Carriers generation

As mentioned earlier, CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. In addition, electron-beam excitation of solids may produce much greater carrier generation rates than typical optical excitation. In such cases, CL microscopy and spectroscopy are valuable methods in identifying various impurities, defects, and their complexes, and in providing a powerful means for the analysis of their distribution, with spatial resolution on the order of 1 pm and less. ... 

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

Combinatorial Chemistry. Figure 2 Chemical libraries are prepared either by parallel synthesis or by the split-and-recombine method. In the latter case, coupling m building blocks in m separated reaction flasks through n synthetic cycles on a beaded polymer carrier generates a combinatorial library with nf individual compounds and one compound per bead. 

The Gartner model simulates charge collection by a potential-dependent space charge layer and considers diffusion into the space charge layer of charge carriers generated deep inside the semiconductor. The well-known Gartner formula for the photocurrent /ph is... 

It follows that /ph is (only photons /0 leading to minority carrier generation are considered)... 

It is interesting to note that PMC peaks depend on the frequency of periodic excess carrier generation. At higher frequencies, the PMC peak... 

Experimental evidence with very different semiconductors has shown that at semiconductor interfaces where limited surface recombination and a modest interfacial charge-transfer rate for charge carriers generate a peak... 

It is important to note that there may be at least two reasons for obtaining deviations from a purely exponential behavior for a PMC transient. These are a too high excess carrier generation, which may cause interfacial rate constants that are dependent on carrier concentration, and an interfacial band bending AU, which changes during and after the flash. For fast charge transfer, a more complicated differential equation has to be solved. 

Carrier generators in molecular conductors have been associated for a long time to a partial charge transfer between the HOMO (or LUMO) electronic band and other chemical species. These systems are known as two-component molecular conductors. Tetrathiofulvalene derivatives are versatile systems for the formation of molecular organic conductors due to their electron donor capacity by transferring one u-electron from the HOMO orbital, and to their planar shape that promotes their stacking as a consequence of the n-n orbital overlap. The electronic properties of these salts are essentially determined by the packing pattern of the donor molecules which, in turn, depends on the counter-ion. 

At the turn of this century it was realized that carrier generation was also possible between the HOMO and LUMO band even in neutral single-component materials assuming that there was a small HOMO-LUMO gap and conduction paths have been associated with the presence of large transverse intermolecular interactions. The most relevant examples of single-component molecular conductors are the mononuclear M(dithiolate)2 (M = Co, Ni, Cu, Au) complexes with... 

This equation presumes that each photon absorbed creates an electron-hole pair if there are other absorption mechanisms, the right-hand side must be multiplied by a quantum efficiency. The total rate of minority carrier generation is obtained by integrating over the space-charge region ... 

Carrier generation increases usually due to the electron accepting property of the sensitizer in its ground state or in its excited state. The detailed mechanism of the carrier generation is the subject to be reported in another symposium of the present meeting (lj. 

C-V and I-V measurements of Si electrodes of different doping density in electrolytes free of fluoride show that in this case the dark current becomes dominated by thermally activated electron transfer over the Schottky barrier rather than by carrier generation in the depletion region [ChlO]. Note that the dark currents discussed above may eventually initiate the formation of breakdown type meso-pores, which causes a rapid increase of the dark current by local breakdown at the pore tips, as shown in Fig. 8.9. This effect is enhanced for higher values of anodic bias or doping density. 

The number of charge carriers generated in the SCR depends on the absorbed flux of incident photons per unit area P, the width W of the SCR and the wave-length-dependent absorption coefficient a of bulk Si. The latter parameter is shown in Fig. 7.6, while the resulting penetration depth for light of different wavelengths is shown in Fig. 10.4a. 

Fig. 10.4 (a) Computed minority carrier generation rate in bulk silicon for different wavelengths of monochromatic illumination of an intensity corresponding to a photocurrent density of 10 mA crrf2. (b) Bulk minority carrier density for carrier collection at the illumi-... 

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