Excitation dynamics in α-sexithiophene single crystals and UHV-grown films

We have investigated the disorder-determined optical properties of α-sexithiophene by comparing measurements of an evaporated film and a single crystal. We observed for the first time sharp lines in absorption and photoluminescence from both samples representing the intrinsic electronic properties of the material. The time-integrated emission spectra of both samples are dominated by radiative trap emission, which is more pronounced in the film due to the higher degree of disorder. Photoluminescence excitation and site-selective photoluminescence measurements show the different origin and character of the traps in the samples. We attribute the single crystal traps to the so-called X-traps while the nature of the traps in the film is still undetermined. Time-resolved photoluminescence measurements of the film show a fast transfer process of the sharp bulk emitters to the unstructured trap emission within the first 50 ps after excitation. A comparable process is observable in the single crystal, showing a slower dynamics due to lower trap density.