Advances in spin-dependent recombination studies of π-conjugated polymers

We have studied photogenerated charged excitations in sexithiophene and a variety of π-conjugated polymers by photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR). 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 photo excitations, with negative effective correlation energy. We have also identified polaron parr species in the PADMR of C60-doped DOO-PPV films by the existence of an anisotropic spin-exchange interaction.