Charge carriers generation

Let us now discuss in some detail the key events that control the photocurrent charge-carrier generation, injection and transport. 

The first step is photoexcitation of the light-absorbing group, followed by migration of the exciton to an acceptor site, where an encounter 

Bound hole-electron pair Geminate recombination [ ...A ] Relaxed exciplex (fluorescence, heat) 

According to this theory, the overall photogeneration efficiency 0(E) is given by 

After neglecting the small terms, 0(E) becomes a relatively simple function of the electric field E, temperature T, the permittivity of the medium, the material-dependent thermalization length rg and 0g, which can also be thought of as the maximum photogeneration efficiency at an infinite field E and high temperature T. 

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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... 

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... 

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. 

For a 6 pm thickness of the nanotube array film annealed at 600 °C the quantum efficiency was calculated to be 81% and 80%, for wavelengths of 337 nm (3.1 mW cm ) and 365 nm (89 mW cm ), respectively. The high quantum efficiency clearly indicates that the incident light is effectively utilized by the nanotube arrays for charge carrier generation. 

Fig. 23 (a) Dependence of the hole mobility in a film of poly-spiro-bifluorene-co-benzothiazole (PSF-BT) as function of the time elapsed after charge carrier generation by a 130 fs laser pulse at different applied voltages. The horizontal lines represent the electron and hole mobilities inferred from ToF experiments, (b) Momentary mobility as a function of the averaged distance that a carrier travelled after a given time. The inset depicts the chemical structure of PSF-BT. From [154] with permission. Copyright (2009) by the American Institute of Physics... 

In some compounds the doping effect may be the result of charge-carrier generation brought about by an intermolecular charge-transfer transition 72,83)... 

So the free charge carrier generation depends on the quantum yield of the creation of the thermalized pairs and the probability of their dissociation. 

The main experimental results of charge carrier generation in PVC was explained in the frame of the Pool-Frenkel model [28-30]. The dependence of the recombination time on electric field was due to the change of the mobility in the electric field. Germinate recombination of the electron-hole pairs was investigated by means of luminescence decay characteristics [31]. 

Many authors have investigated the photoconductivity of the polydiacetylenes [142-171], The main problem discussed concerns the nature of the initial act of the photoeffect. At first, most authors considered the exciton formation to occur at the beginning with consequent dissociation on the free carriers. Then it was shown the broad band existence for directions along the chains. The unification of the excitonic and band model of the free charge carrier generation was developed [146-150],... 

Even when meaningful parameters have been obtained, it is still difficult to relate them to possible electronic, molecular or ionic processes. This point is discussed at length by Garrett (14, 15). However, some recent experiments have yielded more fundamental evidence about the conduction process and charge carrier generation. 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

While this general model for charge carrier generation has developed over the years, it is not without conjecture. As one alternate possibility, the presence of diamagnetic Jt-dimers, resulting from the combination of cation radicals, has... 

Fig. 2 Schematic representation of charge-carrier generation via singlet excitons. (a) Singlet/electrode (b) singlet-singlet (c) intrinsic...

Free charge carriers generated upon optical excitation either get trapped at the surface vacancies or undergo charge recombination (1) to (3). 

The photoconductivity of poly[2-(N-carbazolyl)ethyl vinylether]91,92) and particularly of poly(N-acryloylcarbazole)91) is much inferior to that of PVK. In the case of the acrylic polymer the reported photocurrents are at least two orders of magnitude lower. The poor charge carrier generating efficiency is blamed for low photocurrents91 The relatively poor performance of the vinylether polymer is how ever attributed to charge carrier mobility92). 

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