Abstract Low-density polymer foams of varying sizes, shapes, and densities are of specific interest to the inertial confinement fusion (ICF) program and related high-energy density plasma physics research. Historically, these foams are comprised of polystyrene or other low atomic number materials and have densities in the 30 to 300 mg/cm3 range. However, at the lower end of this density range, these traditional polymer foams become fragile and difficult to cast and machine into the geometries needed. Recently, the need by experimentalists for materials with densities below 30 mg/cm3 has increased. To address these needs, we are developing three-dimensional (3-D) printing techniques to create high-precision, low-density, and repeatable complex lattice structures. Using two-photon polymerization 3-D printing, we recently developed the first 5 mg/cm3 low-density lattice structure having an annular hemispherical shape. These microscale to mesoscale structures were modeled and designed using the nTopology software, specifically utilizing the “Voronoi volume lattice” and “random points in body” option blocks. All printing operations were performed using the Nanoscribe Photonic Professional GT instrument. Characterization of these 3-D structures was conducted using various microscopic and X-ray tomographic imaging techniques. Overall printed part sizes ranged from 1 to 5 mm in diameter and were composed of lattice ligaments having thicknesses in the 3- to 5-µm range. These structures have been incorporated into ICF targets recently shot on both the University of Rochester’s Laboratory of Laser Energetics Omega laser and the National Ignition Facility.

The use of infrared lasers to power accelerating dielectric structures is a developing area of research. Within this technology, the choice of the dielectric material forming the accelerating structures, such as the photonic band gap (PBG) structures, is dictated by a range of interrelated factors including their dielectric and optical properties, amenability to photo-polymerization, thermochemical stability and other target performance metrics of the particle accelerator. In this direction, electronic structure theory aided computational screening and design of dielectric materials can play a key role in identifying potential candidate materials with the targeted functionalities to guide experimental synthetic efforts. In an attempt to systematically understand the role of chemistry in controlling the electronic structure and dielectric properties of organic polymeric materials, here we employ empirical screening and density functional theory (DFT) computations, as a part of our multi-step hierarchal screening strategy. Our DFT based analysis focused on the bandgap, dielectric permittivity, and frequency-dependent dielectric losses due to lattice absorption as key properties to down-select promising polymer motifs. In addition to the specific application of dielectric laser acceleration, the general methodology presented here is deemed to be valuable in the design of new insulators with an attractive combination of dielectric properties.

“Logpile” photonic band gap structures are an attractive option for the construction of laser dielectric accelerators. In principle, these structures can be fabricated using a commercial Nanoscribe 3-D printer, although currently available resins do not meet the materials requirements necessary for a functional dielectric waveguide for laser accelerators. In particular, the requisite optical-frequency dielectric constant is well outside the range of conventional organic materials. This work examines material options for overcoming this barrier, while simultaneously meeting requirements for loss tangent, laser-induced breakdown, and compatibility with two-photon polymerization. We present computational screening of more exotic organics resins, and synthetic options for promising candidates. In addition, we discuss materials approaches involving metal-polymer complexes, as well as germanium and metal-chalcogenide polymer nanocomposites. Prospects, inherent limitations, and initial characterization of these various materials will be discussed in the context of 3D-printed dielectric accelerators.

Conjugated polyelectrolytes and related mixed ionic-electronic conductors (MIECs) are being explored for energy applications including solid-state lighting and photovoltaics. Fundamental models of charge injection into MIECs have been primarily developed for MIECs contacted with highly conductive or metal electrodes (MEs), despite many potential applications involving semiconductors. We theoretically and experimentally demonstrate that an appropriate semiconductor electrode (SE), n-type for electron or p-type of hole injection, can limit injection into MIECs. When the SE is the injecting electrode and is under accumulation, there is little difference from a ME. When the SE acts as the extracting electrode, however, injection into the MIEC can be limited because a fraction of any applied bias must support charge depletion in the semiconductor rather than charge injection into the MIEC. In a ME/MIEC/SE system, this can lead to significant asymmetry in current-voltage and injected charge-voltage behavior.

The incorporation of a cationically functionalized fullerene interfacial layer (NMFP-Br) into an inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction photovoltaic cell results in a significant power conversion efficiency (PCE) improvement of 67%, from 2.1% to 3.5%, relative to cells without an interfacial layer. The incorporation of NMFP-Br as an ITO modifying interfacial layer results in a 190 mV increase in the open-circuit voltage, 13% increase in fill factor, and 250% reduction in series resistance. Cell efficiencies are greater than or comparable to other reported cells based on organic electron injection layers with the same active layer and electrode configuration. The orthogonal solubility afforded by ionic functionality allows for sequential solution phase deposition without the necessity of chemical cross-linking of the fullerene based interfacial layer. Inverted devices incorporating a single phase, ionically functionalized fullerene interfacial layer have not been previously demonstrated. The unusually high conductivity of NMFP-Br films is investigated and shown to be ~3 orders of magnitude higher than PCBM and contributes to the substantial reduction in series resistance.

We discuss the determination of concentration-dependent carrier mobility by measurement of current and charge injection in a conjugated polyelectrolyte thin film. Equilibration under bias of the mobile counterions in a cationically functionalized polyacetylene film leads to steady-state current, while total charge injection is measured through NIR absorbance spectroscopy. The charge-voltage behavior is consistent with a model of charge injection where migration of mobile ions enables injection of electronic carriers in a manner analogous to electrochemical doping, and current is limited by diffusion across the bulk. The relation between injected charge and current density is also used to obtain the functional form of the concentration dependence of mobility, and this in turn is used to model the carrier profile across the film and obtain the mobility mu = 2.8 X 10(-10) + 1.6 x 10(-8)(c/C-0)(1.3) cm(2)/(V s) where C-0 is a reference concentration of 1 C/cm(3). Another method of charge quantification, integration of transient short-circuit current, is investigated and found to serve as an approximation of charge density, though it overestimates at low injection levels and underestimates at higher levels.

Existing photovoltaic junctions typically contain only electronic charge carriers, and they rely on the built-in electronic asymmetry between constituent materials to separate photogenerated electrons and holes. In this Letter we report the observation of a photovoltaic response and photochemically induced near infrared absorption from a mixed ionic electronic junction constructed from two intrinsic polyacetylene ionomers with oppositely charged. mobile counterions. We propose that the asymmetry in ionic charge carriers across the junction leads to the separation of photogenerated carriers and the in situ photochemical doping of both ionomers under illumination. Unlike purely electronic junctions, the doping level of mixed ionic electronic junctions can change adaptively with light intensity due to the interaction between ionic and electronic carriers. The unique working mechanism of mix-conducting junctions could be used for designing solar cells and adaptive photoelectronic devices.