Photogeneration of Charge Carriers in Anthracene Through Two Photon Excitation

Onsager's theoretical description of the diffusive motion of a pair of oppositely charged particles in an applied field appears to be satisfactory for describing the field dependence of carrier generation in low mobility materials. This theory predicts also that the quantum efficiency of carrier generation is dependent upon the energy of excitation. Though this dependation is often not found in organic materials such as anthracene, presumably due to internal conversion thermalization processes, it is found to hold for a-selenium. The field dependence of the quantum efficiency in a-selenium was originally described within the framework of a Poole-Frenkel effect, but more recent results from a two-photon excitation study showed Onsager's theory gave good agreement with experiment over a wider range of applied fields, particularly for low fields, because surface effects were eliminated. Since Onsager's theory gives agreement with single photon experimental studies on anthracene when clean crystal surfaces are used, (no surface effects), it was thought worthwhile to study two photon carrier generation also in anthracene as a function of applied field and excess photon energy.