Disorder influenced optical properties of α-sexithiophene single crystals and thin evaporated films

We report measurements on steady state and time resolved photoluminescence (PL) and PL-excitation (PLE) of sexithiophene single crystals and evaporated thin films. In contrast to the broad spectral features usually observed even in crystals, we have observed for the first time sharp emission peaks from the single crystal superimposed on a broader background. The broad background is due to emission from defect states, which are observable in PLE measurements and site-selective PL. These defect states are visible both in films and crystals, but their density and nature are strongly dependent on the growth conditions. Additionally in the film a fast energy transfer from the primary excited molecular states to defect states is observed on a sub 100 ps timescale whereas the fluorescence lifetime of these defects is around 700 ps.