Ladder polymers with poly(diketopyrrolopyrrole) (DPP) moieties have recently attracted enormous interest for a large variety of opto-electronic applications. Since the rigidity of the backbone increases with ladderization, a strong influence on the self-organization of thin films is expected. We study the molecular orientation of DPP-based ladder polymers in about 50 nm thin films using polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS). Exemplarily, for one polymer, the orientation in thicker films is qualitatively investigated by infrared spectroscopy in transmission. Further, this method allows us to rule out the effects of a possible azimuthal ordering, which would affect the analysis of the orientation by PM-IRRAS. For all polymers, the long axis of the polymer backbone is preferentially oriented parallel to the substrate surface, pointing to a high degree of ordering. It is suggested that the choice of the side chains might be a promising way to tune for face-on and edge-on orientations. The exemplarily performed investigation of interface properties on substrates with different work functions suggests that the choice of the side chains has a minor effect on the interfacial electronic interface structure.
Keywords: FTIR; PM-IRRAS; UV/vis; benzodithiophene; benzothiadiazole; low band gap polymers; orientation.