, also called
, any device that directly converts the
into electrical energy through the
. The overwhelming majority of solar cells are fabricated from
and lowering cost as the materials range from
(noncrystalline) to polycrystalline to crystalline (single
) silicon forms. Unlike
, solar cells do not utilize
or require fuel to produce
, and, unlike
, 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
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
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
. (Solar power is insufficient for space probes sent to the outer planets of the
, however, because of the
with distance from the
.) Solar cells have also been used in consumer products, such as electronic toys, handheld
, and portable
. 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
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
resources shrink. In fact, calculations based on the world’s projected energy
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
, as silicon is the second most abundant element in Earth’s crust. These factors have led solar proponents to
a future “solar economy” in which practically all of humanity’s energy requirements are satisfied by cheap, clean, renewable
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
, 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
that is formed on the cell surface by spin-coating or a vacuum
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
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
. 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
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
, 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 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
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 “
” electrons are in random motion, and so there can be no oriented
. The addition of junction-forming layers, however, induces a
that produces the
. In effect, the electric field gives a
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
and microelectronics (
). Solar cells and microelectronic devices share the same basic
. In solar cell fabrication, however, one seeks to construct a large-area device because the power produced is proportional to the
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
The photovoltaic process bears certain similarities to
, the process by which the energy in light is converted into
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
Solar panel design
Most solar cells are a few square centimetres in area and protected from the
by a thin coating of
. 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
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.
A scientist examines a sheet of polymer solar cells, which are more lightweight, more flexible, and cheaper than traditional silicon solar cells.
Stand-alone systems contain a solar array and a bank of batteries directly wired to an application or load circuit. A
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
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
(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.
solar arrays with
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.
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
or parabolic mirrors that can intensify
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
encountered in Earth’s upper
. 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 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
while experimenting with a solid
solution; he observed that voltage developed when light fell upon the electrode. About 50 years later,
constructed the first true solar cells using junctions formed by coating the semiconductor
with an ultrathin, nearly transparent layer of
. 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
and the semiconductor copper oxide, had been demonstrated. By the 1930s both the
and the copper oxide cell were being employed in light-sensitive devices, such as
, for use in photography. These early solar cells, however, still had energy-conversion
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
, 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
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