Studies of Charged Excitations inπ-Conjugated Oligomers and Polymers by Optical Modulation

We have studied photogenerated charged excitations in sexithiophene and a variety of $\pi$-conjugated polymers by photoinduced absorption (PA) and PA-detected magnetic resonance. We found distinct spin signatures for polarons and bipolarons, which enabled us to separate their contributions to the PA spectrum. In all cases, we find that polarons have two subgap optical transitions, whereas bipolarons have one. Having unambiguously identified the energy levels of polarons and bipolarons, we show that bipolarons are stable photoexcitations, with negative effective correlation energy.