Photon effects photovoltaic

Although the list of photon effects in Table 2.1 is extensive, only the photoconductive, photovoltaic, and photoemissive ones have been widely... 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

If photons of light of a suitable wavelength (usually ultraviolet or x-rays) impinge on a metal surface, electrons are emitted. This effect is photoelectric (or photovoltaic) and can be used to start a flow of electrons in a discharge tube. 

The lead compounds PbS, PbSe, PbTe are narrow-gap semiconductors that have been widely investigated for infrared detectors, diode lasers, and thermo-photovoltaic energy converters. Their photoconductive effect has been utilized in photoelectric cells, e.g., PbS in photographic exposure meters. Integrated photonic devices have been fabricated by their heteroepitaxial growth on Si or III-V semiconductors. 

Similarly, many semiconductors, such as silicon, germanium, etc. produce a flow of electrons (the photovoltaic effect) when photons of a certain wavelength interact with the materials. Figure 11 shows Air Mass 1.5 direct normal, diffuse sky, and total global solar spectral distributions, with indications of the spectral regions where our vision, plants, and various photovoltaic materials interact with the solar spectral distribution. 

For metal, dielectric and semiconductor films fabrication, optical and silica glass are popular substrate materials because of their availability, cost-effectiveness, and inert character, i.e., they are stable in the required temperature range for common photonic, optoelectronic and photovoltaic applications, they do not chemically react with the prepared films, and the hard plane surface makes the formation of optically smooth thin films fairly easy. Generally, it is preferable to form films by a simple, low-temperature, inexpensive and environment friendly method. Sol-gel technique and thermal evaporation is found suitable for the preparation of film parts of efficient solar cells [1], emitters, transformers [2], detectors and modulators of light [3], as well as optically stimulated luminescence dosimeters [4]. Here, we present the experimental data on the resistance to high-power optical and ionizing irradiations of the undoped components of film compositions with nanociystais. 

Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

The use of nanoscale constructs has given a further major boost to solar photon conversion. The scale of nanosized materials such as quantum dots and nanotubes, conventionally taken to lie in the range 1-100 nm, produces very interesting size quantisation effects in optoelectronic and other properties bandgaps shift to the blue, carrier lifetimes increase, potent catalytic properties emerge and constructs with very high surface-to-volume ratios can be made. Incorporation of nanoscale structures in photovoltaic devices allows these unique properties to be exploited, with conversion efficiencies above the detailed balance limit becoming possible in principle. 

The photovoltaic effect in organic solar cells arises, not from formation of free charge carriers in one or both phases, as in a classical siliconp-n junction cell, but from exciton dissociation at the junction between the two phases. Photon absorption in organic semiconductors, whether small molecules or polymers, does not generate free holes and... 

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