Hole: photoelectric effect

The absorption of light by semiconductors cremes electron-hole pairs e h+) which can be separated because their components diffuse in different directions. The energies of these moieties can be stored by several mechanisms or used in photocatalysis or photosynthesis for nitrogen fixation, formation of amino acids, methanol, etc. The efficiencies of such conversions depend almost entirely upon the semiconductor material, and as yet these efficiencies are too low for significant application. Currently the most promise is demonstrated by the use of titania on a platinum substrate or single crystals of strontium titanate. See also Photoelectric Effect. 

In the photoelectrical effect a photon removes an electron from a molecule to produce a radical cation or hole . This effect is utilised in the key imaging step in photocopiers and laser printers, and in solar cells. 

The internal photoelectric effect, just as infrared radiation, was also first observed in the 19th century, when certain minerals such as selenium or lead sulfide were found to increase their electrical conductivity in the presence of light. These photoconductors depend upon the photoexcitation of bound electrons and/or holes into the conduction and/or valence bands of the material. Then, at the turn of the century, external photoemission was discovered in vacuum diodes. As first explained by Einstein, the photoelectric effect was found to have a threshold wavelength determined by the relation hv = he lk>E, where E is the energy required for the electron to exit the material. In the case of a semiconductor, the excitation energy, E, is that of the gap between the valence and conduction bands or the ionization energy of an impurity in the material. The electronic detector family has two main branches, the first being the vacuum photodiode and its more useful... 

For a crystal illuminated by a photoelectrically active light, the quantities j/°, rr, and 77+ have values different from those for a crystal in the dark. Thus, the effect of illumination is to change the relative content of different forms of chemisorption on a surface for particles of each particular species in other words, it changes the population of electron and holes on the surface local levels corresponding to chemisorbed particles. A change in the quanti-... 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

Irrespective of some still unresolved details of the primary process that leads to generation of electron-hole pairs one can determine the effective absorption coefficient, called a], that determines the photoelectric geminate pair yield in the low field limit, i.e. at fields where (j)o is constant. The result is shown in fig.7 for DCH. has a singularity close to the energy where electroreflectance measurements of Sebastian and Weiser (24) locate the onset of the transition between delocalized valence and conduction band states of the chain. The feature near 3.5 eV is due to charge transfer from the carbazole substituent to the polymer chain (49). 

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