in reverse, as advances in PV research and development are sometimes adopted by the semiconductor industry.

Despite these advances, photovoltaics in 1970 was still too expensive for most terrestrial uses. In the mid-1970s rising energy costs, sparked by a world oil crisis, renewed interest in making PV technology more affordable. Since then, the federal government, industry, and research organizations have invested hundreds of millions of dollars in research, development,and production. Often, industry and the federal government work together, sharing the cost of PV research and development (R&D).

Much of this effort has gone into the development of crystalline silicon, the material Bell's scientists used to make the first practical cells.

As a result, crystalline silicon devices have become more and more efficient, reliable, and durable. Industry and government have also explored a number of other promising materials, such as noncrystalline (amorphous) silicon, polycrystalline cadmium telluride and copper indium diselenide, and other singlecrystal materials like gallium arsenide.

Today' commercial PV systems can convert from 5% to 15% of sunlight into electricity. They are highly reliable, and they last 20 years or longer. The cost of PV-generated electricity has dropped 15- to 20-fold, and PV modules now cost around $6 per watt (W) and produce electricity for as little as 25 cents to 30 cents per kilowatt-hour (kWh).

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