Aggregate states and energetic disorder in highly ordered nanostructures of para-sexiphenyl grown by hot wall epitaxy

We report on photoluminescence (PL) and thermally stimulated luminescence (TSL) in highly ordered nanostructures of para-sexiphenyl (PSP) grown by hot wall epitaxy (HWE). A low-energy broad band is observed in the PL spectra that can be attributed to the emission from molecular aggregates. While the intrinsic exciton emission in steady-state PL dominates at low temperatures, the emission from aggregates increases with elevating temperature and its magnitude depends sensitively on film preparation conditions. Time-resolved PL measurements showed that the aggregate emission decays with a life-time of approximate to4 ns, which is approximately an order of magnitude larger than the-lifetime of singlet excitons. TSL data suggests the presence of an energetically disordered distribution of localized states for charge carriers in PSP films, which results from an intrinsic disorder in this material. A low-temperature TSL peak with the maximum at around 30 K evidences for a weak energy disorder in PSP films, and has been interpreted in terms of a hopping model of TSL in disordered organic materials.