Characterization of nanoscale morphology and mechanical properties of conjugated polymer thin films dynamically exposed to a secondary solvent

Solution processing techniques are often used to enhance intra and interchain order in semiconducting conjugated polymer thin films used in the active layer of organic optoelectronic devices. While there has been extensive research into the impact of solution processing on the local structural and electronic properties of these films, the nanomechanical properties of the films are less well understood and are challenging to characterize. We investigate the nanomechanical properties of conjugated polymer thin films arising from solution processing techniques. Our study relies on dynamic secondary solvent dripping to induce subtle changes to the film morphology. We examine thin films of P3HT, PCDTBT, PTB7, and PBDB-T-SF conjugated polymers, commonly used in organic photovoltaics, with the bimodal amplitude modulation-frequency modulation (AM-FM) imaging mode of the atomic force microscopy (AFM) viscoelastic method to probe their local physical and mechanical properties. We find that Young's Modulus data measured by AM-FM AFM can detect additional changes in film properties not discernible by other commonly used bulk thin-film characterization techniques. For PBDB-T-SF, we detect an increase in molecular order that is not noticable by UV-visible absorption spectroscopy, X-ray diffraction or nanoindentation. PCDTBT, the most amorphous of the polymers studied, shows no changes in absorption or X-ray diffraction data, yet clear changes in Young's Modulus were detected by AFM. On the other hand, increases in P3HT order that occurred due to dynamic secondary solvent dripping are detected in both the bulk and AFM measurements. Our study demonstrates the applicability of nanomechanical measurements for characterizing local structural variations in conjugated polymer films. This work is relevant to ongoing efforts to control and understand the complex structure-property-processing relationships of conjugated polymer thin films.