Symposium FH
Recent Developments in the Research and Application of Transparent Conducting and Semiconducting Oxides
ABSTRACTS
Session FH-1 - Fundamentals
FH-1:IL01 New Insights into Electronic and Structural Properties of Zinc Oxide
K. ELLMER, Helmholtz-Zentrum für Materialien und Energie, Dept. Solar Fuels, Berlin, Germany
Despite the fact, that zinc oxide is investigated as a wide band gap semiconductor for more than half a century, many fundamental properties are still not understood suffiently well. This holds especially for polycrystalline doped ZnO, which is increasingly used for optoelectronic devices, especially for thin film solar cells.
In this overview, I will present recent results on the doping and transport in single crystalline and polycrystalline as well as X-ray amorphous ZnO films. The group III element donors, B, Al, Ga, and In, will be compared to the hydrogen donor. The role of crystallographic defects and its interplay with the donors will be highlighted. It was found, that the trap density at grain boundaries, limiting the carrier transport in polycrystalline films, is correlated with the density of dopants. Therefore, an optimum has to be found for the dopant density, which is needed for a high carrier density without detrimentally influencing the carrier mobility. X-ray absorption studies are reported to shed light on the atomic environment of the dopants and of alloying elements like Mg.
FH-1:IL02 Structure and Performance of Oxide TFTs; High Mobility and High Stability
SANG YEOL LEE, Department of Semiconductor Engineering, Cheongju University, Cheongju, Chungbuk, Korea
Since amorphous indium gallium zinc oxide (a-IGZO) was reported by Hosono et al., it has been intensively studied for active channel layer. These excellent performances originate from the unique characteristics of amorphous oxide semiconductors (AOS) which is the direct orbital overlap between s orbitals of neighboring metal cations. Indium is well known materials which make oxygen vacancy to increase the mobility and wide applications in Zn-based oxide TFTs, such as indium-gallium-zinc oxide (IGZO), indium-tin oxide (ITO) and indium-zinc oxide (IZO). Oxygen vacancy plays an important role by providing major carrier in active channel layer in AOS systems. However, indium atoms are not abundant in earth crust and expansive materials. It is necessary to development In-free TFTs for display backplane. More recently, theoretical calculations have indicated that the Si may be an excellent oxygen binder as carrier suppressor in the ZTO system for the stability enhancement. In order to increase electrical characteristics, control of defect states was needed. To overcome these, novel structures of oxide TFTs using various oxide channel materials will be discussed in terms of the enhancement of mobility and stability.
FH-1:IL03 Optoelectronic Properties of CdO
T.D. VEAL, Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool, Liverpool, UK
Cadmium oxide (CdO) was the first transparent conductor to be identified over a century ago, but it has yet to realise its full potential. Here temperature-dependent optical absorption, Hall effect, and infrared reflectance measurements have been performed on as-grown and post-growth annealed CdO films grown by metal organic vapor phase epitaxy on sapphire substrates. The evolution of the absorption edge and conduction electron plasmon energy with temperature has been modeled, including the effects arising from the Burstein-Moss shift and band gap renormalization. The zero-temperature fundamental direct band gap and band edge effective mass have been determined to be 2.31 eV and 0.27m0, respectively. The associated Varshni parameters for the temperature dependence of the band gap are found to be alpha = 8 x 10^4 eV/K and beta = 260 K. Additionally, the electron mobility has been studied by comparing the "optical" intra-grain mobility with the transport inter-grain mobility. While ionized impurity scattering is dominant within the grains, grain boundary scattering is found to limit the overall mobility of the CdO films. The surface electronic properties of CdO studied by x-ray photoemission and angle resolved photoemission spectroscopy will also be reported.
FH-1:IL04 Fundamental Limits on Optical Transparency of Transparent Conducting Oxides
C.G. VAN DE WALLE, D. STEIAUF, H. PEELAERS, Materials Department, University of California, Santa Barbara, CA, USA; E. KIOUPAKIS, Department of Materials Science and Engineering, University of Michigan, USA
Basic information about transparent oxides, particularly about doping and how it affects electronic and optical properties, is often lacking. First-principles calculations are now capable of accurately predicting quantities that are directly relevant for applications. In oxides that can be highly doped, the large carrier concentrations significantly affect optical transparency. While direct absorption (either across the gap or to higher-lying conduction bands) is usually not a problem, indirect processes assisted by electron-phonon scattering create absorption, sometimes with unexpected wavelength dependence. First-principles evaluations of free-carrier absorption [1] provide insight into the factors that limit this key criterion for transparent conducting oxides. The presence of large concentrations of electrons in the conduction band also affects the absorption edge, not only because of conduction-band filling but also through band-gap renormalization-processes that we can now treat consistently and in quantitative detail.
[1] H. Peelaers, E. Kioupakis, and C. G. Van de Walle, Appl. Phys. Lett. 100, 011914 (2012).
FH-1:IL06 Fundamentals of Reliability and Instability Issues in Amorphous Oxide Semiconductor TFTs
J.F. CONLEY Jr., School of EECS, Intercollege Materials Program, Oregon State University, Corvallis, OR, USA
Amorphous oxide semiconductors (AOS) are a relatively new class of materials that exhibit good electron mobilities (up to more than 30 cm2/V-sec) and can be processed at low temperatures. As transparent thin film transistors (TFTs) based on these new materials rapidly approach commercialization for large area electronics (such analog current drivers for active matrix OLED displays and flexible electronics), the reliability and stability of these devices are becoming critical issues. This presentation will include an overview of TFT operation, a discussion of the physics of anticipated reliability and stability problems, and a review of recently emerging work on the reliability and stability of AOS TFTs.
FH-1:IL07 First Principles Theory of Transparent Crystalline and Amorphous Oxide Conductors and Semiconductors
J. MEDVEDEVA, Department of Physics, Missouri University of Science and Technology, Rolla, MO, USA
Carrier generation in wide-bandgap oxides remains one of the major challenges in the area of transparent conductors and semiconductors. The compositional complexity of multicomponent oxides - although appealing technologically due to a possibility to control the electrical, optical, and structural properties over wide ranges - requires in-depth understanding of complex defect chemistry to determine the carrier source and the role played by each constituent.
Systematic density-functional studies of defect formation in crystalline InAMO4, A=Ga or Al, M=Zn or Mg, (i) establish that antisite defects - not oxygen vacancies - are major electron donors; (ii) explain the observed behavior of conductivity in In-rich vs Ga-rich InGaZnO4; and (iii) reveal why some light-metal containing oxide samples are unstable under a wide range of growing conditions.
Presence of several cations is necessary to achieve low-temperature deposition of amorphous oxide semiconductors. Molecular dynamics simulations of amorphous In-X-O, X=Zn, Ga, Sn, etc, reveal that not all bonding preferences are preserved upon amorphization, highlighting the origins of under(over)-coordination; and show that oxygen stoichiometry affects the spatial distribution of X which governs defect formation and carrier generation.
FH-1:L08 Finding Low Hole Effective Masses p-type Transparent Conducting Oxides through High-throughput Computing
G. HAUTIER, A. MIGLIO, G. CEDER1, G.-M. RIGNANESE, X. GONZE, Université Catholique de Louvain, Louvain-la-Neuve, Belgium; 1Massachusetts Institute of Technology, USA
Transparent conducting oxides (TCOs) are essential to many technologies from solar cell to transparent electronics. While n-type TCOs (using electrons as carriers) are widespread in current applications (e.g., indium tin oxides or ITO), their p-type counterparts have been much more challenging to develop and still exhibit carrier mobilities an order of magnitude lower.
The difficulties in developing high mobility p-type TCOs can be related to the intrinsically high effective masses of holes in oxides. In this talk, we will report on a high-throughput computational search for oxides with low hole effective mass and wide band gap. Screening thousands of binary and ternary oxides in the Materials Project Database using state of the art ab initio techniques, we will present several unsuspected compounds with promising electronic structures. Beyond the description of those novel TCOs candidates, we will discuss and chemically rationalize our findings, highlighting several design strategies towards the development of future high mobility p-type TCOs.
FH-1:L09 Atomic and Electronic Structures of Various Phases of Zinc Tin Oxides by Ab-initio Calculations
JOOHWI LEE, JUNG-HAE CHOI, Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul, South Korea; DEOK-YONG CHO, CFI-CES, Institute for Basic Science, and Department of Physics and Astronomy, Seoul National University, Seoul, South Korea; JISIM JUNG, UN KI KIM, SANG-HO RHA, CHEOL SEONG HWANG, WCU Hybrid Materials Program, Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, Seoul National University, Seoul, South Korea
Zinc tin oxide (ZTO) is an n-type transparent semiconducting oxide and has various crystalline phases as well as the high tendency to form amorphous structure. In this study, the atomic and electronic structures of various crystalline and amorphous structures and the thermodynamic stabilities of ZTOs were investigated by ab-initio calculations. The p-T phase diagram in ZnO-SnO2 was determined. In crystalline ZTOs, Zn-O bond lengths in the ternary oxides were longer with larger variation and Zn showed the dual preference on the coordination number (CN). [1] In amorphous phase, both the CN of Zn and the bond length of Zn-O decreased. Meanwhile, the Sn-O bonds showed less distinct changes by amorphization. A higher non-uniform localization of the conduction band minima composed Sn 5s states was observed in the amorphous phase compared with the crystalline counterpart. Both the phase transitions in the ZnO-SnO2 and the amorphization were found to be dominated by the changes in the Zn-O bond rather than changes in the Sn-O bond. Oxygen vacancies will be also discussed.
[1] Joohwi Lee, Jung-Hae Choi et al., J. Mater. Chem. C, 1 6364 (2013).
[2] Joohwi Lee, Jung-Hae Choi et al., Appl. Phys. Lett. 102, 2421111 (2013).
FH-1:IL11 Advancing TCO Technologies via the Application of Atomic Layer Deposition
M.E. PEMBLE1, 2, M. BARDOSOVA2, S. KASSIM2, H. MANLEY2, J. MCGRATH2, S. O'BRIEN2, I.M. POVEY2, C. RYAN2, 1Department of Chemistry, University College Cork, Cork, Ireland; 2Tyndall National Institute, University College Cork, Cork, Ireland
In recent years atomic layer deposition or ALD has emerged as a very powerful means of depositing ultra-thin (often sub-Angstrom) layers of materials over both flat and complex, textured surfaces. In addition the use of ALD to introduce dopant atoms at precise points within a layer has opened up new possibilities in terms of the way that we imagine doping to occur. In this work the use of specific ALD precursors designed for the doping of both ZnO and TiO2 will be described from the perspective of adding these atoms to the growing lattice without the formation of undesirable insulating phases. It will be demonstrated that the use of larger ligands can physically isolate dopant atoms from one another, allowing the dopants to be both structurally and electrically incorporated into the growing film. ALD also allows one to explore the use of a variety of dopants and here we have studied the ALD doping of TiO2 as a potential TCO material, with an overall aim of creating both n and p-type doping in this oxide.
The ability of ALD to coat textured materials has been employed to create textured, doped TCOs. In addition we have created novel light trapping TCO structures by performing ALD growth over colloidal photonic crystals made in our laboratories.
Session FH-2 - Material Design and Characterization
FH-2:IL01 Low-temperature Solution-processible Metal Oxide Nanoparticles for Interfacial Contact Layers in Organic Photovoltaics
YUN-JU LEE, JIAN WANG, D. BARRERA, JULIA W.P. HSU, Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, USA
Solution processed organic photovoltaic (OPV) devices represent a promising route toward low cost, lightweight solar energy conversion. Due to the blended nature of the organic active layer in an OPV device, including interfacial contact layers (ICL) at the active layer/electrode interface significantly improves OPV performance. We utilize microwave synthesis to make stable metal oxide nanoparticle suspensions for ICL materials. Some examples are MoOx, ZnO, Al doped ZnO, and CoOx nanoparticles. Our goal is to develop low-temperature solution processible materials that have the desired electronic properties and are compatible with deposition on top of the active organic layer. For example, our MoOx nanoparticles are < 4 nm, with high work function, and in butanol, enabling room temperature solution deposition of hole transport layers for inverted OPV devices. For ZnO, we achieve independent control of work function and bandgap, through low-work-function organic additives and water concentration in the precursor solutions, respectively. Our approach separates materials synthesis from device processing with the possibility of optimization for each. We will show OPV performance comparable to using standard ICL electrode materials.
FH-2:IL02 In2O3 and ZnO Based Transparent Conductive Oxide Films with High Electron Mobility
T. KOIDA, H. SAI, H. SHIBATA, M. KONDO, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
High mobility is required for TCO films as front electrodes of photovoltaic devices to realize both high conductivity and high transparency. Work function as well as reactivity is also important parameters in the view point of interfacial electronic and chemical properties between the TCO and semiconductors. However, the materials that meet these requirements are limited. Here, we introduce our recent research on TCO films. In In2O3 based TCO, we have investigated bilayer structure composed of solid-phase crystallized In2O3:H and amorphous TCO; High mobility can be realized by the former layer, and work function and reactivity could be controlled by changing metal cations in the latter layer. Structural and electrical properties of crystalline/amorphous TCO interfaces will be discussed. In ZnO based TCO, high mobility is usually obtained by post-deposition hydrogen treatments. However, it is difficult to distinguish the roles of low oxygen partial pressure (PO2) and hydrogen on transport properties. We have characterized ZnO:Al films post-annealed under controlled PO2 using an oxygen pump, and found that mobility and carrier density systematically change with Al composition and PO2. We will discuss the changes in the view point of grain boundary scattering and lattice defects.
FH-2:IL03 The Influence of Doping on Properties of Transparent Semiconducting Oxides
HAO GONG, Department of Materials Science and Engineering, National University of Singapore, Singapore
Transparent semiconducting oxides (TSOs) have a very wide range of applications. Doping has been known as an effective way to improve and modify electronic properties of semiconductors. This talk will present the effects of doping on TSOs. Both n-type and p-type TSOs will be covered in this talk. For the example of p-type, p-type ZnO through doping will be discussed. We will demonstrate that even if the same element is doped in ZnO, a slight variation of experimental conditions can lead to two totally different properties, one is p-type and the other is n-type. For the example of n-type, indium zinc oxides (IZO) will be discussed. It will be shown that the achievement of a conductivity increase is made with a very small amount of foreign elements incorporation (doping) such as Al and Y. Increased conductivity can also be achieved with more than 10% Germanium (Ge) incorporation. Various interesting phenomena observed, including insulator/semiconductor-metal transition and the variation of electronic properties with foreign elements incorporation, will also be presented. The results and phenomena understanding will be discussed.
FH-2:IL04 Semiconductor Nanoheterostructures Based on Wide Gap Metal Oxides and A2B6 Quantum Dots
M.N. RUMYANTSEVA, A.S. CHIZHOV, R.B. VASILIEV, L.I. RYABOVA, D.R. KHOKHOLOV, A.M. GASKOV, Moscow State University, Moscow, Russia; A.M. ABAKUMOV, University of Antwerp, Antwerp, Belgium
One of the major disadvantages of currently available semiconductor gas sensors are relatively high operating temperature and power consumption, which limits their use in a miniaturized gas analyzers and fire detectors In recent years, several reports on the use of photoactivation to increase gas sensitivity of a metal oxide semiconductor appeared. However, the published studies were conducted using a powerful UV radiation, which does not significantly reduce the power consumption of the semiconductor gas sensor. This work reports the study of photoconductivity and visible light activated room temperature gas sensors properties of semiconductor nanoheterostructures based on wide gap metal oxides and A2B6 quantum dots. Nanocrystalline metal oxides SnO2 and ZnO were synthesized by wet chemical method. CdSe QDs were obtained via high temperature colloidal synthesis. Immobilization of sensitizers on metal oxide surface leads to the increase of room temperature metal oxide conductance in 50 - 104 times, which greatly simplifies the measurement of the sensor signal. Sensor measurements demonstrated that sensitized nanocrystalline semiconductor oxides can be used for CO and NO2 detection under visible light illumination at room temperature without any thermal heating.
FH-2:IL05 Molecular Monolayer Tuning of Organic/Metal Oxide Interfaces
T.M. BRENNER, G. CHEN, J. BRAID, T.E. FURTAK, R.T. COLLINS, Colorado School of Mines, Golden, CO, USA; D.C. OLSON, D.S. GINLEY, National Renewable Energy Laboratory, Golden, CO, USA
Transparent metal oxides are used extensively in organic electronic devices as charge selective layers for electron or hole collection. The energetics of the organic/metal oxide interface play a central role in the performance of these devices. This presentation discusses molecular monolayer modification of two oxides of considerable interest, ZnO and NiO. Monolayers of surface-bonded silanes, thiols, and carboxylic acids have been developed on these metal oxide surfaces and characterized with a range of surface sensitive structural, electronic, and optical techniques. The chemistry of the surface bond in each case and tradeoffs between bonding and etching were examined. Control of the surface energy of the metal oxide, which changes the wetting and ordering of the organic at the interface, has been demonstrated. In addition, metal oxide work function can be systematically tuned by as much as 1eV by introducing dipolar functional groups. This work function control is reflected in the open circuit voltage of organic solar cells that include the modified metal oxide layers.
Support of the National Science Foundation and Renewable Energy Materials Research Science and Engineering Center is gratefully acknowledged.
FH-2:IL06 Transparent Conductive and Semiconductive Oxides by Hollow Cathode Gas Flow Sputtering
B. SZYSZKA, R. MUYDINOV, TU Berlin, Berlin, Germany; T. JUNG, K. ORTNER, F. SCHMIDT, FhG-IST, Braunschweig, Germany
The novel method of hollow cathode gas flow sputtering (GFS) of metal oxides has shown to be a versatile tool for deposition of transparent conductive and semiconductive oxide films. GFS is a remote plasma process where the bombardment of the film by unwanted high energetic particles is suppressed and where intense but low energetic ion assistance is provided by means of pulse plasma excitation. We report on our recent findings in the field regarding soft growth deposition of transparent conductive n-type ZnO:Al and In2O3:Sn films, n-type semiconductive ZnSnXOy, p-type semiconductive CuCrO2 and visible light induced photocatalytic TiO2:W films. The technology has been implemented on different scales with ranging from lab type tube sources up to linear sources with 1 m cathode length for in-line coating. An overview on the process technique and recent device applications will be given.
FH-2:L07 New, Highly Conducting Si-doped ZnO Thin Films Prepared by Spray Pyrolysis
V.L. KUZNETSOV, N. RASHIDI, P.P. EDWARDS, Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, UK
In this presentation we report the synthesis and properties of highly conducting and transparent ZnO thin films doped with a new type of dopant, namely silicon dioxide. Si-doped ZnO thin films were prepared, for the first time, by spray pyrolysis and characterised using X-ray diffraction, AFM, SEM, UV-Vis spectroscopy, XPS and Hall effect measurements.
Si-doped ZnO thin films exhibit widening of band gap upon increasing deposition temperature and Si content as a result of increasing electron concentration. The electrical resistivity of Si-doped films can be tuned by changing the deposition conditions to reach values as low as 0.0017 Ωcm for as-deposited films without any additional post-deposition treatment. The carrier concentration is practically temperature-independent over the range of 80-350 K for all the films while the Hall mobility exhibits an activated behaviour which is attributed to the thermionic emission of free electrons across potential barriers formed between crystallites. The nature of Si doping and factors controlling the electrical properties of films is discussed.
The results show that Si-doped ZnO thin films prepared by solution routes are promising low cost TCO material for large area photovoltaic and optoelectronic applications.
FH-2:L08 Conductive Path Formation Using Lattice Defects in Insulating Oxide Crystals
A. NAKAMURA, Nagoya University, Nagoya, Japan; E. TOCHIGI, Y. SATO, T. MIZOGUCHI, N. SHIBATA, Y. IKUHARA, University of Tokyo, Tokyo, Japan; K. TOYOURA, K. MATSUNAGA, Nagoya University, Nagoya, Japan
The lattice defects in insulating oxide crystals such as grain boundaries, domain boundaries, dislocations and so on can bring about unusual physical properties, due to special structures and strain at the defects and/or the segregation of dopants and point defects to the defects. In this study, we would like to show the methods to fabricate the conductive path in insulating oxide crystals using lattice defects and report the features of the electric conductivity. At first, using dislocations is a promising method to fabricate conductive nanowires. A dislocation has a one-dimensional, sub-nanometer-scale lattice discontinuity. Moreover, solute atoms in a crystal tend to segregate to a dislocation. Thus, a dislocation can be converted to a conductive nanowire even in insulating oxide crystals. At next, using domain boundaries in insulating polar oxide crystals is an interesting method to fabricate local and two-dimensional conductive region, too. Charged domain boundaries induce carriers at the boundaries and therefore are promising candidate of local conductor.
FH-2:L09 Chemical Solution Deposition via Ink Jet Printing of Aqueous Al-doped ZnO Precursors
K. VERNIEUWE, P. LOMMENS, K. DE BUYSSER, Ghent University, Department of Inorganic and Physical Chemistry, Belgium
Al-doped ZnO (AZO) is a promising transparent conductive oxide (TCO). Due to its excellent optical and electrical properties, it is a future candidate to replace the nowadays used ITO thin films in devices such as LCDs, LEDs, touch screens, etc. Chemical solution deposition (CSD) is an attractive method to prepare thin films due to its simplicity and low costs. Ink jet printing is selected as deposition technique due to its good scalable process and flexibility towards the deposition of patterns or films.
The commonly used solvents in literature are toxic organic solvents such as 2-methoxyethanol and are in this research replaced by the more environmental friendly solvent water. The formulation of these aqueous precursors is selected with care towards stability, wettability with glass substrates, jettability and the final morphology. Avoiding precipitation in solution during the gellation process can be controlled by the addition of complexing agents, while additives such as tensioactive components control the wettability and jettability. Finally, it's observed that the formed complexes and the chosen thermal process have a big influence on the crystallinity.
FH-2:IL10 Combinatorial Synthesis and Thin Film Deposition of Transparent Conductors and Semiconductors
J.D. PERKINS, A. ZAKUTAYEV, J.J. BERRY, P.A. PARILLA, M.A.F.M. VAN HEST, T. GENNETT, D.S. GINLEY, National Renewable Energy Laboratory, Golden, CO, USA
Thin film transparent conducting oxides (TCOs) are critical to most thin film photovoltaic (PV) technologies as well as displays and organic light emitting diodes. In all of these applications, material properties beyond just the TCO conventional metrics of high transparency and low sheet resistance are becoming ever more critical in device optimization. For PV applications in particular, tunable band gaps and band edge energies are necessary to optimize electrical contact to both current and newly emerging earth-abundant absorbers. Accordingly, band edge energies, work function, dopability and morphology should be considered in addition to transparency and sheet resistance when selecting TCOs. a-InZnO and a-InZnSnO are both work function tunable amorphous materials. Crystalline Ga-doped (Zn,Mg)O is a band gap tunable TCO where the conduction band minimum (CBM) energy can be tuned over a 0.3 eV range by varying the Mg content from 0 to 30%, albeit with reduced conductivity as the Mg content increases. For p-type materials, semi-transparent Co-Ni-Zn-O is a viable hole transport layer for organic photovoltaics. For all of these materials, our approach to TCO materials development begins with a broad exploration of the relevant materials and synthesis space using combinatorial high-throughput synthesis and characterization. Combinatorial composition gradient samples (libraries) are grown on 2”x2” glass substrates either by co-sputtering or pulsed laser deposition. Three to five libraries are generally required to cover the full composition range for a binary tie-line and about 20 to cover a ternary composition space. For 1D composition gradient libraries, an orthogonal temperature gradient of ~ 100 °C across the 2” library can also be used. The libraries are characterized by a variety of automated combinatorial mapping tools including XRF for metals stoichiometry, 4-pt. probe for sheet resistance, UV/VIS/NIR reflection and transmission, FTIR optical reflection and transmission, x-ray diffraction (XRD) using a large area 2D detector and work function mapping. More recently, we have developed Seebeck coefficient mapping to determine carrier type and Rutherford Backscattering mapping to measure anion contents.
FH-2:IL11 Efficiency Improvement of n-Oxide Semiconductor/p-Cu2O Heterojunction Solar Cells Fabricated on Thermally Oxidized Cu2O Sheets
T. MIYATA, Y. NISH, T. MINAMI, Optoelectronic Device System R&D Center, Kanazawa Institute of Technology, Ishikawa, Japan
In this paper, we describe the present status and prospects for further development of p-n heterojunction solar cells fabricated using polycrystalline p-type cuprous oxide (Cu2O) sheets that act as the active layer as well as the substrate. Recently, a substantial improvement of conversion efficiency has been reported in Cu2O-based p-n heterojunction solar cells fabricated by depositing appropriate n-type oxide semiconductor thin films on p-Cu2O sheets. The obtainable photovoltaic properties could be improved not only by forming the n-oxide semiconductor thin-film layer on p-Cu2O sheets using a low-temperature and low-damage deposition method such as a pulsed laser deposition, but also by using high-quality Cu2O sheets. Using Cu2O sheets with a resistivity on the order of 102-103 Ωcm, hole concentration on the order of 1013-1014 cm-3 and Hall mobility in the range of 100-110 cm2/Vs, prepared by a thermal oxidization of copper sheets, a conversion efficiency of 5.38% could be obtained in a solar cell fabricated with an Al-doped ZnO/Ga2O3/Cu2O structure, i.e., using a non-doped Ga2O3 thin film as the n-type semiconductor layer.
FH-2:L12 Structural, Electrical and Optical Properties of Nanostructured Ta-doped TiO2
P. MAZZOLINI1, 2, P. GONDONI1, C.S. CASARI1, 2, A. LI BASSI1, 2, 1Dipartimento di Energia and NEMAS - Center for NanoEngineered Materials and Surfaces, Politecnico di Milano, Milano, Italy; 2Center for Nano Science and Technology @PoliMI, Istituto Italiano di Tecnologia, Milano, Italy
Current research on transparent conducting oxides (TCO) is focusing on new materials and nanostructures to fit the different functional properties required for novel organic and hybrid photovoltaic devices. It has been recently discovered that charge transport properties of TiO2 can be strongly enhanced by doping with Nb (NTO) or Ta (TaTO). This new class of TCOs seems to be promising, even though a complete physical investigation is needed to fully exploit its potentialities.
We here present the synthesis and characterization of TaTO nanostructured thin films by Pulsed Laser Deposition at room temperature, followed by vacuum annealing to obtain single phase anatase. The interplay among doping, oxygen stoichiometry and nanostructure is investigated in terms of optoelectronic properties. A resistivity value of 5.9x10-4 Ωcm was obtained for 150 nm compact polycrystalline TaTO, competitive with the best results of the most investigated NTO. Moreover we show how transmittance can be enhanced by means of light scattering from hierarchically organized TiO2 in the form of a nano-tree forest in an all-TaTO double layer (compact + nanoporous).
Exploiting such approach we aim to realize a fully TiO2-based photoanode-TCO, in view of application in dye sensitized/hybrid solar cells.
FH-2:L13 Structural, Electrical and Optical Characteristics of Spin-coated ZnO-based Transparent Thin Films
N. VOROBYEVA, M. RUMYANTSEVA, R. VASILIEV, V. KOZLOVSKIY, YU. SOSHNIKOVA, D. FILATOVA, A. GASKOV, Chemistry Department, Moscow State University, Moscow, Russia
Zinc oxide is a promising functional wide-gap semiconductor. ZnO-based materials belong to the group of transparent conducting oxides (TCO). Doping of ZnO provides an opportunity to regulate electrical and optical properties over a wide range.
ZnO, ZnO(Ga), ZnO(In) and ZnO(Ga,In) thin films were synthesized by spin-coating technique with further annealing at 723 K. The effect of cation composition on structural, morphological, optical and electrical properties of films was studied.
Only polycrystalline ZnO (wurtzite) phase was observed for all films. Increase of impurity content leads to the decrease of grain sizes. The dependence of resistivity of ZnO(Ga) films on Ga content has minimum at 1.0 at.% Ga. The resistivity of ZnO(In) films decreases with In content. The lowest values of resistivity were obtained for ZnO(Ga,In) thin films. All films are transparent in the visible region. The average transmittance of ZnO film is 89% (at 400-800 nm). Introduction of small Ga amount leads to the increase of this value up to 95%, then it decreases with Ga content. The same tendency is observed for ZnO(In) films. The maximum transmittance was achieved in ZnO(Ga,In) film and was 97%.
The best electrical and optical properties were found for ZnO film containing 1 at.% Ga and 0.7 at.% In.
FH-2:L14 Nano- and Mesoscale Engineering of Al-doped ZnO by Pulsed Laser Deposition: Towards Flexible Scattering Electrodes
P. GONDONI1, P. MAZZOLINI1, 2, V. RUSSO1, C.E. BOTTANI1, 2, A. LI BASSI1, 2, C.S. CASARI1, 2, 1Dipartimento di Energia and NEMAS - Center for NanoEngineered Materials and Surfaces, Politecnico di Milano, Milano, Italy; 2Center for Nano Science and Technology @PoliMI, Istituto Italiano di Tecnologia, Milano, Italy
The employment of Transparent Conducting Oxides in novel organic and hybrid photovoltaic devices demands new functional properties, including large surface area, effective light management and mechanical flexibility.
We present the synthesis of mechanically flexible, light scattering transparent conducting Al-doped ZnO (AZO) structures. The material is synthesized at room temperature, in a single deposition process by Pulsed Laser Ablation, and no post-treatments are required.
Two independent approaches are presented: in the first, hierarchically structured mesoporous AZO nanoforests are grown in a mixed Ar:O2 atmosphere, to allow independent control of cluster assembly (Ar partial pressure) and defect concentration in the material (O2 partial pressure). The mesoscale porosity of the material is beneficial for light scattering, and defect engineering allows to control electrical conductivity. In the second, a functionally graded architecture is developed by varying O2 pressure during the deposition process, obtaining a mesoporous scattering layer which evolves into a compact conducting layer.
The benefits of light scattering are studied by measuring a substantial absorption increase in a low bandgap polymer (PCPDTBT) employed as an electron donor in organic photovoltaic devices.
FH-2:IL15 Rational Design of p-type TCOs
D.O. SCANLON, University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, London, UK
Fabrication of high figure-of-merit p-type transparent conducting oxides (TCOs) is a much sought after goal for optoelectronic devices. Realization of a p-type TCO to rival the current industry standard n-type TCOs (e.g. In2O3:Sn, ZnO:Al, SnO2:F ), would open up the possibility of "transparent electronics". Unfortunately, the discovery of a high performance p-type TCO has been a major stumbling block. In this presentation we will present the results of hybrid density functional theory investigations into the suitability of a range of materials for p-type TCO ability. Specifically, we have investigated binary metal oxides, Cu-Delafossite based TCOs, and Layered oxychalcogenides. The intrinsic limitations to p-type TCO ability for each materials class will be highlighted, with strategies to surpass these limitations discussed.
FH-2:IL16 Site Occupancy in TCOs and the Relationship to Functional Properties
M.F. TONEY, SLAC National Accelerator Laboratory, Menlo Park, CA, USA
We quantify anti-site defect concentrations in p-type oxide based spinels and relate these defects to the spinel conductivity. We confirm that the cation disorder and doping physics in A2BO4 is consistent with that previously predicted and demonstrate that intrinsic cation disorder improves electrical properties of certain classes of spinels. This is achieved by investigating biaxially textured Co2ZnO4 and Co2NiO4 films using both experiment and theory. Cation disorder was quantified by resonant elastic x-ray diffraction (REXD) on as-deposited and annealed films. We find that conductivity decreases with anti-site disorder for Co2ZnO4, confirming predictions that anti-site disorder is a net hole producer for a class of II-III spinels typified by Co2ZnO4. For Co2NiO4, conductivity increases with anti-site disorder, since Co2NiO4 is a half-metal providing an explanation of the opposite trends of conductivity as a function of disorder. We introduce the concept of effective temperature that provides a method to quantify the disorder due to non-equilibrium growth and establishes a link between thermodynamic models and thin film growth techniques. This experimental confirmation of design principles used to guide materials selection is a critical step in a materials-by-design approach.
FH-2:IL18 Highly Diffuse Fluorine-doped SnO2 Thin Films for Photovoltaic Applications
G. GIUSTI1, V. CONSONNI1, D.P. LANGLEY1, 2, G. REY1, S. ZHANG1, Y. PELLEGRIN3, N.D. NGUYEN2, D. BELLET1, 1Laboratoire des Matériaux et du Génie Physique, CNRS - Grenoble INP, Grenoble Cedex, France; 2Laboratoire de Physique des Solides, Interfaces et Nanostructures, Département de Physique, Université de Liège, Liège, Belgique; 3Laboratoire Chimie et Interdisciplinarité, Synthèse, Analyse, Modélisation, CNRS - Faculté des Sciences et des Techniques de Nantes, Nantes Cedex, France
Over the past few decades, polycrystalline fluorine-doped SnO2 (FTO) thin films have received increasing interest due to their promising application in a wide variety of devices such as gas sensors, coatings and front transparent electrodes for solar cells. FTO thin films exhibit good electrical and optical properties and other important physical properties such as a high work function, good thermal and chemical stability. FTO can act as an efficient transparent electrode in solar cells.
In this work, FTO thin films are grown by ultrasonic spray pyrolysis on glass or flexible substrates. The interplay between structural, electrical and optical properties of these FTO layers is investigated with a special emphasis on electron scattering mechanisms. We also demonstrate that FTO layers combined with ZnO nanoparticles exhibit average haze factor as high as 54% in the visible range, whilst maintaining very good electrical and optical properties. The integration into solar cells of such electrodes is also presented and discussed.
References:
V. Consonni, G. Rey, H. Roussel D. Bellet, J. Appl. Phys. 111(2012)33523
V. Consonni, G. Rey, H. Roussel, E. Blanquet D. Bellet, Acta Mater. 61(2013)22
A. Muthukumar, G. Giusti, M. Jouvert, V. Consonni, D. Bellet, Thin Solid Films 545(2013)302
FH-2:L19 Effect of Temperature on Phase Transition of Ni-Co Oxide and Its Application on Optoelectronics
SHU-YI TSAI1, K.-Z. FUNG1, C.-N. WEI2, H.-Y. BOR2, 1Research Center for Energy Technology and Strategy, Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, ROC; 2Metallurgy Section, Materials & Electro-Optics Research Division, Chung-Shan Institute of Science and Technology(CSIST), Gaoping village, Longtan Township, Taoyuan County, Taiwan, ROC
Nickel oxide and cobalt oxide tend to crystallize in a rock salt structure at high temperatures. However, when such a solid solution is annealed at temperature lower than 500℃, the crystal structure of the sample gradually changes to a spinel structure.
The Ni/Co oxide spinel tends to adopt the inverse-spinel arrangement with the tetrahedral sites completely occupied by Co+3 since the ionic radius of Ni+2 is much larger than that of Co+3.
NiCo2O4 spinel has been used as a bifunctional catalyst for oxygen evolution and reduction reaction as electrode in both inorganic and organic electrosyntheses. Recently, NiCo2O4 spinel was found to be an infrared transparent conducting material with potential for optoelectronic applications. By using rf magnetron sputtering technique, a NiCo2O4 solid film was deposited on a glass substrate from a sintered oxide target with the same composition. With annealing at adequate temperature, a thin film of nickel-cobalt oxides showing spinel structure was obtained. The electrical property of spinel film was measured using 4-probe technique. A resistivity as low as 10-2~10-1 ohm/cm was obtained. The optical property of this spinel was also measured as a function of thickness/deposition time in the wavelength range of infrared radiation.
Session FH-3 -Device Development and Applications
FH-3:IL01 Developing New TCOs for Renewable Applications
D. GINLEY, A. ZAKUTAYEV, N. WIDJONARKO, P. NDIONE, A. SIGDEL, P. PARILLA, J. BERRY, S. LANY, T. GENNETT, D. OLSON, J. PERKINS, NREL, Golden, CO, USA
Transparent conducting oxides enable a broad range of optoelectronic technologies. Not only are conductivity and transparency critical but many other factors are critical including: processing conditions, work function, chemical stability, and interface properties. The historical set of materials cannot meet all these needs. This has driven a renaissance in new materials development and approaches to transparent contacts. We present these new developments in general and in the context of photovoltaics specifically. We will present results on new materials and also the development bilayer structrues that enable charge selective contacts. Materials set includes amorphous materials for hybrid solar cells like InZnO, and ZnSnO, it includes Nb and Ta doped TiO2 as a high refractive index TCO, Zn1-xMgxO:Ga as a dopable TCO with an adjustable gap/workfunction and it includes the use of thin n- and p-type oxides as electron and hole selective contacts such as has been demonstrated for organic photovotaics.
U.S. Department of Energy, Office of Science, BES, under Contract No. DE-AC36-08GO28308 to NREL DOE Energy Frontier Research Center "Center for Inverse Design" and through the US Department of Energy under Contract DOE-AC36-08GO28308 the National Center for Photovoltaics
FH-3:IL02 Advances in Photocatalytic Applications Utilizing Metal Oxide Compounds
G. KIRIAKIDIS1, 2, V. BINAS1, 1Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology (FORTH), Vasilika Vouton, Heraklion, Crete, Greece; 2University of Crete, Physics Department, Heraklion, Crete, Greece
In the past decades, TiO2 photocatalyst has been extensively studied for the degradation of different forms of pollutants such as inorganic and/or VOCs (Volatile Organic Compounds) gases as well as liquids (organic dyes) due to its low cost, innoxiousness, chemical inertness, and high photocatalytic (PC) performance under UV light. However, only a small amount, about 3%, of the solar spectrum falls in the UV region providing energy larger than the band gap of TiO2 (3.2eV/ anatase) phase. Recently there are intensive efforts related to the development of active photocatalysts to harvest the additional harmless part (~45%) of the visible spectrum leading to safe indoor and outdoor applications in private, industrial and public buildings.
In this work we present an overview and recent advances on the synthesis and characterization of TiO2 materials doped with transition metals in different concentrations capable to absorb and activate under visible light irradiation. We will report on novel PC materials as effective pollutant reducing agent, harmless to humans and suitable for indoor besides outdoor applications. The crystal structure, particle size, morphology, and porosity along with surface morphology and elemental analysis of the materials are presented.
FH-3:IL03 Preparation and Optical Characterization of ZnO Nanowires for Sensing and Optoelectronic Applications
C. BARATTO1, 2, E. COMINI2, 1, I. CONCINA, M. FERRONI2, 1, G. FAGLIA2, 1, V. GALSTYAN2, 1, R. KUMAR2, 1, A. PONZONI2, 1, A. VOMIERO2, 1, D. ZAPPA2, 1, G. SBERVEGLIERI2, 1, 1CNR INO, SENSOR, Brescia, Italy; 2University of Brescia, Dept. of Information Engineering, Brescia, Italy
Zinc oxide is a very intriguing material studied for a variety of applications ranging from lasing, UV LED, gas sensors, transparent conducting oxides and piezoelectric transducers. ZnO can be prepared as single crystalline nanowires; the electrical and optical properties of such small structures are strongly influenced by the surface, i.e. lattice termination, defects, reconstruction or amorphization of the crystalline arrangement, dangling bonds, and chemisorbed or physisorbed atoms. Due to wide bandgap (3.37 eV) and large exciton binding energy (60 meV) ZnO is one of the most promising materials for photonic devices, such as light-emitting diodes (LED) and lasers. Heterojunctions with easily p-dopable materials were proposed for light emission (GaN, Si, NiO or CuAlO2).
We prepared ZnO nanowires by VLS and VSS mechanisms in tubular furnace; the nanowires are single crystalline with diameter in the 50-100 nm range. An overview of the most recent results we have obtained in the study of electrical and optical properties of ZnO nanowires and heterostructures for LED will be presented, as well as their application as sensing materials.
FH-3:IL04 CMOS Oxides used for Amplifiers and Logic Circuits on Paper
R. MARTINS1, A. NATHAN2, P. BARQUINHA1, L. PEREIRA1, E. FORTUNATO1, 1CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia (FCT), Universidade Nova de Lisboa (UNL), Caparica, Portugal; 2 Department of Engineering, Cambridge University, Cambridge, UK
In this work we report results concerning the performances of low power consumption complementary metal oxide devices (CMOS) used in analogue and digital circuits, such as inverters, amplifiers, NAND and NOR logic gates seamlessly layered on mechanically flexible, low cost, recyclable paper substrates. The ability to build circuits using low temperature processes opens the door to new applications ranging from smart labels and sensors on clothing and packaging to electronic displays printed on paper pages for use in newspapers, magazines, books, signs and advertising billboards. And since the CMOS complimentary circuits reported here constitute fundamental building blocks for analogue and digital electronics, then this development creates the potential to have flexible form factor computers seamlessly layered onto paper. The holistic approach of merging low power circuitry with recyclable substrate poses a significant step forward in the green electronics.
R. Martins, A. Nathan, R. Barros, L. Pereira, P. Barquinha, N. Correia, R. Costa, A. Ahnood, I. Ferreira, E. Fortunato, Advanced Materials 2011, 23, 4491.
E. Fortunato, P. Barquinha, R. Martins, Advanced Materials 2012 (review).
FH-3:IL06 All Amorphous Metal Oxide p-n Diodes
M. GRUNDMANN, Universität Leipzig, Institut für Experimentelle Physik II, Leipzig, Germany
Amorphous oxides are attractive as functional materials due to ease of fabrication at or close to room temperature and their good homogeneity, in particular the absence of grain boundaries. IGZO has been advantageously used for the channel in MISFET. We have demonstrated previously the first Schottky diode with an amorphous oxide n-type semiconductor (Ag/a-IGZO) and MESFETs based on such gate diode [1] and the first pn-diode with amorphous p-oxide (a-ZnCo2O4/ZnO) that exhibits high rectification and JFETs based on such gate diode [2].
Here we report fully amorphous bipolar diodes with high rectification, based on p-ZnO:Co/n-ZnO:Sn. The oxygen partial pressure is used as parameter to tune and optimize the conductivity and transparency of both the materials. Our results pave the way for a complementary oxide transistor technology.
[1] M. Lorenz et al., Appl. Phys. Lett. 97, 243506 (2010)
[2] F.-L. Schein et al., IEEE Electron Device Letters 33(5), 676 (2012)
FH-3:IL07 Preparation of Large Area Graphene by a Roll-to-roll Microwave Plasma Chemical Vapor Deposition for Transparent Electrode Applications
M. HASEGAWA, Nanotube Research Center, National Institute of Advance Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
We developed the large-area microwave plasma CVD of graphene for transparent electrode applications. This technique has been successfully combined with the roll-to-roll process to synthesize graphene on Cu substrates. Fast growth of graphene is a big advantage of plasma CVD which is suitable for the mass production. However, nucleation density of graphene crystal is too high, which suppresses 2D growth of graphene. Consequently, graphene flakes of several nanometer stacks in multiple layers, which causes low electrical conductivity of the plasma CVD graphene.
We have developed a new synthesis method of graphene by plasma treatment using small amount of C atoms contained in Cu foil as the carbon source to reduce the nucleation density and to enhance 2D crystal growth. After the precipitation of C atoms on Cu foil surface by a heat treatment the foil was exposed to hydrogen plasma in a short time to make graphene. Raman spectroscopy and Hall effect measurements revealed the dramatically improvement of the crystalline quality and the electrical conductivity compared with the conventional plasma CVD using methane as a carbon source. This new synthesis method of graphene using carbon atoms in Cu foil is suitable for mass production of graphene by roll-to-roll.
FH-3:L08 Transparent P-type Cu2O/SnO Bilayer Thin Film Transistors
H.A. AL-JAWHARI, J.A. CARAVEO-FRESCAS, M.N. HEDHILI, H.N. ALSHAREEF, Department of Physics, King Abdulaziz University, Jeddah, Saudi Arabia; Materials Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
Novel transparent p-type Cu2O/SnO bilayer channel thin film transistors (TFTs) were fabricated using room temperature sputtered copper and tin oxides. Using Cu2O film produced at a relative oxygen partial pressure Opp of 10% - as an upper layer - and four different Opp SnO films ranging between 0%-9% - as lower layers - we built a matrix of bottom gate bilayer TFTs with various ratios of Cu2O/SnO and different thicknesses. The only TFTs set that showed a p-type polarity was that fabricated using 3% Opp SnO films. The influence of both post annealing temperature and ratio of the Cu2O/SnO stack on the performance of TFTs were then investigated. We found that the bilayer transistor with a total 25 nm thick channel and tCu2O/tSnO =10/15 ratio showed the best condition after being annealed for 30 min at 170°C in air. It exhibited a p-type behavior with field-effect mobility, on/off ratio and threshold voltage of 0.66 cm2/V.s, 1.5×102 and -5.2V, respectively. Compared to the performance of a single SnO layer TFT with the same thickness,(25 nm), deposited at 3% Opp, the bilayer structure reduces the required annealing temperature, shifts the threshold voltage to the negative, enhances the field effect mobility and reduces the device off - current by two orders of magnitude.
Poster Presentations
FH:P01 Oxyfluoride Glasses as Promising Materials
N. SAVCHENKO, L. IGNATEVA, YU. MARCHENKO, Institute of Chemistry FEB RAS, Vladivostok, Russia; V. BOUZNIK, Baikov Institute of Metallurgy and Materials Science, Moscow, Russia
Recently we have synthesized new glasses in the system MnNbOF5-BaF2-BiF3. Samples were studied by the XRD, IR and Raman spectroscopy, differential scanning calorimetry, impedance spectroscopy and atomic force microscopy. Results of IR- and Raman spectroscopy show that the glass structures are formed from Nb(OF)6 polyhedra connected in networks by fluorine or oxygen bridges and BiF3-polyhedra form its own lays or spheres. Thermal treatment of the glasses on the base of MnNbOF5 was carried out and at the defined temperatures we successfully obtained transparent glassceramics having the crystalline phase of the composition Ba1-xBi xF2-x in the glass matrix. The data on transport properties of individual solid solutions with fluorite-type structure of the same composition were described by L. Soubeyroux & al. So, on the base Bi-containing glasses we can obtain glassceramics having in the glass matrix the crystallites with important physical properties. We investigated the ionic conduction of new oxyfluoride glasses in the system 20MnNbOF5-xBaF2-(80-x)BiF3. The electrical conductivity is the highest in the glasses 20MnNbOF5-30BaF2-50BiF3 and 20MnNbOF5-40BaF2-40BiF3 (σ = 7.46 · 10-3S/cm at 533 K and σ = 1.78 · 10-3S/cm at 523 K, respectively); i.e., with the highest BiF3 content.
FH:P04 Facile Synthesis of One-dimensional Bismuth Oxychloride Nanostructures as Potential Visible Light-driven Photocatalysts
LI-CHIA TIEN, YU-CHEN LAI, YU-LIN LIN, SENG-YU CHEN, Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien, Taiwan
Bismuth oxychlorides, V-VI-VII ternary oxide semiconductors, have attracted much attention as highly efficient visible light-driven photocatalysts. It has been reported that the unique layered structure of bismuth oxychlorides is beneficial for the separation of photo-generated electron-hole pairs and could result in good visible-light-induced photocatalytic properties and excellent chemical stabilities. Considerable efforts have been focused on synthesizing nanostructured BiOCl with enhanced catalytic and strong luminescence properties. Bismuth oxychloride (Bi12O17Cl2) nanowires, for the first time, were successfully synthesized by chlorination of α-Bi2O3 nanowires at low temperature. The as-grown sample consists of nanowires with diameters 200-400 nm and lengths of 10-15 μm. XRD results show that the obtained nanowires are of tetragonal structure. Both EDS and XPS confirm the composition and stoichiometry of Bi12O17Cl2 nanowires. The optical band gap of Bi12O17Cl2 nanowires was estimated to be 2.28 eV by absorption measurement. The Bi12O17Cl2 nanowires exhibit intense green (568 nm) and red (746 nm) emissions at room temperature. The obtained Bi12O17Cl2 nanowires may be potentially useful as photocatalysts and visible-light luminescent materials.
FH:P05 Synthesis of Homogeneous Indium-free Conductive Nanomaterials by a Water Controlled-release Solvothermal Process
SHU YIN, C.S. GUO, B. LIU, Q. DONG, T. SATO, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan
The tungsten bronze (AxWO3, A=alkali metalic element) with homogeneous morphology and nanosize were successfully synthesized by a modified water controlled-release solvothermal process. In the mentioned process, the mixed-solution of acetic acid and ethanol were selected as solvent, and the water molecules was released gradually from the esterification reaction between ethanol and acetic acid under the elevated reaction temperature. This process is preferred to synthesis homogeneous inorganic nanoparticles. The synthesized Indium-free tungsten bronze nanomaterials showed excellent electrical conductivity, visible light transparency, and near-infrared light shielding characteristics. The samples treated in 5% H2 -95% N2 mixed gas showed the higher pressed-powder electric conductivity than that of commercial conductive tin-doped indium oxide (ITO). Meanwhile, the samples showed strong absorption of near-infrared light and instantaneous conversion of photo-energy to local heat, indicating the extremely high-potential applications as solar filter and indium-free transparent conductive thin film materials. In addition, the tungsten bronze nanomaterials possessed various novel functionalities such as environmental sensing properties and the ability for thermotherapy to cancer cells.
FH:P07 Effect of Oxygen Partial Pressure on Electrical Properties of Ga,P co-doped ZnO Thin Films
WOO-SEOK NOH, JUNG-A LEE, JOON-HYUNG LEE, YOUNG-WOO HEO, HEE-YOUNG LEE*, JEONG-JOO KIM, Kyungpook National University, Daegu, Korea; *Yeungnam University, Korea
ZnO has been regarded as a promising material for transparent electrodes, solar cell, photo-detectors, diodes, sensors, thin film transistors and wave resonators. ZnO has a hexagonal wurtzite crystal structure, a wide band gap of 3.37eV, and a large exciton binding energy of 60meV. Undoped ZnO films generally exhibit n-type properties. The development of ZnO-based optoelectronic devices has suffered from one major disadvantage: the lack of good and reproducible p-type conduction. Several reports have suggested that co-doping of both acceptor (such as N,P) and donor (such as Ga,Al) can increase the solubility of acceptors in ZnO thin films.
In this study, we investigated the effect of oxygen partial pressure on electrical properties of Ga,P co-doped ZnO thin films by pulsed laser deposition. Highly excess P-doped ZnO showed a decreasing tendency in the carrier concentration as the oxygen partial pressure increased from 1mTorr to 200mTorr, while the carrier concentration begins increasing over 200mTorr which shows an n-p type transition at 200mTorr. On the other hand under 3at% of total concentration of the dopants, the carrier concentration of the films decreased as the oxygen partial pressure increased from 1 to 500 mTorr, which is a typical n-type semiconductive characteristic.
FH:P09 Synthesis of Indium Doped ZnO TCO Films for Organolead Hybrid PV Cells
O. DINNER, G.E. SHTER, G.S. GRADER, Technion-Israel institute of Technology, Haifa, Israel
Transparent conductive oxide layers (TCO) have a major role in the PV cells. General desired TCO properties are resistance less than 100 Ohm/square and a transparency more than 80% in the VIS-NIR. As for today, ITO is mostly in use although the Indium cost is increasing. Therefore an alternative cheaper TCO is an attractive goal for researchers and developers.
In the present work Indium doped ZnO (IZO) TCO sol-gel films were prepared by chemical solution deposition using a spin coating technique. The effect of the precursor synthesis parameters on the morphology of IZO film and its stability during heat treatment at 350 -500°C was investigated. Two precursors were compared: nano-suspension, prepared by solvothermal synthesis and transparent solution prepared by direct Zn and In salts dissolving. The Indium and stabilizer concentrations were varied. The thermal behavior of the precursors was examined by TGA/DTA/MS. The TCO films were thermally treated with different profiles and tested. Final films of 200 - 2000 nm showed high transparency above 80% in the visible and NIR regions of spectrum and low resistance less than 60 Ohm/square. Currently the integration of prepared films in PV cell with organolead hybrid absorbers is under investigation and the results of first tests will be discussed.
FH:P10 In2O3-ZnO-SnO2 Transparent Conducting Oxide Thin Films with Very Low Indium Content
M.A. PUTRI, KI HWAN KIM, CHANG YOUNG KOO, JUNG-A LEE*, JEONG-JOO KIM*, HEE YOUNG LEE, Yeungnam University, Gyeongsan, South Korea; *Kyungpook National University, South Korea
Transparent thin films within the In2O3-ZnO-SnO2 ternary system with very low indium content were studied. The compositions investigated belong to the three-phase region in the equilibrium phase diagram with the mol fraction of In2O3 smaller than 0.6. Both co-doping and off-stoichiometry effects were explored, and it was found that single bixbyite phase was formed even beyond the solubility limit of 20%, which was identified by the use of high resolution transmission electron microscopy. It was also found that the onset of crystallization was affected by the cation stoichiometry, and amorphous films with good electrical and optical properties were possible if processing conditions were carefully controlled. The resulting films were applied to typical organic photovolataic cells deposited on flexible polyimide substrate. In this paper, it will be discussed in detail how the characteristics of transparent conducting oxides would influence the flexible solar cell performance.