The origin of photoluminescence from α-sexithienyl thin films

We have studied photoluminescence (PL) from sublimed films of alpha-sexithienyl (T6) in order to understand the nature of the excited species which give rise to radiative emission. Temperature-dependent measurements of the PL spectra show an apparent 530 cm(-1) blue shift of the emission as the sample temperature decreases, and the integrated PL intensity at 50 K is 1 order of magnitude larger than that at 300 K. Time-resolved PL spectra (at 10 K) indicate a fast energy transfer of excitons to lower lying trap states on the time scale of 250 ps. Site-selective PL (SSPL) measurements made at 25 K show clearly that the peaks in the steady-state PL spectrum are not only due to vibronic structure, but to emission from different defect states. Emission from the highest energy peak disappears when the excitation energy is below 17700 cm(-1), and that from the second peak drops when the excitation is below 17095 cm(-1). PL excitation (PLE) measurements show absorption into a distribution of defect states at about 17700 cm(-1). Our data show that PL in T6 polycrystalline thin films originates from defect states. Comparison is made with the intrinsic excitonic PL of the T6 solid phase, demonstrating the relevance of defects in the radiative processes of rigid-rod-like conjugated systems.