  Through a combination of rheological characterization and
temperature-variable imaging methods, a novel gelation pathway in dilute
solutions of a semiconducting polymer to achieve interconnected, crystalline
networks with hierarchical porosity is reported. Upon rapid cooling, solutions
of regioregular poly(3-hexylthiophene) (RR-P3HT) in ortho-dichlorobenzene
formed thermoreversible gels. Temperature-variable confocal microscopy revealed
cooling-induced structural rearrangement to progress through viscoelastic phase
separation. The phase separation process arrested prematurely during the
formation of micron-sized solvent-rich "holes" within the RR-P3HT matrix due to
intrachain crystallization. Cryogen-based scanning electron microscopy of RR
P3HT gels revealed the existence of an interfibrillar network exhibiting
nano-sized pores. Remarkably, these networks formed to equal gel strengths when
a third component, either small molecule phenyl C61 butyric acid methyl ester
(PCBM) or non-crystallizing regiorandom (Rra)-P3HT, was added to the solution.
Organic solar cells in which the active layers were deposited from
phase-separated solutions displayed 45% higher efficiency compared to reference
cells.
