Charge-carrier dissociation

Deibel, C. Charge carrier dissociation and recombination in polymer solar cells. Phys. Status Solidi A 2009, 206, 2731-2736. 

The variations of dielectric constant and of the tangent of the dielectric-loss angle with time provide information on the mobility and concentration of charge carriers, the dissociation of defect clusters, the occurrence of phase transitions and the formation of solid solutions. Techniques and the interpretation of results for sodium azide are described by Ellis and Hall [372]. 

Lanthanum fluoride (and fluorides of some other lanthanides) has an unusual type of defect (see Section 6.3.2), namely Schottky defects of the molecular hole type (whole LaF3 molecules are missing at certain sites). Charge carriers (F ) are formed as the result of interaction of LaF3 with this hole, leading to dissociation with formation of LaF2+ and F . 

As suggested in Fig. 4.3 the charge carrier formation in step (a) may be compared to a dissociation leading to the following equilibrium... 

Such a chemical approach which links ionic conductivity with thermodynamic characteristics of the dissociating species was initially proposed by Ravaine and Souquet (1977). Since it simply extends to glasses the theory of electrolytic dissociation proposed a century ago by Arrhenius for liquid ionic solutions, this approach is currently called the weak electrolyte theory. The weak electrolyte approach allows, for a glass in which the ionic conductivity is mainly dominated by an MY salt, a simple relationship between the cationic conductivity a+, the electrical mobility u+ of the charge carrier, the dissociation constant and the thermodynamic activity of the salt with a partial molar free energy AG y with respect to an arbitrary reference state ... 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

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

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],... 

Photoexcitation of the complex is accompanied by charge transfer from the donor TBPDA molecule to the acceptor C6o one and CT-exciton is formed. Depending on mutual orientation of spins of components (electrons and holes), CT-exciton can be either a singlet or a triplet one. Free charge carriers are formed in molecular crystals mainly due to thermal or impurity dissociation of triplet CT-excitons [6],... 

We have developed a new technique, Charge Distribution Analysis or CDA. It holds promise of being widely applicable to the study of peroxy and 0 in insulators. It draws on three diagnostic properties of 0" states (i) as long as they are dimerized (= peroxy) they are electrically inactive, (ii) upon dissociation at least some 0 become mobile charge carriers, (iii) 0 which diffuse to the surface cause a positive surface charge. 

---