Quantum yield of charge carrier generation

It has been pointed out above that the deactivation of excitons may result in the formation of geminate electron/hole pairs that can eventually form free charge carriers. This process proceeds with strong competition from charge recombination and can be affected by an external electric field. According to the Onsager theory [31], the probability Pr of recombination can be estimated with the aid of Eq. (2-4). 

e is the elementary charge, F is the electric field strength, k is the Boltzmann constant, T is the temperature, and 0 is the angle between the vector connecting the charges and the direction of the electric field. 

The Onsager theory considers two potentials determining the fate of an electron/hole pair the Coulomb potential e /er (e=dielectric constant) and the thermal energy kT. Pairs having a radial distance r larger than tc will escape recombination. At the critical radial distance r, the thermal energy is equal to the Coulomb potential [see Eq. (2-5)]. 

o denotes the primary quantum yield, f(r, F, T) is the dissociation probability of pairs at radial distance r, and g(r) is the initial spatial distribution 

PBMSi poly(methyl phenyl silylene), PMPSi and poly(dihexyl silylene), PDHeSi. Adapted from Eckhardt [32] with permission from the author. 

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Fig. 1. Quantum yield of charge carrier generation vs. electrical field strength for neat PMPSi and PBMSi, and PMPSi doped with 3 mol% dinitrobenzene (DNB) = 355 nm.

In order to increase the quantum yield of charge carrier generation, doping of PMPS with different... 

Fig. 2.3 Quantum yield for charge carrier generation as a function of the electric field strength determined at 295 K for three poly-silylenes poly(biphenyl methyl silylene),...

Due to the generation of charge carriers, conjugated polymers become electrically conductive when irradiated with UV/visible light. The quantum yield for charge carrier generation is increased by additives, such as fullerene compounds. With the aid of time-resolved THz spectroscopy, the time- and frequency-dependent complex conductivity with its real and imaginary parts can be measured. THz radiation is absorbed by the polymer due to the dielectric... 

Fig. VC-3) [181]. Both the magnitude and the lifetime of transient PC increase by addition of a few percent C o to the pure polymer. Transient photoinduced absorption studies demonstrated rapid (sub-picosecond) photo-induced electron transfer from the polymer to C q thereby minimizing early time recombination and enhancing the quantum yield for mobile carrier generation [175,176]. If neutral excitons were the primary excitations, the observed charge transfer would be thermally activated this is not observed.

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

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. 

The local superficial rate of electron-hole pair generation can be computed considering a wavelength averaged primary quantum yield for the generation of charge carriers on the catalytic surface... 

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