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-2,5-diyl) (P3HT) films doped by each method with the small-molecule dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) 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/mK2. 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 and better suited for polymer films with micrometer thicknesses.
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 localizes the holes created on the polymer, reducing their mobility. Here, a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers is employed. By designing the cluster to have a high redox potential and steric protection of the core‐localized electron density, highly delocalized polarons with mobilities equivalent to films doped with no anions present are obtained. AC Hall effect measurements show that P3HT films doped with these boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with F4TCNQ, 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 F4TCNQ is primarily to keep the counterions far from the polymer backbone.
Chemical doping of conjugated polymers produces polarons via charge transfer. The counterions produced using traditional small‐molecule dopants are proximal to the polaron and thus limit polaron mobility due to Coulomb attraction. A boron cluster‐based dopant that shields the charge on the counterion from the polaron is designed. This allows the creation of nearly 100% free polarons with high mobility.
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.
The effect of dopant redox potential on the chemical doping of conjugated polymers is studied using dodecaborane dopants whose potential can be tuned at constant size and shape. The 2‐nm dodecaborane clusters preferentially intercalate into the polymer crystalline lamellae, and the doping efficiency increases with redox potential. The isolation of the counteranion from the hole leads to improved carrier mobility.
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 J-coupling 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 (poly2-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.
The NASA
Perseverance
rover Mast Camera Zoom (Mastcam-Z) system is a pair of zoomable, focusable, multi-spectral, and color charge-coupled device (CCD) cameras mounted on top of a 1.7 m Remote ...Sensing Mast, along with associated electronics and two calibration targets. The cameras contain identical optical assemblies that can range in focal length from 26 mm (
25.5
∘
×
19.1
∘
FOV
) to 110 mm (
6.2
∘
×
4.2
∘
FOV
) and will acquire data at pixel scales of 148-540 μm at a range of 2 m and 7.4-27 cm at 1 km. The cameras are mounted on the rover’s mast with a stereo baseline of
24.3
±
0.1
cm and a toe-in angle of
1.17
±
0.03
∘
(per camera). Each camera uses a Kodak KAI-2020 CCD with
1600
×
1200
active pixels and an 8 position filter wheel that contains an IR-cutoff filter for color imaging through the detectors’ Bayer-pattern filters, a neutral density (ND) solar filter for imaging the sun, and 6 narrow-band geology filters (16 total filters). An associated Digital Electronics Assembly provides command data interfaces to the rover, 11-to-8 bit companding, and JPEG compression capabilities. Herein, we describe pre-flight calibration of the Mastcam-Z instrument and characterize its radiometric and geometric behavior. Between April 26
t
h
and May 9
t
h
, 2019, ∼45,000 images were acquired during stand-alone calibration at Malin Space Science Systems (MSSS) in San Diego, CA. Additional data were acquired during Assembly Test and Launch Operations (ATLO) at the Jet Propulsion Laboratory and Kennedy Space Center. Results of the radiometric calibration validate a 5% absolute radiometric accuracy when using camera state parameters investigated during testing. When observing using camera state parameters not interrogated during calibration (e.g., non-canonical zoom positions), we conservatively estimate the absolute uncertainty to be
<
10
%
. Image quality, measured via the amplitude of the Modulation Transfer Function (MTF) at Nyquist sampling (0.35 line pairs per pixel), shows
MTF
Nyquist
=
0.26
−
0.50
across all zoom, focus, and filter positions, exceeding the
>
0.2
design requirement. We discuss lessons learned from calibration and suggest tactical strategies that will optimize the quality of science data acquired during operation at Mars. While most results matched expectations, some surprises were discovered, such as a strong wavelength and temperature dependence on the radiometric coefficients and a scene-dependent dynamic component to the zero-exposure bias frames. Calibration results and derived accuracies were validated using a Geoboard target consisting of well-characterized geologic samples.
During its first year of operation, the Perseverance rover explored the cratered and fractured floor of Jezero crater on Mars. Here, we report the use of the Scanning Habitability Environments with ...Raman and Luminescence for Organics and Chemicals (SHERLOC) imaging system that includes two high-resolution cameras, the Autofocus and Contextual Imager (ACI) and Wide Angle Topographic Sensor for Operations and eNgineering (WATSON). ACI is a fixed focus gray scale imager with a resolution of 10.1 μm/pixel whereas WATSON is a variable field of view, variable focus imager capable of resolution down to 14 μm/pixel. WATSON is a reflight of the MArs Hand Lens Imager (MAHLI) imager and has similar capabilities. During first-time activities, WATSON was used to support both science and engineering operations related to sample and abrasion patch assessment and sample collection and caching. WATSON also documented the deployment of the Ingenuity helicopter. The Crater Floor Campaign identified two primary rock units, the Máaz formation and the Séítah formation, which have been interpreted as lava flows and an olivine cumulate, respectively. Interpretation of rock textures with WATSON and ACI images was limited to abraded surfaces because unmodified outcrop surfaces (herein termed “natural surfaces”) show high degrees of dust covering, wind abrasion, and coating by secondary mineral products. WATSON and ACI images support the hypothesis that the material of both the Máaz and Séítah formations consists of largely aqueously altered mafic materials with varying igneous origins.
Throughout World War II, when Saturday nights came around, servicemen and hostesses happily forgot the war for a little while as they danced together in USO clubs, which served as havens of stability ...in a time of social, moral, and geographic upheaval. Meghan Winchell demonstrates that in addition to boosting soldier morale, the USO acted as an architect of the gender roles and sexual codes that shaped the "greatest generation."Combining archival research with extensive firsthand accounts from among the hundreds of thousands of female USO volunteers, Winchell shows how the organization both reflected and shaped 1940s American society at large. The USO had hoped that respectable feminine companionship would limit venereal disease rates in the military. To that end, Winchell explains, USO recruitment practices characterized white middle-class women as sexually respectable, thus implying that the sexual behavior of working-class women and women of color was suspicious. In response, women of color sought to redefine the USO's definition of beauty and respectability, challenging the USO's vision of a home front that was free of racial, gender, and sexual conflict.Despite clashes over class and racial ideologies of sex and respectability, Winchell finds that most hostesses benefited from the USO's chaste image. In exploring the USO's treatment of female volunteers, Winchell not only brings the hostesses' stories to light but also supplies a crucial missing piece for understanding the complex ways in which the war both destabilized and restored certain versions of social order.
Semiconducting polymers are a versatile class of materials that are used in many (opto)electronic applications, including organic photovoltaics. However, they are inherently disordered and suffer ...from poor conductivities due to bends and kinks in the polymer chains along the conjugated backbone, as well as disorder at grain boundaries. In an effort to reduce polymer disorder, we developed a method to straighten polymer chains by creating amphiphilic conjugated polyelectrolytes (CPEs) that self-assemble in water into worm-like micelles. The present work refines our design rules for self-assembly of CPEs. We present the synthesis and characterization of a straight, micelle-forming polymer, a derivative of poly(cyclopentadithiophene-alt-thiophene) (PCT) bearing two ammonium-charged groups per cyclopentadithiophene unit. Solution-phase self-assembly of PCT into micelles is observed by both small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM), while detailed SAXS fitting allows for characterization of intra-micellar interactions and inter-micelle aggregation. We find that PCT displays significant chain straightening thanks to the lack of steric hindrance between its alternating cyclopentadithiophene and thiophene subunits, which increases the propensity for the polymer to self-assemble into straight rod-like micelles. This work extends the availability of micelle-forming semiconducting polymers and points to further enhancements that can be made to obtain homogeneous nanostructured polymer assemblies based on cylindrical micelles.
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