Quantum efficiency measures how photosensitive a device is by creating electron-hole pairs from incoming photons. Different materials absorb and reflect different wavelengths, affecting quantum efficiency. Charge-coupled devices have the highest quantum efficiencies, while solar cells can have poor internal or external efficiency. Higher quantum efficiency generates more energy in solar cells. Most solar cells are designed to maximize efficiency in the visible spectrum.
Quantum efficiency is a measure of how electrically photosensitive a photosensitive device is. Photoreactive surfaces use the energy of incoming photons to create electron-hole pairs, where the photon’s energy raises the energy level of an electron and allows the electron to leave the valence band, where electrons are bonded 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 when they hit the photoreactive surface, the higher its quantum efficiency. Quantum efficiency is an important feature 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 wavelength of the photon, and the quantum efficiency of a device 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 for the substances used in different photosensitive devices. The most common material for solar cells is crystalline silicon, but there are also cells based on other photoreactive substances, such as cadmium telluride and copper, indium and gallium selenide. Photographic film uses silver bromide, silver chloride, or silver iodide, 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 boron-doped epitaxial silicon layer, which creates electric charges that are then moved through a series of capacitors to a charge amplifier. The charge amplifier converts charges into a series of voltages which can be processed as an analog signal or recorded digitally. Charge-coupled devices, which are often used in scientific applications such as astronomy and biology that require great precision and sensitivity, can have quantum efficiencies of 90% or more.
In solar cells, quantum efficiency is sometimes divided into two measurements, external quantum efficiency and internal quantum efficiency. External efficiency is a measure of the percentage of all photons hitting the solar cell that produce an electron-hole pair that is successfully collected by the cell. Quantum efficiency only counts those photons hitting the cell that have not been reflected or transmitted out of the cell. Poor internal efficiency indicates that too many electrons that have been driven to the conduction level are losing their energy and attaching themselves back to an atom in the valence level, a process called recombination. Poor external efficiency can be a reflection of poor internal efficiency or it can mean that large amounts of light reaching the cell are not available for use because it is either reflected from the cell or allowed to pass through it.
Once the electrons begin moving in the conduction band, the solar cell design controls the direction of their movement to create a direct current flow of electricity. Since higher quantum efficiency means more electrons can enter the conduction band and be successfully collected, higher efficiency allows more energy to be generated. Most solar cells are designed to maximize quantum efficiency in the wavelengths of light most common in the Earth’s atmosphere, namely the visible spectrum, although specialized solar cells have also been developed to harness infrared or ultraviolet light .
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