Although chemical doping is widely used to tune the optical and electrical properties of semiconducting polymers, it is not clear how the degree of doping and the electrical properties of the doped materials vary with the bandgap, valence band level, and crystallinity of the polymer. We addressed these questions utilizing a series of statistical copolymers of poly­(3-hexylthiophene) (P3HT) and poly­(3-heptylselenophene) (P37S) with controlled gradients in bandgap, valence band position, and variable crystallinity. We doped the copolymers in our series with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) using solution sequential processing. We then examined the structures of the films using grazing incidence wide-angle X-ray scattering, differential scanning calorimetry, and ellipsometric porosimetry, and the electrical properties of the films via the AC Hall effect. We found that the ability of a particular copolymer to be doped is largely determined by the offset of the polymer’s valence band energy level relative to the LUMO of F4TCNQ. The ability of the carriers created by doping to be highly mobile and thus contribute to the electrical conductivity, however, is controlled by how well the polymer can incorporate the dopant into its crystalline structure, which is in turn influenced by how well it can be swelled by the solvent used for dopant incorporation. The interplay of these effects varies in a nonmonotonic way across our thiophene:selenophene copolymer series. The position and shape of the polaron absorption spectrum correlate well with the polymer crystallinity and carrier mobility, but the polaron absorption amplitude does not reflect the number of mobile carriers, precluding the use of optical spectroscopy to accurately estimate the mobile carrier concentration. Overall, we found that the degree of crystallinity of the doped films is what best correlates with conductivity, suggesting that only carriers in crystalline regions of the film, where the dopant counterions and polarons are forced apart by molecular packing constraints, produce highly mobile carriers. With this understanding, we are able to achieve conductivities in this class of materials exceeding 20 S/cm.

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This work investigates the effect of wall thickness on the thermal conductivity of mesoporous silica materials made from different precursors. Sol–gel- and nanoparticle-based mesoporous silica films were synthesized by evaporation-induced self-assembly using either tetraethyl orthosilicate or premade silica nanoparticles. Since wall thickness and pore size are correlated, a variety of polymer templates were used to achieve pore sizes ranging from 3–23 nm for sol–gel-based materials and 10–70 nm for nanoparticle-based materials. We found that the type of nanoscale precursor determines how changing the wall thickness affects the resulting thermal conductivity. The data indicate that the thermal conductivity of sol–gel-derived porous silica decreased with decreasing wall thickness, while for nanoparticle-based mesoporous silica, the wall thickness had little effect on the thermal conductivity. This work expands our understanding of heat transfer at the nanoscale and opens opportunities for tailoring the thermal conductivity of nanostructured materials by means other than porosity and composition.

This work elucidates the effect of porous structure on thermal conductivity of mesoporous amorphous silica. Sol–gel and nanoparticle-based mesoporous amorphous silica thin films were synthesized by evaporation-induced self-assembly using either tetraethyl orthosilicate or premade silica nanoparticles as the framework precursors with block copolymers Pluronic P123 or Pluronic F127 as template. The films were characterized with scanning- and transmission-electron microscopy, two-dimensional grazing-incidence small-angle X-ray scattering, ellipsometric porosimetry, and UV–vis reflectance spectroscopy. The thermal conductivity of the mesoporous films, at room temperature and in vacuum, was measured by time-domain thermoreflectance. The films were 150 to 800 nm thick with porosities ranging from 9% to 69%. Their pore diameters were between 3 and 19 nm, and their thermal conductivities varied between 0.07 and 0.66 W/m.K. The thermal conductivity decreased strongly with increasing porosity and was also affected by the structure of the silica framework (continuous or nanoparticulate) and the pore size. A simple porosity weighted effective medium approximation was used to explain the observed trend in thermal conductivity. These results give new insight into thermal transport in nanostructured materials, and suggest design rules of the nanoscale architecture to control the thermal conductivity of mesoporous materials for a wide range of applications.

Abstract