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Anomalous photovoltaic effect
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<p>The anomalous photovoltaic effect (APE) is a type of a <a href="/facts/Photovoltaic_effect/huqzdkH2">photovoltaic effect</a> which occurs in certain <a href="/facts/Semiconductor/zFf3mGTE">semiconductors</a> and <a href="/facts/Insulator_(electricity)/SfSXkrh5">insulators</a>. The "anomalous" refers to those cases where the photovoltage (i.e., the open-circuit voltage caused by the light) is larger than the <a href="/facts/Band_gap/IjIeh95l">band gap</a> of the corresponding <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a>. In some cases, the voltage may reach thousands of volts. </p><p>Although the voltage is unusually high, the short-circuit current is unusually low. Overall, materials that exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems. </p><p>There are several situations in which APE can arise. </p><p>First, in polycrystalline materials, each microscopic grain can act as a photovoltaic. Then the grains <a href="/facts/Series_and_parallel_circuits/007qC7j8">add in series</a>, so that the overall open-circuit voltage across the sample is large, potentially much larger than the bandgap. </p><p>Second, in a similar manner, certain <a href="/facts/Ferroelectric/DLVYFNQT">ferroelectric</a> materials can develop stripes consisting of parallel ferroelectric domains, where each domain acts like a photovoltaic and each domain wall acts like a contact connecting the adjacent <a href="/facts/Photovoltaics/5Xefaayl">photovoltaics</a> (or vice versa). Again, domains add in series, so that the overall open-circuit voltage is large. </p><p>Third, a perfect single crystal with a <a href="/facts/Centrosymmetry/nIFJqujU">non-centrosymmetric structure</a> can develop a giant photovoltage. This is specifically called the bulk photovoltaic effect (BPV effect), and occurs because of non-centrosymmetry. Specifically, the electron processes—photo-excitation, scattering, and relaxation—occur with different probabilities for electron motion in one direction versus the opposite direction. </p><p>The compound α-In2Se3 can be made to exhibit the bulk photovoltaic effect and outperform traditional solar cells. </p>