Looking back on the 140-year development path of solar batteries

Currently, the exploitation and use of solar energy is no longer a strange issue for each of us. Solar energy is one of the types of green energy that promises to be widely applied in people's lives in the future. This is a seemingly endless source of energy, easy to exploit and help protect the living environment of people. And of course, solar cells are an important part of using this future energy source.

It all started from the very accidental discovery of Smith's engineer .

Preamble

British engineer Willoughby Smith (1828-1891), who first discovered the photoelectric phenomenon

It all started with Willoughby Smith (1828-1891), an English electrical engineer. In 1848, Smith began working for Gutta Percha Electric Company, whose main job was to develop iron and copper wires. In 1849, he was involved in managing underground wiring cables and he continued to do so during the next few decades. Not until .

In 1873, Smith developed a method to test the continuity of wires that were installed underground. In order to build a test circuit, he needed a kind of high-resistance resistor material and finally, he chose selenium. In Smith's theory, selenium fits perfectly with his requirements. However, Smith discovered a problem arises: At night, selenium bars work in accordance with Smith's requirements. The electrical conductivity of selenium increases significantly when exposed to strong light.

To verify the cause, Smith placed the selenium bar inside the slide-box. When the lid is closed and there is no light entering, the selenium has the highest resistance and performs the right task to prevent electrical current. But when the lid was slid off so that light could enter, the current flowing through was intensified and increased with the intensity of the light coming on.

At the time, Smith published his findings in the journal Nature, "The effect of light on selenium through the transmission of electricity" . The report drew attention to many scientists throughout Europe at that time. With his research, Smith was recognized as the first person to discover the photoelectric nature of selenium . This discovery has paved the way for making solar cells later.

In 1874, the Scottish scientist with the famous electromagnetic laws, James Clerk Maxwell wrote that to one of his associates with the content: "I saw firsthand the effect of light on Selen. That's unexpected. Heated copper can't have the same reaction. It's a great thing of the Sun. "

Discover the photoelectric effect in solid materials

Smith then performed a series of experiments to determine how the nature of sunlight worked on selenium bars? Heat effect or optical effect. In one experiment, he put selenium bars in a shallow trough containing water. The water in the trough works to block the temperature from the sun but retains the effect of light on the selenium bar.

The results of the experiment showed that when the heat problem was eliminated and the light was retained only from the Sun, the reaction of the selenium was the same as the first time Smith discovered it. And finally, he came to the conclusion that selenium resistance changes with light intensity.

After Smith, among the many scientists who continued to study the effect of light on selenium, there were two scientists in England: William Grylls Adams and his student Richard Evans Day. During the late 1870s, two people carried out many experiments with selenium. One of those experiments is to light a candle that is 1 inch away from the used selenium.

When the candle has just been lit, the needle on the electric meter immediately responds. When the light from the candle was covered, the needle on the electric meter immediately returned to the zero position. This quick reaction once again strengthened Smith's conclusion: The light itself is the main agent. affects the electrical conductivity of selenium bars. Because if there is an effect of the heat effect, the needle in the electrical meter will move slowly without increasing dramatically.

The group of two researchers felt they had discovered an entirely new problem that had never been before: Light could cause "an electric current" on a solid. Adams and Day named the electricity produced by light as "photovoltaic".

The first module

American inventor Charles Fritts and the first photovoltaic module

A few years later, the American inventor Charles Fritts made a big step forward in technology when successfully developing the world's first photovoltaic module . With the first module, Fritts covered a thin and wide layer on a metal plate. After that, he used a very thin and semi-transparent gold leaf to cover the plate. According to Fritts's report, the selenium module he created can produce a constant, stable and significant current, . not only with daylight, low light but also active. with light bulbs.

With his success, Frotts was optimistic about predicting that his photovoltaic model could replace the method of generating electricity by burning coal which is now in common use. His statement came about three years after Thomas Edison created a method to produce electricity by burning fossil fuels such as coal, oil, .

Next, Fritts sent a photovoltaic plate to Werner von Siemens, the inventor with a reputation comparable to Edison at the time. Before the electricity generated by Fritts photovoltaic panels, Siemens and German scientists were impressed. They simultaneously presented photovoltaic panels to the Royal Prussian Academy of Sciences. Siemens has reported to the world's scientists: "The module of Americans presented to us, for the first time, can directly convert the energy of sunlight into electrical energy."

Siemens has stated that photovoltaic is the most important and deep scientific discovery. James Clerk Maxwell (1831-1879), a Scottish physicist famous for his fundamental laws of electric fields, concurred with Siemens. Maxwell praised photovoltaic research as "an invaluable contribution to science".

Not only Siemens but also Maxwell have not found an explanation to explain the photovoltaic phenomenon

However, both Siemens and Maxwell still cannot understand the nature of photovoltaic phenomena. Maxwell wondered: "Is the solar radiation the cause of the problem or does it cause chemical changes in the selenium? And of course, Siemens has not explained the nature of the phenomenon and he called for "to conduct a thorough investigation to determine which elements of the photoelectricity of selenium depend on what factors?"

Very few scientists have responded to Siemens' call. The reason is that the discovery of photovoltaic seems to be contrary to human understanding of science. At the time, it was only known that heat could be converted into electricity thanks to Edison's previous discovery. Adams and Day's selenium or Fritts' "magic fork" were considered unscientific and could not be true because they did not use heat to get electricity. Therefore, most scientists refuse to continue studying photovoltaic phenomena.

However, there is still a "brave" scientist: George M. Minchin, professor of applied mathematics at the Royal Indian Technical College. Minchin has begun to research to explain photovoltaic phenomena . Minchin's actions have been deemed unscientific by the scientific community and a crazy thing. In fact, Minchin has come very close to explaining the effect of light on selenium bars. However, no satisfactory explanation has been given yet.

Minchin's scientific community rejected the potential for photovoltaic exploration after seeing results from a Minchin experiment. In the experiment, Minchin placed the photovoltaic module into a dark-glass box and measured the heat inside the box.

The experiment was carried out by placing the photovoltaic module in a black glass case to absorb sunlight made by Minchin. Minchin argued that: "It is clear that the black glass box absorbed all the forms of energy in the sun's rays and converted it into thermal energy in the box. However, this may not be accurate."

Minchin believes that "there may be some forms of solar energy that are not absorbed by black surfaces. And something needs to be discovered. Only when science can measure energy of different wavelengths, the new photovoltaic problem is solved ".

Einstein's important discovery

In line with Minchin, Albert Einstein argues that contemporary science has not yet discovered and measured all the forms of energy transmitted from the Sun to Earth. In a daring study published in 1905, Einstein raised an attribute of light that previous scientists did not recognize. He discovered that light consists of energy "packages" and he calls it quanta (now photons) .

An additional contribution of Einstein for the development of humanity with light quantum theory, paving the way for future photovoltaic studies

True to what Minchin predicts, Einstein argues that the amount of energy that light quanta will be expressed in different forms and depends on the wavelength of light. More specifically, the shorter the wavelength, the greater the energy. The shortest wavelength can carry four times more energy than the longest wavelength.

Einstein's bold description of the nature of light, combined with the discovery of electrons, has led many scientists to begin studying the effects of light. All of this is a turning point for the development of photovoltaics in the 19th century. All the previous mysteries surrounding sunlight and photovoltaics could be explained within the framework of science.

In materials such as selenium, photons carry enough energy to be able to impact weakly bound electrons and control it to move differently from the original orbit. When the wire is attached to the selenium, the electrons released by the photon energy will travel in the wire and form an electric current. Experiments in the 19th century began to call this phenomenon photovoltaic.

Clearly explaining the phenomenon of photovoltaics has stimulated scientists to study further to find ways to create photovoltaics under industrial scale. Since then, the dream of exploiting clean and endless energy sources from the Sun has been fulfilled.

German scientist Bruno Lange, who designed a photovoltaic module similar to Fritt in 1931, also predicted: "In the near future, thousands of photovoltaic modules will be created. In order to convert solar energy into electricity, this could replace hydroelectric or thermal power plants, which can make solar cars and even be used for every household. " .

However, because the battery that Lange built works less efficiently than Fritt's version, only about 1% of the energy from sunlight is converted to electricity. This is not enough to justify the feasibility of harnessing solar energy on an industrial scale.

Pioneers in the creation of photovoltaics have encountered failure compared to the original hope. However, all their efforts are not entirely useless. A contemporary of Minchin also predicts: "there will be times when people will be able to obtain energy from the Sun with high efficiency and even store them. This will make the steam engine and the other types of engines completely extinct ".

In the next period, no breakthrough was recorded in photovoltaic mining. Even the head of Westinghouse Energy Group said: "Solar batteries will not be attractive to engineers until the conversion efficiency from photovoltaics to electricity reaches at least 50%. .

The authors of the Photoelectricity and Its Applications book published in 1949 gave a rather pessimistic prediction: "Only one type of matter will be discovered in the future." new photovoltaic batteries can harness solar energy for useful purposes for humans ".

Solar battery complete first

Gerald Pearson, scientist at Bell Laboratories

Everything around the photovoltaic mining seemed to have ended until researchers discovered the capabilities of silicon. This is a big turning point in the development of solar cells. The researchers unknowingly discovered this possibility in the process of making silicon transistors - the main component of all electronic devices today.

Two scientists, Calvin Fuller and Gerald Pearson of the famous Laboratories Bell Laboratories (now AT&T laboratory), are pioneers in making silicon semiconductor diodes from the formation of original theories. to manufacturing practices. Pearson is described by his colleagues as an "experimental person". Fuller, a chemist, has made a small contribution to the discovery of silicon supplements that make it from a poorly conductive substance to a superior conductor.

In the study, Fuller provided Pearson with a piece of silicon containing a small amount of gallium . The presence of gallium makes silicon available positively charged. According to Fuller's formula, when Pearson immersed the gallium silicon sample in a hot lithium tank, the silicium content in the solution was negatively charged. At the junction between a negative charge and a positively charged part, a durable electric field will be formed. This is the pn structure where all electrical activities take place. The pn transition structure is a central component of semiconductor diodes and of solar cells.

During the testing of the silicon-phase silicon sample, Pearson connected the silicon-gallium sample with wires, placed it under the light bulb to illuminate the sample and used the ammeter to measure. And there was a phenomenon in this experiment that surprised Pearson . An electric current was created when the light shone on the silicon sample. This is a random but extremely important finding for solar cells today.

Pn transition structure, the most important component of semiconductor diodes.The premise of successfully building a solar battery is complete

While Fuller and Pearson are working on improving semiconductor diodes, another scientist from Bell Laboratories, Daryl Chapin began studying how the energy in batteries deteriorates when used in high-density areas. High humidity. In any other climate, the traditional dry battery will perform its function well. Only in hot and humid tropical climates, battery life becomes shorter than when used in other climates.

The laboratory tasked Chapin with finding a more viable energy battery such as wind power, heat and steam generators, etc. Chapin proposed developing a solar battery and the proposal was Laboratory approved.

In late February 1953, Chapin began conducting photovoltaic research. To enable a solar cell to be commercially exploited, Chapin aims to create a battery that can generate a capacity of 4.9 W per square meter and a magnetic conversion efficiency. photovoltaic energy is the highest. Chapin's research spread to Pearson's ears. He told Chapin about his accidental discovery and gave Chaplin a silicon form of gallium.

Immediately, Chapin experimented under sunlight and found Pearson's discovery to be completely accurate. According to Chapin's measurement, solar cells with silicon samples supplied by Pearson have a conversion efficiency of 2.3% from photovoltaic to electricity, 5 times greater than batteries with Selenium. Since then, Chapin has focused on developing silicon solar cells.

Based on his hypothetical calculations, Chapin predicts silicon solar cells can harness solar energy with up to 23% efficiency if the conditions are ideal. However, the initial goal he set was conversion efficiency of about 6%. This is the threshold that engineers at the time set out to create a photovoltaic battery and consider it a real source of electrical energy.

Callvin S. Fuller, covering Bo into Silicon to create the world's first complete solar battery

However, despite a lot of experiments with different methods, Chapin has not progressed from the beginning. There are obstacles that appear and seem insurmountable. And Chapin found Enstein's quantum theory of light as well as previous semiconductor studies of Pearson and Fuller. Finally, he realized that it was necessary to rely on Fuller's help to bring the pn transition structure as close to the battery surface as possible. In addition, Chapin found the surface of the silicon plate too shiny to reflect a significant amount of light. Therefore, he chose to cover a translucent plastic sheet. Next, he coated a layer of Bo on the top surface of the photovoltaic panel to obtain more photons.

And the end result is the solar panels that Chapin set out - with a conversion efficiency of 6%. The group of 3 scientists reported their work to the National Academy of Sciences on the successes achieved.

Group of 3 scientists in an experiment with solar batteries (From left over: Pearson, Chapin and Fuller)

On April 25, 1954, the director of Bell Laboratories officially introduced solar panels to the press. It is a board containing photovoltaic cells that can generate an amount of electricity to spin a 21-inch Ferris ferris wheel. The next day in Washington, scientists at Bell used the photovoltaic power to run a radio, voice transmitter and songs in front of leading scientists from across the United States. The American newspapers have called it an endless fuel and can replace coal, oil and match uranium.

Bell Lab's first solar battery advertising template

Finally, solar cells are officially a new energy source for humans. Since then, solar batteries have continued to be improved and improved to improve working efficiency, but the manufacturing method is simple and low cost. Countless methods have been developed and eventually applied to solar panels as we see them today.