FEATURE ARTICLE

Enjoy a look at the history of Bell Labs and the solar cell

GOOD AS GOLD: The Silicon Solar Cell Turns 50

by John Perlin, Lawrence Kazmerski, Ph.D., and Susan Moon

THEN AND NOW. In 1953, the solar world started with a single 2 cm2 photovoltaic (PV) cell that was about 5 percent efficient and generated 5 milliwatts of electricity. In 2002, that world shipped over 500 million PV cells totaling 4 billion cm2 that were from 15 percent to 20 percent efficient and produced more than 500 megawatts (MW) of electricity.

In April 1954, the media spotlight shone brightly on a new invention, a bit of a curiosity really, from Bell Telephone Laboratories in Murray Hill, New Jersey. The invention was a strip of boron-doped silicon about the size of a razor blade. Connected in series, these strips comprised the Bell Solar Battery, which converted about 6 percent of sunlight into electricity. In a sense, all the solar cells in existence today are direct descendents of this invention.

During its first public demonstration at a press conference in New York, the Bell Solar Battery powered a radio transmitter that carried both voice and music. It electrified the New York media audience.

U.S. News & World Report speculated that one day such silicon strips “may provide more power than all the world’s coal, oil and uranium.” The New York Times stated on page one of its April 26, 1954, issue that it marked “the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams—the harnessing of the almost limitless energy of the sun for the uses of civilization.”

Today, we are on that path toward our clean-energy solar future. It is up to us to make sure it happens.

Getting Started
Daryl Chapin, Calvin Fuller and Gerald Pearson of Bell Labs were not planning to invent a solar cell that would revolutionize the PV industry. There wasn’t even a PV industry to revolutionize when their work commenced in 1952.

The three scientists were simply trying to solve problems within the Bell telephone system. Traditional dry-cell batteries worked fine in mild climates but degraded too rapidly in the tropics and ceased to work when needed. So the company asked its famous research arm—Bell Laboratories—to explore alternative sources of freestanding power. Chapin got the assignment. At that time, his job was to test wind machines, thermoelectric generating sets and steam engines. Being a solar energy enthusiast, he suggested that the investigation include solar cells. His supervisor approved the idea.

Chapin began work in February 1952, but his initial research with selenium was unproductive. Selenium solar cells, the only type on the market, produced too little power—a mere 5 watts per square meter—and converted less than 0.5 percent of the incoming sunlight into electricity.

At the time, March 1953, Pearson was engaged in pioneering semiconductor research with Fuller. They took silicon solidstate devices from the experimental stage to commercialization. Fuller, a chemist, had discovered how to control the introduction of the impurities necessary to transform silicon from a poor to a superior conductor of electricity. To build a solid-state rectifier—a device that changes DC electricity to AC—Fuller provided Pearson with a piece of silicon containing a small concentration of gallium. The introduction of gallium made it positively charged. Pearson then dipped the gallium-rich silicon into a hot lithium bath, according to Fuller’s instructions. The spot where the lithium penetrated created an area of poorly bound electrons and became negatively charged.

Pearson conducted various tests on the rectifier. In one experiment, he shone light from a lamp onto the lithium-gallium silicon. An ammeter (an instrument for measuring electric current in amperes) connected to the silicon recorded a significant electrical flow. Much to his surprise, Pearson had made a solar cell superior to any other available at the time.

Switching to Silicon
Pearson went directly to Chapin’s office and advised him to switch to silicon, rather than wasting another moment on selenium. Chapin’s tests on this new material proved Pearson right. Exposing Pearson’s silicon solar cell to strong sunlight, Chapin found that it performed significantly better—five times more efficiently, in fact—than selenium. Theoretical calculations brought even more encouraging news. An ideal silicon solar cell, Chapin figured, could convert 23 percent of sunlight into electricity. Developing a silicon solar cell with 6 percent conversion efficiency, though, would satisfy Chapin and rank as a viable power source. His colleagues concurred, and all his work focused on this goal.

However, try as he might, Chapin could not improve on Pearson’s accomplishment. “The biggest problem appears to be making electrical contact to the silicon,” Chapin reported. Not being able to solder the leads directly to the cell forced Chapin to electroplate a portion of the negative and positive silicon layers in order to tap into the electricity generated by the cell. Unfortunately, no metal plate would adhere very well, thus presenting a seemingly insurmountable obstacle to collecting more of the electricity generated. Chapin also had to cope with the inherent instability of the lithium-bathed silicon, because the lithium “traveled” through the cell at room temperature. This caused the p-n junction to shift from its original location near the surface to a greater depth within the silicon. (The p-n junction is a region within a semiconductor where a positive-type and negative-type material are in direct contact. This junction is where all electrical activity occurs within the cell, and as such, is the core of any PV device.)

Then, an inspired guess changed Chapin’s tack. He correctly hypothesized that “it appears necessary to make our p-n junction very near to the surface so that nearly all the photons are effective.” He turned to Fuller for advice on creating a solar cell that would permanently fix the p-n junction very close to the top of the cell. Coincidentally, Fuller had done that very thing two years earlier while trying to make a transistor. He therefore replicated his prior work to satisfy his colleague’s need. Instead of doping the cell with lithium, Fuller vaporized a small amount of phosphorous onto the otherwise positive silicon. The new concoction almost doubled previous performance records. Still, the lingering failure to obtain good contacts frustrated Chapin from reaching the 6 percent efficiency goal.

The Competition Heats Up
While Chapin’s work reached an impasse, Bell’s competitor, RCA Laboratories, with headquarters in Princeton, New Jersey, announced that its scientists had invented a nuclear-powered silicon cell dubbed the Atomic Battery. Its development coincided with America’s Atoms for Peace program that promoted the use of nuclear power throughout the world. Instead of photons supplied by the sun, the Atomic Battery ran on photons from strontium-90 (which is now classified as one of the more hazardous constituents of nuclear waste). To showcase the new invention, RCA decided to put on a dramatic presentation in
New York City. David Sarnoff, founder and president of RCA, who had initially gained fame as the telegraph operator who tapped out the announcement to the world that the Titanic had sunk, hit the keys of an old-fashioned telegraph powered by the Atomic Battery to send the message: “Atoms for Peace.”

The New York Times called Sarnoff’s demonstration “prophetic,” and predicted that power from the Atomic Battery would allow “hearing aids and wrist watches [to] run continuously for the whole of a man’s useful life."

Proof of Concept
RCA’s success put the pressure on the Bell solar researchers to hurry up and produce something newsworthy. Luckily for them, Fuller had busied himself in his lab and discovered an entirely new way to make more efficient solar cells. He began with silicon cut into long, narrow strips modeled after Chapin’s best-performing cells. But that’s where the similarity ended. Instead of adding gallium to the pure silicon and producing positive silicon, Fuller introduced a minute amount of arsenic to make the starting material negative. Then he placed the arsenic-doped silicon into a furnace to coat it with a layer of boron. The controlled introduction of an ultra-thin layer of boron gave the cell a p-n junction that was extremely close to the surface. The Bell team had no trouble making good contacts to the boron-arsenic silicon cells, resolving the main obstacle in extracting electricity when exposing them to sunlight.

It Still Works! by John Perlin, Lawrence Kazmerski, Ph.D., and Susan Moon ——————————— HALF A CENTURY has gone by and the world of photovoltaics (PV) has seen some impressive strides. But one thing seemed to amaze just about everyone who visited the “Bell Cell” golden anniversary exhibit at the Third World Conference on Photovoltaic Energy Conversion in Osaka, Japan, in May 2003—that the 50-year-old Bell Solar Battery still works. The Golden, Colorado-based National Renewable Energy Laboratory (NREL) and New York-based Institute of Electrical and Electronics Engineers (IEEE) produced this exhibit to honor Bell Labs and Pearson, Chapin and Fuller for this “golden moment” in the history of PV. A glass case displays actual historical solar pieces: the first two silicon cells produced at Bell Labs; the first solar module; a scale replica of the Vanguard (the first solar-powered satellite); and some archival photos. Visitors can also peruse copies of key lab notebook pages from the three inventors describing important experiments and the results of their research. A four-sided kiosk tells the Bell Cell story: “The Inventors” provides a brief biography of each inventor. “The Beginning at Bell” highlights key research activities and discoveries that led to the 1954 announcement. “Making a Bell Solar Battery” shows the eight basic steps in going from silicon material to an actual useable solar cell. And “50 Years of Progress” puts the invention in its historical context and chronicles the 50 years of progress in the technology and its applications. Pearson, Chapin and Fuller are no longer living, but schoolchildren and scientists alike have delighted in having their pictures taken looking over the shoulders of these giants—via a life-sized photo silhouette of the three inventors. Another big draw was an audio/visual show playing vintage documentary clips from Bell Labs, including interviews with the inventors and discussions with a lab technician and former Bell Labs director about the importance of the invention. Morton Prince, who was a colleague of the three inventors and contributed to developing the first silicon cell, was an honored guest at the exhibit and conference. His first-person recitation of the historic events added immeasurably to the event. All in all, Photon International called the Bell Cell exhibit “the stand that spurred the most interest among visitors.” But back to that “working” solar cell. In 1954, various Bell Labs press releases and ads made a bold statement: “Since it has no moving parts and nothing is consumed or destroyed, the Bell Solar Battery should theoretically last indefinitely.” Well, 50 years is not exactly forever … but so far, so good. One of the original cells, which was about 6 percent efficient in 1953, now has a conversion efficiency of about 1.4 percent. The cell is not encapsulated and has taken a beating during the last five decades, as evidenced by one end having been chipped off. A 1955 Bell cell, however, takes home the “Ironman” prize. This one is encapsulated and is still a thing of beauty and strength. And, 49 years later, the original 6 percent cell boasts an NREL-verified efficiency of 5.1 percent. •

All cells built according to Fuller’s new method did much better than previous cells. One, however, outperformed the rest, and reached the 6 percent efficiency goal that Chapin had set. Chapin confidently referred to the silicon solar cells the lab now produced as “power photocells intended to be primary power sources.” Assured of success, the Bell solar team began putting together modules for a public demonstration of this exciting breakthrough.

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 electrically, the Bell solar cells deliver power from the sun at the rate of 50 watts per square yard, while the atomic cell announced recently by the RCA Corporation merely delivers a millionth of a watt” over the same area.

In 1954, the world had less than a watt of solar cells capable of running electrical equipment. Fast-forward through 50 years of continued discovery and development of silicon and other solar PV materials and this is what you’ll see. Today, a billion watts of electricity generated by solar cells help to: power the satellites so necessary for modern life; ensure the safe passage of ships and trains; bring abundant water, lighting and telephone service to many who have done without; and supply clean power to those already connected to the grid.

With each passing year, the expectation triggered by the pioneering work of Chapin, Fuller and Pearson—the harnessing of almost limitless energy from the sun—comes closer to being fulfilled. But the revolution is not yet won. The hope for the next 50 years is to see solar cells providing power throughout the world and being used in ways we can’t even imagine today. •

John Perlin served as a consultant to the National Renewable Energy Laboratory for the “Bell Cell” golden anniversary exhibit. He is the author of the book From Space to Earth: The Story of Solar Electricity, 2002, Harvard University Press. He can be reached at 805.569.2740, e-mail: solarperlin@aol.com.

Lawrence Kazmerski, Ph.D., is director of the National Center for Photovoltaics at the National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, 303.384.6600, FAX 303.384.6601, e-mail: kaz@nrel.gov, web site: www.nrel.gov.

Susan Moon is a senior writer at the National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado, 80401, 303.384.6631, FAX 303-384-6530, e-mail: susan_moon@nrel.gov, web site: www.nrel.gov.

Acknowledgements: The historic photos in this article are the property of AT&T Archives and are reprinted with permission of AT&T. John Perlin is grateful to Chip Larkin of AT&T Archives in Warren, New Jersey. He also thanks Ed Eckert of Lucent Technologies for providing helpful insight into the AT&T Archives.