Ho Nyung Lee Oak Ridge National Laboratory
Akira Ohtomo Tokyo Institute of Technology
Gervasi Herranz Institut de Ciencia de Materials de Barcelona ICMAB-CSIC
John Perkins National Renewable Energy Laboratory
Oak Ridge National Laboratory
Rocky Mountain Vacuum Tech Inc
G1: Transparent Conducting Oxides I
Tuesday AM, April 26, 2011
Room 2006 (Moscone West)
9:30 AM - **G1.1
Fundamental Properties and Applications of TiO2-based Transparent Conducting Thin Films.
Tetsuya Hasegawa 1 2 Show Abstract
1 Department of Chemistry, University of Tokyo, Tokyo Japan, 2 , Kanagawa Academy of Science and Technology, Kasawaki, Kanagawa, Japan
Transparent conducting oxides (TCOs) are key materials in opto-electronic devices. Recently, antase TiO2 doped with Nb (TNO) has joined the TCO family consisting of ITO, ZnO, SnO2: epitaxial films of TNO exhibit low resistivity of 2x10-4 Ohm cm and high transparency of >95% in the visible region. Here, we discuss the mechanism of transparent conductivity in TNO, focusing on the roles of Nb and excess oxygen. From first principles band calculations, it has been shown that doped Nb atoms hybridize very well with the Ti 3d-nature conduction band, resulting in high ionization efficiency. Meanwhile, excess oxygen atoms occupy interstitial sites form deep p-type levels, which compensate carriers and thus increase the resistivity.To establish TNO as a practical TCO material, it is needed to fabricate TNO films on conventional substrates, such as glass and plastics. However, the TNO films directly grown on such substrates are in polycrystalline form with very high resistivity of ~0.1 Ohm cm. We recently develop a new process via crystallization of amorphous into anatase phase. The polycrystalline TNO films obtained by this process show a minimum resistivity of the order of 10-4 Ohm cm.Optical measurements revealed that TNO has relatively large electron mass anisotropy: the electron mass along the c-axis is 3-6 times larger than that along the a-axis. This indicates that control of crystallographic orientation is of crucial importance for achieving high conductivity in polycrystalline TNO films. We found that inorganic nanosheets, such as Ca2Nb3O10, could serve as seed layers on which TNO with c-axis orientation grows.Besides high conductivity and transparency, TNO features unique properties, such as high chemical stability, particularly in reducing conditions, and high refractive. Our recently attempts to develop new applications of TNO will also be presented.
10:00 AM - G1.2
Anisotropy of Electrochemical Properties of TiO2 (Anatase) in Orientations (101) and (001).
Ladislav Kavan 1 Show Abstract
1 , J. Heyrovsky Institute of Physical Chemistry, Prague 8 Czech Republic
The (101) face is dominating on the usual TiO2 (anatase) materials. Hence, great deal of earlier studies of polycrystalline anatase electrodes is addressing the effects occurring virtually on this face only. Previous experiments on single crystal electrodes have shown that the (001) face had more negative flatband potential, and this conclusion was recently confirmed also for polycrystalline electrodes. Polycrystalline TiO2 anatase with predominant (001) face was prepared hydrothermally in F-containing medium in the form of rectangular platelets with a thickness of about 5 to 8 nm. Cyclic voltammetry and chronoamperometry of Li insertion proved its higher activity compared to that of reference nanomaterials materials with dominating (101) face. Lithium diffusion coefficient calculated from cyclic voltammetry was by one order of magnitude higher. The same tendency, although not so large difference, exhibited the chronoamperometric diffusion coefficients and rate constants. The enhanced activity of anatase (001) face for Li-insertion stems from synergic contributions of faster interfacial charge-transfer at this surface and facile Li transport within a more open structure of the anatase lattice in the direction parallel to the c-axis. The anisotropy of Li+-transport normal to the (101) and (001) faces was a consequence of different numbers the Li+ hopping events between pseudo-octahedral positions in the anatase lattice. The presented study represents an upgrade of previously published results on single crystal anatase electrode and on polycrystalline materials as well. The main new finding of this work is the proof that the crystal morphology, rather than F-termination of the surface is decisive for the improved Li-insertion behavior of nanocrystalline materials.
10:15 AM - G1.3
Anomalous X-ray Diffraction Studies of Novel Transparent Conducting Spinel Oxides.
Joanna Bettinger 1 , Yezhou Shi 1 2 , Nicola Perry 3 , Arpun Nagaraja 3 , Thomas Mason 3 , John Perkins 4 , David Ginley 4 , Tula Paudel 4 , Alex Zunger 4 , Michael Toney 1 Show Abstract
1 1Stanford Synchrotron Radiation Lightsource , SLAC National Accelerator Laboratory, Menlo Park, California, United States, 2 Materials Science and Engineering , Stanford University, Stanford, California, United States, 3 Materials Science and Engineering , Northwestern University, Evanston, Illinois, United States, 4 National Center for Photovoltaics , National Renewable Energy Laboratory, Golden, Colorado, United States
The development of new transparent conducting materials is essential for the scale-up of solar cell technology. Indium-tin-oxide (ITO) is currently used almost exclusively as a transparent conducting oxide (TCO) material. This limits both the usage and the scientific advancement of TCOs. Indium is very scarce and expensive, and ITO is only an n-type conductor. Thus new TCO materials must be developed as an alternative to ITO. In this work, we examine (Zn,Co)3O4, a novel p-type TCO made out of earth-abundant materials. Bulk (Zn,Co)3O4 has a bandgap of 2.26 eV and thin films have a bandgap of approximately 2.63 eV, making it transparent to most visible light. If the conductivity is enhanced, it poses promise as a TCO material.We have performed Anomalous X-ray Diffraction (AXRD) studies at SSRL beamline 2-1 to probe the crystal structure and cation distribution in these spinel (AB2X4) materials. Using this technique, we perform diffraction scans at a variety of energies – below, through, and above the absorption edge of the cations. If the cation is present in a particular geometry than we will see a decrease in the diffracted intensity as we pass through the absorption edge, while if the cation is not present than we will not see such a change. In a spinel oxide, AB2O4, the A and B cations can sit on either the tetrahedral or octahedral sites. Which site they sit on helps to determine the electronic and other properties of the material. Using structure factor calculations we have determined that in spinel materials, the 222 reflection probes only octahedral sites while the 422 probes only tetrahedral sites. We have probed bulk and thin film (Zn,Co)3O4 spinels to determine the effect of stoichiometry and growth conditions on the cation distribution. We find that bulk samples grown under equilibrium conditions are less than 5% inverse. Within the stoichiometric limitations, almost all Co is in octahedral sites and Zn is in tetrahedral sites. However, thin films grown using non-equilibrium methods (sputtering and pulsed laser deposition) display a different cation distribution, with Zn found on octahedral sites and Co found on tetrahedral sites in excess of that necessitated by stoichiometry. We have found deviations of approximately 15% from the bulk cation distribution. This can lead to enhanced conductivity as a Zn2+ on a Co3+ site allows for hole conduction. This conductivity, in combination with a bandgap close to the UV/visible border, shows that this material has promise for a p-type, earth abundant TCO material. M. Dekkers, G. Rijnders, and D.H.A., Blank Appl. Phys. Lett. 90, 021903 (2007) H.J. Kim, I.C. Song, J.H. Sim, H. Kim, D. Kim, Y.E. Ihm, and W.K. Choo, J. Appl. Phys. 95, 7387 (2004)
10:30 AM - **G1.4
Closing the Loop between Experiment, Defect Models, and Theory for Transparent Conductive Oxides.
Stephan Lany 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Being a vital materials component in many optoelectronic technologies, including solar cells, transparent conductive oxides (TCO) deserve a firm scientific underpinning of the physical mechanisms that control their properties. Some of such mechanisms have been initially assigned according to simple models, which over time became legend that is rarely questioned. Ultimately, however, we want to close the loop between experimental observations, defect models, and theory prediction. One example is the wide-held belief that vacancies of oxygen atoms (VO
) in the lattice would release free-electrons and cause the high conductivity, e.g., of In2
. Here, recent theory , finding an unexpected large ionization energy of VO
, requests to re-think the paradigm. Indeed, a careful analysis, using thermodynamic modeling of defect densities and carrier concentrations, shows that theory compares favorably with conductivity and thermogravimetry under equilibrium conditions at elevated temperature, but that the behavior of In2
thin-films is inconsistent with both theory and the behavior of bulk crystals. Attempting to explain the thin-film conductivity with bulk-defect models runs inevitably into contradictions. New experiments and calculations point to a much higher importance of the surface for thin-film properties than previously anticipated.
 S. Lany and A. Zunger, Phys. Rev. Lett. 98, 045501 (2007).
This work was funded by the US Department of Energy, Basic Energy Sciences, through the EFRC "Center for inverse Design".
11:30 AM - **G1.5
Development of New Transparent Conducting Oxides: Materials Design, Electronic Structure, Carrier Transport and Device Applications.
Toshio Kamiya 1 , Hideo Hosono 1 Show Abstract
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Many optoelectronic devices such as flat-panel displays (FPDs), solar cells and touch panels require transparent conducting oxides (TCOs) as window electrodes. There has been much effort devoted to developing improved or new TCO materials because higher mobility improves energy conversion efficiency in the long wavelength region > 800 nm for multi-junction solar cells and reduction of expensive and rare elements such as In and Ga is getting much important due to the global resource issue. On the other hand, electro-active transparent materials have attracted much interest also for active layers in semiconductor devices because it has been demonstrated that the wide bandgap (Eg > 3.0 eV) of oxide semiconductors have advantages over conventional semiconductors such as Si for applications to light-emitting diodes (LEDs) and thin-film transistors (TFTs). For example, TFTs using transparent amorphous oxide semiconductor (TAOS) channels are considered to be most promising for practical products of organic LED displays. Here, we should notice that many such breakthrough have been achieved by finding new materials; however, it is not easy to find a new TCO materials because large bandgaps are, in general, not compatible with high electronic conductivity and large carrier mobilities.In this lecture, we will explain how we can design new TCO materials based on consideration of electronic structures, carrier doping and carrier transport in wide bandgap materials. An important key factor is to utilize spatially-spread s orbitals in heavy metal cations for n-type TCOs, and to employ Cu+ or chalcogenide ions to enhance hybridization of valence band orbitals for p-type TCOs. Utilizing natural nanostructures of oxide crystals is an another good approach, which actually found a representative ‘non-rare-element based TCO’, 12CaO-7Al2O3. At the conference, these will be explained with an assistance of visualization by first-principles calculations.
12:00 PM - G1.6
Rational Synthesis of Indium Gallium Zinc Oxide Nanowires.
Michael Moore 1 2 , Sean Andrews 1 2 , Melissa Fardy 1 2 , Shaul Aloni 2 , Velimir Radmilovic 2 3 , Peidong Yang 1 2 Show Abstract
1 Department of Chemistry, University of California, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Indium zinc oxide-based alloys (In2-xMxO3(ZnO)n, M=In, Ga, Fe) have been examined for a variety of applications such as transparent conducting oxides, thermoelectric devices, and photoelectrochemical cells. Compared to ZnO, alloys such as indium gallium zinc oxide exhibit improved n-type conductivity, decreased thermal conductivity, and improved absorption of visible light. Some of these new properties arise from the natural superlattice structure of the alloy: single atomic layers of MO2– in ZnO, with periodicity dependent on the alloy concentration. While previous reports have grown ZnO alloy nanowires (NWs) by chemical vapor transport, we utilize a new method for synthesizing ZnO alloy NWs by depositing metal onto ZnO NW arrays and sintering in oxygen at high temperature. Our method provides a synthetic pathway for designing ZnO alloy NWs with control of nanowire dimensions, array density and orientation, and alloy composition. Using transmission electron aberration-corrected microscopy, we study the structure of indium gallium zinc oxide in detail and predict a dislocation-driven diffusion mechanism of formation.
12:15 PM - G1.7
Structural and Electrical Properties of Layer-by-Layer Grown Al-doped ZnO Films by Atomic Layer Deposition.
Do-Joong Lee 1 , Hyun-Mi Kim 1 , Jang-Yeon Kwon 1 , Hyoji Choi 1 , Soo-Hyun Kim 2 , Ki-Bum Kim 1 Show Abstract
1 Department of Materials Science and Engineering, Seoul National University, Seoul, Seoul, Korea (the Republic of), 2 School of Materials Science and Engineering, Yeungnam University, Gyeongsan-si, Gyeongsangbuk-do, Korea (the Republic of)
Transparent conducting oxides (TCOs) are an essential component of various optoelectronic devices for the emerging energy technologies. Recently, atomic layer deposition (ALD) has been considered as a potential method to deposit TCO films for electrodes of light-emitting diodes and advanced solar cells. The surface-saturated and digitally-controlled reaction mechanism allows conformal coating of ALD-TCO films into extremely high-aspect-ratio structures at low process temperature. In addition, the composition of TCO films is simply varied via alternate deposition of various oxides. These aspects make ALD as a promising tool for the deposition of complex and layered TCO films. For the integration of ALD-TCO films into the emerging energy-related applications, understanding on the physical properties of those films is highly required. In this talk, we investigate structural and electrical properties of layer-by-layer grown Al-doped ZnO (AZO) films as transparent conducting electrodes. AZO films with various doping concentrations were deposited by ALD via in-situ alternate stacking of ZnO and Al2O3. Due to the inherent nature of sequential deposition, these films exhibit a unique layer-by-layer structure consisting of a ZnO matrix and Al2O3 dopant layers from the analysis of Auger electron spectroscopy and transmission electron microscopy. From the combined understanding of the structural and electrical properties, we found that the carrier concentration of those films is closely related to the number of ZnO/Al2O3 interfaces. For instance, a single Al2O3 dopant layer deposited for 1 ALD cycle could provide a constant number of free electrons (~4.5×1013 cm-2) to the ZnO. In addition, the carrier generation behavior of these films is comprehended with respect to the interval between adjacent Al2O3 layers. To explain the decrease of carrier concentration in ALD-AZO films which has narrow interval (< 2.3 nm), an effective field model is proposed. Finally, the extrinsic doping mechanism of layered AZO films is suggested based on the effective field model. Detailed features of ALD-AZO films will be presented.
12:30 PM - **G1.8
Nanomaterial -based Transparent Conductive Coatings.
Ilia Ivanov 1 , Matthew Garrett 1 , Rosario Gerhardt 1 2 , Alex Puretzky 1 , Dave Geohegan 1 Show Abstract
1 Center for Nanophase Material Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 School of Materials Science and Engineering , Georgia Institute of Technology , Atlanta, GA, Georgia
A review of current state of transparent conductive coatings (TCC) based on carbon nanotubes (CNT) and other nanomaterials will be presented.For CNTs based TCC it is well established that the resistance across junctions is higher than the resistance through the bundles themselves, and therefore, the transport through junctions is the major barrier to obtaining high network conductivity.(1,2) Reported values of the resistance of metallic SWNTs vary from nearly as low as the theoretical one-dimensional resistance, 15 kohm, to several Mohm,(3,4) due to the ballistic conduction mechanism in place, and measurements of semiconducting SWNT resistance range anywhere from 160 kohm to 20 Moh.(5,6) Junction resistances have been reported to range from 90 kohm to 3 Mohm for metallic SWNT and from 300 kOm to 160 Mohm for semiconducting SWNT as conductivity occurs via a tunneling mechanism.We discuss a macroscopic approach to assess properties of nanotubes bundles and junctions from the impedance specroscopy of CNT TCC.(7)1.S. Kumar, J. Murthy, and M. Alam, Phys. Rev. Lett. 95, 066802 2005.2. A. Kaiser, V. Skakalova, and S. Roth, Physica E 40, 2311 2008.3. A.. Bezryadin, A. Verschueren, S. Tans, and C. Dekker, Phys. Rev. Lett. 80, 4036 1998.4. J. Lefebvre, R. Antonov, M. Radosavljevic,15J. Lefebvre, R. Antonov, M. Radosavljevic, J. Lynch, M. Llaguno, and A. Johnson, Carbon 38, 1745 2000.5. V. Skákalosvá, A. Kaiser, Y. Woo, and S. Roth, Phys. Rev. B 74, 085403 2006.6. Z. Yao, H. Postma, L. Balents, and C. Dekker, Nature London 402, 273 1999.7. M. P. Garrett, I. N. Ivanov, R. A. Gerhardt, A. A. Puretzky, and D. B. Geohegan, Appl. Phys. Lett. 97, 163105, 2010
G2: Oxides for Energy Conversion and Photochemical Processing
Tuesday PM, April 26, 2011
Room 2006 (Moscone West)
2:30 PM - **G2.1
Advances in High Mobility Amorphous Oxide Semiconductors.
Elvira Fortunato 1 Show Abstract
1 Materials Science, FCT-UNL, Caparica Portugal
Transparent electronics is growing so fast and is today one of the most advanced topics for a wide range of device applications, where the key component are wide band gap semiconductors, and oxides of different origin play an important role, not only as passive component but also as an active component similar to what we observe in conventional semiconductors with crystalline or amorphous like structure. As passive components they include the use of these materials as dielectrics for a wide range of electronic devices and also as transparent electrical conductors for a wide variety of optoelectronic applications, such as liquid crystal displays, organic light emitting diodes, solar cells, optical sensors, etc. Besides that, these films have been also used for gas sensing and for photo-catalytic purification of gases and fluids. As active material, they exploit the use of truly electronic semiconductors where the main emphasis is been put on transparent thin film transistors, light emitting diodes, lasers, uv sensors, biosensors, etc.The present paper aims to address all aspects related to the physics and structure of these emerging materials, with special attention on the oxide ones, presenting structures ranging from poly/crystalline, to amorphous, where nanocrystalline and nanostructured films are also included.Complementing these topics, devices and their different aspects related to applications, will be also addressed.
3:00 PM - G2.2
High-dielectric Constant (K) Al2O3 / TiO2 Atomic Scale Multilayers for Supercapacitors for Energy Storage.
Orlando Auciello 1 , Wei Li 2 Show Abstract
1 Materials Science & Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, United States, 2 Marerials Science Division, Argonne National Laboratory, Lemont, Illinois, United States
Dielectric materials exhibiting high dielectric constants have recently gained considerable attention for their potential applications in microelectronics such as capacitors and memory devices. Al2O3 and TiO2 have been investigated as high-K materials to replace SiO2 as gate and for high-capacitance oxide-based capacitors for electronics. The dielectric constants of Al2O3 and TiO2 are approximately 7 and 60-70, respectively. Our previous studies indicate that amorphous TixAl1-xOy films (Ti:Al=75:25 at %, a trading-off between high-K and offset barrier height) demonstrate a dielectric constant of ~ 30. Other work has shown that the dielectric constant of mixtures of Al2O3 and TiO2 varies between 7 and 60-70 depending on the mixture ratio. Here, we report here that high dielectric constants (> 800) can be achieved in Al2O3/TiO2 multilayers with sub-layer thickness ≤ 1 nm, fro frequencies ≤ 104 Hz. A step-like cecrease in dielectric constant to ~ 50 occurs between 10 4 – 10 5Hz. The high dielectric constant can be attributed to M-W relaxations, which occurs not due to the orientation of dipole but to the electrical heterogeneity of the multilayers. The Ellingham diagram indicates that the Al oxidation is more favored than Ti-oxide since the Al Gibbs free energy for oxidation is more negative than for Ti. Therefore, O atoms will have a diffusion tendency from TiO2 to Al2O3 sublayers,that can result in reduction of oxygen content in the TiO2 sublayers, leading to TiOx stoichiometry, thus electrical conductivity. As a consequence, the conductivities of TiO2 and Al2O3 sublayers become so different that surface charges would accumulate at the interfaces when electric current pass through. The surface charges relax with a.c. field and cause MW relaxation (the equations expressing the dispersion are completely identical to Debye relaxation although the origins are quite different). A discussion will be presented on the use of the ALD process to produce large area capacitors via conformal coating of large area ridge arrays fabricated on Si surfaces. These capacitors can yield ≥ 10 µF capacitance. We are exploring these capacitors for applications such as energy storage embedded capacitors in a Si microchip implantable in the human retina, as part of an artificial retina to restore sight to people blinded by genetically-induced degeneration of photoreceptors, and for super-capacitors integrated with ferroelectric-based high-efficiency photovoltaic devices for energy generation/storage systems.This work was supported by US Department of Energy, Office of Science, Office of Basic Energy Sciences-Materials Science, under contract DE-AC02-06CH11357 and DARPA under contracts MIPR 06-W238. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
3:15 PM - G2.3
Thermoelectricity in the Ultra-thin Limit.
Jayakanth Ravichandran 1 2 , Pim Rossen 3 , Vincent Wu 3 , Arun Majumdar 4 , R. Ramesh 2 3 5 Show Abstract
1 Applied Science and Technology, University of California, Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 4 ARPA-E, Department of Energy, Washington, District of Columbia, United States, 5 Department of Physics, University of California, Berkeley, Berkeley, California, United States
Hicks and Dresselhaus  predicted an enhanced thermoelectric power factor due to quantum confinement. In the past, superlattices have been employed to demonstrate this effect but the results have remained controversial. Sustained efforts on surface termination and treatment of single crystalline oxide substrates has enabled growth of high quality thin films using techniques like pulsed laser deposition and molecular beam epitaxy. In this work, we explore the nature of thermoelectric response for ultra thin layers (~ 1 – 100 nm) of model thermoelectric oxides such as doped SrTiO3 and Bi2Sr2Co2Oy grown by pulsed laser deposition. Thermopower, resistivity and Hall measurements were carried out as a function of thickness to understand the effect of quantum confinement and other extraneous effects like surface depletion etc. on the thermoelectric response.References: L.D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).
4:15 PM - G2.5
Doped SrTiO3 Anodes for Photoelectrochemical Water Splitting.
Kavaipatti Balasubramaniam 1 , Shuzhi Wang 1 , Esther Llado 1 , Tyler Matthews 2 , Luca Corbellini 2 , Jayakanth Ravichandran 1 , Lin-Wang Wang 1 , Ramamoorthy Ramesh 1 2 3 , Joel Ager 1 Show Abstract
1 Materials Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California, United States, 2 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Department of Physics, University of California, Berkeley, Berkeley, California, United States
The valence and conduction band edges of SrTiO3 (STO) straddle the water splitting redox potentials making this material the subject of many investigations to split water spontaneously, albeit under UV irradiation. Here, we exploit our ability to grow perovskite-structure STO epitaxially by pulsed laser deposition to investigate the potential for improving its photoanodic performance by doping. STO is a stable, spontaneous photoelectrode material for water-splitting under UV light. Engineering of its band structure and carrier density is achieved via doping on the anion and/or cation sites. This modulates the absorption and conductivity characteristics to make a useful photocatalyst in the visible range. Aliovalent cation doping of the Sr site can modulate the conductivity from insulating to metallic. Ab initio and band anti-crossing model calculations indicate that isovalent anion doping with Se in the range of x = 0.03 to 0.1 results in a 0.8 to 1.0 eV red shift in the STO optical absorption edge. Thin films of the Se-doped STO grown by pulsed laser deposition on insulating substrates were characterized for their structural (XRD), chemical (RBS and EDS), optical, and electrical properties. The experimentally determined band structure evolution is compared with theoretical calculations. Initial results of the photoelectrochemical measurements with our doped STO films will also be discussed.Supported by the Helios Solar Energy Research Center, DOE/BES/DMSE.
4:30 PM - G2.6
Performance-limiting Factors in Spray-deposited Thin Film BiVO4 Photoanodes.
Fatwa Abdi 1 , Hans van 't Spijker 1 , Roel Van de Krol 1 Show Abstract
1 Chemical Engineering / Materials for Energy Conversion and Storage, Delft University of Technology, Delft Netherlands
BiVO4 has received widespread attention recently due to its promising photocatalytic properties under visible light illumination . Monoclinic BiVO4 (Scheelite) has a bandgap of ~2.4 eV, and is able to absorb ~11% of the solar spectrum, compared to 4% for the ‘standard’ UV-sensitive photocatalyst TiO2. The quantum efficiencies reported so far are promising, but still too low (<50% [2-3]) for practical energy conversion applications. In our attempts to improve this, we have developed a novel recipe for spray pyrolysis of monoclinic BiVO4 resulting in an Incident Photon-to-Current Efficiency (IPCE) of up to 55%. Integrating this leads to a predicted current density of 1.6 mA/cm2 under AM1.5 illumination, which corresponds to a solar-to-hydrogen conversion efficiency of ~2% (theoretical efficiency ~7%). However, the observed photocurrent under simulated AM1.5 illumination is much lower (~0.2 mA/cm2) than the predicted value due to extensive recombination at high light intensities. In order to understand the recombination mechanism, we have indentified and studied the performance-limiting factors as a function of light intensity. By comparing front- vs. back-side illumination, electron transport is found to be the main performance-limiting factor at low light intensities (few μW/cm2). This is confirmed by a time-of-flight study using a pulsed LED light source, which revealed an electron mobility value that is significantly lower than that of typical semiconducting metal oxides. Under high intensity illumination (AM1.5, ~100 mW/cm2), front- and back-illumination photocurrents are similar. However, addition of hydrogen peroxide into the electrolyte shows a significant increase of the photocurrent density, indicating hole transfer limitations. This effect was not observed at low light intensities. Based on these observations, we conclude that bulk electron transport is rate-limiting at low light intensities, and that hole transfer across the semiconductor/electrolyte interface limits the performance at high light intensities. A detailed model that describes these rate-limiting steps will be presented. Finally, possible strategies to improve the performance, such as doping and nanostructuring to solve the electron transport problem, and application of oxidation catalysts (Co, IrO2, etc.) to overcome the hole accumulation issue, will be outlined.References A. Kudo, K. Omori, and H. Kato, J. Am. Chem. Soc., 121 (1999) 11459. K. Sayama, A. Nomura, T. Arai, T. Sugita, R. Abe, M. Yanagida, T. Oi, Y. Iwasaki, Y, Abe, and H. Sugihara, J. Phys. Chem. B, 110 (2006) 11352 M. Long, W. Cai, and H. Kisch, J. Phys. Chem. C, 112 (2008) 20211.
4:45 PM - G2.7
Mechanism of Visible-light Photocatalysis in Nitrogen-doped TiO2.
Joel Varley 1 , Anderson Janotti 2 , Chris Van de Walle 2 Show Abstract
1 Physics, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Material, University of California, Santa Barbara, Santa Barbara, California, United States
TiO2 is a wide-band-gap semiconducting oxide of exceptional interest due to its rich bulk and surface properties, with applications in solar cells, high-κ dielectrics, and most notably photocatalysis. Although very promising as a photocatalyst, the efficiency of TiO2 at utilizing the light due to solar irradiation and other lower-frequency light-sources is low due to its large band gap, corresponding to wavelengths below 400 nm. Using first-principles methods based on the screened hybrid functional HSE06, we investigate the effects of nitrogen doping on the optical properties of the rutile and anatase phases of TiO2. Using a supercell approach with explicit finite-size corrections, we determine the formation energies of a number of N-related configurations and find that N can be incorporated into the bulk with appreciable solubility in the form of a substitutional or an N2 split-interstitial defect. Substitutional N gives rise to a deep acceptor state, while the split interstitial behaves as an amphoteric impurity. We find that substitutional N on the O site leads to impurity-band transitions which extend the absorption of light to the visible range for both rutile and anatase, reducing the onset by 0.6-0.7 eV with respect to their band gaps of 3.1 and 3.3 eV, in good agreement with recent experiments. Our results indicate that the N2 split-interstitial defects do not contribute to the observed onset of visible-light absorption but play a role in the pinning of the Fermi Level. Additionally, we find that hydrogen passivation can explain the conflicting experimental reports on the efficacy of N-doping. This work was supported by the NSF MRSEC Program under Grant No. DMR05-20415.
5:00 PM - G2.8
Ultrafast Anodic Growth of TiO2-nanotubes with Semi-metallic Properties.
Robert Hahn 1 , Himendra Jha 1 , Doohum Kim 1 , Patrik Schmuki 1 Show Abstract
1 Material Science and Engineering, University Erlangen, Erlangen Germany
Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium , have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, solar energy conversion and as host for Li storage) in a nanotubular morphology. The conventional way to grow anodic TiO2 nanotubes is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an recent overview see ).In the first part of the work we report on an entirely novel approach , the growth of nanotubes by so-called RBA (rapid breakdown anodization). This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition this method is capable to produce mixed oxides with various transition metals in form of nanotubular powders and suspensions on a large scale .The second part of the presentation will demonstrate that the electronic  and mechanical properties  of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior and high mechanical strength by a high temperature reduction treatment in acethylene. Literature: V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Electrochim. Acta, 45, 921 (1999). S. Berger, R. Hahn, P. Roy, P. Schmuki Phys. Status Solidi B247, 10, 2424 (2010). R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Commun.,9, 947 (2007). H. Jha, R. Hahn, P. Schmuki Electrochimica Acta 55, 8883 (2010). R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y.Y. Song, J. Kunze,V.-P. Lehto, P Schmuki, Angewandte Chemie - International Edition 48 (39), 7236 (2009). F. Schmidt-Stein, S. Thiemann, S.°Berger, R. Hahn, P. Schmuki, Acta Materialia, 58, 6317 (2010).
5:15 PM - G2.9
Photochemical Reduction of CO2 Using CuGaO2 and CuGa1-xFexO2 (x=0.05, 0.01, 0.15, 0.2) Delafossite Oxides.
Jonathan Lekse 1 , Kylee Underwood 2 , Christopher Matranga 1 , James Lewis 2 Show Abstract
1 , National Energy Technology Laboratory, Pittsburgh, Pennsylvania, United States, 2 Physics, West Virginia University, Morgantown, West Virginia, United States
The photochemical reduction of CO2 in the presence of H2O to form CO, CH4, and other light gases with industrial value is an interesting technical approach for dealing with CO2 emissions. This approach can also generate tangible revenue to help offset carbon capture and storage costs by generating a product stream with industrial demand from a CO2 feedstock. Delafossite materials of the general stoichiometry ABO2 are a new class of photocatalysts being considered for this application. Recent theoretical calculations have indicated that B-site alloying in these systems breaks the inversion symmetry of the crystal giving rise to symmetry forbidden optical transitions across the band structure of the material. B-site alloying can also be used to modulate the delafossite band structure to create new, low energy, band gaps, as well as to align band-edge positions for the photochemical redox reactions needed specifically for CO2 applications. The photochemical activity of CuGaO2 and CuGa1-xFexO2 (x=0.05, 0.01, 0.15, 0.2) for the reduction of CO2 will be presented. The CuGaO2 materials studied have an optical gap at ~3.7 eV in agreement with previous literature reports. Our results also see an optical feature at ~2.6 eV which is not as commonly reported in the thin film literature due to sample thickness effects. Alloying at the B-site with Fe to form CuGa1-xFexO2 (x=0.05, 0.01, 0.15, 0.2) creates new features in the visible and near-infrared optical spectra of the substituted material. The photoreduction of CO2 in the presence of H2O vapor using CuGaO2 and CuGa1-xFexO2 produces CO with little evidence for other products such as H2 or hydrocarbons. The observed optical spectra will be compared to the calculated density of states in this system to better understand how B-site alloying can be used to control the optical activity of this material for photochemical applications. Likewise, the calculated band edge positions will be examined to determine which atomic species would primarily participate in CO2 reduction and H2O oxidation potentials to better understand the reaction products and yields observed experimentally.
Ho Nyung Lee Oak Ridge National Laboratory
Akira Ohtomo Tokyo Institute of Technology
Gervasi Herranz Institut de Ciencia de Materials de Barcelona ICMAB-CSIC
John Perkins National Renewable Energy Laboratory
Oak Ridge National Laboratory
Rocky Mountain Vacuum Tech Inc
G8: Poster Session: Complex Oxide Materials for Emerging Energy Technologies
Thursday PM, April 28, 2011
Salons 7-9 (Marriott)
1:00 AM -
G8.23 Transferred to G3.27
G7: Transparent Conducting Oxides II: Photovoltaics
Thursday PM, April 28, 2011
Room 2006 (Moscone West)
2:30 PM - **G7.1
Functional Oxide Materials for Photovoltaic Applications.
Ajaya Sigdel 1 , Edwin Widjonarko 1 , Yi Ke 1 , Xerxes Steirer 1 , Paul Ndione 1 , Andriy Zakutayev 1 , Jens Meyer 2 , Erin Ratcliff 3 , Dana Olson 1 , Matthew Lloyd 1 , Neal Armstrong 3 , Thomas Gennett 1 , Antoine Kahn 2 , David Ginley 1 , Joseph Berry 1 Show Abstract
1 NCPV, NREL, Golden, Colorado, United States, 2 Dept. of Electrical Engineering, Princeton University , Princeton, New Jersey, United States, 3 Dept. of Chemistry, College of Optical Sciences, University of Arizona, Tuscon, Arizona, United States
Oxide based semiconducting materials are an increasingly important constituent of a number of optoelectronic technologies. The ability to perform semiconductor band engineering in these materials has prompted a number of well known oxide material systems to be reexamined. The improved or novel synthesis approaches combined with detailed materials analysis have permitted the application of these materials to emerging thin film solar conversion technologies such as organic photovoltaics (OPV). In this presentation we discuss our efforts to use a range of oxide materials to perform band engineering of the contacts to bulk heterojunction based organic photovoltaics. In this application the contacts and electrodes must both create the asymmetry in the devices as well as provide charge selectivity. The use of oxide materials to perform this demanding task is motivated by the relatively large band gaps of oxide semiconductors as well as the ability to modify the oxide/organic interface. We will present our work on the use of large work function transparent conducting oxide materials such as InZnSnOx in OPV and thin film PV technologies. We will also present work on use of selective oxide contact layers based on p-type oxides (i.e. NiOx, AxB2-xO4 spinels) for engineering the hole carrier extraction in OPV devices. Similar approaches to electron harvesting contacts based on low work function oxides will also be discussed.
3:00 PM - G7.2
MoO3/Ag/MoO3 Transparent Electrodes for Coherent Light Trapping in Thin Film Organic Solar Cells.
Nicholas Sergeant 1 , Barry Rand 2 , Paul Heremans 2 3 , Peter Peumans 1 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 , Imec vzw, Leuven Belgium, 3 Electrical Engineering, K.U.Leuven, Leuven Belgium
Tin-doped Indium oxide (ITO) requires elevated substrate temperatures to achieve optimal combinations of transparency and sheet resistance, making it incompatible with low-cost plastics. In addition, the brittleness of ITO and the elevated cost of indium has lead to increased interest to replace ITO by a low-cost transparent electrode with similar optical and electrical properties. We have thermally evaporated molybdenum trioxide/silver/molybdenum trioxide (MoO3/Ag/MoO3) stacks on glass as transparent anodes for thin-film organic solar cells. The growth conditions for the Ag evaporation were optimized in terms of evaporation rate and substrate temperature. We report on the excellent properties of MoO3 as a seed layer for silver nucleation, resulting in smooth conductive silver films of nominal thickness below 5 nm. The silver layer inside the stack dominates the lateral conductance of the anode and leads to sheet resistances below 10 Ohm/sq. Due to its high workfunction, molybdenum trioxide serves as an excellent hole transport layer (HTL) for a wide range of solution-processed or vacuum evaporated organic photoactive materials. In this work we have metal-doped the MoO3 films to enhance their vertical conductance by co-evaporating MoO3 with a small concentration of Ag (between 0.5-1% by weight). We have fabricated bilayer organic solar cells on these multilayer anodes using thermal evaporation of chloroaluminum phthalocyanine (ClAlPc) as a donor material and C60 as an acceptor. The reflective properties of the thin Ag film inside the stack can create coherent light trapping effects to enhance photon absorption for this specific photoactive material. We have optimized the resonant optical cavity created between the multilayer anode and a 200nm thick silver back-contact, by tuning the thickness of the silver (1 wt%) doped MoO3 hole transport layer (HTL). Coherent light trapping in these cells leads to a 42% increase in AM1.5 solar weighted absorption efficiency (AE) compared to the ITO reference in the absorption spectrum of ClAlPc (550-850nm). The enhancement in AM1.5 solar weighted external quantum efficiency (EQE) for the devices fabricated on multilayer anodes is as high as 59% when going from thin (2nm) to optimized (50nm) HTL layer. In order to further analyze the experimental results we compare the EQE and spectral reflection of the fabricated cells with optical and exciton diffusion models and show excellent agreement. The best ClAlPc/C60 bilayer cells fabricated on these electrodes have power conversion efficiencies of 2.0%, compared to 2.4% for the devices fabricated on a reference ITO anode. We make suggestions on pathways to further improve these promising multilayer anodes and their applicability to other photoactive materials.
3:15 PM - G7.3
P-type Spinel Oxides as Hole Transport Layers for Organic Photovoltaic Devices.
Paul Ndione 1 , Andres Garcia 1 , Nicodemus Widjonarko 1 2 , Ajaya Sigdel 1 3 , Andriy Zakutayev 1 , John Perkins 1 , Philip Parilla 1 , Dana Olson 1 , David Ginley 1 , Joseph Berry 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Physics, University of Colorado, Boulder, Colorado, United States, 3 Department of Physics and Astronomy, University of Denver, Denver, Colorado, United States
Organic photovoltaics (OPV) have attracted considerable interest as a potential low cost alternative to current silicon solar cells and a mechanically flexible energy conversion device. The control of carrier transport at metal/organic interfaces in OPV device is very important and has been the subject of intensive investigation. For example, the insertion of a functional thin film in the electrode/active layer interface is very well known to increase the charge collection efficiency, leading to improved overall device performance. Our study reports the use of p-type spinel oxides such as cobalt nickel oxide (Co2NiO4) and cobalt zinc oxide (Co2ZnO4) as a hole transport layer (HTL) in OPV devices. Co2NiO4 and Co2ZnO4 HTLs were deposited by pulsed laser deposition on ITO coated glass substrates to create ITO/HTL/P3HT:PCBM/Ca organic solar cells. The alteration of the HTL physical properties by varying the film composition and the processing conditions such as substrate temperature (32 - 400 °C), oxygen pressure (2.8x10-3 - 200 mTorr) and thickness (3.5 - 50 nm) is discussed. The role and influence of the HTL’s physical properties on the devices performance are also investigated. For example, at 32 °C of substrate deposition temperature and 50 mTorr of oxygen deposition pressure, a work function of 4.45 eV and an absorption onset at 3.5 eV are measured (respectively by Kelvin probe and ellipsometry techniques) for a 7 nm thick Co2NiO4 HTL, leading to a power conversion efficiency of 2.8%. Also, a comparison of P3HT:PCBM and PCDTBT:PC70BM devices employing these HTLs will be discussed.
3:30 PM - G7.4
Graded-crystallinity Transition-metal Oxide Blocking Layers for Dye-sensitized and Polymer Solar Cells.
Robert Gunning 1 , Pablo Docampo 1 , Michael Lee 1 , Henry Snaith 1 Show Abstract
1 Condensed Matter Physics, University of Oxford, Oxford, Oxon, United Kingdom
Photovoltaic devices using relatively inexpensive and scalable methods frequently utilize a transition-metal oxide (TMO) layer sensitized with a dye as the active layer. Electrons are typically collected with a transparent conductive oxide (TCO) such as fluorine-doped tin oxide or indium-doped tin oxide, while some form of hole transporting material (HTM) is used to collect the holes. However, the porous nature of this TMO layer makes contact between the HTM and TCO possible, giving rise to recombination between the electrons from the latter and holes from the former; this can be detrimental to the device properties. Of vital importance for device performance optimization are hole-blocking layers, typically a compact layer of a TMO. These are placed between the porous layer and the transparent electrode, screening the electrons in the TCO from the photo-generated holes in the active region. Layers which can perform this function effectively without incurring large rise in device cost are desirable. Here, we present work on the use of low-cost, facile and scalable SnO2 and TiO2 compact layers deposited by spray pyrolysis of discrete and graded crystallinity as hole-blocking layers. Crystallinity and surface morphology is investigated using XRD, X-ray reflectivity and electron microscopy. Significant improvement in suppression of recombination is observed with the use of crystalline and amorphous bilayers. This results in greatly enhanced fill factor, open-circuit voltage and efficiency of the ensuing solar cells.
4:15 PM - **G7.5
Highly Efficient Dye-sensitized Solar Cells.
Liyuan Han 1 Show Abstract
1 Advanced Photovoltaic Center, National Institute for Materials Science, Tsukuba-city, Ibaraki, Japan
Dye-sensitized solar cells (DSCs) have been widely investigated as a next-generation solar cell because of low manufacturing costs. A dye-sensitized solar cell comprises a nanocrystalline titanium dioxide electrode modified with a dye fabricated on a transparent conducting oxide, a platinum counter electrode, and an electrolyte solution with a dissolved iodide ion/tri-iodide ion redox couple between the electrodes. The internal resistances of DSCs were investigated through electrochemical impedance spectroscopy measurement. Four internal resistance elements related to the charge transfer processes at the Pt counter electrode (R1), the charge transportation at the TiO2/dye/electrolyte interface (R2), ionic diffusion in the electrolyte (R3), and the sheet resistance of TCO (Rh) were observed. According to the results obtained, the equivalent circuit for DSCs was proposed. The method for improvement of shirt circuit density (Jsc), open circuit voltage (Voc) and fill factor (FF) were investigated based on the equivalent circuit. The decrease of the series-internal resistance was studied in order to improve of fill factor. It was found that the series-internal resistance decreases with increase of the roughness factor of the counter electrodes, the decrease of the thickness of the electrolyte layer and the sheet resistance of the transparent conducting oxide.TO improve Jsc, dependence of incident photon to current conversion efficiency (IPCE) spectra on haze of TiO2 film was investigated. IPCE is significantly increased with increase of the haze of TiO2 film, especially in IR region. Jsc of 21 mA/cm2 was obtained with haze of over 67%. A cell with the series-internal resistance of 1.8Ωcm2 and high haze factor was fabricated. Current-voltage characteristics were measured by Research Center for Photovoltaic, National Institute of Advanced Industrial Science and Technology using a metal mask and with an aperture area of 0.219 cm2 under standard AM 1.5 sunlight (100.0 mW/cm2). An overall conversion efficiency of 11.2% was achieved which is the highest confirmed efficiency.
4:45 PM - G7.6
High Performance Transparent Photosensor Array Utilizing Triple Oxide Semiconductor Thin Film Transistor.
Seung-Eon Ahn 1 , Sanghun Jeon 1 , Ihun Song 1 Show Abstract
1 , Samsung Advanced Institute of Technology, Suwon Korea (the Republic of)
A zinc oxide based semiconductor has a wide band-gap of about 3.2-3.8eV depending on the composition of semiconductor. However, it still has sensitivity under near-UV light illumination due to sub-gap states of semiconductor. For the case of amorphous indium zinc oxide (a-IZO), it shows high sensitivity even in near UV-visible light due to a large number of sub-gap states. In addition, the characteristic time for the rise of photocurrent for an oxide-based photosensor is less than 0.1s, while that for the fall of the photocurrent is relatively long, about 1-100s due to oxygen-related defects, which has been a long standing issue of oxide based photosensor for more than a few decades.In this study, we propose a novel sensor architecture utilizing different oxide semiconductor transistors as both switch and sensor elements. In addition, our study presents the optimum active structure for a workable three-terminal photosensor and the operation principle of an oxide sensor TFT proposed in this article provides to realize high frame rate (>150Hz) sensor array, which enables to solve intrinsically slow response issue of oxide-based photosensor.
5:00 PM - G7.7
Instabilities of Amorphous-InGaZnO TFTs Characterized by Photo-excited Charge-collection Spectroscopy: Channel Thickness Effects.
Youn-Gyoung Chang 1 2 , Dae-Hwan Kim 2 , Gunwoo Ko 1 , Jae Hoon Kim 1 , Seongil Im 1 Show Abstract
1 Institute of Physics and Applied Physics, Yonsei University, Seoul Korea (the Republic of), 2 R&D Center, LG Display, Paju-si Korea (the Republic of)
Amorphous (a)-InGaZnO thin-film transistors (IGZO-TFTs) have recently attracted a great deal of attention as driving device for large area display panels and transparent electronics. In order to realize the industrialization of the a-IGZO-TFT the operational reliabilities of the device should be secured while the reliabilities mainly depend on the nature and density of charge traps present at the channel/dielectric interface or thin-film channel itself. In particular, securing the photoelectric stabilities of the oxide TFTs is very critical since the oxide TFT devices have aimed at functioning as the pixel driver of display panel. Very recently, photo-excited charge-collection spectroscopy (PECCS) was reported with ZnO- and organic pentacene-TFTs, as a direct probe for the deep traps which cause the photo-induced threshold voltage (Vth) response of a working TFT device.1,2 In this presentation, we characterized the interfacial trap DOS of a-InGaZnO TFTs with different channel thickness by PECCS, while each a-InGaZnO TFT also went through bias temperature stress (BTS) along with photo illumination as another type of stability test.We prepared the inverted stagger type a-IGZO TFTs with etch stopper layer as protecting a channel layer for the experiments. The width/length (W/L) length of our TFTs is 36/8 μm and the a-IGZO (In:Ga:Zn = 1:1:1) layer was deposited by DC sputtering at room temperature for the thicknesses of 30, 50 and 70 nm. The condition of BTS test is ±30 V homogeneous gate voltage (S/D electrodes were sustained to be ground or 0 V) at 60 °C for 1000 s along with the halogen lamp exposure. PECCS measurement were carried out with a semiconductor parameter analyzer and a light source setup (including 500 W Hg(Xe) arc lamp, a grating Monochrometor covering the spectral range of 254 ~ 1000 nm, and optical fiber probe: power density ~0.2 mW/cm2).The ±30 V BTS test leads to almost the same degree of Vth shift regardless of the a-IGZO layer thickness while the (-) BTS test showed Vth shift toward negative direction and the (+) BTS made it toward positive direction. The similar Vth shift indicates that the TFTs have the similar quality of channel/dielectric interface regardless of the a-IGZO layer thickness. However, under PECCS measurements with several drain bias, the trap density-of-states (DOS) intensities of the three TFTs with different a-IGZO thickness displayed significant differences one another. It is because our PECCS analysis includes the interfacial traps and bulk-type traps in the channel as well. The PECCS analysis is thus regarded as a promising technique to quantitatively measure the traps in the channel and at the interface. We hope that more detailed discussion will be shared in the meeting.
5:15 PM - **G7.8
New (Old?) Methodologies for Analyzing High-performance Oxide Semiconductors for Energy Conversion Applications.
Qimin Zhu 1 , E. Mitchell Hopper 1 , Brian Ingram 2 , Thomas Mason 1 Show Abstract
1 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States
Since the development of oxide-based chemical sensors (e.g., SnO2-based) and voltage-dependent resistors or “varistors” (e.g., ZnO-based) circa 1970, there has been a rapid rise of interest in oxide semiconductors. For example, publications dealing with ZnO have doubled each half-decade since 1990 to more than 26,000 papers (2006-2010). This talk will focus on “medium band gap” (~3 eV) post-transition metal oxides, the basis set of which include CdO, ZnO, In2O3, and SnO2. Ga2O3 is also of interest, although its band gap is significantly larger, and also Ti-based materials, like SrTiO3. These compounds and their numerous binary, ternary and multinary compounds and solid solutions are known for their rare combination of high electronic conductivity (when degenerately doped) and optical transparency, and are collectively referred to as transparent conducting oxides or TCOs. TCOs find application as transparent electrodes in various photovoltaic systems. When non-degenerately doped, many of the same compounds/solid solutions can serve as thermoelectric oxides or TEOs for direct conversion of heat (solar, commercial, vehicular) to electricity. This talk “dusts off” two long-standing (but under-utilized) semiconductor analysis procedures, so-called “Jonker” and “Ioffe” analyses, and applies them to the characterization/optimization of high-performance oxides for energy conversion applications. An additional analysis procedure is also introduced for the prediction of optimum TCO conductivities from bulk ceramic electrical property measurements via so-called “TCO BLADE plots.” Ramifications for the use of TCO and TEO materials in energy conversion systems will also be discussed
G8: Poster Session: Complex Oxide Materials for Emerging Energy Technologies
Thursday PM, April 28, 2011
Salons 7-9 (Marriott)
9:00 PM - G8.1
Electron Beam Generated Oxide Nanostructures Arrays on Insulating Single Crystal Substrates for Flexoelectric Applications.
Josh Malowney 1 2 , Gustau Catalan 4 , Albert Calleja 1 , Jordi Arbiol 1 3 , Roger Guzman 1 , Francesc Belarre 1 , Jordi Llobert 2 , Narcis Mestres 1 , Teresa Puig 1 , Xavier Borrise 2 , Joan Bausells 2 , Xavier Obradors 1 Show Abstract
1 Superconductors, Institute of Materials Science of Barcelona (ICMAB-CSIC), Barcelona Spain, 2 , National Center of Microelectonics (CNM-CSIC), Barcelona Spain, 4 , Center of Investigation in Nanoscience and Nanotechnology (CIN2-CSIC)), Barcelona Spain, 3 , Catalan Institution for Research and Advanced Studies (ICREA), Barcelona Spain
Controlled arrays of oxide nanostructures have been formed by spin coating a novel water developable precursor onto a single crystal substrate, then irradiating via an electron beam and followed by a final pyrolysis step to form the crystals. The precursor consists of a stoichiometric amount of La, Sr, and Mn in an aqueous nitrate solution, yielding La0.7Sr0.3MnO3 (LSMO), along with 5wt% polyvinyl alcohol. The reason for the PVOH is to form locally polymerized areas where the spun film is irradiated with doses greater than 0.5 mC/cm^2 . The insulating single crystal substrates of SrTiO3, LaAlO3, and yttria stabilized ZrO2 (YSZ) were used to promote various amounts of localized strains on the grown nanoislands and nanowires. The precursor solution was confirmed to give epitaxial LSMO and yield a magnetic response in line with bulk values when spun and pyrolized on LaAlO3 as a thin film. The nanostructures which were written via electron beam range from nanoislands for radiation doses on the order of 1mC/cm^2 to nanowires at dosages larger than 220mC/cm^2. The nanoislands were investigated with AFM to show dimensions which are dosage dependent and on the order of 25 nm in height with a diameter of 150 nm and for the nanowires 30nm in height and 10μm in length. The TEM analysis showed an epitaxial growth and the EELS analysis showed the nanostructures to be depleted of manganese which falls in line with previously reported results derived from other methods [2, 3]. By using SEM images at various stages of thermal treatment, the nanowires growth mechanism was studied and revealed to form at 800C and have a fast non-linear growth rate of 3 μm/hr for the first thirty minutes. This technique’s applicability was also confirmed by writing nanoislands and nanowires with a SrTiO3 precursor solution. This is of interest to the field of flexoelectricity due its effect being directly proportional their small island size and high dielectic constant . The system's flexoelectic nature is currently being investigated. AcknowledgementsThis work has been financed by the AGAUR grant resolution IUE/2681 and Consolider NANO-SELECT CSD 2007 00041References1.C. Chuang et al Nanotechnology 17 4399 (2006)2.A. Carretero et al Adv Func Mat 20 892 (2010)3.C. Moreno et al Adv Func Mat 19 2139 (2009)4. W. Zhu et al App Phys Lett 89 192904 (2006)
9:00 PM - G8.10
Continuous Supercritical Hydrothermal Synthesis of Lithium Iron Phosphate (LiFePO4) Nanoparticles and Their Electrochemical Properties.
Jaehoon Kim 1 Show Abstract
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Nanosize lithium iron phosphate (LiFePO4) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25 MPa and 400 °C under various flow rates. The properties of LiFePO4 synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200-800 nm, higher BET surface area ranging 6.3-15.9 m/g and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3-15 micron meter; 2.4 m/g). LiFePO4 synthesized in supercritical water exhibit higher discharge capacity of 70-80 mAh/g at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO4 (60 mAh/g), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 150 to 145 mA/g is observed during the 30 cycles in carbon-coated LiFePO4 synthesized using supercritical water.
9:00 PM - G8.11
TiO2 Nanotube Arrays Sensitized with Bi2MoO6 for Hydrogen Energy.
Ulugbeck Shaislamov 1 , BeeLyong Yang 1 Show Abstract
1 Dept of Information Nano Materials, Kumoh National Institute of Technology, Gumi, Geongbuk Korea (the Republic of)
Visible light-driven photocatalytic technology can help to alleviate both problems by splitting water for green energy hydrogen production and degrading toxic pollutants. At present, titanium dioxide (TiO2) is the most widely used and most intensely studied photocatalyst, due to its low cost, nontoxicity, and high efficiency. However, TiO2 can only be activated with UV light, which accounts for only 4% of the solar energy that reaches the Earth’s surface. There is therefore an urgent need to develop photocatalysts that respond to visible light, and this important field of research has attracted considerable attention in recent years. Bi2MoO6 with the Aurivillius structure is active for an O2 evolution reaction under visible light irradiation. Its activity for H2 evolution is relatively low due to its lower conduction band level (the band gap: 2.7 eV). Moreover the binary systems Bi2O3–MoO3 have received special attention due to superior photocatalytic properties and Bi2MoO6 is the most studied compound within this family due to its interesting properties such as ion conductive, dielectric capacity, and luminescent and catalytic properties. We report investigation results of photocatalytic properties of TiO2 nanotube arrays sensitized with Bi2MoO6 metal-oxides to address the issues of improving the efficiency of water-splitting under visible-light irradiation for hydrogen energy. XRD and TEM investigations showed well crystallized Bi2MoO6 nanoparticles on the TiO2 nanotube wall. Measurements of photo conversion efficiency and monochromatic absorption in the range of visible light indicate superior properties of photolysis water-splitting for the composite samples. The GC and band gap measurement, and photo-conversion efficiency of the TiO2 nanotube arrays sensitized with Bi2MoO6 for visible light water-splitting will be also discussed.
9:00 PM - G8.13
Heterojunction Diodes for Solar Cell Applications Using Atomic Layer Deposition (ALD) of p-V2O5 and n-ZnO.
Parag Banerjee 1 2 , Xinyi Chen 1 2 , Keith Gregorczyk 1 2 , Laurent Lecordier 4 , Gary Rubloff 1 2 3 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Institute for Systems Research, University of Maryland, College Park, Maryland, United States, 4 , Cambridge Nano Tech Inc., Boston, Massachusetts, United States, 3 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland, United States
Oxide-based solar cells offer an economical, versatile and robust alternative for energy harvesting applications. Furthermore, Atomic Layer Deposition (ALD) processing allows one the ability to deposit ultra-thin films with superb conformality inside high aspect ratio pores creating 3D devices with unique functionalities and performance enhancements. In solar cells for example, nanowire photovoltaics offer the ability to decouple light absorption and charge separation within the device. In this work, we report on the electrical characteristics of a planar pn heterojunction diode prepared using a low temperature (<150oC) ALD scheme. We find that amorphous p-V2O5 is surprisingly p-type as confirmed by Hall measurements. This material is deposited using vanadyl triisopropoxide and water at a temperature of 120oC. The low deposition temperature causes the V2O5 to remain amorphous, as confirmed using x-ray diffraction and Raman spectroscopy. For the n-type material, we use ZnO up to 90nm thick deposited using ALD of diethyl zinc and water at 150oC. UV-Vis spectroscopy yields a band gap of 2.2eV for V2O5 and 3.2eV for ZnO, respectively.The diode characteristics are strongly dependent on the bottom electrode (BE) material. For example, a relatively low diode ideality factor (n = 29) and a high Ion/Ioff ratio (~600) is obtained for Pt whereas a high ideality factor (n = 60) and a low Ion/Ioff ratio (~19) are obtained for conducting indium-tin oxide (ITO) bottom electrodes. Pt surfaces are oxygen free whereas ITO surfaces have a large availability of oxygen atoms and their diffusion (or lack thereof) into V2O5 and ZnO significantly affects device performance. These results indicate the important role of oxygen atoms and vacancies and their relative concentration profiles within the device in determining device characteristics of oxide based semiconductor materials. The observance of diode-like behavior in heterojunctions comprising of p-V2O5 and n-ZnO has many promising applications including sensors, transparent and flexible electronics and oxide-based solar cells.
9:00 PM - G8.14
Band Structure Engineering of ZnO Nanowire Arrays by Hydrothermal Synthesis Technique.
Paresh Shimpi 1 , Kuo-ting Liao 1 , Joshua Leibowitz 1 , Pu-Xian Gao 1 Show Abstract
1 Institute of Material science, IMS., University of connecticut, Storrs, Connecticut, United States
Band structure engineering of ZnO nanowire arrays has been successfully achieved using a low temperature hydrothermal process, which are potentially useful for the applications in blue/UV light emitting devices, laser diodes and solar cell. Vertically aligned, single crystalline ZnO/ZnCuO/CuxO and ZnO/ZnMgO heterostructure nanowire arrays were successfully synthesized by the sequential hydrothermal synthesis method on silicon, quartz and GaN substrates. Room temperature photoluminescence (PL) spectra have shown that near band edge (NBE) emission for ZnO/ZnMgO nanowire arrays have blue shifted ~6 nm compared to ZnO nanowire arrays when annealed at 900 oC for 5 minutes under vacuum. PL spectra also indicated that the defect related visible emission was completely suppressed; the intensity of NBE peak increased as a result of possible point-defects reduction as suggested by a crystalline quality improvement of the annealed nanowire array. This combined effect clearly suggests its possible application as UV/blue light emitting devices. On the other hand, ZnO/ZnCuO/CuxO nanowire arrays were also synthesized. After annealing at 200 oC for 30 minutes under oxygen flow, well-defined single crystalline ZnO/CuO heterostructured nanowire was achieved. The wide band gap (3.34 eV) ZnO n-type core alloyed with narrow band gap (~1.2 eV) CuO based p-type shell might enable more efficient optical absorption by covering a wide UV-to-red solar spectrum. The successful localized doping/alloying in these nanowire arrays using low cost, low temperature and environmental friendly hydrothermal method would help in realizing low cost electronic and optoelectronic devices in future.
9:00 PM - G8.15
The Effect of Codoping on the Structural and Optical Properties of TiO2 Films Prepared by Pulsed Laser Deposition.
Murari Regmi 1 , Gyula Eres 1 Show Abstract
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
It has been demonstrated that a new doping scheme that consists of simultaneous doping (codoping) with a pair of dopants with unbalanced charge states such as Cr-N has the ability to narrow the bandgap in TiO2 particles [1-2]. In this work, we report recent results on the preparation of epitaxial TiO2 thin film samples on LaAlO3 (LAO) and SrTiO3 (STO) doped with Cr alone, N alone, and codoped with the Cr-N dopant pair using pulsed laser deposition (PLD). The films were characterized using X-ray diffraction and optical absorption to understand the effect of growth parameters and the incorporation of dopants. The optical absorption in the films depends strongly on the growth pressure but weakly on the growth temperature. The samples codoped with Cr-N show stronger optical absorption than the samples doped with Cr alone or N alone. In addition to optical properties, the phase of codoped TiO2 also changes. By changing the growth rate samples that are pure rutile, pure anatase or containing mixed phase can be prepared.*Work done in collaboration with the authors in Refs. 1 and 2  W. Zhu, X. Qiu, V. Iancu, X. Chen, H. Pan, W. Wang, N. Dimitrijevic, T. Rajh, H. M. Meyer III, M. P. Paranthaman, M. Stocks, H. Weitering, B. Gu, Gyula Eres, and Z. Zhang, “Bandgap narrowing of titanium oxide semiconductors by non-compensated anion-cation codoping for enhanced visible-light photoactivity”, PRL 103, 226401 (2009). H. Pan, B. Gu, Gyula Eres, and Z. Zhang, Ab initio study on noncompensated CrO codoping of GaN for enhanced solar energy conversion, JCP 132, 104501 (2010).
9:00 PM - G8.16
Ab Initio Calculations of Defects in Gallium Oxide.
Tobias Zacherle 1 , Peter Schmidt 2 , Manfred Martin 1 Show Abstract
1 Institute of Physical Chemistry, RWTH Aachen University, Aachen Germany, 2 Institute of Physical Chemistry, Technical University of Darmstadt, Darmstadt Germany
Gallium Oxide is an interesting material for various applications like optoelectronic or gas detecting devices. The thermodynamically stable modification, β-Ga2O3, crystallizes in a monoclinic crystal structure (C2/m). But due to its low symmetry and the resulting large number of possible defect positions - especially for the interstitials - only few theoretical calculations concerning the defect structure of β-Ga2O3 have been performed so far. In our work we study β-Ga2O3 within the GGA-approximation of density-functional theory. We determine formation energies for numerous defects in different charge states, also taking into account several interstitial positions, and we extrapolate to infinite dilution via finite size scaling. The thermodynamic conditions are explicitly incorporated in our approach, i.e. defect concentrations for gallium-rich and gallium-poor conditions can be derived and compared with experimental data. We also discuss some corrections like e.g. the band-gap correction.
9:00 PM - G8.17
Multi-generation Hierarchical ZnO Nanowire Growth for High Efficiency DSSC Fabrication.
Hyun Wook Kang 1 , Seung Hwan Ko 1 , Daeho Lee 2 , Koohyun Nam 1 , Hyung Jin Sung 1 , Cstas P. Grigoropoulos 2 Show Abstract
1 Applied Nano Tech & Science Lab, KAIST, Daejeon Korea (the Republic of), 2 Laser Thermal Lab, University of California, Berkeley, California, United States
Nano scale, wide-bandgap oxide semiconductor materials (TiO2, ZnO) have been extensively studied as a dye sensitized solar cell (DSSC) photoelectrode due to their increased surface area with adsorbed dye molecules as the photosensitized anode. TiO2 nanoparticle based DSSC has recorded high conversion efficiency (11.2%) larger than obtained for ZnO based DSSC (0.4~5.8%). However, further increase in the TiO2 DSSC conversion efficiency has been limited by energy loss due to recombination between electrons and either the oxidized dye molecules or electron-accepting species in the electrolyte during the charge transport processes. ZnO possess energy band structure and physical properties similar to those of TiO2, but its electron mobility is higher by ~2-3 orders of magnitude and easy to crystallization and anisotropic growth. Therefore, ZnO is expected the most promising alternative to TiO2. In this study, we present that “nano-forest” of high density, long branched multi generation hierarchical ZnO nanowire (NW) photoanodes grown via a simple selective hierarchical growth approach. Depending on the growth condition, we defined lengthwise growth (LG) and branched growth (BG). We performed parametric study to improve the efficiency of ZnO NW DSSC by combining length-wise growth and branched growth. LG can yield ZnO NWs of increased length by extending the growth at the tip of the backbone ZnO NW while BG produces highly branched ZnO NW by multiple generation hierarchical growth on ZnO NW side surfaces. The backbone ZnO NWs are grown from ZnO quantum dot seeds deposited on a substrate in an aqueous precursor solution containing 25mM zinc nitrate hydrate, 25mM hexamethylenetetramine (HMTA) and 5-7mM polyethylenimine (PEI). Longer ZnO NW can be grown by repeating the hydrothermal growth (LG mode). Instead of LG, BG of ZnO NW in the first generation ZnO NW sidewall could be obtained by adding seed NPs and hydrothermal growth. The heating (350°C, 10mins) of ZnO NW induces polymer (HMTA, PEI) removal and highly branched ZnO NWs can be grown with NP seed at polymer removed surface. This signifies that both polymer removal and seed layer are important for high density hierarchical branced ZnO NW forest growth. The diameter of branched ZnO NW is 30~50nm, which is smaller than that of first generation backbone ZnO NW (130~200nm). The length of branched ZnO NW is 2~10μm after single growth step. High efficiency DSSCs are made from multiple length-wise growths ZnO NW and densely branched ZnO NW “nano-forest” on a FTO glass substrate with Pt counter electrodes and ruthenium dye solution for electrolyte. The characteristics of cells are measured under AM1.5G 100mW/cm2. The short circuit current density (Jsc) and the overall light conversion efficiency increased as the length of the LG ZnO NW increased (7μm: 0.45%, 13μm: 0.7%, 16μm: 0.85%). The conversion efficiency of high density hierarchical branched ZnO NW is significantly increased by 350~500% (~2.51%).
9:00 PM - G8.18
3D-transparent Conductive Oxides with SnO2 Nanorod Arrays.
Hui Huang 1 , Chiew Keat Lim 1 , Man Siu Tse 1 , Ooi Kiang Tan 1 Show Abstract
1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Singapore
Transparent conductive oxides (TCO) such as indium tin-oxide (ITO) and fluorine-doped tin oxide (FTO) films coated on glass are widely used as TCO electrode (TCOE) for thin film and dye sensitized solar cells (DSCs). In the DSCs, the thick mesoporous TiO2 film provides a large surface area for anchoring the light-harvesting dye molecules, but the structural disorder at the contact between two crystalline nanoparticles, oxygen defects, and TiO2 amorphous layer lead to enhanced scattering of free electrons, thus reducing diffusion coefficients of electrons in the mesoporous TiO2 film. Therefore, electron transport is a limiting factor in the performance of these nanocrystalline electrodes, hindering progress in achieving higher efficiencies. Currently, most of the works using nanorod electrodes in DSCs focused on the interfacial area of the oxide layer for dyes while the TCOE is still flat. The electron capture through the limited interface between the 2D-TCOE and nanoparticles of the mesoporous film is not efficient. Herein, we report a novel 3D-TCOE (Fig. 1) with SnO2 nanorod arrays grown by PECVD at temperature below 300 oC. The 3D-TCOE shows higher transmittance in visible light due to optical coupling while still maintains the good electrical conductance and electron mobility. The incorporation of 3D-TCOE into solar cells is very attractive. The 3D-TCOE provides a direct and short conduction pathway from the point of electron-hole pair generation to the collecting electrode and would significantly improve the electron transport efficiency. Additionally, this 3D-TCOE is excellent for other applications such as display, photochemical and photocatalyst.
9:00 PM - G8.19
Solvothermal Synthesis of Cd2SnO4 For Dye Sensitized Solar Cells.
Reshma Bhosale 1 , Sarika Phadke 1 Show Abstract
1 Physical and Materials Chemistry Division, National Chemical Laboratory, Pune, Maharashtra, India
Dye Sensitized Solar Cells (DSSCs) have attracted extensive attention due to their low fabrication cost and relatively high efficiency . In DSSC, a porous electrode made of a wide- band gap semiconductor with long electron diffusion length is needed for supporting dye molecules and transporting photoinjected electrons. Earlier research has been limited to simple binary oxides like TiO2, ZnO, SnO2 and Nb2O5 but the application of multication oxides has been rarely explored. In comparison with the simple binary oxides, complex metal oxides offer a wider scope for tuning the chemical and physical properties of the materials by altering the cations and their relative ratios,. Here we report the promising application of Solvothermally Synthesized Cd2SnO4 nanoparticles in DSSC. Cd2SnO4 is a n- type transparent conducting oxide(TCO) material with a band gap of 2.3- 3.6eV and transparency of about 80%.In previous research Cd2SnO4 has been synthesized by various methods like spray pyrolysis, dip method, electrochemical synthesis and by magnetron sputtering but solvothermal synthesis of pure phase Cd2SnO4 nanoparticles has not been reported yet. In this work solvothermally prepared Cd2SnO4 particles were characterized with use of X-ray diffraction. It showed a mixed phase of as synthesized powder but after heating at 450¬oC for 1 hour a pure Cd2SnO4 phase was obtained. Transmission electron microscopy (TEM) showed uniform particles of 10-15 nm. Diffuse reflectance spectroscopy showed band gap of 2.9 eV . Cd2SnO4 films were prepared by doctor blade method and its Dye Sensitized Solar Cell performance was measured under 1sun air mass 1.5. It showed the best efficiency of 2% that was obtained from a 13μm thick film Cd2SnO4 dipped for 10 hrs in N719 dye as is acid sensitive. I-V characteristics of a Cd2SnO4 solar cells showed the short circuit current (Isc)=1.35mA, open circuit voltage (Voc)=0.61V and fill factor (ff)=61.Cyclic Voltammetry and Impedance of electrode film was also studied in detail.  Oregan, B.; Gratzel, M. Nature 1991, 353, 737–740.Gayatri Natu and Yiying Wu, J. Phys. Chem. C, 2010, 114 (14), pp 6802–6807. Bing Tan, Elizabeth Toman, Yanguang Li, and Yiying Wu, J. Am. Chem. Soc., 2007, 129(14), pp 4162–4163.  Radhouane Bel Hadj Tahar, Takayuki Ban, Yutaka Ohya, and Yasutaka Takahashi, J. Am.Ceram. Soc., 84  85–91 (2001). Gintaras Valincius, Vytautas Reipa, Vincent Vilker, John T. Woodward and Mark Vaudin, Journal of The Electrochemical Society, 148 (8) E341-E347 (2001).This work is supported by Department of Information and Technology,Government of India
9:00 PM - G8.2
Structural and Magnetic Properties of La0.7Sr0.3Mn1-xNixO3 (x=0.5, 0.1, 0.2, 0.3, 0.4).
Thomas Creel 1 , Oran Pringle 1 , William James 2 , William Yelon 2 , Satish Malik 3 , Sylvio Quezado 3 , Jinbo Yang 4 , Jagat Lamsal 5 , Mehmet Kahveci 5 Show Abstract
1 Physics, Missouri University of Science and Technology, Rolla, Missouri, United States, 2 Chemistry, Missouri University of Science and Technology, Rolla, Missouri, United States, 3 , International Institue of Phusics(IIP), UFRN, Natal, Brazil, 4 , State Key Laboratory for Artificial Microstructure and Mesoscopic Physics and School of Physics, Beijing China, 5 , Department of Physics and Astronomy, Columbia, Missouri, United States
As part of a systematic study of perovskites containing the 3d transition elements Fe, Co, Cr, Cu and Zn, we have studied the structural and magnetic properties of La0.7Sr0.3Mn1-xNixO3 (x=0.05, 0.1, 0.20, 0.30, and 0.40) using x-ray and neutron diffraction and magnetic measurements. To our knowledge, there exists no neutron-diffraction data available for this group of perovskite compositions. Neutron (λ = 1.479Å) and x-ray (λ = 1.5481Å; Cu Kα) powder diffraction indicate for x >0.05 all samples have the main phase rhombohedral perovskite structure (space group R-3c) and a NiO impurity phase (space group Fm3m). Neutron diffraction data for the perovskite structure at 12K and 300K indicate ferromagnetic ordering for x 0.2 and antiferromagnetic ordering for x = 0.4. However, for x = 0.3, neutron diffraction at 12K suggests coexisting ferromagnetic and antifferomagnetic ordering while at 300K no magnetic ordering is found. Magnetic measurements for all values of x indicate deviations from simple ferromagnetic ordering and suggest ferromagnetic and antiferromagnetic transitions. These magnetic measurements also indicate that Curie temperature decreases with increasing Ni content, and show a very small amount of residual magnetic moment above room temperature that is likely due to the magnetic NiO. The magnetic measurements are consistent with the neutron diffraction measurements and for the perovskite main phase, indicate all samples have long-range magnetic ordering at low temperature and at room temperature exhibit transitions from ferromagnetic to no magnetic to antiferromagnetic ordering.
9:00 PM - G8.20
Inorganic Light Absorbing Semiconductor for Solid State Self-sensitized Solar Cells.
Chiew Keat Lim 1 , Ooi Kiang Tan 1 , Man Siu Tse 1 Show Abstract
1 , Nanyang Technological University, Singapore Singapore
In recent years, many advances in the solar cells with nanostructured oxide electrodes based on dye-sensitized solar cell (DSSC) have been intensively studied due to its simple structure and the potential to provide a low cost alternative as compared to the conventional silicon based solar cells. However, the limited availability of noble ruthenium complexes will become a problem for the wide applications of DSSCs. The recent success of nanocomposites heterojunction solar cells have proven that inorganic semiconductor can be used as light sensitizer replacing the organic dye. Here we report on the feasibility of a p-type perovskite semiconductor, strontium titanate ferrite (SrTi1-xFexO3, in short called STFX) as a candidate for light absorbing semiconductor in solar cells. By altering the substitution concentration of Fe, these nanostructured STFX materials have found to have band gap energies ranging from 3.17 eV to 1.8 eV. Thus, these STFX materials have good absorption range over the visible light region. Hence, STFX materials are the possible candidature to act as an inorganic sensitizer and hole conductor in the photovoltaic devices. In this work, the solid state self-sensitized solar cells consist of the nanostructured n-type TiO2 electrode and the p-type semiconductor STFX, that acts as a light absorbing material and hole conductor. A simple, low cost and low vacuum process will be adopted to fabricate the photovoltaic devices. The STFX materials synthesized using high temperature solid state reaction will be deposited using spin-coating method onto the platinum substrate. After that, the hydrothermally growth TiO2 nanoparticles will be used to infiltrate into the mesoporous layer of STFX to create the interpenetrating p-n networks. The photovoltaic responses of the devices fabricated tested using solar simulator with 100 mW/cm2 light intensity, shows high open circuit photovoltage of more than 0.50 V even though the short circuit photocurrent is relatively low.In short, using STFX as light absorbing semiconductor shows promising potential to be used in the solid state self-sensitized solar cells. However there are still rooms for improvement to achieve comparative photovoltaic energy conversion efficiency as the Ru-based dye-sensitized solar cells. References1.Nanu, M., J. Schoonman, and A. Goossens, Inorganic Nanocomposites of n-and p-Type Semiconductors: A New Type of Three-Dimensional Solar Cell. Advanced Materials, 2004. 16(5): p. 453-456.2.Rothschild, A., et al., Electronic structure, defect chemistry, and transport properties of SrTi1-xFexO3-y solid solutions. Chemistry of Materials, 2006. 18(16): p. 3651-3659.
9:00 PM - G8.21
Metal-doped Cobalt Oxide Nanostructures for Solution-processed Solid-state Dye-sensitized Solar Cells.
Chun-Te Ho 1 , Tri-Rung Yew 2 Show Abstract
1 Materials Science and Engineering, National Tsing Hua University, Hsinchu City Taiwan, 2 Materials Science and Engineering, National Tsing Hua University, Hsinchu City Taiwan
In this study, nanostructures of stable metal-doped cobalt oxide and self-assembled titanium dioxide were used to enhance the short circuit current and power conversion efficiency (PCE) of solid-state dye-sensitized solar cells (SS-DSSCs). Various methods of solution-processing were also utilized to fabricate the low-cost SS-DSSCs for large-scale applications. The metal-doped cobalt oxide nanostructures were hydrothermally synthesized through serials of process optimization and used as hole transport layers for SS-DSSCs. The metal-doping on cobalt oxide was optimized, especially to improve carrier concentration and sunlight absorption. The morphology and structure of the SS-DSSCs were characterized by scanning electron microscope (SEM) and high resolution transmission electron microscopy (HR-TEM). The crystallinity of metal-doped cobalt oxide nanostructures was also measured by X-ray diffraction spectroscopy (XRD). The band-gaps and absorption spectra were analyzed using UV-visible optical absorption spectroscopy and the PCE of SS-DSSCs and their electrical properties were measured using solar simulators.
9:00 PM - G8.24
An Amorphous Indium Gallium Zinc Oxide Solar-blind Thin Film Phototransistor.
Chiu-Jung Chiu 1 , Wen-Yin Weng 1 , Sheng-Po Chang 1 , Shoou-Jinn Chang 1 Show Abstract
1 , Nation Cheng Kung University, Tainan Taiwan
The optical and electrical properties of the amorphous indium gallium zinc oxide thin film transistor (a-IGZO TFT) with Ta2O5 dielectric layer were studied. We observed the a-IGZO thin film transistor reported thus far that the short wavelength effect obtained was found for the first time. a-IGZO TFT channel starts to absorb the light when the photon energy is approaching or near UV region and deep UV region that the IDS off current increases along with a negative shift of Vth. The subthreshold swing increases and becomes smooth when we illuminated the TFT at shorter wavelength. With an incident light wavelength of 230 nm and an applied gate bias of -0.4 V, it was measured responsivity of the device was 1.91 A/W. The results demonstrate that the a-IGZO thin film transistor allows for UV light sensor and solar-blind sensor applications.
9:00 PM - G8.4
Ab initio Study of the Hydrogen Molecule on ZnO Surfaces.
Po-Liang Liu 1 , Yen-Ting Wu 1 , Yu-Jin Siao 1 Show Abstract
1 Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung Taiwan
The wurtizite ZnO, a II-VI compound semiconductor, is a wide band gap of 3.37 eV compared to 3.39 eV for wurtizite GaN and large exciton binding energy (60 meV) for leading to the lasing action even upon room temperature. Various epitaxy technologies have been employed to grow high quality ZnO thin layer materials on GaN templates, including MOCVD, MOVPE, ALD, and so on. Hydrogen molecules (H2) is often employed as a carrier gas in MOCVD, MOVPE, and ALD. Unfortunately, it has been found that ZnO films are unstable and tend to disintegrate in H2 ambient, which is commonly ubiquitous during epilayer growth processes. Therefore, it is of significant interest to carry out a detailed study of H2 on ZnO surfaces. The dissociation of H2 and the adsorption of H on ZnO surfaces must be taken into account to estimate the role of H2 on ZnO surfaces. In order to provide further insight into the reaction of a H2 gas on (0001)-, (10-10)-, (2-1-10)-, and (11-23)-oriented ZnO surface, the surface phase diagram in the presence of H2 will be evaluated by using thermodynamic functions of the chemical potentials of the constituents in Models.In this study, we consider five surface models of Zn-terminated (0001)-, O-terminated (0001)-, (10-10)-, (2-1-10)-, and (11-23)-oriented ZnO layers. To elucidate the interaction of H2 on ZnO surfaces, we employ five surface models with H2 on the on-top, hcp-hollow, and fcc-hollow site of various surface models and determine their corresponding energies as a function of the adsorbate position of H2. This program employs ultrasoft pseudopotentials derived from the generalized gradient approximation (GGA) functional using the Perdew−Wang exchange and correlation functional (PW91), as implemented in the Vienna Ab Initio Simulation Package (VASP). The calculated result showed that the clean (2-1-10) oriented ZnO surface is more stable than others over a wide range of allowed chemical potentials. Only surfaces of O-terminated (0001)-oriented ZnO models exhibit active sites for the dissociation of H2, which in turn enables the formation of water from dissociative chemisorption of 2H on the surface oxygen atom of ZnO surfaces. A negative surface energy for O-terminated ZnO(0001) surface in the presence of water was observed under O-rich and H-rich condition. The finding of a negative surface energy of O-terminated ZnO(0001) surface suggests that O-terminated ZnO(0001) surface in the presence of water become more and more stable with increasing the surface area. High surface area means that the porous ZnO will be formed. Our result is consistent with the experimental observations that ZnO epitaxial layers are highly unstable and easily etched by hydrogen at typical growth temperatures. Apart from of O-terminated (0001)-oriented ZnO models, other four surface models exhibit that H2 desorbed from stable surfaces because of a nondissociatively adsorbed H2.
9:00 PM - G8.5
Ab initio Based Multiscale Modeling of the Effect of Kinetic Demixing on Sr Segregation in La1-xSrxMnO3.
Brian Puchala 1 , Yueh-Lin Lee 1 , Dane Morgan 1 Show Abstract
1 Materials Science and Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
Strontium surface segregation has been widely reported and implicated in decreased cathode performance in solid oxide fuel cells utilizing La1-xSrxMnO3 (LSMO) as the cathode material. In this work, we have considered the effect kinetic demixing may play in Sr surface segregation. We use a multiscale modelling approach integrating ab initio calculations of defect interactions and kinetic barriers, kinetic Monte Carlo simulations to determine diffusion constants, and a LSMO defect model for chemical potential gradients. The combined model allows us to calculate the relative fluxes of Sr and La and predict the tendency for Sr to demix as a function of operating conditions. We find that the migration barrier for Sr-vacancy exchange is significantly smaller than the La-vacancy exchange barrier, which would suggest much faster Sr diffusion and possibly significant demixing. However, we also find a significant repulsion between Sr and A-site vacancies which works to slow Sr diffusion.
9:00 PM - G8.6
Separation of Ionic and Electronic Contributions to Charge Transport in Complex-oxide Thin Films Using c-AFM and Near-field Scanning Microwave Microscopy.
Alexander Tselev 1 , Amit Kumar 1 , Mike Biegalski 1 , Sergei Kalinin 1 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Many complex oxides for oxide electronics and energy-related applications (manganites, cobaltites, titanates) are known as mixed ionic and electronic conductors. Knowledge of contributions of these different change transport mechanisms in the material response in different setting would be highly desirable for correct interpretation of the experimental data and for understanding of the material performance. While for bulk samples, contributions of ionic and electronic currents can be separated in principle due to significantly different mobilities of electrons and positively changed ions (oxygen vacancies) using impedance spectroscopy, it becomes a challenging and currently unresolved task when one deals with local measurement at micrometer and smaller length scale. In this presentation we demonstrate how near-field scanning microwave microscopy (SMM) can be applied in combination with dc-based c-AFM to identify local differences in the ionic and electronic contributions to the charge transport in thin films of different oxides grown by pulsed laser deposition. The SMM is performed at a frequency of about 2 GHz, which allows sufficiently strong capacitive coupling between an AFM-cantilever-like probe and a sample. The SMM imaging mechanism can be understood as arising due to change of electrical impedance of the probe that is capacitively coupled to a sample, and the image contrast is directly related to the sample complex dielectric permittivity (where the imaginary part is in turn directly related to the material conductivity). Ionic current contributions at these frequencies is negligible due to a low mobility of ions. The spatial resolution of the technique is ~100nm. The conductivity maps obtained at GHz frequencies are compared with local I-V maps obtained with c-AFM at dc. Research at ORNL's CNMS was sponsored by the Scientific User Facilities Division, BES, U.S. DOE.
9:00 PM - G8.7
Combinatorial Conductivity Analysis of Solid Oxide Fuel Cell Electrolyte Candidates.
Natalie Becerra 1 Show Abstract
1 Material Science and Engineering, Cornell University, Ithaca, New York, United States
Fuel cells are an improved form of energy storage because, unlike batteries, they never need to be re-charged and the by-products are mostly benign. Unfortunately, current fuel cells must operate at 800 degrees Celsius or higher. To find an efficient, low-temperature electrolyte for Solid Oxide Fuel Cells we use combinatorial analysis and off-axis sputtering. We have made samples of Yttria-stabilized Zirconia and Scandia-doped YSZ, both of which have a range of compositions. We have made conductivity vs. composition graphs and noted that the conductivity peaks around 3x10 -5 S/cm, at a composition of 30% Yttria. Compared to the literature on thin film YSZ conductivity, our measurements probe a considerably larger composition phase space. While there is a discrepancy between our peak conductivity measurements and current literature values, this is likely due to the space charge effect. To address this, we have deposited interdigitated contacts on our samples, allowing us to have more YSZ between our contacts, thus making the impedance readings larger and more accurate. Additionally, we have optimized analysis methods using an original program that visually compares models of raw impedance data.
9:00 PM - G8.8
Engineering LixAlySizO Thin Films as a Solid Electrolyte for 3D Microbatteries.
Ya-Chuan Perng 1 , Jea Cho 1 , Daniel Membreno 2 , Bruce Dunn 2 , Michael Toney 3 , Jane Chang 1 Show Abstract
1 Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 3 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Lithium-ion batteries have been widely used in portable electronic devices due to their high energy density. By engineering 3D electrode architectures based on high aspect ratio pillars, these batteries have the potential to be down-sized even further to power micro-systems on a chip. In order to realize this potential, however, an ultra-thin and highly conformal solid electrolyte layer coating on the 3D electrode array is required. The solid electrolyte, lithium aluminosilicate (LiAlSiO4), is a promising candidate for this application due to high ionic conductivity along its c-axis resulting from channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O). Atomic layer deposition (ALD) was employed to synthesize a stoichiometric thin film LiAlSiO4. The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating suitable for 3D micro-battery applications. The metal precursors used in this work are tetraethyl orthosilicate (TEOS), trimethylaluminum (TMA) and lithium t-butoxide (LTB) with water vapor as the oxidant. The overall deposition rate of stoichiometric LiAlSiO4 was ~5Å/cycle at a substrate temperature of 290°C. The cation ratios of each metal element in LixAlySizO (LASO) thin films are found to correlate closely to ALD cycles and the associated incubation times. The as-deposited films were amorphous but became crystalline after rapid thermal annealing at 900oC. The epitaxial relationship to silicon, a potential anode material, is beta-LiAlSiO4 (1bar 2 1bar 0) || Si (400) and beta-LiAlSiO4 (1 0 1bar 0) || Si (004). The Li-ion conductivities of as-deposited, amorphous LASO thin films were determined by impedance measurements to be a strong function of lithium content with values in the range of 10^-7 to 10^-8 S/cm. The crystalline films are expected to have higher conductivity due to the presence of 1D channels in the crystalline structure. The films were also grown on Si nanowires (NWs) to test as a possible prototype of a single NW based battery. The coating was confirmed to be conformal and uniform by transmission electron microscopy (TEM) imaging.
9:00 PM - G8.9
Direct Observation of Point Defects and Impurities in LiFePO4.
Wu Zhou 1 2 , Jaekwang Lee 1 2 , Juan-Carlos Idrobo 1 2 , Sokrates Pantelides 1 2 , Stephen Pennycook 2 1 Show Abstract
1 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States, 2 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Solid state Li-ion batteries have undergone rapid development in the past four decades. However, significant challenges still exist in searching for new electrode materials to increase the stored energy density, charge/discharge rate, safety, and service life and to decrease the cost for mass production. The olivine-type LiFePO4 material is now considered a promising new-generation cathode material due to its high energy density, high safety and low cost.It is well accepted that the ionic conductivity in LiFePO4 is highly orientation-dependent, and lithium diffusion tends to occur along the b-axis (i.e. lithium channels) , which could be affected by point defects located along these lithium channels . It has also been reported that the electrical conductivity of LiFePO4 can be increased by several orders of magnitude by doping . Therefore, a detailed atomic-level understanding of the intrinsic point defects, cation non-stoichiometry and doping in LiFePO4 is desirable in order to further improve the electrochemical performance of LiFePO4. In this work, we use aberration-corrected scanning transmission electron microscopy (STEM) and first-principles methods, based on density functional theory (DFT), to locate and identify point defects and impurities in LiFePO4. Antisite defects, where the Li sites are occupied by Fe ions, were directly identified from STEM imaging. Systematic studies along different crystallographic orientations suggest that these Fe(Li)-type antisite defects tend to segregate along the b-axis of LiFePO4. DFT calculations show that the clustering of Fe ions can be attributed to the strong bonding interaction of the Fe(Li)-V(Li) (Li vacancy) pairs along the b-axis. Using atomically resolved STEM-EELS mapping, we found that Fe(Li) antisites show an oxidation state slightly higher than that of the bulk material (+2), which can be expected from the segregation of Fe(Li)-V(Li) pairs along the b-axis as suggested by the DFT calculations. References: S. Nishimura et al. Nat. Mater. 7, 707-711 (2008) S. Chung et al. Angew. Chem. Int. Ed. 48, 543-546 (2009) S. Chung et al. Nat. Mater. 1, 123-128 (2002)This research was partially supported by the National Science Foundation under Grant No. DMR-0938330 (J-CI, WZ), by ORNL's Shared Research Equipment (SHaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy (J-CI) and the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (SJP, JL), DOE grant DE- F002-09ER46554 (STP), and by the McMinn Endowment (STP) at Vanderbilt University.