Spin signature of photoexcitations in sexithiophene

We present photoexcitation studies of vacuum-deposited neutral films of sexithiophene (6T) using photoinduced absorption spectroscopy and photoinduced absorption detected magnetic resonance (PADMR) techniques. We find evidence for photoinduced polarons (photoexcited ‘radical ions’) having spin 1/2, with two absorption bands at 0.80 and 1.54 eV, respectively, and with negative PADMR signal at g≅2. Similar absorption bands are observed in lightly p-doped 6T, and are interpreted as due to 6T+ radical cations. In addition, a photoinduced absorption band is found at about 1.1 eV, which is shorter lived and decreases faster with increasing temperature than the polaron bands. We identify this band as due to spinless bipolarons (photoexcited ‘di-ions’). A similar di-cation band has been identified in heavily p-doped 6T. Finally, we have identified triplet excitons (S=1) with triplet-triplet transition energy at about 1.45 eV.