Quantum efficiency

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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.
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[1]. 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 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 measures the external quantum efficiency (EQE, sometimes also simply referred to as QE), which is the current obtained outside the device per incoming photon.

\text{Efficiency} = \frac{\text{output}}{\text{input}}
\text{EQE} = \frac{\text{electrons/sec}}{\text{photons/sec}}= \frac{\text{current}/\text{(charge of 1 electron)}}{(\text{total power of photons})/(\text{energy of one 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.

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[edit] Spectral responsivity

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 and wavelength[2]. 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): QE_\lambda=\frac{R_\lambda}{\lambda}\times\frac{h c}{e}\approx\frac{R_\lambda}{\lambda} {\times} (1240\;{\rm W}\cdot {\rm nm/A}) 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.

[edit] Determination

QE_\lambda=\eta =\frac{N_e}{N_\nu}

where Ne = number of electrons produced, Nν = number of photons absorbed.

N_\nu/t = \Phi_o \frac{\lambda}{hc}

Assuming each photon that is absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not,

N_e/t = \Phi_{\xi}\frac{\lambda}{hc}

where t is the measurement time (in seconds) Φo = incident optical power in watts, Φξ = optical power absorbed in depletion layer, also in watts.

[edit] See also

[edit] References

  1. ^ Definition of quantum efficiency in Photonics Dictionary
  2. ^ Definition of responsivity in Photonics Dictionary