Unusual luminescence of thin vapour-deposited oligothiophene films

Thin films of two different oligothiophenes, end-capped quaterthiophene (EC4T) and α-sexithiophene (α6T) were investigated by optical absorption and photoluminescence (PL) spectroscopy. Films of different thicknesses (2–200 nm) were deposited on glass at different substrate temperatures by thermal vapour deposition. For α6T, a large Stokes shift (1645 cm−1) is observed, indicating a trap-dominated luminescence of the polycrystalline bulk material. For thin films (less than 200 nm) and temperatures below 100 K, additional extraordinary PL lines appear in the luminescence spectra of EC4T and α6T. These are red-shifted by 730 cm−1 and blue-shifted by 545 cm−1 for EC4T and α6T, with respect to the ordinary PL lines, respectively. Their relative intensity increases for decreasing film thicknesses. For both molecules, we thus suggest that these extra lines are due to specific traps, which stem from the nucleation process of the crystallites on the glass substrate.