Structural characterization of highly oriented naphthalene-diimide-bithiophene copolymer films via vibrational spectroscopy.

Epitaxially grown highly oriented crystalline films, named form I and form II, and spin-coated films 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), have been investigated through infrared vibrational spectroscopy techniques (infrared absorption in double transmission at normal incidence (IRA-TR) and reflection absorption infrared spectroscopy at grazing angle incidence (RAIRS)) to get access to polymer chain orientation and structure. An analytic model to correlate the experimental intensities of the IR bands with structural parameters has been developed and applied for the three film morphologies. While spin-coated and form I films show P(NDI2OD-T2) chains lying parallel to the substrate in the face-on arrangement, form II films feature a structure with chains tilted out from the surface. The combined experimental and theoretical methodology gives insights into the local molecular orientations of naphthalene diimide (NDI2OD) and bithiophene (T2) counits. This approach can be easily extended to a variety of organic polymer semiconductors, allowing one to directly correlate molecular structure to properties such as charge transport, which is of fundamental relevance for developing quantitative models for applications in organic electronics and photovoltaics.