How Do Photovoltaics Work
Content
HOW DO P HOTOVOLTAICS W ORK?
BY
G IL K NIER
Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials
exhibit a property known as the photoelectric effect that causes them to absorb photons of light
and release electrons. When these free electrons are captured, an electric current results that
can be used as electricity.
The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who
found that certain materials would produce small amounts of electric current when exposed to
light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which
photovoltaic technology is based, for which he later won a Nobel prize in physics. The first
photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and
was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the
space industry began to make the first serious use of the technology to provide power aboard
spacecraft. Through the space programs, the technology advanced, its reliability was
established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic
technology gained recognition as a source of power for non-space applications.
The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell.
Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the
microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form
an electric field, positive on one side and negative on the other. When light energy strikes the
solar cell, electrons are knocked loose from the atoms in the semiconductor material. If
electrical conductors are attached to the positive and negative sides, forming an electrical
circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This
electricity can then be used to power a load, such as a light or a tool.
A number of solar cells electrically connected to each other and mounted in a support structure
or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain
voltage, such as a common 12 volts system. The current produced is directly dependent on how
much light strikes the module.
Multiple modules can be wired together to form an array. In general, the larger the area of a
module or array, the more electricity that will be produced. Photovoltaic modules and arrays
produce direct-current (dc) electricity. They can be connected in both series and parallel
electrical arrangements to produce any required voltage and current combination. Depending
on the size of the installation, multiple strings of solar photovoltaic array cables terminate in one
electrical box, called a fused array combiner. Contained within the combiner box are fuses
designed to protect the individual module cables, as well as the connections that deliver power
to the inverter. The electricity produced at this stage is DC (direct current) and must be
converted to AC (alternating current) suitable for use in your home or business.
Today's most common PV devices use a single junction, or interface, to create an electric field
within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose
energy is equal to or greater than the band gap of the cell material can free an electron for an
electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the
portion of the sun's spectrum whose energy is above the band gap of the absorbing material,
and lower-energy photons are not used.
One way to get around this limitation is to use two (or more) different
cells, with more than one band gap and more than one junction, to
generate a voltage. These are referred to as "multijunction" cells (also
called "cascade" or "tandem" cells). Multijunction devices can achieve
a higher total conversion efficiency because they can convert more of
the energy spectrum of light to electricity.
As shown below, a multijunction device is a stack of individual singlejunction cells in descending order of band gap (Eg). The top cell
captures the high-energy photons and passes the rest of the photons
on to be absorbed by lower-band-gap cells.
Much of today's research in multijunction cells focuses on gallium
arsenide as one (or all) of the component cells. Such cells have
reached efficiencies of around 35% under concentrated sunlight.
Other materials studied for multijunction devices have
been amorphous
silicon and copper
indium diselenide.
As an example, the multijunction device below uses a top cell of
gallium indium phosphide, "a tunnel junction," to aid the flow of
electrons between the cells, and a bottom cell of gallium arsenide.
BY
G IL K NIER
Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials
exhibit a property known as the photoelectric effect that causes them to absorb photons of light
and release electrons. When these free electrons are captured, an electric current results that
can be used as electricity.
The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who
found that certain materials would produce small amounts of electric current when exposed to
light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which
photovoltaic technology is based, for which he later won a Nobel prize in physics. The first
photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and
was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the
space industry began to make the first serious use of the technology to provide power aboard
spacecraft. Through the space programs, the technology advanced, its reliability was
established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic
technology gained recognition as a source of power for non-space applications.
The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell.
Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the
microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form
an electric field, positive on one side and negative on the other. When light energy strikes the
solar cell, electrons are knocked loose from the atoms in the semiconductor material. If
electrical conductors are attached to the positive and negative sides, forming an electrical
circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This
electricity can then be used to power a load, such as a light or a tool.
A number of solar cells electrically connected to each other and mounted in a support structure
or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain
voltage, such as a common 12 volts system. The current produced is directly dependent on how
much light strikes the module.
Multiple modules can be wired together to form an array. In general, the larger the area of a
module or array, the more electricity that will be produced. Photovoltaic modules and arrays
produce direct-current (dc) electricity. They can be connected in both series and parallel
electrical arrangements to produce any required voltage and current combination. Depending
on the size of the installation, multiple strings of solar photovoltaic array cables terminate in one
electrical box, called a fused array combiner. Contained within the combiner box are fuses
designed to protect the individual module cables, as well as the connections that deliver power
to the inverter. The electricity produced at this stage is DC (direct current) and must be
converted to AC (alternating current) suitable for use in your home or business.
Today's most common PV devices use a single junction, or interface, to create an electric field
within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose
energy is equal to or greater than the band gap of the cell material can free an electron for an
electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the
portion of the sun's spectrum whose energy is above the band gap of the absorbing material,
and lower-energy photons are not used.
One way to get around this limitation is to use two (or more) different
cells, with more than one band gap and more than one junction, to
generate a voltage. These are referred to as "multijunction" cells (also
called "cascade" or "tandem" cells). Multijunction devices can achieve
a higher total conversion efficiency because they can convert more of
the energy spectrum of light to electricity.
As shown below, a multijunction device is a stack of individual singlejunction cells in descending order of band gap (Eg). The top cell
captures the high-energy photons and passes the rest of the photons
on to be absorbed by lower-band-gap cells.
Much of today's research in multijunction cells focuses on gallium
arsenide as one (or all) of the component cells. Such cells have
reached efficiencies of around 35% under concentrated sunlight.
Other materials studied for multijunction devices have
been amorphous
silicon and copper
indium diselenide.
As an example, the multijunction device below uses a top cell of
gallium indium phosphide, "a tunnel junction," to aid the flow of
electrons between the cells, and a bottom cell of gallium arsenide.
