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The Silicon Solar Cell Turns 50

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The Silicon Solar Cell Turns 50 TELLING THE WORLD On April 25, 1954, proud Bell executives held a press conference where they impressed the media with the Bell Solar Battery powering a radio transmitter that was broadcasting voice and music. One journalist thought it important for the public to know that “linked together Daryl Chapin, Calvin Fuller, and Gerald Chapin began work in February 1952, electrically, the Bell solar cells deliver Pearson likely never imagined inventing but his initial research with selenium power from the sun at the rate of a solar cell that would revolutionize the was unsuccessful. Selenium solar cells, 50 watts per square yard, while the photovoltaics industry. There wasn’t the only type on the market, produced atomic cell announced recently by even a photovoltaics industry to revolu- too little power—a mere 5 watts per the RCA Corporation merely delivers tionize in 1952. square meter—converting less than a millionth of a watt” over the same 0.5% of the incoming sunlight into area. An article in U.S. News & World The three scientists were simply trying electricity. Word of Chapin’s problems Report speculated that one day such to solve problems within the Bell tele- came to the attention of another Bell silicon strips “may provide more phone system. Traditional dry cell researcher, Gerald Pearson. The two power than all the world’s coal, oil, batteries, which worked fine in mild scientists had been friends for years. and uranium.” The New York Times climates, degraded too rapidly in the They had attended the same university, probably best summed up what Chapin, tropics and ceased to work when needed. and Pearson had even spent time on Fuller, and Pearson had accomplished. The company therefore asked its famous the Chapins’ tulip farm. On page one of its April 26, 1954, issue, research arm—Bell Laboratories—to the Times stated that the construction of The inventors of the Bell explore alternative sources of freestand- At the time, March 1953, Pearson was the first solar module to generate useful Solar Battery, from left, ing power. Daryl Chapin got the assign- engaged in pioneering semiconductor amounts of power marks “the beginning Gerald Pearson, Daryl ment. At that time, his job was to test research with Calvin Fuller. They took of a new era, leading eventually to the Chapin, and Calvin wind machines, thermoelectric gensets, silicon solid-state devices from their realization of one of mankind’s most Fuller, check devices and steam engines. Being a solar energy experimental stage to commercializa- cherished dreams—the harnessing of for the amount of solar enthusiast, he suggested that the investi- tion. Fuller, a chemist, had discovered the almost limitless energy of the sun electricity derived from gation include solar cells. His supervisor how to control the introduction of the for the uses of civilization.” sunlight, here simulated approved the idea. impurities necessary to transform silicon by a lamp. from a poor to a superior conductor of In 1954, the world had less than a watt electricity. Fuller provided Pearson with of solar cells capable of running elec- a piece of silicon containing a small trical equipment. Fast-forward through concentration of gallium. The introduc- 50 years of continued discovery and tion of gallium made it positively development of silicon and other PV charged. Pearson then dipped the materials and this is what you’ll see. gallium-rich silicon into a hot lithium Today, a billion watts of electricity bath, according to Fuller’s instructions. generated by solar cells help to power The spot where the lithium penetrated the satellites so necessary for modern created an area of poorly bound elec- life, ensure the safe passage of ships and trons and became negatively charged. trains, bring abundant water, lighting, and telephone service to many who had Then came the test. Pearson shined light done without, and supply clean power from a lamp onto the lithium-gallium to those already connected to the grid. silicon. One can only guess whether or not he crossed his fingers. An ammeter The worldwide market for solar electric connected to the silicon recorded a energy has grown by 20%–25% per year significant electrical flow. Much to over the past 10 years. According to his surprise, Pearson had made a solar Solarbuzz, the international solar electric cell superior to any other available at industry now generates around $3–$4 the time. billion (U.S.) in revenues each year. SWITCHING TO SILICON “The biggest problem appears to be making Pearson went directly to Chapin’s office electrical contact to and advised him to switch to silicon, the silicon,” Chapin rather than wasting another moment reported. The problem on selenium. Chapin’s tests on this new was solved when Fuller material proved Pearson right. Exposing incorporated an ultra- Pearson’s silicon solar cell to strong sun- thin layer of boron, light, Chapin found that it performed which gave the cell a significantly better—five times more p-n junction that was efficiently, in fact—than selenium. extremely close to the Theoretical calculations brought even surface. more encouraging news. An ideal silicon solar cell, Chapin figured, could convert 23% of sunlight into electricity. Developing a silicon solar cell with 6% conversion efficiency, though, would close to the top of the cell. Coincidentally, satisfy Chapin and rank as a viable Fuller had done that very thing two years The Atomic Battery, according to RCA, power source. His colleagues concurred, earlier while trying to make a transistor. would some day power homes, cars, and and all his work focused on this goal. He therefore replicated his prior work locomotives with radioactive waste— to satisfy his colleague’s need. Instead strontium-90—produced by nuclear However, try as he might, Chapin could of doping the cell with lithium, Fuller reactors. What its public relations people not improve on Pearson’s accomplish- vaporized a small amount of phospho- failed to mention, however, was why the ment. “The biggest problem appears to rous onto the otherwise positive silicon. room’s blinds had to be closed during be making electrical contact to the sili- The new concoction almost doubled Sarnoff’s demonstration. Years later, one con,” Chapin reported. Not being able previous performance records. Still, the of the lead scientists involved in the to solder the leads directly to the cell lingering failure to obtain good contacts project told the rest of the story: If the forced Chapin to electroplate a portion frustrated Chapin from reaching the 6% silicon device had been exposed to the of the negative and positive silicon efficiency goal. sun’s rays, solar energy would have over- layers in order to tap into the electricity powered the contribution of the stron- generated by the cell. Unfortunately, no THE COMPETITION HEATS UP tium-90. Had the nuclear component metal plate would adhere very well, thus While Chapin’s work reached an impasse, been removed, the battery would have presenting a seemingly insurmountable Bell’s competitor, RCA, announced that continued to work on sunlight if obstacle to collecting more of the elec- its scientists had come up with a nuclear- allowed to stream into the building. tricity generated. Chapin also had to powered silicon cell dubbed the Atomic “Who cares about solar energy?” said cope with the inherent instability of Battery. Its development coincided with the director of RCA Laboratories. “Look, the lithium-bathed silicon, because America’s Atom’s for Peace program, what we have is this radioactive waste the lithium migrated through the cell which promoted the use of nuclear converter. That’s the big thing that’s at room temperature. This caused the power throughout the world. Instead going to catch the attention of the pub- location of the p-n junction, the core of photons supplied by the sun, the lic, the press, the scientific community.” of any photovoltaic device, to shift Atomic Battery ran on photons from from its original location near the strontium-90 (which is now classified as The director had gauged the media well. surface, making it more difficult for one of the more hazardous constituents The New York Times, for example, called light to penetrate the junction where of nuclear waste). To showcase the new Sarnoff’s demonstration “prophetic,” all electrical activity occurs. invention, RCA decided to put on a and predicted that power from the dramatic presentation in New York City. Atomic Battery would allow “hearing Then an inspired guess changed Chapin’s David Sarnoff, founder and president of aids and wrist watches [to] run continu- tack. He correctly hypothesized that “it RCA, who had initially gained fame as ously for the whole of a man’s useful life.” appears necessary to make our p-n junc- the telegraph operator who tapped out tion very near to the surface so that the announcement to the world that PROOF OF CONCEPT nearly all the photons are effective.” the Titanic had sunk, hit the keys of RCA’s success stirred management at Bell He turned to Calvin Fuller for advice an old-fashioned telegraph powered by Laboratories to pressure the solar investi- on creating a solar cell that would the Atomic Battery to send the message: gators to hurry up and produce some- permanently fix the p-n junction very “Atoms for Peace.” SWITCHING TO SILICON thing newsworthy. Luckily for them, Fuller had busied himself in his lab and Pearson went directly to Chapin’s office discovered an entirely new way to make and advised him to switch to silicon, more efficient solar cells. He began with rather than wasting another moment silicon cut into long, narrow strips mod- on selenium. Chapin’s tests on this new eled after Chapin’s best-performing cells.
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