From Wikipedia, the free encyclopedia

A graph showing variation of quantum efficiency with wavelength of the CCD chips in the Hubble Space Telescope's Wide Field and Planetary Camera 2.

Quantum efficiency (QE) is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device (CCD) as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's sensitivity. It is often measured over a range of different wavelengths to characterize a device's efficiency at each 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 white light. 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 meaures the external quantum efficiency, i.e. 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-quality material is crucial to avoid charge recombination and therefore a drop in the quantum efficiency.)

The spectral responsivity is a similar measurement, but it has different units, amperes per watt (A/W); i.e how much current comes out of the device for an incoming light beam of a given power. Both the quantum efficiency and the responsivity are functions of the photons' wavelength.

To convert from responsivity to QE in %: where R is the responsivity in A/W at the wavelength λ, and λ, the wavelength, is in nm.

Equations:

η = Ne / Np

where Ne = No. of Electrons, Np = No. of photons

Ne = φo / (hc / λo)

Np = φxi / (hc / λo)

where

φo = incident optical power, φxi = optical power absorbed in depletion layer

• Definition of Quantum Efficiency in Photonics Dictionary
• Definition of Responsivity in Photonics Dictionary