solar cell | Definition, Working Principle, & Development

Solar cell

, also called

photovoltaic cell

, any device that directly converts the

energy

of

light

into electrical energy through the


photovoltaic effect

. The overwhelming majority of solar cells are fabricated from

silicon

—with increasing

efficiency

and lowering cost as the materials range from

amorphous

(noncrystalline) to polycrystalline to crystalline (single

crystal

) silicon forms. Unlike

batteries

or

fuel cells

, solar cells do not utilize

chemical reactions

or require fuel to produce

electric power

, and, unlike

electric generators

, they do not have any moving parts.

Solar cells can be arranged into large groupings called

arrays. These arrays, composed of many thousands of individual cells, can function as central electric power stations, converting sunlight into electrical energy for distribution to industrial, commercial, and residential users. Solar cells in much smaller configurations, commonly referred to as solar

cell

panels or simply solar panels, have been installed by homeowners on their rooftops to replace or augment their conventional electric supply. Solar cell panels also are used to provide electric

power

in many remote terrestrial locations where conventional electric power sources are either unavailable or prohibitively expensive to install. Because they have no moving parts that could need maintenance or fuels that would require replenishment, solar cells provide power for most space installations, from communications and weather

satellites

to

space stations

. (Solar power is insufficient for space probes sent to the outer planets of the

solar system

or into

interstellar space

, however, because of the

diffusion

of

radiant energy

with distance from the

Sun

.) Solar cells have also been used in consumer products, such as electronic toys, handheld

calculators

, and portable

radios

. Solar cells used in devices of this kind may utilize artificial light (e.g., from incandescent and fluorescent lamps) as well as sunlight.

International Space Station


International Space Station

The International Space Station (ISS) was built in sections beginning in 1998. By December 2000 the major elements of the partially completed station included the American-built connecting node Unity and two Russian-built units—Zarya, a power module, and Zvezda, the initial living quarters. A Russian spacecraft, which carried up the station’s first three-person crew, is docked at the end of Zvezda. The photograph was taken from the space shuttle Endeavour.

National Aeronautics and Space Administration

While total photovoltaic energy production is minuscule, it is likely to increase as

fossil fuel

resources shrink. In fact, calculations based on the world’s projected energy

consumption

by 2030 suggest that global energy demands would be fulfilled by solar panels operating at 20 percent efficiency and covering only about 496,805 square km (191,817 square miles) of Earth’s surface. The material requirements would be enormous but

feasible

, as silicon is the second most abundant element in Earth’s crust. These factors have led solar proponents to

envision

a future “solar economy” in which practically all of humanity’s energy requirements are satisfied by cheap, clean, renewable

sunlight

.

solar cell


solar cell

Learn about efforts to increase the efficiency of solar cells.

Contunico © ZDF Enterprises GmbH, Mainz

Solar cell structure and operation

Solar cells, whether used in a central power station, a


satellite

, or a calculator, have the same basic structure. Light enters the device through an optical coating, or

antireflection layer, that minimizes the loss of light by reflection; it effectively traps the light falling on the solar cell by promoting its transmission to the energy-conversion layers below. The antireflection layer is typically an oxide of

silicon

,

tantalum

, or

titanium

that is formed on the cell surface by spin-coating or a vacuum

deposition

technique.

solar energy; solar cell


solar energy; solar cell

A solar energy plant produces megawatts of electricity. Voltage is generated by solar cells made from specially treated semiconductor materials, such as silicon.

Courtesy of the National Renewable Energy Laboratory

The three energy-conversion layers below the antireflection layer are the

top junction layer, the

absorber layer, which

constitutes

the core of the device, and the

back junction layer. Two additional

electrical contact layers are needed to carry the electric current out to an external load and back into the cell, thus completing an

electric circuit

. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good

conductor

such as a metal. Since metal blocks light, the grid lines are as thin and widely spaced as is possible without impairing collection of the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer also must be a very good electrical conductor, it is always made of metal.

Since most of the energy in sunlight and artificial light is in the visible range of

electromagnetic radiation

, a solar cell absorber should be efficient in absorbing radiation at those wavelengths. Materials that strongly absorb visible radiation belong to a class of substances known as


semiconductors

. Semiconductors in thicknesses of about one-hundredth of a centimetre or less can absorb all incident visible light; since the junction-forming and contact layers are much thinner, the thickness of a solar cell is essentially that of the absorber. Examples of

semiconductor

materials employed in solar cells include silicon, gallium arsenide, indium phosphide, and copper indium selenide.

When light falls on a solar cell, electrons in the absorber layer are excited from a lower-energy “ground state,” in which they are bound to specific atoms in the solid, to a higher “excited state,” in which they can move through the solid. In the absence of the junction-forming layers, these “


free

” electrons are in random motion, and so there can be no oriented

direct current

. The addition of junction-forming layers, however, induces a

built-in

electric field

that produces the

photovoltaic effect

. In effect, the electric field gives a

collective

motion to the electrons that flow past the electrical contact layers into an external circuit where they can do useful work.

The materials used for the two junction-forming layers must be dissimilar to the absorber in order to produce the built-in electric field and to carry the electric current. Hence, these may be different semiconductors (or the same semiconductor with different types of conduction), or they may be a metal and a semiconductor. The materials used to construct the various layers of solar cells are essentially the same as those used to produce the

diodes

and

transistors

of solid-state

electronics

and microelectronics (

see also


electronics: Optoelectronics

). Solar cells and microelectronic devices share the same basic

technology

. In solar cell fabrication, however, one seeks to construct a large-area device because the power produced is proportional to the

illuminated

area. In microelectronics the goal is, of course, to construct electronic components of ever smaller dimensions in order to increase their density and operating speed within semiconductor chips, or

integrated circuits

.

The photovoltaic process bears certain similarities to

photosynthesis

, the process by which the energy in light is converted into

chemical energy

in plants. Since solar cells obviously cannot produce electric power in the dark, part of the energy they develop under light is stored, in many applications, for use when light is not available. One common means of storing this electrical energy is by charging electrochemical storage batteries. This sequence of converting the energy in light into the energy of excited electrons and then into stored chemical energy is strikingly similar to the process of

photosynthesis

.

Solar panel design

Most solar cells are a few square centimetres in area and protected from the

environment

by a thin coating of

glass

or transparent

plastic

. Because a typical 10 cm × 10 cm (4 inch × 4 inch) solar cell generates only about two watts of electrical power (15 to 20 percent of the energy of light incident on their surface), cells are usually combined in series to boost the voltage or in parallel to increase the current. A solar, or photovoltaic (PV), module generally consists of 36 interconnected cells laminated to glass within an aluminum frame. In turn, one or more of these modules may be wired and framed together to form a solar panel. Solar panels are slightly less efficient at

energy conversion

per surface area than individual cells, because of inevitable inactive areas in the assembly and cell-to-cell variations in performance. The back of each solar panel is equipped with standardized sockets so that its output can be combined with other solar panels to form a solar array. A complete photovoltaic system may consist of many solar panels, a power system for accommodating different electrical loads, an external circuit, and storage batteries. Photovoltaic systems are broadly classifiable as either stand-alone or grid-connected systems.

solar cell


solar cell

A scientist examines a sheet of polymer solar cells, which are more lightweight, more flexible, and cheaper than traditional silicon solar cells.

Patrick Allard—REA/Redux

Stand-alone systems contain a solar array and a bank of batteries directly wired to an application or load circuit. A

battery

system is essential to compensate for the absence of any electrical output from the cells at night or in overcast conditions; this adds considerably to the overall cost. Each battery stores

direct current

(DC)

electricity

at a fixed voltage determined by the panel specifications, although load requirements may differ. DC-to-DC converters are used to provide the voltage levels demanded by DC loads, and DC-to-AC inverters supply power to

alternating current

(AC) loads. Stand-alone systems are ideally suited for remote installations where linking to a central power station is prohibitively expensive. Examples include pumping water for feedstock and providing electric power to lighthouses, telecommunications repeater stations, and mountain lodges.

Grid-connected systems

integrate

solar arrays with

public utility

power grids in two ways. One-way systems are used by utilities to supplement power grids during midday peak usage. Bidirectional systems are used by companies and individuals to supply some or all of their power needs, with any excess power fed back into a utility power grid. A major advantage of grid-connected systems is that no storage batteries are needed. The corresponding reduction in capital and maintenance costs is offset, however, by the increased complexity of the system. Inverters and additional protective gear are needed to interface low-voltage DC output from the solar array with a high-voltage AC power grid. Additionally, rate structures for reverse metering are necessary when residential and industrial solar systems feed energy back into a utility grid.

A grid-connected solar cell system.

A grid-connected solar cell system.

Encyclopædia Britannica, Inc.

The simplest deployment of solar panels is on a tilted support frame or rack known as a fixed mount. For maximum efficiency, a fixed mount should face south in the Northern Hemisphere or north in the Southern Hemisphere, and it should have a tilt angle from horizontal of about 15 degrees less than the local latitude in summer and 25 degrees more than the local latitude in winter. More complicated deployments involve motor-driven tracking systems that continually reorient the panels to follow the daily and seasonal movements of the Sun. Such systems are justified only for large-scale utility generation using high-efficiency concentrator solar cells with

lenses

or parabolic mirrors that can intensify

solar radiation

a hundredfold or more.

Although sunlight is free, the cost of materials and available space must be considered in designing a solar system; less-efficient solar panels imply more panels, occupying more space, in order to produce the same amount of electricity. Compromises between cost of materials and efficiency are particularly evident for space-based solar systems. Panels used on satellites have to be extra-rugged, reliable, and resistant to

radiation damage

encountered in Earth’s upper

atmosphere

. In addition, minimizing the liftoff weight of these panels is more critical than fabrication costs. Another factor in solar panel design is the ability to fabricate cells in “thin-film” form on a variety of substrates, such as glass, ceramic, and plastic, for more flexible deployment. Amorphous silicon is very attractive from this viewpoint. In particular, amorphous silicon-coated roof tiles and other photovoltaic materials have been introduced in architectural design and for recreational vehicles, boats, and automobiles.

thin-film solar cell


thin-film solar cell

Thin-film solar cells, such as those used in solar panels, convert light energy into electrical energy.

Anson Lu—Panther Media/age fotostock

Development of solar cells

The development of solar cell technology stems from the work of French physicist

Antoine-César Becquerel in 1839. Becquerel discovered the


photovoltaic effect

while experimenting with a solid

electrode

in an

electrolyte

solution; he observed that voltage developed when light fell upon the electrode. About 50 years later,


Charles Fritts

constructed the first true solar cells using junctions formed by coating the semiconductor

selenium

with an ultrathin, nearly transparent layer of

gold

. Fritts’s devices were very inefficient converters of energy; they transformed less than 1 percent of absorbed light energy into electrical energy. Though inefficient by today’s standards, these early solar cells fostered among some a vision of abundant, clean power. In 1891 R. Appleyard wrote of

the blessed vision of the Sun, no longer pouring his energies unrequited into space, but by means of photo-electric cells…, these powers gathered into electrical storehouses to the total extinction of steam engines, and the utter repression of smoke.

By 1927 another metal-semiconductor-junction solar cell, in this case made of

copper

and the semiconductor copper oxide, had been demonstrated. By the 1930s both the

selenium cell

and the copper oxide cell were being employed in light-sensitive devices, such as

photometers

, for use in photography. These early solar cells, however, still had energy-conversion

efficiencies

of less than 1 percent. This impasse was finally overcome with the development of the

silicon solar cell by

Russell Ohl in 1941. Thirteen years later, aided by the rapid commercialization of silicon technology needed to fabricate the

transistor

, three other American researchers—Gerald Pearson, Daryl Chapin, and Calvin Fuller—demonstrated a silicon solar cell capable of a 6 percent energy-conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as well as cells made of

gallium

arsenide, with efficiencies of more than 20 percent had been fabricated. In 1989 a

concentrator solar cell in which sunlight was concentrated onto the cell surface by means of lenses achieved an efficiency of 37 percent owing to the increased intensity of the collected energy. By connecting cells of different semiconductors optically and electrically in series, even higher efficiencies are possible, but at increased cost and added complexity. In general, solar cells of widely varying efficiencies and cost are now available.


Stephen Joseph Fonash


Raymond T. Fonash


S. Ashok

Power Efficiency Guide, Solar Power Efficiency Guide, Solar Power Guide

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