Harry Radousky Lawrence Livermore National Laboratory
Univ. of California-Davis
James Holbery Pacific Northwest National Laboratory
Bob O'Handley Massachusetts Institute of Technology
Nicholas Kioussis California State University-Northridge
LL6: Energy Harvesting, All Topics II
Thursday AM, March 27, 2008
Room 2016 (Moscone West)
9:30 AM - **LL6.1
Nanogenerators for Harvesting Mechanical Energy.
Zhong Wang 1 Show Abstract
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Developing novel technologies for wireless nanodevices and nanosystems are of critical importance for in-situ, real-time and implantable biosensing, biomedical monitoring and biodetection. An implanted wireless biosensor requires a power source, which may be provided directly or indirectly by charging of a battery. It is highly desired for wireless devices and even required for implanted biomedical devices to be self-powered without using battery. Therefore, it is essential to explore innovative nanotechnologies for converting mechanical energy (such as body movement, muscle stretching), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as body fluid and blood flow) into electric energy that will be used to power nanodevices without using battery. We have demonstrated an innovative approach for converting nano-scale mechanical energy into electric energy by piezoelectric zinc oxide nanowire (NW) arrays. By deflecting the aligned NWs using a conductive atomic force microscopy (AFM) tip in contact mode, the energy that was first created by the deflection force and later converted into electricity by piezoelectric effect has been measured for demonstrating nano-scale power generator [1, 2]. The operation mechanism of the electric generator relies on the unique coupling of piezoelectric and semiconducting dual properties of ZnO as well as the elegant rectifying function of the Schottky barrier formed between the metal tip and the NW . Based on this mechanism, we have recently developed DC nanogenerator driven by ultrasonic wave in bio-fluid, which is a gigantic step towards applications in practice [4,5]. This presentation will introduce the fundamental principle of nanogenerator and its potential applications. Finally, a new field is introduced on nano-piezotronics, which uses piezoelectric-semiconducting coupled property for fabricating novel and unique electronic devices and components . Z.L. Wang and J.H. Song “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, 312, 242 (2006). P.X. Gao, J.H. Song, J. Liu and Z.L. Wang “Nanowire Nanogenerators on Plastic Substrates as Flexible Power Source”, Adv. Mater., 19, 67 (2007). Yifan Gao and Z.L. Wang “Electrostatic Potential in a Bent Piezoelectric Nanowire – The Fundamental Theory of Nanogenerator and Nanopiezotronics”, Nano Letters, 7, 2499 (2007). X.D. Wang, J.H. Song J. Liu, and Z.L. Wang “Direct current nanogenerator driven by ultrasonic wave”, Science, 316, 102 (2007). Xudong Wang, Jin Liu, Jinhui Song, Zhong Lin Wang “Integrated Nanogenerators in Bio-Fluid”, Nano Letters, 7, 2475 (2007). Z.L. Wang “Nano-piezotronics”, Adv. Mater., 19, 889 (2007). For details visit: http://www.nanoscience.gatech.edu/zlwang/
10:00 AM - **LL6.2
Bi2Te3 Nanowire Composites for Thermoelectric Devices.
Kalapi Biswas 1 2 , Timothy Sands 1 2 3 Show Abstract
1 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 2 Birck Nanotechnology Center, Purdue University, West Lafayette , Indiana, United States, 3 School of Electrical and Computer Science Engineering, Purdue University, West Lafayette , Indiana, United States
10:30 AM - LL6.3
The Power Behavior of a Resonant Frequency Tunable Piezoelectric Vibration Harvester.
John Youngsman 1 , Dylan Morris 1 , Michael Anderson 2 , David Bahr 1 Show Abstract
1 Mechanical and Materials Engineering, Wahington State University, Pullman, Washington, United States, 2 Mechanical Engineering, University of Idaho, Moscow, Idaho, United States
10:45 AM - LL6:Mech/Mag
11:15 AM - LL6.4
Vibrational Energy Scavenging: Ferroelectric Domain Configurations on Strained Benders.
Padraic Shafer 1 , Lindsay Miller 2 , Pu Yu 3 , Nathan Emley 4 , Peter Minor 2 , Eli Leland 2 , Daniel Steingart 1 , Paul Wright 2 , R. Ramesh 1 3 Show Abstract
1 Materials Science & Engineering, University of California, Berkeley, California, United States, 2 Mechanical Engineering, University of California, Berkeley, California, United States, 3 Physics, University of California, Berkeley, California, United States, 4 Electrical Engineering & Computer Science, University of California, Berkeley, California, United States
Ferroelectric materials provide the high piezoelectric coupling needed to efficiently produce electric power from scavenged vibrational energy. However without proper processing they tend to form nanoscale domains of polarized charge that are not aligned with each other, thus reducing the efficacy of piezoelectric energy conversion. Misalignment of domains is further exacerbated by applied stress (or strain) such as that experienced by vibrational scavengers during operation. A method of imaging these electromechanical domains in the dynamic environment inherent to vibrational energy scavenging is presented. Furthermore, such a characterization technique provides feedback to device processing for optimizing the efficiency of energy collection.
Piezoelectric force microscopy paired with magnetostatic actuation of the piezoelectric energy scavenger (henceforth, the “piezobender”) is used to image ferroelectric domain patterns in the active layer as a function of the piezobender’s deflection. We investigate the evolution of both written and as-grown domain patterns in epitaxial ferroelectric films of lead zirconate-titanate [Pb(Zr1-x,Tix)O3 or PZT] on silicon cantilevers as the piezobender cycles through its operational range of motion. The emphasis of this study is on identifying the critical factors that enhance the stability of PZT domain walls in piezobenders without significantly decreasing the macroscopic polarization of the film. We consider processing parameters of the PZT film growth (e.g., oxygen partial pressure, temperature, or epitaxial strain), film thickness, size and orientation of the ferroelectric domains, and piezobender geometry in order to increase the mechanical-to-electrical energy conversion and extend the lifetime of the device.
11:30 AM - LL6.5
Improving Bandwidth of Piezoelectric Energy Harvester.
Yi-Chung Shu 1 , I-Ching Lien 1 Show Abstract
1 Institute of Applied Mechanics, National Taiwan University , Taipei Taiwan
11:45 AM - LL6.6
The Biobattery – Harvesting Energy from the Biological Ion Gradient.
Jian Xu 1 , David LaVan 2 Show Abstract
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States, 2 Mechanical Engineering, Yale University, New Haven, Connecticut, United States
The electric eel is able to produce a powerful electrical pulse from the transmembrane ion gradients of the cells of its electric organ. This energy conversion is realized by concerted stimulus-induced ion transport. Based on the same mechanism, we have designed an artificial ion-transport system to power implanted devices, such as neural prostheses. As a guide to the design of a future nanoscale system, a mathematic model has been created of energy transduction in these polarized electrogenic cells. The model tracks the conversion of transmembrane ion concentration gradients into electrical energy in the form of an action potential. The model was used to quantify a number of unknown channel parameters for the electrocyte of the electric eel, extrapolating from measured action potentials. With this model, the design of biomimetic system was numerically optimized, and the models of the optimally designed cells are compared to the models and experimental action potentials from natural electrocytes and a model of an axon. We have shown that the design numerically optimized for maximum power output density has similar behavior to the natural electrocyte, but a comparison of the two models shows that the optimally designed cell has 11% greater power output density and 12% higher energy conversion efficiency than the natural electrocyte. These findings are the foundation for the further design and fabrication of artificial cells.
12:00 PM - LL6.7
Micro Power Generator Based on Piezoelectric PZT Thin Film.
Paul Muralt 1 , Marcin Marzencki 2 , Brahim Belgacem 1 , Skandar Basrour 2 , Florian Calame 1 , Mikaël Colin 3 Show Abstract
1 Materials Science, TIMA Laboratory, Grenoble France, 2 , TIMA Laboratory, Grenoble France, 3 , MEMSCAP, Crolles France
12:15 PM - LL6.8
Hierarchically-Structured ZnO Nanoparticle Film for Dye-Sensitized Solar Cells.
Qifeng Zhang 1 , Samson Jenekhe 1 , Guozhong Cao 1 Show Abstract
1 Material Science and Engineering, University of Washington, Seattle, Washington, United States
12:30 PM - LL6.9
Limits to the Efficiency of Dye Sensitized and Organic Solar Cells: The Role of Interfacial Bimolecular Recombination.
Brian O'Regan 1 , Piers Barnes 1 , Sara Koops 1 , Chris Shuttle 1 , Andrea Maurano 1 , Ana Morandiera-Lopez 1 , James Durrant 1 , Assaf Anderson 1 Show Abstract
1 Chemisty, Imperial College London, London United Kingdom
In photovoltaic cells, bimolecular recombination "competes" with other loss mechanisms, such as shunts and sub-optimum charge separation (e.g. geminate recombination). It is important to understand which of these loss mechanisms dominates a given cell parameter (Voc, Jsc, FF) in order to have a reasonable strategy for improvement. Perhaps even more important, it is critical to examine the loss routes when new materials fail to preform as desired, in order to develop design rules for future efforts. Concurrent charge density and charge lifetime measurements, along with transient absorption, can quickly and convincingly determining the major contribution to losses for both existing and new cell materials. This paper will discuss two general findings from this type of measurement. The "holy grail" of dye sensitized cells is a new dye that has strong absorption between 700 and 850 nm and yet functions as well as present best dyes which are limited to λ<750 nm. The number of dyes designed, synthesized and tested now runs into the thousands, representing a very large research effort. Despite this effort ‘design rules’ for optimized dye function remain poorly understood. By comparing charge density and recombination rates, and by examination of the literature, we have come to the conclusion that many, if not most, new dyes are catalysts for the recombination of the electron with the electrolyte. This catalysis of the recombination can strongly decrease the Voc and fill factor relative to the standard, N3, which does not catalyze recombination. We will present new recombination measurements covering a range of dyes and dye classes, with the hope that motifs that encourage and discourage recombination will be identifiable. For polymer/PCBM cells it has been observed that that the "1 sun" Voc is always ~300mV lower than the band offset, independent of the polymer used. There is still debate concerning which loss route is the major contributor to this observation. By measurement of the charge density across the IV, and by measurement of the charge density at Voc for a range of Vocs we have put together a model of bimolecular recombination losses as a function of voltage. The results show that the main determinate of the "1 sun" Voc and FF is bimolecular recombination. Shunts and geminate recombination appear to be less important. The 300mV rule of thumb indicates that the recombination losses for a given Fermi level position are not dependent on the polymer used. This might seem to show that there is little hope for improvement. However, recombination losses are the product of the bimolecular rate constant and the charge. We plan further measurements to elucidate whether both these values are independent of the polymer, or whether there is scope to pick polymer motifs which decrease the rate constant, and others which decrease the charge.
12:45 PM - LL6.10
Optimization of Inorganic Nanostructured Solar Cell Design Using Drift-Diffusion and Hydrodynamic Simulations.
Evan Pickett 1 , Jeffrey King 1 , Michael Rowell 1 , Bruce Clemens 1 , Michael McGehee 1 , James Harris 1 , Stacey Bent 1 Show Abstract
1 , Stanford University, Stanford, California, United States
LL7: Solar II
Thursday PM, March 27, 2008
Room 2016 (Moscone West)
2:30 PM - LL7.1
CdS-Modified TiO2 Nanocrystalline Photoelectrode for Dye-Sensitized Solar Cell and Hydrogen Generation Applications.
Yuh-Lang Lee 1 , Yaw-Chai Yang 1 Show Abstract
1 Chemical Engineering Department, National Cheng Kung University, Tainan Taiwan
Cadmium sulfide (CdS) quantum dots (QDs) were used as a sensitizer to enhance the light harvest of a TiO2 film. Various assembly methods, including self-assembly monolayer (SAM), chemical bath deposition (CBD), and SAM/CBD coupling technique, were employed to assemble the CdS-QDs onto mesoscopic TiO2 films. For the SAM technique, bifunctional linker molecules including mercaptosuccinic acid (MSA), 3-mercaptopropyl trimethoxysilane (MPTMS), and 3-aminopropyl-methyl diethoxysilane (APMDS) were used to incorporate QDs onto the TiO2 surface. For the dye-sensitized solar cell (DSSC) application, MSA was proved to be an efficient linker. However, the incorporated amount and coverage ratio of CdS-QDs on the TiO2 surface is not sufficient due to the high resistance for QDs to transport in the mesopores. Therefore, CBD was introduced to replenish the incorporation of CdS-QDs. The pre-assembled CdS-QDs act as nucleation sites in the CBD process, forming a CdS nanofilm with an interfacial structure capable of inhibiting the recombination of injected electrons. An efficiency as high as 1.35% was achieved using the SAM/CBD coupling technique. In an alternative CBD process, alcohol, instead of water, was used as a solvent for the in-situ synthesis of CdS-QDs onto mesoporous TiO2 films. Due to low surface tension, the alcohol solutions have high wettability and superior penetration ability on the mesoscopic TiO2 film, leading to a well-covered CdS QDs on the surface of mesopores. The efficiency of a CdS QDs-sensitized solar cell prepared using the present method is increased to 1.84 % under the illumination of one sun (AM1.5, 100 mW/cm2). The CdS-modified TiO2 film was also used as a photoanode in a photoelectrochemical cell for water splitting. The saturated photocurrent under the visible light illumination (100 mW/cm2) was ca. 5.5 mA/cm2 at an applied potential 0.6 V versus OCP, which is equivalent to a conversion efficiency of 3.67 %. The hydrogen generation rate obtained at this condition is 95.5μmol/h.
2:45 PM - LL7.2
Computational Design of Nanostructured Materials for Photovoltaic Energy Conversion.
Qinghui Shao 1 , Denis Nika 1 , Evgenii Pokatilov 1 , Alexander Balandin 1 , Alex 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
We report on the development of new theoretical models and computer simulation tools, which allow for an efficient computational design of new nanostructured materials for the photovoltaic energy harvesting. Our models have been applied to investigate the electron (hole) energy spectra and light absorption in the quantum dot superlattices (QDS). The calculations were performed for QDS with the substantial electron (hole) wave function overlap using the one-band Hamiltonian for the electrons and 6-band Hamiltonian for the holes . It has been shown that the energy spectra of the electrons and holes in the ordered QDS are distinctively different from that in the single quantum dot or conventional quantum well superlattices. The obtained results were compared with the predictions of the simplified models for the uncoupled heavy, light, and split-off holes. The charge carrier dispersion is very sensitive to the quasi-crystallographic directions defined by the dots, which play the role of atoms in such a quantum dot supra-crystal. We found that in the ordered QDS the oscillator strength for the inter-band optical transitions can be high for a relatively wide range of the photon energies, contrary to some previous suggestions. We have used the computational design approach to determine the parameters of the photovoltaic QDS structure required for the implementation of the intermediate-band (IB) solar cells design . It has been demonstrated that the optimized IB-QDS solar cell can deliver a higher efficiency than the Schockley - Queisser limit. This work has been supported by the AFOSR contract FA9550-07-C-0059 and NASA contract NNC07CA20C. D. L. Nika, E. P. Pokatilov, Q. Shao and A. A. Balandin, Phys. Rev. B 76, 125417 (2007). Q. Shao, A. A. Balandin, A. I. Fedoseyev and M. Turowski, Appl. Phys. Lett. 91, 163503 (2007).
3:00 PM - LL7.3
Simulation of Dye and Coadsorbent Organization on Anatase Surfaces.
Stefan Adams 1 , Zhongliang Peng 1 Show Abstract
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore
3:15 PM - LL7.4
Dielectric Band Edge Enhancement of Energy Conversion Efficiency in Photonic Crystal Dye-Sensitized Solar Cell.
Chan-Hoe Yip 1 , Yet-Ming Chiang 2 , Chee-Cheong Wong 1 Show Abstract
1 AMM&NS, Singapore-MIT Alliance, Singapore Singapore, 2 DMSE, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Photonic band edge effect on the light-to-electrical energy conversion efficiency of an inverse opal dye-sensitized solar cell (DSC) was demonstrated using angle-resolved spectral measurements, theoretical analysis and numerical simulation. The DSC includes a bilayer electrode consisting of a nanocrystalline TiO2 film and an inverse opal TiO2 photonic crystal. To ensure similar volume of TiO2 nano-particles for comparison across samples, a section of inverse opal structure was collapsed in toluene/ultrasonic treatment. Thus, within a same cell, energy conversion efficiency measurement can be carried out between an inverse opal bilayer electrode and a collapsed inverse opal bilayer electrode.The inverse opal TiO2 photonic crystal acts as a back reflector of the DSC. Using angle-resolved transmission spectroscopy and angle-resolved monochromatic irradiance along Gamma-L-U direction, the incident photon-to-current conversion efficiency (IPCE) was observed to be greater in the red-edge of the photonic band edge (dielectric band). On the other hand, the IPCE at the photonic bandgap center is relatively lower than that at the dielectric band. From this observation, the photonic crystal has a relatively lower enhancement effect as a back reflector in DSC. More evidence on dielectric band enhancement is revealed when the enhancement peak shifted in accordance with a blue shift in dielectric band in the L-U direction.The highly dispersive dielectric band causes the anomalously slow group velocity of light propagation through the inverse opal. The slow group velocity, which relates to a high density of states, in turn indicates the presence of high propagating wave field in the photonic crystal’s band edges. From our numerical analysis, the electric field is greater in the vicinity of the dielectric band and this leads to the enhanced photon-dye interaction. A model of the enhanced photogeneration of current due to band edge’s effect is built using a Maxwell-Bloch Two-level atom model. From this analysis, the photogenerated current is proportional to the intensity of the propagating wave field, in agreement with our experimental result.In progress, DSC with several thickness of TiO2 inverse opal is carried out. This is to vary the strength of band edge effect on conversion efficiency of DSC. Numerical simulation will also be carried out and investigated for different thickness and orientation of inverse opal photonic crystal.
3:30 PM - LL7.5
Multi-Component Nanostructured Semiconductors for Photoelectrocatalysis - Progress Towards Efficient and Stable Solar-to-Chemical Energy Conversion.
Arnold Forman 1 , Alan Kleiman-Shwarsctein 2 , Yong-Sheng Hu 3 , Galen Stucky 1 2 , Eric McFarland 3 4 Show Abstract
1 Chemistry, University of California, Santa Barbara, California, United States, 2 Materials Dept., University of California, Santa Barbara, California, United States, 3 Chemical Engineering, University of California, Santa Barbara, California, United States, 4 Electrical & Computer Engineering, University of California, Santa Barbara, California, United States
Using photoelectrocatalysis (PEC) the energy in sunlight can be used to drive electrochemical interconversions. If PEC can be made cost effective at large scales the production of commodity chemicals such as fuels may be practical. Progress towards efficient and stable solar-to-chemical energy conversion will be discussed. The development of potentially low cost, multi-component nanostructured PEC systems using solution chemistry will be described. Co-assembly of nanostructured metal oxide semiconductor absorbers with appropriate nanoparticle electrocatalysts afford enhanced solar-to-chemical efficiencies. We will show that inert, thin layer encapsulants (such as SiO2) can be used to protect the semiconductor surface from photocorrosion while leaving portions of the metallic electrocatalysts in contact with the electrolyte and hence, still electrochemically active.
3:45 PM - LL7.6
Organic:PbS-nanocrystal:Fullerene Hybrid Photovoltaics.
Nanditha Dissanayake 1 , Ross Hatton 1 , Cristina Giusca 1 , Thierry Lutz 1 , Richard Curry 1 , Ravi Silva 1 Show Abstract
1 Advanced Technology Institute, University of Surrey, Guildford, Surrey, United Kingdom
A hybrid photovoltaic system comprising of hole transporting organic dyes (acenes), infrared sensitive PbS nanocrystals and fullerenes is demonstrated as a potential route to a high efficiency device. Using this system a comprehensive study of the fundamental photo-physical processes of exciton dissociation, through favourable donor-acceptor energy level offset, and organic-quantum dot interfacial effects, important for all hybrid optoelectronic devices is presented.These devices demonstrate energy harvesting from the ultraviolet to the near infrared (1600 nm) region covering the majority of the terrestrial solar spectrum. The material systems were selected for increased photon absorption (>100000 cm-1 absorption coefficient), enhanced exciton dissociation and for higher carrier transport (hole mobility between 1.0 - 0.1 cm2V-1s-1). The performances of the hybrid photovoltaic devices were characterized by current-voltage measurements under one Sun simulated solar irradiation. External quantum efficiency measurements were carried out to quantify the contribution from each photoactive layer. The initial un-optimized devices demonstrated greater than 10% external quantum efficiency and 0.3% monochromatic power conversion efficiency at 400 nm. It is important to note that all above measurements were carried out under the ambient atmosphere and pressure conditions. A near 1% external quantum efficiency was demonstrated at the infrared wavelengths by these devices justifying the infrared photoharvesting capability of the PbS nanocrystals. The fundamental process of photoinduced charge transfer between the organic materials and nanocrystrals were studied by photoluminescence quenching, time resolved photoluminescence and spectral response measurements. It was observed that the photoinduced electron and hole transfer between the organic-nanocrystal heterojunctions were strongly affected by the relative molecular energy level offset and interfacial dipole effects hitherto unreported at such hybrid interfaces. Energy level measurements of all materials involved in this study were directly carried out by ultraviolet photoelectron spectroscopy. It is believed that the work presented in this paper will provide crucial information facilitating the fabrication and analysis of future high performance hybrid photovoltaics.
4:00 PM - LL7:Solar
4:45 PM - LL7.8
Synthesis of Solar Cells using High-Density TiO2 Nanowire Heterostructures.
Jung-Chul Lee 1 , Kang-Jin Kim 2 , Yun-Mo Sung 1 Show Abstract
1 Materials Science & Engineering, Korea University, Seoul Korea (the Republic of), 2 Chemistry, Korea University, Seoul Korea (the Republic of)
Over the past few decades, a plethora of research has been done concerning solar cells based on nanostructured materials. Among these materials, dye-sensitized solar cells (DSCs), which are a combination of organic dyes and inorganic (TiO2) nanoparticles, have been most widely studied due to their low-cost production process. However, DSCs have critical problems for practical applications. The first problem is their low efficiency, which results from the agglomeration of nanoparticles. These agglomerations reduce the surface areas, decreasing the amount of dye absorbed. As a consequence, the probability of trapping electrons increases due to the long path for electron transport. The second problem with DSCs made from TiO2 nanoparticles is their poor long-term stability. The use of an organic dye and a liquid electrolyte has been known to be very unstable in long-term photovoltaic reactions. For this reason, we replace the TiO2 nanoparticles and Ru-organic dye frequently used in DSCs with TiO2 nanowires and CdSe nanocrystals, respectively. In our previous research high-density, single-crystalline TiO2 nanowires with a diameter of ~20-50 nm were successfully grown on quartz, sapphire and Ti substrates at a remarkably low temperature (700 oC) by a vapor-liquid-solid (VLS) mechanism. We combined CdSe semiconductor nanocrystals with single-crystal TiO2 nanowires to demonstrate quantum dot-sensitized solar cells. The process for fabricating TiO2/CdSe/CuSCN solar cells is as follows. The first step is synthesis of an array of TiO2 nanowires on quartz glass substrates coated with conductive SnO2:F, which will act as a negative electrode. The second step is attachment of CdSe nanocrystals to the nanowire surface via dip coating. The third step is injection of p-type solid CuSCN between the nanowires via electrochemical deposition or solution casting. The fourth step is coating the Pt catalyst by sputtering. This Pt layer can form an ohmic contact with the SnO2:F, which acts as an opposite electrode. When the DSCs are illuminated by visible light, the excited electrons from the CdSe quantum dots are injected to the surface TiO2 nanowires across the nanocrystal-nanowire interface. These TiO2 heterostructured nanowires reveal a high potential to be used for further development of high-efficiency inorganic-dye sensitized solar cells.
5:00 PM - LL7.9
Transport Properties and Surface Defect Structures of Anatase TiO2 Nanotubes: Toward High Efficient Dye-Sensitized Solar Cells.
Changdeuck Bae 2 1 , Sihyeong Kim 2 1 , Hyunjun Yoo 2 1 , Jooho Moon 3 , Hyunjung Shin 2 1 Show Abstract
2 National Research Lab for Nanotubular Structures of Oxides, Kookmin University, Seoul Korea (the Republic of), 1 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of), 3 Department of Advanced Materials Engineering, Yonsei University, Seoul Korea (the Republic of)
5:15 PM - LL7.10
Single Carrier Resonant Tunneling Design for Improving Carrier Collection in Quantum Confined Solar Cells.
Andenet Alemu 1 , Alex Freundlich 1 Show Abstract
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Promising nanostructured device concepts with staggering theoretical efficiencies where quantum confined states are embedded in the intrinsic region of conventional p-i-n solar cells have been proposed. However, practical realizations remain inefficient as these devices suffer from an inherent difficulty in the extraction of photo-generated carriers from the confined states. Within the framework of a "single particle in the box" theory, such shortcomings could be addressed by the use of resonant quantum tunneling designs that can expedite carrier escape. Nonetheless, in material systems studied thus far, the implementation of such design becomes elusive as band offsets between the nanostructure and the host material are distributed between the conduction and valence band leading to the confinement of both holes and electrons (i.e. two particle problem). Our studies of such p-i-n Multi-Quantum Well (MQW) solar cells, only differing by their MQW region composition and geometry, have shown a strong dependence of device performance on quantum wells composition and thickness. Leveraging on the special property of dilute nitrides and using a carefully chosen material system and device design we show the possibility of circumventing this problem by separating the optimization of the valence and conduction band and reducing the issue to a single particle problem. Band structure calculations including strain effects, band anti-crossing models and transfer matrix methods are used to theoretically demonstrate optimum conditions for enhanced vertical transport. High electron tunneling escape probability, together with a free movement of quasi-3 D holes, is predicted to result in enhanced PV device performance. Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive properties, ideal for PV application.
5:30 PM - LL7.11
Development of a High-Temperature, Oxidation-Resistant, Solar-Selective Coating for Concentrating Solar Power Parabolic Trough Receivers.
Cheryl Kennedy 1 Show Abstract
1 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States
Increasing the operating temperature of parabolic trough solar fields from 400°C to greater than 450°C can increase the overall solar-to-electricity efficiency and reduce the cost of electricity from parabolic trough power plants. Current solar-selective coatings do not have the stability and performance necessary to move to higher operating temperatures. The objective is to develop new, more-efficient solar-selective coatings with both high solar absorptance (α>0.96) and low thermal emittance (ε<0.07 at 400°C) that are thermally stable above 450°C, ideally in air, with improved durability and manufacturability, and reduced cost. Using computer-aided optical design software, a multilayer solar-selective coating was modeled with α=0.959 and ε=0.061 at 400°C, composed of materials with high-temperature stability. This exceeds the specified goal by about 1% overall, because 1% in emittance equates to about 1.2% in absorptance. Improvements are expected in the future by incorporating hydrogen barrier coatings, improved antireflective coatings, cermets, and textured surfaces; however, trade-offs exist between optimizing both low emittance and high absorptance. The key issue is depositing the modeled coating. Ion-beam-assisted (IBAD) electron-beam (e-beam) co-deposition was used to deposit the individual layers and the prototype modeled solar-selective multilayer structure because of its versatility and lower material costs. Dielectrics can be evaporated directly or reactively, and the ion gun can be used to improve the quality, composition, and density of the coating. Experimental work has focused on modeling high-temperature, solar-selective coatings; depositing the individual layers and modeled coatings; measuring the optical, thermal, morphology, and compositional properties and using the data to validate the modeled and deposited properties; re-optimizing the coating; and testing the coating performance and durability. The progress toward developing a durable advanced solar-selective coating will be described.