Bruce Dunn University of California-Los Angeles
Gang Li Solarmer Energy Inc.
Jeffrey W. Long Naval Research Laboratory
Eli Yablonovitch University of California-Berkeley
P1: 3-D Battery Architectures: From Design Concepts to Functional Devices
Tuesday AM, April 14, 2009
Room 2022 (Moscone West)
9:30 AM - **P1.1
Three-Dimensional Architectures: Establishing a New Paradigm for Power Source Materials and Device Design.
Michele Anderson 1 Show Abstract
1 , US Office of Naval Research, Arlington, Virginia, United States
The concept of three-dimensional (3D) power source architectures was proposed to challenge the paradigm of conventional micrometer-scaled multilayer power source designs, which are limited by long ionic diffusion lengths and labor-intensive manufacturing. Understanding and controlling ion and electron transport and power source architectures at the nanoscale, and rational assembly of electroactive nanometer-scale structures, should allow for simultaneous increases in energy and power densities in energy-storage devices. Through major investments beginning in 2001, ONR facilitated the enabling science that has now established a new field of 3D power source architectures. Over the past eight years, progress in the theory of charge storage and transfer at the nanoscale has demonstrated the necessity of re-thinking the design paradigms for nanostructured devices. The development and demonstration of 3D power source architectures based on Si micromolded structures, self-assembly, macroporous templating, and origami, to name a few, have established the versatility of synthetic and fabrication techniques available for the embodiment of functional 3D architectures. This paper will summarize the history of 3D power source architectures for energy-storage applications, including the scientific challenges that have been overcome and those that remain, key theoretical insights that have influenced approaches to designing nanostructured power sources, significant milestones in 3D power source device demonstrations, and the future impact of this new scientific and technical field.
10:00 AM - **P1.2
Improving Centuries-Old Electrical Energy-Storage Devices by Rethinking Multifunction on the Nanoscale and in 3D
Debra Rolison 1 , Jeffrey Long 1 , Lytle Justin 1 , Jennifer Dysart 1 , Anne Fischer 1 , Katherine Pettigrew 1 , Amanda Barrow 1 , Jean Wallace 2 Show Abstract
1 Surface Chemistry, U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 , Nova Research, Inc., Alexandria , Virginia, United States
Electrical energy storage in batteries and electrochemical capacitors (ECs) buoys any future success in the global effort to shift energy usage away from fossil fuels. A marked improvement in the performance of these power sources is critical for this effort, yet both are mature technologies that have always disregarded Moore’s Law. Improved performance requires redesigning the reaction interphases in which occur the fundamental processes that store energy in batteries and ECs. Energy researchers are now rethinking the requisite multifunction—mass and charge transport, electronic and ionic conductivity, and electron-transfer kinetics—in light of nanoscience and architectural design in three dimensions [1,2,3]. The design and fabrication of three-dimensional multifunctional architectures from the appropriate nanoscale building blocks for chemical, physical, and physicochemical charge storage will be highlighted, including the use of “nothing” (void space) and deliberate disorder as design components  as well as the importance of reexamining in a nanoscopic form those materials that yielded poor energy-storage performance when used in a macroscopic form.  J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem. Rev. 104 (2004) 4463. D.R. Rolison, J.W. Long, Acc. Chem. Res. 40 (2007) 854. D.R. Rolison, J.W. Long, J.C. Lytle, A.E. Fischer, C.P. Rhodes, T.M. McEvoy, M.E. Bourg, A.M. Lubers, Chem. Soc. Rev. 12 (2009) in the press. D.R. Rolison, Science 299 (2003) 1698.
10:30 AM - **P1.3
Towards High Energy Density 3D-integrated Lithium-ion Microbatteries.
Peter Notten 1 , Jos Oudenhoven 1 Show Abstract
1 , Eindhoven University of Technology, Eindhoven Netherlands
In our modern-day society electronics play an ever increasing role. Traditionally separate devices are used for lighting, control of temperature and entertainment. However, there is a strong tendency towards more complete and intelligent systems. A crucial role in this Ambient Intelligence is played by various sensing devices, preferentially Autonomous Devices, which combine a sensor function with wireless communication. To make independent operation possible an energy supply needs to be included. Energy can, for example, be harvested from photovoltaic cells, temperature differences, vibrations etc. To ensure a stable power supply, it is necessary to also include an energy storage device, for which all-solid-state lithium ion micro-batteries are a promising candidate.It has been shown that planar thin-film lithium ion batteries can successfully be produced, and that these show an excellent reversible electrochemical response . To increase the energy density of these thin-film batteries a novel approach was proposed . This concept is based on the etching of 3D structures into a silicon substrate, which increases the effective surface area of the thin film battery without increasing its footprint area in the device it is powering. Using this method an increase in energy storage capacity of a factor 25 is predicted . Silicon wafers are common substrates in semiconductor industry, and also the anisotropic etching of several 3D geometries (e.g. pores, trenches and pillars) using reactive ion etching is a relatively mature technique . The step conformal deposition of battery layers into these structures require non-line of sight techniques, which are on one hand established in the production of integrated devices, but are on the other hand still mostly unexplored for the deposition of battery materials. These methods include Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) . In this presentation the concept of the 3D integrated battery will be discussed and the highlights in the investigations will be shown. The focus of this contribution will be on the exploration of CVD and ALD as deposition techniques for thin-film micro-batteries. Moreover, the (electro)-chemical characterization of several active battery layers will be discussed.
11:30 AM - **P1.4
Designing Architecture and Composition of Templated Electrodes For Energy Storage.
Andreas Stein 1 , Zhiyong Wang 1 , Melissa Fierke 1 , Anh Vu 1 Show Abstract
1 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
Targeting miniaturized batteries with sufficient geometric power and energy densities to support ultrasmall devices, we designed and developed interpenetrating electrochemical cell architectures based on three-dimensionally ordered macroporous (3DOM) electrode structures. We previously demonstrated the rate advantages of nanostructured, 3DOM carbon electrodes in half-cell designs and assembled a complete lithium ion electrochemical cell, in which a conformal electrolyte/separator coating on the surface of a 3DOM carbon anode isolated the anode from a continuous vanadia cathode phase that filled the remaining pore space. While reversible lithiation/delithiation was possible with this design, the capacity of the first prototypes was limited by the electronic resistivity of the two electrode components and ionic resistivity of the separator. To address these limitations we are studying the effects of architecture and composition of each cell component on its electronic and mechanical properties. In particular, we will discuss the effects of carbon pore texture, composite formation of porous glassy carbon anodes with graphitic carbon, tin oxide or silicon, and doping of vanadia cathodes with ruthenia or silver on these properties.
12:00 PM - **P1.5
The Challenge of Modelling the 3D-Microbattery
Daniel Brandell 1 , Vahur Zadin 1 , John Thomas 1 Show Abstract
1 Materials Chemistry, Uppsala University, Uppsala Sweden
Parallel to current popular interest in novel battery technologies for large-scale applications (like EV/HEV/P-HEVs, UPS and renewable-energy storage), there is the somewhat less well publicized but no less pressing demand at the opposite end of the dimension-scale for miniaturized power sources to satisfy the ever-increasing needs of the microelectronics and bio-MEMS industries. Research into solid-state Li-based thin-film batteries has so far focused mainly on flat 2D-configurations, with their intrinsic limitations in performance. The need for better battery performance within a confined space has provoked the exploration of ways of exploiting the third dimension in so-called 3D-MB concepts, where Area Gain (A.G.) factors in excess of 40 are now considered realistic. A common feature of this research has been the implementation of novel 3D current-collector and electrode fabrication techniques to realize short ion-diffusion lengths and thereby high power densities. These efforts have often exploited nano-porous Al2O3 templates; some practical examples will be given.In the design of such 3D-MB architectures (and not least in their subsequent device implementation), it is very clear that modelling has a vital rôle to play, especially since we are here exploring the nano-scale regime, where counter-intuitive electrochemical phenomena can appear. Clearly, atomic-level Molecular Dynamics (MD) simulation in combination with Finite Element Analysis (FEA) techniques will be especially relevant in this context. However, there is an underlying problem in their use: current upper space/time limits for standard MD methods lie in the range 10nm/10ns; while the FEA concept usually breaks down above these limits. We are currently endeavouring to bridge this gap, so that information derived from atomic-level MD can be used both to help pinpoint the types of local 3D-MB geometry which should be avoided – or encouraged – and also for use as input parameters to FEA to probe the performance of different micro-battery architectures. The FEA approach delivers information on overall system properties: heat flow, mass transport, stress energy, etc. Some relevant examples taken from our recent work involving the modelling of nano phenomena in 3D-MB design will be given; e.g., self-assembled short-chain polymer nano-coatings, nano-fillers, etc. The enhancement achieved has been characterized at the atomic level in our MD treatment, and this information then transferred to an FEA analysis of their implementation in nano- and microbattery architectures. Our work in this area and in it subsequent use in modelling real 3D-MB configurations will be described.
12:30 PM - P1.6
High Volumetric Energy Density Microbatteries.
Can Erdonmez 1 , Wei Lai 1 , Thomas Marinis 2 , Caroline Bjune 2 , Fan Xu 3 , Nancy Dudney 3 , Yet-Ming Chiang 1 Show Abstract
1 , M.I.T., Cambridge, Massachusetts, United States, 2 , Draper Laboratory, Cambridge, Massachusetts, United States, 3 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Advances in microfabrication allow for the construction of compact, stand-alone systems integrating multiple device functionalities. Lack of accompanying power sources of similar size and sufficient energydensity, however, has been a roadblock to proliferation of such devices. With decreasing size, all existing bulk battery designs exhibit rapidly decreasing energy density due to a growing fraction of packaging materials. While significant advances have been made in solid-state thin film batteries, many applications require other form factors to achieve areal power density requirements. Here, we demonstrate rechargeable batteries of ~ 5 mm3 volume that exhibit energy densities exceeding those of commercial primary batteries 2 orders of magnitude larger in volume. Volumetric energy densities up to 650 Wh/L have been obtained at C/3 rates (~200 W/L) for packaged batteries. To our knowledge, such energy densities are unprecedented for batteries at the ~ mm3 size scales. The high energy density corresponds to exceptionally high volumetric utilization by active materials, enabled by a thick 3D cathode design. Li1-xCoO2 is used as the active cathode material for its high electronic conductivity across most of the composition range of interest, and metallic lithium is used as the anode. A novel packaging design was developed and will be discussed in the presentation.
12:45 PM - P1.7
Probing Li-ion Battery Electrode Architectures with a Focused Ion Beam and Modeling.
Arnold Stux 1 , David Rowenhorst 2 , Edward Gorzkowski 2 , David Stephenson 3 , Dean Wheeler 3 Show Abstract
1 Nanopower Research Labs, Rochester Institute of Technology, Rochester, New York, United States, 2 Materials Science and Technology, Naval Research Laboratory, Washington, District of Columbia, United States, 3 Chemical Engineering, Brigham Young University, Provo, Utah, United States
A dual beam Focused Ion Beam (FIB) has been used in conjunction with porous electrode modeling and modifications to electrode fabrication as an integrated effort to tune morphologies of Li-ion battery electrodes. Recent advances in automation, as well as computing power, have rendered serial sectioning a viable solution for determining the three-dimensional (3D) structure of complex microstructures. Using the ion beam, thin sections were milled from a Li-ion battery electrode while the electron beam was used for high-resolution imaging of the cross section. The information from the FIB can be entered into a model that describes the chemical, physical, and electronic phenomena. Porosity is a commonly used variable to characterize aggregate microstructure and morphology and is considered in the model. Pore structure and overall porosity affect ion and electron transport in combination with particle size and shape distributions. Micron- and submicron-size constituents can also influence transport in these electrode film architectures. We use 3D reconstruction techniques as a tool for further understanding of these morphologies. This opens up possibilities to develop more rigorous models and further understand the structure-performance relationships pertaining to ion and electron transport and surface reactions in porous battery electrodes. The development of these analytical and predictive techniques is envisioned for implementation into the materials and process design cycle for battery electrodes.
P2: Multiple Junction and Novel Structure Solar Cells
Tuesday PM, April 14, 2009
Room 2022 (Moscone West)
2:30 PM - **P2.1
Three-Dimensional Photovoltaic System Architecture for Very High Efficiency Modules.
Allen Barnett 1 , Christiana Honsberg 2 Show Abstract
1 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States, 2 Electrical and Computer Engineering, Arizona State University, Tempe, Arizona, United States
Very High Efficiency Solar Cell (VHESC) modules are being designed and developed for portable applications. The design goal is for modules that operate at greater than 50 percent efficiency. The high-efficiency module is based on co-design of the optics, interconnects, and solar cells. Low concentration (5 to 20X) is used to capture much of the performance benefit of concentration, reduce the material costs and lead to systems with no moving parts (static concentrators). The new architecture significantly increases the design space for high-performance photovoltaic modules in terms of materials, device structures, and manufacturing technology. It affords multiple benefits, including increased theoretical efficiency, new architectures that circumvent material/cost trade-offs, improved performance from non-ideal materials, device designs that can more closely approach ideal performance limits, reduced spectral mismatch losses, and increased flexibility in material choices. The design approach focuses first on performance, enabling the use of existing state-of-the-art photovoltaic technology to design high performance multiple junction III-Vs for the high and low energy photons and a new silicon solar cell for the mid-energy photons, all while circumventing existing cost drivers through novel solar cell architectures and optical elements. Our approach is driven by proven quantitative models for the solar cell design, the optical design and the integration of the two.A test bed for rapid development and verification of performance of subsystems has been developed. The results and analysis of the first complete integrated optics and solar cells on this test bed shows module efficiency greater than 36%. Analysis shows a direct path to efficiencies greater than 40%. These initial results have not been verified by any 3rd party. We have previously reported the sum of the solar cell efficiencies to be over 42%, and optical subsystem efficiency greater than 93%. The new system architecture is based on a “parallel” or lateral optical concentrating system, which splits the incident solar spectrum into several bands and allows different optical and photovoltaic elements in each band. The optics and the solar cells are co-designed to achieve the maximum conversion efficiency of the module. The design rules, initial designs, solar cell and module results will be presented. The lateral solar cell architecture increases the choice of materials for multiple junction solar cells, by allowing the solar cell in each spectral band to be optimized independently of the others. In this way, the lattice and current matching constraints are reduced. Further, since the devices do not need to be series connected, spectral mismatch losses are reduced, which is important for tandems in terrestrial environments.
3:00 PM - **P2.2
Efficient Light Harvesting in Multiple-device Stacked Structure for Polymer Solar Cells.
Srinivas Sista 1 , Hsiang-Yu Chen 1 , Jianhui Hou 2 , Vishal Shrotriya 2 , Gang Li 2 , Yan Yao 1 , Yang Yang 1 Show Abstract
1 Materials Science & Engineering, University of California-los angeles, Los Angeles, California, United States, 2 , Solarmer Energy, Inc. , El Monte, California, United States
Efforts are being made to increase the efficiency of polymer solar cells. One of the strategies to increase the efficiency is to make use of tandem architecture. In this presentation we will discuss a multiple-device stacked structure in which two polymer solar cells are stacked together and are connected either in series or parallel for efficient light harvesting. The two sub cells of the stacked structure were based on poly(2-methroxy-5, 2’-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The bottom cell has a multilayer transparent conducting layer made of a lithium fluoride (LiF)/ aluminium (Al)/ gold (Au), so that the unabsorbed photons can be transmitted through and absorbed by the top cell. The stacked structure showed an almost doubled open circuit voltage (VOC) when connected in series and doubled short circuit current (JSC) when connected in parallel. We extend this concept to stacked cells with sub cells made from two different polymer blend systems. The two polymers are chosen such that one is a low band gap polymer and the other a large band gap polymer. This results in an increase in absorption range and a larger part of the solar spectrum is covered. We expect to achieve very high efficiencies from this multiple-device stacked structure.
3:30 PM - P2.3
Absorption Enhancement in Si Wire Arrays for Photovoltaic Applications.
Michael Kelzenberg 1 , Jan Petykiewicz 1 , Morgan Putnam 1 , Josh Spurgeon 1 , Daniel Turner-Evans 1 , Chaitanya Rastogi 1 , Brendan Kayes 1 , Michael Filler 1 , Nathan Lewis 1 , Harry Atwater 1 Show Abstract
1 , California Institute of Technology, Pasadena, California, United States
Photovoltaic devices based on arrays of vapor-liquid-solid (VLS) grown Si nano- or micro-wires are being investigated as potential low-cost alternatives to wafer-based Si solar cells. A fundamental requirement is that they absorb nearly all above-bandgap photons in the solar spectrum. While others have observed and simulated light absorption in VLS-grown Si nanowires for photovoltaic applications [1,2], our prior experimental data and simulations  indicate that, for the 2-10 µm minority carrier diffusion lengths observed in our wires, optimal photovoltaic efficiency favors substantially larger wire diameters than previously studied. For this reason we have characterized the optical absorption of several Si wire arrays to study the effects of incident beam angle and wavelength, wire dimensions, long-range and short-range order in the wire array lattice, packing fraction, and the metal catalyst tips.Si wires were grown by a photolithographically-patterned VLS process which produced high-fidelity arrays of vertical Si wires over several mm2. Wires of 3 µm nominal diameter and 80 µm nominal length were patterned in quasi-random, quasi-periodic, and periodic lattices with packing fractions ranging from 4.9% to 17%. The as-grown wire arrays were then encased in a transparent PDMS polymer; peeled intact from the Si growth substrate, and transferred upside-down to a quartz slide to enable substrate-free observation the reflection and transmission of light through the wire arrays. Optical absorption measurements were performed from 460-1800 nm in a custom designed absorption apparatus featuring an integrating sphere assembly capable of angular rotation, with swept monochromatic illumination provided by a supercontinuum laser. The absorption spectra of each Si wire array were compared in terms of the percentage of photons, within the above-bandgap spectral range considered, that would be absorbed from the AM 1.5G spectrum. For the arrays studied, this ranged from 17% to 58%, representing an absorption enhancement of 2.3 to 3.5 times the geometrical packing fraction alone, or a spectral enhancement of 3 to tens of times that expected from planar Si slab of equal volume as the wires comprising the array. Although none of the wire arrays absorbed as much as an efficient Si solar cell, this result has two potentially useful implications for Si wire array-based photovoltaics. Firstly, it suggests that full absorption might be achievable with higher but physically realizable packing fractions. Secondly, this absorption enhancement could increase injection levels within a photovoltaic device by reducing the volume of Si; which could enable higher efficiencies analogous to light trapping or optical concentration in planar photovoltaic devices.  Tsakalakos, L., et. al. Journal of Nanophotonics 2007 Hu, L.; Chen, G. Nano Lett. 2007 Kelzenberg, M., et. al. 33rd PVSC
3:45 PM - P2.4
Efficient Photovoltaic Devices Employing Ternary PbSxSe1-x Nanocrystals.
Wanli Ma 1 , Joseph Luther 1 , Haimei Zheng 1 , Yue Wu 1 , Paul Alivisatos 1 Show Abstract
1 Chemistry, UC Berkeley, Berkeley, California, United States
Solar power, as one of the most abundant and easily accessible energy sources, has attracted more and more interests. In recent years, extensive studies have been carried out to search for novel photovoltaic materials with easier processing abilities and large-volume production potentials, such as semiconducting polymers and inorganic colloidal nanoparticles.Compared to polymer/fullerene composites, colloidal quantum dots (QD) can have increased infrared photon harvesting and improved charge transport due to their tunable bandgap and highly crystalline structures. Among them, low bandgap lead chalcogenides with superior carrier mobilities are currently the most promising photovoltaic QD materials. PbS and PbSe QD based solar cells have already obtained reasonable efficiency of 1.1%~2.1% due to large photocurrent densities. Recently, ternary QD based on cadmium and lead chalcogenides have been synthesized and studied. However, the utilization of novel alloyed QD in solar cells has not yet been systematically reported.In order to investigate the photovoltaic property of ternary QD, we synthesized monodispersed and highly crystalline PbSxSe1-x nanoparticles with different selenium and sulfur composition ratios. According to the observation of high resolution TEM with elemental mapping, selenium and sulfur were uniformly distributed in each QD, which indicated the formation of alloyed ternary nanoparticles. Rutherford Back Scattering Spectra were recorded to investigate the actual elemental composition ratios of nanoparticles in comparison with the precursor ratios used during QD synthesis. When studying the optical properties, we observed the gradual absorbance and photoluminescence (PL) spectra changes with different selenium and sulfur ratios in alloyed nanoparticles. The PL and PL lifetime measurements indicated that alloyed QD had less surface charge traps and hence better transport property. The photovoltaic properties of ternary nanoparticles were thoroughly investigated. Schottky junction solar cells with a structure of ITO/QD/Al were fabricated and tested under standard AM 1.5 illumination conditions. PbSe based solar cells had higher short circuit current (ISC) while PbS devices had larger open circuit voltage (VOC). Compared to pure PbSe and PbS QD, improved VOC and ISC were both obtained for PbSxSe1-x QD based devices. At a certain sulfur to selenium ratio, the best photovoltaic performance has achieved a current density of 16mA/cm2, a Voc of 0.45eV, a fill factor of 50%, and an energy conversion efficiency of 3.5%. This is the highest efficiency obtained so far for colloidal QD based solar cells. Our photovoltaic devices were fabricated at room temperature without any sintering process. This efficiency can be further improved with P-N junction architectures. The better performance of ternary QDs is assumed to arise from the reduced surface traps and improved QD/metal interface energy band alignment.
4:30 PM - **P2.5
Future Terrestrial III-V Multijunction Solar Cells: Practical Efficiency Ceiling and Technology Pathways
Daniel Law 1 , Richard King 1 , Hojun Yoon 1 , Melissa Archer 2 , Andreea Boca 1 , Christopher Fetzer 1 , Shoghig Mesropian 1 , Taka Isshiki 1 , Kenneth Edmondson 1 , Dhananjay Bhusari 1 , William Hong 1 , Harry Atwater 2 , Nasser Karam 1 Show Abstract
1 , Boeing - Spectrolab, Sylmar, California, United States, 2 , California Institute of Technology, Pasadena, California, United States
Increasing number of concentrator photovoltaic systems being built now use III-V multijunction cells, due to the high efficiency levels that this cell technology achieved. The rapid growth in cell efficiency and its strongly leveraging effect to lower system cost are enabling system manufacturers to produce concentrator systems competitive with other technologies and offering potential cost advantages. It is important to keep pushing the theoretical efficiency ceiling through the use of better band gap combinations in future multijunction cell designs to reach still higher efficiencies. First-principle efficiency limits are examined for current and potential solar cell architectures under the terrestrial spectrum. The fundamental loss mechanisms of non-absorption of low-energy photons, thermalization of electrons and holes to their respective band edges, and the difference between band edge energies and quasi-Fermi levels are quantified and contrasted for selected cell designs. Spectrolab is developing technology pathways for future high efficiency terrestrial multijunction cells in several research and development programs, with the goals of reaching average production concentrator cell efficiencies of 40% in 2010, and 43% by 2015. These future terrestrial concentrator cells will likely utilize new technology approaches such as highly metamorphic materials, inverted crystal growth, direct-wafer bonding, 4- or more junction architectures, and their combinations to achieve the desired bandgaps while maintaining excellent device material quality for optimal solar energy conversion. Recent experimental results will be discussed for prototype terrestrial concentrator cells using an upright metamorphic approach, as in the 40.7% efficient cell, 4-junction GaInP/ AlGaInAs/ GaInAs/ Ge terrestrial concentrator cells, metamorphic ~1-eV subcells & inverted GaInP/ 1.4-eV GaInAs/ 1.0-eV GaInAs 3-junction structure, and multijunction cells on wafer-bonded, layer transferred epitaxial templates.
5:00 PM - P2.6
Outdoor characterisation of High Efficiency Luminescent Solar Concentrators for Smart Windows
Mauro Pravettoni 1 2 , Rahul Bose 2 , Amanda Chatten 2 , Robert Kenny 1 , Keith Barnham 2 Show Abstract
1 Institute for Energy - Renewable Energy Unit, European Commission DG JRC, Ispra, Varese, Italy, 2 Blackett Laboratory, Imperial College London, London United Kingdom
Luminescent Solar Concentrators (LSCs) have been studied since 70s [i] and typically consist of glass or transparent polymer slabs, doped with organic dyes that act as luminescent centres. Incident light is partially absorbed, re-radiated by luminescent species and then partially total-internal-reflected towards the edges of the slab, where photovoltaic cells are glued to convert light in electricity. Interest in LSCs has recently grown thanks to recent reports of high efficiency devices [ii]. The increase in lifetimes of organic dyes together with some results in the production of inorganic luminescent species such as quantum dots or quantum rods helped also to boost the development in this technology.The high efficiency value of 7.1% was measured in 2008 at the European Solar Test Installation, Joint Research Centre of the European Commission (Ispra, IT) and reported in literature [iii].Due to their own nature, LSCs convert both direct and diffuse radiation to electrical power [iv]: no tracking system is necessary and therefore they represent a potentially cheap solution, complementary to conventional concentrating systems, particularly well-fitted to building integration. In this work we present recent results in outdoor current/voltage characterisation of LSCs, with particular highlight of the performance under various irradiating conditions. The impact of the presence of a backside reflector on the module efficiency is also highlighted.[i] W. H. Weber - J. Lambe, Appl. Optics 15, 2299 (1976).[ii] M. J. Currie et al., Science 321, 226 (2008).[iii] L. H. Sloof et al., Phys. Stas. Sol. (RRL), to be published (online Sep 29 2008).[iv] M. Pravettoni et al., Proc. 23rd EU PVSEC, Valencia (2008).
5:15 PM - P2.7
Atomic Layer Deposition of Copper Sulfides for 3-D Photovoltaic Devices.
Qiaoer Zhou 2 , Lily Yang 1 , Tong Ju 1 , G. Alers 1 Show Abstract
2 Electrical Engineering, University of California, Santa Cruz, Santa Cruz, California, United States, 1 Physics, University of California, Santa Cruz, Santa Cruz, California, United States
Photovoltaic devices with nanostructured TiO2 electrodes and an inorganic absorber layer of CuxS deposited with atomic layer deposition (ALD) have been constructed and tested. The goal is to combine the high surface area and enhanced scattering of a porous electrode with a very thin conformal inorganic absorber deposited by a surface-controlled ALD technique. The incorporation of self-assembled quantum dots onto the TiO2 surface is also being investigated to harvest electrons from multiple-exciton generation within the quantum dots. The basic un-optimized solar cell devices used a heterojunction structure of glass/ITO/sol-gel TiO2/porous-TiO2/ALD CuxS/P3HT/Au. The purpose of the thin layer of spin coated P3HT is to collect the holes and prevent shorting. The porous TiO2 layer was deposited with a solution of ~40nm or ~100nm nanoparticles of TiO2, baked at 100C and then sintered at 450C. A hydrogen plasma treatment of the TiO2 was evaluated for the removal of residual carbon from the surface and to facilitate nucleation of the ALD layer. A b-diketonate-type metal-organic compound Cu(thd)2 and H2S were used as precursors for ALD deposition. The temperature dependence of the growth rate was determined for films deposited on corning glass, silicon, TiO2 sol-gel films and TiO2 nanoparticle (NP). The films deposited at low temperature had low resistivity of 1x10-3 ohm-cm. The resistivity increases as samples were annealed at 250C or by depositing films at higher temperature. The absorption data showed that films with a thickness of 12nm had a bandgap of 1.6eV making them ideal for efficient photovoltaic response. AFM analysis indicated that the film deposited at low temperature had polycrystalline morphology with a grain size of ~50nm and a film roughness of ~1nm. SEM showed conformal coating of a 300nm thick layer of TiO2 nanoparticle network formed with ~40nm nanoparticles. The penetration of the CuxS into the TiO2 matrix was complete forming a solid composite structure of CuxS wth an embedded TiO2 electrode matrix. Results for the optimization of open circuit voltage and short circuit current will be presented.
5:30 PM - P2.8
Three-dimensional Carbon Nanotube Based Photovoltaics.
Jack Flicker 1 , Jud Ready 2 1 Show Abstract
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia, United States
The production of cheap energy from the sun will be a major research objective in the coming years. Major strides must be made in solar cell efficiency, including increasing the absorbance efficiency of a cell by etching or texturing. In order to increase the absorbance efficiency of solar cells, we have developed a three dimensional solar cell structure by depositing a cadmium telluride thin film overtop carbon nanotube towers. These towers act as both a scaffolding and electrical interconnect. Multiple photon interactions as they reflect between these towers increase the absorption efficiency. We have developed a theoretical model and computer simulation to maximize the number of photon interactions due to the geometrical characteristics of the system (aspect ratio, spacing, size, shape, etc). Simulated modeling has shown that by optimization of parameters a three dimensional cell can obtain up to a 300% increase in power production over traditional planar cells.
5:45 PM - P2.9
Three-Dimensional Quantum Dot Supra-Crystals for Photovoltaic Energy Conversion.
Qinghui Shao 1 , Alexander Balandin 1 , Alexander Fedoseyev 2 , Marek Turowski 2 Show Abstract
1 Nano-Device Laboratory, Department of Electrical Engineering and Materials Science and Engineering Program, University of California - Riverside, Riverside, California, United States, 2 , CFD Research Corporation, Huntsville, Alabama, United States
Three-dimensional (3D) ordered arrays of quantum dots – quantum dot supra-crystals (QDS) – where the role of atoms is played by quantum dots – have been shown to benefit the thermoelectric energy conversion [1-2]. The potential of 3D regimented QDS in thermoelectrics is related to mini-band formation for both electrons and acoustic phonons in way similar to photonic crystals. In this talk we argue that 3D supra-crystals can be beneficial for the photovoltaic (PV) energy generation by serving as intrinsic layers in the intermediate-band (IB) solar cells. It was predicted that IB solar cells have the PV efficiency for energy conversion exceeding the Shockley-Queisser limit for a single junction cell of ~33%. The IB energy level helps to collect photons with the energy less than the band gap of the host material, which allows one to improve the short-circuit current without degrading the open-circuit voltage. QDS with appropriately tuned parameters such as dot size, inter-dot distance, shape and barrier height, can act as the IB solar cell material. The QDS parameters have been determined using the first-principle theory . It has been found that the energy spectra of the electrons and holes in the ordered QDS are distinctively different from those in the single quantum dots or conventional quantum-well superlattices. The charge-carrier dispersion and charge localization are very sensitive to the quasi-crystallographic directions defined by the dots. The analysis of the PV solar cells performance was carried out on the example of the InAsN/GaAsSb material system. By calculating the actual electron dispersion for different QDS we were able to select the quantum dot parameters, which place the first electron mini-band, i.e. subband, in the band-gap region at the energy suitable for this mini-band to serve as the IB energy level . Using the detailed balance theory we determined the efficiency of such QDS-based IB solar cells. The upper-bound theoretical PV efficiency for QDS with the quantum-dot size of ~5 nm is substantially higher than the theoretical efficiency for a single-junction single-band material. This work has been supported by the AFOSR contract FA9550-07-C-0059 and NASA contract NNC07CA20C. A. A. Balandin and O. L. Lazarenkova, "Mechanism for thermoelectric figure-of-merit enhancement in regimented quantum dot superlattices," Appl. Phys. Lett., 82, 415 (2003). Y. Bao, A. A. Balandin, J. L. Liu and Y.H. Xie, "Experimental investigation of Hall mobility in Ge/Si quantum dot superlattices," Appl. Phys. Lett., 84, 3355 (2004). D. L. Nika, E. P. Pokatilov, Q. Shao and A. A. Balandin, "Charge carrier states and light absorption in the ordered quantum dot superlattices," Phys. Rev. B, 76, 125417 (2007). Q. Shao, A. A. Balandin, A. I. Fedoseyev and M. Turowski, "Intermediate-band solar cells based on quantum dot supra-crystals," Appl. Phys. Lett., 91, 163503 (2007).
Bruce Dunn University of California-Los Angeles
Gang Li Solarmer Energy Inc.
Jeffrey W. Long Naval Research Laboratory
Eli Yablonovitch University of California-Berkeley
P3: 3-D Battery Architectures: Materials and Fabrication Methods
Wednesday AM, April 15, 2009
Room 2022 (Moscone West)
9:30 AM - **P3.1
Electrodeposition as a Versatile Tool for the Fabrication of Three-Dimensional Lithium-ion Rechargeable Batteries.
Amy Prieto 1 , James Mosby 1 , Timothy Arthur 1 Show Abstract
1 Chemistry Department, Colorado State University, Fort Collins, Colorado, United States
The two main limitations to the rate of charging and discharging in Li-ion batteries are the slow diffusion of Li+ into the anode and the cathode and the slow diffusion between them. A successful method to decreasing the diffusion length of Li+ in intercalation reactions has been to fabricate electrode materials as high surface area nanowire arrays. The fabrication of nanowire arrays of both carbon based anodes and several common cathode materials has been shown to dramatically enhance electrode performance. The problem of decreasing the Li+ diffusion length between the cathode and anode has not yet been solved. We are incorporating nanowire arrays of Cu2Sb anodes into a new battery architecture. Each nanowire anode is conformally coated with a polymer electrolyte via reductive electropolymerization, and then surrounded by the cathode electrode synthesized using sol-gel chemistry. The significant advantage to this geometry is that the diffusion length between the electrodes has been dramatically reduced. Electrodeposition of the anode and the polymer electrolyte is key for the final three-dimensional architecture.
10:00 AM - **P3.2
Electrodeposition of Polymer Electrolytes as Battery Separator Layers on Convoluted Surfaces.
John Owen 1 , Phil Johns 1 , Matt Lacey 1 , Supputra Visetpotjanakit 1 , Gaber El-Enany 2 Show Abstract
1 School of Chemistry, University of Southampton, Southampton United Kingdom, 2 Physics and Math Department, Faculty of Engineering, Suez Canal University, Port Said Egypt
Sequential deposition of the three components of a battery onto an arbitrarily shaped surface offers a radical departure from conventional battery fabrication methods. By contrast with the laminate construction, which places a sheet of electrolytic separator in between two electrode-coated foils or plates, the sequential deposition route can, in principle, be applied to a general, convoluted surface such as the inside of a porous metal. From the several methods available for the deposition of electrodes, e.g. electrodeposition of metals and metal oxides, chemical vapour deposition and sol-gel deposition of compounds, electrodeposition has the advantages of self-levelling and pinhole-filling if the resistance of the product can be made to be control the deposition rate. Therefore, electrodeposition is an ideal method for producing the homogeneous, conformal coatings required of the separator layer to encourage a uniform current distribution during battery operation. Electrodeposition of electrolyte separators is relatively novel, and unusual because the separator lacks, by definition, the electronic conductivity normally required for deposition of a layer of non-infinitesimal thickness. This paper will outline examples of two ways in which this apparent impossibility can be circumvented. The first method uses electropolymerization as previously used in corrosion protection of metals. Vinyl polymerization has been achieved through the formation of polymerization initiators by electron transfer from the substrate, followed by diffusion of initiator and reactants through the film. Examples will be given for polyacrylonitile and oligo-ether substituted vinyl monomers. Surface conductance values of around 10 Ohm cm2 have been achieved after plasticization of layers up to 10 microns in thickness with added liquid electrolytes.The second method described uses oligoether-substituted pyrrole as the precursor to the growth of an electronically conducting film by electrodeposition. This allows the electrodeposition of an electrode layer on top before finally activating the battery structure by chemically or electrochemically destroying the electron path through the conjugated structure of the polypyrrole. In this way, complete battery structures can be made entirely by sequential electrodeposition.The presentation will report progress on the application of both methods to planar and 3D substrates and characterization of the electrolyte layers in each case. The initial work was presented at ISPE-11, Ofir, Portugal, Sept 2008.G. M. El Enany†, P. Johns, S. Visetpotjanakit and J. R. Owen* † on leave from the Department of Physics and Math.,Faculty Of Engineering, Suez Canal University, Port Said, Egypt
10:30 AM - P3.3
A High Aspect Ratio Cu2Sb Electrode Coated With a Silane-based Polymer Film to be used in a 3D Micro-battery.
Emilie Perre 1 2 , Pierre-Louis Taberna 1 , Torbjorn Gustafsson 2 , Patrice Simon 1 , Kristina Edstrom 2 Show Abstract
1 Centre Inter-universitaire de Recherche et d’Ingénierie des MATériaux , Université Paul Sabatier, Toulouse France, 2 Ångström Advanced Battery Centre, Dept of Materials Chem., Uppsala Universitet, Uppsala Sweden
Research into energy storage devices is undergoing great changes especially for powering micro/nano applications. New micro battery concepts based on the Li-ion battery technology such as 3-dimensional nano-architecturing are now being explored. The expectation is to maintain the advantage of high kinetics shown by thin-film Li-ion batteries while increasing the energy delivered per foot print area by building a 3D structure. Applicability of the 3D model has already been demonstrated and full or half-cells have been experimentally obtained[2,3,4]. We will present our work on the synthesis of a 3D Li-ion micro battery using chemical/electrochemical methods. Three-dimensionally organised current collectors made of copper or aluminium were grown by direct electrodeposition through an alumina template[4,5], and coated with active material and polymer separator. Such an approach permits the achievement of high aspect ratio columns having diameters below the micrometer range. Herein we present the behaviour of antimony electrodeposited onto copper current collectors, subsequently coated by a polymer separator filled with electrolyte. Sb alloys reversibly with lithium but endeavours high volume variations during the charge/discharge processes. In order to mechanically buffer these changes, alloying of Sb with the copper current collectors has been performed by heat treatment. The addition of a silane-based polymer film onto the so obtained active material will be presented. The obtained cell did not only show an increased capacity per foot print area and a more stable cycling compared to planar systems, but it also maintained its rate capability. 1. Long J. W., Dunn B., Rolison D. R., White H. S. Chemical Reviews 104 (2004) 4463.2. Golodnitsky D., Yufit V., Nathan M., Shechtman I., Ripenbein T., Strauss E., Menkin S., Peled E. J. Power Sources 153 (2006) 281.3. Wang C., Taherabadi L., Jia G., Madou M., Yeh Y., Dunn B. Electrochem. Solid-State Lett. 7 (2004) A435.4. Taberna P. L., Mitra S., Poizot P., Simon P., Tarascon J. M. Nature Mater. 5 (2006) 567.5. Perre E., Nyholm L., Gustafsson T., Taberna P.-L., Simon P., Edström K. Electrochemistry Communications 10 (2008) 1467.
10:45 AM - P3.4
Nanostructured Anodes for 3D Li-ion Microbatteries based on Copper Nano-architectured Current Collector
Laurent Bazin 1 , Marie-Joelle Menu 1 , Patrice Simon 1 Show Abstract
1 CIRIMAT UMR-5085, Université Paul Sabatier, Toulouse Cedex 4 France
Recent breakthrough in the field of nano-technology will hopefully lead to the replacement of traditional electronic systems by microelectronic devices, designed at the nanoscale. These advances, in association with the increasing need for autonomy in portable device applications dragged the energy storage field toward a new goal: to design battery systems capable to be integrated in micrometric objects. In this optic, 3D microbatteries is a promising approach, based on a clever use of the third dimension of batteries. This strategy has been successfully used in various work [1, 2], leading to high power and energy volumic densities. In this work, we propose nano-architectured electrodes for 3D Li-ion batteries, using electrodeposited Sn as active material. Also, a similar coating was achieved by EPD using SiO2 particles as model compound. High surface area nano-architectured copper current collectors have been designed based on simple electrodeposition method . It consists in arrays of pure Cu pillars (2µm long, 200 nm diameter). The nano-architectured electrode design not only increases the effective surface area of the electrode but it is also very suitable for sustaining the mechanical and structural stain during electrochemical reaction. It is also providing better electronic and ionic conduction. A nano-architectured Sn anode for Li-ion battery, based on Li-Sn alloying reaction, was prepared by coating this nanostructure by pulsed ELD. It delivers very high cycle life and good power performance compared to planar tin films. This electrode should be successfully used in the field of 3D microbatteries. Following this work, we also achieved a versatile electrophoretic coating of the nano-architectured current collector . Silica nanoparticles were used as a model compound, leading to well covering, thin film on the Cu rods.. The use of this technique open the way for the deposition of a large variety of materials that cannot be deposited by ELD, with controllable cristalinity and particles size.Ref: 1)J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem Rev 104 (2004) 4463 2)M. Nathan, D. Golodnitsky, V. Yufit, E. Strauss, T. Ripenstein, I. Shectman, S. Menkin, E. Peled, Journal of microelectromechanical systems 14 (5) (2005) 8793)P.L. Taberna; S. Mitra, P. Poizot, P. Simon, J.-M Tarascon, Nature Material, 5 (2006) 5674)L. Bazin, M. Gressier, P.-L. Taberna , M.-J. Menu, P. Simon, Chem. Commun (2008) 5004
11:30 AM - **P3.5
Lithium Phosphorous Oxynitride (Lipon) Electrolyte for Rechargeable Batteries with Three-Dimensional Architectures
Nancy Dudney 1 , Fan Xu 1 , Can Erdonmez 2 , Wei Lai 2 , Yet-Ming Chiang 2 Show Abstract
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The glassy lithium phosphorous oxynitride electrolyte, known as Lipon, has been successfully applied in the development of thin film rechargeable lithium and lithium-ion batteries. The modest lithium ion conductivity, ~2µS/cm, is sufficient for most devices when deposited by RF sputtering as thin films with a thickness of ~1µm. The stability of the Lipon films allows for high voltage lithium batteries, high temperature processing, and exposure to air during battery fabrication. In addition, the Lipon has a high electronic resistivity that prevents self-discharge. Recent work is exploring whether Lipon can be used as an electrolyte for some of the more complex 3D architectures being investigated for solid state batteries. Using a planar magnetron sputtering source and nitrogen process gas, the composition, morphology, and conductivity of the film has been characterized for surfaces positioned at grazing angles and at angles completely shadowed from the line-of-sight to the magnetron source. Although the film composition varied, a dense morphology and good lithium ion conductivity were achieved. In addition, Lipon coverage has been evaluated for several different surfaces with regular 10-100 µm features, including porous membranes and arrays of posts. Acknowledgement: This work was supported by DARPA Defense Sciences Office and by the Division of Materials Sciences and Engineering, U.S. Department of Energy.
12:00 PM - P3.6
Microporous vs. Planar Intermetallic Anodes for Li-Ion Batteries.
Lynn Trahey 1 2 , John Vaughey 1 , Harold Kung 2 , Michael Thackeray 1 Show Abstract
1 , Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical and Biological Engineering, Northwestern University, Evanston, Illinois, United States
The future of lithium-ion batteries, the staple of portable energy storage, relies on improving the anode in terms of safety and energy. Most state-of-the-art batteries use graphitic carbon, although alternative materials such as Sn-based compounds and Si are being advocated because they offer larger volumetric and gravimetric capacities. In general, the drawback of Sn-based and Si materials is that they tend to disintegrate on cycling as a result of the large volume changes incurred on cycling, which leads to short electrode lifetimes. By using intermetallic compounds that show strong structural relationships before and after reaction with lithium, such as Cu6Sn5 and Cu2Sb, the volume expansion is lowered significantly and the cycle life improved; however, further improvements in electrochemical performance of these anode materials are still required .In efforts to enhance the electrochemical properties of Cu-based intermetallic electrodes, we have adopted an approach using electrodeposition techniques to engineer 3-D porous copper foam current collecting architectures with sufficient void volume to accommodate the expansion of the active electrode particles; our recent work on Sn-Cu6Sn5 anodes has led to promising results . In order to raise the average operating potential of the anode, we have extended this work to composite Cu6Sn5 and Cu2Sb electrode systems. Thin film (2-D) and microporous (3-D) architectures, synthesized by direct and pulsed electrodeposition, have been explored. Electrodes were analyzed by scanning electron microscopy, energy dispersive spectrometry and powder X-ray diffraction before and after electrochemical coin cell cycling to assess the changes in electrode morphology, composition and crystallinity. The effects of Cu-foam sintering and active material annealing, as well as key differences in electrode stability and cycling kinetics between 2-D and 3-D architectures will be discussed.AcknowledgmentsFinancial support from the Office of Vehicle Technologies of the U.S. Department of Energy under Contract DE-AC02-06CH11357 and Center for Energy Efficient Transportation at Northwestern University is gratefully acknowledged. ReferencesM. M. Thackeray, J. T. Vaughey, C. S. Johnson, A. J. Kropf, R. Benedek, L. M. L. Fransson, K. Edstrom, Journal of Power Sources, 113 (2003) 124-130.L. Trahey, J. T. Vaughey, H. H. Kung, M. M. Thackeray, Electrochem. Comm. (2008). Submitted.
12:15 PM - P3.7
Metal-Oxides-Coated LiCoO2 with Atomic Layer Deposition for All-Solid-State Lithium Secondary Batteries
Yoon Seok Jung 1 , James Trevey 1 , Andrew S. Cavanagh 2 , Markus D. Groner 2 , Steven M. George 2 , Sehee Lee 1 Show Abstract
1 Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado, United States, 2 Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado, United States
Lithium ion batteries (LIBs) using liquid electrolyte have been widely used in various portable electronic devices. Even further, their development for hybrid electric vehicles (HEV) is a current focus. However, current LIBs suffer from intrinsic safety problem concerning flammable liquid electrolyte. In this regard, all-solid-state Li secondary batteries can be the ultimate solution for the safety issue. As sulfide-based superionic electrolytes which represent high conductivity (10-4~10-3 S cm-1) at room temperature close to that of liquid electrolyte (10-3~10-2 S cm-1) have been developed, all-solid-state “composite” batteries which may even compete with current LIBs on the aspect of high capacity have been studied.[1,2] In spite of high capacity achieved by this “composite” configuration, the rate capability of all-solid-state lithium secondary batteries is generally very poor. Takada group reported that the high interfacial resistance between active powders and solid electrolyte (SE) is the origin of still low power density even when high-conductivity SE is used. And it was reported that the high interfacial resistance can be dramatically reduced by coating electrically insulating metal-oxide layer on the active powders. Recently we have developed an atomic layer deposition (ALD) process for the metal-oxide coating on electrode materials for all-solid-state lithium secondary batteries. Compared with the sol-gel method which requires an excess amount of precursors, solvent, and post-heat-treatment process, and etc, ALD is much more cost-effective. Furthermore, precise control of atomic scale coating thicknesses and almost 100% surface coverage contribute to the outstanding advantages of ALD. Various metal oxides including Al2O3 were coated on the LiCoO2 by ALD and with controlled coating thicknesses from ~0.2 to ~20 nm. Using xLi2S-(100-x)P2S5 (x = 70-80) glass prepared by mechanical milling as SE, all-solid-state cells In / SE / coated LiCoO2 were fabricated. Compared with bare LiCoO2, Al2O3-coated LiCoO2 represents higher reversible capacity when cycled with 75 μA cm-2 in the potential range of 2.5-4.3 V (vs. Li/Li+) at 50oC: First discharge capacity is 60 mA h g-1 and 100 mA h g-1 for bare LiCoO2 and Al2O3-coated LiCoO2, respectively. This could be attributed to the decreased interfacial resistance by introducing Al2O3 layer between SE and LiCoO2. The electrochemical response of LiCoO2 varied by metal oxide species and thickness of layers will be discussed in detail, wherein major focus will be paid on the interfacial kinetics.
References F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Adv. Mater., 17, 918 (2005). M. Tatsumisago, F. Mizuno, A. Hayashi, J. Power Sources, 159, 193 (2006). N. Ohta, K. Takada, L. Zhang, R. Ma, M. Osada, T. Sasaki, Adv. Mater., 18, 2226 (2006). S. M. George, A. W. Ott, J. W. Klaus, J. Phys. Chem., 100, 13121 (1996).
12:30 PM - P3.8
Microstructure Design of Portable Power Sources through Finite Element Analysis.
Madeleine Smith 0 , R. Edwin Garcia 0 Show Abstract
0 Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Economical and practical considerations for new technologies result in an increase of demand for electrical power sources with higher energy and power densities than those currently available. As a result, crucial material challenges arise, and material non-idealities, conceived chemistries, and inherent ohmic losses have motivated the development of new scientific methodologies and out-of-the box engineering approaches to create advanced power sources. The present paper presents a theoretical and numerical framework that spatially resolves the thermodynamic and kinetic properties of the constituent materials of rechargeable lithium ion batteries microstructures. For traditional topologies, bottleneck microstructural mechanisms and limiting rates are identified. Improved traditional and three-dimensional architectures are proposed, and the location of undesirable microstructural features are identified for real and computer-generated electrode architectures.
12:45 PM - P3.9
Aligned Carbon Nanotubes for Nanostructured Lithium-ion Battery Electrodes
Daniel Welna 1 2 , Barney Taylor 1 2 , Liangti Qu 3 , Liming Dai 3 , Michael Durstock 1 Show Abstract
1 , Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 , Universal Technology Corporation, Dayton, Ohio, United States, 3 Department of Chemical and Materials Engineering, University of Dayton, Dayton, Ohio, United States
Nanostructured electrodes offer an exciting solution to current battery technology limitations related to their size and power to weight ratio. Their increased surface area allows for two fundamental improvements over conventional flat electrode designs: 1) higher lithium uptake leading to increased storage capacity and 2) increased rate capability allowed through faster interfacial kinetics. Although nanostructured electrodes can lead to significantly increased performance, there is a need to understand how the nanoscale morphology affects electrochemical behavior. This work focuses on understanding the relationship between electrochemical behavior (capacity, cyclability, and rate capability) and nanoscale morphological control of lithium-ion battery electrode materials, specifically analyzing carbon-based electrode materials. The current state-of-the-art of lithium-ion batteries utilizes graphite as a negative electrode with a maximum theoretical specific capacity of 372 mAh/g and a practical specific capacity ranging from 150-370 mAh/g . Vertically-aligned multi-walled carbon nanotube (VAMWNT) electrodes, which were aligned in the direction of current flow, were examined in this work. By aligning the nanotubes in this manner, increased access and interfacial dynamics between lithium-ions and the interstitial spaces of the MWNTs as well as the internal and external surfaces of the MWNTs were allowed. These electrodes were able to produce a stable and reversible capacity of 650 mAh/g. Excellent rate capability was also shown as the VAMWNTs were able to achieve 500 mAh/g at a discharge rate of ~400 mA/g (1.3C). Furthermore, moderate specific capacities were obtained at very high discharge rates. X-ray diffraction and Raman spectroscopy were utilized to provide a means of relating the electrode performance characteristics to the morphological changes occurring during the lithium-ion insertion and de-insertion process.1. Linden, D.; Reddy, T.B., Handbook of Batteries, 3rd ed. McGraw-Hill Co., Inc.: New York, 2005.
P4: Solar Cells with 3D Architectures
Wednesday PM, April 15, 2009
Room 2022 (Moscone West)
2:30 PM - **P4.1
Si Wire Array Solar Cells
Harry Atwater 1 , Nathan Lewis 1 , Michael Kelzenberg 1 , Morgan Putnam 1 , Joshua Spurgeon 1 , Daniel Turner-Evans 1 , Jan Petykiewicz 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Silicon wire array solar cells are three dimensional photovoltaic absorbers which enable orthogonalization of light absorption and carrier collection in dense arrays of high aspect ratio micron-scale wires, enabling efficient collection even in relatively impure silicon material characterized by micron-scale minority carrier diffusion lengths. Silicon wire arrays are grown by vapor-liquid-solid growth on a lithographically patterned array of catalyst particles. Following growth on a crystalline (111) Si wafer, Si wire arrays are embedded in a polymethyldisiloxane (PDMS) film and can be peeled off the growth template substrate, yielding an unusual photovoltaic material: a flexible, bendable, wafer-thickness Si absorber. Following wire array peel off, the original growth template substrate can be reused for subsequent array growth without further lithography. In this paper, I will describe the observation of enhanced absorption in wire arrays relative to planar Si cells of equivalent material thickness as well as photovoltaic cell results obtained to date and directions for future cell processes and designs.
3:00 PM - P4.2
Solution Processed Cu2O/ZnO Nanowire Solar Cell.
John Joo 1 , Jacob Richardson 1 , Frederick Lange 1 2 , Evelyn Hu 1 3 Show Abstract
1 Materials Department, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California, United States, 3 Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
The integration of nanostructured materials in solar cell design offers new opportunities for device optimization. The planar solar cell structure has been the conventional structure; but the needs for both large area photon collection and efficient carrier collection provide a challenge to device performance, even when the absorption length and the minority carrier diffusion length of the semiconductor are similar in magnitude. This situation is aggravated for materials with short minority carrier diffusion lengths. Solar cell structures based on vertically aligned arrays of semiconductor nanowires do not require the minority carrier diffusion length to be as large as the absorption length, because they separate the absorption direction from the carrier collection direction. Consequently, the nanowire structure provides a natural means of device optimization and opens the possibility of using low cost semiconductors not traditionally used in solar cells. We report on the fabrication and characterization of a solar cell created by electrodepositing Cu2O on an array of ZnO nanowires grown in water at 90°C. The aqueously grown ZnO is n-type with a band gap of 3.4 eV and serves as the window layer; the electrodeposited Cu2O is p-type with a smaller band gap of 2.0 eV and serves as the absorber layer. The bottom up growth of the ZnO nanowire array eliminates the need for templates or lithography, keeping the process simple and inexpensive. We examine the growth of Cu2O on the nanostructured ZnO and show complete filling and planarization of the ZnO nanowire array. I-V measurements of these devices are compared to similar devices made using planar ZnO. Initial results indicate Voc of 0.13 V, Jsc of 1.1 mA/cm2, and FF of 43% for the nanostructured device tested using an Oriel solar simulator with Xe lamp. These results demonstrate a pathway toward low cost solar cells.
3:15 PM - P4.3
Quantum Dot Sensitized TiO2 Nanotube Solar Cells.
Jun Wang 1 , Jun Xu 1 , Zhiqun Lin 1 Show Abstract
1 Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
We report the rational design and engineering of quantum dot sensitized nanostructured solar cells (QDSNSC) by infiltrating quantum dots (e.g., CdSe QDs) into highly ordered anodic TiO2 nanotubes. A water soluble bifunctional group (i.e., dithiocarbamate) capped CdSe QDs was synthesized via a biphasic ligand exchange. The -COOH group at the dithiocarbamate-functionalized CdSe QDs surface reacts with the -OH group at the TiO2 nanotubes surface, thereby facilitating electronic interaction between the electron donor (i.e., CdSe) and electron accepter (i.e., TiO2). Oxygen plasma treatment was carried out on the TiO2 nanotubes for improving coverage of CdSe quantum dots by saturating the –OH group on the TiO2 nanotube walls. The effects of the size of CdSe QDs, the aspect ratio of TiO2 nanotubes, and the oxygen plasma treatment on the performance of resulting QDSNSC were studied.
3:30 PM - P4.4
Vertical Phase Separation in Poly(3-hexylthiophene):Fullerene Derivative Blends and its Advantage for Inverted Structure Solar Cells.
Li-Min Chen 1 , Zheng Xu 1 , Guanwen Yang 1 , Jianhui Hou 2 , Yue Wu 2 , Gang Li 2 , Yang Yang 1 Show Abstract
1 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 , Solarmer Energy Inc., El Monte, California, United States
We utilized a unique method, using water to lift-off the polymer blend films to investigate the buried polymer/substrate interface without altering the film properties. The PCBM/P3HT ratios were evaluated using carbon/sulfur (C/S) atomic ratios obtained from XPS analysis, which revealed spontaneous vertical phase separation upon spin-coating, as well as enrichment of the donor and acceptor components at the top and bottom surfaces, respectively. This vertical phase separation is attributed to the surface energy differences of P3HT and PCBM, as well as induced dipole-dipole interactions between PCBM and the substrates. This inhomogeneous phase distribution, with P3HT-rich layer at the metal electrode and PCBM-rich at the ITO side is contrary to the ideal morphology for the regular device structure; however, polymer films with this vertical phase separated morphology are beneficial for charge collection in the inverted configuration. By varying the substrate surface property, the distribution of the donor and acceptor materials can be manipulated, and the PCBM concentrations at the polymer/Cs2CO3 interfaces are higher than at the polymer/glass or polymer/PEDOT interfaces, indicating enhanced vertical segregation on Cs2CO3-coated ITO substrates. Our work unveiled the advantage of the inverted configuration, validated by the I-V characteristics and EQE results, with 4.2% PCE and EQE maximum at 72%. This device performance is comparable to the regular structure based on the same system, and provides a promising alternative for structure design flexibility in tandem cell application.
3:45 PM - P4.5
Three-Dimensionally Configured Dye-Sensitized Solar Cells.
Cyrus Rustomji 1 2 , Christine Cobb 2 , Michael Tauber 1 2 , Sungho Jin 2 Show Abstract
1 Chemistry and Biochemistry, University of California at San Diego, San Diego, California, United States, 2 Materials Science and Engineering, University of California at San Diego, La Jolla, California, United States
Dye sensitized solar cells (DSSC) based on a mesoporous TiO2 titanium dioxide nanoparticle layer and ruthenium sensitizers have been the subject of intense research as an alternative to traditional solid-state cells. Modest increases in efficiency have been demonstrated in recent years, however the open circuit voltage (~0.7 V) , short circuit current (~20mA/cm2), fill-factor (~0.70) and overall efficiency (~11%) of the best DSSC’s are all significantly below desired values in standard AM 1.5 solar conditions. Our research focuses on a new cell that is based upon ~10 micrometer long and 100 nm diameter TiO2 nanotubes that are arranged in a three-dimensional architecture, exhibiting orders of magnitude larger surface reaction area. Unlike prior designs that utilize vertically aligned tubes[2,3], our cell is comprised of nanotubes that are arranged radially from a grid of fine titanium wires. The tubes and Ti metal substrate to which the nanotubes have solid contacts provide a direct and low-resistance conduit for capture of electrons at the anode, while still maintaining very high surface area for covalently binding inorganic or organic sensitizers. Light can be effectively absorbed by our cell, even into the near-IR region, because the path-length of the TiO2 nanotube layer is significantly greater than the thickness of the mesoporous TiO2 layer of traditional DSSCs. Furthermore, the new architecture does not require any transparent conductive glass at either electrode, which results in an increase in efficiency and simplification of our cell that is not possible in the traditional design. Based on our preliminary measurements and estimations, it is anticipated that the nanotube 3-D architecture explored in our work, especially with multilayered or hierarchically vertical arrayed configurations, will lead to cell efficiencies that will exceed the current record of 11% that has been demonstrated for the best DSSC to date.  Mohammad K. Nazeeruddin, Michael Gratzel, J Am. Chem Soc. 2001, 123: 1613-1624 Karthik Shankar, Gopal K Mor, Haripriya E Prakasam, Craig A Grimes, Nanotechnology 2007, 18, 065707 Kai Zhu, Nathan R Neale, Alexander Miedaner, Arthur J Frank, Nano Letters, 2007, v.7, 1, 69-74
4:30 PM - **P4.6
Silicon Microcell Photovoltaics
John Rogers 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States
Silicon, in amorphous or various crystalline forms, is used in >90% of all installed photovoltaic (PV) capacity. The high natural abundance of silicon, together with the excellent reliability and good efficiency of solar cells made with it suggest its continued use, on massive scales, for the foreseeable future. In this talk, we describe the use of this relatively old material in the form of micro-cells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable levels of transparency and ultra-low profile micro-optic concentrator designs. Detailed studies of the processes for creating and manipulating such micro-cells, together with investigations of the electrical, mechanical and optical characteristics of several types of modules that incorporate them illuminate the key aspects. The results represent strategies that might expand the application possibilities for monocrystalline silicon PV.
5:00 PM - P4.7
Improved Short-Circuit Current in Hybrid Photovoltaics by Lithium Doping of Zinc Oxide.
Matthew Lloyd 1 , Yun-ju Lee 1 , Julia Hsu 1 Show Abstract
1 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States
While ZnO/polymer heterojunctions offer a route to low temperature, solution processed nanostructured photovoltaics, these devices typically suffer from very low efficiencies stemming from low short-circuit current (Jsc). Typically, hybrid devices utilize ZnO films or nanorod arrays that are not intentionally doped. Consequently, there is no control over the conductivity or the position of the Fermi-energy level in ZnO. By loading sol-gel derived ZnO films with 5% (atomic) lithium, we observe a 45% increase in Jsc compared to undoped ZnO/poly-3-hexylthiophene (P3HT) photovoltaic devices. Kelvin probe results show a monotonic decrease in the work function of ZnO as a function of lithium concentration. This suggests that the role of lithium might be to decrease the barrier to charge extraction between the ZnO electron acceptor layer and the ITO electrode, leading to increased photocurrent. We also investigate the relative position of a thin lithium doped layer within the device architecture. Compared to undoped ZnO/P3HT, we a significant increase in efficiency when the ZnO/polymer interface is doped with lithium; however, the improvement is even greater when the ITO/ZnO interface is doped with lithium. To further understand the origin of the performance enhancement, we characterize the chemical and electrical properties of lithium doping in thin films of ZnO. Experiments are also underway to extend lithium doping to 3D nanostructured solar cells. Preliminary results suggest that the performance of ZnO nanorod based devices is enhanced when the ITO and ZnO seed layer interface is doped with lithium. Incorporation of lithium doped ZnO nanorods as the electron acceptor will also be investigated. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
5:15 PM - P4.8
Wafer-scale Photovoltaic Application of Radial p-n Junction Silicon Nanowire Arrays Prepared by Metal-induced Electroless Etching with Plasma Conformal Doping.
Jin-Young Jung 1 , Sang-Won Jee 1 , Han-Don Um 1 , Jong-Yeoul Ji 2 , Chung-Tae Kim 2 , Jung-Ho Lee 1 Show Abstract
1 Chemical Engineering, Hanyang University, Ansan, Kyounggi, Korea (the Republic of), 2 , ADP engineering CO., LTP, Seongnam Korea (the Republic of)
Silicon nanowire (SiNW)-based solar cells offer new opportunities for addressing the efficiency and cost issues of photovoltaic since one-dimensional (1D) nanowires with a p-n junction in a radial direction can allow for separation of light absorption and carrier extraction into two orthogonal directions, thus giving rise to high efficiency. Since SiNWs of controlled properties, such as diameters, electronic properties, crystallographic orientation and location, are needed for solar cell application, much effort has been devoted to the fabrication of SiNWs by various techniques. In general, the bottom-up method for synthesis of SiNWs is vapor-liquid-solid (VLS) growth using Au as catalysts, because the Au-catalyzed-grown SiNWs have a superior crystalline morphology and good controllability. But VLS growth method is not easy to control the doping gradient and crystallographic orientation.Here, we report the fabrication of wafer-scale arrays of p-type SiNWs directly from heavily doped silicon wafers prepared by metal-induced electroless etching, and then n-type ultra-shallow junction can be formed by using plasma ion doping for making a radial p-n junction of nanowire solar cells. Metal-induced etching has the several advantages of simplicity, low-cost, wafer-scale fabrication of SiNWs without the need of doping. However, it is difficult to control over a wire diameter and spacing due to random distribution of metal. To overcome this hurdle, we have developed the metal-induced etching combined with nanopatterning techniques such as block copolymer assembly and anodic aluminum oxide (AAO) template which can be successfully applied for fabrication of vertically aligned SiNWs. Then, we have investigated the electrical characteristics and a junction depth of ultra-shallow junctions conformally formed by plasma doping inside the SiNWs surface. Phosphine (PH3) plasma ion doping process is used to convert the p+ doped SiNWs surface to the n+ SiNWs surface. Electron holography and scanning capacitance microscopy (SCM) were adopted to obtain two-dimensional doping profile of radial p-n junction SiNWs.
5:30 PM - P4.9
Silicon Nanowires on Glass Catalyzed by Gold Colloids for Thin Film Solar Cells: Growth, Materials Integration and Characterization.
Gerald Broenstrup 1 , Vladimir Sivakov 1 , Thomas Stelzner 1 , Samuel Hoffmann 3 , Christoph Niederberger 3 , Johann Michler 3 , Silke Christiansen 1 2 Show Abstract
1 , Institute of Photonic Technology, Jena Germany, 3 , Empa - Swiss Federal Laboratories for Materials Testing and Research, Thun Switzerland, 2 , Max-Planck-Institute of Microstructure Physics, Halle Germany
Making use of silicon nanowires (SiNWs) in solar cells essentially requires their integration onto cheap glass substrates. This is realized by bottom-up gold nanoparticle (colloids between 10nm and 250nm or droplets from annealing thin gold layers) catalyzed vapour-liquid-solid (VLS) growth of SiNWs from silane by chemical vapour deposition (CVD) at glass compatible temperatures (<600°C).The SiNWs grow in different directions on the glass substrate favouring, but not exclusively, low index growth directions (preferably <111>, <110>, <112>) as determined by electron backscatter diffraction (EBSD) measurements in a scanning electron microscope (SEM). Optical characterization of SiNW carpets grown with different gold colloids using a UV-vis/NIR spectrometer equipped with an integrating sphere suggests that the highest absorption occurs for large SiNW diameters. Passivation of SiNW surfaces is successfully carried out by atomic layer deposition (ALD) of aluminium oxide. Furthermore, passivation and contact formation using transparent conductive oxide is tested by ALD of doped ZnO:Al layers.Axial and radial SiNW doping strategies using co-doping during CVD, ion implantation and thermal activation of dopants as well as dopant diffusion from highly doped spin-on suspensions will be presented, and I-V-measurements will be carried out for single SiNWs in a scanning electron microscope (SEM) and for SiNW ensembles in a probe station.First NW solar cell parameters of contacted SiNW ensembles as well as of single SiNWs will be presented.Additional characterization of structural properties of these SiNWs is carried out using transmission electron microscopy.
5:45 PM - P4.10
Crystalline-amorphous Si Core-shell Nanowires for Energy Storage.
Li-Feng Cui 1 , Riccardo Ruffo 2 , Candace Chan 3 , Hailin Peng 1 , Yi Cui 1 Show Abstract
1 Materials Science & Engineering, Stanford University, Stanford, California, United States, 2 Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Milan Italy, 3 Department of Chemistry, Stanford University, Stanford, California, United States
Silicon is an attractive alloy-type anode material for lithium ion batteries because of its highest known capacity (4,200 mAh/g). However silicon’s large volume change upon lithium insertion and extraction, which causes pulverization and capacity fading, has limited its applications. Designing nanoscale hierarchical structures is a novel approach to address the issues associated with the large volume change. In this letter we introduce a core-shell design of silicon nanowires for high power and long life lithium battery electrodes. Silicon crystalline-amorphous core-shell nanowires were grown directly on stainless steel current collectors by a simple one-step synthesis. Amorphous Si shells instead of crystalline Si cores can be selected to be electrochemically active due to the difference of their lithiation potentials. Therefore, crystalline Si cores function as a stable mechanical support and an efficient electrical conducting pathway while amorphous shells store Li ions. We demonstrate here that these core-shell nanowires have high charge storage capacity (~1000 mAh/g, 3 times of carbon) with ~90% capacity retention over 100 cycles. They also show excellent electrochemical performance at high current charging and discharging (6.8 A/g, ~20 times of carbon at 1 hour rate).