CHARGED PHOTOEXCITATIONS IN THIOPHENE-BASED MOLECULAR SEMICONDUCTORS

We applied the steady-state photomodulation spectroscopy to vacuum-dep osited films of thiophene oligomers with controlled mesoscopic organiz ation in order to understand the relation between structural order and charge photogeneration. We studied alpha,omega-dihexylsexithiophene f ilms, which are highly ordered, and beta,beta'-dihexylsexithiophene fi lms, which are amorphous. We observed pairlike excitations in the form er (bipolarons and pi-dimers), triplets and polarons in the latter. To explain the results we propose a topological dependent charge separat ion model in which both dimensionality and carrier mobility play a rol e.