Mobility-lifetime product

Besides cTph and the photoresponse, the quantum-efficiency-mobility-lifetime product, r]gfj.r, is used as a figure of merit. Usually this product is measured at a wavelength of 600 nm, and typical valuesof (r/i /ur)6oo are lO cm A/ or higher. 

In the presence of an electric field the drift length is the mobility-lifetime product times the electric field A.mfp = prE [576]. With typical values of pz and E the mean free path usually exceeds by far the thickness of the solar cell, and virtually all photogenerated carriers can be collected. However, under certain operating conditions, field-free regions in the / -layer may exist, and the collection efficiency is decreased because the diffusion lengths of the carriers are much smaller than the thickness of the solar cell [11, 577]. 

H Antoniadis, MA Abkowitz, and BR Hsieh, Carrier deep-trapping mobility — lifetime products in poly(p-phenylene vinylene), Appl. Phys. Lett., 65 2030-2032, 1994. 

Figure 5.9 Hole and electron drift mobility lifetime product /xt and residual potential versus Te content in a-Scj- Te films. The /xt product was xerographically measured by Abkowitz and Markovics [14].

The low defect density in compensated material is apparent from the optical data in Fig. 5.18, which show a much reduced defect absorption band. The same result is deduced from time-of-flight and ESR data. Although the drift mobility is low, the mobility-lifetime product is comparable with the best undoped material, confirming the low defect density (see Fig. 8.24). The dangling bond density in the dark ESR experiment is about 4x10 cm" , with little dependence on the doping... 

Valerian and Nespurek (1993) determined values of the electron range (mobility-lifetime product) of vapor-deposited a-H2Pc from measurements of the photocurrent action spectra. The values were about 6 x 10-12 cm2/V, considerably lower than 10-9 cm2/V reported earlier by Popovic and Sharp (1977) for /J-H2Pc. For further discussions of photoconductivity in n-type phthalocyanies, see Schlettwein et al. (1994, 1994a), Meyer et al. (1995), and Karmann et al. (1996,1997). 

A finite carrier range (3) refiects the infiuence of deep traps and is controlled by the mobility-lifetime product ( xt). By using the numbers just given, (XT in practical polymeric TL should exceed 10 cm /V substantially. 

Figure 6.18 Results from junction-recovery measurements on Sn02/Ti02/Au diodes. The electron drift mobility and the mobility-lifetime product are determined for various injection conditions and forward bias. The electron mobility is found to increase with increasing injection level, while the mobility-lifetime product remains approximately constant. These findings can be consistently explained in a transport model based on trap filling and a transport-limited recombination mechanism. An alternative explanation can, however, also be based on a tunnelling transport model (Konenkamp, 2000a).

Internal photoemission of holes and electrons into KN3 was studied by de Panafieu and Royce [66]. Analysis of the results provided values for a number of important material parameters The band-gap value of 8.44 0.25 eV is consistent with the 8.55 eV obtained from Deb s absorption measurements [48] and with the theoretical model of Section C.2.b. At room temperature the mobility-lifetime product (range) /zr = (1.8 0.2) X 10" cm V" for electrons, and (6 2) X 10" cm V for holes. The electron affinity value 0.3 eV is consistent with Deb s photoemission data [142] if one interprets the rapid rise in his emission at 8.7 eV as being the threshold. 

In CO2 gas, the density-normalized electron mobility /ig fe is independent of temperature (2 X 10 molecule/cm V sec [25]), although the apparent mobility steadily decreases with the pressure free electrons are trapped by neutral (C02) clusters ( = 6) with nearly collisional rates, and the electron affinity of these clusters > 0.9 eV. When extrapolated to solvent densities of (2-15) x 10 cm typical for sc CO2, these estimates suggest that the free electron mobility is ca. 1 cm /V sec and its collision-limited lifetime Xg < 30 fsec [18]. If the lifetime were this short, the electrons would negligibly contribute either to the conductivity or the product formation. However, this extrapolation is not supported by experiment [18,20]. 

The residual potential is due to trapped electrons in the bulk of the specimen. The simplest theoretical model, which is based on range limitation and weak trapping (Vj drift mobility and lifetime r product) via the Warter equation [19] ... 

When guest molecules are able to explore more space during their transformation to products than is available in the cavity in which they are accommodated at the time of excitation (initial reaction cavity), their behavior may depend upon the effective space explored. The effective reaction cavity, the space explored, will depend on the lifetime of the excited state, the nature of the mobility, and the structure of the guest molecule and the intermediate(s) derived therefrom. The initial and effective reaction cavity... 

The calculations demonstrate a fundamental property of thin film solar cells made from low mobility materials the film thickness has to match the product of the mobility and the lifetime (/it) for the semiconductor. 

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