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I came across the following un-bylined news story from our June 1954 issue which I thought solarheads would enjoy. Not only does it recount the invention of the photovoltaic cell at Bell Labs, it provides one of the most elegant explanations I've seen of how the device works, though the predictions about its limited usefulness are charmingly dated. A brief excerpt from this story also appeared in the 50, 100 and 150 Years Ago column of our June 2004 issue.
Power From the Sun
A little wafer of adulterated silicon which converts sunlight directly into electrical energy was unveiled last month by Bell Telephone Laboratories. This solar battery is an outgrowth of transistor research. It works at an efficiency of 6 percent, which is comparable to the performance of an ordinary gasoline engine. Bell scientists believe that the figure can be raised to 10 per cent through straightforward engineering improvements. An assembly of wafers covering a square yard could turn out 50 watts of power. The device is not likely to replace large-scale power plants—a 30,000-kilowatt battery would have to cover some 100 acres—but the company expects it to be useful as a small power source for such applications as rural telephone systems.
The battery operates on the same principle that underlies the junction transistor, described by Morgan Sparks in Scientific American for July 1952. A "p-n junction" is set up in a silicon crystal. This means that the crystal is divided into two zones, one containing an impurity which produces an excess of movable electrons, the other an impurity which entraps electrons and produces movable "holes" (spaces where electrons should be). Across the junction between two such zones there is always a small voltage. A quantum of light falling on the junction may knock loose an electron from one of the crystal atoms, creating an electron-and-hole pair. Because of the existing voltage difference, the electron is pushed one way and the hole the other. If the zones are connected by an external circuit, a current will flow.
What has made possible the extraction of usable quantities of power from the arrangement is the discovery of a way to make large area junctions. In the solar battery a strip, about the size of a razor blade, of n-type silicon (containing arsenic) is treated with a gas containing a p-type impurity (boron). Bell engineers have found a way to control precisely the amount of boron taken up and the depth to which it penetrates. They produce a p-type layer less than one 10,000th of an inch thick, with a sharp boundary, over the entire strip. Thus, the whole surface becomes a source of current when sunlight falls on it.
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It makes me nostalgic for the glory days of Bell Labs.
Solar cell inventors Gerald L. Pearson, Daryl M. Chapin, and Calvin S. Fuller in 1954. Courtesy of Alcatel-Lucent Bell Labs.
