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 between 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'-dimethylperylenetetracarboxylic bisimide (MePe, n-type photoconductor) and copper(II) phthalocyanine (CuPc, p-type photoconductor) dispersed in poly(vinyl butyrale) (PVB) as binder. The models used are valid both for a neglectable and for a real dark charge carrier concentration. Nevertheless, recombination is the most important pathway of charge carrier disappearance in both photoconductors. The models may also describe the influence of laser flash intensity if the sample is illuminated by continuous white xenon light additional to the laser flash and also if the laser is focused on a very small sample area. The model valid for a real dark charge carrier concentration yields the best reproduction of the experimental data. In the case of neglectable dark charge carrier concentrations this model may be simplified. Then it permits 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 mu (n)/mu (p) amounts to 1.35 for MePe and 0.73 for CuPc.