page_title: Anomalous photovoltaic effect
section_title: Simple example
hierarchical_section_title: Anomalous photovoltaic effect / Bulk photovoltaic effect in a non-centrosymmetric single crystal / Simple example
context_page_description: The anomalous photovoltaic effect, also called the bulk photovoltaic effect in certain cases, is a type of a photovoltaic effect which occurs in certain semiconductors and insulators. The "anomalous" refers to those cases where the photovoltage is larger than the band gap of the corresponding semiconductor. In some cases, the voltage may reach thousands of volts.
  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.
  There are several situations in which APE can arise.
  First, in polycrystalline materials, each microscopic grain can act as a photovoltaic. Then the grains add in series, so that the overall open-circuit voltage across the sample is large, potentially much larger than the bandgap.
  Second, in a similar manner, certain ferroelectric 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 photovoltaics.
context_section_description: Shown at right is an example of a simple system that would exhibit the bulk photovoltaic effect. There are two electronic levels per unit cell, separated by a large energy gap, say 3 eV. The blue arrows indicate radiative transitions, i.e. an electron can absorb a UV photon to go from A to B, or it can emit a UV photon to go from B to A. The purple arrows indicate nonradiative transitions, i.e. an electron can go from B to C by emitting many phonons, or can go from C to B by absorbing many phonons.
  When light is shining, an electron will occasionally move right by absorbing a photon and going from A to B to C. However, it will almost never move in the reverse direction, C to B to A, because the transition from C to B cannot be excited by photons, but instead requires an improbably large thermal fluctuation. Therefore, there is a net rightward photocurrent.
  Because the electrons undergo a "shift" each time they absorb a photon (on average), this photocurrent is sometimes called a "shift current".
caption: An example of a simple system that would exhibit the bulk photovoltaic effect. See text for description.