The Invention Of The Solar Cell
60 years ago this week, the modern solar cell came into being. Here's how.
The great Scottish scientist James Clerk Maxwell wrote in 1874 to a colleague: “I saw conductivity of Selenium as affected by light. It is most sudden. Effect of a copper heater insensible. That of the sun great.”
Maxwell was among many European scientists intrigued by a behavior of selenium that had first been brought to the attention of the scientific community in an article by Willoughby Smith, published in the 1873 Journal of the Society of Telegraph Engineers. Smith, the chief electrician (electrical engineer) of the Gutta Percha Company, used selenium bars during the late 1860s in a device for detecting flaws in the transatlantic cable before submersion. Though the selenium bars worked well at night, they performed dismally when the sun came out. Suspecting that selenium’s peculiar performance had something to do with the amount of light falling on it, Smith placed the bars in a box with a sliding cover. When the box was closed and light excluded, the bars’ resistance — the degree to which they hindered the electrical flow through them — was at its highest and remained constant. But when the cover of the box was removed, their conductivity — the enhancement of electrical flow — immediately “increased according to the intensity of light.”
Discovering the Photovoltaic Effect in a Solid Material
Let It Shine
To determine whether it was the sun’s heat or its light that affected the selenium, Smith conducted a series of experiments. In one, he placed a bar in a shallow trough of water. The water blocked the sun’s heat, but not its light, from reaching the selenium. When he covered and uncovered the trough, the results obtained were similar to those previously observed, leading him to conclude that “the resistance [of the selenium bars] was altered…according to the intensity of light.”
Among the researchers examining the effect of light on selenium following Smith’s report were two British scientists, Professor William Grylls Adams and his student Richard Evans Day. During the late 1870s they subjected selenium to many experiments, and in one of these trials they lit a candle an inch away from the same bars of selenium Smith had used. The needle on their measuring device reacted immediately. Screening the selenium from light caused the needle to drop to zero instantaneously. These rapid responses ruled out the possibility that the heat of the candle flame had produced the current (a phenomenon known as thermal electricity), because when heat is applied or withdrawn in thermoelectric experiments, the needle always rises or falls slowly. “Hence,” the investigators concluded, “it was clear that a current could be started in the selenium by the action of the light alone.”5 They felt confident that they had discovered something completely new: that light caused “a flow of electricity” through a solid material. Adams and Day called current produced by light “photoelectric.”
The First Module
A few years later, Charles Fritts of New York moved the technology forward by constructing the world’s first photoelectric module. He spread a wide, thin layer of selenium onto a metal plate and covered it with a thin, semitransparent gold-leaf film. This selenium module, Fritts reported, produced a current “that is continuous, constant, and of considerable force[,]…not only by exposure to sunlight, but also to dim diffused daylight, and even to lamplight.” As to the usefulness of his invention, Fritts optimistically predicted that “we may ere long see the photoelectric plate competing with [coal-fired electrical-generating plants],” the first fossil-fueled power plants, which had been built by Thomas Edison only three years before Fritts announced his intentions.
Fritts sent one of his solar panels to Werner von Siemens, whose reputation ranked on a par with Edison’s. The panels’ output of electricity when placed under light so impressed Siemens that the renowned German scientist presented Fritts’s panel to the Royal Academy of Prussia. Siemens declared to the scientific world that the American’s modules “presented to us, for the first time, the direct conversion of the energy of light into electrical energy.”
The blessed vision of the Sun, no longer pouring unrequited into space.
Siemens judged photoelectricity to be “scientifically of the most far-reaching importance.” James Clerk Maxwell agreed. He praised the study of photoelectricity as “a very valuable contribution to science.” But neither Maxwell nor Siemens had a clue as to how the phenomenon worked. Maxwell wondered, “Is the radiation the immediate cause or does it act by producing some change in the chemical state?” Siemens did not even venture an explanation but urged a “thorough investigation to determine upon what the electromotive light-action of [the] selenium depends.”
Few scientists heeded Siemens’s call. The discovery seemed to counter all of what science believed at that time. The selenium bars used by Adams and Day, and Fritts’s “magic” plate, did not rely on heat to generate energy as did all other known power devices, including solar motors. So most dismissed them from the realm of further scientific inquiry.
One brave scientist, however, George M. Minchin, a professor of applied mathematics at the Royal Indian Engineering College, complained that rejecting photoelectricity as scientifically unsound — an action that originated in the “very limited experience” of contemporary science and in “a ‘so far as we know’ [perspective —] is nothing short of madness.” In fact, Minchin came closest among the handful of nineteenth-century experimentalists to explaining what happens when light strikes a selenium solar cell. Perhaps, Minchin wrote, it “simply act[s] as a transformer of the energy it receives from the sun, while its own materials, being the implements used in the process, may be almost wholly unmodified.”
The scientific community during Minchin’s time also dismissed photoelectricity’s potential as a power source after looking at the results obtained when measuring the sun’s thermal energy in a glass-covered, black-surfaced device, the ideal absorber of solar heat. “But clearly the assumption that all forms of energy of the solar beam are caught up by a blackened surface and transformed into heat is one which may possibly be incorrect,” Minchin argued. In fact, he believed that “there may be some forms of [solar] energy which take no notice of blackened surfaces[, and] perhaps the proper receptive surfaces” to measure them “remain to be discovered.” Minchin intuited that only when science had the ability to quantify “the intensities of light as regards each of [its] individual colours [that is, the different wavelengths] could scientists judge the potential of photoelectricity.”
Einstein’s Great Discovery
Albert Einstein shared Minchin’s suspicions that the science of the age failed to account for all the energy streaming from the sun. In a daring paper published in 1905, Einstein showed that light possesses an attribute that earlier scientists had not recognized. Light, he discovered, contains packets of energy, which he called light quanta (now called photons). He argued that the amount of power that light quanta carry varies, as Minchin suspected, according to the wavelength of light — the shorter the wavelength, the more power. The shortest wavelength, for example, contains photons that are about four times as powerful as those of the longest.
Einstein’s bold and novel description of light, combined with the discovery of the electron and the ensuing rash of research into its behavior — all happening at the turn of the nineteenth century — provided photoelectricity with a scientific framework it had previously lacked and that could now explain the phenomenon in terms understandable to science. In materials like selenium, the more powerful photons carry enough energy to knock poorly linked electrons from their atomic orbits. When wires are attached to the selenium bars, the liberated electrons flow through them in the form of electricity. Nineteenth-century experimenters called the process photoelectric, but by the 1920s scientists referred to the phenomenon as the photovoltaic effect.
This new legitimacy stimulated further research into photovoltaics and re-vived the dream that the world’s industries could hum along fuel- and pollution-free, powered by the inexhaustible rays of the sun. Dr. Bruno Lange, a German scientist whose 1931 solar panel resembled Fritts’s design, predicted that, “in the not distant future, huge plants will employ thousands of these plates to transform sunlight into electric power…that can compete with hydroelectric and steam-driven generators in running factories and lighting homes.” But Lange’s solar battery worked no better than Fritts’s, converting far less than 1 percent of all incoming sunlight into electricity — hardly enough to justify its use as a power source.
The pioneers in photoelectricity failed to attain the goals they had hoped to reach, but their efforts were not in vain. One contemporary of Minchin’s credited them for their “telescopic imagination [that] beheld the blessed vision of the Sun, no longer pouring unrequited into space, but by means of photo-electric cells…[its] powers gathered into electric storehouses to the total extinction of steam engines and the utter repression of smoke.” In his 1919 book on solar cells, Thomas Benson complimented these pioneers’ work with selenium as the forerunner of “the inevitable Solar Generator.” Maria Telkes, too, felt encouraged by the selenium legacy, writing, “Personally, I believe that photovoltaic cells will be the most efficient converters of solar energy, if a great deal of further research and development work succeeds in improving their characteristics.”
With no breakthroughs on the horizon, though, the head of Westinghouse’s photoelectricity division could only conclude, “The photovoltaic cells will not prove interesting to the practical engineer until the efficiency has increased at least fifty times.” The authors of Photoelectricity and Its Applications agreed with the pessimistic prognosis, writing in 1949, “It must be left to the future whether the discovery of materially more efficient cells will reopen the possibility of harnessing solar energy for useful purposes.”
The First Practical Solar Cell
Bell executives presented the Bell Solar Battery to the press on April 25, 1954.
Just five years later the beginning of the silicon revolution spawned the world’s first practical solar cell and its promise for an enduring solar age. Its birth accidentally occurred along with that of the silicon transistor, the principal component of every electronic device in use today. Two scientists, Calvin Fuller and Gerald Pearson of the famous Bell Laboratories, led the pioneering effort that took the silicon transistor from theory to working device. Pearson was described by an admiring colleague as the “experimentalist’s experimentalist.” Fuller, a chemist, learned how to control the introduction of the impurities necessary to transform silicon from a poor to the preeminent conductor of electricity. As part of the research program, Fuller gave Pearson a piece of silicon containing a small concentration of gallium. The introduction of gallium had made the silicon positively charged. When Pearson dipped the rod into a hot lithium bath, according to Fuller’s formula, the portion of the silicon immersed in the lithium became negatively charged. Where the positive and negative silicon met, a permanent electrical field developed. This is the p-n junction, the heart of the transistor and solar cell, where all electronic activity occurs. Silicon prepared this way needs but a certain amount of outside energy for activation, which lamplight provided in one of Pearson’s experiments. The scientist had the specially prepared silicon connected by wires to an ammeter, which, to Pearson’s surprise, recorded a significant electrical current.
While Fuller and Pearson worked on improving transistors, another Bell scientist, Daryl Chapin, had begun work on the problem of providing small amounts of intermittent power in remote humid locations. In any other climate, the traditional dry-cell battery would do, but “in the tropics [it] may have too short a life” due to humidity-induced degradation, Chapin explained, “and be gone when fully needed.” Bell Laboratories had Chapin investigate the feasibility of employing alternative sources of freestanding power, including wind machines, thermoelectric generators, and small steam engines. Chapin suggested that the investigation include solar cells, and his supervisors approved.
In late February 1953, Chapin commenced his photovoltaic research. Placing a commercial selenium cell in sunlight, he recorded that the cell produced 4.9 watts per square meter. Its efficiency, the percentage of sunlight it could convert into electricity, was a little less than 0.5 percent. Word of Chapin’s solar power studies and dismal results got back to Pearson. He told Chapin, “Don’t waste another moment on selenium,” and gave him the silicon solar cell that he had made. Chapin’s tests, conducted in strong sunlight, proved Pearson right. The silicon solar cell had an efficiency of 2.3 percent, about five times greater than the selenium cell’s. Chapin immediately dropped selenium research and dedicated his time to improving the silicon solar cell.
His theoretical calculations of its potential were encouraging. An ideal unit, Chapin figured, could use 23 percent of the incoming solar energy to produce electricity. However, he set a goal of obtaining an efficiency of nearly 6 percent, the threshold that engineers of the time felt it was necessary to reach if photovoltaic cells were to be seriously regarded as electrical power sources.
Chapin, doing most of the engineering, had to try new materials, test different configurations, and face times of despair when nothing seemed to work. At several junctures, seemingly insurmountable obstacles arose. One major breakthrough came directly from knowledge of Einstein’s light quanta (photon) work. “It appears necessary to make our p-n [junction] very next to the surface,” Chapin realized, so that the more powerful photons belonging to light of shorter wavelengths could effectively move electrons to where they could be harvested as electricity. To build such a cell required collaboration with Fuller. Chapin also observed that silicon’s shiny surface reflected a good deal of sunlight that could be absorbed and used, so he coated its surface with a dull transparent plastic. Adding boron to the top of the cell permitted better photon harvesting by allowing for good electrical contact on the silicon strips while keeping the p-n junction close to the surface. Chapin finally triumphed, reaching his 6 percent goal. He could now confidently call the cells he built “power photocells…intended to be primary power sources.” Assured of the cells’ reproducibility and sufficient efficiency, the trio built a number of arrays and demonstrated them at a press conference and the annual meeting of the National Academy of Sciences.
Proud Bell executives presented the Bell Solar Battery to the press on April 25, 1954, displaying a panel of cells that relied solely on light power to run a 21-inch Ferris wheel. The next day the Bell scientists ran a solar-powered radio transmitter, which broadcast voice and music to America’s top scientists gathered at a meeting in Washington, DC. The press took notice. U.S. News & World Report speculated excitedly in an article titled “Fuel Unlimited”: “The [silicon] strips may provide more power than all the world’s coal, oil and uranium….Engineers are dreaming of silicon-strip powerhouses.” The New York Times concurred, stating on page one that the work of Chapin, Fuller, and Pearson, which resulted in the first solar cell capable of generating useful amounts of power, “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.”