One of the most effective ways to tune the electronic properties of conjugated polymers is to dope them with small-molecule oxidizing agents, creating holes on the polymer and molecular anions. Undesirably, strong electrostatic attraction from the anions of most dopants localize the holes created on the polymer, reducing their mobility. Here, we employ a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers. By designing the cluster to have a high redox potential and steric protection of the corelocalized electron density, we obtain highly delocalized polarons with mobilities equivalent to films doped with no anions present. AC Hall effect measurements show that P3HT films doped with our boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with F 4 TCNQ, even though the boron-cluster-doped films have poor crystallinity. Moreover, the number of free carriers approximately matches the number of boron clusters, yielding a doping efficiency of ∼100%. These results suggest that shielding the polaron from the anion is a critically important aspect for producing high carrier mobility, and that the high polymer crystallinity required with dopants such as F 4 TCNQ is primarily to keep the counterions far from the polymer backbone.

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.

For thermoelectric and other device applications there has been great interest in the chemical doping of conjugated polymer films. Solution doping followed by film deposition generally produces poor-quality films, but this issue can be alleviated by sequential doping: a pure polymer film is deposited first, and the dopant is then added as a second processing step, preserving the quality and structure of the original polymer film. In this paper, we compare two methods for sequential doping of conjugated polymer films: evaporation doping, where a controlled thickness of dopant is added via thermal sublimation to a temperature-controlled polymer film, and sequential solution doping, where the dopant is spin cast from a solvent chosen to swell but not dissolve the underlying polymer film. To compare these two different types of sequential doping, we examine the optical, electrical, and structural properties of poly(3-hexylthiophene-2,5diyl) (P3HT) films doped by each method with the small-molecule dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) as a function of the polymer film thickness. Although each method intercalates dopant in fundamentally unique ways, we find that both vapor and solution doping methods produce films that share many of the same properties. Interestingly, both methods can produce doped P3HT films with conductivities of ∼5 S/cm and comparable thermoelectric properties, even for films as thick as 400 nm. For the evaporation method, an "overhead" dopant film thickness of ∼6 nm is required, either to promote reorganization of existing crystallites or to fill preexisting trap states in the polymer film. After the overhead amount has been deposited, the thickness of the dopant layer that must be evaporated to reach the optimal electrical conductivity is ∼1/3 that of the underlying polymer film. For a given P3HT film thickness, the amount of evaporated dopant needed to produce the highest conductivity corresponds to a thiophene monomer to ionized dopant ratio of ∼8.5:1. For solution processing, with the appropriate choice of solvent and dopant concentration, we show that P3HT films as thick as 2 μm can be doped to achieve conductivities of ∼5 S/cm and thermoelectric power factors approaching 2 μW/mK 2 . For either method, if excess dopant is applied, it remains in neutral form either in the amorphous regions or on top of the film, reducing the conductivity by increasing the film thickness. For both methods, UV−vis absorption can be used as a quick proxy to easily monitor whether saturation doping levels have been reached or exceeded. Fourier transform infrared spectroscopy (FTIR) and grazing-incidence wide-angle X-ray scattering (GIWAXS) both show that vapor-doped films and thicker solution-doped films have improved morphologies that result in more mobile carriers. Overall, we demonstrate that it is a straightforward process to select a sequential doping method for a desired application: evaporation doping is more amenable to large-area films, while solution doping is lower cost ...

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil−coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear.Here, we report a detailed study to elucidate the rich selfassembly behavior of conjugated thiophene−selenophene (P3AT-b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, cocrystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50−60) and high N (>80), the thiophene and selenophene blocks cocrystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Although there has been significant interest in the nature of exciton spatial coherence in conjugated polymers, it is usually not possible to control the extent of H-or Jcoupling between polymer chains because the polymers have a naturally preferred morphology when spin-cast into films. In this work, we explore the nature of exciton spatial coherence in conjugated polymer chains that have been straightened by encapsulation in the channels of a macroscopically aligned mesoporous silica host. The small size of the pores hinders the formation of polymer crystallites, allowing us to control the polymer chain conformation without crystallinity. This provides a way for us to study the H-and J-coupling on straight polymer chains that are not crystalline, something that cannot be done with traditional film casting methods. We specifically prepare two different sets of host/guest composites using both P3HT (poly(3-hexylthiophene)) and MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) chains as the guests. We then perform a series of polarized absorption and emission experiments on the encapsulated polymer chains, which reveal that the composite samples each contain two distinct polymer populations: a predominantly aligned population of straightened polymer chains and an isotropic population of coiled and generally isolated polymer chains. The aligned population is more ordered than the isotropic population for both polymers, an effect that is particularly pronounced for P3HT. We find that the same types of spectral changes observed when P3HT crystallizes are also present in the straightened P3HT chains and that P3HT can still exhibit H-dominant aggregation, even without extended crystallinity. In contrast, MEH-PPV exhibits enhanced J-character when the polymer backbone is aligned and straightened by the pores, with an exciton coherence size that is larger than that observed in spin-cast polymer films. This study thus provides a more fundamental understanding of the effects of backbone straightening on the aggregation behavior of both H-and J-type conjugated polymers.