In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes). (US Dept. of Energy)


A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron. (US Dept. of Energy)

Activated shelf life

The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level. (US Dept. of Energy)

Activation voltage(s)

The voltage(s) at which a charge controller will take action to protect the batteries. (US Dept. of Energy)

Adjustable set point

A feature allowing the user to adjust the voltage levels at which a charge controller will become active. (US Dept. of Energy)


See amperage interrupt capability. (US Dept. of Energy)

Air mass (sometimes called air mass ratio)

Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness). (US Dept. of Energy)

Ambient temperature

The temperature of the surrounding area. (US Dept. of Energy)

Amorphous semiconductor

A non-crystalline semiconductor material that has no long-range order. (US Dept. of Energy)

Amorphous silicon

A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon a polycrystalline silicon. (US Dept. of Energy)

Amperage interrupt capability (AIC)

Direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current. (US Dept. of Energy)

Angle of incidence

The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.(US Dept. of Energy)


The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode. (US Dept. of Energy)

Antireflection coating

A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission. (US Dept. of Energy)


See photovoltaic (PV) array. (US Dept. of Energy)

Array current

The electrical current produced by a photovoltaic array when it is exposed to sunlight. (US Dept. of Energy)

Array operating voltage

The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load. (US Dept. of Energy)

Azimuth angle

The angle between true south and the point on the horizon directly below the sun.

Barrier energy

The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier. (US Dept. of Energy)


See building integrated photovoltaics. (US Dept. of Energy)

Blocking diode

A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards. (US Dept. of Energy)

Boron (B)

The chemical element commonly used as the dopant in photovoltaic device or cell material. (US Dept. of Energy)


A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth. (US Dept. of Energy)

Building integrated photovoltaics

A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems. (US Dept. of Energy)

Bypass diode

A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light. (US Dept. of Energy)

Cadmium (Cd)

A chemical element used in making certain types of solar cells and batteries. (US Dept. of Energy)

Cadmium telluride (CdTe)

A polycrystalline thin-film photovoltaic material. (US Dept. of Energy)

Cell (battery)

A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). See also photovoltaic (PV) cell. (US Dept. of Energy)

Cell barrier

A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge. (US Dept. of Energy)

Cell junction

The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone. (US Dept. of Energy)

Charge controller

A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status. (US Dept. of Energy)

Charge factor

A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate. (US Dept. of Energy)

Chemical vapor deposition (CVD)

A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate. (US Dept. of Energy)

Cloud enhancement

The increase in solar intensity caused by reflected irradiance from nearby clouds. (US Dept. of Energy)

Concentrating photovoltaics (CPV)

A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells. (US Dept. of Energy)

Concentrating solar power (CSP)

A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator. (US Dept. of Energy)


A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times. (US Dept. of Energy)

Conduction band (or conduction level)

An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge. (US Dept. of Energy)

Crystalline silicon

A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon. (US Dept. of Energy)

Cutoff voltage

The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery. (US Dept. of Energy)

Dangling bonds

A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface. (US Dept. of Energy)

Dendritic web technique

A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools. (US Dept. of Energy)

Diffuse insolation

Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation. (US Dept. of Energy)

Diffuse radiation

Radiation received from the sun after reflection and scattering by the atmosphere and ground. (US Dept. of Energy)

Diffusion furnace

Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material. (US Dept. of Energy)

Direct insolation

Sunlight falling directly upon a collector. Opposite of diffuse insolation. (US Dept. of Energy)

Distributed generation

Localized or on-site power generation. (US Dept. of Energy)

Distributed power

Generic term for any power supply located near the point where the power is used. Opposite of central power. See also stand-alone systems. (US Dept. of Energy)

Distributed systems

Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system. (US Dept. of Energy)


A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell. (US Dept. of Energy)

Electron hole pair

The result of light of sufficient energy dislodging an electron from its bond in a crystal, which creates a hole. The free electron (negative charge) and the hole (positive charge) are a pair. These pairs are the constituents of electricity. (US Dept. of Energy)

Epitaxial growth

The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal. (US Dept. of Energy)


The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox). (US Dept. of Energy)

Fill factor

The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance. (US Dept. of Energy)

Fixed tilt array

A photovoltaic array set in at a fixed angle with respect to horizontal. (US Dept. of Energy)

Flat-plate array

A photovoltaic (PV) array that consists of non-concentrating PV modules. (US Dept. of Energy)

Flat-plate module

An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight. (US Dept. of Energy)

Flat-plate photovoltaics (PV)

A PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array. (US Dept. of Energy)

Fresnel lens

An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point. (US Dept. of Energy)

Full sun

The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter). (US Dept. of Energy)

Guarantee of Origin

An electronic certificate representing 1 MWh of electricity production used for the purpose of proving to final customers the share or quantity of renewable energy that was supplied to them. (Directive 2009/28/ec)

III-V cell

A high-efficiency solar cell made from materials including Group III and Group V elements from the periodic table. (US Dept. of Energy)

Incident light

Light that shines onto the face of a solar cell or module. (US Dept. of Energy)

Infrared radiation

Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light. (US Dept. of Energy)


A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell. (US Dept. of Energy)


The solar power density incident on a surface of stated area and orientation.


A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells. (US Dept. of Energy)

Intrinsic layer

A layer of semiconductor material, used in a photovoltaic device, whose properties are essentially those of the pure, undoped, material. (US Dept. of Energy)

Inverted metamorphic multijunction (IMM) cell

A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency. (US Dept. of Energy)


A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid. (US Dept. of Energy)


An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged. (US Dept. of Energy)

Junction box

A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary. (US Dept. of Energy)

Junction diode

A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes. (US Dept. of Energy)


The width of a cut used to create wafers from silicon ingots, often resulting in the loss of semiconductor material. (US Dept. of Energy)

Langley (L)

Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2. (US Dept. of Energy)


The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material. (US Dept. of Energy)


The period during which a system is capable of operating above a specified performance level. (US Dept. of Energy)

Life-cycle cost

The estimated cost of owning and operating a photovoltaic system for the period of its useful life. (US Dept. of Energy)

Light trapping

The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers. (US Dept. of Energy)

Light-induced defects

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light. (US Dept. of Energy)


A small groove scribed into the surface of a solar cell, which is filled with metal for contacts. (US Dept. of Energy)


The use of multiple inverters connected in parallel to service different loads. (US Dept. of Energy)

Module derate factor

A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module. (US Dept. of Energy)


Solar silicon solidified into a uniform, cylinder-shaped crystal as a result of conditions during the crystallization process. (Deutsche Solar)


Fabricated as a single structure. (US Dept. of Energy)


The simplest form of silane (silicon analogues of alkane hydrocarbons). It is used in solar silicon production. (Deutsche Solar)


A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline. (US Dept. of Energy)


Negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons. (US Dept. of Energy)

N-type semiconductor

A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity (e.g., phosphorus in silicon).

N-type silicon

Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon. (US Dept. of Energy)

One-axis tracking

A system capable of rotating about one axis. (US Dept. of Energy)


Solar power systems which are connected to a regional electricity grid. When a large amount of electricity is produced due to optimum levels of sunlight, electricity is fed into the grid; electricity can be drawn from the grid if necessary (when it is dark). (Deutsche Solar)

Open-circuit voltage (Voc)

The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing. (US Dept. of Energy)

Operating point

The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array. (US Dept. of Energy)

Parallel connection

A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage. (US Dept. of Energy)

Peak sun hours

The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2. (US Dept. of Energy)

Peak watt

A unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified. (US Dept. of Energy)


An electric current induced by radiant energy. (US Dept. of Energy)

Photoelectric cell

A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers. (US Dept. of Energy)

Photoelectrochemical cell

A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use. (US Dept. of Energy)

Photovoltaic (PV)

Pertaining to the direct conversion of light into electricity. (US Dept. of Energy)

Photovoltaic (PV) array

An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module. (US Dept. of Energy)

Photovoltaic (PV) cell

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell. (US Dept. of Energy)

Photovoltaic (PV) conversion efficiency

The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device. (US Dept. of Energy)

Photovoltaic (PV) device

A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms. (US Dept. of Energy)

Photovoltaic (PV) effect

The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available. (US Dept. of Energy)

Photovoltaic (PV) generator

The total of all PV strings of a PV power supply system, which are electrically interconnected. (US Dept. of Energy)

Photovoltaic (PV) module

The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). (US Dept. of Energy)

Photovoltaic (PV) panel

Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current). (US Dept. of Energy)

Photovoltaic (PV) system

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components. (US Dept. of Energy)

Photovoltaic-thermal (PV/T) system

A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system or solar thermal system. (US Dept. of Energy)

Physical vapor deposition

A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate. (US Dept. of Energy)


A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices. (US Dept. of Energy)

Plug-and-play PV system

A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation. (US Dept. of Energy)

Point-contact cell

A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection. (US Dept. of Energy)


See multicrystalline. (US Dept. of Energy)

Polycrystalline silicon

A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon. (US Dept. of Energy)

Polycrystalline thin film

A thin film made of multicrystalline material. (US Dept. of Energy)

P-type semiconductor

A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon). (US Dept. of Energy)


See photovoltaic(s).


An instrument used for measuring global solar irradiance. (US Dept. of Energy)


An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc. (US Dept. of Energy)

Quantum efficiency (QE)

The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight. (US Dept. of Energy)

Rankine cycle

A thermodynamic cycle used in steam turbines to convert heat energy into work. Concentrating solar power plants often rely on the Rankine cycle. In CSP systems, mirrors focus sunlight on a heat-transfer fluid. This is used to creates steam, which spins a turbine to generate electricity. (US Dept. of Energy)


A device that converts alternating current to direct current. See also inverter. (US Dept. of Energy)

Remote systems

See stand-alone systems. (US Dept. of Energy)

Resistive voltage drop

The voltage developed across a cell by the current flow through the resistance of the cell. (US Dept. of Energy)

Reverse current protection

Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode. (US Dept. of Energy)

Ribbon (photovoltaic) cells

A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material. (US Dept. of Energy)

Sacrificial anode

A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure. (US Dept. of Energy)

Schottky barrier

A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal. (US Dept. of Energy)

Secondary silicon

Recycled silicon. (Deutsche Solar)


Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process. (US Dept. of Energy)


See multicrystalline. (US Dept. of Energy)

Series connection

A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage. (US Dept. of Energy)

Series controller

A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery. (US Dept. of Energy)

Series regulator

Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array. (US Dept. of Energy)

Series resistance

Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections. (US Dept. of Energy)

Shunt controller

A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less. (US Dept. of Energy)

Siemens process

A commercial method of making purified silicon. (US Dept. of Energy)

Silicon (Si)

A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide). (US Dept. of Energy)

Sine wave

A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle. (US Dept. of Energy)

Sine wave inverter

An inverter that produces utility-quality, sine wave power forms. (US Dept. of Energy)

Single-crystal material

A material that is composed of a single crystal or a few large crystals. (US Dept. of Energy)

Single-crystal silicon

Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon. (US Dept. of Energy)

Soft costs

Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection. (US Dept. of Energy)

Solar cell

See photovoltaic (PV) cell.

Solar constant

The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 watts per square meter or 492 Btu per square foot. (US Dept. of Energy)

Solar cooling

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners. (US Dept. of Energy)

Solar energy

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners. (US Dept. of Energy)

Solar noon

The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky. (US Dept. of Energy)

Solar resource

The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours. (US Dept. of Energy)

Solar spectrum

The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation. (US Dept. of Energy)

Solar thermal electric systems

Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough. (US Dept. of Energy)

Solar-grade silicon

Silicon crystals which have a sufficiently high grade of purity to be used in photovoltaic applications. The chemical element silicon is a semi-conductor and forms crystals with a resistant diamond structure. It is the second most common element in the earth's crust after oxygen. For processing in the solar sector, raw silicon is purified to achieve solar-grade silicon and then cast into blocks for further processing into wafers. (Deutsche Solar)

Split-spectrum cell

A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See also mulitjunction device. (US Dept. of Energy)


A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate. (US Dept. of Energy)

Square wave

A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics. (US Dept. of Energy)

Square wave inverter

A type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency. (US Dept. of Energy)

Staebler-Wronski effect

The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light. (US Dept. of Energy)

Stand-alone system

An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage. (US Dept. of Energy)

Standard reporting conditions (SRC)

A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions. (US Dept. of Energy)

Standard test conditions (STC)

Conditions under which a module is typically tested in a laboratory. (US Dept. of Energy)

Standby current

This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low. (US Dept. of Energy)

Stand-off mounting

Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle. (US Dept. of Energy)


A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load. (US Dept. of Energy)


The physical material upon which a photovoltaic cell is applied. (US Dept. of Energy)


Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load). (US Dept. of Energy)


The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero. (US Dept. of Energy)


The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum. (US Dept. of Energy)

Temperature factors

It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures. (US Dept. of Energy)

Thermophotovoltaic cell (TPV)

A device where sunlight concentrated onto an absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation. (US Dept. of Energy)

Thick-crystalline materials

Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons. (US Dept. of Energy)

Thin film

A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells. (US Dept. of Energy)

Thin film photovoltaic module

A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See also amorphous silicon. (US Dept. of Energy)

Tilt angle

The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection. (US Dept. of Energy)

Tin oxide

A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass. (US Dept. of Energy)

Total internal reflection

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor. (US Dept. of Energy)

Tracking array

A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy. (US Dept. of Energy)

Transparent conducting oxide (TCO)

A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials. (US Dept. of Energy)

Two-axis tracking

A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal). (US Dept. of Energy)


Electromagnetic radiation in the wavelength range of 4 to 400 nanometers. (US Dept. of Energy)

Underground feeder (UF)

May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures. (US Dept. of Energy)

Vacuum evaporation

The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum. (US Dept. of Energy)


A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground. (US Dept. of Energy)


A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot. (US Dept. of Energy)


A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath. (US Dept. of Energy)

Zenith angle

The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead). (US Dept. of Energy)