Wiebeler, C.C.WiebelerTautz, R.R.TautzFeldmann, J.J.FeldmannHauff, E. vonE. vonHauffComo, Enrico daEnrico daComoSchumacher, S.S.Schumacher2022-03-042022-03-042013https://publica.fraunhofer.de/handle/publica/23309410.1021/jp30848692-s2.0-84875457190We study electronic and optical properties of the low-bandgap co-polymer PCPDT-BT (poly-cyclopentadithiophene-co-benzothiadiazole) and compare it with the corresponding homo-polymer PCPDT (poly-cyclopentadithiophene). We investigate the linear absorptivity in these systems for neutral molecules and for their singly charged ions based on quantum-chemical calculations and experiments. One of our main findings is that the ions of the homo-polymer show a polaron absorption that is symmetric between anion and cation, whereas for polaron excitations in the co-polymer this symmetry is strongly lifted. We demonstrate that this asymmetry can be attributed to the absence of symmetry between the high-lying occupied and low-lying unoccupied molecular orbitals in the co-polymer with type-II orbital alignment between the moieties constituting the chain. This notion is of importance for the qualitative and quantitative interpretation of spectroscopic polaron data and is not specific t o the system studied here but similarly applies to other co-polymers.en621541journal article