Recombination effects

A further application of the coplanar cell configuration showed in Fig. 3.1(c) concerns the study of the time dependence of the photocurrent following carrier excitation by means of a short pnlse of illnmination. This transient photodecay technique enables the examination of the interaction of initially free carriers with varions localized states. In principle, the decay of photocnrrent measured in this manner should (in the absence of recombination effects and phenomena associated with drift close to the surface of a thin film) correspond to the behavior in the initial pre-transit regime of a TOF pnlse. Becanse it allows measurements to be performed on very thin films under conditions appropriate to their nse in many device applications, and because the photocurrent may be examined over several decades of time withont the complications associated with carrier extraction, the techniqne has become rather popular over recent years. 

Bakale et al. [397] pulse irradiated the hydrocarbons cyclopentane, cyclohexane and n-hexane with 0.9 MeV electrons of duration 10 or 100 ns. The transient conductivity decreased approximately exponentially with time for low doses of radiation. The first-order decay of the conductance is probably due to electrons reacting with impurities. With higher doses, the conductance decays approximately as inverse time, characteristic of a second-order recombination of free ions. No evidence for time-dependent geminate ion-pair recombination effects was observed. 

In the case of a metal-semiconductor junction the semiconductor surface is closely coupled to the metal. As a result electrons in the conduction band at the surface see a high density of empty metal states into which they can cross the interface isoenergetically. Similarly, valence band holes see a high density of filled electron states in the metal. As a result, electron transfer is usually treated as direct transfer in a thermionic process and surface states a re-rjs legated to a minor role in surface generation and recombination effects. -1—... 

Additional work is needed to refine these models. For example, rapid vacancy generation and vacancy-interstitial recombination are assumed. These effects combine to modulate the self-interstitial supersaturation. Thus, the supply of self-interstitials at the oxidizing interface cannot keep up with recombination effects. Rapid recombination may be justified at high doping levels, because species such as V+ and I or V and I+ may be plentiful. 

Following Vieth and Huneke [19], it can be stated that the RIY is determined only by transport and ioni-zation/recombination effects ... 

Schroder, K. and Memming, R. 1985. Analysis of trapping and recombination effects at n-GaAs. Ber. Bunsenges. Phys. Chem., 89, 385-392. 

Let us note that the introduction of (io accounts for the recombination effect on the current. Its relation to the carrier... 

The surface recombination of a-Si H is greatly enhanced in multilayer structures which constrain the electron-hole pair to be near an interface. For example a thin a-Si H film sandwiched between a-Sig N4 layers, has a luminescence intensity which drops rapidly when the layer thickness is less than 200 A. These results are discussed further in the next chapter (Section 9.4.1) and show that surface recombination effects extend 100-200 A into the film, and that electron-hole pairs created farther from the surface are not influenced by the surface at low temperature. As the carriers become more mobile at higher temperatures, the diffusion length increases and surface recombination is more significant. 

The charge-induced defect creation mechanism is too slow to be significant at low temperature and the electronic recombination effects reestablish themselves. Low temperature measurements (0-100 K) have been performed using an IR probe beam to modulate the excess carrier density that is in the band tail states (Hundhausen, Ley and Carius... 

Removal of free ions occurs by two mechanisms ion-ion recombination (essentially saturated ternary ion-ion recombination effective binary coefficient a = 2 10 cm s ) and ion-attachment to aerosol particles. The latter process leads to so-called large ions which are, in fact, electrically-charged aerosols rather than ions in a strictly physical sense [60]. Usually, ion-attachment is the most important sink for free ions throughout the troposphere as the tropospheric aerosol content is relatively large (Fig. 2). In this respect, the tropospheric ionization-deionization balance differs greatly from the stratospheric one. 

A variety of refinements have been made to take into account the surface recombination effect [93, 211-225]. The earliest of these [212, 213, 216] involve some simplifying assumptions ... 

Low temperature bolometers have also been used for detecting neutral particles in high resolution scattering experiments (Cavallini et al, 1971c). Especially the detection of H-atoms proved to be very efficient due to recombination effects on the surface (Marenco et al, 1972). 

Several papers have been published to explain the sensitivity saturation shown in the high mole fraction region of N. In those reports, the saturation is interpreted by the steric hindrance effect ( 8 ) or the recombination effect K)). Assumption (II) made in this paper means that these two effects are neglected. However, the calculated results explain the experimental data on sensitivities well. Therefore, the contribution of these effects seems to be small. 

Equation (7) is valid if the field in the space-charge region is sufficient to prevent recombination but, in disordered materials, we might expect some recombination effects even when migration is rapid. Space-charge recombination will also be important if slow interfacial electron transfer leads to a build-up of minority carriers. 

Although the effects of spin coherence have been mainly studied using radiation-chemical processes as an example, published are the first works on the MARY spectra of radical ion pairs produced in solutions by photoionization. Probably, there are no principle obstacles to the application of the method of quantum beats to these systems. Interpretation of results is expected to be more simple, in this case, because of the use of monochromatic sources of ionization and the absence of cross recombination effects typical of the ionization track. Another manifestation of spin coherence, observed experimentally but omitted in this review, is the beats induced by resonance microwave pumping [36-38]. The range of applications of this phenomenon for studying spin-correlated radical ion pairs has yet to be outlined. 

Solid curves are without and dashed curves are with space charge region recombination. The approximate relation tp = 5.21 x 10 6d 177 was assumed to simulate grain boundary recombination effects. 

J. F. McCann and D. Haneman, Recombination effects on current-voltage characteristics of illuminated surface barrier cells, J. Electrochem. Soc. 129 (1982) 1134-1145. 

Remove surface and interface states or at least ehminate them in the energy interval of the band gap. Because the presence of surface states is conducive for charge-trapping and recombination effects, moving them out of the band gap is necessary for the construction of high-speed semiconductor devices like the ones that are based on GaAs andInP. 

It should be emphasized that the previous models only describe the current due to minority carriers. Under near-flatband conditions, the majority carriers start to contribute to the overall current. This can be recognized by an increase in the dark current, and under those conditions (2.64) and (2.65) are no longer useful. Another implicit assumption is the absence of mass transport hmitations in the electrolyte, but this is rarely an issue in photoelectrochemistry due to the high electrolyte concentrations and modest efficiencies reported thus far. Finally, it should be realized that the models described above cannot accormt for recombination at the semiconductor/electrolyte interface. Wilson pubhshed an extension of G tner s original model that included interfacial recombination effects [56]. Although this somewhat complicated model is not often used because it does not account for the dark current and recombination in the space charge, it is of value for metal oxides that show extensive surface recombination. 

The importance of recombination effects in solar cell performance and methods of treating them should be clear from these three Parts. 

For a fuller treatment of bulk recombination in solar cells one needs to introduce the electron-hole pair generation rate due to the absorption of radiation in (9.10). As our main interest is here in recombination effects, however, this term was omitted for the sake of simplicity. The effect of its inclusion, together with the other items noted at the end of section 8, in a spirit closely related to that of the present exposition was described in Among other attacks on these problems we can note only a small selection from the very large literature ... 

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