Small-Angle Neutron Scattering of P(NDI2OD-T2) Solutions: Importance of Network Structure for Data Interpretation and Film Morphology

Small-angle neutron scattering (SANS) is used to study the solution-phase behavior of the well-studied n-type semiconducting polymer P-(NDI2OD-T2). To provide a global overview of polymer behavior, four different molecular weight samples are studied in three different solvents and at three different temperatures. The SANS data are interpreted in terms of a hierarchical model combining a cylinder model to explain scattering from individual rigid chains/aggregates at high scattering vector, q, and a Guinier-Porod model to explain the upturn in scattering at low q that appears in the SANS patterns of the higher molecular weight samples. In this way, the scattering patterns of P-(NDI2OD-T2) solutions with different molecular weights, different solvents, and at different temperatures can all be adequately modeled and parametrized in terms of varying cylinder length, cylinder width, and mass fractal dimension. To connect the SANS results to thin film microstructure and charge transport, thin films are prepared and studied with atomic force microscopy and organic field effect transistor (OFET) measurements. Significantly, excess network formation in solution is associated with a decrease in the in-plane alignment of polymer chains and decrease in OFET mobility. Thus, while increased aggregation can enhance chain ordering and charge transport in thin films, excess aggregation in the form of a compact entangled network of these aggregates can be detrimental to the formation of ordered structures during solution deposition.