Quantum efficiency
The quantum yield (also quantum efficiency or fluorescence yield) describes the relationship between the number of absorbed photons ( light quanta ) and a subsequent event such as fluorescence or a chemical reaction. Usually, the quantum efficiency is less than 1
In fluorescence spectroscopy, the quantum yield of a fluorophore indicates the ratio between the number of emitted and absorbed photons. The difference is related to the competition Auger effect. The ratio of generated holes to the generated photons is also called fluorescence yield (English fluorescence yield ). The fluorescence yield is usually assigned to one of the original ionization corresponding shell and is thus always less than or equal to one. The total fluorescence yield ( sum over all shells with cascading effects ) can be greater than one in consequence so.
In photon detectors ( photomultiplier, semiconductor detectors such as photodiodes and CCDs), the quantum yield is in the probability with which an electron is released by the photoelectric effect, and thus the photon can be detected. In solar cells, the quantum yield for the energy efficiency is crucial.
In light-induced chemical reactions, the quantum yield is the number of unreacted molecules per number of absorbed photons. Here, the quantum efficiency of the energy of the photon and hence the wavelength of light ( or electromagnetic radiation ) depends. In chain reactions (eg, polymerization reactions ) may secondarily be greater than one.
Quantum efficiency of photodetectors, phosphors and semiconductor light sources
In photovoltaics, with photodiodes and other photoreceptors, the quantum efficiency (QE ) refers to at a certain wavelength of light, the ratio of electrons that contribute to the photocurrent, the number of incoming photons:
Here is the elementary charge, the photoelectric current, the number of photons per unit time and the radiation power.
According referred to in LEDs and laser diodes, the QE, the ratio of photons emitted to the number of recombining electron-hole pairs and in phosphors, the ratio between the number of emitted photons to the new wavelength absorbed photons of the excitation wavelength.
Spectral sensitivity
The same size, among others, photodiodes, solar cells or photo cathode in unit amperes per watt measured, as spectral response (SR - for engl spectral response. ) Referred to:
Wherein the light output is at a particular wavelength.
The connection with the quantum efficiency is:
The factor is for a spectral sensitivity in A / W and wavelength in m.
Principle of measurement
For the measurement of the quantum efficiency, the precise knowledge of the (absolute ) of the incident light power / number of photons necessary. This is usually achieved by a measuring device on the quantum efficiency of a known ( calibrated ) Comparative receiver is calibrated. We then have:
Being measured for the test cell current and the measured for the reference cell current are.
Measurement setup
For lighting a light source (xenon and / or halogen) and a monochromator for selecting wavelength intervals is needed. As a monochromator come Filtermonochromatoren or grating monochromators in question. The monochromatic light is directed homogeneously to the receiver sheet to be tested.
The measurement of the signal is often done using lock-in amplifiers to improve the signal -to-noise ratio; for the light signal with an optical chopper must periodically modulated ( pulsed) are.
Quantum efficiency vs. quantum yield
There are two factors limiting a quantum- induced process in its efficiency:
Practical significance
The quantum yield is among other things for the characterization of photodiodes, photocathodes of photocells, image intensifiers and photomultiplier, but also of phosphors, fiber lasers and other ( light- pumped ) solid-state lasers is important.
The quantum yield of the photocathode can reach values of more than 50 %. Current peak values are:
- Cs2Te at 213 nm: ~ 20 %
- GaAsP to the 460 ... 540 nm: ~ 50 %
- GaAs to the 550 ... 720 nm: ~ 25 %
- InP -InGaAsP just over 1000 nm: ~ 1 %
The quantum yield of single-crystal photodiode can reach 90%; monocrystalline silicon photodiodes reach at the optimum receiving wavelength around 900 nm, a spectral sensitivity of about 0.5 A / W; Solar cells achieve this value usually not - they are polycrystalline or amorphous, and their efficiency is optimized for the widest possible range in the visible spectral range ( sunlight).
Add to quantum yields found for the analysis of used fluorescent dyes 2-42 %, which depend strongly on the solution used. In a value of 28 % for the dye indocarbocyanine place at a Anregungswellenänge of 678 nm ( red) and a fluorescence maximum at 703 nm
The quantum efficiency of phosphors used for lighting purposes (CCFL, fluorescent lamps, white LEDs ) is according to various sources close to 100 %. In quantum yields of 70 to 90 % at the excitation wavelengths of 245 nm ( mercury vapor gas discharge ) and 450 nm ( Blue LED ) are indicated.
The quantum efficiency also plays a role in photosynthesis and productivity of agricultural crops.