Snapshots from Physics History
April 25, 1954: Bell Labs demonstrates the first practical silicon solar cell
The story of solar cells dates back to an observation of the photovoltaic effect in 1839. French physicist Alexandre-Edmond Becquerel, son of physicist Antoine Cesar Becquerel and father of physicist Henri Becquerel, was working with metal electrodes in an electrolyte solution when he noticed that small electric currents were being produced when the metals were exposed to light, but he couldn’t explain the effect.
Several decades later, in 1873, Willoughby Smith, an English engineer, discovered the photoconductivity of selenium while testing materials for underwater telegraph cables. In 1883, American inventor Charles Fritts made the first solar cells from selenium. Though Fritts had hoped his solar cells might compete with Edison’s coal-fired power plants, they were less than 1 percent efficient at converting sunlight to electricity, and thus not very practical. Some research on selenium photovoltaics continued for the next several decades, and a few applications were found, but they were not put to widespread use.
The next major advance in solar cell technology was developed in 1940 by Russell Shoemaker Ohl, a semiconductor researcher at Bell Labs. He investigated some silicon samples, one of which had a crack in the middle. He noticed that current flowed through this particular sample when it was exposed to light. This crack, which had probably formed when the sample was made, actually marked the boundary between regions containing different levels of impurities, so one side was positively doped and the other side negatively doped. Ohl had inadvertently made a p-n junction, the basis of a solar cell. Excess positive charge builds up on one side of the p-n barrier, and excess negative charge builds up on the other side, creating an electric field. When the cell is hooked up in a circuit, an incoming photon that hits the cell can then give an electron a kick and start current flowing. Ohl patented his solar cell, which was about 1 percent efficient.
The first practical silicon solar cell was created 13 years later by a team of scientists working at Bell Labs.
In 1953, engineer Daryl Chapin, who had previously worked on magnetic materials at Bell Labs, tried to develop a source of power for telephone systems in remote, humid locations, where dry cell batteries degraded too quickly. Chapin investigated several alternative energy sources and decided that solar power was one of the most promising. He tried selenium solar cells, but found them too inefficient.
Meanwhile, Calvin Fuller, a chemist, and Gerald Pearson, a physicist, worked on controlling the properties of semiconductors by introducing impurities. Fuller gave Pearson a piece of silicon containing gallium impurities. Pearson dipped it in lithium, creating a p-n junction. Pearson then hooked up an ammeter to the piece of silicon and shined a light on it. The ammeter jumped significantly, to their surprise.
Pearson, who was aware of Chapin’s work, told his friend not to waste any more time on selenium solar cells, and Chapin immediately switched to silicon.
The three then worked for several months on improving the properties of their silicon solar cells. One problem was the difficulty in making good electrical contacts with the silicon cells. Another problem was that, at room temperature, lithium migrated through the silicon over time, moving the p-n junction farther away from the incoming sunlight. To solve that problem, they tried different impurities, and eventually settled on arsenic and boron, which created a p-n junction that stayed near the surface. They also found they were able to make good electrical contacts with the boron-arsenic silicon sells. After making some other improvements to the design, they linked together several solar cells to create what they called a “solar battery.”
Bell Labs announced the invention on April 25, 1954, in Murray Hill, New Jersey. They demonstrated their solar panel by using it to power a small toy Ferris wheel and a radio transmitter.
Those first silicon solar cells were about 6 percent efficient at converting the energy in sunlight into electricity, a huge improvement over any previous solar cells.
The New York Times wrote that the silicon solar cell “may mark 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.”
The first silicon solar cells were expensive to produce, and early efforts at commercialization were not very successful. But within a few years, solar cells were commonly used to power satellites, and other applications followed.
Chapin simplified the process of making silicon solar cells and even developed a solar cell science experiment for high school students. Chapin, Fuller and Pearson were inducted into the National Inventors Hall of Fame in 2008.
Today, solar cells are used in many devices, from handheld calculators to rooftop solar panels. Improved designs and advanced materials have made it possible to build solar cells that reach over 40 percent efficiency. Research and development continues with the goal of decreasing the solar cells’ cost and improving their efficiency to make solar power more competitive with fossil fuels.
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