PerspectiveMaterials Science

Unraveling Charge Transport in Conjugated Polymers

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Science  06 Sep 2013:
Vol. 341, Issue 6150, pp. 1072-1073
DOI: 10.1126/science.1242935

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Polymer semiconductors have picked up the pace in the constant search for new and better electronic materials. High carrier mobility is an essential figure-of-merit for their applications as thin-film transistors (TFTs), solar cells, and light-emitting diodes. Polymers are disordered materials with partial crystallinity, and disorder introduces localized electronic states at the band edges—the so-called band tail—that trap electrons and reduce mobility. Reaching a mobility of 1 cm2 V−1 s−1 was the result of engineering polymers with increased crystallinity to reduce the effect of band tail states. Reported mobilities have recently surged to between about 5 and 10 cm2 V−1 s−1 (1), but the polymers are less ordered, according to some structural measurements. Two recent papers help resolve this puzzle. Zhang et al. (2) use a combination of structural probes to show that the high-mobility polymers gain longer-range structural order, if not high crystallinity, by having a more rigid backbone. Noriega et al. (3) provide a careful analysis of the role of disorder in charge transport for the specific case of polymers, and conclude that high crystallinity is not essential for high mobility.