Solar Cell
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.
When research into electricity began and simple batteries were being made and studied, research into solar electricity followed amazingly quickly. As early as 1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce voltage. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.
With Albert Einstein’s explanation in 1905 of the photoelectric effect—metal absorbs energy from light and will retain that energy until too much light hits it—hope soared anew that solar electricity at higher efficiencies would become feasible. Little progress was made, however, until research into diodes and transistors yielded the knowledge necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a silicon solar cell of four percent efficiency in 1954.
Further work brought the cell’s efficiency up to 15 percent. Solar cells were first used in the rural and isolated city of Americus, Georgia as a power source for a telephone relay system, where it was used successfully for many years.
A type of solar cell to fully meet domestic energy needs has not as yet been developed, but solar cells have become successful in providing energy for artificial satellites. Fuel systems and regular batteries were too heavy in a program where every ounce mattered. Solar cells provide more energy per ounce of weight than all other conventional energy sources, and they are cost-effective.
Only a few large scale photovoltaic power systems have been set up. Most efforts lean toward providing solar cell technology to remote places that have no other means of sophisticated power. About 50 megawatts are installed each year, yet solar cells provide only about. 1 percent of all electricity now being produced. Supporters of solar energy claim that the amount of solar radiation reaching the Earth’s surface each year could easily provide all our energy needs several times over, yet solar cells have a long way to go before they fulfill Charles Fritts’s dream of free, fully accessible solar electricity.
Raw Materials
The basic component of a solar cell is pure silicon, which is not pure in its natural state.
To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification.
Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.
The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.
The Manufacturing Process
Purifying the silicon
- 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
- 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure drags the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.
Making single crystal silicon
- 3 Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or boule of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.
Making silicon wafers
- 4 From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer- 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to ‘drag the impurities toward one end of the rod. The impure end can then be removed. Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
Doping
- 6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms burrow into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth. A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.
Placing electrical contacts
- 7 Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated
This illustration shows the makeup of a typical solar cell. The cells are encapsulated in ethylene vinyl acetate and placed in a metal frame that has a mylar backsheet and glass cover.
Why This Matters
US generation of electricity from solar energy could grow six-fold by 2050. Alternatives to commonly used crystalline silicon cells may reduce material usage, manufacturing capital expenditures, and lifecycle greenhouse gas emissions. Many of these new materials, however, are under development, and more research is needed to better understand their potential.
What is it? Most solar cells (the components that generate electricity from sunlight) are currently produced with crystalline silicon in a process that is complex, expensive, and energy-intensive. Alternative materials—such as cadmium telluride, amorphous silicon, perovskites, and organic (carbon-containing) compounds—applied in thin layers of film may perform better and be easier and cheaper to manufacture.
How does it work? As sunlight shines on a solar cell, some of the energy is absorbed to generate electricity either for immediate use or for storage in batteries (see fig. 1). The more readily a given solar cell absorbs light and transforms it into electricity, the higher its efficiency. The electric current generated by sunlight flows through wires that connect the front and back contacts of the solar cell.
Figure 1. A simplified representation of how a solar cell generates an electric current.
Some alternative materials absorb light 10 to 100 times more strongly than crystalline silicon, allowing them to produce electricity using less material. In turn, solar cells made with these materials are typically thinner and weigh less. In addition, these thin film solar cells can be manufactured quickly, reducing cost.
How mature is it? The maturity of these alternative materials varies widely, with some currently used to manufacture solar cells and others in the early stages of research and development. For example, cadmium telluride cells and copper indium gallium diselinide cells together account for roughly 10 percent of current solar cells and they are already cost-competitive with crystalline silicon cells.
Novel solar cells under development use a variety of materials. Among them is amorphous silicon, which is non-crystalline and can be deposited as a thin film. Perovskites are an emerging class of materials with rapidly increasing efficiencies. Organic materials offer yet another option for thin films. They consist of carbon-containing compounds, either long chains or molecules, tailored to absorb specific wavelengths of light. Researchers are also investigating the use of quantum dots—microscopic particles of compounds such as cadmium telluride, cadmium selenide, indium phosphide, or zinc selenide, that are able to produce electricity from light.
Although these diverse materials differ in their chemical composition, they all fall under the category known as thin films because of the extremely thin layer—comparable in thickness to a red blood cell—in which they are applied (see fig. 2). In addition to being easy to produce and relatively inexpensive, these materials can be deposited on a variety of substrates, including flexible plastics in some cases.
Figure 2. Current and potential alternative materials for solar cells are applied in extremely thin layers, with emerging materials being the thinnest.
In addition to absorbing light, solar cells must convert it to electricity. While promising, commercial thin film solar cells currently average a conversion efficiency in the range of 12 to 15 percent, compared to 15 to 21 percent for crystalline silicon, according to a Massachusetts Institute of Technology (MIT) study. In addition, they require appropriate sealing materials to protect them from ambient oxygen and moisture. As a result, many alternative solar cell materials are currently under development or limited to specialized applications.
Policy Context and Questions
As alternative materials for solar cells continue to evolve and mature, some key questions that policymakers could consider include:
- What steps could help encourage further development and use of alternative energy sources such as solar cell materials?
- What analyses of incentives and barriers can determine whether government stimulus may be needed for private sector investment in solar cell materials and what are the trade-offs of such a stimulus?
- What actions could help ensure sufficient understanding of the human health and environmental impacts of various solar cell materials across the full lifecycle?
For more information, contact Karen Howard at 202-512-6888 or HowardK@gao.gov.
III Solar cell structure
Silicon heterojunction solar cells are formed by n-type c-Si absorber wrapped with intrinsic and doped layers of a-Si forming a p/i/n/i/n stack. Bare a-Si is highly defective. By hydrogenating the a-Si the defects density decreases dramatically. Hydrogenated amorphous silicon (a-Si:H) films of a few nanometers thick are suitable candidates for buffer layers: their bandgap is slightly wider than c-Si and they can be doped relatively easily, either n– or p-type, enabling the fabrication of electronic heterojunctions [8]. The intrinsic a-Si:H layers provide surface passivation, the p-doped a-Si:H forms the PN junction, and the n-doped a-Si:H layer provides surface passivation. The conductivity of a-Si:H films is very poor and insufficient to provide a good carrier collection by the metal contacts. For this reason a transparent conductive oxide (TCO) film is deposited on top of the amorphous layers. The TCO facilitates lateral carrier transportation, promotes a good ohmic-contact, and works as antireflecting coating (similar to SiNx). There are many options of TCO, indium tin oxide (ITO) is one of the most widely used TCOs. Due to its quasi-symmetrical structure these cells are particularly suitable for bifacial applications. In Figure 1 a simple structure of a SHJ with silver back reflector is shown.
Figure 1. Example of one of the SHJ cell structures manufactured at Arizona State University [9]
IV Manufacturing process
The wafers are textured using alkaline wet etching (KOH or NaOH solution), and followed by wet chemical cleaning. The junction is formed using plasma enhanced chemical vapor deposition (PECVD) to grow few nm of intrinsic and doped a-Si:H layers on both sides of the c-Si wafer, forming a p/i/n/i/n stack. Subsequently, the ITO layer is sputtered on both sides of the wafers. In Figure 1 we can see that the TCO thickness at the front and rear is different. The front ITO thickness and oxygen content are optimized to promote a suitable sheet resistance for carriers’ lateral transportation, a good transparency to avoid parasitic light absorption, and to enhance light trapping. The rear ITO is optimized for low absorption in the infrared region. The contacts are then screen printed on the front or on both sides of the cell depending if the aim is to obtain a bifacial solar cell. Finally, the samples are annealed at 200-250 °C for 30-60 min. Figure 2 shows a simplified flowchart of the manufacturing process of SHJ solar cells including photographs of the partly processed wafer after each process step.
Figure 2. Process steps to manufacture a silicon heterojunction solar cell
A short video of the silicon heterojunction solar cell fabrication process at ASU is shown in the video below.
Silicon Solar Cells
The vast majority of today’s solar cells are made from silicon and offer both reasonable and good efficiency (the rate at which the solar cell converts sunlight into electricity). These cells are usually assembled into larger modules that can be installed on the roofs of residential or commercial buildings or deployed on ground-mounted racks to create huge, utility-scale systems.
Another commonly used photovoltaic technology is known as thin-film solar cells because they are made from very thin layers of semiconductor material, such as cadmium telluride or copper indium gallium diselenide. The thickness of these cell layers is only a few micrometers—that is, several millionths of a meter.
Thin-film solar cells can be flexible and lightweight, making them ideal for portable applications—such as in a soldier’s backpack—or for use in other products like Windows that generate electricity from the sun. Some types of thin-film solar cells also benefit from manufacturing techniques that require less energy and are easier to scale-up than the manufacturing techniques required by silicon solar cells.
III-V Solar Cells
A third type of photovoltaic technology is named after the elements that compose them. III-V solar cells are mainly constructed from elements in Group III—e.g., gallium and indium—and Group V—e.g., arsenic and antimony—of the periodic table. These solar cells are generally much more expensive to manufacture than other technologies. But they convert sunlight into electricity at much higher efficiencies. Because of this, these solar cells are often used on satellites, unmanned aerial vehicles, and other applications that require a high ratio of power-to-weight.
Solar cell researchers at NREL and elsewhere are also pursuing many new photovoltaic technologies—such as solar cells made from organic materials, quantum dots, and hybrid organic-inorganic materials (also known as perovskites). These next-generation technologies may offer lower costs, greater ease of manufacture, or other benefits. Further research will see if these promises can be realized.
Reliability and Grid Integration Research
Photovoltaic research is more than just making a high-efficiency, low-cost solar cell. Homeowners and businesses must be confident that the solar panels they install will not degrade in performance and will continue to reliably generate electricity for many years. Utilities and government regulators want to know how to add solar PV systems to the electric grid without destabilizing the careful balancing act between electricity supply and demand.
Materials scientists, economic analysts, electrical engineers, and many others at NREL are working to address these concerns and ensure solar photovoltaics are a clean and reliable source of energy.