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Quantum efficiency is a measurement of how electrically photosensitive a photosensitive device is. Photoreactive surfaces use the energy from incoming photons to create electron-hole pairs, in which the photon's energy increases the energy level of an electron and allows the electron to leave the valence band, where electrons are bound to individual atoms, and enter the conduction band, where it can move freely through the entire atomic lattice of the material. The higher the percentage of photons that produce an electron-hole pair upon striking the photoreactive surface, the higher its quantum efficiency. Quantum efficiency is an important characteristic of a number of modern technologies, most notably photovoltaic solar cells used to generate electricity, as well as photographic film and charge-coupled devices.
Photon energy varies with the photon's wavelength, and a device's quantum efficiency can vary for different wavelengths of light. Different configurations of materials vary in how they absorb and reflect different wavelengths, and this is an important factor in what substances are used in different photosensitive devices. The most common material for solar cells is crystalline silicon, but cells based on other photoreactive substances, such as cadmium telluride and copper indium gallium selenide, also exist. Photographic film uses silver bromide, silver chloride, or silver iodide, either alone or in combination.
The highest quantum efficiencies are produced by charge-coupled devices used for digital photography and high-resolution imaging. These devices collect photons with a layer of epitaxial silicon doped with boron, which creates electric charges that are then shifted through a series of capacitors to a charge amplifier. The charge amplifier converts the charges into a series of voltages that can be processed as an analog signal or recorded digitally. Charge-coupled devices, which are used often in scientific applications such as astronomy and biology that require great precision and sensitivity, can have quantum efficiencies of 90 percent or more.
In solar cells, quantum efficiency is sometimes divided into two measurements, external quantum efficiency and internal quantum efficiency. External efficiency is a measurement of the percentage of all photons striking the solar cell that produce an electron-hole pair that is successfully collected by the cell. Quantum efficiency counts only those photons striking the cell that were not reflected away or transmitted out of the cell. Poor internal efficiency indicates that too many electrons that had been raised up to the conduction level are losing their energy and again becoming attached to an atom in the valence level, a process called recombination. Poor external efficiency can be either a reflection of poor internal efficiency or can mean that large amounts of the light reaching the cell are unavailable for use because it is being reflected away by the cell or allowed to pass through it.
Once electrons begin moving into the conduction band, the design of the solar cell controls the direction of their movement to create a flow of direct current electricity. As higher quantum efficiency means that more electrons can enter the conduction band and be successfully collected, higher efficiency makes it possible to generate more power. Most solar cells are designed to maximize quantum efficiency in the wavelengths of light most common in Earth's atmosphere, namely the visible spectrum, although specialized solar cells to exploit infrared or ultraviolet light have also been developed.
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