A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 volts open-circuit (VOC). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 volts per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (Isc). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This output can be up to 70% of the VOC x ISC product. The short-circuit current (Isc) from a cell is nearly proportional to the illumination, while the open-circuit voltage (VOC) may drop only 10% with a 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 VOC at 1/2 ISC. The usable power output could thus drop from 70% of the VOC x ISC product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as VOC x ISC, without giving load curves, can be seriously distorting their actual performance.
A solar cell may operate over a wide range of (V) and (I). By increasing the resistive load on an irradiated cell continuously from zero (a ) to a very high value (an ) one can determine the point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes).
The threshold frequency is different for different materials. It is for alkali metals, near-ultraviolet light for other metals, and extreme-ultraviolet radiation for nonmetals. The photoelectric effect occurs with photons having energies from a few electronvolts to over 1 MeV. At the high photon energies comparable to the electron rest energy of 511 keV, Compton scattering may occur pair production may take place at energies over 1.022 MeV.
A solar cell is a device that converts the energy of sunlight directly into by the . Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight such as solar panels and solar cells, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make , , or . is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of (also known as ).
The term "photovoltaic" comes from the φῶς (phōs) meaning "light", and "voltaic", meaning electric, from the name of the physicist , after whom a unit of electro-motive force, the , is named. The term "photo-voltaic" has been in use in English since 1849.
Tiny -sized photovoltaic cells (from 14 to 20 thick) could have intelligent controls, and even storage built in at the chip level. Glitter photovoltaic cells use 100 times less silicon to generate the same amount of electricity. They have 14.9 percent efficiency and commercial range from 13 to 20 percent efficient.
Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si.
These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.
High-efficiency multijunction cells were originally developed for special applications such as and , but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. These multijunction cells consist of multiple thin films produced using . A triple-junction cell, for example, may consist of the semiconductors: , , and . Each type of semiconductor will have a characteristic energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
When gallium is substituted for some of the indium in CIS, the material is referred to as CIGS, or , a solid mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical formula CuInxGa(1-x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction (see under ), these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light or by using multi-junction tandem solar cells. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage, but decreasing the short circuit current. In another point of view, gallium is added to replace indium due to gallium's relative availability to indium. Approximately 70% of indium currently produced is used by the flat-screen monitor industry. However, the atomic ratio for Ga in the >19% efficient CIGS solar cells is ~7%, which corresponds to a bandgap of ~1.15 eV. CIGS solar cells with higher Ga amounts have lower efficiency. For example, CGS solar cells (which have a bandgap of ~1.7 eV have a record efficiency of 9.5% for pure CGS and 10.2% for surface-modified CGS. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2 GW of CIGS cells (roughly the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2004. For comparison, silicon solar cells used up 33% of the world's electronic grade silicon production in 2006.
use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. High concentration means a hundred or more times direct sunlight is focused when compared with crystalline silicon panels. Most commercial producers are developing systems that concentrate between 400 and 1000 suns. All concentration systems need a one axis or more often two axis tracking system for high precision, since most systems only use direct sunlight and need to aim at the sun with errors of less than 3 degrees. The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of high efficiency photovoltaic cells. Despite the advantages of CPV technologies their application has been limited by the costs of focusing, sun tracking and cooling equipment. On October 25, 2006, the federal government and the state government together with photovoltaic technology company announced a project using this technology, , planned to come online in 2008 and be completed by 2013. This plant, at 154 MW, would be ten times larger than the largest current photovoltaic plant in the world.