Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.

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Resistivity, thermopower, and Hall-effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n-type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

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