Photovoltaics from Milliwatts to Gigawatts. Tim Bruton
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Название: Photovoltaics from Milliwatts to Gigawatts
Автор: Tim Bruton
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119130062
isbn:
Figure 1.2 Charles Fritts’ first photovoltaic array, produced in New York City in 1884 [16]
(Courtesy New World Library)
The underlying science of photovoltaics was given a big boost by the parallel discoveries and developments in photoemission. Hertz observed in 1887 that ultraviolet light caused a significant increase in the sparks in an air gap between electrodes and that it was a function of the wavelength of the light rather than its intensity [17]. While a number of physicists worked on the effect, it was Albert Einstein in 1905 who explained it in terms of different wavelengths behaving as particles of energy, which he called ‘quanta’ but which were later renamed ‘photons’. These quanta had different energies depending on their wavelength. Einstein was awarded the Nobel Prize in 1921 for this work [15]. While these discoveries and other advances in quantum mechanics at the start for the twentieth century did not directly explain photovoltaic effects, they did provide a scientific basis for understanding the interaction of light and materials.
Although research continued on developing solar cells, little progress was made. However, photovoltaics still had its advocates in the 1930s. Ludwig Lange, a German physicist, predicted in 1931 that ‘in the distant future huge plants will employ thousands of these plates to transform sunlight into electric power … that can compete with hydroelectricity and steam driven generators in running factories and lighting homes’ [15]. A more pragmatic view was taken by E.D. Wilson at Westinghouse Electric, who stated that the efficiency of the photovoltaic cell would need to be increased by a factor of 50 in order for them to be of practical use, and this was unlikely to happen [15]. Actually, as will be shown in later chapters, a factor of 20 was achieved, and this was sufficient to create the current global markets.
While progress in other areas of technology was immense in the nineteenth and early twentieth centuries, little real advancement in photovoltaics had been made since Becquerel’s discovery a hundred years previously. Entering into the second half of the twentieth century, everything would change.
1.2.2 The Breakthrough to Commercial Photovoltaic Cells
It is well known that the birth of the commercially successful photovoltaic cell dates back to April 1954, when Pearson, Chapin, and Fuller demonstrated the first 6% efficient cell using a p/n junction in silicon. It is no surprise that this discovery occurred at Bell Telephone Laboratories, which was one of the world’s premier research laboratories until its forced break‐up in 1984. As the research arm of the American Telephone and Telegraph Company, it had a long history of successful innovation, with nine Nobel Prizes awarded over time for work done there. Perhaps its most notable success was the demonstration in 1948 of the point‐contact germanium transistor. This illustrates the strength and depth of both the theoretical understanding and expertise in semiconductor processing at Bell labs [18].
Figure 1.3 Ohl’s patented solar cell structure [20]
Source: R.S. Ohl: US Patent Application filed 27th May 1941
Russel Ohl, a Bell Labs scientist interested in exploring the crystallisation of silicon, is recognised as the discoverer of the p/n junction in this material, in 1941 [19]. In directionally solidifying 99.85% pure silicon, Ohl noted a change in the structure of the solidified ingot, with the upper portion becoming columnar and the lower portion showing no structure; a striated region appeared between the two, forming a barrier to conduction. The upper zone was p type while the lower zone was n type [20]. This can be easily understood as a result of the segregation of dopants during the crystallisation process. While measuring the resistance of rods containing the barrier, Ohl noted a sensitivity to light, which he termed a ‘photo electromotive force’. He proceeded to patent this as a solar cell, although its efficiency was similar to that of the selenium cells, at about 1% [20]. Figure 1.3 shows Ohl’s silicon structure, the n type region being fine‐grained crystallites and the cell contacts plated rhodium. The low efficiency is not surprising given the relatively impure starting material, its multicrystalline nature, and the fact that the n type region was 0.5 mm thick. The relatively low efficiency meant little further work was done until a new approach at Bell Labs.
Success came in the 1950s. The first transistor had been demonstrated at Bell in 1948 using germanium, and had entered commercial production in 1951 [21]. However, germanium had some disadvantages in its fragility and stability, and silicon offered a better option – although a working silicon transistor was not demonstrated until 1954. Two scientists working on this were Calvin S. Fuller and Gerald L. Pearson. Fuller was an expert in doping silicon, while Pearson was an experimentalist. There were three iterations before a good working solar cell was demonstrated [22]. Initially, while not looking for a solar cell, Fuller produced a p type gallium‐doped silicon sample, which Pearson dipped into a lithium bath to form a shallow n type region. When Pearson exposed the sample to light, he found to his surprise that a current was generated. At the same time, in a different department, another scientist, Daryl M. Chapin, was looking for a power source for telecommunications repeaters in hot humid locations where conventional dry cell batteries rapidly failed. Chapin concluded that solar cells were a good option, but his experiments with commercial selenium cells of low efficiency were disappointing. He and Pearson knew each other, and Pearson offered Chapin his lithium‐doped ‘solar cell’. Chapin tested it and found it 2.3% efficient – an enormous improvement on selenium, justifying further investigations into silicon’s potential. The next step was to replace the lithium with phosphorus. A small amount of phosphorus was evaporated on to the p type silicon to make a shallow n type region. Initial results weren’t particularly good, but then Chapin applied a thin plastic layer to act as an antireflection coating (ARC) on the otherwise highly reflecting silicon surface. This gave the encouraging result of around 4% cell efficiency, which was good progress toward Chapin’s target of 5.7% for a viable power source. However, further progress was slow, and forming a good electrical contact proved to be an ongoing problem. A spur to further activity came from Bell’s competitor, RCA, which was developing an ‘atomic battery’ using a strontium 90 source to irradiate a silicon solar cell, although efficiencies were poor. The breakthrough came when Fuller, who had been experimenting with boron to give a p type silicon emitter which offered a new configuration, demonstrated that heating an n type silicon wafer for 5.5 hours at 1000 °C in a boron trichloride atmosphere under reduced pressure could produce a 0.25 μm‐deep diffusion with a resistivity of .001 Ω/cm [23]. This equated to 40 Ω per square sheet resistance emitter, which is a typical figure for later commercial silicon solar cells. Arsenic was used to dope the silicon base n type to 0.1 Ω/cm, and this was then cut into long narrow strips in accordance with the best previous cell results. The emitter was formed using the new boron diffusion process, with the emitter itself wrapping around the cell as shown in Figure 1.4. It was then partially removed on the rear to expose the n type base. Contacts were made by electroplating rhodium to the exposed base and emitter [24]; the relative ease in forming these contacts represented a significant advance. A polystyrene layer with refractive index 1.6 was used as the ARC. The solar cell efficiency was approximately 6% [25]. Chapin had proposed СКАЧАТЬ