Photogeneration temperature dependencies

Figures 1 through 3 show the field and temperature dependencies of r]/7]0 calculated from Eq. (20) for different values of rQ. The dielectric constant was assumed to be 3.0. The Onsager formalism leads to strongly field-dependent photogeneration efficiencies that approach a limiting value at high fields. At...

Figure 2 The temperature dependencies of the photogeneration efficiency predicted from the Onsager theory for different values of the thermalization distance. The field was l.Ox 10 V/cm.

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. 

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. 

Several completely different experiments support our interpretation of the time-of-flight transport process and the conclusions we have drawn about the distribution of band-tail states. The time-resolved photoinduced absorption experiments of Ray etal. 9% ) support the view that the photogenerated holes are concentrated in the vicinity of an energy E, which moves deeper into the localized state distribution, linearly with temperature and logarithmically with time. Furthermore, the time decay of the photoinduced absorption, which is controlled by the more mobile of the two carriers (electrons), has the t form expected from the multiple trapping model (see, for example, Orenstein eta/., 1982). Thea = r/300°K temperature dependence for a reported by Tauc (1982) is in excellent agreement with the electron time-of-flight results. 

Two conclusions arise from Eq. (10). First, it is seen that efficiency can be maximized if electrochemical losses can be made small as compared with the Gibbs free energy change for water electrolysis, AGhjO- Second, it is seen that AT/hjO. the enthalpy of water electrolysis (with little temperature dependence), is involved in determining the overall efficiency as well as A//ph, the enthalpy of photogenerated charge carriers. If these two can be properly matched, a maximum overall efficiency may be accomplished. 

Strong field dependence of the yield and a simultaneously weak temperature dependence of exciton dissociation is at variance of Onsager s classic description of photogeneration predicting that limf F) = lim7-- oo (T) = being the... 

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