Christof W. Schneider, Paul Scherrer Institut
Craig B. Arnold, Princeton University
Nini Pryds, Technical Univesity of Denmark
Symposium Support CrysTec GmbH Kristalltechnologie
F2: Photo Catalysis
Monday PM, November 26, 2012
Hynes, Level 3, Room 309
2:30 AM - F2.01
Synthetic Biology Pathways of Carbon Dioxide Photosynthesis for Life Support and Energy Generation
Carolina I. Ragolta 2 Bin Chen 1
1NASA Ames Research Center Moffett Field USA2Harvard College Cambridge USAShow Abstract
Photosynthesis converts carbon dioxide (CO2) into hydrocarbons that can be utilized as food and fuel. We have developed a synthetic photosynthesis system that combines plant pigments and a titanium dioxide (TiO2) photocatalyst to convert CO2 into hydrocarbons using a broader range of solar energy. Synthetic pigment components were assembled by partially replacing magnesium ions in chlorophyll with TiO2. The TiO2 photocatalyst utilizes both UV and green light, changing the landscape of photosynthesis by allowing plants to scavenge additional wavelengths. TiO2 nanowires and nanotubes also resemble the natural antenna systems formed by plant pigments that concentrate light energy. Hybrid pigment structures were formed by self-assembly on silk protein substrates and analyzed using UV-Vis absorption spectroscopy, tensile strain, and structure analysis. The hybrid pigment structures were also confirmed for biocompatibility and physiology compatibility. Water and CO2 were converted to hydrocarbons using a microfluidic and capillary approach under simulated light and atmospheric and compressed gas extraction pressure. The synthetic photosynthesis system efficiently produces hydrocarbons not possible in natural plants with unprecedented efficiency and applicability.
2:45 AM - F2.02
Ab initio Study of Anatase TiO2 Surfaces for Solar Hydrogen Production
Frederico Sanches 1 3 Giuseppe Mallia 1 3 Leandro Liborio 3 Nicholas Harrison 1 2 3
1Imperial College London London United Kingdom2STFC- Daresbury Laboratory Daresbury United Kingdom3Imperial College London London United KingdomShow Abstract
Photolytic water splitting as a method of hydrogen production has attracted a lot of attention since Honda et al. first demonstrated this concept with TiO2 in 1972 . TiO2 has since been used as a reference material in numerous experimental as well as in theoretical studies in the field of solar hydrogen production[2-4]. The most promising performance is observed with nanostructured films consisting of predominantly the anatase TiO2 phase. The atomic scale structure of such films is not well understood, and could be essential to further the understanding of the mechanisms involved in this complex process. Being able to predict which anatase surfaces are likely to be exposed in experimental conditions is, therefore, essential to achieve this goal. With the aim of gaining a better understanding of anatase surfaces, Li et al studied a vicinal surface of anatase. The experimental techniques used did not allow for the determination of the atomic structure of the surface or distinguish between the (514) and (516) planes. In this work we present hybrid-exchange density functional theory calculations of low-index anatase surfaces as well as the vicinal surfaces possibly observed by Li et al.. We show that the (516) surface is very stable and, in fact, has a surface formation energy comparable to the (101) surface. The (514) surface is also relatively stable with respect to other low-index surfaces and both could provide ways of forming low energy steps. These could be important when forming nanostrucured films. Electronic structure calculations were used to simulate STM images (in the constant current mode) to allow for the comparison of the computed structures with experimentally obtained images. References 1. Fujishima K. and Honda, A. Nature, 1972 238, 37. 2. Graetzel, M. Nature, 2001, 414, 338-344 3. Lazzeri, M. and Selloni, A. Phys. Rev.Lett. 2001, 87, 266105 4. Labat, F., Baraneka, P. Domain, C., Minot, C., Adamo, C. J. Chem. Phys. 2007 126, 154703 5. Li, S.C., Dulub, O. and Diebold, U., J.Phys.Chem.C Lett. 2008 112, 16166-16170
3:00 AM - F2.03
Electronic and Optical Properties of TaO1-xN1+x Alloys
Nabil Al-Aqtash 1 Florin Apostol 1 Wai-Ning Mei 1 Renat Sabirianov 1
1University of Nebraska Omaha USAShow Abstract
TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON alloys. Specifically, we show that the position of conduction and valence band can be modified by varying the oxygen and nitrogen concentrations in TaO1-xN1+x. We find that the band gap decreases monotonically with the increase of N/O ratio. The band gap energy is decreased in monoclinic TaON from near 2.7eV to just over 1.1eV (i.e. by 230%) when N/O ratio is reduced from 1/3 to 3/1. The band gap reduction is mostly associated with the change in the position of the valence band due to the hybridization of N 2p states, while the conduction band consisting mostly of Ta 5d-states is not sensitive to N content. The calculated optical absorption spectra show reduction in the optical band gap with increasing N/O ratio. Our calculations show that the band gap reduces as function of the N/O ratio in a series of experimentally fabricated alloys ZrTa3O5N3->TaON->YTa7O7N8 where the latter has the cubic, and the formers have the baddeleyite crystal structure.
3:15 AM - F2.04
Extending the Limits of Hematite (Fe2O3)-based Water Splitting by Using Homojunctions and Heterojunctions
Matthew T. Mayer 1 Chun Du 1 Yongjing Lin 1 Yang Xu 1 Dunwei Wang 1
1Boston College Chestnut Hill USAShow Abstract
Photoelectrochemical water splitting using semiconductor electrodes offers a promising route towards harvesting solar energy and storing it in chemical bonds. However, efficient performance requires several traits which one single material cannot itself satisfy, foremost being the generation of a photovoltage sufficient to drive both the reduction and oxidation half-reactions of electrolysis (1.23 V plus overpotentials). For instance, the metal oxide hematite (α Fe2O3) has a band gap suitable for substantial solar absorption, yet with a conduction band too positive to achieve water reduction, hematite-only electrodes require applied bias to achieve water splitting. No degree of catalyst treatments, doping, or nanostructuring can solve this fundamental problem of band misalignment. We have therefore performed studies on the under-investigated aspect of hematite electrode energetics, using combinations of materials design and high-quality synthesis by atomic layer deposition (ALD). Both n-p homojunction hematite and silicon nanowires / hematite heterojunction devices produced enhanced photovoltages, manifested as cathodic shifts in the photocurrent onset potentials signifying a decreased requirement for applied anodic bias. These results represent some of the lowest turn-on potentials observed on hematite devices and were achieved without hematite doping, catalysts, or surface treatments, pointing towards an important new direction of study for enhancing the efficiency of metal oxide photoelectrodes.
3:30 AM - F2.05
Design of Advanced Photocatalysts: Perspective of Janus Type Heterostructures
Wolfram Jaegermann 1
1Darmstadt University of Technology Darmstadt GermanyShow Abstract
The anticipated production of fuels as H2 or CH-compounds with solar light would provide a sustainable and secure primary energy source by producing a storable and transportable fuel. For this reason oxide photocatalysts to mimic photosynthesis have intensively been studied in the past. However, the systems identified so far are limited in their conversion efficiencies. Advanced heterostructure photocatalysts are suggested in this presentation based on nanosized Janus type structures. We suggest the combination of a wide band gap light absorber (bandgap 2-3eV) and adapted contact materials (co-catalysts) in a photovoltaic arrangement which breaks the spherical symmetry of the absorber and provide the conditions for quantum efficiencies approaching 100%. Central part is the (nanoscale) synthesis of Janus type heterostructures either by combining two different semiconductor materials with a well defined mis-alignment of band edges (type II alignment) or by forming asymmetric contacts by different metals providing different work functions. Efficient charge carrier separation is provided by a well-defined vectorial separation of electron-hole pairs and transport of electrons and holes to the different co-catalyst particles ideally deposited on opposite sides of the semiconductor. The additionally needed minimization of photovoltage (chemical potential) losses can be expected for isoenergetic electronic coupling of the highly efficient HER/OER catalyst to the conduction/valence band of the semiconductor. Possible ways to manufacture such Janus structures will be presented.
3:45 AM - F2.06
Atomic Layer Deposition of Platinum on Conductive Titanium Oxide Films for Hydrogen Fuel Cell Catalysts
Robin Paul Hansen 1 Eric Eisenbraun 1
1College of Nanoscale Science and Engineering at SUNY Albany Albany USAShow Abstract
Current fuel cell technology requires the use of large amounts of platinum in the cathode for the oxygen reduction reaction (ORR). A major roadblock to the commercialization of fuel cells is the high cost of Pt. Current efforts to reduce the cost of producing fuel cells have focused on the minimization of the amount of Pt, thus bringing down the overall cost. Prior work has demonstrated the creation of stable conductive titanium oxide films grown using atomic layer deposition (ALD). ALD is a self-limiting technique used to grow single layers of a film, or well dispersed particles of a material. These ALD-grown supports have demonstrated themselves to be stably conductive in oxidizing environments. This allows for noncontinuous coatings of Pt catalysts for the ORR. Current work focuses on ALD Pt on titanium oxide grown in a multi-chamber ALD cluster tool. Pt was deposited using methylcyclopentadienyltrimethyl platinum (IV) (MeCpMe3Pt) as the precursor with NH3 and H2 as reactant gases, and a recipe was developed to optimize the the Pt with respect to growth rate and conductivity. In addition, post-deposition annealing in a reducing atmosphere was used to increase conductivity of ALD Pt films. Films were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS), four point probe analysis, and cyclic voltammetry (CV). It was found that Pt nanoparticle size could be controlled by adjusting the number of ALD growth cycles. In the first 50 cycles of Pt deposition, particles in the 3-5 nm regime are visible. After 200 cycles, the particles have grown and coalesced into full films. After this point no further increase in catalytic activity is expected.
F3: Oxide Thin Films for PV
Monday PM, November 26, 2012
Hynes, Level 3, Room 309
4:30 AM - F3.01
The Effect of Atmospheric Exposure on the Electronic Properties of ZnO/Cu2O Thin Film Heterojunctions
Robert Waddingham 1 Ahmed Kiani 1 Andrew Flewitt 1
1University of Cambridge Cambridge United KingdomShow Abstract
Cuprous oxide (Cu2O) has emerged as a promising p-type thin film semiconductor for solar cells as its bandgap is well matched to the solar spectrum. However, as it is not possible to stably dope the material n-type, it is necessary to use heterojunction cells rather than homojunctions. There are several published reports that heterojunctions of Cu2O deposited onto ZnO have on/off current rectification ratios of over 100 [Gershon, T., et al., Sol. Energy Mater. Sol. Cells, 96, 148-154 (2012); Mittiga, A., et al., Appl. Phys. Lett., 88, 163502 (2006)]. In these previous studies, a number of deposition techniques have been employed, including sputtering, thermal oxidation and electrodeposition. In all cases, the junction has been exposed to atmosphere between the deposition of the two layers. In this work, ZnO/Cu2O heterojunctions were fabricated using a low temperature sputtering technique (HiTUS) which allows the ZnO and Cu2O thin films to be deposited consecutively without the need to break vacuum [Li, F.M., et al., Thin Solid Films, 520, 1278-1284 (2011)]. These heterojunctions were found to have a rectification ratio of 10, which compares poorly to cells published in literature fabricated with atmospheric exposure to the heterojunction interface. Hence, a series of experiments were performed to investigate this apparent effect. It is known that ZnO is prone to adsorption of both oxygen and water vapour. Therefore this study aimed to understand if either or both of these were responsible for the improved cell rectification. It was found that when the ZnO thin film was exposed to atmospheric conditions briefly (8 minutes) before the Cu2O layer was deposited, the resulting heterojucntions had improved rectification ratios of up to 300. This improvement came from a reduction in the reverse saturation current, suggesting that the ZnO/Cu2O interface defect density had been reduced by the exposure. Further devices were also fabricated with prolonged exposure to atmosphere, but this did not noticeably improve the junction quality. In order to investigate whether H2O or O2 was the likely cause of the improvement, cells were fabricated where the ZnO layer was dipped in water before the Cu2O deposition. These were found to produce diodes with a rectification ratio of over 28000. Control cells were also fabricated with exposure to atmosphere for 362 hours followed by annealing at 393 K for 15 minutes and this resulted in the rectification ratio reducing once more to 10. These results demonstrate for the first time that exposure to H2O is important for controlling interface states in the ZnO/Cu2O heterojunction. This result has implications for the fabrication of ZnO based thin film devices where the active interface is exposed to atmosphere during processing.
4:45 AM - F3.02
Structural Modulation of Indium Zinc Tin Oxide Thin Film Using Radio Frequency Magnetron Sputtering
Ajaya K Sigdel 1 2 Paul F. Ndione 2 John D. Perkins 2 Kim Jones 2 Maikel van Hest 2 Thomas Gennett 2 Sean E Shaheen 1 2 David S. Ginley 2 Josheph J. Berry 2
1University of Denver Denver USA2National Renewable Energy Laboratory (NREL) Golden USAShow Abstract
Transparent conducting oxides (TCOs) with tunable morphology and tunable work functions are of general interest in a range of optoelectronic systems. In this study, we have examined variation of opto-electronic properties such as crystallinity, transparency, conductivity, work functions and surface roughness of Indium Zinc Tin Oxide (IZTO) thin films. Specifically we have focused on controlled radio frequency magnetron sputter deposition from a fixed composition In0.5 Zn0.25Sn0.25Ox target. Our focus was to evaluate the effect(s) of various regions of empirical parameter space during deposition of the thin films including: power density applied to the target (ρrf); substrate temperature (Ts); and process gas pressure (P). The results indicate that films grown at room temperature (30 oC) are largely amorphous while those grown at elevated temperature of 150 oC and 250 oC show varying crystallinity which is multivariate dependent upon other deposition conditions. Cross-sectional TEM data shows the amorphous IZTO (a-IZTO) films obtained for a higher temperature of 250 oC suffered some degree of phase segregation. Crystalline IZTO (c-IZTO) films have x-ray diffraction with the bixbyite In2O3 structure suggesting it is primarily In2O3 with SnO2 and ZnO as co-substitutional dopant. The films grown with increased substrate temperature yielded more efficient doping in both a-IZTO and c-IZTO resulting in high conductivity films. The highest σ to date for this composition of c-IZTO and a-IZTO film are respectively 2260 ± 30 S/cm and 1470 ± 20 S/cm. Higher temperature deposited TCOs have broader optical gap due to a Burstein-Moss shift resulting from the increase in carrier concentration and improved doping efficiency. Films have work functions ranging from -5.6 eV to -6.1 eV and optical transparency of >80% in the visible is observed for both high conductivity a-IZTO and c-IZTO films. Both c-IZTO and a-IZTO with similar conductivity show significant variation in surface conductivity as measured by conductive AFM. The impact of these changes to surface conductivity is examined in the context of organic photovoltaic devices in which we demonstrate the use of IZTO as a transparent contact. Acknowledgements: Materials development including oxide deposition and characterization equipment was provided by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory. Support for characterization and analysis work was provided as part of the Center for Interface Science: Solar-Electric Materials (CIS:SEM), an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award Number DE-SC0001084
5:00 AM - F3.03
Mobility Enhancement in CdO by Rapid Thermal Annealing
Sylwia Anna Grankowska 1 2 Douglas M. Detert 2 3 Lothar A. Reichertz 4 Derrick T. Speaks 2 3 Kin M. Yu 2 Wladek Walukiewicz 2
1Warsaw University Warsaw Poland2Lawrence Berkeley National Laboratory Berkeley USA3University of California Berkeley Berkeley USA4Rose Street Laboratories of Energy Phoenix USAShow Abstract
It has been recently demonstrated that intentionally doped cadmium oxide (CdO) can have very high mobility of around 300 cm2/Vs at electron concentration of 3x1020 cm-3. These properties, combined with a wide transmission window extending from 400 nm to above 1500 nm, make CdO a promising transparent conductive material for solar cell application as well as for photodiodes and gas sensors . It should be noted that these exceptional electrical properties were obtained in polycrystalline CdO synthesized by variety of methods including pulsed laser deposition (PLD) and sputtering. Here, we use thermal annealing as a tool to better understand the influence of native defects on the electrical and optical properties of undoped and intentionally doped CdO films. Samples were synthesized by pulsed laser deposition and sputtering techniques at growth temperatures ranging from 25-320 °C. The films were treated with rapid thermal annealing (RTA) for 30 seconds at 300-600 °C in nitrogen and oxygen atmospheres. The electron concentrations and mobilities were determined by Hall effect measurements and the optical characteristics were measured using optical absorption and photoluminescence (PL). We find that both the annealing temperature and atmosphere affect the properties of the films. For nominally undoped samples that received RTA treatments in an oxygen atmosphere, a decrease in the electron concentration and an increase in the mobility is observed at RTA temperatures up to 400 °C, with the electron concentration and mobility reaching saturation values of n~5x1019 cm-3 (as grown: n~2x1020 cm-3) and µ~200 cm2/Vs (as grown µ~75 cm2/Vs). Such trends are not observed in samples annealed in nitrogen atmosphere. Collectively, these results can be explained within the model that native defects serve as donors within CdO: with increasing RTA temperature in an oxygen ambient, the carrier concentration decreases as oxygen vacancies are healed. Moreover, the removal of native defects reduces ionized impurity scattering and leads to an increase in the mobility. We will also present the results of the absorption and PL measurements as well as our results of the effects of thermal annealing on the properties of intentionally doped CdO.  K.M Yu, et al., J. Appl. Phys. 111 123505 (2012) This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
5:15 AM - F3.04
Engineering the Interface of Inverted Organic Solar Cells Comprising Al-doped ZnO to Enhance Photo-voltage and Fill Factor
Abay Gadisa Dinku 1 Yingchi Liu 2 Edward T. Samulski 1 Rene Lopez 2
1The University of North Carolina at Chapel Hill Chapel Hill USA2The University of North Carolina at Chapel Hill Chapel Hill USAShow Abstract
Inverted organic solar cells have attracted great attention due to their exceptional environmental stability compared to the conventional solar cell architecture.1 Metal-oxides are commonly employed in inverted solar cells as electron collector/injector layers. Inverted solar cells are subjected to losses, in particular in photo-voltage, due to imperfect injection levels and interfacial charge traps at the metal-oxide interfaces. Here, we demonstrate a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. Interface characterization using ultraviolet photoemission spectroscopy shows that the interface modification with N719 results into modification of charge injection levels. The efficiency of inverted solar cells with a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester has increased from 2.80 to 3.80 percent upon employing the modified electrodes.2 References: 1. Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter, and A. J. Heeger, Adv. Mater. 23, 1679 (2011). 2. A. Gadisa, Y. Liu, E. T. Samulski, and R. Lopez, Appl. Phys. Lett. 100, 253903 (2012). # This work is supported by the NSF SOLAR Grant (DMR-0934433).
5:30 AM - F3.05
Indium Tin Oxide-based Multi-layers: A Solution toward Transparent Conducting Electrodes of High Electro-mechanical Durability
Zheng Jia 1 Cheng Peng 3 Henry Neilson 3 Teng Li 1 2 Jun Lou 3
1University of Maryland College Park USA2University of Maryland College Park USA3Rice University Houston USAShow Abstract
Indium tin oxide (ITO)-based transparent conducting electrodes have been widely used in electronic devices. However, such brittle oxide electrodes suffer from poor mechanical durability, which poses significant challenge to their successful application in large area flexible electronics, such as paper like displays and flexible solar cells. Inspired by recent development of inorganic/organic hybrid permeation barriers for flexible electronics, we design and fabricate ITO-based multilayer electrodes with enhanced electro-mechanical durability. In situ electro-mechanical experiments of five structural designs of ITO-based multilayer electrodes are performed to investigate the evolution of crack density and the corresponding variance of electrical resistance of such electrodes. A coherent mechanics model is established to determine the driving force for crack propagation in the ITO layer in these electrodes. The mechanics model suggests that a top protective polymeric coating above and an intermediate polymeric layer below the ITO layer can effectively enhance the mechanical durability of the ITO electrodes by reducing the crack driving force up to ten folds. The modeling results offer mechanistic understanding of the in situ experimental measurements of the critical fracture strains of the five types of ITO-based multilayer electrodes. The findings in this work provide quantitative guidance for the material selection and structural optimization of ITO-based multilayer transparent electrodes of high mechanical durability.
5:45 AM - F3.06
TaOx Film Based MIS Photovoltaic Device
Pawan Tyagi 2 1
1University of the District of Columbia Washington DC USA2University of Kentucky Lexington USAShow Abstract
Application of the economical metal oxide thin-film photovoltaic devices is hindered by the poor energy efficiency and complexity of device fabrication approach. This paper investigates the metal-insulator-semiconductor (MIS) type photovoltaic cell with an ultrathin tantalum oxide (TaOx) tunnel barrier, formed by the plasma oxidation of a pre-deposited tantalum (Ta) film. These ~3 nm TaOx tunnel barriers showed approximately 160 mV open circuit voltage and 3-5% energy efficiency, for varying light intensity. The ultrathin TaOx (~3 nm) could absorb approximately 12% of the incident light radiation in 400-1000 nm wavelength range; this strong light absorbing capability was found to be associated with the dramatically large extinction coefficient. Spectroscopic ellipsometry revealed that extinction coefficient of 3 nm TaOx was ~0.2, two orders higher than that of the stochiometeric Ta2O5. Interestingly, refractive index of this 3 nm thick TaOx was comparable with that of stochiometeric Ta2O5. However, heating and prolonged high-intensity light exposure deteriorated the photovoltaic effect in TaOx junctions. This study provides the basis to explore the photovoltaic effect in a highly economical and easily manufactured ultrathin metal oxide tunnel barrier or analogous systems.
F4: Poster Session
Monday PM, November 26, 2012
Hynes, Level 2, Hall D
9:00 AM - F4.01
Imaging Columnar Hematite Thin Film for Understand it Photoelectrochemical Behavior for Water Splitting
Edson Roberto Leite 1 Ricardo Henrique Gonsalves 1
1Federal University of Samp;#227;o Carlos Samp;#227;o Carlos BrazilShow Abstract
Energy from the sun can provide sufficient power for all of our energy needs if it can be efficiently harvested. An elegant and potentially efficient route to storing solar energy is to convert light into chemical energy in the form of chemical bonds, which is a form of an artificial photosynthesis process. Considering the abundance of H2O on the planet, water splitting is a natural pathway for artificial photosynthesis. Hematite (α-Fe2O3) is a candidate material to be used as a photoanode for water splitting due to intrinsic properties such as suitable band gap (2.0-2.2 eV), chemical and photoelectrochemistry stability, abundance and low cost. By combining high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with analytical capability, we investigated the nanostructure of a textured hematite photoanode with columnar grains obtained by the colloidal deposition of magnetite nanocrystals. This initial report describes in detail the structure and chemistry of the α-Fe2O3/FTO interface by identifying semi-coherent and incoherent interfaces as well as a localized inter-diffusion layer of Sn and Fe at the interface (~100 nm in length). Our study indicates unintentional doping by tin at a high sintering temperature is not significant in enhancing hematite photoanode performance for water oxidation. The correlation of nanoscale morphology with photoelectrochemical characterization facilitated the identification of the beneficial effect of a preferential growth direction of a hematite film along the  axis for water-splitting efficiency. In addition, the colloidal process combined with high sintering temperature resulted in achieving a photocurrent of 1.85 mA.cm-2 at 1.23 VRHE which is one of the best photoelectrochemical performances reported to date in the literature for pure hematite.
9:00 AM - F4.04
The Band Offsets between Copper Oxide, Zinc Oxide and Gallium Nitride
Benedikt Kramm 1 Andreas Laufer 1 Daniel Reppin 1 Achim Kronenberger 1 Philipp Hering 1 Angelika Polity 1 Bruno K. Meyer 1
1Justus Liebig Universitaet Giessen GermanyShow Abstract
The band offset between two semiconductor materials is essential for the behavior of the charges at the heterointerface. As known from Minemoto et al.  it affects the electron transport in device applications. Using photoelectron spectroscopy (XPS) we investigate the band offsets of the heterointerfaces Cu2O/ZnO and Cu2O/GaN. For the first one we found a conduction band offset (CBO) value of 0.97 and for the other one of 0.24 eV. Out of this information one can see, that the large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells. For good photovoltaic performance a low conduction band offset is necessary. Thus gallium nitride seems to be a more suitable candidate for the front contact of Cu2O based solar cells.  Minemoto, T. et al., Solar Energy Materials and Solar Cells, 2001, 67(1-4):83-88.
9:00 AM - F4.05
Growth, Structure, and Electrical Properties of Metastable, Room Temperature delta;-Bi2O3 Thin Films
Danielle Proffit 1 2 Matthew Highland 1 Seong-Keun Kim 1 Chad Folkman 1 Guo-Ren Bai 1 Peter Baldo 1 Paul Fuoss 1 Dillon Fong 1 Thomas Mason 2 Jeffrey Eastman 1
1Argonne National Laboratory Lemont USA2Northwestern University Evanson USAShow Abstract
Oxygen ion conductors are critical components in many important energy conversion devices, including solid oxide fuel cells (SOFCs), oxygen separation membranes, and oxidation catalysts. Enhancements in performance may be possible using thin film growth to produce properties not seen in bulk samples, such as using epitaxy or strain to grow metastable phases. One model ionic conductor is the high temperature phase, δ-Bi2O3, which exhibits the highest oxygen ion conductivity of any oxide material but is stable only in a very narrow temperature range, 730-825°C. The study of δ-Bi2O3 thin films can provide insight into the key structural factors for high ionic conductivity as well as the challenges in enlarging the stability region of such a material. In our research, we have stabilized δ-Bi2O3 to room temperature via synthesis on (001) perovskite single crystal surfaces. Growth by magnetron sputtering using a Bi2O3 target in an Ar/O2 gas mixture at substrate temperatures higher than 300°C produced (001)-oriented δ-Bi2O3 nanoislands coherently strained to the (001) substrates. Synchrotron x-ray scattering observations at controlled temperatures and oxygen partial pressures revealed that the δ-Bi2O3 nanostructures exhibit a superstructure that may arise from ordering of the vacant oxygen sites. Continuous (001)-oriented single crystal films were achieved by sputtering at substrate temperatures of 300°C and below in a 100% oxygen environment. Synchrotron studies show the continuous films do not exhibit the same superstructure as the nanoislands grown at higher temperature. The potential for achieving high ionic conductivities at low temperatures in δ-Bi2O3 films and for elucidating the origin of superionic conductivity in oxide materials in general will also be discussed.
9:00 AM - F4.06
Thin-film K0.15MnO2 Anodic Materials for Pseudocapacitors
Matthew Philip Yeager 1 Wenxin Du 1 Rui Si 2 Dong Su 3 Nebojsa Marinkovic 4 Xiaowei Teng 1
1University of New Hampshire Durham USA2Brookhaven National Laboratory Upton USA3Brookhaven National Laboratory Upton USA4University of Delaware Newark USAShow Abstract
In addition to clean and renewable energy generation, the subsequent storage and delivery of such energy is likewise a critical facet to achieving long-term sustainable energy practices. An emerging technology known as electrochemical capacitors (ECs) has exhibited efficient, high-power energy storage in both the commercial and academic realm. ECs are physically analogous to traditional capacitors (i.e. charge storage occurs in opposing electrodes), but couple an organic/aqueous electrolyte with high surface area electrodes to enhance energy storage by means of electrical double layer phenomena. Pseudocapacitors (PCs) are a subcategory of ECs that strive to further enhance energy storage by depositing or substituting redox active materials onto the electrode surfaces, thereby permitting the aqueous/organic electrolyte to interact with the electrodes via redox processes that yield greater net charge transfer. Ruthenium dioxide, RuO2, has been the most promising candidate for PC electrode materials, but the implementation of RuO2-based PCs has been hampered by its high market price. Manganese dioxide, MnO2, has displayed auspicious redox characteristics, particularly with respect to its market cost, but suffers from long-term redox instabilities and generally prohibitive electrical resistance. The goal of this research is to explore the use of thin-film manganese dioxide for incorporation into composite PC anode materials. This desired morphology promotes redox activity with an increased active surface area while simultaneously reducing the electron diffusion paths for rapid power delivery. We report the facile synthesis of birnessite K0.15MnO2 nanosheets, which were fully characterized by transmission electron microscopy (TEM), scanning TEM (STEM), high-resolution TEM (HRTEM), X-ray diffraction spectroscopy (XRD), energy dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA). A K0.15MnO2 composite material was tested in a three-electrode cell with Na2SO4 electrolyte, resulting in a specific capacitance in excess of 300 F/g that significantly increased baseline carbonaceous materials. To further elucidate the properties of the evolving Mn(III/IV) valence state during discharge processes, we constructed an in situ X-ray absorption near edge spectroscopy (XANES) cell, which permitted for transmission and fluorescence analyses while simultaneously applying an external electrical potential. These results suggested a region of principal Mn(III/IV) oxidation/reduction for optimal operating voltage ranges of MnO2-based PCs.
9:00 AM - F4.07
Impact of Tungsten Oxide Colloidal Deposition on the Photoeletrochemical Water Oxidation
Edson Roberto Leite 1 Lucas Daniel Tognoli Leite 1 Ricardo Henrique Goncalves 1
1Federal University of Samp;#227;o Carlos Samp;#227;o Carlos BrazilShow Abstract
N-type oxide semiconductors are of great technological importance, mainly when these oxides show photoactivity for water oxidation via photoeletrochemistry cell (PEC). Among the n-type semiconductors oxides, the tungsten oxide (VI) shows a promising behavior for photoanode, due to the maximum theoretical efficiency conversion of 8% considering a band gap of 2.6 eV. Here we report an approach to prepare tungsten oxide (VI) photoanode by a non-hydrolytic route where tungsten oxide was synthesized in oleylalcohol at 150 oC. The colloidal stability in hexane was achieved by addition of oleylamine, which have the ability to bind strongly to the acid sites on the surface of the oxide. In this way, the tungsten oxide colloid was obtained in hexane, forming a yellowish solution in a single synthetic step. Herein this stable precursor was easily deposited on an FTO glass substrate. The thin film prepared was thermally treated at different temperatures, in order to study the effect of temperature in the grain growth and consequently on the WO3 photoresponse. The relevant result obtained in the process was a photocurrent (at 1.23 VRHE) of 1.09 mA cm-2 in a standard condition of AM 1.5 G illumination (in H2SO4 electrolyte - 1.0 mol). The morphology and thickness was characterized with scanning electron microscopy (SEM). Besides, the cross-sectional TEM image was acquired with sample prepared with Focus Ion Beam (FIB-SEM) that reveled a mesoporous thin film and excellent interface. The film microstructure and the photocurrent showed clear sintering temperature dependence.
9:00 AM - F4.08
Formation of a Core-shell Structure Using Ba-Si-Ti-B-Al-O Glass on BaTiO3 Nanoparticles by Pulsed Laser Deposition for Applications in Capacitive Energy Storage
Brian Ozsdolay 1 B. C. Riggs 1 X. Su 1 M. Tomozawa 1 D. B. Chrisey 1 2
1Rensselaer Polytechnic Institute Troy USA2Rensselaer Polytechnic Institute Troy USAShow Abstract
Nanoparticles having core-shell morphology have found use in disparate applications including improved biocompatibility for magnetic nanoparticles, reducing agglomeration, or chemical functionalization. In this study, BaTiO3 nanoparticles were coated with a crystallizable glass shell (29.6% BaO-7.4% SiO-37% TiO2-8% SiO2-16% B2O3-2% Al2O3) by pulsed laser deposition (PLD) and were used as improved dielectric starting materials for capacitors. Utilizing the extremely high dielectric constant of ferroelectric titanates and the high breakdown strength of alkali-free glass, the core-shell morphology further increases the composite dielectric homogeneity and the hermetic seal formed at the interface. 50 nm diameter BaTiO3 particles were coated with amorphous thin film glass by a KrF excimer laser at 0.24 J/cm2 for 15 minutes for a desired 6nm layer. The core-shell particles were then sintered at 1200C for 4 hrs to form a pellet for testing. The resulting composite material consisted of a glass matrix surrounding the core ferroelectric nanoparticles. The composite mixture was further improved for capacitive energy storage because the glass stoichiometry used would crystallize into BaTiO3, i.e., BaTiO3 in the glass was then crystallized at 1000C for 2 hrs, adding to the original core particle by heterogeneous nucleation and increasing the dielectric constant of the overall composite material. The remaining glass increased the breakdown strength of the material, allowing for the creation of especially high energy density capacitors. Electron microscopy was used to characterize the core-shell structure of the nanoparticles.
9:00 AM - F4.09
Enhancing Oxygen Permeation Performance of Electronically Short-circuited Oxygen-ion Conductor by Decorating with Mixed Ionic-electronic Conducting Oxide Thin Films
Lei Wang 1 Susumu Imashuku 1 Alexis Grimaud 1 Dongkyu Lee 1 Yang Shao-Horn 1
1Massachusetts Institute of Technology Cambridge USAShow Abstract
The mixed ionic-electronic conducting (MIEC) oxides are effective catalysts towards oxygen reduction reaction (ORR) and therefore they have been widely studied as potential solid oxide fuel cell cathode materials [1-3]. In this study, we show that the oxygen permeation flux of Au-short-circuited oxygen ion conductor membranes is dramatically enhanced (approximately 2 orders of magnitude) by decorating the surface with porous MIEC La0.8Sr0.2CoO3-δ (LSC113) or LaSrCoO4±δ (LSC214) thin films prepared by pulse laser deposition (PLD). The oxygen permeation performance of these membranes is further improved by changing the short-circuiting material from Au to Ag. Crystal structure and surface morphology were studied by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Using gas chromatography (GC) measurements the oxygen permeation properties were examined. Based on the feed and permeate side oxygen partial pressure dependence of the oxygen permeation flux, the feed side ORR is determined to be rate-limiting. The aforementioned enhancement of the oxygen permeation flux is then believed to be mainly due to the accelerated ORR at the feed side. Consequently further enhancement can be achieved by varying the morphology and/or the composition of the decoration MIEC oxide thin films.  S. B. Adler, Chem. Rev. 104 (2004) 4791.  A. Tarancon, M. Burriel, J. Santiso, S. J. Skinner, J. A. Kilner, J. Mater. Chem. 20 (2010) 3799.  G. J. la O', S. J. Ahn, E. Crumlin, Y. Orikasa, M. D. Biegalski, H. M. Christen, Y. Shao-Horn, Angew. Chem. Int. Ed. 49 (2010) 5344.
9:00 AM - F4.10
Dynamic Properties of Spectrally Selective Reactively Sputtered Transition Metal Oxides
Stephanie D. Worsley 1 Terence L. Baker 1 Aswini K. Pradhan 2 Adetayo Victor Adedeji 1
1Elizabeth City State University Elizabeth City USA2Norfolk State University Norfolk USAShow Abstract
Thermochromic properties of nanolayers of titanium and vanadium oxides and their alloys have been investigated. The ultra thin oxides layer was deposited by reactive magnetron sputtering from pure titanium and vanadium targets. The sputtering was done in Argon-Oxygen mixture at elevated substrate temperatures. The development of “smart window” materials for efficient energy consumption in buildings and automobiles are of interest in this project. The optical and electrical properties of the thin films as a function of temperatures are reported. The composition and thickness of the films was determined with Rutherford Backscattering Spectroscopy (RBS) and Energy Dispersive Spectrometer (EDS) data. The surface morphology of the films from SEM and AFM are reported also.
9:00 AM - F4.11
Predicting the Electrocaloric Behavior of BaTiO3 from Effective Hamiltonian Methods
Scott P Beckman 1 Jordan A Barr 1 Liwen F Wan 1 Takeshi Nishimatsu 2
1Iowa State University Ames USA2Tohoku University Sendai JapanShow Abstract
A pyroelectric crystal develops a spontaneous electrical polarization when its temperature changes. It is possible to cycle the temperature and electric field to drive the crystal through an order/disorder phase transition to convert between heat and electric energies. The phenomena known as the electrocaloric effect (ECE), in which electrical energy is used to induce a temperature change, is directly related to this thermodynamic cycle. The ECE holds great potential for future technologies such as solid-state refrigeration. In this presentation I will demonstrate the calculation of the pyroelectric properties of a perovskite crystal using an effective Hamiltonian model within a molecular dynamics framework. In this presentation we focus on the perovskite BaTiO3 (BTO) compound for a variety of reasons: it is a well studied archetypical perovskite crystal that is relatively easy to produce, it exhibits an good pyroelectric response, it does not contain the toxin lead, and its ferroelectric properties have the potential to be tailored by alloying or the creation of superlattice structures. The ECE in BTO has been observed and reported for a variety of geometries including thin and thick films, nanostructures, and surprisingly off-the-shelf BTO multilayer capacitors. Previous theoretical studies of BTO have largely relied on thermodynamic models, for example the Ginzburg-Landau-Devonshire model. In this presentation we directly calculate the pyroelectric response using a molecular dynamics approach and study the impact of epitaxial strain on the magnitude of the ECE. The results presented here demonstrate that BTO can exhibit a moderate sized pyroelectric and ECE response, with a ΔT around 5-6 K, for a relatively small electric field gradient, less than 100 kV/cm. Unlike the Pb-containing alloys that have been the focus of most giant-ECE studies, it is unnecessary to apply a large electric field gradient because there is no antiferroelectric ground state that must be avoided through the use of large fields.
9:00 AM - F4.12
Growth and Characterisation of Cerium Oxide Thin Films by Pulsed DC Magnetron Sputtering: Influence of Oxygen Partial Pressure and Post Deposition Annealing
Asmaa Eltayeb 1 Rajani K. Vijayaraghavan 2 Colm T Mallon 3 Stephen Daniels 1 2 Enda McGlynn 4
1Dublin City University Dublin 9 Ireland2Dublin City University Dublin 9 Ireland3Dublin City University Dublin 9 Ireland4Dublin City University Dublin 9 IrelandShow Abstract
Cerium oxide (CeO2, ceria) is a rare earth metal-oxide which is scientifically important because of its unique properties and various applications, in particular as a redox active material for two-step thermochemical cycling. In this study, nanostructured cerium oxide films are grown on Si(100) and quartz substrates by pulsed DC magnetron sputtering from a cerium oxide target. The influence of oxygen partial pressure in both the sputtering chamber and post-deposition annealing on the film properties are studied. The structural and optical properties of the films are examined using X-ray diffractometry (XRD) and UV-visible (UV-Vis) absorption spectroscopy, respectively. XRD results show that the films have preferential orientation along the  direction and the crystalline quality of the films, as measured by grazing incidence XRD, can be improved by post-deposition annealing in an O2 ambient. Morphological studies of the films using atomic force microscopy (AFM) indicated grain formation and an increase in surface roughness as a result of the annealing process. The chemical nature of the cerium oxide films is determined using secondary ion mass spectrometry (SIMS) and the electrochemical charge storage properties of the films are examined using cyclic voltammetry experiments.
9:00 AM - F4.13
Crystal Polarity and Electrical Properties of Heavily Doped ZnO Films
Yutaka Adachi 1 Naoki Ohashi 1 Isao Sakaguchi 1 Hajime Haneda 1
1National Institute for Materials Science Tsukuba JapanShow Abstract
ZnO has attracted significant attention for applications in transparent electrodes for solar cells. Since a ZnO crystal has a wurzite-type structure, it shows spontaneous polarization along the c-axis. Therefore, ZnO has polar surfaces corresponding to the c(+)-face and c(-)-face. It is well known that various properties of ZnO depend on its polarity, such as chemical stability of the surface, incorporation effciency of nitrogen and growth rate. Therefore, it is important for device applications of ZnO to investigate the effects of crystal poarlity on poroperies of ZnO films. In this study, we investigated the crystal polarity and electorical properties of heavily doped ZnO films. We found that the films grown at a low temperature by pulsed laser deposition using a heavily Al-doped ZnO target had a c(-)-face, whereas the films prepared at a high temperature had a c(+)-face. The room-temperature resistivities of the films were 4.50×10-4 and 1.94×10-3Omega;cm, respectively. More details will be presented at the conference site.
9:00 AM - F4.14
The Nature of Radiative Transitions in TiO2-based Nanosheets
Giacomo Giorgi 1 Maurizia Palummo 2 Letizia Chiodo 3 Angel Rubio 4 Koichi Yamashita 5
1The University of Tokyo Tokyo Japan2Universita' di Roma ``Tor Vergata" Rome Italy3Universita' di Lecce Lecce Italy4Universidad del Paamp;#237;s Vasco UPV/EHU San Sebastian Spain5The University of Tokyo Tokyo JapanShow Abstract
The possibility of splitting water via light irradiated TiO2 reported by Fujishima and Honda in 1972  has tremendously boosted the interest of researchers for this very "intriguing" material. Its relevance in many industrial and technological applications, in catalysis, in the field of solar-to-energy conversion, in photocatalysis (and in many others), is testified by the ever increasing number of published scientific reports. Nowadays, the availability of more accurate experimental techniques has lead to the synthesis of nanostructured TiO2-based materials: their surface area enhancement and the inherent improvement of their photochemical activity make them subject of further deep analysis. Among all the possible morphological shapes , the study of (001)-oriented nanosheets is of wide interest for the reported improved performances of systems with such facet exposure . In particular, a double morphological nature characterizes such layered structures. The first is the “ordinary” reconstruction obtained from the "cut" of bulk anatase along . The second is derived from the previous which, after the gliding of the upper part of the film over the lower along the Ti-O-Ti chain direction, evolves towards the so-called lepidocrocite  (barrierless path due to stress reduction). Such nanosheets are precursors of titania nanotubes (NTs) which in turn can be used for further assembling of nanostructured materials with different morphologies and applications . Despite the strong interest, due to different preparation methods and chemical environments, conflicting reports about their photo-excited properties exist and even their crystalline structures are still under debate. Here we show by means of first-principles simulations, the unambiguous relation among atomic structure, electronic bandstructure and optical properties of several TiO2-based nanosheet. Results of GW/BSE calculations on top DFT simulations on the electronic and optical properties of TiO2-based nanosheets reveal that the inclusion of many-body effects in the theoretical description is of primary importance to get a comparison between experiment and theory. The excitonic nature of the main optical peaks clearly comes out from the analysis of the theoretical spectra of isolated nanosheets. The packing of layers influences both the electronic and optical gaps of such nanosystems, while the inclusion of lattice vibrations in the optical spectra calculation provides a clear evidence of the Stokes-shifts experimentally observed. The optical spectra excitonic nature of the 2D TiO2-based nanostructures is confirmed via finite-temperature (300 K) simulations. Honda, K. et al., Nature, 1972, 238, 37.  See for example Chen, X. et al. Chem. Rev. 2007, 107, 2891 and refs. therein.  Yang H.G. et al., J. Am. Chem. Soc., 2009, 131, 3152.  Orzali T., et al. Phys. Rev. Lett., 2006, 97, 156101.  Pradhan, S. K. Chem. Mater. 2007, 19, 61.
9:00 AM - F4.15
Elevated-temperature Photoelectrochemical Cell for Water Splitting
Xiaofei Ye 1 Zhuoluo Feng 2 John Melas-Kyriazi 1 Nicholas A. Melosh 1 William Chueh 1
1Stanford University Stanford USA2Stanford University Stanford USAShow Abstract
The conversion of solar energy into chemical fuels, which can be easily stored and transported, is being pursued via a wide range of processes. In particular, photoelectrochemical cell (PEC), which produces hydrogen under illumination by splitting water, has attracted enormous attention. However, the reported efficiency is still rather low mainly due to low reaction kinetics at room temperature. Here, we propose to develop a PEC of new design that can work at temperatures significantly above room temperature. This can be achieved by creating a heterostructure consisting of a photon absorber and a solid electrolyte. As the temperature increases, the rate of thermally-activated chemical reactions is substantially enhanced over that at room temperature, reducing or eliminating the need for expensive catalysts. A device-level model is created to describe the thermal enhancement mechanisms in processes ranging from light absorption to surface reaction. Finally, proof-of-concept devices are fabricated and characterized with and without illumination under controlled gas environment and temperature.
9:00 AM - F4.16
Al-doped ZnO Film as a Transparent Conductive Substrate in Indoline-sensitized Nanoporous ZnO Solar Cell
Boateng Onwona-Agyeman 1 Motoi Nakao 2 Gamaralalege Rajanya Asoka Kumara 3 Takuya Kitaoka 1
1Kyushu University Fukuoka Japan2Kyushu Institute of Technology Kitakyushu Japan3Peradeniya University Peradeniya Sri LankaShow Abstract
Transparent conducting oxide (TCO) films such as tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO) and aluminum-doped zinc oxide (AZO) have high transmittance in the visible region of the electromagnetic spectrum, combined with reasonable electrical conductivity. TCO films are therefore used as components in opto-electronic devices for solar cell applications, light emitting diodes and flat panel displays. In a dye-sensitized solar cell (DSC), a TCO film is coated with a nanoporous oxide semiconductor (photo-anode) which is sensitized with an organic or inorganic dye. The role of the TCO is to allow significant light transmittance into the DSC and also to “collect” injected electrons from the sensitized photo-anode into the external circuit. Therefor