Herein, we propose a model to describe picosecond-nanosecond charge separation and nongeminate recombination in organic semiconductors. The model is used to explain time-resolved electroabsorption (EA) measurements performed on diodes made from phenyl-C61-butyric acid methyl ester. We find that the measured shape of the EA transient is due to a combination of microscopic carrier dynamic effects such as carrier trapping, as well as macroscopic effects such as band bending caused by the nonuniform poloron generation profile across the device. We demonstrate that the initial fast phase of the EA transient is due to hot free carriers being able to move freely within the device; over time these hot free carriers cool and become trapped giving rise to the second slower phase of the transient. We further show that the commonly observed dependence of the EA signal on probe wavelength can be explained in terms of the spatial overlap of electrostatic potential within the device and the optical mode of the probe light. Finally, we discuss the implications of these results for pump-probe experiments on thin organic films.

• Received 2 December 2013
• Revised 18 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.195307