Solar Photovoltaic Technologies
Abstract
The solar photovoltaic cell is a solid-state device that exploits properties inherent in semiconducting materials to capture photons of light and use them to generate an electric current. They are constructed using microchip technology and have the same efficiency, whether in a tiny pocket calculator or in a central power station generating hundreds of megawatts. Economies of scale in the production process have seen prices fall continuously since the 1980s. They are likely to become one of the cheapest forms of electricity generation within a decade, and probably offer the best long-term renewable source of energy and replacement for fossil fuel generation on the planet. Much of the global power generation capacity based on solar cells is found in rooftop installations on domestic and commercial buildings. Silicon crystalline solar cells dominate the market while some alternative thin film materials take a small market share.
Keywords
Solar cell; solar photovoltaic device; solid-state p-n junction; rooftop solar installation; single crystal silicon; thin film; cadmium telluride; semiconductor bandgap
Of all the forms of renewable power generation, the solar cell is perhaps the most elegant. The cell is a solid-state device so it has no moving parts; it simply exploits the physical properties of the elements from which it is made to capture sunlight and convert it into electricity. Although it is based on extremely sophisticated technology, a solar cell is easy to deploy. Size, both large and small, is no constraint. At their smallest, solar cells can be used to provide power to tiny consumer devices such as calculators or watches. At their largest, massive arrays are used to create grid-connected power plants that can generate tens or even hundreds of megawatts of electric power. The efficiency is the same in each case.
Solar cells (or more precisely, solar photovoltaic devices, because they rely on the photovoltaic effect) are manufactured using the same type of technology used to make transistors and microchips. While the technology is costly, economies of scale have allowed the cost of the solar cell to fall to such a level that it is becoming competitive with other forms of power generation. The key transition point will be reached when solar cells reach economic parity with mainstream generating technologies. When that will happen is a subject of hot debate, but researchers will certainly be approaching that point by the end of the second decade of the 21st century. Indeed, in some situations and regions, the economic parity barrier has already been breached.
The simplicity and ease of deployment of this technology means that over the long-term this is probably the most promising means of replacing large-scale fossil fuel-based power generation with a renewable resource. That may take several decades to achieve, but with a global total of 40 GW of new capacity installed in 2014 (see Table 1.1), and an aggregate global capacity of close to 180 GW at the end of that year, the process is well underway.
Origins
The history of the solar cell begins with the discovery of the photovoltaic effect by French scientist Alexandre Edmond Becquerel in 1839. Becquerel was experimenting with an early battery comprising two metal electrodes in an electrolyte solution. During this work he observed that the amount of electricity generated increased when the electrodes were exposed to light. Becquerel’s observation was followed in 1873 by the discovery that the electrical conductivity of selenium increased when it was exposed to light, results that were published by the British engineer Willoughby Smith. Finally, in 1883, American inventor Charles Fritts coated selenium with a thin layer of gold to create the first photovoltaic cell. It had a light-to-electrical energy conversion efficiency of about 1%.
Although these advances were used in photocells and other simple light-sensitive devices, it was the discovery of the p-n semiconductor by Russell Ohl in 1939 that lead to the development of the first modern solar cell, based on a silicon p-n junction. This revolutionary silicon solar cell was demonstrated at Bell Labs in the United States by Daryl Chapin, Calvin Fuller, and Gerald Pearson in 1954. The first devices achieved about 6% efficiency, much higher than had been possible with simple selenium photocells. The new silicon solar cells were used in the U.S. Vanguard satellite in 1958.
Early photovoltaic cells were expensive. Consequently, while they were used in space programs where cost was no object but reliability was vital, they found little application elsewhere. Prices remained high during the 1960s and 1970s, but the evolution of methods to grow large single crystals of silicon for use in integrated circuits gradually brought the price of the raw material down, and the price of silicon solar cells started to fall.
During the 1970s and 1980s solar cells were still considered expensive, but they were more readily available and they found a use in niche markets such as navigational buoys and remote telecommunications stations. As production volumes increased and costs continued to fall, the number of applications increased. However, it was not until the 1990s that large-scale application of solar cells for power generation began to advance, particularly in domestic and commercial rooftop applications.
While single crystal silicon was, and remains, the most important base for solar cells, other technologies have also been developed. Polycrystalline silicon became popular during the 1990s because it was much cheaper to produce and appeared capable of challenging single crystal silicon in performance. Even cheaper thin film amorphous silicon solar cells were also developed. During the first decade of the 21st century, thin film solar cells based on a number of other materials, cadmium telluride in particular, attracted attention and appeared to offer an extremely cheap and practical alternative to silicon. However, the cost of silicon cells continued to fall, and their high efficiency, coupled with low price, has allowed their continued domination of the market.
There is also a niche market for high-efficiency solar cells, some based on more complex silicon solar cell structures, others exploiting materials like gallium arsenide and indium telluride, which have an inherently higher efficiency. However, they are generally only used in specialized applications such as concentrating solar cells.
Solar Photovoltaic Basics
Modern solar cells are made from a variety of different semiconductors, all of which have the ability to absorb light from the visible spectrum. The operation of these solar cells depends upon a fundamental property of these semiconductors called the bandgap, a part of the quantum-level structure of the material relating to the distribution of electrons within the solid.
The bandgap is a function of the specific semiconductor’s electron energy levels, which results in an energy gap between the top layer of full electron energy levels and the first set of empty energy levels. In a conductor, this empty band of energy levels is so close in energy to the full band below it that electrons can easily jump from one to the other as a result of thermal activation. The electrons in the upper level can then move easily across the material in the mostly empty energy band of energy levels, conferring electronic (or at the macroscopic scale, electrical) conductivity on the material.
In a semiconductor the energy gap is too large for electrons to jump from the lower to the upper level as a result of thermal activation at normal temperatures. However, electrons in the lower level can become promoted from the lower to the upper level, across the bandgap, by absorbing photons of electromagnetic radiation. For this to be possible, the photon must contain at least as much energy as the size of the energy gap between the two sets of energy levels in the semiconductor.
The range of electromagnetic radiation that the cell can absorb is determined by the size of its bandgap. Semiconductors that are useful for solar cells have bandgaps that make them capable of absorbing photons within the visible region of the solar spectrum. All those with an energy greater than the bandgap can be absorbed.1 However, any photon with an energy lower than the bandgap (such as infrared radiation) will not be absorbed. Table 8.1 shows the bandgaps of some semiconductors commonly used for solar cells. The bandgap of silicon is 1.11 eV.
Table 8.1
The Bandgap of Some Common Solar Cell Semiconductors
| Semiconductor | Bandgap (eV) |
| Silicon | 1.11 |
| Cadmium telluride | 1.44 |
| Gallium arsenide | 1.43 |
| Copper indium gallium diselenide | 0.9–1.7 |
Each photon of light energy absorbed by the semiconductor is captured by an electron within the material. In absorbing the energy, the electron acquires an electrical potential relative to the electrons around it because it has a higher energy. The special structure of a photovoltaic cell created by the p-n junction allows this potential to be exploited to provide an electric current. The current is produced at a specific fixed voltage called the cell voltage. The cell voltage is, again, a property of the semiconducting material. For silicon it is around 0.6 V.
The energy contained in light increases as the frequency increases from infrared through red to blue and ultraviolet light. However, a solar cell must throw away some of these frequencies since it can only absorb light above its cell threshold, defined by the semiconductor bandgap. Light that is of an energy below this threshold simply passes through the material.
It might seem sensible, therefore, to use a semiconductor with a low threshold or bandgap. However, this would lead to a cell with a low output voltage because these factors are also directly related to the threshold for absorption. There is another drawback to using a semiconductor with a small bandgap. When a photon of light with energy much greater than the threshold energy is absorbed, it loses all the energy above the threshold value. The surplus energy is essentially thrown away (it emerges as heat) and cannot be used for electricity production.
These two factors mean that the lower the bandgap and absorption threshold, the greater the number of photons absorbed but the more energy thrown away as heat; the higher the bandgap, the more energy simply passes through the material without being absorbed. The optimum conditions are therefore a compromise between the two competing effects.
The optimum bandgap for a solar cell semiconductor is 1.43 eV. As Table 8.1 shows, this is exactly matched by gallium arsenide. However, this material is much more expensive than silicon, and the presence of arsenic has environmental implications. The bandgap of silicon, at 1.11 eV, is less than optimum, but it has proved to be the most effective solar cell material to date, and has the largest market share. Silicon has been used in three different forms: crystalline, semi-crystalline, and amorphous. Crystalline silicon is the most efficient, but also the most expensive to produce, while amorphous silicon is both the least efficient and the cheapest. Alternatives to silicon include cadmium telluride, which is cheaper to produce and is always in amorphous form. Its bandgap is close to optimum. Meanwhile, copper indium gallium diselenide has a bandgap that varies with composition, allowing a degree of tailoring. Once again, however, the material is much more expensive than silicon.