Polaron Confinement in n-Doped P(NDI2OD-T2) Unveiled by Vibrational Spectroscopy

Through specific marker bands, IR and Raman spectra of chemically doped polyconjugated polymers allow investigation of doping and monitoring of its effectiveness. The vibrational modes associated with the doping-induced features provide information about the polymer units affected by the transferred charge and the structure relaxation associated with the formation of the polaron. Here, we doped the P(NDI2OD-T2) copolymer with three differently substituted 1H-benzimidazoles, which allow for doping in solution, leading to an increase of conductivity values up to four orders of magnitude. Careful inspection of the IR and Raman spectra of P(NDI2OD-T2) while varying the dopant concentration and the kind of dopant proves that the polaron markers are almost independent from the dopant species. The IR intensity of the polaron markers is a very sensitive probe of charge delocalization upon doping: for n-doped P(NDI2OD-T2), these bands show absorption intensities of the same strength as those of the pristine species. In other words, they are very weak in comparison to the so-called IRAV bands of doped polyacetylene, polythiophenes, and related materials. This experimental observation provides evidence of the strong confinement of the polaron on the NDI2OD unit. Multiwavelength Raman spectra of n-doped P(NDIOD-T2) further corroborate this point, showing that the T2 moiety is almost unaffected by doping. The analysis of the experimental data is complemented by DFT calculations which fully support the diagnosis of the formation of localized polarons. Hence, vibrational spectroscopy is an effective tool to characterize charge carriers induced by doping P(NDIOD-T2): it indicates that the observed conductivity enhancement is ascribed to an efficient interchain hopping involving charged NDI2OD units, whereas polaron diffusion along the chain is unlikely.