Molecular Level Investigation of the Film Structure of a High Electron Mobility Copolymer via Vibrational Spectroscopy

Vibrational spectroscopy is adopted to investigate the film structure of poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) at the molecular level. Both Raman and IR spectra are measured for P(NDI2OD-T2) solutions and films. A good match with density functional theory (DFT) calculations at the B3LYP/6-311G** level is obtained, so that the main spectral features could be assigned. No significant spectral shifts are recorded when passing from very diluted solutions to the solid state, while clear variations in the relative intensity of specific spectral markers are observed The comparison of the spectral patterns shown by IR spectra recorded with reflection-absorption IR spectroscopy (RAIRS) and in normal transmission experiments allows to derive a structural model of the polymer. In as-cast films, or in films subjected to mild thermal treatments, below the melting point, the backbone of the polymer chains lies preferentially in the substrate plane, with the T2 units lying flat parallel to the substrate and the NDI2OD unit featuring a dihedral angle theta with the T2 unit (theta approximate to 38 degrees). This structure and polymer orientation is consistent with reported good bulk electron mobility in vertical diodes structures and high field-effect mobility in lateral field-effect transistors Furthermore, we observe that upon a melt annealing treatment, a clear modification of the RAIRS spectrum occurs suggesting either a loss of the preferential orientational order of the film or a flip of some domains featuring the polymer segments tilted out of the substrate.