Controlling condensed state structures of different polar conjugated polymer polyfluorenes (PFs) by applying an external electric field across a solution with polar solvent THF

Abstract

In this work, the control of condensed state structures of different polar conjugated polymer polyfluorenes (PFs) by applying an external electric field across a solution containing polar solvent tetrahydrofuran (THF), was investigated. We used photoluminescence (PL) spectroscopy, light scattering (LS), Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution-transmission electron microscopy (HR-TEM). An external electric field was applied across the optoelectronic conjugated polymer solutions for the first time. Interestingly, it was found that from dilute solutions (0.1 mg mL⁻¹) to concentrated solutions (5 mg mL⁻¹), the chain conformation of the weakest polar PFO changed the most dramatically and always transformed towards the more planar geometry, along with effective conjugated length increase. These findings subverted the conventional thought that weakly polar polymer chains cannot be easily affected by an external electric field, and only polar conjugated polymers can. Under this condition – applying an external electric field across solutions containing the polar solvent THF – the PFO β conformation content increased obviously in a concentrated 5 mg mL⁻¹ solution, and by this simple physical method, the PFO chains oriented and self-assembled to directly form β-phase crystal films. Other β-phase crystal preparation methods are very difficult. Astoundingly, using this method, the long-wavelength green emission of PPFOH aggregation, which is an obstacle to the application of PFs, was also suppressed, and this has been a difficult issue for a long time. The mechanism of these changes were revealed; increasing the dielectric constant gap between the polymer chain and solvent molecules is key, rather than simply the polymer polarity. The above conclusions are also applicable to other conjugated polymer systems. This research is significant not only to enrich the understanding of the physical processes of conjugated polymer solution behaviours to enhance optoelectronic device performance, but also to control condensed state structures to achieve chain ordered packing as well as multi-color larger-area optoelectronic devices with high carrier mobility and efficiency.