Photogeneration Theories

This chapter reviews theories proposed to describe photogeneration in organic solids. Experimental results are described in the following chapter. 

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The recombination of photogenerated electrons and holes is the bane of all solar cells and a major reason for their less than ideal efficiencies. Excitonic solar cells, in which the electrons and holes exist in separate chemical phases, are subject primarily to interfacial recombination. There is, as yet, no theoretical model to accurately describe interfacial recombination processes, and this is an important area for future research. Wang and Suna [91] have laid a possible foundation for such a model by combining Marcus theory with Onsager theory. 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

The main scheme is shown in Fig. 17. The photogenerated electron hole pairs transfer to the soliton-antisoliton pairs in 10 13s. Two kinks appeared in the polymer structure, which separates the degenerated regions. Due to the degeneration, two charged solitons may move without energy dissipation in the electric field and cause the photoconductivity. The size of the soliton was defined as 15 monomer links with the mass equal to the mass of the free electron. In the scheme in Fig. 17, the localized electron levels in the forbidden gap correspond to the free ( + ) and twice occupied ( — ) solitons. The theory shows the suppression of the interband transitions in the presence of the soliton. For cis-(CH)n the degeneration is absent, the soliton cannot be formed and photoconductivity practically does not exist. 

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. 

The Onsager theory of geminate recombination was qualitatively consistent for aryl-substituted thiapyrylium salt and dialkylamino-substituted triphenyl-methane dispersed in polycarbonate film [301]. The quantum yield of Hie photogeneration was equal to 0.5 at the electric field strength of 106 V cm-1, mobility of I0 12m2 V-1 s-1. Hole and electron conductivity was established. In a triphenylamine-lexan system doped with a boron diketone acceptor, the... 

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

Figure 4 Photogeneration quantum efficiencies for hole and electron generation in compound 1. The solid lines are Onsager theory fittings with parameters listed in the figure. (Reprinted from Ref. 27. Copyright 2000 The American Physical Society.)...

The behaviour of p-InP discussed in Section 8.9 was judged to be nonideal on the basis of a simplified theory for one electron transfer reactions. However, the reduction of protons to hydrogen by photogenerated electrons must be a two step process. There are two possible mechanisms ... 

Theory of the transient photocurrent behaviour at the ORDE. The stationary optical disc electrode is assumed to be uniformly illuminated by parallel light which is switched on at time t = 0, and which produces a measurable concentration of photogenerated electrons on the particles denoted by c. The differential equation for the generation and transport of these electrons to the electrode surface, with concurrent homogeneous back reaction is set up with the following assumptions. 

Kochi and co-workers have recently identified and characterized the weak charge transfer complexes between tropylium ion and a series of substituted arenes in acetonitrile solution [74], Photoexcitation of these electron donor acceptor (EDA) complexes leads to an electron transfer from the arene donors to the tropylium ion in accord with Mulliken s theory [75]. Time resolved spectroscopic observation of the arene radical cations (formation within the 30 ps laser pulse) has confirmed their intermediacy. The subsequent decay of the photogenerated radical cation and the concomitant regeneration of the ground state EDA complex occurs with a rate constant, kBET > 4 x 1010 s 1 (Scheme 11). This fast back electron transfer... 

Figure 1 The field dependencies of the photogeneration efficiency predicted from the Onsager theory for different values of the thermalization distance. The temperature was 295 K.

The applicability of the Onsager theory can be determined from the field dependence of the photogeneration efficiency at low fields. From Eq. (20), the efficiency should have a linear field dependence. From plots of the efficiency versus field, the slope-to-intercept ratio is... 

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