Spectroscopic study of electron and hole polarons in a high-mobility donor-acceptor conjugated copolymer

Recent advances in developing donor-acceptor conjugated copolymers have led to great performance improvement in both organic photovoltaic cells and field-effect transistors. In contrast to the extensive spectroscopic studies on the photoinduced charge transfer and separation processes in these copolymers, little has been done to probe their charge-transport properties on a microscopic scale. In this work, we combine charge modulation spectroscopy (CMS), photoinduced absorption, and chemical doping spectroscopy to interrogate separately the nature of hole and electron transport in a recently developed high-mobility copolymer poly(N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2- b]thiophene) (DPP-DTT). It is found that both hole and electron polarons exhibit two-dimensional delocalization in the DPP-DTT films due to the strong intermolecular coupling. A new subgap charge induced optical transition is observed, which has not been reported before in homopolymers. The origin of this transition is studied by combing the temperature and gate voltage dependent CMS and quantum chemical calculation. The results reveal that this transition could be related to the existence of staggered dimer stacking in copolymers due to the asymmetrical volume fractions of donor and acceptor moieties.