Triplet Excitations in Sexithiophene Films

In earlier publications we have presented detailed optical studies of sexithiophene (6T), including absorption of films and solutions (doped and undoped), photoinduced absorption (PA) and PA detected magnetic resonance (PADMR) of undoped films [1, 2, 3]. Through these measurements we have established the existence of photoinduced polarons (or 'radical ions') having spin 1/2 and two absorptionbands at 0.80 and 1.54 eV, respectively, and photoinduced bipolarons (or 'di-ions') with zero spin and a single absorption band at 1.2 eV. The long time dynamics of these charged excitions were studied by measuring the dependence of the PA and PADMR on the pump intensity and modulation frequency and temperature. The overall picture that emerges from these studies is that the polarons whichcontribute to the PA and PADMR signals are bound to shallow traps (more or less 0.05 eV) from which they are released via a monomolecular process. The characteristic time constants for this process are widely distributed with a mean time of about 0.1 msec. The bipolarons are bound to shallower traps (more or less 0.03 eV), they are therefore more dominant at lower temperatures. In addition tothese photoinduced charged species, which dominate the spectrum above 10 K, we have identified triplet excitons (S=1) at lower temperatures. In this work we extend our measurements to temperatures as low as 2 K in order to study in more detail the triplet excitons. We observe the triplet vibronic transitions and are able to estimate the energy of the ground triplet manifold level (1 to the thirdBu).