Session: Self-assembly in Polymer Systems:

Conjugated polymers are promising materials for the next generation of optical and electronic devices due to their unique charge transport properties. To date, poly(3-alkylthiophenes) (P3ATs) are among the most intensely studied semiconducting polymers for a variety of organic electronics applications. Despite their charge-transport advantages, the rigidity of the conjugated backbone introduces constraints which limit processibility and structure formation of these materials. Here, we report a newly synthesized block copolymer, poly(3-hexylthiophene)-block-poly(oligo-oxyethylene methacrylate) (P3HT-b-POEM), made by attaching a flexible coil block to the rigid conjugated block using a “grafted to” synthetic method. The results of the investigation of thermoelectric performance of this material upon solution processing with the ionic dopant, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), followed by vapor phase doping 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) are discussed. Our results demonstrate that the co-processing of the block copolymer with LiTFSI is a straightforward and promising method to improve both the molecular order and thermoelectric performance, which were further improved upon the exposure to F4TCNQ vapor. The improved molecular order was rationalized by the formation of P3HT aggregates upon interaction of the block copolymer with the LiTFSI salt in solution before film formation. It is worth noting that doping P3HT-b-POEM with LiTFSI salt unexpectedly caused electronic doping, a phenomenon that has not been previously observed. Further doping of P3HT-b-POEM-LiTFSI with F4TCNQ vapor produced a maximum conductivity of 31 S/cm with a LiTFSI blend concentration ratio of r = [Li⁺]/[EO] = 0.02 which is 50 times higher than the undoped sample and led to a power factor of 13.0 μWm^-1K^-2 , 20 times higher than the neat sample.

Paul Nealey

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