Free charge carrier generation which is photoinduced by excitation of charge transfer complexes or by formation of exciplexes has often been interpreted in terms of large “initial” separations between geminate carrier pairs. Such large separations are in contrast to the contact geometry of ground state charge transfer complexes and the relatively short range electron transfers expected in exciplex formation. The large separations appear to be required by Onsager analysis of the electric field dependence of the free carrier yield. Almost invariably the analysis is made under the assumption that the initial distribution of pair radii with respect to the applied field is spherically symmetric. Here we propose the possibility that the angular distribution function might be nonspherical and suggest a simple modification of the Onsager analysis to include a field-dependent angular distribution of pair radii that is Boltzmann-weighted. The resulting electric field dependence is shown to be much steeper than for the corresponding spherical distribution and provides the opportunity to interpret experimental data in terms of substantially smaller initial separations.
Sergei N. Smirnov, Charles L. Braun, "Photoinduced Free Carrier Yields in High Fields: Field Dependent Distribution Functions" in Journal of Imaging Science and Technology, 1999, pp 425 - 429, https://doi.org/10.2352/J.ImagingSci.Technol.1999.43.5.art00005