A solar cell (also called photovoltaic cell or photoelectric cell) is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect.
Assemblies of cells used to make solar modules which are used to capture energy from sunlight, are known as solar panels. The energy generated from these solar modules, referred to as solar power, is an example of solar energy.
Photovoltaics is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight.
Cells are described as photovoltaic cells when the light source is not necessarily sunlight. These are used for detecting light or other electromagnetic radiation near the visible range, for example infrared detectors, or measurement of light intensity.
The term "photovoltaic" comes from the Greek φῶς (phōs) meaning "light", and "voltaic", from the name of the Italian physicist Volta, after whom a unit of electro-motive force, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.
The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. However, it was not until 1883 that the first photovoltaic cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov built the first photoelectric cell (based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel prize in Physics in 1921. Russell Ohl patented the modern junction semiconductor solar cell in 1946, which was discovered while working on the series of advances that would lead to the transistor.
The modern photovoltaic cell was developed in 1954 at Bell Laboratories. The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a diffused silicon p-n junction. At first, cells were developed for toys and other minor uses, as the cost of the electricity they produced was very high - in relative terms, a cell that produced 1 watt of electrical power in bright sunlight cost about $250, comparing to $2 to $3 for a coal plant.
Solar cells were rescued from obscurity by the suggestion to add them to the Vanguard I satellite. In the original plans, the satellite would be powered only by battery, and last a short time while this ran down. By adding cells to the outside of the fuselage, the mission time could be extended with no major changes to the spacecraft or its power systems. There was some skepticism at first, but in practice the cells proved to be a huge success, and solar cells were quickly designed into many new satellites, notably Bell's own Telstar.
Improvements were slow over the next two decades, and the only widespread use was in space applications where their power-to-weight ratio was higher than any competing technology. However, this success was also the reason for slow progress; space users were willing to pay anything for the best possible cells, there was no reason to invest in lower-cost solutions if this would reduce efficiency. Instead, the price of cells was determined largely by the semiconductor industry; their move to integrated circuits in the 1960s led to the availability of larger boules at lower relative prices. As their price fell, the price of the resulting cells did as well. However these effects were limited, and by 1971 cell costs were estimated to be $100 a watt.
In the late 1960s, Elliot Berman was investigating a new method for producing the silicon feedstock in a ribbon process. However, he found little interest in the project and was unable to gain the funding needed to develop it. In a chance encounter, he was later introduced to a team at Exxon who were looking for projects 30 years in the future. The group had concluded that electrical power would be much more expensive by 2000, and felt that this increase in price would make new alternative energy sources more attractive, and solar was the most interesting among these. In 1969, Berman joined the Linden, New Jersey Exxon lab, Solar Power Corporation (SPC).
His first major effort was to canvas the potential market to see what possible uses for a new product were, and they quickly found that if the dollars per watt was reduced from then-current $100/watt to about $20/watt there was significant demand. Knowing that his ribbon concept would take years to develop, the team started looking for ways to hit the $20 price point using existing materials.
The first improvement was the realization that the existing cells were based on standard semiconductor manufacturing process, even though that was not ideal. This started with the boule, cutting it into disks called wafers, polishing the wafers, and then, for cell use, coating them with an anti-reflective layer. Berman noted that the rough-sawn wafers already had a perfectly suitable anti-reflective front surface, and by printing the electrodes directly on this surface, two major steps in the cell processing were eliminated. The team also explored ways to improve the mounting of the cells into arrays, eliminating the expensive materials and hand wiring used in space applications with a printed circuit board on the back, acrylic plastic on the front, and silicone based glue between the two, potting the cells. But the largest improvement in price point was Berman's realization that existing silicon was effectively "too good" for solar cell use; the minor imperfections that would ruin a boule (or individual wafer) for electronics would have little effect in the solar application.
Putting all of these changes into practice, the company started buying up "reject" silicon from existing manufacturers at very low cost. By using the largest wafers available, thereby reducing the amount of wiring for a given panel area, and packaging them into panels using their new methods, by 1973 SPC was producing panels at $10 per Watt and selling them at $20 per Watt, a fivefold decrease in prices in two years.
SPC approached companies making buoys as a natural market for their products, but found a curious situation. The primary company in the business was Automatic Power, a battery manufacturer. Realizing that solar cells might eat into their battery profits, Automatic purchased the rights to earlier solar cell designs and suppressed them. Seeing there was no interest there, SPC turned to Tideland Signal, another battery company formed by ex-Automatic managers. Tideland introduced a solar-powered buoy and was soon ruining Automatic's business.
The timing could not be better; the rapid increase in the number of offshore oil platforms and loading facilities produced an enormous market among the oil companies. As Tideland's fortunes improved, Automatic started looking for their own supply of solar panels. They found Bill Yerks of Solar Power International (SPI) in California, who was looking for a market. SPI was soon bought out by one of its largest customers, the ARCO oil giant, forming ARCO Solar. ARCO Solar's factory in Camarillo, California was the first dedicated to building solar panels, and has been in continual operation from its purchase by ARCO in 1977 to this day.
This market, combined with the 1973 oil crisis, led to a curious situation. Oil companies were now cash-flush due to their huge profits during the crisis, but were also acutely aware that their future success would depend on some other form of power. Over the next few years, the major oil companies started a number of solar firms, and were for decades the largest producers of solar panels. Exxon, ARCO, Shell, Amoco (later purchased by BP) and Mobil all had major solar divisions during the 1970s and 80s. Technology companies also had some investment, including General Electric, Motorola, IBM, Tyco and RCA.
In the time since Berman's work, improvements have brought production costs down under $1 a watt, with wholesale costs on the order of $2. "Balance of system" costs are now more than the panels themselves, with large commercial arrays falling to around $5 a watt, fully commissioned, in 2010.
As the semiconductor industry moved to ever-larger boules, older equipment became available at fire-sale prices. Cells have grown in size as older equipment became available on the surplus market; ARCO Solar's original panels used cells with 2 to 4 inch diameter. Panels in the 1990s and early 2000s generally used 5 inch wafers, and since 2008 almost all new panels use 6 inch cells. Another major change was the move to polycrystalline silicon. This material has less efficiency, but is less expensive to produce in bulk. The widespread introduction of flat screen televisions in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, used on the front of the panels.
Other technologies have also come to market. First Solar has grown to become the largest panel manufacturer, in terms of yearly power produced, using a thin-film cell sandwiched between two layers of glass. This was the first product to beat $1 a watt for production costs. Since then a glut of polycrystalline silicon has pushed prices of conventional panels into the same range.
Solar cell development is often considered to have taken place in three successive generations, although one of them, the third, is still undergoing research and is not fully developed. The two previous generations are still in use and are also being developed further.
The first generation technologies are the most commonly used ones in commercial production and account for nearly 90% of all cells produced. They are often described as high-cost and high-efficiency. They involve high energy and labor inputs, which has prevented major progress in reducing production costs.
These solar cells are manufactured from silicon semiconductors and use a single junction for extracting energy from photons. They are approaching the theoretical limiting efficiency of 33% and achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years. Nevertheless, due to very capital intensive production, it is generally not thought that first generation cells will be able to provide energy more cost effective than fossil fuel sources.
The second generation of solar cells has been under intense development for the 1990’s and 2000’s. They are often described as low-cost and low-efficiency cells. Second generation materials have been specifically developed to address energy requirements and production costs of first generation cells. These include copper-indium-gallium-selenide, cadmium-telluride, amorphous silicon and micromorphous silicon. Alternative manufacturing techniques such as vapor deposition, electroplating, and use of ultrasonic nozzles are used to reduce needs for energy-intensive production processes significantly.
A commonly cited example of second generation cells are printed cells that can be produced at an extremely fast rate. Though these cells have only 10-15% conversion efficiency, the decreased costs mean that, per unit of energy produced, the tradeoff is favorable. Second generation technologies have been gaining market share since 2008 and it is thought that second generation solar cells will surpass first generation cells in market share sometime during the 2010’s. Second generation solar cells have the potential to become more cost effective than fossil fuels.
Third generation solar cells are currently just being researched. No actual products exist yet. Third generation technologies aim to combine the high electrical performance of the first generation with the low production costs of the second generation. The goal is thin-film cells that obtain efficiencies in the range of 30-60% by using new technologies. Some say that third generation cells could start to be commercialized sometime around 2020, but it is too early to say for sure. Technologies associated with third generation solar cells include multijunction photovoltaic cells, tandem cells, nanostructured cells for improved incident light usage and even infrared collection during night, and excess thermal generation caused by UV light to enhance voltages or carrier collection.
Solar cells are often electrically connected and encapsulated as a module. Photovoltaic modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the semiconductor wafers from abrasion and impact due to wind-driven debris, rain, hail, et cetera. Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher current. Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current.
To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected photovoltaic systems); in stand-alone systems, batteries are used to store the energy that is not needed immediately. Solar panels can be used to power or recharge portable devices.
The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies.
Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency, integrated quantum efficiency, VOC ratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under "external quantum efficiency". Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio.
Crystalline silicon devices are now approaching the theoretical limiting efficiency of 29%.
Different materials display different efficiencies and have different costs. Materials for efficient solar cells must have characteristics matched to the spectrum of available light. Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms.
Materials presently used for photovoltaic solar cells include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulfide.
Many currently available solar cells are made from bulk materials that are cut into wafers between 180 to 240 micrometers thick that are then processed like other semiconductors.
Other materials are made as thin-films layers, organic dyes, and organic polymers that are deposited on supporting substrates. A third group are made from nanocrystals and used as quantum dots (electron-confined nanoparticles). Silicon remains the only material that is well-researched in both bulk and thin-film forms.