Viscoelastic Phase Separation to Form Gel Networks of Semicrystalline Conjugated Polymers

Controlling the solution structure of polymer nanocomposite systems prior to thin film deposition is a promising strategy to enhance the efficiency of organic solar cells in a manner compatible with continuous processing methods. Using a model system comprising poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) dissolved in ortho-dichlorobenzene, we employed rheological characterization to characterize the thermoreversible gelation of these solutions upon rapid cooling to sub-ambient temperatures.[1] Rapid cooling was found to induce gelation in P3HT solutions, with the maximum gel strength independent of the presence or absence of PCBM. Interestingly, temperature-variable confocal microscopy imaging of P3HT fluorescence revealed the formation of micron-sized solvent “holes” during cooling, a signature of viscoelastic phase separation. This phenomenon originates from the dynamic asymmetry between slow-moving polymer chains and fast-moving solvent molecules and was found to arrest during the early stages of phase separation due to P3HT interchain crystallization. Cryogen-based scanning electron microscopy images of the gels further uncovered an interfibrillar network with characteristic pore sizes tens of nanometers in diameter. These interconnected polymer structures with hierarchical porosity were deposited as thin films via doctor blading, resulting in a 45% enhancement of light conversion efficiency compared to organic solar cells comprising active layers deposited from uncooled solutions.

1. He, J., Kong, X., Wang, Y., Delaney, M., Kalyon, D. M., and Lee, S. S. “Crystallization-Arrested Viscoelastic Phase Separation in Semiconducting Polymer Gels” ACS Applied Polymer Materials 1, (2019): 500–508.