Quantum efficiency

Quantum Efficiency (“QE” for its acronym in English “quantum efficiency”). It is a quantity defined for a photosensitive device such as photographic film or a CCD as the percentage of photons that collide with the photoreactive surface that will produce an electron-hole pair. It is an accurate measure of the sensitivity of the device.

Summary

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  • 1 Features
  • 2 solar cell
  • 3 types
  • 4 Quantum efficiency of image sensors
  • 5 EQE mapping
  • 6 Spectral responsibility
  • 7 Determination
  • 8 Spectral sensitivity
  • 9 Principle of measurement
  • 10 Measurement settings
  • 11 Quantum efficiency versus quantum performance
  • 12 Practical meaning
  • 13 Sources

characteristics

The term quantum efficiency (QE) can be applied to the ratio of incident photon to converted electron (IPCE), of a photosensitive device or it can refer to the TMR effect of a magnetic tunnel junction.

This article deals with the term as a measure of the electrical sensitivity of a device to light. In a charge coupled device (CCD) it is the percentage of photons hitting the photoreactive surface of the device that produce charge carriers.

It is measured in electrons per photon or 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 the efficiency of a device at each energy level of the photon. The QE for photons with energy below the band space is zero. Photographic film generally has a QE of much less than 10%, while CCDs can have a QE of more than 90% at some wavelengths.

Solar cell

The quantum efficiency value of a solar cell indicates the amount of current the cell will produce when it is irradiated by photons of a certain wavelength. If the quantum efficiency of the cell is integrated into the entire solar electromagnetic spectrum, the amount of current the cell will produce when exposed to sunlight can be evaluated. The ratio of this power output value to the highest possible power output value for the cell (i.e., if the QE was 100% across the spectrum) provides the overall power conversion efficiency value of the cell.

Note that in case of multiple exciton generation (MEG), quantum efficiencies of over 100% can be achieved as the incident photons have more than double the band gap energy and can create two or more electron pairs for each photon incident.

Types

Two types of quantum efficiency of a solar cell are often considered:

  1. External Quantum Efficiency (EQE):It is the ratio between the number of charge carriers collected by the solar cell and the number of photons of a given energy that shines in the solar cell from the outside (incident photons).
  2. Internal Quantum Efficiency (IQE):It is the ratio between the number of charge carriers collected by the solar cell and the number of photons of a given energy that shine in the solar cell from the outside and are absorbed by the cell.

The IQE is always bigger than the EQE. A low IQE indicates that the active layer of the solar cell cannot make good use of the photons. To measure the IQE, first the EQE of the solar device is measured, then its transmission and reflection is measured, and this data is combined to infer the IQE.

External quantum efficiency, therefore, depends on both light absorption and charge collection. Once a photon has been absorbed and generated an electron-hole pair, these charges must separate and collect at the junction. A “good” material prevents charge recombination. Charge recombination causes a drop in external quantum efficiency.

The ideal quantum efficiency graph has a square shape, where the QE value is fairly constant across the spectrum of measured wavelengths. However, the QE for most solar cells is reduced due to the effects of recombination, where the charge carriers cannot move to an external circuit. The same mechanisms that affect collection probability also affect QE.

As an example, modifying the front surface can affect carriers generated near the surface. And because high-energy (blue) light is absorbed very close to the surface, considerable recombination on the front surface will affect the “blue” part of the QE. Similarly, lower energy light (green) is absorbed in most of a solar cell, and a low diffusion length will affect the probability of collection of the solar cell mass, reducing QE in the green portion of the solar cell. spectrum. In general, solar cells on the market today do not produce much electricity from ultraviolet and infrared light (<400nm and> 1100nm wavelength, respectively); these wavelengths of light are filtered or absorbed by the cell, thus heating the cell.

Quantum efficiency of image sensors

Quantum efficiency (QE) is the fraction of the photon flux that contributes to the photocurrent in a photodetector or pixel. Quantum efficiency is one of the most important parameters used to evaluate the quality of a detector and is often called a spectral response to reflect its dependence on wavelength. It is defined as the amount of signal electrons created per incident photon. In some cases, it may exceed 100% (i.e. when Orange Alloy.

EQE mapping

Conventional EQE measurement will give the overall device efficiency. However, it is often useful to have an EQE map over a large area of ​​the device. This mapping provides an efficient way to visualize homogeneity and / or defects in the sample. It was carried out by researchers from the Institute of Researchers and Development of Photovoltaic Energy (IRDEP) who calculated the EQE mapping from electroluminescence measurements taken with a hyperspectral imager.

Spectral responsibility

Spectral responsiveness is similar, but has different units: amps per watt (A / W); (i.e. how much current leaves the device per incoming photon of a given energy and wavelength). Both quantum efficiency and responsiveness are functions of the wavelength of the photons (indicated by the subscript λ). To convert from responsiveness (Rλ, in A / W) to QEλ (on a scale of 0 to 1): more than one electron is created per incident photon). Where λ is the wavelength in nm, h is Planck’s constant, c is the speed of light in vacuum and is the elementary charge.

Determination

Where = number of electrons produced = number of photons absorbed.

Assuming that each absorbed photon in the depletion layer produces a viable electron-hole pair, and the rest of the photons do not, where t is the measurement time (in seconds), = incident optical power in watts, = absorbed optical power in the depletion layer, also in watts.

Spectral sensitivity

The same size, measured among others for photodiodes, solar cells, or photocathodes in units of amps per watt, is called the spectral response (SR):

  • In which the light output is at a specific wavelength.

The connection to quantum efficiency is:

  • The factor is for A / W spectral sensitivity and wavelength in m.

Measurement principle

For the quantum efficiency measurement, it is necessary to know exactly the number of power of light irradiated (absolute) / photon. This is generally accomplished by a measuring device that has the known quantum efficiency of a comparison (calibrated) receiver, it is calibrated. Then it is applied: In which the measured current for the test cell and are the measured current for the comparative cell.

Measurement settings

For lighting, a light source (xenon and / or halogen lamp) and a monochromator are required to select wavelength ranges. Suitable monochromators are filter monochromators or lattice monochromators. Monochromatic light is passed as homogeneously as possible on the surface of the receiver to be tested. Signal measurement is often done with blocking amplifiers to improve the signal-to-noise ratio; For this purpose, the light signal must be periodically modulated (boosted) with an optical chopper.

Quantum efficiency vs. quantum performance

There are two factors that limit a quantum-induced process in its efficiency:

  • The photon rate that really takes effect (the rest is absorbed differently)
  • The proportion of the energy of the photon that is transferred (apart from the absorption of the multiphoton, only one photon will be involved): the energy of the emitted photon will be less by the Stokes change than that of the incident photon.

Practical meaning

Among other things, quantum performance is important for the characterization of photodiodes, photocell photocathodes, image intensifiers, and photomultipliers, but also of phosphors, fiber lasers, and other solid-state (light-pumped) lasers. The quantum efficiency of the photocathodes can reach values ​​of more than 50%. The current peak values ​​are:

  • Cs 2 Te at 213 nm: ~ 20%
  • GaAsP around 460… 540 nm: ~ 50%
  • GaAs around 550… 720 nm: ~ 25%
  • InP – InGaAsP just over 1000nm: ~ 1%

The quantum efficiency of single crystal photodiodes can reach 90%; Monocrystalline silicon photodiodes achieve a spectral sensitivity of approximately 0.5 A / W at the optimum reception wavelength around 900 nm; Solar cells generally do not reach this value: they are polycrystalline or amorphous, and their efficiency is optimized over the widest possible range in the visible spectral range (sunlight).

There are quantum yields of fluorescent dyes used for analysis from 2 to 42%, which strongly depend on the solution used. The indocarbocyanine dye has a value of 28% at an excitation wavelength of 678 nm (red) and a maximum of fluorescence at 703 nm.

The quantum efficiency of the matches used for lighting purposes (cold cathode fluorescent lamps (CCFL), fluorescent lamps, white LEDs) is close to 100% according to different sources. According to Henning Höppe, there are quantum yields of 70 to 90% at excitation wavelengths of 253.65nm (mercury vapor gas discharge) and 450nm (blue LED).

 

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