Robert C. Tenent

Transparent Conductive Single-Walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes

We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube-tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube-tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films.

Metal-oxide films for electrochromic applications: present technology and future directions

Many transition metal-oxide films exhibit an electrochromic (EC) effect as they change their optical transmittance upon charge insertion or extraction. These materials may be integrated into multilayer devices, and the optical modulation is then produced by application of a small electrical voltage. Electrochromic films are therefore being developed for application in dynamic or “smart” windows that are at the forefront of emerging energy-saving advances in building technologies. Here we will describe the state-of-the-art technology that is being implemented in commercial applications. It predominantly relies on the use of tungsten oxide-based films (coloring with ion insertion) and nickel oxide-based films (coloring with ion extraction). We also suggest future research directions that are motivated by the need to reduce the production costs of large-area EC windows. Specifically, we describe the possibility of alternative less expensive manufacturing processes, as well as the development of flexible EC devices that allow for an inexpensive “retrofit” installation to existing structures.

Carbon nanotube network electrodes enabling efficient organic solar cells without a hole transport layer

We report on the effects of replacing both In2O3:Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA/cm2. Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.

Electrical characterization of redox-active molecular monolayers on SiO2 for memory applications

Hybrid silicon capacitors have been successfully fabricated by attaching monolayers of redox-active molecules via self-assembly to ultrathin silicon dioxide layers. Capacitance, conductance, and cyclic voltammetric measurements have been used to characterize these capacitors. The presence of distinct capacitance and conductance peaks associated with oxidation and reduction of the monolayers at low gate voltages indicates discrete electron storage states for these capacitors, suggesting their feasibility in memory devices. The inherent molecular scalability and low-power operation coupled with existing silicon technology support the approach of hybrid molecule-silicon devices as a strong candidate for next generation electronic devices.

High-Performance Hydrogen Production and Oxidation Electrodes with Hydrogenase Supported on Metallic Single-Wall CarbonNanotube Networks

We studied the electrocatalytic activity of an [FeFe]-hydrogenase from Clostridium acetobutylicum (CaH2ase) immobilized on single-wall carbon nanotube (SWNT) networks. SWNT networks were prepared on carbon cloth by ultrasonic spraying of suspensions with predetermined ratios of metallic and semiconducting nanotubes. Current densities for both proton reduction and hydrogen oxidation electrocatalytic activities were at least 1 order of magnitude higher when hydrogenase was immobilized onto SWNT networks with high metallic tube (m-SWNT) content in comparison to hydrogenase supported on networks with low metallic tube content or when SWNTs were absent. We conclude that the increase in electrocatalytic activities in the presence of SWNTs was mainly due to the m-SWNT fraction and can be attributed to (i) substantial increases in the active electrode surface area, and (ii) improved electronic coupling between CaH2ase redox-active sites and the electrode surface.

Multiple-bit storage properties of porphyrin monolayers on SiO2

Hybrid molecule-silicon capacitors have been fabricated by the self-assembly of a monolayer of porphyrin molecules on a silicon oxide surface. The porphyrin employed [5-(4-dihydroxyphosphorylphenyl)-10,15,20-trimesitylporphinatozinc(II)] attaches to silicon oxide via a phosphonate linkage. Cyclic voltammetry current and capacitance/conductance measurements have been used to characterize the capacitors. The presence of multiple distinct peaks in current density and capacitance/conductance measurements are associated with oxidation and reduction of the molecular monolayer. The charge-storage states of the capacitor indicate applicability for use in multiple-bit memory devices.

Controlling the Optical Properties of Plasmonic Disordered Nanohole Silver Films

Disordered nanohole arrays were formed in silver films by colloidal lithography techniques and characterized for their surface-plasmon activity. Careful control of the reagent concentration, deposition solution ionic strength, and assembly time allowed generation of a wide variety of nanohole densities. The fractional coverage of the nanospheres across the surface was varied from 0.05-0.36. Electrical sheet resistance measurements as a function of nanohole coverage fit well to percolation theory indicating that the electrical behavior of the films is determined by bulk silver characteristics. The transmission and reflection spectra were measured as a function of coverage and the results indicate that the optical behavior of the films is dominated by surface plasmon phenomena. Angle-resolved transmission and reflection spectra were measured, yielding insight into the nature of the excitations taking place on the metal films. The tunability of the colloidal lithography assembly method holds much promise as a means to generate customized transparent electrodes with high surface plasmon activity throughout the visible and NIR spectrum over large surface areas.

Homeotropic alignment and director structures in thin films of triphenylamine-based discotic liquid crystals controlled by supporting nanostructured substrates and surface confinement.

We explore the effects of nanoscale morphology of supporting solid substrates on alignment, defects, and director structures exhibited by thin films of triphenylamine-based discotic liquid crystals. Fluorescence confocal polarizing microscopy and intrinsic polarized fluorescence properties of studied molecules are used to visualize three-dimensional director fields in the liquid crystal films. We demonstrate that, by controlling surface anchoring on supporting or confining solid substrates such as those of carbon nanotube electrodes on glass plates, both uniform homeotropic and in-plane (edge-on) alignment and nonuniform structures with developable domains can be achieved for the same discotic liquid crystal material.

Switchable photovoltaic windows enabled by reversible photothermal complex dissociation from methylammonium lead iodide

Materials with switchable absorption properties have been widely used for smart window applications to reduce energy consumption and enhance occupant comfort in buildings. In this work, we combine the benefits of smart windows with energy conversion by producing a photovoltaic device with a switchable absorber layer that dynamically responds to sunlight. Upon illumination, photothermal heating switches the absorber layer—composed of a metal halide perovskite-methylamine complex—from a transparent state (68% visible transmittance) to an absorbing, photovoltaic colored state (less than 3% visible transmittance) due to dissociation of methylamine. After cooling, the methylamine complex is re-formed, returning the absorber layer to the transparent state in which the device acts as a window to visible light. The thermodynamics of switching and performance of the device are described. This work validates a photovoltaic window technology that circumvents the fundamental tradeoff between efficient solar conversion and high visible light transmittance that limits conventional semitransparent PV window designs.Conventional smart windows with tunable transparency are based on electrochromic systems that consumes energy. Here Wheeler et al. demonstrate a halide perovskite based photo-switchable window that dynamically responds to sunlight and change colors via reversible phase transitions.

Optimizing carbon nanotube contacts for use in organic photovoltaics

Transparent electrical contacts (TCs) play an important role in thin film photovoltaics (PV), including in organic devices (OPV). OPV devices have different TC requirements than traditional thin film devices, due to the need to make them flexible, cost and manufacturing constraints, and the potential benefits that may be realized from using a hole conducting TC. Additionally, the active layer in an OPV device is far thinner than in an inorganic “thin-film” device, causing TC morphology to have a significant effect on device performance. Here, we present data on optimizing carbon nanotube networks for use as TCs in OPV. We look at network deposition techniques and required post-treatment methods along with electrical, optical, and morphological data to devise a fabrication process for a high quality network that is compatible with OPV device production.