Quantum efficiency
Encyclopedia
Quantum efficiency is a quantity defined for a photosensitive device such as photographic film
or a charge-coupled device
(CCD) as the percentage of photon
s hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to light. Since the energy of a photon
depends on (more precisely, is inversely proportional to) its wavelength
, QE is often measured over a range of different wavelengths to characterize a device's efficiency at each photon energy. Photographic film typically has a QE of much less than 10%, while CCDs can have a QE of well over 90% at some wavelengths.
The quantum efficiency of a solar cell
is a very important measure for solar cells as it gives information on the current that a given cell will produce when illuminated by a particular wavelength. If the quantum efficiency is integrated (summed) over the whole solar electromagnetic spectrum, one can evaluate the current that a cell will produce when exposed to the solar spectrum. The ratio between this current and the highest possible current (if the QE was 100% over the whole spectrum) gives the electrical efficiency of the solar cell. With solar cells, one often measures the external quantum efficiency (EQE, sometimes also simply referred to as QE), which is the current obtained outside the device per incoming photon.
The external quantum efficiency therefore depends on both the absorption
of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination and therefore a drop in the external quantum efficiency. EQE should not be confused with internal quantum efficiency which is the ratio of current to absorbed photons.
is a similar measurement, but it has different units: ampere
s per watt
(A/W); i.e., how much current comes out of the device for an incoming light beam of a given power and wavelength. Both the quantum efficiency and the responsivity are functions of the photons' wavelength (indicated by the subscript λ).
To convert from responsivity (Rλ, in A/W) to QEλ (on a scale 0 to 1):
where λ is in nm, h is the Planck constant
, c is the speed of light
in a vacuum, and e is the elementary charge
.
where
= number of electrons produced, 
= number of photons absorbed.
Assuming each photon that is absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not,
where t is the measurement time (in seconds)
= incident optical power in watts,
= optical power absorbed in depletion layer, also in watts.
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
or a charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
(CCD) as the percentage of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to light. Since the energy of a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
depends on (more precisely, is inversely proportional to) its wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, QE is often measured over a range of different wavelengths to characterize a device's efficiency at each photon energy. Photographic film typically has a QE of much less than 10%, while CCDs can have a QE of well over 90% at some wavelengths.
The quantum efficiency of a solar cell
Quantum efficiency of a solar cell
Quantum efficiency is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell. QE therefore relates to the response of a solar cell to the various wavelengths in the spectrum of light shining on the cell. The QE is...
is a very important measure for solar cells as it gives information on the current that a given cell will produce when illuminated by a particular wavelength. If the quantum efficiency is integrated (summed) over the whole solar electromagnetic spectrum, one can evaluate the current that a cell will produce when exposed to the solar spectrum. The ratio between this current and the highest possible current (if the QE was 100% over the whole spectrum) gives the electrical efficiency of the solar cell. With solar cells, one often measures the external quantum efficiency (EQE, sometimes also simply referred to as QE), which is the current obtained outside the device per incoming photon.
The external quantum efficiency therefore depends on both the absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination and therefore a drop in the external quantum efficiency. EQE should not be confused with internal quantum efficiency which is the ratio of current to absorbed photons.
Spectral responsivity
The spectral responsivityResponsivity
Responsivity measures the input–output gain of a detector system. For a system that responds linearly to its input, there is a unique responsivity. For nonlinear systems, the responsivity is the local slope ....
is a similar measurement, but it has different units: ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s per watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
(A/W); i.e., how much current comes out of the device for an incoming light beam of a given power and wavelength. Both the quantum efficiency and the responsivity are functions of the photons' wavelength (indicated by the subscript λ).
To convert from responsivity (Rλ, in A/W) to QEλ (on a scale 0 to 1):
where λ is in nm, h is the Planck constant
Planck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...
, c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in a vacuum, and e is the elementary charge
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
.
Determination
where
= number of electrons produced, = number of photons absorbed.Assuming each photon that is absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not,
where t is the measurement time (in seconds)
= incident optical power in watts, = optical power absorbed in depletion layer, also in watts.