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The Onsager theory of dissociation has been used to explain the electric field, excitation wavelength, and temperature dependence of photogeneration in amorphous selenium. The Onsager theory was formulated to explain the departure from Ohmic behavior in either weak electrolytes or solid dielectrics, and the analysis of charge separation was carried out using the theory of Brownian motion of one particle under the action of Coulomb attraction and the collecting field. Both the absolute magnitude and functional dependence on electric field of the photogeneration efficiency in amorphous selenium at any excitation wavelength can be unambiguously explained using a single parameter which is the initial separation between thermalized electron-hole pairs. This initial separation varies from 7.0 nm at 400-nm excitation to 0.84 nm at 620-nm excitation. The application of the theory to the measured photogeneration data also leads to the important conclusion that each absorbed photon creates a pair of thermalized carriers bound by their mutual Coulomb attraction. The low quantum efficiency measured for long-wavelength excitation is due to the smaller initial separation between oppositely charged thermalized pairs of carriers resulting in smaller dissociation efficiency. Good agreement is also obtained between the measured temperature dependence of the photogeneration efficiency and that predicted by the theory.

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Transport of holes through thin films of poly(N-vinylcarbazole) was studied using transient photoconductivity techniques. The photogeneration depends on the electric field with quantum efficiency of generation approaching a value of 0.1 at an applied electric field of 108 V/m. The motion of carriers cannot be characterized by a drift mobility but can be fully represented by an “effective transit time” given by (Teff)−1 = (mτ0)−1exp(γE1/2 − ε0 / kT), where m = d / d0 (d being the thickness of the film and d0 a characteristic length), τ0 is a characteristic release time from a localized state, E is the applied electric field, ε0 is an activation energy, k is the Boltzmann constant, and T is the absolute temperature. This relationship suggests a model in which holes jump from one localized state to the next one, the “effective transit time” being the sum of release times from the localized states. Bulk trapping occurs at low fields, and the release from these traps depends on both temperature and electric field.