Paul W. Leu

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates

Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

Nanowire active-matrix circuitry for low-voltage macroscale artificial skin

Large-scale integration of high-performance electronic components on mechanically flexible substrates may enable new applications in electronics, sensing and energy. Over the past several years, tremendous progress in the printing and transfer of single-crystalline, inorganic micro- and nanostructures on plastic substrates has been achieved through various process schemes. For instance, contact printing of parallel arrays of semiconductor nanowires (NWs) has been explored as a versatile route to enable fabrication of high-performance, bendable transistors and sensors. However, truly macroscale integration of ordered NW circuitry has not yet been demonstrated, with the largest-scale active systems being of the order of 1 cm(2) (refs 11,15). This limitation is in part due to assembly- and processing-related obstacles, although larger-scale integration has been demonstrated for randomly oriented NWs (ref. 16). Driven by this challenge, here we demonstrate macroscale (7×7 cm(2)) integration of parallel NW arrays as the active-matrix backplane of a flexible pressure-sensor array (18×19 pixels). The integrated sensor array effectively functions as an artificial electronic skin, capable of monitoring applied pressure profiles with high spatial resolution. The active-matrix circuitry operates at a low operating voltage of less than 5 V and exhibits superb mechanical robustness and reliability, without performance degradation on bending to small radii of curvature (2.5 mm) for over 2,000 bending cycles. This work presents the largest integration of ordered NW-array active components, and demonstrates a model platform for future integration of nanomaterials for practical applications.

Ordered arrays of dual-diameter nanopillars for maximized optical absorption.

Optical properties of highly ordered Ge nanopillar arrays are tuned through shape and geometry control to achieve the optimal absorption efficiency. Increasing the Ge materials filling ratio is shown to increase the reflectance while simultaneously decreasing the transmittance, with the absorbance showing a strong diameter dependency. To enhance the broad band optical absorption efficiency, a novel dual-diameter nanopillar structure is presented, with a small diameter tip for minimal reflectance and a large diameter base for maximal effective absorption coefficient. The enabled single-crystalline absorber material with a thickness of only 2 μm exhibits an impressive absorbance of ∼99% over wavelengths, λ = 300-900 nm. These results enable a viable and convenient route toward shape-controlled nanopillar-based high-performance photonic devices.

Tunable and selective resonant absorption in vertical nanowires

We demonstrate that vertical subwavelength diameter nanowires exhibit tunable and selective resonant absorption using numerical simulations and optical waveguide theory. Incident light on vertical nanowires only excites hybrid TM-dominant HE(1m) leaky modes due to symmetry matching requirements. The transverse resonances associated with these hybrid modes result in strong absorption enhancements that may be adjusted by changing the nanowire diameter. In particular, the fundamental HE(11) transverse resonance may be tuned across a wide range of wavelengths and is separated from that of the HE(12) mode by a large spectral gap, which is advantageous for wavelength selectivity. Leaky longitudinal resonances result in weaker absorption peaks at larger wavelengths. We further study the effect of incident light angle on the absorption spectra.

Enhanced absorption in silicon nanocone arrays for photovoltaics

Silicon nanowire arrays have been shown to demonstrate light trapping properties and promising potential for next-generation photovoltaics. In this paper, we performed systematic and detailed simulation studies on the optical properties of silicon nanocone arrays as compared to nanowires arrays. Nanocone arrays were found to have significantly improved solar absorption and efficiencies over nanowire arrays. Detailed simulations revealed that nanocones have superior absorption due to reduced reflection from their smaller tip and reduced transmission from their larger base. The enhanced efficiencies of silicon nanocone arrays were found to be insensitive to tip diameter, which should facilitate their fabrication. Breaking the vertical mirror symmetry of nanowires results in a broader absorption spectrum such that overall efficiencies are enhanced. We also evaluated the electric field intensity, carrier generation and angle-dependent optical properties of nanocones and nanowires to offer further physical insight into their light trapping properties.

Nanoscale doping of InAs via sulfur monolayers

One of the challenges for the nanoscale device fabrication of III-V semiconductors is controllable postdeposition doping techniques to create ultrashallow junctions. Here, we demonstrate nanoscale, sulfur doping of InAs planar substrates with high dopant areal dose and uniformity by using a self-limiting monolayer doping approach. From transmission electron microscopy and secondary ion mass spectrometry, a dopant profile abruptness of ∼3.5 nm/decade is observed without significant defect density. The n+/p+ junctions fabricated by using this doping scheme exhibit negative differential resistance characteristics, further demonstrating the utility of this approach for device fabrication with high electrically active sulfur concentrations of ∼8×1018 cm−3.

Uniform and Ordered Copper Nanomeshes by Microsphere Lithography for Transparent Electrodes

We report a comprehensive simulation and experimental study on the optical and electronic properties of uniform and ordered copper nanomeshes (Cu NMs) to determine their performance for transparent conductors. Our study includes simulations to determine the role of propagating modes in transmission and experiments that demonstrate a scalable, facile microsphere-based method to fabricate NMs on rigid quartz and flexible polyethylene terephthalate substrates. The fabrication method allows for precise control over NM morphology with near-perfect uniformity and long-range order over large areas on rigid substrates. Our Cu NMs demonstrate 80% diffuse transmission at 17 Ω/square on quartz, which is comparable to indium tin oxide. We also performed durability experiments that demonstrate these Cu NMs are robust from bending, heating, and abrasion.

Hybrid Core−Shell Nanowire Forests as Self-Selective Chemical Connectors

Conventional connectors utilize mechanical, magnetic, or electrostatic interactions to enable highly specific and reversible binding of the components (i.e., mates) for a wide range of applications. As the connectors are miniaturized to small scales, a number of shortcomings, including low binding strength, high engagement/disengagement energies, difficulties with the engagement, fabrication challenges, and the lack of reliability are presented that limit their successful operation. Here, we report unisex, chemical connectors based on hybrid, inorganic/organic nanowire (NW) forests that utilize weak van der Waals bonding that is amplified by the high aspect ratio geometric configuration of the NWs to enable highly specific and versatile binding of the components. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (approximately 163 N/cm(2)) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio approximately 25), reusability (approximately 27 attach/detach cycles), and efficient binding for both micro- and macroscale dimensions.

Hierarchical Graphene/Metal Grid Structures for Stable, Flexible Transparent Conductors

We report an experimental study on the fabrication and characterization of hierarchical graphene/metal grid structures for transparent conductors. The hierarchical structure allows for uniform and local current conductivity due to the graphene and exhibits low sheet resistance because the microscale silver grid serves as a conductive backbone. Our samples demonstrate 94% diffusive transmission with a sheet resistance of 0.6 Ω/sq and a direct current to optical conductivity ratio σdc/σop of 8900. The sheet resistance of the hierarchical structure may be improved by over 3 orders of magnitude and with little decrease in transmission compared with graphene. Furthermore, the graphene protects the silver grid from thermal oxidation and better maintains the sheet resistance of the structure at elevated temperature. The graphene also strengthens the adhesion of the metal grid with the substrate such that the structure is more resilient under repeated bending.

Strong broadband absorption in GaAs nanocone and nanowire arrays for solar cells

We studied the influence of geometric parameters on the optical absorption of gallium arsenide (GaAs) nanocone and nanowire arrays via finite difference time domain simulations. We optimized the structural parameters of the nanocone and nanowire arrays to maximize the ultimate efficiency across a range of lengths from 100 to 1000 nm. Nanocone arrays were found to have improved solar absorption, short-circuit current density, and ultimate efficiencies over nanowire arrays for a wide range of lengths. Detailed simulations reveal that nanocones have superior absorption due to reduced reflection from their smaller tip and reduced transmission from their larger base. Breaking the vertical mirror symmetry of nanowires results in a broader absorption spectrum such that overall efficiencies are enhanced for nanocones. We also evaluated the electric field intensity, carrier generation and angle-dependent optical properties of nanocones and nanowires. The carrier generation in nanocone arrays occurs away from the surface and is more uniform over the entire structure, which should result in less recombination losses than in nanowire arrays.