The photovoltaic effect is the generation of voltage and
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
in a material upon exposure to
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
. It is a
physical
Physical may refer to:
*Physical examination
In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally co ...
and
chemical
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
phenomenon.
The photovoltaic effect is closely related to the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
. For both phenomena, light is absorbed, causing excitation of an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
or other
charge carrier
In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
to a higher-energy state. The main distinction is that the term ''photoelectric effect'' is now usually used when the electron is ejected out of the material (usually into a vacuum) and ''photovoltaic effect'' used when the excited charge carrier is still contained within the material. In either case, an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
(or voltage) is produced by the separation of charges, and the light has to have a sufficient energy to overcome the potential barrier for excitation. The physical essence of the difference is usually that photoelectric emission separates the charges by
ballistic conduction
In mesoscopic physics, ballistic conduction (ballistic transport) is the unimpeded flow (or transport) of charge carriers (usually electrons), or energy-carrying particles, over relatively long distances in a material. In general, the resistivity ...
and photovoltaic emission separates them by diffusion, but some "hot carrier" photovoltaic devices concepts blur this distinction.
History
The first demonstration of the photovoltaic effect, by Edmond Becquerel in 1839, used an electrochemical cell. He explained his discovery in '' Comptes rendus de l'Académie des sciences'', "the production of an electric current when two plates of platinum or gold immersed in an acid, neutral, or alkaline solution are exposed in an uneven way to solar radiation."
The first solar cell, consisting of a layer of
selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
covered with a thin film of gold, was experimented by
Charles Fritts
Charles Fritts (1850 – 1903) was the American inventor credited with creating the first working selenium cell in 1883.
The world's first rooftop solar array, using Fritts' selenium cells, was installed in 1884 on a New York City rooftop.
Fritt ...
in 1884, but it had a very poor efficiency. However, the most familiar form of the photovoltaic effect uses solid-state devices, mainly in
photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
s. When sunlight or other sufficiently energetic light is incident upon the photodiode, the electrons present in the
valence band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
absorb energy and, being excited, jump to the conduction band and become free. These excited electrons diffuse, and some reach the rectifying junction (usually a diode
p–n junction
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...
) where they are accelerated into the n-type semiconductor material by the built-in potential (
Galvani potential
In electrochemistry, the Galvani potential (also called Galvani potential difference, or inner potential difference, Δφ, delta phi) is the electric potential difference between two points in the bulk of two phases. These phases can be two differ ...
). This generates an
electromotive force
In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transd ...
and an electric current, and thus some of the light energy is converted into electric energy. The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect.
Physics
In addition to the direct photovoltaic excitation of free electrons, an electric current can also arise through the
Seebeck effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
. When conductive or semiconductive material is heated by absorption of electromagnetic radiation, the heating can lead to increased temperature gradients in the semiconductor material or differentials between materials. These thermal differences in turn may generate a voltage because the electron energy levels are shifted differently in different areas, creating a potential difference between those areas which in turn create an electric current. The relative contributions of the photovoltaic effect versus the Seebeck effect depend on many characteristics of the constituent materials.
All above effects generate direct current, the first demonstration of the alternating current photovoltaic effect (AC PV) was done by Dr. Haiyang Zou and Prof. Zhong Lin Wang at the
Georgia Institute of Technology
The Georgia Institute of Technology, commonly referred to as Georgia Tech or, in the state of Georgia, as Tech or The Institute, is a public research university and institute of technology in Atlanta, Georgia. Established in 1885, it is part of ...
in 2017. The AC PV effect is the generation of alternating current (AC) in the nonequilibrium states when the light periodically shines at the junction or interface of material. The AC PV effect is based on the capacitive model that the current strongly depends on the frequency of the chopper. The AC PV effect is suggested to be a result of the relative shift and realignment between the quasi-Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions. The electrons flow in the external circuit back and forth to balance the potential difference between two electrodes. The organic solar cell, which the materials have no initial carrier concentration, does not have the AC PV effect.
Effect of the temperature
The performance of a photovoltaic module depends on the environmental conditions, mainly on the global incident irradiance G on the module plane. However, the temperature T of the p–n junction also influences the main electrical parameters: the short-circuit current ISC, the open-circuit voltage VOC, and the maximum power Pmax. The first studies about the behavior of PV cells under varying conditions of G and T date back several decades ago.1-4 In general, it is known that VOC shows a significant inverse correlation with T, whereas for ISC that correlation is direct, but weaker, so that this increment does not compensate for the decrease of VOC. As a consequence, Pmax reduces when T increases. This correlation between the output power of a solar cell and its junction working temperature depends on the semiconductor material,2 and it is due to the influence of T on the concentration, lifetime, and mobility of the intrinsic carriers, that is, electrons and holes, inside the PV cell.
The temperature sensitivity is usually described by some temperature coefficients, each one expressing the derivative of the parameter it refers to with respect to the junction temperature. The values of these parameters can be found in any PV module data sheet; they are the following:
– β Coefficient of variation of VOC with respect to T, given by ∂VOC/∂T.
– α Coefficient of variation of ISC with respect to T, given by ∂ISC/∂T.
– δ Coefficient of variation of Pmax with respect to T, given by ∂Pmax/∂T.
Techniques for estimating these coefficients from experimental data can be found in the literature. Few studies analyse the variation of the series resistance with respect to the cell or module temperature. This dependency is studied by suitably processing the current–voltage curve. The temperature coefficient of the series resistance is estimated by using the single diode model or the double diode one.
Solar cells
In most photovoltaic applications the
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
is sunlight, and the devices are called
solar cells
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physics, physical and Chemical substance, chemical phenomenon.The photovoltaic effect Scienzagiovane.unibo.it (2006-12-01). Retrieved on 2010-12-12. The AC PV is operated at the non-equilibrium conditions. The first study was based on a p-Si/TiO2 nanofilm. It is found that except for the DC output generated by the conventional PV effect based on a p–n junction, AC current is also produced when a flashing light is illuminated at the interface. The AC PV effect does not follow Ohm's law, being based on the capacitive model that the current strongly depends on the frequency of the chopper, but voltage is independent of the frequency. The peak current of AC at high switching frequency can be much higher than that from DC. The magnitude of the output is also associated with the light absorption of materials.
See also
*
Theory of solar cells The theory of solar cells explains the process by which light energy in photons is converted into electric current when the photons strike a suitable semiconductor device. The theoretical studies are of practical use because they predict the fundame ...
Photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...