Nicola Ferralis

Nanocarbon-Based Photovoltaics

Carbon materials are excellent candidates for photovoltaic solar cells: they are Earth-abundant, possess high optical absorption, and maintain superior thermal and photostability. Here we report on solar cells with active layers made solely of carbon nanomaterials that present the same advantages of conjugated polymer-based solar cells, namely, solution processable, potentially flexible, and chemically tunable, but with increased photostability and the possibility to revert photodegradation. The device active layer composition is optimized using ab initio density functional theory calculations to predict type-II band alignment and Schottky barrier formation. The best device fabricated is composed of PC(70)BM fullerene, semiconducting single-walled carbon nanotubes, and reduced graphene oxide. This active-layer composition achieves a power conversion efficiency of 1.3%-a record for solar cells based on carbon as the active material-and we calculate efficiency limits of up to 13% for the devices fabricated in this work, comparable to those predicted for polymer solar cells employing PCBM as the acceptor. There is great promise for improving carbon-based solar cells considering the novelty of this type of device, the high photostability, and the availability of a large number of carbon materials with yet untapped potential for photovoltaics. Our results indicate a new strategy for efficient carbon-based, solution-processable, thin film, photostable solar cells.

Solar energy generation in three dimensions

We formulate, solve computationally and study experimentally the problem of collecting solar energy in three dimensions. We demonstrate that absorbers and reflectors can be combined in the absence of sun tracking to build three-dimensional photovoltaic (3DPV) structures that can generate measured energy densities (energy per base area, kWh/m2) higher by a factor of 2–20 than stationary flat PV panels for the structures considered here, compared to an increase by a factor of 1.3–1.8 for a flat panel with dual-axis sun tracking. The increased energy density is countered by a larger solar cell area per generated energy for 3DPV compared to flat panels (by a factor of 1.5–4 in our conditions), but accompanied by a vast range of improvements. 3DPV structures can mitigate some of the variability inherent to solar PV as they provide a more even source of solar energy generation at all latitudes: they can double the number of peak power generation hours and dramatically reduce the seasonal, latitude and weather variations of solar energy generation compared to a flat panel design. Self-supporting 3D shapes can create new schemes for PV installation and the increased energy density can facilitate the use of cheaper thin film materials in area-limited applications. Our findings suggest that harnessing solar energy in three dimensions can open new avenues towards Terawatt-scale generation.

Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices

Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp(2) content, localization length, and disorder. The measured hopping energies demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide. Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285 °C with excellent stability were achieved.

Catalyst Self-Assembly for Scalable Patterning of Sub 10 nm Ultrahigh Aspect Ratio Nanopores in Silicon

Nanoporous silicon (NPSi) has received significant attention for its potential to contribute to a large number of applications, but has not yet been extensively implemented because of the inability of current state-of-the-art nanofabrication techniques to achieve sufficiently small pore size, high aspect ratio, and process scalability. In this work we describe the fabrication of NPSi via a modified metal-assisted chemical etching (MACE) process in which silica-shell gold nanoparticle (SiO2-AuNP) monolayers self-assemble from solution onto a silicon substrate. Exposure to the MACE etchant solution results in the rapid consumption of the SiO2 spacer shell, leaving well-spaced arrays of bare AuNPs on the substrate surface. Particles then begin to catalyze the etching of nanopore arrays without interruption, resulting in the formation of highly anisotropic individual pores. The excellent directionality of pore formation is thought to be promoted by the homogeneous interparticle spacing of the gold core nanocatalysts, which allow for even hole injection and subsequent etching along preferred crystallographic orientations. Electron microscopy and image analysis confirm the ability of the developed technique to produce micrometer-scale arrays of sub 10 nm nanopores with narrow size distributions and aspect ratios of over 100:1. By introducing a scalable process for obtaining high aspect ratio pores in a novel size regime, this work opens the door to implementation of NPSi in numerous devices and applications.

Strain-induced accelerated asymmetric spatial degradation of polymeric vascular scaffolds

Significance Bioresorbable scaffolds (BRS) were thought to represent the next cardiovascular interventional revolution yet they failed compared with metal stents. When BRS were tested using methods for MS, no signal of concern emerged––perhaps because BRS are not metal stents. BRS not only degrade, they also possess significant localized structural irregularities that cause asymmetric degradation. We posit these microstructural irregularities are responsible for variability in device performance in first-generation BRS. We correlated nonuniform degradation with variation in polymer microstructure and tolerance to integrated strain generated during fabrication and implantation. Differentiating failure modes in metallic and polymeric devices explains clinical results and suggests optimization strategies for the design and fabrication of next-generation BRS, indeed all devices using degradable materials. Polymer-based bioresorbable scaffolds (BRS) seek to eliminate long-term complications of metal stents. However, current BRS designs bear substantially higher incidence of clinical failures, especially thrombosis, compared with metal stents. Research strategies inherited from metal stents fail to consider polymer microstructures and dynamics––issues critical to BRS. Using Raman spectroscopy, we demonstrate microstructural heterogeneities within polymeric scaffolds arising from integrated strain during fabrication and implantation. Stress generated from crimping and inflation causes loss of structural integrity even before chemical degradation, and the induced differences in crystallinity and polymer alignment across scaffolds lead to faster degradation in scaffold cores than on the surface, which further enlarge localized deformation. We postulate that these structural irregularities and asymmetric material degradation present a response to strain and thereby clinical performance different from metal stents. Unlike metal stents which stay patent and intact until catastrophic fracture, BRS exhibit loss of structural integrity almost immediately upon crimping and expansion. Irregularities in microstructure amplify these effects and can have profound clinical implications. Therefore, polymer microstructure should be considered in earliest design stages of resorbable devices, and fabrication processes must be well-designed with microscopic perspective.

Plasma enhanced atomic layer deposition of molybdenum carbide and nitride with bis(tert-butylimido)bis(dimethylamido) molybdenum

Molybdenum carbonitride films were deposited using plasma enhanced atomic layer deposition techniques with (tBuN)2(NMe2)2Mo at temperatures ranging from 80 to 300 °C. The elemental composition of the molybdenum carbonitride films were analyzed using x-ray photoelectron spectroscopy with a MoCxNy composition extending from carbide, MoC0.45N0.08 to nitride MoC0.06N1.40 (x: 0.06–0.45; y: 0.08–1.40). The film composition, electrical properties, and optical properties are strongly dependent upon the % N2 in H2 of the plasma gas stream, as well as the process temperature. The molybdenum carbide film (MoC0.45N0.08) deposited at 150 °C achieved an electrical resistivity (ρ) value of 170 μΩ cm and exhibited superconducting behavior with a transition temperature (Tc) of 8.8 K. Nitrogen rich molybdenum carbonitride films (MoC0.28N0.44) deposited at 250 °C with 6% N2 in the plasma gas showed a microcrystalline fine grained structure with a measured ρ = 200 μΩ cm. Film thickness and optical properties were characteriz...