Vibrational Spectroscopy of a Low-Band-Gap Donor-Acceptor Copolymer and Blends

The molecular vibrations of the polymer PCDTBT are examined with resonant Raman spectroscopy accompanied by DFT calculations. By comparison of the building blocks (DTBT and carbazole), the monomer unit, and the polymer, most of the strongest vibrations can be identified and assigned. This systematic study reveals that the polymer vibrations are dominated by the monomer modes. Blending with PC71BM at varying fullerene load exhibits no influence on the Raman signature of PCDTBT. This interesting finding is discussed in the context of yet changing morphology, which is investigated by atomic force microscopy and grazing-incidence wide-angle X-ray scattering measurements, and solar cell performance. Therefore, Raman spectroscopy may not be generally suitable to study the molecular order in very amorphous low-band-gap polymers, as shown here for PCDTBT.