Models for description of the influence of light intensity on the parameters of the transient photo-EMF of organic photoconductors

The aim of this work is a quantitative description of the connections betwe en the excitation laser flash intensity and the parameters of the transient photo-EMF. The models suggested in this paper were checked by experimental data obtained from layers of the organic photoconductors N,N'-dimethylpery lenetetracarboxylic bisimide (MePe, n-type photoconductor) and copper(II) p hthalocyanine (CuPc, p-type photoconductor) dispersed in poly(vinyl butyral e) (PVB) as binder. The models used are valid both for a neglectable and fo r a real dark charge carrier concentration. Nevertheless, recombination is the most important pathway of charge carrier disappearance in both photocon ductors. The models may also describe the influence of laser flash intensit y if the sample is illuminated by continuous white xenon light additional t o the laser flash and also if the laser is focused on a very small sample a rea. The model valid for a real dark charge carrier concentration yields th e best reproduction of the experimental data. In the case of neglectable da rk charge carrier concentrations this model may be simplified. Then it perm its an estimation of the mobility ratio of electrons and holes mu (n)/mu (p ): for example at excitation wavelength lambda (flash) = 580 nm the ratio m u (n)/mu (p) amounts to 1.35 for MePe and 0.73 for CuPc.