Adv.Funct.Mater.http://doi.org/fz9kfr(2012)

Credit: © 2012 WILEY

With many different applications — ranging from solar cells to display technologies — there is continued interest in the development of semiconducting polymers. The compatibility of these polymers with cheap and rapid solution-phase processing is attractive, but improvements in the efficiencies of devices made from these materials are required for them to compete with established technologies. The polymer forms a significant fraction of the total cost of a device, and although efficient syntheses have been designed, the transition from a research route to production scale remains a problem. Now, John de Mello, Martin Heeney and co-workers from Imperial College London have developed a droplet-based flow synthesis suitable for the synthesis of poly-3-hexylthiophene (P3HT).

The electrical properties of semiconducting polymers such as P3HT are critically dependent on the regioregularity of the polymer. The standard route for making this polymer starts with an unsymmetrical dibromothiophene, in which one bromine site is selectively converted to the corresponding Grignard reagent. This AB-type monomer can be polymerized in a nickel-catalysed cross-coupling reaction to give the desired polymer. On a small scale this synthetic route is well developed, but as reaction volumes are increased, mixing of reagents and setting the temperature for a successful outcome become difficult to control. In the method developed by de Mello and co-workers, thiophene-based Grignard reagents dissolved in tetrahydrofuran (THF) are co-injected into narrow-diameter tubing along with a dispersion of the nickel catalyst in a perfluorinated polymer fluid.

These two fluids are immiscible and the THF solution forms small droplets in the polymer carrier fluid. The reaction tubing is heated by immersing it in a temperature-controlled oil bath, and reaction time can be carefully controlled by the flow rate of the droplets through the tube. Each droplet is essentially a small-scale reaction in its own right, and scale-up of polymer production can be achieved simply by increasing the total amount of reagent introduced to the system. The method produces polymers with consistent properties on anything from a sub-gram scale to multiple tens of grams.