Materials Science

The Power of Annealing

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Science  02 Apr 2010:
Vol. 328, Issue 5974, pp. 19
DOI: 10.1126/science.328.5974.19-b

Combine a conducting polymer with a small molecular acid and you get high conductivity but poor processability. A polymeric acid improves processability, but the conductivity plummets. Yoo et al. show that you can have the best of both worlds by using a polymeric acid as the dopant and then performing a post-processing solvent annealing step that substantially improves the conductivity. Polyaniline becomes conductive through proton doping with poly(2-acrylamido-2-methly-1-propanesulfonic) acid. Exposure to dichloroacetic acid (DCA) decreases film roughness by allowing the colloidal particles that form during polymerization to blend, leading to chain conformations with improved charge transport. A somewhat different effect occurs in poly(ethylene dioxythiophene) films doped with poly(styrene sulfonic) acid (PSS). Treatment with DCA increases conductivity as well as surface roughness. The authors attribute this result to disruption by DCA of the insulating surface layer of PSS that forms during spin casting. The DCA-treated films were tested in organic thin-film transistors, solar cells, and light-emitting diodes. In all cases, the treated films showed much better characteristics than the untreated ones. Although the properties did not match those of devices made with indium tin oxide (the current benchmark material), the annealed films could be considered as practical alternatives to a range of metals and metal oxides, especially if cost is a concern.

Proc. Nat. Acad. Sci. U.S.A. 107, 10.1073/pnas.0913879107 (2010).

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