Photogeneration thermalization distances

Over the temperature interval 165 K to 300 K, the calculations of Silinsh and Jurgis (1985) indicate that the thermalization rate in pentacene decreases from 3 X 1012 to 0.8 X 1012 s 1. The trend is opposite to what would be expected in liquid hydrocarbons and may be attributed to the rapid increase of mcff with temperature. The calculated mean thermalization distance increases with incident photon energy fairly rapidly, from 3 nm at 2.3 eV to 10 nm at 2.9 eV, both at 204 K. With increasing temperature, (r) decreases somewhat. These thermalization distances have been found to be consistent with the experimental photogeneration quantum efficiency when Onsager s formula for the escape probability is used. 

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.

For = 3.0, the ratio is 3.5 x 10-5 cni/V at 296 K. Although based on the assumption that g(r,0) is spherically symmetric, the ratio is independent of the function selected to represent the distribution of thermalized pair separations and contains no adjustable parameters. It thus provides a very critical test of the theory. Batt et al. (1968,1969) were the first to demonstrate the applicability of the Onsager formalism by use of the low-field slope-to-intercept ratio. The primary quantum yield and the thermalization distance can be determined by comparing experimental and theoretical values of the field dependence of the photogeneration efficiency at high fields, or by the temperature dependence of the zero-field quantum efficiency. The latter technique is based on the assumption that the primary quantum yield is independent of temperature. In most cases, thermalization distances and primary quantum yields have been determined from the field dependencies of photogeneration efficiencies at high fields. 

In materials where the photogeneration involves the surface-enhanced dissociation of an exciton, as is generally the case for the phthalocanines, the photogeneration efficiency defined by Kanemitsu and Imamura represents the fraction of photons that create exitons that diffuse to the interface between the generation and transport layers. The injection efficiency then represents the fraction of pairs that dissociate into free electrons and free holes. The field dependence of the photogeneration efficiency was described by the Onsager theory. A primary quantum yield of 0.50 was reported. Values of the thermalization distance and the injection efficiency were not cited. 

Melz (1972) measured photogeneration efficiencies of charge-transfer complexes fonned between PVK and 2,4,7-trinitro-9-fluorenone (TNF). The thermalization distance varied from 25 A for the 1.0 0.06 PVK TNF molar ratio to 35 A for the 1.0 1.0 molar ratio. For all concentrations, the primary quantum... 

Photogeneration efficiencies of vapor-deposited 2,4,7-trinitro-9-fluorenone (TNF) were reported by Bulyshev et al. (1984). For photon energies greater than 3.0 eV, the results were described by a primary quantum yieild of 0.20 and a thermalization distance of 25 A. The geminate pair dissociation was described by an autoionization mechanism due to Balode et al. (1978). In agreement with model, the photogeneration efficiency was only weakly temperature dependent. 

Kaul and Haarer (1987) measured photogeneration efficiencies of PVK. In agreement with Hughes (1971a) and Schechtman (1976), the low-field results agreed with predictions of the Onsager theory. For 337 nm excitation, the thermalization distance was 18 A, compared to 20 to 26 A reported by Pfister and Williams (1974), Borsenberger and Ateya (1978), and Stolka and Pai (1978). 

The fundamental change consists in the assumption that the thermalization process takes place by internal conversion. Transition to the bound pair is always performed from state Si, so the separation distance of the charge carriers is independent of light energy. Electric field dependence of photogeneration efficiency is similar in this case to that obtained on the basis of Onsager model, but the thermalization distance ro is considerably lower. ... 

If the polymer photoconduaor is doped with the high elearon affinity dopant (acceptor) or with low ionization potential dopant (donor), then one can usually observe a new absorption band in the visible range of light. This band results from the formation of CT complexes between the photoconduaor and the dopant molecules. These complexes become the centers of photogeneration. The thermalization distance determined from the conventional Onsager theory is of the order of 2-3 nm, so it is 1 order of magnitude bigger than the distance between donor and acceptor in the excited complex. 

Amster s apparatus is an on-line ESI-MS technique for the study of photochemical reactions that greatly reduces the transit time of photogenerated species [34]. Figure 5.5 shows sample solutions that are irradiated directly in the optically transparent nanospray tip of the ESI source. Subsequent thermal reactions of the primary photoproducts take place in the region between the photolysis zone and the tip end. The transit time of a photoproduct depends on the volumetric flow rate of the sample, the inner diameter of the tip, and D, the distance between the midpoint of the irradiated zone and the tip end. For example, with D=0.84 mm, a tip diameter of 40 im, and a flow rate of 40 tLh , products require 95 ms to arrive at the tip end for spraying. All chemical reactions are quickly quenched (ms time scale) once the... 

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