ON CHARGE CARRIER PHOTOGENERATION MECHANISMS IN ORGANIC MOLECULAR-CRYSTALS

Photoconductivity spectral characteristics in polyacene crystals are reported which may be interpreted in terms of two possible autoionization mechanisms, viz. direct ionization of photoexcited neutral molecular states and/or autoionization from fixed electronic levels after intramolecular vibronic relaxation. It is shown that the step-like character of the spectral dependence of the photogeneration activation energy E(a)ph (hv) allows the determination of the dominant autoionization mechanism in a given spectral range, while the structure of the intrinsic photoconductivity quantum efficiency-eta-(hv) curves indicates the energy values at which the excitation from lower-lying valence levels begin to switch on. The relaxation kinetics of both competitive autoionization channels are discussed.