UNUSUAL LUMINESCENCE OF THIN VAPOR-DEPOSITED OLIGOTHIOPHENE FILMS

Thin films of two different oligothiophenes, end-capped quaterthiophen e (EC4T) and alpha-sexithiophene (alpha 6T) were investigated by optic al absorption and photoluminescence (PL) spectroscopy. Films of differ ent thicknesses (2-200 nm) were deposited on glass at different substr ate temperatures by thermal vapour deposition. For alpha 6T, a large S tokes shift (1645 cm(-1)) is observed, indicating a trap-dominated lum inescence of the polycrystalline bulk material. For thin films (less t han 200 nm) and temperatures below 100 K, additional extraordinary PL lines appear in the luminescence spectra of EC4T and alpha 6T. These a re red-shifted by 730 cm(-1) and blue-shifted by 545 cm(-1) for EC4T a nd alpha 6T, with respect to the ordinary PL lines, respectively, Thei r relative intensity increases for decreasing film thicknesses. For bo th molecules, we thus suggest that these extra lines are due to specif ic traps, which stem from the nucleation process of the crystallites o n the glass substrate.