Symposium OrganizersZhong Lin (ZL) Wang, Georgia Institute of Technology
Xudong Wang, University of Wisconsin-Madison
Thomas G. Thundat, University of Alberta
Magnus Willander, Linkoping University
N3: One-Dimensional Nanomaterial Based Photocatalysis
Tuesday PM, April 10, 2012
Moscone West, Level 2, Room 2010
2:45 AM - N3.1
Development of High Efficiency Colloidal Quantum Dot Solar Cells through Improved Solution Processing and Nanoparticle Surface Modification
Lisa Rollny 1 Kang Wei Chou 3 4 Aram Amassian 4 Edward Sargent 1 2
1University of Toronto Toronto Canada2University of Toronto Toronto Canada3King Abdullah University of Science and Technology (KAUST) Thuwal Saudi Arabia4Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
Colloidal quantum dot (CQD) solar cells promise higher efficiencies than existing thin film devices due to their inherent infrared bandgap â?" a trait more difficult to obtain in organic chromophores. CQD solar cells have sufficient tunability allowing for the construction of tandem and triple-junction cells reaching beyond the single-junction (Shockley-Quiesser) limit. The first reports of infrared colloidal quantum dot solar cells were published in 2005. In six years, solar power conversion efficiency has increased to 6%. This indicates rapid progress in the direction of commercial viability, the threshold of which is often estimated at > 10% for low-cost, flexible PV technologies. This talk will detail our progress making colloidal light absorbing layers via layer-by-layer methods in which a thin (typically 5-30 nm) layer of organic-ligand-passivated colloidal quantum dots is deposited on the substrate; and that layer is then treated using a solvent in which the nanoparticles are not redispersible (e.g. methanol) and which contains a reagent (such as Br- anions) that will diffuse into the layer and displace the more weakly-bound organic ligands with the final inorganic passivant. The formerly bound ligands will be removed by subsequent washing steps. GI-SAXS is used to detect degree of crystallization in CQD films and we relate this parameter to performance of CQD based photovoltaics. Cation-exchange techniques will also be discussed wherein a very thin shell (monolayer), lattice-matched to the PbS core, is grown on the surface of nanoparticles in order to enhance passivation. We will pursue lanthanide (La, Ce, Eu) chalcogenides (S, Se, Te) lattice-matched within 5% to PbSâ?T rocksalt structure; many of which should form type-I heterointerfaces with PbS CQDs.
3:00 AM - N3.2
Improving Redox Couple Mass Transport in High Aspect Ratio Silicon Wire-array Liquid Junctions
Chengxiang Xiang 1 Andrew C Meng 1 Nathan S Lewis 1
1Caltech Pasadena USAShow Abstract
Silicon wire-array liquid junctions show promise in photoelectrosynthetic devices for solar to fuel applications such as water splitting. However, high aspect ratio wire array geometries present additional mass transport complications relative to planar analogs. A carrier collection loss due to redox couple depletion at the bottom of the wire arrays is predicted by simulations and has been observed in experiments. In this work, a device architecture to facilitate redox couple mass transport is proposed, realized experimentally, and demonstrated to achieve high efficiencies in liquid junction solar cells. In this study, Ag films were deposited onto the bottom of Si wire arrays by physical vapor deposition, to serve as counter electrodes. To prevent direct contacts to the Si wire array from the Ag films, electrically insulating layers of SiO2 and Al2O3 were prepared by thermal oxidation and physical vapor deposition, respectively. During cell operation, the Ag film counter electrode regenerated the redox couple species where molecular depletion would otherwise occur. Computational modeling suggested that the supply of redox species from the Ag films minimized the concentration overpotential and the solution resistance. The energy-conversion properties of p-Si wire-array photocathodes with Ag film counter electrodes were characterized in contact with the cobaltocene (CoCp2+/0) redox couple. Under high illumination intensities or low stirring conditions, the electrode design demonstrated >20% improvements in fill factor and >2Ã- improvement in short circuit current density relative to regular wire array photocathodes. Evaluating the Ag film and Si wire-array in a two-electrode configuration in contact with a thin electrolyte layer, a short circuit current density of 89 mA cm-2 was observed at 900 mW cm-2 of W-halogen ELH illumination. The demonstrated structure, which essentially minimized the effective ion transport path length to the photoelectrode, also provides design guidance for future tandem cells for water splitting reactions.
3:15 AM - N3.3
Solution Grown Hematite Nanowire Arrays for Photoelectrochemical Water Splitting
Nripan Mathews 1 2 Morgan Stefik 2 Subodh Mhaisalkar 1 Michael Gratzel 2
1Nanyang Technological University Singapore Singapore2Ecole Polytechnique Feacute;deacute;rale de Lausanne Lausanne SwitzerlandShow Abstract
The photoelectrolysis of water using semiconductors is a very attractive yet challenging approach towards storing solar energy within the chemical bonds of H2. This necessitates the utilization of materials that are stable, have good optical absorption and are abundant in nature. Within this context, hematite (Fe2O3) which has a bandgap of ~2eV, is of great interest and have been widely explored. Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination due to the short diffusion lengths of photoexcited carriers. This is due to the poor charge transport properties of iron oxide. One approach towards improving the performance of these devices is the utilization of nanostructured morphologies. Hematite nanowire arrays are thus promising due to the large electrolyte-semiconductor interface (which allows for higher injection currents to the water) while enabling short transport lengths for the charge carriers to the electrodes. In this contribution, we describe the fabrication and characterization of hematite nanowire arrays grown through solution processed methods by the controlled hydrolysis of FeCl3 by urea. The photoelectrochemical water splitting performance of these nanowire arrays indicate current densities of 1mA/cm2 at 1.43V vs RHE. The influence of nanowire morphology, doping as well as oxygen evolution catalysts will be presented.
3:30 AM - N3.4
Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production
In Sun Cho 1 Zhebo Chen 2 Arnold J Forman 2 Dong Rip Kim 1 Pratap M Rao 1 Thomas F Jaramillo 2 Xiaolin Zheng 1
1Stanford University Stanford USA2Stanford University Stanford USAShow Abstract
Vertically oriented single-crystalline nanorod arrays, owing to their superior charge transport properties as well as light trapping capabilities, are desirable architectures for efficient photoelectrochemical devices. However, nanorod arrays, compared to nanoparticle films, have low internal surface area that leads to small contact area with electrolyte and thus hinders efficient charge transfer. Herein, We report a hierarchically branched TiO2 nanorod structure which serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. The array of nanorods with cone-shaped branches was synthesized via a simple two-step solution phase method. The nanorods were grown by hydrothermal method at 170 Â°C with ~2 Î¼m in length and diameters of ~70 nm, and the cone-shaped branches were grown subsequently at 80 Â°C by a solution phase growth method with approximately 90 nm in length and a base diameter of 15 nm. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm2 total power density), the branched TiO2 nanorod array produces a photocurrent density of 0.83 mA/cm2 at 0.8 V vs. RHE. The incident photon-to-current conversion efficiency reaches 67 % at 380 nm with an applied bias of 0.6 V vs. RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of i) improved charge separation and transport within the branches due to their small diameters, and ii) a four-fold increase in surface area which facilitates the hole transfer at the TiO2/electrolyte interface. Though it is unlikely that a pure TiO2 photo-electrode alone could ultimately serve as an efficient solar water-splitting device due to its wide bandgap, the approach and device geometry demonstrated with TiO2 in this work can be leveraged to other, more promising semiconducting materials to greatly improve their efficiencies.
3:45 AM - N3.5
p-InP Nanopillar Photocathodes for Efficient, Solar-driven Hydrogen Production
Min Hyung Lee 1 2 3 Joel Ager 1 2 Ali Javey 1 2 3
1Lawrence Berkeley National Laboratory Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USA3University of California, Berkeley Berkeley USAShow Abstract
The production of hydrogen from water using sunlight via photoelectrochemical (PEC) water splitting could provide an abundant, renewable, green source of energy for the future. Here, the roles of nanostructuring, crystalline orientation, oxide passivation, and co-catalysts on the photoelectrochemical evolution of hydrogen using p-InP photocathodes are investigated. Arrays of InP nanopillars are produced by a modification to a deep reactive ion etching (RIE) process applied to (100)-oriented p-InP wafers. Photoluminescence analysis indicates that the low surface recombination velocity of InP is retained after the etching treatment. TiO2 protection layers are applied by atomic layer deposition. Ru nanoparticles are used a co-catalyst. Optimized nanopillar/TiO2/Ru photocathode arrays have AM 1.5G photocurrents of up to 35 mA cm-2 with current onsets +600 mV vs. normal hydrogen electrode. The power conversion efficiencies for the photocathode (relative to the reversible potential in the dark) reach 15%; these are among the highest values reported for a solar-driven photocathode. Flat band potential measurements of wafers, nanopillar structures, and grating structures suggest that both exposure of non-100 oriented surfaces and, possibly, near-surface carbon doping from the RIE process are responsible for the larger open circuit voltage observed for the nanopillar structures. The flat band potential is not affected by the ALD TiO2 layers, and excellent photocurrent and stability are observed for TiO2 coatings up to 5 nm.
JCAP is a DOE Energy Innovation Hub supported through the Office of Science of the U.S. DOE under Award Number DE-SC0004993.
4:30 AM - N3.6
Colloidal Gallium Phosphide Nanowires: Surfactant-free Synthesis and Visible-light-driven Hydrogen Production from Water Reduction
Jianwei Sun 1 2 Peidong Yang 1 2
1University of California at Berkeley Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
Practical solar-to-fuel conversion via semiconductor-based photocatalytic water splitting and CO2 reduction requires production of high-surface-area semiconductors on a large scale. Compared to bulk materials and nanoscale structures of other morphologies, one-dimensional single-crystalline nanowires are advantageous because the longitudinal dimension allows efficient light absorption and the reduced radial dimension and the increased surface-to-volume ratio facilitate rapid diffusion of photogenerated charge carriers to nanowire surfaces. Traditional solution-liquid-solid (SLS) method generally produces colloidal semiconductor nanowires having surface organic capping surfactants/ligands, which prevent efficient injection of photogenerated carriers to electrolytes. We report a surfactant-free, large-scale, SLS growth of semiconducting GaP nanowires. The wire growth is catalyzed by in situ generated Ga nanodroplets, which can be completely removed by selective etching using hydrochloric acid. Purified, Ga-removed GaP nanowires are used for visible-light-driven hydrogen production from water reduction. Although the conduction band of GaP is ~1 V more negative than the standard water reduction potential, the surface-unmodified GaP nanowires only exhibit low hydrogen evolution activity. Upon photodeposition of Pt nanoparticles on the nanowire surfaces, significantly enhanced hydrogen production is observed, which should be due to the improved separation of photogenerated charge carriers and the reduced kinetic barrier for proton reduction. Our results indicate that colloidal surfactant-free GaP nanowires combined with potent surface electrocatalysts could serve as promising photocathodes for Z-scheme artificial photosynthesis.
4:45 AM - N3.7
Solar Fuel Generation from Earth-abundant Silicon Microwire/Ni-Mo Photocathodes
Emily L Warren 1 James R McKone 1 Bruce S Brunschwig 1 Harry B Gray 1 Harry A Atwater 2 Nathan S Lewis 1
1Caltech Pasadena USA2California Institute of Technology Pasadena USAShow Abstract
Solar-driven water splitting is a promising technology for the generation of hydrogen as a renewable fuel. One of the challenges in the development of a water splitting system is finding stable, earth abundant materials to serve as absorbers and catalysts for both the oxidation (O2-producing) and reduction (H2-producing) reactions. Our approach to designing such a system involves the use of a separate microwire based photoanode and photocathode and connecting them with a membrane that can conduct both protons and electrons. Here we discuss the development of photocathodes for the hydrogen evolution reaction (HER) using silicon microwires (SiMW) and earth abundant Ni-Mo alloy catalysts. SiMWs have been demonstrated as a promising earth-abundant photocathode material for the water splitting reaction. The radial geometry of microwire arrays decouples the direction of light absorption and carrier collection, enabling the use of materials with shorter minority-carrier diffusion lengths than would be acceptable in a planar geometry liquid junction. When coupled to a noble metal Pt HER catalyst, radial pn junction SiMW photocathodes have achieved efficiencies up to 6%. However, due to the high cost and low abundance of Pt, it is desirable to use a more abundant material. Prior work has focused on the development of pn junction Si microwire arrays as efficient photovoltaic devices, but using these materials for solar-driven chemical reactions presents additional challenges. In this presentation we will discuss our progress in combining earth-abundant HER catalysts based on Ni-Mo alloys with radial pn junction Si microwire arrays. SiMW arrays tested in a three-electode electrochemical cell under 1-sun illumination achieved Vocs of 490 mV (relative to the hydrogen evolution potential) and overall photocathode conversion efficiencies >2%. By optimizing the photovoltaic properties of the wire arrays, the deposition of nanostructured catalyst, and the incorporation of light trapping features, we have demonstrated SiMW HER photocathodes composed of only cheap, abundant materials, and with efficiencies comparable to devices using expensive semiconductor and catalyst materials. Progress towards the creation of a flexible photocathode membrane will also be discussed.
5:00 AM - N3.8
Photoelectrochemistry of Solution-grown Gallium Phosphide Nanowire Photocathode
Chong Liu 1 Jianwei Sun 1 2 Jinyao Tang 1 Peidong Yang 1 2
1UC Berkeley Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
Via Solid-Liquid-Solution (SLS) grown mechanism, large-scale gallium phosphide nanowires are synthesized via a solution method. Furthermore, its photoelectrochemical (PEC) properties are investigated. By introducing Zn precursor in the synthesis, the doping levels and the corresponding PEC activities could be improved. Optimization of material properties leads to GaP NW photocathode of activity comparable with single-crystalline p-GaP wafer. And the GaP NW shows much higher internal quantum efficiency despite its reduced optical absorption.
5:15 AM - N3.9
Atomic-scale Design of Novel Carbon-based Hybrid Nanostructures for Solar Energy Harvesting and Storage
Alexie Kolpak 1 Timothy J Kucharski 1 Jeffrey C Grossman 1
1MIT Cambridge USAShow Abstract
We use first-principles density functional theory computations to design and explore the properties of hybrid nanostructures composed of photo-switchable organic molecules covalently bound to a range of carbon-based templates. We show that the templates induce formation of novel 1D and 2D crystalline phases of the photo-active molecules, introducing a number of independent tuning parameters for optimizing their ability to function as solar thermal fuels with high energy storage capacity, storage lifetime, and good photo-absorption properties. However, the volumetric energy density of these fuels can be limited by their solubility in water or other common solvents. By employing additional chemical modifications, we show that the hybrid nanostructures can potentially self-assemble to form new, solvent-free solid state solar thermal fuels with increased volumetric energy density. Furthermore, we predict that these solid state fuels will also be able to release stored solar energy as higher-temperature heat than their solution-based counterpart, and we present recent experimental work from our lab testing these predictions. Our work shows that these materials, both in solution and solid state forms, have the potential to provide an efficient, customizable, and emissions-free renewable energy technology.
5:30 AM - N3.10
Nanostructured Black Silicon Anti-reflection for 18.2% Photovoltaics and High-current Photocathodes for Hydrogen Production
Jihun Oh 1 Todd G Deutsch 1 Hao-Chih Yuan 1 Howard M Branz 1
1NREL Golden USAShow Abstract
One-dimensional (1-D) silicon nanostructures can provide good performance in highly-efficient photovoltaic (PV) cells and as photocathodes for solar hydrogen production because of their excellent anti-reflection (AR) properties. Here, we report an independently-confirmed 18.2%-efficient nanostructured black silicon PV cell under simulated AM 1.5 G illumination, which is the most efficient among any solar cells with intentional nanostructuring. Our nanostructured black silicon is fabricated by a simple metal-assisted etching technique, and consists of vertically aligned nanopores of random depths of more than 300 nm, and with feature sizes smaller than about 50 nm. These nanostructures are further modified by applying additional chemical etch steps. Our nanostructured solar cells therefore have a density-graded layer that reduces the reflection of silicon in air to below 4% across the usable solar spectrum with Î» <1.2 Âµm. Black silicon can therefore provide high solar conversion efficiency, while significantly reducing manufacturing costs by eliminating the industry-standard Si3N4 AR coatings. However, increased photocarrier recombination in 1-D nanostructures can limit the spectral response at short wavelengths in PV cells and in photocathodes for solar H2 generation. In this talk, we will present a novel process that enables us to control these recombination mechanisms and fabricate an 18.2% efficient nanostructured black Si without any AR coating. Because reflection is reduced without the need for an insulating AR coating, use of similar 1-D nanostructured surfaces in photocathodes for water splitting provides more than 10 % increase in the rate of solar hydrogen production. Nanostructured photocathodes also facilitate H2 bubble evolution and reduce the overpotential required for the water-splitting half-reaction by increasing the surface density of reaction sites. The photovoltaic work was entirely supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, through both the Solar Energy Technologies Program and a DOE American Recovery and Reinvestment Act (ARRA) grant. The PEC H2 work was entirely supported by the SISGR program of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences. Yuan was supported by SETP and ARRA programs. Deutsch was supported by the SISGR program. All work was performed under DOE Contract No. DE-AC36-08-GO28308
5:45 AM - N3.11
WO3 Nanotubes for Effective Photoelectrochemical Water-splitting
Pratap Mahesh Rao 1 In Sun Cho 1 Xiaolin Zheng 1
1Stanford University Stanford USAShow Abstract
We report the effective photoelectrochemical splitting of water by arrays of tungsten trioxide (WO3) nanotubes (NTs) synthesized by rapid, atmospheric flame vapor deposition. Sub-stoichiometric W18O49 (WO2.72) NTs were first synthesized by evaporating WOx vapor onto fluorinated tin oxide (F:SnO2) coated glass substrates in a flame. The flame vapor deposition method grows W18O49 NTs with typical axial growth rates of ~0.1Î¼m/minute and diameters in the range of 100-200nm. The as-grown W18O49 NTs were further oxidized to WO3 at 550Â°C for 2 hours in air before testing their light absorption properties and PEC water-splitting performance. These densely packed, vertically aligned arrays of WO3 NTs had an absorption edge at 460nm (Eg = 2.7 eV), similar to that of dense WO3 nanoparticle (NP) films. Moreover, WO3 NT and NP films of the same thickness showed similar levels of integrated light absorption in the wavelength range of 280-460nm. Despite the similarities in light absorption, the WO3 NT film outperformed the WO3 NP film in PEC water-splitting. Specifically, under simulated solar light illumination (AM 1.5G, 100 mW/cm2), the NT array of 0.55Î¼m thickness showed 47% higher photocurrent at a bias potential of 1V vs. RHE compared to a dense NP film of the same thickness, and a steeper increase of photocurrent with applied potential. The higher photocurrent observed for the NT film is mainly due to more efficient charge separation and transport processes in the NT structure. Critically, the diffusion length for minority holes in WO3 is approximately 150nm, which is similar to the radius of the WO3 NTs, so that holes can reach the WO3/electrolyte interface for water oxidation with less electron-hole recombination. Moreover, the large, accessible surface area of the NTs also facilitates efficient hole extraction to the electrolyte, which further reduces recombination near the solid/electrolyte surface. Finally, the NT geometry, unlike the NP film geometry, allows electrons to reach the current collector by vectorial transport, without hopping through interparticle grain boundary resistances that can also act as recombination centers.
N4: Poster Session: One-Dimensional Nanomaterials for Solar Energy Harvesting
Tuesday PM, April 10, 2012
Moscone West, Level 1, Exhibit Hall
6:00 AM - N4.1
Investigation of Thin Film and Nano-patterned Amorphous Silicon as Anode for Lithium Ion Battery
Rajamouly S Omampuliyur 1 Maruf Bhuiyan 1 Han Zheng 2 Jing Zhu 3 Hui Xia 3 Li Lu 3 Eugene Fitzgerald 4 Wee K Choi 1 2
1Singapore- MIT Alliance, National University of Singapore Singapore Singapore2National University of Singapore Singapore Singapore3National University of Singapore Singapore Singapore4Singapore- MIT Alliance, Massachusetts Institute of Technology Cambridge USAShow Abstract
Silicon is increasingly being investigated as an alternative anode material in lithium ion rechargeable batteries due to its very high theoretical specific capacity (~4200mAh/g). However, volume change associated with lithiation and delithiation (~400%) makes it extremely difficult to be used in commercial Li-ion batteries. We will report in this paper the anode performance of as-deposited and nano-patterned amorphous silicon on stainless steel substrate. An adhesion layer of titanium between the stainless steel substrate and Si has proved to be critical in improving the integrity of Si thin film. When the thickness of the as-deposited Si thin film was less than 250nm, very good cyclability was achieved. This thickness constraint acts as a ceiling in increasing the specific capacity per area. Such problem may be circumvented by the use of nano-patterning the amorphous silicon film thereby increasing the surface to volume ratio, accommodating the volume expansion and achieving sustainable higher specific capacity per area. The nano-patterning was achieved by (i) glancing angle deposition (GLAD) of gold followed by catalytic etching (for the creation of Si nanowires of diameter 10-100nm) and (ii) laser-interference lithography followed by catalytic etching (for the creation of Si nanopillars of diameter of 200-300nm). We examine the effect of geometry and size on the anode performance of amorphous silicon nanostructures.
6:00 AM - N4.10
Au-decorated ZnSe*0.5(N2H4) Nanocrystals: Correlation of Photocatalysis with Carrier Dynamics
Yu-Chih Chen 1 Tao-Cheng Liu 1 Yung-Jung Hsu 1
1National Chiao Tung University Hsinchu TaiwanShow Abstract
ZnSe*0.5(N2H4) nanocrystals with various morphologies including nanowires, nanobelts and nanosheets were successfully prepared with a hydrothermal synthetic approach. It was found that the ratio of N2H4 to H2O employed in synthesis played an important role in nanocrystal growth, affecting the resultant composition and morphology of product. The chemical composition and crystallographic structure of ZnSe*0.5(N2H4) were examined and confirmed with XRD, TGA, FTIR, XPS and TEM analyses. With a relatively long exciton lifetime, ZnSe*0.5(N2H4) nanowires performed much better in the photodegradation of rhodamine B than the other four counterpart products. As compared to the relevant commercial products like N-doped P-25 TiO2 and ZnSe powders, the as-synthesized ZnSe*0.5(N2H4) nanowires exhibited superior photocatalytic performance under visible light illumination, demonstrating their potential as an efficient photocatalyst in relevant redox reactions. A further enhancement in the photocatalytic activity can be achieved for ZnSe*0.5(N2H4) nanocrystals when Au nanoparticles were deposited on their surfaces. This improvement resulted from the pronounced charge carrier separation occurring at the interface of ZnSe*0.5(N2H4)/Au, which can be quantitatively interpreted with the time-resolved photoluminescence analysis. References:  M. Chen, and L. Gao, Mater. Chem. Phys, 2005, 91, 437.  S. Xiong, J. Shen, Q. Xie, Y. Gao, Q. Tang, and Y. Qian, Adv. Funct. Mater., 2005, 15, 1787.  T.-T. Yang, W.-T. Chen, Y.-J. Hsu, K.-H. Wei, T.-Y. Lin, T.-W. Lin, J. Phys. Chem. C 2010, 114, 11414.
6:00 AM - N4.11
Copper Tungstate Thin Film and Nanowire Photoanodes for Visible Light Water Splitting
Tyler Matthews 1 2 3 K. R Balasubramaniam 2 3 Shiyou Chen 2 3 Esther Alarcoacute;n-Lladoacute; 2 3 Lin-Wang Wang 1 2 3 Junqiao Wu 1 2 Joel W Ager 2 3
1University of California, Berkeley Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USA3Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
Copper tungstate (CuWO4), an n-type semiconductor with an indirect bandgap of ~2.3 eV, is a promising photoanode for electrochemical water oxidation. Our band structure calculations indicate that the Cu 3d orbitals form an intermediate band located near the conduction band minimum, which is responsible for the lower band gap compared to pure WO3 (~2.7 eV). We investigated the growth of (1) CuWO4 thin films via reactive ion co-sputtering and (2) highly-ordered CuWO4 nanowire arrays using templated electrochemical deposition. Structural and electronic characterization was performed with scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and Hall effect. Photoelectrochemical characterization was performed using a standard 3-electrode setup with simulated solar radiation. Average current density was found to be ~80 uA/cm^2 at an applied bias of +1.2 V, with an onset potential of +0.3 V vs NHE. We also evaluated the performance of CuWO4 photoanodes for complete photo-driven water splitting by coupling them with suitable photocathodes (such as p-InP and p-Si) in a tandem or â?oz-schemeâ? geometry. These measurements employ a two-electrode photoelectrochemical setup both with and without an applied bias. Our results demonstrate that the films are stable over long periods of time, absorb in the visible part of the spectrum, and are active in the photo-oxidation of water. We find that a tandem device with CuWO4 as the photoanode does spontaneously split water, although the energy conversion efficiency is low (<1%). Optimization of the growth conditions and a detailed study into the energy conversion dependency upon illumination wavelength, electrolyte pH, film thickness, and nanowire dimensions will be discussed. JCAP is a DOE Energy Innovation Hub supported through the Office of Science of the U.S. DOE under Award Number DE-SC0004993.
6:00 AM - N4.14
Semiconductor and Metallic Single Walled Carbon Nanotubes in TiO2 Photoanodes for Hybrid Solar Cells
German Alvarado Tenorio 1 M. E Rincoacute;n 1
1Centro de Investigacioacute;n en Energiacute;a Temixco MexicoShow Abstract
1Centro de InvestigaciÃ³n en EnergÃa â?" Universidad Nacional AutÃ³noma de MÃ©xico, Priv.Xochicalco s/n, Col. Centro, Temixco, Mor. 62580, MÃ©xico. email@example.com* The incorporation of single walled carbon nanotubes (SWCNTs) with mesoscopic semiconductor as TiO2 has shown improvement in the charge separation and transport of carriers to collector electrode, decreasing the recombination in photoelectrochemical solar cells and dye sensitized solar cells [1-3]. However, these works reports the use of pristine SWCNTs consisting of nearly equal amounts of semiconducting and metallic carbon nanotubes. In this contribution an experimental study is presented to compare photoanodes separately elaborated with semiconductor and metallic carbon nanotubes coated with TiO2 compact film. Commercial semiconductor and metallic carbon nanotubes pristine were dispersed separately in aqueous surfactant solution at 0.25 Î¼g/ml concentration for to elaborate s-SWCNTs and m-SWCNTs thin films onto glass and ITO substrates by filtration. These films were characterized by AFM and Scanning Electrochemical Microscopy (SECM) to measure spreading resistance and work function respectively. The s-SWCNTs and m-SWCNTs thin films obtained were coated with TiO2 by sol gel method, optical and morphological techniques were used and their electrical properties compared those photoanodes elaborated with semiconductor and metallic mixed. The samples were coated with conjugated polymer poly(3-hexilthiophene) to be explored in hybrid solar cells. Keywords: Single Walled Carbon Nanotubes, TiO2, Photoanodes, Conjuagted Polymer, Hybrid Solar Cell.  A. Kongkanand, R. MartÃnez, P. V. Kamat, anoletters., 7, 3, 676 (2007).  P. Brown, K. Takechi, P. V. Kamat, J. Phys. Chem. C., 112, 4776 (2008).  T. Hasobe, S. Fukuzumi, P. V. Kamat, Angew. Chem. Int. Ed., 45, 755 (2006).
6:00 AM - N4.15
Vertically Aligned WO3 Nanorod Array Photoanode for Efficient Photoelectrochemical Water Splitting
Junjun Zhang 1 2 3 Min Hyung Lee 1 2 3 Kuniharu Takei 1 2 3 Zhibin Yu 1 2 3 Max Zheng 1 2 3 Tyler Matthews 2 Esther Alarcon Llado 2 Joel W Ager 2 Weixin Zhang 4 Ali Javey 1 2 3
1University of California at Berkeley Berkeley USA2Lawrence Berkeley National Laboratory Berkeley USA3University of California at Berkeley Berkeley USA4Hefei University of Technology Hefei ChinaShow Abstract
We report on a dense vertical tungsten trioxide (WO3) nanorod array as photoanode for photoelectrochemical water splitting. Photocorrosion stable WO3 nanorod array in monoclinic structure are grown on Si substrates directly by chemical vapor deposition (CVD) method without using any catalysts. Synthesis details are discussed, with structure and morphologies characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL) spectrum and UV-vis diffused reflectance spectra. This bare nanorod array with diameter around 80 nm have significant photo anticorrosion ability with stable photocurrent in 4 hours simulated solar illumination, and show novel photoelectrochemical energy conversion properties with a photocurrent of 1.3 mA/cm2 under AM 1.5G illumination due, we believe, to the high surface area, good contact to Si substrate and efficient separation ability of the photogenerated charge at the WO3 nanostructured surface.
6:00 AM - N4.16
Tandem Wire Array Solar Cells: Novel Growth of Si1-xGex Wires and Optoelectronic Modeling
Daniel Turner-Evans 1 Julie Jester 1 Christopher Chen 1 Hal Emmer 1 Emily Warmann 1 Harry Atwater 1
1CALTECH Pasadena USAShow Abstract
In an effort to make low cost, high efficiency, tandem solar cells, we have both synthesized Si1-xGex wire arrays through a novel, Cu catalyzed, high T, atmospheric VLS growth process and performed a full slate of optoelectronic simulations on GaAsxP1-x on Si1-xGex solar cells with Ga1-xInxP window layers. These devices offer the potential of >30% efficiencies in a flexible, lightweight, inexpensive form factor. Most conventional multijunction photovoltaics are grown on expensive Ge substrates. By instead growing Si1-xGex wires on a Si substrate, we can create bottom cells with a tunable lattice parameter and bandgap that are unavailable commercially. Furthermore, the wire geometry allows for strain relaxation in the base of the wire and high quality material in the upper, active region. Additionally, these arrays can be embedded in a flexible polymer and peeled off,(1) allowing the growth substrate to be reused.(2) By introducing GeCl4 into our Si wire array growth process,(3) we have grown Si1-xGex wires from a Cu catalyst. To the best of our knowledge, this is the first time Cu has been used to catalyze Si1-xGex wire growth. The alloy composition ranges from x = 0 to x = 0.5 as confirmed by XRD and EDS. Using a modeling approach that combines full field optics simulations and an FEM carrier transport device physics model,(4) we have explored a range of architectures for a multijunction structure featuring GaAs0.9P0.1 /Si0.1Ge0.9 tandem wire cells coated with Ga0.56In0.44P window layers. The optical generation rates are exported from the electromagnetic simulations into the device physics model, and light IV curves simulated for GaAs0.9P0.1 lifetimes ranging from 50 psec to 5 nsec. The structures investigated consist variously of conformally GaAs0.9P0.1 coated Si0.1Ge0.9 wire arrays and arrays with GaAs0.9P0.1 grown as â?~tip-coatedâ?T quasi-axial heterostructures on top of the Si0.1Ge0.9 wires. Optical modeling reveals that these two cells can be successfully current matched in a tandem configuration with current densities approaching 20 mA/cm2. The device performance reveals that, for the 5 ns lifetimes the smaller volume of GaAs0.9P0.1 material used in the tip-coated designs leads to lower dark current than for the conformal GaAs0.9P0.1 coatings and thus that relatively high open circuit voltages can be preserved despite low minority carrier lifetimes. In contrast, the thin shell of the uniformly coated array allows it to retain higher currents at low lifetimes, despite the voltage drop. Thus, combined optoelectronic/carrier transport simulations have yielded unexpected insights about tandem wire arrays design that are useful for achievement of high efficiency devices. 1. K. E. Plass et al., Advanced Materials 21, 325 (Jan, 2009). 2. J. M. Spurgeon et al., Applied Physics Letters 93, (Jul, 2008). 3. B. M. Kayes et al., Applied Physics Letters 91, 103110 (2007). 4. M. D. Kelzenberg, et al, in 2009 34th IEEE PVSC (2009), 1948-1953.
6:00 AM - N4.17
Comparison of Graphene Anode versus Metallic Single Walled Carbon Nanotube Anode for Organic Solar Cells
Isaac Ruiz 1 Aaron S George 2 Maziar Ghazinejad 3 Wei Wang 2 Adan Velasco 3 Mihrimah Ozkan 1 Cengiz Ozkan 2
1University of California Riverside Riverside USA2University of California Riverside Riverside USA3University of California Riverside Riverside USAShow Abstract
Graphene and metallic single-walled carbon nanotubes (SWCNTâ?Ts) are separately grown by chemical vapor deposition and then each used as transparent conducting anodes for organic solar cells. Performance of single layer, bilayer and multilayer graphene solar cells are compared to metallic SWCNT solar cells. Although the transparencies of the single layer and bilayer graphene anodes are greater compared to the metallic SWCNTâ?Ts it is demonstrated that open circuit voltage, short circuit current density and power conversion efficiency are improved by using metallic SWCNTâ?Ts. The ratio of metallic SWCNTâ?Ts compared to semiconducting SWCNTâ?Ts changes these properties greatly and the affects are discussed.
6:00 AM - N4.18
Controlled Growth of Hierarchical ZnO Nanorods on Si Nanowire Backbone for Efficeint Photoanode in Semiconductor-sensitized Solar Cells
Yeryung Jeon 1 Taeseup Song 2 Jaehwan Ha 2 Won Il Park 2 Ungyu Paik 1 2
1Hanyang University Seoul Republic of Korea2Hanyang University Seoul Republic of KoreaShow Abstract
Semiconductor or quantum dot (when quantum confinement is manifested) sensitized solar cells (QDSCs) are receiving currently an increasing attention as alternative to conventional dyes. The efficiencies of QDSCs have experienced a fast growth in the last years, mainly due to an increase of the reported photocurrent and fill factors. Despite this increase further enhancement of QDSCs needs an improvement of the obtained photovoltage, Voc, being the current main challenge in these devices. Here, we show that an appropriated nanostructure of wide band gap semiconductor electrode allows reducing the recombination process, with a significant enhancement of Voc. Voc as high as 0.77 V has been demonstrated for ZnO nanowires array electrodes. The performance of the cell can be even increased to a promising 3%, using a novel photoanode architecture of â?opine treeâ? ZnO nanorods (NRs) on Si NWs hierarchical branched structure, with high light scattering properties. Most importantly, we show the necessity of exploring new electrode architectures in order to improve the current efficiencies of QDSCs.
6:00 AM - N4.3
Atomic Layer Deposition of TiO2 Ultrathin Films on 3D Substrates for Energy Applications
Audrey Soum-Glaude 1 Virginie Brize 1 Laurent Cagnon 2 Gael Giusti 3 Rached Salhi 3 Stephane Daniele 4 Celine Ternon 5 Elisabeth Blanquet 1 Daniel Bellet 3
1SIMAP Saint-Martin d'Hegrave;res France2Institut Neacute;el Grenoble France3LMGP Grenoble France4IRCELYON Lyon France5LTM Grenoble FranceShow Abstract
In the present global environmental context, it becomes more and more critical to find efficient solutions to lower our energy consumption on one hand, and to produce energy from clean renewable sources on the other hand. Consequently, research efforts on materials for energy applications are intensifying. The present work aims at developing optoelectrical components usable for both energy saving (light emitting diodes) and renewable energy production (solar cells) by fabricating p-n heterojunctions based on a single semiconductor, titanium dioxide. TiO2 is indeed a very promising candidate: it is chemically and physically stable under irradiation, transparent to visible and near-infrared light (Eg = 3 - 3.5 eV), presents photocatalytic activity, is non-toxic and low cost, which permits to envisage its large scale use. The proposed architecture for solar cells and LEDs is original and unique for both applications: a three-dimensional architecture based on anodic alumina nanoporous membranes which serve as nanomasks for TiO2 growth in order to enlarge the effective surface of the components. TiO2 is synthesized by Atomic Layer Deposition (ALD), a technique particularly well adapted to the deposition of ultrathin films (from one monolayer to few tens of nanometers) on 3D porous substrates patterned with low diameter-to-height aspect ratio. In this work, TiO2 ultrathin films (10 to 100 nm) were grown by ALD on flat, micropatterned, microporous and nanoporous AAM substrates. The films were highly conformal, as confirmed by SEM and TEM imaging. EDS and XPS analyses validated the dioxide film stoichiometry. Now that the capacity of synthesizing the expected 3D nanostructures is successfully demonstrated, the possibilities of electrically doping these TiO2 films to realize p-n heterojunctions are discussed. Novel specific ALD precursors are particularly considered.
6:00 AM - N4.4
Vertically Aligned Single-walled Carbon Nanotubes as Low-cost and High-electrocatalytic Counter Electrode for Dye-sensitized Solar Cells
Pei Dong 1 Jing Zhang 1 Cary Pint 2 Mel Hainey 1 Yongjie Zhan 1 Robert Hauge 2 Hong Lin 3 Jun Lou 1
1Rice University Houston USA2Rice University Houston USA3Tsinghua University Beijing ChinaShow Abstract
A novel Dye-sensitized solar cell (DSSC) structure using vertically aligned single-walled carbon nanotubes (VASWCNTs) as the counter electrode has been developed. In this design, the VASWCNTs serve as a stable high surface area and highly active electrocatalytic counter-electrode that could be a promising alternative to the conventional Pt analogue. Utilizing a scalable dry transfer approach to form a VASWCNTs conductive electrode, the DSSCs with various lengths of VASWCNTs were studied. VASWCNTs-DSSC with 34Î¼m original length was found to be the optimal choice in the present study. The highest conversion efficiencies of VASWCNTs-DSSC achieved 5.5%, which rivals that of the reference Pt DSSC. From the electrochemical impedance spectroscopy analysis, it shows that the new DSSC offers lower interface resistance between the electrolyte and the counter electrode. This reproducible work emphasizes the promise of VASWCNTs as efficient and stable counter electrode materials in DSSC device design, especially taking into account of low-cost merit of this promising material. Finally, our recent efforts on direct growth of VASWCNTs on quartz substrate for DSSC applications will be discussed.
6:00 AM - N4.5
Multi-branched TiO2/Si Nanostructures for Enhanced Photoelectrochemical Water Splitting
Sun Young Noh 1 Ke Sun 2 Chulmin Choi 1 Sungho Jin 1 Deli Wang 1 2
1University of California, San Diego San Diego USA2University of California, San Diego San Diego USAShow Abstract
Silicon has desirable band gap and electrical properties useful for photoelectrochemical (PEC) reactions. However, long-term stability of Si is a well-known issue. On the other hand, TiO2 offers superior chemical stability as well as photochemical and photocatalytic properties but it only absorb UV light. In order to resolve these issues, we have fabricated three-dimensional (3D) nanostructures consisting of TiO2 nanobranches on vertical Si nanopillars. The heterogeneous integration of Si and TiO2 not only combines their properties allowing good chemical stability for PEC application, but also leads to enhanced efficiency due to efficient light absorption, enhanced separation and transportation of photo-induced charges, improved gas evolution from the nanoscale surface, etc. We report the fabrication of 3D multi-branched TiO2/Si nanostructures by a combination of nanoimprinting lithography, reactive ion etching, and hydrothermal reactions, and their application as photoelectrodes in PECs. By using RF sputtered TiO2 seeding layer, the TiO2 nano-branches on Si nanopillars have very small dimensions of ~25 nm in diameter and ~150 nm in length. PEC cells with these nanostructured photoelectrodes were constructed and the water splitting performances were evaluated. The 3D nanostructures, by virtue of their extremely large surface area and durability of TiO2, are beneficial for substantially improved PEC performances. The effects of dimensions, density, and periodicity or Si â?onanotrunksâ? and TiO2 â?onanobranchesâ?, etc. to PEC performance, the PEC testing conditions such as electrolytes, pH values, etc. to nanostructure surface will be discussed.
6:00 AM - N4.6
Synthesis and Electrochemical Performance of SiOC-Carbon Nanotube Composite Nanowires
Romil Bhandavat 1 Marco Cologna 2 Rishi Raj 2 Gurpreet Singh 1
1Kansas State University Manahattan USA2University of Colorado Boulder USAShow Abstract
Rechargeable battery anodes made from crystalline Si-based nanostructures have been shown to possess high experimental first cycle capacities (~3000 mAh/g), but face challenges in sustaining these capacities beyond initial cycles mainly due to large volume expansion (~400 %) and chemical degradation (pulverization). Polymer-derived ceramic SiOC due to its high thermodynamic stability and nano domain structure could present a viable alternative. Additionally, functionalization of SiOC with carbon nanotubes could result in increased electronic and ionic conductivities in the ceramic. Here, we demonstrate synthesis and electrochemical characterization of SiOC/CNT (shell/core) composite nanowires for use in Li-ion battery anode. Materials characterization performed using electron microscopy, Infrared (FT-IR), and X-ray photoelectron spectroscopy suggests non-covalent functionalization of CNT with oxygen moieties in SiOC. Sustained battery capacities of ~700 mAh/g @ at least 100 mA/g and first cycle columbic efficiencies of ~75 % were achieved. Future work will involve determination of lithium ion intercalation sites characterized by electron microscopy whereas cyclic voltammetry analysis will access the sequential change in anode chemistry.
6:00 AM - N4.7
Enhancement of Electrochemical Properties of Molybdenum Oxide Nanowires
Lin Xu 1 Shuo Li 1 Liqiang Mai 1 Chunhua Han 1 Liang Chang 1
1Wuhan University of Technology Wuhan ChinaShow Abstract
Rechargeable lithium-ion batteries(LIBs) have many advantages such as low-cost, high-efficiency and environmentally friendly performance, and is playing a dominant role in portable electronic devices. The performance of rechargeable LIBs depends intimately on the properties of their electrode materials. Compared with bulk materials, nanostructured electrode materials have a short Li-ion insertion/extraction distance, facile strain relaxation upon electrochemical cycling, enhanced electron transport, and very large surface to volume ratio, which have much potential in the application of LIBs. Coating a conductive layer is an effective way to protect the active molybdenum oxides from being damaged. Polythiophene(PTh) is a promising candidates motivated by its high environmental stability, moderate band gap, low redox potential, high conductivity, general facility for structural modfications and controllable for electrochemical behavior. The MoO3 NWs have an initial capacity of 210 mAh/g, but it decreased rapidly. When it comes to the 45th cycle, the capacity retention of MoO3 NWs is only 49%. After a rational coating of PTh with desired amount that the mass ratio of MoO3:Th(1:0.2), the capacity retention increased to 83% after 45 cycles, but with a decrease of capacity. This phenomenon results from the introduction of low-capacity component PTh, and can be verified by calculation and control experiment. A simple mixed PTh/MoO3 product has a capacity retention of only 34%, which indicate that coating PTh layer is an effective way to increase materialsâ?T capability. Compared with our previous prelithiation method which also increase the cyclability performance with a cost of capacity decrease, the PTh coating retains the integrity of the crystal structure, and can be extended to many other materials. This work was partially supported by National Basic Research Program of China (973-program) (2012CB933000) and the National Nature Science Foundation of China (51072153), Program for New Century Excellent Talents in University (NCET-10-0661). Thanks to Professor C.M. Lieber of Harvard University, Professor Z.L. Wang of Georgia Institute of Technology, Professor J. Liu of Pacific Northwest National Laboratory, and Professor Q.J. Zhang of Wuhan University of Technology for strong support and stimulating discussion.
6:00 AM - N4.8
Functionalized Au Nanorods with Thienly-derivative Fullerene for Bulk Heterojunction Organic Solar Cells
Yong Seok Kim 1 Yo-Han Suh 2 Won Bae Kim 1 2
1Gwangju Institute of Science and Technology (GIST) Gwangju Republic of Korea2Gwangju Institute of Science and Technology (GIST) Gwangju Republic of KoreaShow Abstract
Bulk heterojunction organic solar cells are very attractive photovoltaic device owing to their low cost and easy fabrication. However, to achieve high power conversion efficiency of the bulk heterojunction organic solar cells, charge transport properties should be improved from the low charge mobility of organic materials (ca. 10-4 cm2 V-1 S-1). Because one-dimensional nanostructures like nanorods or nanowires have outstanding electrical conductivity caused from their anisotropic morphologies, they can enhance the charge transport properties and reduce the recombination of holes and electrons. In this study, we synthesized Au nanorods by electrodeposition method, and then the Au nanorods were functionalized with thienyl-derivative fullerenes by forming gold-sulfur bonding, which can be served as electron acceptors. After functionalizing them, the bulk heterojunction solar cells were fabricated and the J-V characteristics were examined under 100 mW/cm2 irradiation. The device containing functionalized Au nanorods showed remarkably increased short circuit current density and fill factor resulting in improved power conversion efficiency. The improvement should be attributed to enhanced electron transport features from the Au nanorods functionalized with electron acceptors in bulk heterojunction organic solar cells. Acknowledgment This research was supported by Basic Science Research Program (no. R15-2008-006â?"03002-0), Global Frontier R&D Program on Center for Multiscale Energy System and the National Research Foundation of Korea (NRF) grant [nos. 201000001888 and 2011-0016600 (Mid-career Researcher Programs)] funded by the Korea government (MEST).
N1: One-Dimensional Nanomaterial Based Photovoltaics
Tuesday AM, April 10, 2012
Moscone West, Level 2, Room 2010
9:30 AM - *N1.1
Heterojunction Wire Array Structures Photovoltaics and Solar Fuels
Harry Atwater 1 Dan Turner-Evans 1 Chris T Chen 1 Hal S Emmer 1 Katherine T Fountaine 1 Matthew Shaner 1 Anna Beck 1
1California Institute of Technology Pasadena USAShow Abstract
Semiconductor heterojunction wire structures represent a path for high efficiency solar energy conversion. Building upon the Si wire array photovoltaic technology that has demonstrated 8% efficiency in array-fabricated cells and single wire measurements indicating efficiencies higher than 17%, we have designed III-V/Si and III-V/SiGe wire array heterojunction tandem photovoltaic structures. Conformal GaP/Si and GaInP/Si on Si wire array heterostructures have been synthesized by metallorganic chemical vapor deposition and offer an opportunity for a wide bandgap top cell for series-connected tandem multijunction wire array photovoltaics. The large surface area and high aspect ratio of Si and other semiconductor wire arrays also offers an attractive architecture for solar-driven photoelectrochemical reduction and oxidation of water and carbon dioxide to generate solar fuels. I will describe WO3/Si Z-scheme tandem heterostructure wire array designs for integrated photoreduction and photo-oxidation of water.
10:00 AM - N1.2
Planar Waveguide-nanowire Integrated Three-dimensional Dye-sensitized Solar Cells
Wenzhuo Wu 1 Chen Xu 1 Zhong Lin Wang 1
1Georgia Institute of Technology Atlanta USAShow Abstract
We present a new approach to fabricate three-dimensional (3D) dye-sensitized solar cells (DSSCs) by integrating planar optical waveguide and nanowires (NWs). The ZnO NWs are grown normally to the quartz slide. The 3D cell is constructed by alternatively stacking a slide and a planar electrode. The slide serves as a planar waveguide for light propagation. The 3D structure effectively increases the light absorbing surface area due to internal multiple reflections without increasing electron path length to the collecting electrode, resulting in a significant improvement in energy conversion efficiency by a factor of 5.8 on average compared to the planar illumination case. Our approach demonstrates a new methodology for building large scale and high-efficient 3D solar cells that can be expanded to organic- and inorganic-based solar cells.
10:15 AM - N1.3
Design of Nanowire Arrays for Photovoltaic Applications
Olivier Demichel 1 Martin Heiss 1 Anna Fontcuberta i Morral 1
1EPFL Lausanne SwitzerlandShow Abstract
Third generation solar cells are nowadays the topic of intense research and different ways are explored to reduce the cost-to-efficiency ratio. Nanowire based solar cells are one of these ways. Indeed, their geometry allows to dissociate the light absorption direction (vertical) and the photo-generated carrier diffusion direction (radial). The decoupling of these two directions allows to tune both axial and radial dimensions to optimize both ligth absorption and carrier extraction. Simulations appear to be the key for optimizing the design of nanowire based solar cells. Usually, the approach to simulate such cells is to model the electric behavior of solar cells taking into account an exponentially decaying vertical absorption along the nanowire axis like in the planar situation. Here, we present 3D simulations of the vertical light absorption of single free standing GaAs nanowires in the whole visible range, for diameters in the 50-400 nm range. We demonstrate that nanowires absorb photons on an absorption cross section whose surface can be up to 70 times the nanowire surface (pi.r^2), and the absorption rate on the whole solar spectrum is about 10 times higher than the absorption of a thin film with equivalent surface. Also we explored the spatial distribution of light absorption to understand where carriers are more efficiently generated. We demonstrate that the absorption rate is quasi-homogeneous along the nanowire axis contrarily to the exponential decaying absorption model, this is a consequence to the fact that nanowires absorb light on their sides and thus, the absorption rate is also length dependent even for length much higher than the absorption length. These results induce important consequences for solar applications: first, nanowires act as light concentrators which will increase the open-circuit voltage of nanowire-based solar cell compare to the planar case, second, the total amount of photo-generated carriers is really higher than the one estimated with a classical exponential absorption indicating that with such approaches the short-circuit current is underestimated. We thus demonstrate that the full wave equation resolution is absolutely necessary to really simulate the behavior of nanowire based solar cells. From these simulations we extract optimized parameters for nanowire arrays acting as solar cells. And we finally demonstrate that the inter-wire distance is directly linked to the spatial distribution of photo-generated carriers: for close nanowires, the absorption trends to follow the exponential decaying whereas when they are far one from each other the absorption is similar to isolated wires. Thus an optimal inter-wire distance can be defined to reduce the cost-to-efficiency ratio and the used materials in nanowire array baser solar cells.
10:30 AM - N1.4
Si Wire Array Solar Cells with Highly Flexible Contacts
Daniel Turner-Evans 1 Harry Atwater 1
1CALTECH Pasadena USAShow Abstract
We demonstrate Si wire array solar cells with highly flexible contacts. The cells are polymer-embedded Si wire arrays with hundreds of thousands of Si wires contacted in parallel,and with front contacts consisting of metalized wires contacted by Ag nanowire networks. The cells demonstrate efficiencies in excess of 2% with the potential to reach >17% efficiency(1) as the process is refined, all while being able to be rolled to a radius of 3 mm without loss of performance. Since polymer-embedded Si wire arrays are lightweight, waferless cells grown from SiCl4 and use 100 times less Si material than conventional Si cells, the absorber material is inherently flexible. While previous wire array contact designs used brittle transparent conductors such as indium tin oxide, the flexible Ag nanowire contacts reported here enable truly flexible Si solar cells. We have grown p-type, Cu catalyzed, <111> oriented, Si microwire arrays using a 1000oC, atmospheric chemical vapor deposition process.(2) The arrays are cleaned, oxidized, and doped with an emitter using a unique polymeric masking process.(1) Next, the arrays are covered with a SiNx anti-reflection coating/surface passivation layer and infilled with a Al2O3 nanoparticle embedded polymer. The combination has been demonstrated to lead to 85% day integrated absorption in the array.(3) A thin layer of either indium tin oxide or electrolessly deposited Ni was used to make direct contact to the tops of the wires which were subsequently coated with ClearOhm Ink-N (Cambrios Technologies Corp.), an aqueous solution of Ag nanowires that form a conductive Ag nanowire network for an overall flexible, transparent top contact . Finally, the Si wire arrays were removed from the growth substrate and a thin layer of either Au or Al was evaporated on the back to serve as both a contact and a reflector. The fabricated cells were characterized electronically and optically to gain insight into their performance. Current-voltage curves demonstrated relatively high open circuit voltages (in excess of 500 mV) and suggested short circuit current density as the main performance limitation. Light beam induced current mapping revealed the failure of a number of the wire top contacts while spectral response measurements suggested that longer wire lengths and optimized Al2O3 scatterer distribution could lead to enhanced collection and hence higher efficiencies. Efforts to improve contacting and optical absorption are underway and will be presented along with mechanical performance testing. 1. M. D. Kelzenberg et al., Energy & Environmental Science 4, 866 (2011). 2. B. M. Kayes et al., Applied Physics Letters 91, 103110 (2007). 3. M. D. Kelzenberg et al., Nat. Mater. 9, 239 (Mar, 2010).
10:45 AM - N1.5
Heterojunction Silicon Wire Solar Cells
Majid Gharghi 1 Ehsanollah Fathi 2 Siva Sivoththaman 2 Xiang Zhang 1
1University of California Berkeley Berkeley USA2University of Waterloo Waterloo CanadaShow Abstract
We report heterojunction amorphous silicon crystalline silicon (a-Si/c-Si) solar cell, using arrays of vertical silicon wire structures, with strong light absorption, efficient charge collection, and high open circuit voltage. One dimensional nanowire and microwire solar cells have been widely considered for efficient simultaneous photon absorption and charge collection in materials with low quality (i.e. high minority carrier recombination rate). By forming radial junctions, the optical absorption depth is separated from the charge diffusion path, leading to more efficient carrier collection, even in the case of short diffusion length. The major problem in such structures is the increased saturation current due to increased junction area, which leads to poor open circuit voltages. We fabricate vertical silicon wire arrays and implement a heterojunction with higher bandgap p-type a-Si to provides a possibility of low saturation current cells, and by including a thin intrinsic layer, a high quality junction interface is achieved similar to the planar Heterojunction with Intrinsic Thin layer (HIT) structure. Besides the passivation of regular diffused homojunction wire cells , effort have been made to address the issue of the saturation current by including poly silicon layers on microwires . However, in order to achieve high open circuit voltages, heterjunction band structure is crucial, as is evident from the low voltages observed in homojunction poly-Si/c-Si devices. Moreover, the potential to decrease the saturation current has limitations that make it achievable only on wires with radii comparable to the diffusion length (micrometer range) , and therefore, the previous attempts to passivate nanowire Si solar cells with the same HIT concept have still resulted in low voltages . With the high quality interface from the intrinsic layer and the band misalignment from the higher bandgap a-Si window, the saturation current of the devices reported in this work is greatly suppressed, resulting in high open circuit voltages of above 610mV in spite of the increased junction area due to wire structure.  Kelzenberg et al, Energy & Environmental Science, 2011.  Kim et al, Nano Letters, 2011.  Gharghi, in review, Journal of Applied Physics, 2011.  Jia et al, Solar Energy Materials and Solar Cells, 2011.
N2: Conversion between Light and Electricity in 1D Nanostructures
Tuesday AM, April 10, 2012
Moscone West, Level 2, Room 2010
11:30 AM - *N2.1
Applications of III-V and III-N Nano Wires for the Conversion of Light-to-electricity (Solar Cells) and Electricity-to-light (Light Emitting Diodes)
Lars Samuelson 1
1Lund University Lund SwedenShow Abstract
Semiconductor nanowires can today be grown with a high degree of perfection and can be turned into realistic devices using various methods for lithographic patterning as well as processing and contacting technologies. Initially, I will present the present degree of control of growth and top-down guided bottom-up growth of nano wires by self-assembly. I will then present our recent progress in the field of developing state-of-the-art nanowire devices for conversion of sun-light into electricity, i.e. in solar cells, and for the opposite process for conversion of electrical energy into light, i.e. in light emitting diodes.
12:00 PM - *N2.2
1-D ZnO Nanostructured Materials for Optoelectronic Nanodevices
Yue Zhang 1
1University of Science and Technology Beijing Beijing ChinaShow Abstract
Optoelectronic devices â?" from light-emitting diodes (LEDs) and solar cells to lasers and detectors â?" play very important roles in everyday life. Nano-structured zinc oxide has a wide direct band gap and a large exciton binding energy, which make it a promising candidate for optoelectronic nanodevices. We have realized blue light emission diodes based on GaN/ZnO nanowire heterojunction arrays with a turn-on voltage of 3.0 V . For UV detecting, bending ZnO nanowires would enhance their UV responsivity . Also, It is highly required that devices work well without external power source. In our group, self-driven UV detectors were fabricated using Sb doped ZnO nanobelts with sensitivity up to 2200% and a response time less than 100 ms . Additionally, in dye-sensitized solar cells, we have used the surface modification method to solve the surface destruction and performance reduction of the ZnO electrode, which occur frequently in the dye sensitization process [4, 5]. Furthermore, we have also utilized novel ZnO nanostructures and Sn-doped ZnO nanomaterials for the photoelectrode in order to improve the cell performance . Reference  X. M. Zhang, M. Y. Lu, Y. Zhang, et al. Adv. Mater. 2009, 21, 2767  Y. Yang, J. J. Qi, Y. Zhang, et al. Phys. Chem. Chem. Phys. 2010, 12, 12415  Y. Yang, W. Guo, Y. Zhang, et al. Appl. Phys. Lett. 2010, 97, 223113  Z. Qin, Y. H. Huang, Y. Zhang, et al. Colloids and Surfaces A 2011, 386(1-3): 179-184  Z. Qin, Y. H. Huang, Y. Zhang, et al. Materials Letters, 2011, 65: 3506-3508  N. Ye, J. J. Qi, Y. Zhang, et al. J. Power Sources, 2010, 195: 5806-5809
12:30 PM - N2.3
Investigations of Lightly Doped Silicon Microwire Arrays under High Level Injection Conditions
Elizabeth A Santori 1 Nicholas C Strandwitz 1 Harry A Atwater 2 Nathan S Lewis 1
1California Institute of Technology Pasadena USA2California Institute of Technology Pasadena USAShow Abstract
Si wire arrays grown by the vaporâ?"liquidâ?"solid (VLS) process have emerged as a promising technology for the fabrication of efficient, scalable photovoltaics. However, to ultimately achieve efficiencies comparable to wafer-based solar cells, higher open circuit voltages must be obtained. One strategy to improve the attainable photovoltage of Si microwire arrays is to operate under conditions of high level injection, where the bulk lifetimes and photovoltages of planar cells exceed those generated in p-n junction solar cells operating under low level injection conditions. To this end, lightly doped, selectively contacted Si wire arrays were fabricated, in a device configuration similar to that of a silicon point contact solar cell operating under high level injection conditions. Si microwires were grown by a Cuâ?"catalyzed VLS process without dopants, on a n+ or p+-Si<111> substrate as a selective back contact. The resulting wires possessed controlled electronically active dopant concentrations of ~ 1 x 1013 cm-3. Arrays of such wires were radially contacted using non-aqueous 1,1â?T-dimethylferrocene (Me2Fe)+/0 and cobaltocene (CoCp)+/0 redox couples, which produce high barrier height contacts to n-Si and p-Si, respectively. In particular, arrays with a n+ contact measured in radial contact with (Me2Fe)+/0 exhibited photovoltages of 0.45 V and an energy-conversion efficiency of 1.4% under simulated AM 1.5 G illumination, with diode quality factors of 1.85 Â± 0.05 and external quantum yields of 0.86 at large incident angles of illumination. Similar behavior was observed for an identical array with a p+ back contact in radial contact with (CoCp)+/0, demonstrating that the arrays were operating under high level injection conditions, in which kinetic asymmetries at the back contact determined the charge separation in the device. Additionally, complementary device physics simulations of wires in radial contact with (Me2Fe)+/0 showed that the lightly doped wires are completely depleted of electrons, with a hole-rich inversion layer in the near surface region, ~100 nm in depth into the wires. As a consequence, small diameter (D < 200 nm) wires suffer extremely low quantum yield values, due to strong inversion throughout the radial dimension. Larger diameter wires (D > 2 Âµm) are not strongly inverted in the core of the wire, providing a collection pathway for electrons that is relatively free of holes, and resulting in near-unity quantum yield for wire lifetimes exceeding 5 Âµs. These numerical simulations can be further leveraged to optimize the device geometry of lightly doped, 1-D semiconductors operating under high level injection conditions.
12:45 PM - N2.4
Tuning Light Absorption in Core/Shell Si Nanowire Photovoltaics via Controlled Synthesis
Robert Watson Day 1 Sun-Kyung Kim 1 James F Cahoon 2 Thomas J Kempa 1 Hong-Gyu Park 3 Charles M Lieber 1
1Harvard University Cambridge USA2University of North Carolina at Chapel Hill Chapel Hill USA3Korea University Seoul Republic of KoreaShow Abstract
Semiconductor nanowires are promising candidates to serve as the building blocks for next-generation solar cells and other optoelectronic devices. Strong light confinement effects in these mesoscale materials lead to enhanced absorption compared to planar counterparts. We report how absorption in silicon core/shell nanowires can be systematically tuned by synthetic manipulation of nanowire size and morphology. Experimental polarization-resolved EQE spectra obtained on single-nanowire devices with distinct morphologies are in good agreement with absorption spectra calculated from finite-difference time-domain (FDTD) simulations. These results permit assignment of specific peaks in EQE spectra to Fabry-Perot, whispering-gallery, or complex high-order resonant absorption modes. Single-nanowire device measurements reveal that the number and spectral density of modes increases with nanowire diameter. Notably, by modifying the cross-sectional morphology from hexagonal to rectangular, it is possible to increase light absorption at longer wavelengths. Our fundamental understanding of the tunable optical properties of semiconductor NWs provides a framework for the design and realization of novel devices for photovoltaic and photonic applications.
Symposium OrganizersZhong Lin (ZL) Wang, Georgia Institute of Technology
Xudong Wang, University of Wisconsin-Madison
Thomas G. Thundat, University of Alberta
Magnus Willander, Linkoping University
N7: Piezoelectric Nanogenerators
Wednesday PM, April 11, 2012
Moscone West, Level 2, Room 2010
2:30 AM - *N7.0
Coupling between Piezodectric Effect and Electrochemical Reactions or a New Route of Energy Conversion
Xudong Wang 1
1University of Wisconsin Madison USAShow Abstract
3:00 AM - *N7.1
High-Performance Transparent, Flexible, Stretchable, Foldable Nanogenerators Based on Multi-dimensional ZnO Nanostructures
Sang-Woo Kim 1
1Sungkyunkwan University (SKKU) Suwon Republic of KoreaShow Abstract
Nanogenerators based on piezoelectric semiconductor nanostructures are very promising for the miniaturization of power packages and self-powering of nanosystems used in implantable bio-sensing, environmental monitoring, and personal electronics. New strategies for the dramatic enhancement of the power generation to commercialize the nanogenerators are indispensable for not only self-powered body-implantable nano/micro-systems, but also portable devices such as commercial LCDs, LEDs, etc with low operating power consumption. For realizing highly efficient nanogenerators, morphology control of piezoelectric semiconducting nanostructures is one of the most important issues. Graphene could be a platform to serve as a substrate for both morphology control and direct use of electrodes due to its ideal monolayer flatness with Ï? electrons. As a first issue systematic studies regarding vertically well-aligned ZnO nanowires and nanowalls obtained by controlling Au catalyst thickness and growth time without inflicting significant thermal damage on the graphene layer during thermal chemical vapor deposition of ZnO at high temperature of about 900 C will be presented. Further, I demonstrate that a piezoelectric nanogenerator that was fabricated from the vertically aligned nanowire-nanowall ZnO hybrid/graphene structure generates a new type of direct current through the specific electron dynamics in the nanowire-nanowall hybrid. As a second issue, the first use of thermally stable cellulose paper and stretchable fiber as substrates for foldable, stretchable and thermally stable piezoelectric nanogenerators to overcome the problem of unstable electrical output from plastic-based nanogenerators due to thermal induced-stress. Finally, ultrahigh power output nanogenerators will be introduced at the presentation site.
3:30 AM - N7.2
Self-powered System with Wireless Data Transmission
Youfan Hu 1 Yan Zhang 1 Chen Xu 1 Long Lin 1 Robert L Snyder 1 Zhong Lin Wang 1
1Georgia Institute of Technology Atlanta USAShow Abstract
The emerging development of nanotechnology today moves from inventing individual components to the integrated system that can perform one or more designed function by integrating a group of nanodevices with modern microelectronics technologies. A general integrated system is a package of components such as sensors, transducers, data processor, control unit, and communication system. As the size of the devices shrinks to the nano- or microscale, the power consumption also drops to a much lower level. At such a low power consumption level, it is entirely possible to drive the devices by scavenging energy from the sources in the environment such as gentle airflow, vibration, sonic wave, solar, chemical, and/or thermal energy.We demonstrate the first self-powered system driven by a nanogenerator (NG) that works wirelessly and independently for longdistance data transmission. The NG was made of a free cantilever beam that consisted of a five-layer structure: a flexible polymer substrate, ZnO nanowire textured films on its top and bottom surfaces, and electrodes on the surfaces.When it was strained to 0.12% at a strain rate of 3.56% S-1, the measured output voltage reached 10 V, and the output current exceeded 0.6 Î¼A (corresponding power density 10 mW/cm3). A system was built up by integrating aNG, rectification circuit, capacitor for energy storage, sensor, and RF data transmitter. Wireless signals sent out by the system were detected by a commercial radio at a distance of 5-10 m. This study proves the feasibility of using ZnO nanowire NGs for building self-powered systems, and its potential application in wireless biosensing, environmental / infrastructure monitoring, sensor networks, personal electronics, and even national security.
3:45 AM - N7.3
PZT Nanogenerator Based on Graphene Electrodes
Junggou Kwon 1 Wanchul Seung 1 Sang-Woo Kim 1 Jong-Hyun Ahn 1
1Sungkyunkwan University Suwon Republic of KoreaShow Abstract
Mechanical energy scavenging from human body is an effective approach for various applications such as medical implants and sensors powered by heartbeats or breathing. One of the representative forms of energy harvesting is the conversion of mechanical energy into electrical energy using the piezoelectric characteristic. Piezoelectric harvesting system has been researched and there have been attempts to transfer the perovskite materials onto flexible substrates and use a metal as electrodes; however, metal electrodes have limited use in a flexible and transparent device. Graphene is a 2D material with outstanding electrical and mechanical properties and its sheets have extremely high mobility at room temperature and high mechanical elasticity. Here, we have demonstrated the CVD grown graphene as interdigitated electrodes and combine with perovskite PZT film on a plastic substrate to realize semi-transparent nanogenerators. The device fabrication began with the deposition of Pt/Ti and SiO2 on Si substrate by sputtering and spincoating PZT by sol gel process. The PZT film was then patterned and transferred onto a flexible substrate by a dry transfer. To measure the voltage and current signals, graphene was transferred onto the PZT and patterned as the electrodes. We measured the output voltage and current generated from the PZT nanogenerator by applying the bending force. The generated output voltage and current were approximately 0.3V and 40nA, respectively.
4:30 AM - N7.4
Low Temperature Solution-processed ZnO Thin Film Based Piezoelectric Nanogenerator on Plastic Substrates
Sungyun Chung 1 Joohee Kim 1 Jihyun Lee 1 Hanju Jo 1 Kieun Seong 1 Youn Sang Kim 1 2
1Seoul National University Seoul Republic of Korea2Advanced Institutes of Convergence Technology Suwon Republic of KoreaShow Abstract
Energy harvesting from ambient environment is an attractive issue in scientific fields for purpose of building self-powered systems. Piezoelectric nanogenerator is one way to scavenge energy sources, including body movement, wave, sound, air/liquid pressure and heartbeat. Converting systems of those mechanical energies into electrical power can be applied for fabrication of various nano-scale devices such as mobile electronics, muscle-driven element and body-implantable devices. ZnO is particularly an appealing material because of its coupling effect of piezoelectrical and semiconducting properties, abundant configurations of nanostructures, transparency and biocompatibility. Prior reports on nanogenerators with ZnO nanowire/nanorod (NW/NR) were demonstrated by many research groups. Otherwise, recent studies on thin film-type piezoelectric elements have shown superior performances when compared to NW/NR based piezoelectric nanounits. However, these thin films were mostly deposited by radio freqeuncy radio frequency magnetron sputtering (rf-sputtering) and chemical vapor deposition for epitaxial growth of crystal structure at high temperature. In this work, we present a low temperature, solution-processed ZnO thin film based nanogenerator on a flexible substrate, which can effectively convert mechanical energy to electricity when continuous bending motion was applied. The ZnO solution was prepared by simple inorganic hydroxo-condensation method and deposited at low temperature of 120Â°C. Solution-based method has advantages of simple and low-cost process and large-area deposition with mass production as well as the air-stable method doesnâ?Tt require any vacuum conditions. Moreover, solution process allows low-temperature process, thereby unlike the conventional methods, there is no need to transfer the thin film from a inorganic substrate such as a silicon wafer to a flexible substrate. Epitaxial ZnO NRs along c-axis orientation was detected in several characteristic measurements, indicating that the solution-processd ZnO thin film is constituted with closely-packed ZnO NRs. Accordingly, the operational mechanism of ZnO thin film based nanogenerator can be explained by the vertical-type ZnO nanogenerator model. Otherwise, the dense structure of ZnO thin film diminishes electrical leakages or losses from a poor contact between an electrode and a piezoelectric material caused by a non-uniform height of vertically aligned ZnO nanorods. The average output current and voltage of the nanogenerator were approximately 1.6nA and 100mV, which are superior values when compared to vertically aligned ZnO based nanogenerators. This film-type nanogenerator is versatile to acquire irregular or random mechanical energy sources from variable environmental conditions for powering future energy harvesters such as wearable human patches, shoe-sole power generators and embedded heart-driven energy scavengers.
4:45 AM - N7.5
Nanostructured Zinc Oxide Piezoelectric Energy Generators Based on Semiconductor P-N Junctions
Joe Briscoe 1 Mark Stewart 2 Melvin Vopson 2 Markys Cain 2 Steve Dunn 1
1Queen Mary, University of London London United Kingdom2National Physical Laboratory London United KingdomShow Abstract
The first evidence that a piezoelectrically induced voltage and current can be generated in a p-n junction-based ZnO nanorod device is presented. Low temperature solution-based techniques are used to produce both all-inorganic and inorganic-organic hybrid diodes on flexible plastic substrates. Bending of devices leads to maximum voltages of 60 mV and current densities of 10 ÂµA/cm2 in single devices. It has been demonstrated previously that the piezoelectric polarisation of ZnO nanorods can be used to generate a voltage in both single nanorods and arrays of rods. It has been suggested that a rectifying structure is required in these devices for a voltage to be generated in the external circuit from such a device, which to date has been achieved using a Schottky diode at one of the metal contacts. In this work it is demonstrated that this rectification can be achieved using a p-n diode built into the device, and that this leads to voltages or current being produced. Scanning-electron microscope images show that the ZnO nanorods grow in a dense array on the substrate, and can be grown 1-3 Âµm long and 50-100 nm wide by controlling the synthesis conditions. The inorganic p-type layer penetrates completely between the nanorods filling them intimately and producing a compact layer on top preventing shorts to the top contact. The organic p-type layer does not fill completely between the rods leaving a cavity, but does form a compact layer on top of the rods. Current-voltage measurements show rectification in both the all-inorganic and inorganic-organic hybrid diodes. Voltage and current peaks are generated when the devices are bent and released. Hydrostatic pressure measurements of the devices provide strong evidence that the effect results from a piezoelectric response in the material. It is shown that the dielectric displacement increases linearly with the pressure applied to the sample, which is expected for a piezoelectric effect. The measurements are also performed when the devices are illuminated, which leads to a large reduction in the piezoelectric coefficients. This is attributed to the photo-induced conductivity of the ZnO, which is known to reduce the piezoelectric coefficient due to screening by conduction electrons, providing further evidence that the voltage generated by the devices results from a piezoelectric effect. ZnO nanorods are grown directly onto 2 cm2 substrates in solution, and both inorganic and organic p-type layers are deposited from solution. Combined with the use of inexpensive plastic substrates this technology has the potential to produce low cost devices for harvesting of energy from vibrations and movement in the environment. The technology could easily be scaled up to produce large area devices to harvest large amounts of energy from the environment, or scaled down to power microdevices from small movements or vibrations.
5:00 AM - *N7.6
Energy Harvesting with ZnO Nanowires
Rusen Yang 1
1University of Minnesota Minneapolis USAShow Abstract
Being the richest family with nanowires (NWs)/nanobelts, nanosprings, nanorings, nanohelixes, and nanotubes, ZnO nanostructures are unique not only for the fabrication of nanosensors, but also for its ability to harvest mechanical energy enabled with the piezoelectric property. The energy harvesting unit with nanomaterials is critical to achieve independent and sustainable operations of nanodevices. Innovative nanotechnologies with ZnO nanowires will be discussed for converting mechanical energy (such as body movement, muscle stretching) and vibration energy (such as acoustic/ultrasonic wave) into electric energy. The generated electricity from ZnO nanowires can power directly a nanowire pH sensor or a nanowire UV sensor. The electric energy from ZnO nanowire can also be stored to power light emitting diode. This nanotechnology demonstrated with ZnO nanowires implies feasible approach for a power unit of a self-powered nanosystem, and it also indicates a novel approach for powering small size personal electronics.
N5: One-Dimensional Thermoelectric Nanomaterials and Nanodevices
Wednesday AM, April 11, 2012
Moscone West, Level 2, Room 2010
9:30 AM - *N5.1
Thermal Transport in Nanostructured Materials: Fundamental Limits and Engineering Approaches toward Macroscale Thermal Energy Conversion and Storage Applications
Li Shi 1
1University of Texas at Austin Austin USAShow Abstract
This presentation reviews the upper and lower limits in thermal transport properties of solids, including the ballistic limit, the Casimir limit, the alloy limit, and the amorphous limit. Some of these long-held limits have been challenged by recent experimental and theoretical studies of nanostructured materials including turbostratically disordered layered films, nanowires, carbon nanotubes, and graphene. Besides well-established knowledge, an attempt will be made to highlight in this review the wide range of experimental results and diverse theoretical findings in this vibrant topical area. The experimental results to be discussed include recent thermal and thermoelectric measurement data obtained by a number of the presenterâ?Ts collaborators on individual one-dimensional (1D) semiconductor nanowires, two-dimensional (2D) graphene and hexagonal boron nitride, as well as three-dimensional (3D) architectures of 2D and 1D building blocks. The challenges and possible engineering approaches for the applications of nanostructured materials in thermoelectric vehicle waste heat recovery devices and high-energy density heat batteries will be examined.
10:00 AM - N5.2
Nanowire Based High Performance Thermal Rectifier
Jia Zhu 1 Sheng Shen 1 Kedar Hippalgaonkar 1 Kevin Huang 1 Arun Majumdar 1 Junqiao Wu 1 Xiang Zhang 1
1University of California, Berkeley San Jose USAShow Abstract
Thermal Rectifier (diode), in which thermal conductance depends on the sign of the thermal gradient, representing an advanced thermal management, is critical for thermal energy conversion and the future electronic cooling. Here for the first time, thermal rectification (20%) is observed in a variety of semiconductor nanowires. More strikingly, the thermal rectification can be turned on/off by a third terminal. Therefore it represents a significant advancement towards a thermal transistor, also can serve as a novel building block for the future development of thermal circuit.
10:15 AM - N5.3
Self-organized Quasi-one-dimensional Nano-structure in Cu-Ni and Fe-Cu Alloys
Nguyen Dang Vu 1 Sato Kazunori 1 Katayama-Yoshida Hiroshi 1
1Osaka University, Graduate School of Engineering Science Toyonaka JapanShow Abstract
Metallic alloys such as Ni-Cu, Fe-Cu... are popular and are commonly used in life because Fe, Cu, Mg and many other metals are cheap and have large quantities in the Earth. With the strong development of society, the demand of environment-friendly, non-toxic, cheap, economical and high effect materials is increasing. Thus, studying metallic alloys and discovering their new properties is very important. The Peltier effect is a thermoelectric phenomenon which can be used in cooling devices. Giant Peltier effect was observed experimentally in metallic junctions by Sugihara et al. in 2010. The observed Peltier coefficient in a submicron-sized Cu-Ni/Au was evaluated to be 480 mV at room temperature and this is 40 times larger than that expected from the bulk value of 12 mV. Ni and Cu are neighbour elements with the same crystal structure (fcc) so it is easy to think that this alloy is homogeneous. Thus, discovering the phase separation of this Cu-Ni alloy is new. Based upon ab initio electronic structure calculations by the Korringa-Kohn-Rostoker coherent potential approximation and Monte Carlo simulation under a layer-by-layer crystal growth condition, we simulate the two-dimensional spinodal nano-decomposition of Cu-Ni alloy (Cu 40%), which is quite reasonably in agreement with the experimental observations. Moreover, we also simulate the formation of a self-organized quasi-one-dimensional nano-structure (Konbu-Phase) of Cu-Ni alloy with Cu 20%. From these calculated results, we propose a new mechanism of the giant Peltier coefficient dramatically enhanced by the one-dimensional singular density of states in the Konbu-Phase. We continue to perform the calculation with Fe-Cu alloy in two cases: alloy with Fe 20% (fcc structure with Cu-rich condition) and alloy with Cu 20% (bcc structure with Fe-rich condition). The latter is predicted to be a promising candidate for producing giant Peltier cooling devices and the former is expected to be a strong ferromagnetic material with large magneto-crystal anisotropy or shape anisotropy. We obtain the Konbu-phase of Fe-Cu alloy for both cases. By using Monte Carlo method, we can control the shape of nanowires such as simulating the dumb-bell shape structure. 1. A. Sugihara et al., Appl. Phys. Express 3, 065204 (2010). 2. Nguyen Dang Vu et al., Appl. Phys. Express 4, 015203 (2011).
10:30 AM - N5.4
The Optimal Seebeck Coefficients and Minimal Length Scales, for Obtaining the Maximum Power Factor in Silicon Nanowire Thermoelectrics
Prabhakar Bandaru 1 Paothep Pichanusakorn 1
1UC, San Diego La Jolla USAShow Abstract
The efficiency of thermoelectric device is measured by the Figure of merit,Z=(S2Ïf)/(ke+kL ), and is constituted from both electronic (the Seebeck coefficient: S, the electrical conductivity: Ïf, and the electronic part of the thermal conductivity: ke) and lattice properties (lattice thermal conductivity: kL) However, the electrical properties can vary by orders of magnitude and depend mainly on the carrier concentration. We show, through extensive analytical work and numerical simulations that there exists an optimum reduced Fermi level (Î·opt), where the power factor: S2Ïf is maximized, and which is largely independent of material, device geometry, and temperature. Consequently, given the well-determined relationship between S and Î·, we will show that there exists an optimal Seebeck coefficient, Sopt, which can now be set as a practical and direct measure for finding the optimal carrier concentration in any material at any temperature. The relationship between the electrical properties and the electronic band structure will be intuitively explained through the Boltzmann transport equation. We also consider the influence of the characteristic scattering exponent (r) in the range of -0.5 to +1.5. For example, given a constant relaxation time (r=0), Sopt in bulk material, quantum well, and quantum wire were calculated to be approximately 130, 167 and 186 ÂµV/K, respectively. However, if acoustic phonon scattering is dominant, then Sopt is always equal to 167 ÂµV/K. Given the optimum Fermi level and Seebeck coefficient, we then determine the minimum quantum well/wire thickness required to achieve an enhancement in S2n over bulk values. We then show, through a critical comparison of the electron density distribution in the bulk and nanostructured forms for a variety of thermoelectrics, e.g., Bi2Te3, PbTe, SrTiO3, Si, and Si1-xGex, etc. that there exists an optimal length scale only below which the (S2Ïf of nanostructures is enhanced over the bulk value. We calculate that the minimum required quantum well and wire thickness for n-Si are both approximately 6.5 nm. We will also discuss the issue of breaking of valley degeneracy in nanostructures, which can reduce the power factor and cause major deviation from the ideal value. It is then concluded that it is the increase in the magnitude of the integrated density of states (DOS) and not the change of shape, as is commonly believed, to be most responsible for the increase of the power factor. Our results lay the foundation for future research into the synthesis and characterization of nanostructured thermoelectric materials
10:45 AM - N5.5
Experimental Studies of Thermoelectric Properties and Local Heating in One-dimensional Molecular Junctions
Woochul Lee 1 Kyeongtae Kim 1 Wonho Jeong 1 Pramod Reddy 1 2
1University of Michigan Ann Arbor USA2University of Michigan Ann Arbor USAShow Abstract
We present experimental results from our study of energy dissipation and thermoelectric properties of one-dimensional (1D) molecular junctions. Using custom-fabricated scanning thermal microscopy (SThM) probes, which have thermocouples integrated into the tips of atomic force microscope probes, we have simultaneously probed the thermoelectric properties and energy dissipation characteristics of molecular junctions. Specifically, our studies performed in an ultra high vacuum environment elucidate the dependence of thermoelectric properties of junction on their molecular structure and end group chemistry. Further, our studies of energy dissipation describe the local temperature rise in the contacts of molecular junctions as a function of molecular length, and contact chemistry. Finally, we will briefly describe our recent efforts into experimentally probing thermal transport in single molecule junctions.
N6/V5: Joint Session: Opportunities and Innovation in Solar Energy Conversion
Wednesday AM, April 11, 2012
Moscone West, Level 3, Room 3022
11:30 AM - *N6.1/V5.1
SunShot Initiatives at Department of Energy (DOE)
Ramesh Ramamoorthy 1
1DOE Washington USAShow Abstract
The SunShot Initiativeâ?Ts mission is to develop solar energy technologies through a collaborative national push to make solar Photovoltaic (PV) and Concentrated Solar Power (CSP) energy technologies cost-competitive with fossil fuel based energy by reducing the cost of solar energy systems by ~ 75 percent before 2020. Reducing the total installed cost for utility-scale solar electricity to roughly 6 cents per kilowatt hour (1$/Watt) without subsidies will result in rapid, large-scale adoption of solar electricity across the United States and the world. Achieving this goal will require significant reductions and technological innovations in all PV system components, namely modules, power electronics, and balance of systems (BOS), which includes all other components and costs required for a fully installed system including permitting and inspection costs. This investment will re-establish American technological and market leadership, improve the nation's energy security, strengthen U.S. economic competitiveness and catalyze domestic economic growth in the global clean energy race. SunShot is a cooperative program across DOE, with a significant portion of the budget coming out of EERE (as requested above). We closely coordinate our activities with those at the Office of Science and ARPA-E.
12:00 PM - N6.2/V5.2
Photovoltaic Technologies: Initial Efficiency Gain or Sustainable Power Output?
Sudip Mukhopadhyay 1
1Honeywell Berkeley USAShow Abstract
World records in sports are often made to be broken after years of hard work at a later date by someone else, but when it is in the area of solar electricity, records are made and broken almost several times in a month, even for a mere decimal point increase in power output efficiency. Sometimes it is hard to keep up with all efficiency records made in a month because there are so many of them. This is due to the fact that solar panels are sold on dollars/power output basis. More efficiency means more power, which brings in a higher dollar amount for a panel sold and everybody wants to be the winner. These types of claims on efficiency are also affected by volatility in the prices of the silicon raw materials used to make commercial solar panels. When silicon prices are high, a decimal point gain in efficiency makes a significant impact. But, when silicon is available to manufacturers at a relatively lower cost, decimal point increases are not as significant. While currently available commercial panels typically come with a 25-year warranty, at odds with the â?odecimal point gainsâ? in efficiency is the fact that panels will lose 5% to 10% of their efficiency over the first 10 to 15 years. Now the question remains whether consumers really care about a few decimal points initial increases in power output and are willing to pay the upfront price for it, when they know that the product is not going to produce sustainable power in the long run. Yet, in last 35 yrs, c-Si cell efficiency went up by 12% and most of these gains are accomplished by process engineering and design changes. Power output gains by process tweaking are becoming expensive and c-Si cell efficiency nearing close to the thermodynamic limits. Thus there is a real need to look for simple but alternative approaches to improve efficiency as well as sustainability, which are cost effective, give higher return on investment and make a good business case. This talk will elucidate some new approaches. The physics, chemistry and material science aspects of these approaches will be discussed. The future for PV and other new materials for panels for sustainable output will also be discussed in this talk.
12:15 PM - N6.3/V5.3
Photoelectrochemical and Photovoltaic Solar Energy Conversion Using Earth-abundant Semiconductor Nanowires
Song Jin 1
1U. of Wisconsin-Madison Madison USAShow Abstract
The scale and significance of solar energy challenge not only calls for efficient photovoltaic (PV) or photoelectrochemical (PEC) devices but also abundant, inexpensive, and stable photoactive materials that can simultaneous enable efficient light harvesting, charge separation and collection, and chemical transformations (for solar fuels). One-dimensional (1D) nanowires can be advantageous in solar harvesting compared with conventional planar photoelectrodes because they have a long axis to absorb incident sunlight effectively yet with a short radial distance to efficiently separate the photo-generated carriers. As a result, solar conversion efficiency could be improved, or does not significantly suffer if lower quality (and less expensive) semiconductor materials are used. Unconventional semiconductors of earth abundant elements with less ideal properties prepared through inexpensive synthesis might then become feasible for solar applications. We have developed rational synthetic methods to 1D nanowire materials of earth abundant semiconductors such as hematite (Î±-Fe2O3), pyrite (FeS2), and cuprous oxide (Cu2O). Many of them were grown from aqueous solutions via inexpensive and highly scalable screw dislocation-driven nanowire growth. The photovoltaic and photoelectrochemical solar cell devices using these materials have been fabricated and their performance evaluated. 3D hierarchical nanocomposites or branching nanostructures are further prepared to overcome the conflicting requirements by light harvesting and carrier collection.
12:30 PM - *N6.4/V5.4
Nanomaterials for Photon Management and Electrodes in Solar Cells
Yi Cui 1
1Stanford University Stanford USAShow Abstract
Advanced solar cell manufacturing requires that each component of solar cells performs well and can be processed in low-cost. Here I will present our recent progress on how nanomaterials can be utilized for advanced photon management and for electrodes used in solar cells. I will present two examples of photon management at the nanoscale: nanocone and hollow sphere solar cells. Nanocone solar cells are simple structures combing an efficient antireflection and light trapping across a broad band of spectra and over a wide range of incident angles while enhancing the charge carrier collection and maintaining low dark current. Using amorphous Si, we demonstrate high power efficiency for both substrate and superstrate configurations. Nanocone cells opens up exciting opportunities for a variety kinds of photovoltaic devices, to improve the performance, reduce materials usage, and relieve the materials abundance limitation. Hollow nanospheres represented another exciting structure for light trapping using Whispering-Gallery-like effects. I will also present novel metal nanowire networks as transparent conducting electrodes to replace the existing indium tin oxides. Metal nanowires with diameters smaller than and with separations larger than the wavelength of the light can allow the sunlight pass through without significant reflection or scattering back. We show that these metal nanowire networks provide very competitive optical transmittance at very low sheet resistance. They are flexible and stretchable. The low cost processing makes them attractive for solar cells used in large scale.