Quantum efficiency
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio[1] of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction.
This article deals with the term as a measurement of a device's electrical sensitivity to light.
In a charge-coupled device (CCD) or other photodetector, it is the ratio between the number of charge carriers collected at either terminal and the number of photons hitting the device's photoreactive surface.
As a ratio, QE is dimensionless, but it is closely related to the responsivity, which is expressed in amps per watt.
Since the energy of a photon 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 level.
For typical semiconductor photodetectors, QE drops to zero for photons whose energy is below the band gap.
Note that in the event of multiple exciton generation (MEG), quantum efficiencies of greater than 100% may be achieved since the incident photons have more than twice the band gap energy and can create two or more electron-hole pairs per incident photon.
Two types of quantum efficiency of a solar cell are often considered: The IQE is always larger than the EQE in the visible spectrum.
A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons, most likely due to poor carrier collection efficiency.
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.
The ideal quantum efficiency graph has a square shape, where the QE value is fairly constant across the entire spectrum of wavelengths measured.
However, the QE for most solar cells is reduced because of the effects of recombination, where charge carriers are not able to move into an external circuit.
Highly doped front surface layers can also cause 'free carrier absorption' which reduces QE in the longer wavelengths.
Similarly, lower energy (green) light is absorbed in the bulk of a solar cell, and a low diffusion length will affect the collection probability from the solar cell bulk, reducing the QE in the green portion of the spectrum.
[4] Quantum efficiency (QE) is the fraction of photon flux that contributes to the photocurrent in a photodetector or a pixel.
Quantum efficiency is one of the most important parameters used to evaluate the quality of a detector and is often called the spectral response to reflect its wavelength dependence.
It is defined as the number of signal electrons created per incident photon.
In some cases it can exceed 100% (i.e. when more than one electron is created per incident photon).
It was realized by researchers from the Institute of Researcher and Development on Photovoltaic Energy (IRDEP) who calculated the EQE mapping from electroluminescence measurements taken with a hyperspectral imager.
[citation needed] Both the quantum efficiency and the responsivity are functions of the photons' wavelength (indicated by the subscript λ).
where λ is the wavelength in nm, h is the Planck constant, c is the speed of light in vacuum, and e is the elementary charge.
