Onsager mechanism of photogeneration in amorphous selenium

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Abstract
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.