The origin of the mid-infrared (IR) spectral features for hole absorption in doped poly­(3-hexylthiophene) (P3HT) films is investigated theoretically and experimentally. Using a Holstein-style Hamiltonian to treat vibronic coupling involving the prominent vinyl stretching mode, the low-energy peak (A) occurring in the spectral interval 0.1–0.15 eV is found to contain a substantial (sometimes dominant) intrachain-polarized component, in contrast to the predictions of the more conventional treatments based on self-trapped, mid-gap polaron states where peak A is entirely interchain in origin. A higher-energy peak (B) located between 0.35 and 0.7 eV and largely intrachain-polarized is also obtained and associated with the conventional P1 polaron transition. Spectral signatures for polaron coherence based on peaks A and B are identified and used to analyze the molecular weight dependence of the IR spectral line shapes of P3HT doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Generally, the polaron coherence lengths along the chain and stacking axes increase with molecular weight, which is consistent with similar studies conducted on hole absorption in undoped P3HT.

Charge transport in semiconducting polymers is inextricably linked to their microstructure, making the characterization of polymer morphology at all length-scales essential for understanding the factors that limit mobility in these materials. Indeed, charge transport depends both on the ability of polarons to delocalize at the approximately nanometer length-scale and navigate a complex energetic and morphological mesoscale landscape. While characterization of the mesoscale morphology of polymers is wellestablished, studies of the local chain packing and nanoscale disorder, which affect delocalization, can be significantly more difficult to carry out. Through infrared charge modulation spectroscopy and theoretical modeling, the effect of the local chain environment on polaron delocalization is directly measured and quantified. Using a series of polymers based on the model system, poly(3-hexylthiophene), the link between disorder and polaron localization is systematically explored. Polaron delocalization is correlated with known trends in mobility, revealing that while charge delocalization is always beneficial, the formation of tie-chains is necessary to reach the highest mobilities in semicrystalline polymers. The results provide direct evidence for the importance of both nanoscale (charge carrier delocalization) and mesoscale (tie-chains) orders, demonstrating the need to distinguish the key length-scale limiting charge transport in the design of new, high mobility polymers.

The field of stretchable electronics has recently gained significant interest from the academic community, with a focus on producing materials that demonstrate reliable electrical performance with improved response to mechanical deformation. This review highlights the recent progress in understanding the relationships between the mechanical behavior and electrical performance of such devices. Potential solutions can take the form of intrinsically elastic polymers, polymer semiconductor/ elastomer blends and alternative engineering-oriented approaches, which are discussed herein. Trends and design strategies are beginning to manifest in this early stage of the stretchable electronics field. The development of stretchable electrical systems can provide unique applications of organic electronics.

An honest discussion on the strengths and limitations of molecular dynamics force fields for P3HT through neutron scattering.