Photogeneration of charge carriers

The explanation of the nonmonotonic increase in the quantum efficiency (Fig. 4) may be connected with the high concentration of the intermediate exiplex sites [4], The nature of these sites may be connected with deep acceptor traps located at 1.0—1.3 eV above the valence band. The confirmation of the short lived 

The impurity generation route is shown in Fig. 5 to the right of the line. The interaction of the relaxed states formed from m ami unexcited impurity t results in exiplex t formation. This exiplex can decay thermally or form coupled ion-radical pairs t, which may dissociate in the electric field. For explanation of the absorption and photoconductivity spectrum correlation it is necessary to assume a very high concentration of exiplex sites. 

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]. 

In the treatment of photoconductivity in Sect. 8.4.1, we simply mentioned in passing the primary process the generation of the excess charge carriers using the internal photoeffect. In this section, we will treat the details of this process and choose again as an example the anthracene crystal. 

We initially restrict ourselves to the simplest process the linear intrinsic photogeneration of charge-carrier pairs. Their production rate is proportional to the absorbed intensity of photons of the excitation light in the crystal, and requires neither excitonic processes at the crystal surface nor at the contacts, nor does it involve biexcitonic processes. 

The absolute value of the quantum yield r] in anthracene crystals at room temperature is of the order of 10 [26] Fig. 8.14 shows its electric-field dependence 

For holes, the experimental data for the electric-field dependence are nearly identical ri (F) = rj (F). This holds both for pristine and for non-pristine crystals [26]. 

In the final step, these bound charge-carrier pairs dissociate by thermal activation. The necessary thermal activation energy for this is = -Ucp = Ecp - E and is reduced by an applied electric field which is superposed onto the Coulomb potential, that is by the Stark effect (see Fig. 8.17). 

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Materials with large E ex values would not be indicated for e.g., photovoltaic devices, since the photogeneration of charge carriers would be inefficient, but could in principle be used as light emitting diodes. Section 4.2 will show examples of the experimental determination of the band diagrams of selected organic semiconductors. El is usually obtained from o vs. T measurements, since a oc ... 

Kallmann H, Pope M (1959) Photovoltaic effect in organic crystals. J Chem Phys 30 585 Geacinto N, Pope M, Kallmann H (1966) Photogeneration of charge carriers in tetracene. J Chem Phys 45 2639... 

Recall that the concept of Fermi quasilevels, suggested by Shockley (1950), can be introduced as follows. Under steady state photogeneration of charge carriers, a dynamic equilibrium arises in a semiconductor between generation and recombination of electron-hole pairs. As a result, certain steady state (but not equilibrium ) concentration values nj and p are established. The quasiequilibrium concentrations ng and pg are defined by the relations ng = n0 + A and Po = Po + Ap> and since photogeneration of carriers occurs in pairs, we have An = Ap = A. Let the following inequalities be satisfied ... 

The most authors consider that the Onsager theory cannot be applied to the photogeneration of charge carriers in polydiacetylenes and transport processes are controlled by deep traps. 

Photoconductivity in a solid is defined as an increase of conductivity caused by radiation. The phenomenon of photoconductivity involves the processes of absorption of radiation, photogeneration of charge carriers, their separation, diffusion and drift in an applied electric field, their temporary immobilization at sites known as trapping rites, release from traps and finally their recombination. The phenomenological relationships covering all these processes were primarily developed in connection with the study of crystalline covalent solids which dominated the early scientific literature on photoconductivity. Concurrent with the basic understanding of the phenomena was the development of several experimental techniques to study the fundamental processes and the specific identity of the defects and impurities that control these processes. 

If the rate of charge-carrier generation at a given point in the bulk is proportional to the light irradiance (/) at that point, and the irradiance obeys exponential decay kinetics, the spatial distribution g(x) of photogeneration of charge carriers is given by... 

Schwarzburg K. and Willig F. (1997), Modeling of electrical transients in the semiconductor/electrolyte cell for photogeneration of charge carriers in the bulk , J. Phys. Chem. B 101, 2451-2458. 

Photogeneration of charge carriers in blends of conjugated polymers and semiconducting nanoparticles. Thin Solid Films 451-452 48... 

Fig. 8.16 Scheme of the individual steps in the process of intrinsic photogeneration of charge-carrier pairs, Mp - M, in a molecular C7stal. The charge carriers are polarons (p). a Sq = neutral ground state S], S2, S3 are singlet excitons. Rate constants k/ i for autoionisation, kn for radiationless and kr for radiative intramolecular recombination (fluorescence), b bound charge-carrier pairs... 

Here, n is the concentration of the charge carriers, [S] is the concenhation of singlet excitons, I the intensity of the UV radiation, Uj the constant for photoionisation of the singlet excitons, and k2 is the rate constant for charge-carrier production by two-photon absorption. AU three processes are proportional to 1. For the separation into individual processes, the spectral dependence of the quadratic photogeneration of charge-carrier pairs was determined and analysed [31]. 

Fig. 8.18 The dependence of the number of photo charge carriers in an anthracene crystal on the intensity of the irradiation at high intensities. The wavelengths of the excitation light were in the range of the absorption of the singlet excitons in which single-photon photogeneration of charge carriers is negligible...

Photogeneration of charge carriers 51 Table 2.2 Chemical structures of typical photoconducting polymers. 

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