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Anomalous photovoltaic effect
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Anomalous photovoltaic effect
The anomalous photovoltaic effect (APE) is a type of a photovoltaic effect which occurs in certain semiconductors and insulators. The "anomalous" refers to those cases where the photovoltage (i.e., the open-circuit voltage caused by the light) 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 (or vice versa). Again, domains add in series, so that the overall open-circuit voltage is large.
Third, a perfect single crystal with a non-centrosymmetric structure 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.
The compound α-In2Se3 can be made to exhibit the bulk photovoltaic effect and outperform traditional solar cells.
This effect was discovered by Starkiewicz et al. in 1946 on PbS films and was later observed on other semiconducting polycrystalline films including CdTe, Silicon, Germanium, ZnTe and InP, as well as on amorphous silicon films and in nanocrystalline silicon systems. Observed photovoltages were found to reach hundreds, and in some cases even thousands of volts. The films in which this effect was observed were generally thin semiconducting films that were deposited by vacuum evaporation onto a heated insulating substrate, held at an angle with respect to the direction of the incident vapor. However, the photovoltage was found to be very sensitive to the conditions and procedure at which the samples were prepared. This made it difficult to get reproducible results which is probably the reason why no satisfactory model for it has been accepted thus far. Several models were, however, suggested to account for the extraordinary phenomenon and they are briefly outlined below.
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Anomalous photovoltaic effect
The anomalous photovoltaic effect (APE) is a type of a photovoltaic effect which occurs in certain semiconductors and insulators. The "anomalous" refers to those cases where the photovoltage (i.e., the open-circuit voltage caused by the light) 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 (or vice versa). Again, domains add in series, so that the overall open-circuit voltage is large.
Third, a perfect single crystal with a non-centrosymmetric structure 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.
The compound α-In2Se3 can be made to exhibit the bulk photovoltaic effect and outperform traditional solar cells.
This effect was discovered by Starkiewicz et al. in 1946 on PbS films and was later observed on other semiconducting polycrystalline films including CdTe, Silicon, Germanium, ZnTe and InP, as well as on amorphous silicon films and in nanocrystalline silicon systems. Observed photovoltages were found to reach hundreds, and in some cases even thousands of volts. The films in which this effect was observed were generally thin semiconducting films that were deposited by vacuum evaporation onto a heated insulating substrate, held at an angle with respect to the direction of the incident vapor. However, the photovoltage was found to be very sensitive to the conditions and procedure at which the samples were prepared. This made it difficult to get reproducible results which is probably the reason why no satisfactory model for it has been accepted thus far. Several models were, however, suggested to account for the extraordinary phenomenon and they are briefly outlined below.