Determining the photoelectric parameters of an organic photoconductor by the photoelectromotive-force technique

We report on a theoretical model to describe the non-steady-state photoelectromotive-force (photo-EMF) effect in organic photoconductors. Unlike the conventional theory of the photo-EMF effect developed for crystalline materials, this model accounts for the field dependence of the charge generation quantum efficiency and charge carrier mobility. To verify our findings a detailed experimental study of the charge carrier generation and transport processes in a organic photorefractive composite was performed using the photo-EMF effect and ac photocurrent measurements. The investigated composite was based on a conjugated triphenyldiamine based polymer (TPD-PPV) sensitized with a highly soluble fullerene derivative (PCBM). Our results show that at zero and low dc field the dependence of the photo-EMF signal on frequency, grating period and external electric field is well described by the standard model originally developed for an inorganic monopolar photoconductor with finite charge carrier lifetime. In this regime the photo-EMF effect was used to determine zero- and low-field photoelectric material parameters including the low-field hole mobility (μ0,h=1.3×10−4cm2/Vs), the effective charge carrier lifetime (τ=45μs), the diffusion length (LD=114nm), and the primary charge carrier generation efficiency (Φ=5.3×10−3%). In addition, the validity of the Einstein relation (D∕μ=25meV) was verified for the low-field regime. At high electric fields the signal behavior deviates significantly from the trend predicted by the standard model for inorganic photoconductors. This behavior which is mainly attributed to the strong field dependence of the charge generation rate is well described by our model.