Symposium Organizers
Markus Winterer University Duisburg-Essen
Wayne L. Gladfelter University of Minnesota
Daniel R. Gamelin University of Washington
Shunri Oda Tokyo Institute of Technology
T1: Gas Phase Synthesis
Session Chairs
Tuesday PM, April 06, 2010
Room 2024 (Moscone West)
9:30 AM - **T1.1
Synthesis and Surface Modification of Silicon Nanocrystals for Photovoltaics.
Mark Swihart 1 2 , Folarin Erogbogbo 1 2 , Chen-An Tien 2 , Sung Jin Kim 1 3 , Alexander Cartwright 1 3
1 Institute for Lasers, Photonics, and Biophotonics, The University at Buffalo (SUNY), Buffalo, New York, United States, 2 Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York, United States, 3 Electrical Engineering, The University at Buffalo (SUNY), Buffalo, New York, United States
Show AbstractSolution-based printing and coating processes have the potential to dramatically reduce production costs of photovoltaics. These include not only organic photovoltaics, but also a variety of devices based on metal and semiconductor nanocrystals inks. Advantages of semiconductor nanocrystals include the potential for multi-exciton generation as well as greater durability compared to organics. Research on semiconductor nanocrystal-based and hybrid organic-nanocrystal photovoltaic devices has primarily used nanocrystals of compound semiconductors (e.g. CdSe, InP, PbS, PbSe, etc.). However, ultimate large-scale application of many of these compound semiconductors is problematic, either because they contain highly regulated toxic heavy metals (e.g. Cd, Pb) whose use raises environmental concerns, or because they contain rare or expensive elements (e.g. In). Thus, our group and several others are investigating silicon nanocrystal-based photovoltaic devices. Silicon is non-toxic, abundant, and the dominant material currently used in photovoltaics. For silicon nanocrystals less than about 5 nm diameter, the band gap is size dependent, due to quantum confinement. This creates the possibility of building multiple-junction devices using silicon quantum dots of different sizes. SiGe alloy nanoparticles have the potential to extend absorption into the infrared (beyond the bulk Si bandgap). However, successful application of these nanocrystals requires scalable, cost-effective processes for their manufacture, along with methods of dispersing them in solvents, without leaving them coated with an insulating organic material. Our group has developed a laser pyrolysis method for producing Si nanocrystals with a primary particle size as small as 5 nm. The as-synthesized particles are aggregated and partially sintered. These can be separated and reduced in size by acid etching, and can be made soluble in organic solvents by using hydrosilylation reactions to attach organic molecules to their surface. This talk will present recent advances in producing particles of controlled morphology and surface chemistry. The effects of particle size, morphology, and surface state on their behavior in a silicon nanocrystals / P3HT distributed heterojunction solar cell will be described.
10:00 AM - T1.2
Silicon Quantum Dots Composites for Photovoltaic Applications.
Xavier Paquez 1 , Yann Leconte 1 , Olivier Sublemontier 1 , Philippe Thony 2 , Nathalie Herlin-Boime 1 , Cecile Reynaud 1
1 DSM/IRAMIS/SPAM, CEA, Gif sur Yvette France, 2 INES, CEA, Le Bourget du Lac France
Show AbstractThe efficiency of amorphous Si-based tandem solar cells is limited by their poor conduction properties. Replacing the amorphous layer by a nanocomposite film containing Si quantum dots (Si-QD) could help overcoming this problem. Transport properties can indeed be increased while keeping the bandgap of the layer close to 1.7 eV thanks to the efficient quantum confinement that appears when the nanocrystal size is decreased below 5 nm. Finally, such a nanostructured tandem cell (Si-QD cell on single-crystalline cell) could reach a theoretical 42% efficiency.Single-crystalline silicon nanoparticles were produced in large amount from silane by laser pyrolysis and collected as free-standing powders. With this technique, a mean crystallite size as low as 3 nm can be obtained with a narrow size distribution (10 % around the mean value). A production rate of more than 200 mg/h of these Si-QD can be achieved for 4 nm particles. As a proof of the efficient quantum confinement effect, the obtained nanocrystals showed strong photoluminescence (PL) in the visible range with a position of the PL peak depending on their size.In order to elaborate the nanocomposite layer containing these nanopowders, two approaches are studied in parallel. The laser pyrolysis set up is equipped with a high vacuum extraction of the particle flux. A supersonic jet can be formed and pure Si-QD thin films can be deposited with very high deposition rates (up to 250 nm.min-1) by placing a substrate in front of the jet. Beside this in situ deposition technique, the second approach considers an ex situ deposition by spin coating of silica sol-gel precursor containing Si-QD. For this approach, Si-QD in high concentration are first dispersed in ethanol in order to obtain a Si-QD ink. The obtained layers result in a nanocomposite silica matrix containing a high concentration of Si-QD. In order to achieve the doping of the nanocomposite layers, phosphorus or boron precursors are added in the sol-gel mixture. Activation of the doping elements was studied by resistivity measurements after different annealing treatments. The effects of the thermal step were observed on the structure and on the optical properties of the composite films, showing that the quantum confinement effect was conserved while decreasing the layer resistivity.
10:15 AM - T1.3
Gas Phase Synthesis of Highly Specific Silicon Nanoparticles on the Pilot Plant Scale for Optoelectronic Applications.
Tim Huelser 1 , Christoph Rier 2 , Sophie Schnurre 1 , Dieter Jaeger 2 4 , Christof Schulz 3 4 , Hartmut Wiggers 3 4
1 Nano-Energy & Nano Particle Synthesis, Institute of Energy and Environmental Technology (IUTA), 47229 Duisburg Germany, 2 Center for Semiconductor Technology and Optoelectronics (ZHO), University of Duisburg-Essen, 47057 Duisburg Germany, 4 Center for NanoIntegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, 47057 Duisburg Germany, 3 Institute for Combustion and Gas Dynamics (IVG), University of Duisburg-Essen, 47057 Duisburg Germany
Show AbstractWe report on the gas phase synthesis of highly specific silicon nanoparticles on the pilot plant scale in a hot wall reactor and on a first step to introduce these materials into the field of optoelectronics.The formation of nanoparticles can be divided into two generally differing methods: wet-chemical and gas phase synthesis. While wet chemical synthesis usually ends up in materials grown by thermodynamic control, gas phase processes enable for a kinetic control of nanoparticle formation. Pure nanomaterials from the gas phase with a small size distribution are typically available in minute quantities that are not suited for the investigation of subsequent processing steps since they typically need non-standard formation processes. Therefore, many nanomaterials that require sophisticated technologies have not yet found their way into practical applications.For optoelectronic applications we synthesized Silicon nanoparticles in a hot wall reactor using silane as precursor material. A nozzle mounted directly above the reaction zone injected the precursor SiH4 in a N2/H2 atmosphere. The pressure and temperature of the synthesis process were kept constant at 500 mbar and 1173 K, respectively. Using these parameters we achieved a production rate of approximately 0.9 kg powder per hour.For structural and morphological analysis, x-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations were performed. The XRD measurements reveal a crystalline Si structure and a crystallite size of 15 nm using Scherrer’s equation. From TEM images, loosely agglomerated aggregates consisting of particles in the size regime of 100 nm were observed, which is in very good agreement with the BET investigation, that reveals a specific area of 25.5 m2/g corresponding to a particle diameter of 101 nm.Highly-specific inks based on nanoparticles are potentially prime aspirants for prospective printed (opto)electronics. In order to make stable dispersions, the silicon powder was dispersed in ethanol and inks of up to 8 % silicon by weight were realized through ultrasonic treatment. Silicon layers were spin-coated on several substrates like fused silica, fused silica with a transparent conducting oxide layer on top, and silicon-substrates. The thickness of the spin-coated layers was in the range of a few hundred nanometer and scanning electron microscopy revealed the formation of compact and homogenous layers. Thermally evaporated metal contacts on top of the spin coated layers enabled the measurement of IV-characteristics of the Silicon nanoparticle layers.
10:30 AM - T1.4
Synthesis and Electrical Characterization of n-doped Silicon Nanocrystal Films / Crystalline Silicon Junctions for Low-cost Solar Cells via Inertial Impaction.
David Rowe 1 , Zachary Holman 1 , Rebecca Anthony 1 , Ryan Gresback 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractTheoretical efficiency limits imposed by single crystal materials may ultimately restrict the use of photovoltaics for energy generation. Therefore, decreasing cell fabrication cost and increasing cell efficiency should be targeted simultaneously to make photovoltaics more accessible to the general public. The unique optical and electrical properties of silicon nanocrystals (Si-ncs) combined with novel nanocrystal generation and deposition techniques make an interesting candidate for accomplishing both of these goals. The aim of this study is to investigate the synthesis and electrical characterization of solar cells fabricated from n-doped Si-ncs inertially impacted onto a p-doped silicon wafer. The nanocrystals are produced using a non-thermal capacitively-coupled RF argon plasma, with silane as the semiconductor precursor and phosphine (in hydrogen) as the dopant precursor. Using a non-thermal plasma to synthesize the nanocrystals enables efficient use of the precursor gases and convenient control of the doping levels. Silicon nanocrystals approximately 5 nm in diameter are accelerated through an orifice due to a pressure drop, and collected via inertial impaction on a p-doped silicon wafer with a gold back contact. Deposition via impaction can be easily extended to low cost substrates such as glass or polyimide. Furthermore, no liquid phase processing is necessary, which allows all processing to be done under vacuum or an inert ambient. Films are annealed up to 900°C to study the passivation of surface trap states and nanocrystal sintering. ITO is sputtered on the nanocrystalline film to produce the top contact. Solar cell performance is measured for different doping levels and film thickness using an AM1.5 source. Doping concentrations of approximately 0.1% exhibit optimal performance. X-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM) are used to characterize film structure. Film density is measured using SEM and Rutherford backscattering, and film densities approaching 50% of bulk silicon are recorded.This work was supported by NSF under grant CBET-0756326 and under MRSEC grant DMR-0819885.
10:45 AM - T1.5
Chemical Vapor Functionalization of ZnO Nanocrystals.
Moazzam Ali 1 , Marty Donakowski 2 , Markus Winterer 1
1 Nanoparticle Process Technology, University Duisburg-Essen, Duisburg 47057 Germany, 2 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractIn the last few years, the interest in printable electronics has substantially increased. The advantage of printable electronics is the feasibility of large-scale production of finely tuned patterns, even on flexible substrates. The ink required for printing, especially for inkjet printing, plays an important role in the quality and reliability of the final products. Chemical Vapor Functionalization (CVF) is a method, which is used to generate functionalized nanoparticles in the form of inks with a high production rate. In CVF, two reactors are used in series. First reactor consists of a hot quartz tube (600 °C) where ZnO nanocrystals are synthesized in the gas phase from diethylzinc and oxygen. Second reactor, connected at the exit of the first one and kept at lower temperature (400 °C), is used as functionalization chamber. At the connecting point of the two reactors, vapors of organic functionalizing agents are injected which react with the surface of ZnO nanocrystals in the vapor phase. ZnO nanocrystals have been functionalized by 1-hexanol, hexanoic acid, hexanal and 1-hexylamine. In-situ analysis of the functionalization has been performed by quadrupole mass spectrometery. Functionalized ZnO nanocrystals have been characterized by Dynamic Light Scattering, Transmission Electron Microscopy, Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Nuclear Magnetic Resonance Spectroscopy.
11:00 AM - T1:Gas Phase Syn
BREAK
T2: Processing of ZnO and Related Materials
Session Chairs
Tuesday PM, April 06, 2010
Room 2024 (Moscone West)
11:30 AM - **T2.1
High Performance Solution-processed Indium Zinc Oxide and Indium Gallium Zinc Oxide Thin Film Transistors.
Rebecca Peterson 1 , Kulbinder Banger 1 , Yoshi Yamashita 2 , Kiyotaka Mori 2 , Henning Sirringhaus 1
1 , University of Cambridge, Cambridge United Kingdom, 2 , Panasonic, Osaka Japan
Show AbstractTransparent metal oxide semiconductors have recently drawn attention for use in thin film transistors (TFTs), conductive electrodes, UV-opto-electronics, and sensors. A great variety of conductive or wide-band gap semiconducting materials with binary, ternary and quaternary composition, such as ZnO, InSnO and InGaZnO, are accessible. Since the electron conduction band is formed by overlap of the metal cations' spherical s-orbitals, amorphous metal oxides can exhibit excellent electronic properties and crystalline films are not required. Most work has been focused on materials deposited by sputtering, and TFTs with high electron mobilities (~10 cm2V-1s-1) and good electrical stress stability have been demonstrated. For large-area applications or fabrication on flexible substrates solution-processed materials are preferable, however. Here we demonstrate InZnO and InGaZnO n-type TFTs made by solution processing of metal alkoxide precursors with performance comparable to sputtered devices. Mobilities up to 14 cm2V-1s-1 and ON/OFF ratios greater than 107 were achieved, and devices are well-behaved from room temperature to 5K and exhibit good bias stress stability. The indium to zinc ratio is critical to device performance, and the addition of gallium enables enhancement-mode devices. By careful choice of the metal precursor molecules, we lower the process temperature to 350°C while still obtaining a mobility of 2 cm2V-1s-1.
12:00 PM - T2.2
TCO Thin Films from Nanoparticles for Optoelectronic Devices.
Roland Schmechel 1 , Gabi Schierning 1 , Ralf Theissmann 1 , Simon Bubel 1 , Norman Mechau 2 , Anna Prodi-Schwab 3
1 Faculty of Engineering, University Duisburg-Essen and CeNIDE, Duisburg Germany, 2 Institute of Nanotechnology, Forschungszentrum Karlsruhe, Eggenstein-Lepoldshafen Germany, 3 Creavis Technologies and Innovation, Evonik Degussa GmbH, Marl Germany
Show AbstractTCO nanoparticles are considered for printing processes as an alternative to plasma or vacuum based coating technologies. In this talk, the properties of ITO nanoparticle thin films for printable transparent electrodes as well as ZnO nanoparticles for thin film transistors will be presented. The presentation summarizes our work on two alternative procedures: (1) ITO nanoparticle thin films prepared by annealing steps in order to remove organic additives and (2) Nanoparticle thin films where the particles are embedded in a polymer matrix without subsequent annealing. In case (1) the effect of post-heat treatment of indium-tin-oxide nanopowders in reducing atmosphere on defect structure, electrical resistivity and transparency will be considered. The formation of Indium segregation under reducing atmosphere has been detected very sensitively by susceptibility measurements utilizing the superconducting properties of indium. In case(2), a hybride system of ITO and conducting polymer PEDT/PSS was investigated. A decrease in electrical conductivity of PEDT with increasing ITO content up to a volume fraction of about 16 vol % is observed. The results are discussed with respect to changes in the infrared polaron and bipolaron absorption of PEDT and morphological changes. Despite a charge transfer between the n-type ITO particles and the p-type PEDT/PSS seems possible, the strong reduction in conductivity is mainly assigned to morphological changes. Above the volume fraction of 16 vol % the conductivity increases very steeply, most probably due to percolation between the ITO-nanoparticles. A phase separation at higher ITO content makes this system non-applicable for thin film preparation. Finally, thin films of ZnO nanoparticles with PVP are investigated in thin film transistors. Despite PVP is an insulating polymer, the device parameters are enhanced under certain conditions. The effect of PVP content and solvent on the device performance will be discussed.
12:15 PM - T2.3
Inkjet Printed, Electrochemically-gated Field-effect Transistors With ITO Nanoparticles as Active Layer.
Subho Dasgupta 1 , Norman Mechau 1 , Jooyoung Lee 1 , Robert Kruk 1 , Horst Hahn 1
1 Institute of Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Baden-Würtemberg, Germany
Show AbstractIndium tin oxide (ITO) is a degenerate semiconductor with near metallic conductivity. Recently, a solution-processed field-effect transistor device is demonstrated with ITO nanoparticles as the channel material and a polymer based solid electrolyte as gate dielectric [1]. The device principle is based on the variation of drain current induced by the capacitive double layer charging at the electrolyte/nanoparticle interfaces. Exploiting the metallic nature of conducting nanoparticles, a high on state current and a large transconductance is obtained. On the other hand, high local field of the electrochemical gating ensures sufficient field-effect and an on/off ratio larger than 10^3. However, the depletion of the charge carriers completely resulting in such a low off-state current is not expected for conducting oxides with a carrier concentration higher than 10^20 /cm^3. Further studies show that an increase in electronic roughness at the surface and grain/inter particle boundary occur during electrochemical charging. This contributes to majority of the observed effect by changing the surface and the grain boundary scattering of conducting electrons [2]. The large field-effect mobility (24 cm^2/Vs) of this entirely solution-processed device encourages ink-jet printing of such transistors as an inorganic alternative of the organic printable electronics. In fact, the printed transistors show quite interesting transistor characteristics. It is therefore believed that this result opens a new possibility of all-printed devices with conducting oxide nanoparticles as the channel element. [1] S. Dasgupta, S. Gottschalk, R. Kruk, H. Hahn, Nanotechnology 19 (2008) 435203[2] S. Dasgupta, M. Lukas, K. Dössel, R. Kruk, H. Hahn, Phys. Rev. B 80 (2009) 085425
12:30 PM - T2.4
An in-situ Dynamic ZnO Electrophoretic Deposition Technique.
Chunwei Wu 1 , Sandip Mitra 1 , Michael Zachariah 1
1 Department of Mechanical Engineering, University of Maryland, College Park, Maryland, United States
Show AbstractElectrophoretic deposition (EPD) has been developed into a highly versatile colloidal processing technique for fabricating nanostructured inorganic metallic or ceramic films or patterns on substrate, in which the deposition is induced by the interaction between applied electric field and the electrically charged colloidal nanoparticles. In this work, rather than using pre-synthesized colloidal suspensions, we presented a brand-new in-situ EPD technique, in which EPD was manipulated to accompany the process of nanoparticle generation in solution phase. By controlling the reaction rate and electric field parameters, we were able to selectively deposit ZnO quantum dots at their dynamic growth period from initial nucleation to ripening stage on electrode covered by carbon nanotubes. Various microscopic characterizations have been used to analyze the nanostructures and film properties. The new strategy aims to provide high performance electrode materials with custom properties that will be used for photoelectrochemical energy production.
12:45 PM - T2.5
Ink-jet Printing of ZnO Nanoink.
Ahmed Khalil 1 , Moazzam Ali 1 , Markus Winterer 1
1 NPPT& CeNIDE, Duisburg-Essen University, Duisburg Germany
Show AbstractFor the fabrication of printable devices based on ZnO nanoparticles (ZnO NPs), the formulation of stable colloidal dispersions of these materials is a prerequisite. Here, ZnO NPs have been synthesized by Chemical Vapor Synthesis. The particles have spherical shape with narrow size distribution (about 20 nm). Aqueous dispersions of ZnO NPs have been prepared successfully after the addition of a polymeric stabilizer. The prepared dispersions are stable for at least two months without observable sedimentation. Ultrasonication has been used to disperse the particles as well as to break the agglomerates. The average particle size in dispersions was 30-35 nm. These dispersions are filtered through filters with 200 nm pores and are used as nanoink to prepare ZnO NP films on different substrates using ink-jet printing. The viscosity and the surface tension of the dispersion as well as the printing parameters have been optimized for forming layers with high quality. Dense and low porosity films of ZnO NPs with thicknesses between 100-250 nm have been prepared on glass, ITO-coated glass, and silicon. The interface roughness between the printed films and these hydrophilic substrates is low. The printed films have been tested as active layers for potential applications in thin film transistors, gas sensors and light emitting diodes and showed promising results.
T3: Optical Properties
Session Chairs
Tuesday PM, April 06, 2010
Room 2024 (Moscone West)
2:30 PM - **T3.1
Optical Properties of Impurity Doped Si Nanocrystals.
Minoru Fujii 1
1 Department of Electrical & Electronic Engineering, Kobe University, Kobe Japan
Show AbstractOptical properties of Si nanocrystals are strongly modified by doping n- and/or p-type impurities. Especially, photoluminescence properties are very sensitive to the doping. For example, n- and p-type impurities co-doped Si nanocrystals exhibit strong PL below the band gap energy of bulk Si crystals at room temperature, probably due to the transition from the donor to acceptor states. Impurity doping also affects nonlinear optical properties of Si nanocrystals. The nonlinear refractive index and the two photon absorption coefficient are strongly enhanced by phosphorus doping. Therefore, in addition to the size and the shape, impurity doping is another parameter to control optical properties of Si nanocrystals and by this additional freedom for the material design, the application field of Si-nanocrystal-based materials may be significantly extended. In this presentation, optical properties of impurity-doped Si nanocrystals are summarized with emphasis on the nonlinear optical properties.
3:00 PM - T3.2
Silicon Nanoparticles for Photovoltaic and Optoelectronic Applications.
Axel Lorke 1 , Andreas Gondorf 1 , Matthias Offer 1 , Jens Theis 1 , Nadine van der Schoot 1 , Cedrik Meier 2 , Hartmut Wiggers 3
1 Physics Dept. and CeNIDE, University of Duisburg-Essen, Duisburg Germany, 2 Physics Department and CeOPP, University of Paderborn, Paderborn Germany, 3 Institute for Combustion and Gas Dynamics and CeNIDE, University of Duisburg-Essen, Duisburg Germany
Show AbstractSilicon nanoparticles, directly synthesized from the gas phase, offer a promising route to in-expensive and easy-to-fabricate photovoltaic applications: They can be produced in large quantities and with high purity and crystal quality. As powders or in dispersions, they offer many of the advantages of organic semiconductors (easy spin-on fabrication, low process temperatures, flexible substrates) while keeping the advantages of present Si technology, con-cerning cost, availability and toxicity. We have fabricated photovoltaic test structures based on Si nanoparticles. Preparation starts from commercial glass substrates, coated with a transparent conducting layer (ITO). Silicon nanoparticles, dispersed in ethanol are applied by spin-coating, dried under ambient conditions and contacted by evaporated Au pads. IV-measurements clearly show that electrical transport is possible through these layers and that they exhibit diode-like characteristics. More importantly, a strong photoconductivity is observed. A weak but clearly detectable photovoltage gives a promising outlook on the possibility of fabricating photovoltaic cells using Si nanoparticles. Improvements of the sample properties can be achieved by different measures: Etching of the particles in hydrofluoric acid leads to a reduction of the resistance and an increase of the photovoltage by roughly a factor of two. Also, suitable doping improves the properties of the devices. Bulk Silicon is generally assumed to be an inappropriate material for optoelectronic applications. Si nanoparticles, however, show very promising properties in the quest for full integration of optoelectronic devices into Si-based CMOS technology. The photoluminescence (PL) of Si-nanoparticles exhibits two remarkable properties. First, due to the quantum confinement effect, the luminescence is shifted to the visible range. Second, the luminescence efficiency of the nanoparticles is orders of magnitude higher than that of bulk material. The quantum size effect allows us to tune the luminescence from green to red by variation of the particle size, either through synthesis or by etching. Time-resolved PL measurements reveal an exceptionally long radiative decay time of up to 250 μs. We observe a single exponential decay, which demonstrates the high structural quality of the material and indicates –together with the quantum size effect– that the luminescence is not caused by defects or other spurious effects. Absorption measurements show that the size effect does not lead to a considerable lifting of the crystal momentum conservation and that the nature of the optical transition is still indirect. Temperature-dependent measurements, together with the time-resolved spectroscopy reveal an intriguing exciton fine structure and with a surprisingly strong light emission from ‘dark’ exciton states. Finally, first experiments show that electroluminescence devices can be realized, based on Si nanoparticles from the gasphase
3:15 PM - T3.3
Temperature Dependent Properties of Gallium, Indium, and Tin Doped CdSe Quantum Dots.
Christopher Tuinenga 1 , Viktor Chikan 1
1 Chemistry, Kansas State University, Manhatan, Kansas, United States
Show AbstractThe effects of gallium, indium, and tin dopeing on in-situ photoluminescence and conductivity of CdSe quantum dots is reported. Temperature dependant photoluminescence studies of gallium, indium, and tin doped CdSe indicate that gallium and indium have stronger emission quenching effects than tin. Modeling of PL quenching with respect to donor level energy gives donor levels of 280meV and 100meV below the conduction band for indium and tin, respectively. This model predicts that gallium, indium, and tin doped quantum dots will have strongly temperature dependent conductive properties. Doped quantum dots were found to have higher conductivity at room temperature after pyridine ligand exchange when deposited on a Pt electrode at low bias (2V) compared to undoped quantum dots. This increase in charge carriers makes doped quantum dots a viable material for more efficient solar cells.
3:30 PM - T3.4
Effect of Air Exposure on Carrier Relaxation Dynamics in Colloidal Quantum Dots.
Milan Sykora 1 , Alexey Koposov 1 , John McGuire 1 , Roland Schulze 1 , Victor Klimov 1
1 , LANL, Los Alamos, New Mexico, United States
Show AbstractUnderstanding carrier relaxation processes in colloidal Quantum Dots (QDs) is essential for a number of applications. For example, optical amplification in QDs depends strongly on the efficiency of nonradiative relaxation processes, such as Auger recombination and surface trapping. The ability to control relaxation pathways in NCs is also critical for photovoltaic applications. One process that has been extensively studied in this context is carrier multiplication (CM), whereby multiple excitons are generated following a single photon absorption event. Successful exploitation of CM in practical technologies is strongly dependent on our ability to eliminate or minimize competing relaxation processes. Recent theoretical and experimental studies suggest that changes in the surface properties of QDs can have a pronounced effect on the efficiency of nonradiative relaxation as well as the efficiency of the CM process.In the present work, we study the effect of air exposure on PbSe QDs suspended in hexane. We show that in air-exposed solution the QDs undergo rapid oxidation that has pronounced effect on their chemical composition, electronic structure and carrier relaxation dynamics. We show that dramatic variations in PL quantum yield (QY), observed following air exposure, can be explained in terms of changes in the efficiencies of two relaxation processes: surface carrier trapping and nonradiative interband relaxation. We also show that air exposure induced enhancement of surface carrier trapping can lead to difficulties in accurate determination of Auger relaxation rates and overestimation of CM efficiencies. After accounting for enhanced carrier trapping and oxidation-induced reduction in NC core size, we demonstrate that the dramatic changes in the surface properties of oxidized NCs do not significantly affect the dynamics of the Auger relaxation or the efficiency of CM. Finally, we show that the effects of air exposure can be partially inhibited in core/shell structures whereby the PbSe core is protected by a thin shell of CdSe.
3:45 PM - T3.5
Biexciton Quantum Yield of Single Semiconductor Nanocrystals.
Jing Zhao 1 , Gautham Nair 1 , Tara Sarathi 1 , Moungi Bawendi 1
1 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe luminescence efficiency of biexcitons in single semiconductor nanocrystals (NCs) has an important role in their emission characteristics and in determining their suitability for many applications. In this work, we demonstrate theoretically and experimentally, using colloidal semiconductor NCs as a test material system, that the ratio of biexciton and exciton quantum yield (QY) can be accurately and simply estimated from the normalized size of the 0-time coincidence feature in the second order emission intensity correlation function of a single emitter under weak excitation. Using this method, we explore the biexciton QY of single CdSe/CdS core(shell) NCs. Furthermore, we investigate the effect of shell thickness on photoluminescence blinking and biexciton QYof these NCs. The biexciton QYs report on the Auger decay rates in the NCs therefore providing insight into blinking mechanisms.
4:00 PM - T3:Optical Prop.
BREAK
T4: Hybrid Photovoltaic Devices
Session Chairs
Tuesday PM, April 06, 2010
Room 2024 (Moscone West)
4:30 PM - **T4.1
Nanocrystal and Nanowire Hybrid Organic Semiconductor Photovoltaics.
Cherie Kagan 1
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractHybrid materials combine the low-cost, large-area processing notable of organic materials and the tunable, optical and electronic properties found in nanoscale inorganic materials. Wet-chemical synthetic methods yield macroscopic (~0.1-1 g) quantities of semiconductor nanocrystals, nanorods, and nanowires that are tailored in size, shape, and composition. We combine nanostructures with solution-processable organic semiconductors such as poly-3-hexyl thiophene and a solution-processable precursor that is thermally retro-converted to red-absorbing, high mobility, pentacene that allows us to fabricate organic-inorganic bulk heterojunctions. Optical spectroscopy, electrical measurements, and electrochemical measurements are used to probe the fundamental electronic and optical properties of the organic and inorganic components and the interfacial electronics important in solar energy conversion. These optoelectronic properties are correlated with detailed structural studies and with the characteristics and efficiency of fabricated organic-inorganic solar cells.
5:00 PM - T4.2
Novel 3D Composite Photoanode for Enhanced Efficiency in Photovoltaics.
Nicolas Tetreault 1 , Jeremie Brillet 1 , Geoffrey Ozin 2 , Michael Graetzel 1
1 SB ISIC LPI, EPFL - École Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 2 Chemistry Department, University of Toronto, Toronto, Ontario, Canada
Show AbstractWe shall present an innovative strategy that simultaneously amplifies the harvesting of photons and improves charge collection in a new kind of three-dimensional, nanostructured composite electrode for dye sensitized solar cells. As noted by Grätzel et al. in 2006 [1], charge percolation back to the transparent conductive (TCO) electrodes takes milliseconds. This slow charge extraction increases chances of electron-hole recombination at the mesoporous TiO2 - electrolyte interface. This limitation has proven long lasting over the last 15 years and limited efficient DSCs to be used with only a handful of electrolytes that offer low recombination rates. Herein, we propose to use an innovative three-dimensional charge-collecting network to improve the efficiency of charge transport in photovoltaics in general and in DSCs in particular. Therefore, the essence of the idea is predicated upon a composite cathode in which current collector, photoactive element, sensitizer and electrolyte are integrated into a single unit without sacrificing light harvesting capabilities. We will show how this novel composite photoanode affects charge transport, charge recombination, light harvesting and, most importantly, the overall efficiency of liquid and solid-state dye sensitized solar cells.[1]Wang et al. Characteristics of high efficiency dye-sensitized solar cells. J Phys Chem B (2006) vol. 110 (50) pp. 25210-25221.
5:15 PM - T4.3
Inorganic Nanocrystal Three-dimensional TiO2/PbS Solar Cells.
Tong Ju 1 , Qiaoer Zhou 2 , Lily Yang 1 , Glenn Alers 2 , Alison Breeze 3 , Sue Carter 1
1 Physics, University of California, Santa Cruz, Santa Cruz, California, United States, 2 Electrical Engineering, Unversity of California,Santa Cruz, Santa Cruz, California, United States, 3 , Solexant Inc, San Jose, California, United States
Show AbstractIn the past several years, schottky solar cells composed of PbSxSey, PbS or PbSe quantum dot thin-films and TiO2/In(OH)xSy/PbS/PEDOT:PSS devices with PbS films deposited via chemical bath deposition have been reported. However, heterojuction devices combining TiO2 and colloidal PbS quantum dots have not been demonstrated. Today, we present results on low-cost, all-inorganic, ultra thin heterojunction TiO2/PbS nanocrystal three-dimensional (3D) solar cells where PbS (1.1eV band gap) is the absorber and TiO2 serves as an electron transport layer. With the air stable top electrode and the ability to achieve sufficient absorption in ultrathin PbS films to improve charge extraction, TiO2/PbS heterojunction solar cells have the capability to achieve higher efficiency than Schottky PbS devices. With the aid of spin coating and dip coating techniques, we have been able to obtain 3D ultra thin solar cells with an energy conversion efficiency of 1.5% and Jsc was above 10 mA/cm2. Performance of 3D TiO2/PbS solar cells was determined by TiO2 particle size, cell thickness, different back contacts and buffer layer In2S3. The best overall performance of 1.5% efficiency at air mass (AM) 1.5 was achieved from a device with 250 nm thick TiO2 composed of 37nm TiO2 nanoparticles, a 150 nm thick PbS layer, and using indium tin oxide (ITO) and gold as the electrodes. EQE showed significant IR absorption from PbS nanoparticles and peak EQE was about 55% for the best device. With the same thin PbS layer, the performance of devices made using either larger TiO2 particles sizes or planar TiO2 solgel layers was inferior to that of the 37nm particle TiO2 devices. For the planar cell, the decrease in efficiency may be due to the decrease in the TiO2/PbS junction surface area compared to that of the 3D device. Devices made with larger TiO2 particle sizes showed an increase in the leakage conductance between the contacts, which decreased the performance. Au contacts worked better than Al contacts due to the energy barrier caused by unfavorable alignment of the Al workfunction with the PbS HOMO level. All device efficiencies suffer from low Voc and low fill factor. To overcome these limitations, we study the addition of a super thin buffer layer In2S3 between TiO2 and PbS, in an attempt to increase the separation of free carriers before they recombine at the interface. A further area of improvement is suggested by HR-SEM images that reveal limited penetration of the PbS into the TiO2 mesoporous layers, possibly restricting Jsc.
5:30 PM - T4.4
Infrared Solar Cells Based on a Colloidal Quantum Dots/Organic Bilayer Structure.
Ni Zhao 1 , Tim Osedach 1 , Liang-Yi Chang 2 , Maddalena Binda 3 , Scott Geyer 2 , Darcy Wanger 2 , Moungi Bawendi 2 , Vladimir Bulovic 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Electronics and Information Technology, Politecnico of Milan, Milan Italy
Show AbstractThe potential to harvest sunlight beyond wavelengths of 1000nm has recently spawned significant interest in colloidal PbS and PbSe quantum dot (QDs) based solar cells. Oftentimes these cells have comprised a charge separating Schottky interface between a metal cathode and QDs and exhibited low open-circuit voltages (VOC). Only recently was the first PbSe QD based excitonic solar cell demonstrated [1], incorporating a ZnO-nanoparticle electron transporting layer (ETL) between the QDs and the metal cathode. In order to achieve high VOC, small QDs with relatively large band gaps are required, limiting the solar spectral range harvestable by these cells. In this work, we fabricate PbS QD based excitonic solar cells comprising a fullerene derivative as the ETL. Variations in oxidation treatment, rather than QD size, are used to modify the electronic structure of the QDs. Devices with VOC of 0.47V and fill factor of 62% have been demonstrated, which are the highest among reported values for PbS and PbSe QD based solar cells. The power conversion efficiency reaches 2.4% under monochromatic infrared (wavelength of λ = 1310nm) light illumination. The mechanism for device operation will be discussed, shedding light on new approaches to the optimization of solar cell performance via engineering of the interfaces between the QDs and their neighboring charge transport layers.[1] Choi J J. et al., Nano Lett, article ASAp
5:45 PM - T4.5
Germanium Nanocrystal Solar Cells.
Zachary Holman 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractSemiconductor nanocrystals (NCs) show promise for cheap multi-junction photovoltaic devices. In order to compete with photovoltaic materials that are currently commercially available, NCs will need to be inexpensively cast into dense thin films with bulk-like electrical mobilities and absorption spectra that can be tuned by altering the NC size. The Group II-VI and IV-VI NC communities have had some success in achieving this goal by drying and then chemically treating colloidal particles, but Group IV NCs have proven more challenging. We report on thin films of plasma-synthesized Ge NCs deposited using two different techniques, and preliminary solar cells based on these films.Germanium tetrachloride is dissociated in the presence of hydrogen in a nonthermal plasma to nucleate Ge NCs. Transmission electron microscopy and X-ray diffraction (XRD) indicate that the particles are nearly monodisperse (standard deviations of 10-15% the mean particle diameter) and the mean diameter can be tuned from 4-15 nm by changing the residence time of the Ge NCs in the plasma. In the first film deposition scheme, a Ge NC colloid is formed by dispersing hydrogen-terminated Ge NCs in 1,2-dicholorobenzene (DCB) without further surface modification. While these “bare” NCs quickly agglomerate and flocculate in nearly all non-polar solvents, they remain stable in DCB. Thin-film field-effect transistors (FETs) have been fabricated by spinning Ge NC colloids onto substrates and they have been subjected to various annealing procedures. The devices show n-type, p-type, or ambipolar behavior depending on the annealing conditions, with Ge NC films annealed at 300C exhibiting electron saturation mobilities greater than 10^-2 cm^2/Vs and on-off ratios of 10^4. We have verified by XRD that this performance is not due to sintering of the NCs. We believe this is the first report of FETs based on Ge NCs, and the measured mobilities compare with those of popular organic semiconductors. The second film deposition scheme involves the impaction of Ge NCs onto substrates downstream of the synthesis plasma via acceleration of the particles through an orifice. This technique produces highly uniform films with densities greater than 50% of the density of bulk Ge. By varying the size of the Ge NCs, we have measured films with band gaps ranging from the bulk value of 0.7 eV to over 1.1 eV for films of 4 nm Ge NCs. As far as we know, this is the first report Ge NC films with tunable optical properties. Having deposited dense thin films with tunable band gaps and respectable mobilities, we have begun fabricating solar cells consisting of p-n junctions formed between p-type Si wafers and annealed, n-type Ge NC films. Initial devices exhibit open-circuit voltages and short-circuit currents as large as 0.3 V and 4 mA/cm2, respectively.This work was supported by NSF under grant CBET-0756326 and IGERT grant DGE-0114372. The UMN Center for Nanostructure Applications also provided support.
T5: Poster Session: Synthesis and Processing
Session Chairs
Wayne Gladfelter
Markus Winterer
Tuesday PM, April 06, 2010
Exhibition Hall (Moscone West)
6:00 PM - T5.1
Nonlinear Optical Absorption and Scattering of Lead Chalcogenide Nanocrystals.
Daniel Asunskis 1 , Igor Bolotin 1 , Ali Jawaid 1 , Frank Pleticha 1 , Preston Snee 1 , Luke Hanley 1
1 Chemistry, University of Illinois at Chicago, Chicago, Illinois, United States
Show AbstractThe nonlinear optical properties of lead sulfide (PbS) and lead selenide (PbSe) nanocrystals in both toluene suspension and as films have been probed by the Z-scan method using 532 nm, ~5 ns long laser pulses. Nanocrystals were synthesized using various wet chemical strategies to generate different shapes, sizes, and surface properties [1]. Cluster beam deposition was also employed to prepare films of nanocrystals embedded in organic matrices [2]. Nanocrystal size, shape, and chemistry were characterized by various methods in transmission electron microscopy, X-ray photoelectron spectroscopy, UV/Vis linear absorption, and fluorescence.Z-scan results indicated a clear difference in nonlinear absorption for different shapes and surface chemistries of nanocrystals. Prior work found that the surface chemistry played a central role in nonlinear absorption that occurred via an excited state absorption mechanism [3]. Nonlinear absorption was observed for nanocrystals both in toluene suspension and as films. Nonlinear scattering was also observed to occur for at least some of these lead chalcogenide nanocrystalline materials. References[1] D. J. Asunskis, I. L. Bolotin, and L. Hanley, J. Phys. Chem. C 112, 9555-9558 (2008).[2] A. M. Zachary, I. L. Bolotin, D. J. Asunskis, A. T. Wroble, and L. Hanley, ACS Appl. Mater. Interf. 1, 1770-1777 (2009).[3] D. J. Asunskis, I. L. Bolotin, J. E. Haley, A. Urbas, and L. Hanley, J. Phys. Chem. C, in press.
6:00 PM - T5.10
Effects of Gallium Doping on CdSe Quantum Dots.
Christopher Tuinenga 1 , Brett Vaughn 1 , Viktor Chikan 1
1 Chemistry, Kansas State University, Manhatan, Kansas, United States
Show AbstractCdSe quantum dots doped with gallium (Ga:CdSe) show similar behavior to indium doped dots (In:CdSe) in the low temperature heterogeneous growth regime. Gallium doping activates quantum dot growth resulting in the consumption of magic-sized quantum dots as well as accelerates the size focusing on the ensemble during low temperature growth at 120°C. Temperature dependant photoluminescence (TD-PL) studies indicate that gallium has a strong quenching effect on the photoluminescence, suggesting n-type electron donation to the conduction band. TD-PL studies also demonstrate dopant quenching decreases as the thickness of a ZnS shell increases. Gallium doping resulted in larger particles than indium and tin doping. This suggests that dopant selection plays an important role in determining the final size of doped particles.
6:00 PM - T5.11
Self-organized Formation of Ge Nanocrystals Out of (GeOx-SiO2) Superlattice Structures.
Manuel Zschintzsch 1 , Nicole Jeutter 1 , Johannes von Borany 1 , Arndt Muecklich 1
1 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden, Sachsen, Germany
Show AbstractBandgap engineered Si and Ge nanocrystal solar cells are supposed to be a candidate for high effective 3rd generation thin film solar cells. Photoluminescence studies of the quantum confinement effect in Si and Ge nanocrystals showed the feasibility of this approach [1,2]. However the design and the fabrication of a high density of well separated, monodispersed nanoclusters remains a great challenge.We want to present our investigations [3] on Ge nanocrystals formation in GeOx-SiO2 multilayer structures based on the phase separation of GeOx during annealing. The size of the laterally self-ordered Ge nanocrystals is vertically limited by the SiO2 separation layer. The final goal is to achieve well confined, graded, equally sized and dense nanocrystal superlattices only by the variation of the layer thicknesses and the oxygen content in the GeOx layer.The GeOx-SiO2 stacks were deposited via reactive DC magnetron sputtering. A process window for the oxygen partial pressure in the O2/Ar sputtering gas mixture can be defined which allows both, SiO2 formation for the separation layers as well as GeOx films with tunable stoichiometry in the range of x = 0.2 … 2.In-situ X-ray studies at synchrotron beamlines were performed to monitor the phase separation (XANES) of GeOx and the Ge nanocrystal formation (GIXRD, XRR, GISAXS) which was proofed in addition via TEM, Raman scattering and Absorbance.Separated Ge nanocrystals of 2 … 6 nm in size can be formed at temperatures < 600°C. Very smooth interfaces with roughnesses below 1 nm allowed the separation of the Ge nanocrystal layers by SiO2 films < 2 nm which enables interesting possibilities for charge transport via tunnelling.[1] G. Conibeer et al. TSF 511-512, 654 (2006)[2] Y. M. Niquet et al. APL 77, 1182 (2000)[3] M. Zschintzsch et al. JAP, accepted
6:00 PM - T5.12
ZnO Nanorods Functionalized With Supramolecular Porphyrin-Fullerene Complexes.
Syed Mujtaba Shah 1 , Aiko Kira 2 , Hiroshi Imahori 2 , Frederic Fages 1 , Joerg Ackermann 1
1 , Centre Interdisciplinaire de Nanoscience de Marseille, CINAM UPR-CNRS 3118 , Marseille France, 2 Department of Molecular Engineering, Graduate School of Engineering, Engineering, Kyoto University, Kyoto Japan
Show AbstractSupramolecular assembly of donor-acceptor complexes into functional nanomaterials is a promising approach for future low cost photovoltaics. Inspired by the assembly of porphyrin-fullerene functionalized gold nanoparticles into efficient molecular photovoltaics1, we started a project towards supramoleculare assembly of cografted porphyrin and fullerene onto zinc oxyde (ZnO) nanorods into stable charge transfer complexes. The use of anisotropic semiconductors such as ZnO nanorods should open new possibilities due to their unique electronic, optical and self-assembly properties. Here we report the synthesis and the optical properties of such donor-acceptor complex bearing nanorods. Absorption and fluorescence spectroscopy were used to investigate the influence of solvent and fullerene-porphyrin ratio on the complex formation on the surface of ZnO nanorods. Furthermore we demonstrate that the grafting of porphyrin and fullerene introduce specific self-assembly properties to the hybrid nanoassemblies which leads to the formation of ordered aggregates with parallel oriented nanorods. Additionally some preliminary results on the use of such modified ZnO nanorods as acceptor material in dye sensitized bulk heterojunction solar cells will be shown. [1] H. Imahori, A. Fujimoto, S. Kang, H. Hotta, K. Yoshida, T. Umeyama, Y. Matano, S. Isoda, M. Isosomppi, N. Tkachenko, H. Lemmetyinnen, Chem. Eur. J. 2005, 11, 7265-7275
6:00 PM - T5.13
Synthesis of Nano Porous CdS Thin Films for Hybrid Solar Cells.
Bharath Reddy 1 , Vignesh Gr 1 , Rajeev Jindal 1
1 R&D, Moserbaer Photovoltaic Ltd, Greater NOIDA, Uttar Pradesh, India
Show AbstractOrganic solar cells based on polymer/fullerene bulk hetero-junctions have shown good progress and efficiencies around 6% have been reported. However these efficiencies and stability are well below the conventional inorganic solar cells efficiencies. To improve these issues hybrid solar cell concepts are evolving which comprises of both inorganic and organic semiconductors. Inorganic nano particle and nano rods were widely investigated for hybrid solar cells and the conversion efficiencies of 2-3 % are reported. Here the efficiency is mainly limited by the structural traps due to poor percolation and poor coverage of polymers to nano particles / nano rod network. To solve these percolation issues we propose open voids like nano porous inorganic structures to have better filling and connectivity. Nano porous CdS thin films are prepared by simple chemical bath deposition technique by using different complexing agents and by varying Cd2+ ion concentration. Triethanolamine [TEA] is used as complexing agent to achieve nano crystalline growth, because it is a strong complexing agent with high viscosity. Due to the viscous nature of TEA, bath solution became colloidal in nature thereby resulted in reduction of crystallite size. Uniform CdS thin films with 20-30nm sized grains are achieved and the crystallinity is found to decrease with increase in TEA concentration. Porosity is achieved by increasing the Cd2+ ion concentration in the bath solution. Normally, excess Cd2+ results in formation of Cd(OH)2 in the solution. In presence of TEA Cd(OH)2 particles formed as suspended network in the solution which acted as mask on substrate surface and resulted in nano porous CdS growth. Nano structured films with porosity ~100 nm were achieved. These nano structured films are used to prepare hybrid solar cells using poly hexyl thiophene (P3HT) as absorber layer. Preliminary results have shown that these devices exhibit better device current in comparison to bulk heterojunction solar cells based on CdS nano particles. The detailed growth mechanism of nano structured CdS thin films and characterization results of CdS film and hybrid solar cells will be presented in the main manuscript.
6:00 PM - T5.14
Silver Doping of Semiconductor Nanocrystals.
Ayaskanta Sahu 1 , Moon Sung Kang 1 , Andrew Wills 2 , C. Daniel Frisbie 1 , David Norris 1
1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractColloidal semiconductor nanocrystals are a promising technological material because their size-dependent optical and electronic properties can be exploited for a diverse range of applications, such as light-emitting diodes, bio-labels, transistors, and solar cells. The intentional incorporation of impurities (or doping) allows additional control over the electrical and optical properties of these nanocrystals. However, while impurity doping in bulk semiconductors is now routine, doping of nanocrystals remains challenging. In particular, evidence for electronic doping, in which additional electrical carriers are introduced into the nanocrystals, has been very limited. Here, we adopt a new approach to electronic doping of nanocrystals. We utilize a partial cation exchange to introduce silver impurities into cadmium selenide (CdSe) and lead selenide (PbSe) nanocrystals. Results indicate that the silver-exchanged CdSe nanocrystals show a change in fluorescence, as compared to pure CdSe nanocrystals. The silver-exchange also results in a change in the conductance of both PbSe and CdSe nanocrystals and the magnitude of this change depends on the amount of silver incorporated into the nanocrystals.
6:00 PM - T5.15
Synthesis of Ultrabright Fluorescent Mesoporous Silica Particles.
Igor Sokolov 1 2 3 , Dmytro Volkov 1
1 Physics, Clarkson University, Potsdam, New York, United States, 2 Chemical and Biomolecular Science, Clarkson University, Potsdam, New York, United States, 3 Nanoengineering and Biotechnology Laboratories Center (NABLAB), Clarkson University, Potsdam, New York, United States
Show AbstractHere we describe recent advances in the development of ultrabright fluorescent silica particles. The unusually high brightness is achieved through physical incorporation of fluorescent organic dyes into mesoporous (nanoporous) silica material, which is synthesized by templated self-assembly. For an example of micron size colloidal particles, the particles are about 200x brighter than polymeric particles of comparable size assembled with quantum dots. Comparing with the maximum fluorescence of free dye in the same volume, the particles can show fluorescence which is higher by a factor of ~5,000 (rhodamine 6G dye example). We discuss the nature of high brightness of these particles, existing problems, and recent developments.
6:00 PM - T5.16
High Luminance YAG:Ce Nanoparticles Fabricated by Soft-Chemical Route.
Yi-Wen Kao 1 , Kuo-Chuang Chiu 1
1 Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractHigh luminance Y3Al5O12:Ce3+ (YAG:Ce) nanoparticles were prepared by Soft-chemical route. The as-prepared nanoparticles are hexagonal YAlO3, that are nearly rough on the surface and dense—and they can be converted to YAG:Ce after being annealed at 1000 oC for 1 h. The heat-treated particles are single crystalline, smooth in surface and dense with an average size around 30~40 nm. The optimum cerium-doping concentration of YAG:Ce nanoparticles is 4.0 mol.%. The efficient emission of YAG:Ce nanoparticles also originates from a relatively good distribution of Ce ions incorporated into the host material of YAG as evidenced from the elemental mapping analysis.
6:00 PM - T5.17
Fabrication and Characterization of Si Nanocrystals Embedded in SiC Matrix by Magnetron Sputtering for Third Generation Solar Cell Applications.
Arife Imer 1 , Ilker Yildiz 1 , Rasit Turan 1
1 Physics, Middle East Technical University, Ankara Turkey
Show AbstractSiC containing with Si nanocrystals has been proposed as a promising material for the fabrication of third generation solar cells. Since the bandgap of Si nanocrystals can be tuned by quantum size effect solar cell devices having different nanocrystals can absorb the solar radiation more efficiently. Si nanocrystals are usually fabricated in dielectric matrix such as SiO2 and Si3N4 with relatively high bandgap that makes the electronic transport rather difficult through the device. The use of SiC with relatively low bandgap as a matrix material for solar cell absorber is expected to make the electronic transport easier than other dielectrics. The fabrication of Si nanocrystals in a SiC matrix in a well controlled way is necessary for an efficient device operation. In this study, Si rich SiC films with different Si content were prepared by RF magnetron sputtering deposition technique. The fabricated films were characterized using various diagnostics techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Raman, X ray photoelectron Spectroscopy (XPS), Transmission electron Microscopy (TEM). Optical properties of the films were studied by photoluminescence and UV- visible spectrometry. Si nanocrystal formation kinetics was studied as a function of process parameters such as Si content, annealing temperatures and durations. The SiC film with and without excess Si showed good stoichiometric behavior. In the SiC films having excess Si, nanocrystal formation was clearly identified with Raman spectroscopy and the TEM. High resolution images of Si nanocrystals were obtained by High Resolution Electron Microscopy (TEM). Si nanocrystals having a mean size of 2 nm was imaged by TEM in the samples annealed at 1100 oC for 1h. The presence of Si-Si bonds was also detected by XPS through a series of experiments including the depth profiling of chemical bonds of O, Si, and C as a function of depth from the surface.
6:00 PM - T5.19
Laser Welding of Nanocrystalline Titania and Transparent Conducting Oxide Electrodes for High-efficiency Solar Cells.
Myeongkyu Lee 1 , Jinsoo Kim 1 , Jonghyun Kim 1
1 Dept. of Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractSince the discovery of dye-sensitized solar cell (DSSC), a great deal of efforts have been made to achieve a high energy conversion efficiency by increasing its short-circuit current density. Two main approaches were to enhance the light harvesting and to facilitate the charge transport within the nanoparticulate TiO2 electrode. Here we show that an interfacial resistance arising from the poor contact between TiO2 and transparent conducting oxide (TCO) electrodes takes a considerable portion in the total resistance of DSSC and the efficiency can be greatly improved by welding the interface with an ultraviolet pulsed laser. We find that a thin continuous TiO2 layer is formed at the interface as a result of the local melting of TiO2 nanoparticles and this layer completely bridges the gap between two electrodes, improving the current flow with a reduced contact resistance. A conversion efficiency of 11.2 % was achieved with a short-circuit current density as high as 24 mA/cm2. The presented laser-welding process is simple, fast, and more importantly, is additive to any other efficiency-enhancing schemes.
6:00 PM - T5.2
A Novel Way of Improving Light Harvesting in Dye Sensitized Solar Cells - Electrodeposition of Titania.
Shih-Yuan Lu 1 , Tsung-Yu Tsai 1
1 Chemical Engineering, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractLight harvesting is one of the key issues in efficiency improvement of dye sensitized solar cells. Traditionally, this is done by casting an extra layer of sub-micron sized TiO2 particles on top of the titania photo-anode to serve as a light scattering layer to enhance the light harvesting. Here, we developed a brand new way of creating this scattering layer by electrodepositing TiO2 onto the TiO2 photo-anode. This treatment created TiO2 patches of sub-micron sizes, composed of closely-packed TiO2 nanoparticles of 10-15 nm, functioning as the scattering layer. These sub-micron sized patches in the top portion of the photo-anode layer, serve as effective light scattering centers to enhance the light harvesting, functionally similar to the traditional extra layer of larger sized TiO2 particles on top of the photo-anode layer. The existence of this denser nanoparticle layer in the top portion of the titania photo-anode layer, although enhancing the light harvesting of the cell, retarded the diffusion of electrolyte. Consequently, an optimum extent of titania deposition was necessary to achieve a maximum improvement in the light to electricity conversion efficiency of the cell. We demonstrated a 52% increase in the cell efficiency (from 4.12 to 6.27%) achieved with this treatment at an optimum treatment condition.
6:00 PM - T5.20
Synthesis and Characterization of Zn2SiO4:Mn2+ Nanophosphors Prepared by Flame Spray Pyrolysis.
Jae Seok Lee 1 , Myoung Hwan Oh 1 , Aniruddh Khanna 1 , Purushottam Kumar 1 , Madhav Ranade 2 , Rajiv Singh 1
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 2 Particle Engineering Research Center, Univsrsity of Florida, Gainesville, Florida, United States
Show AbstractMn-doped zinc silicate (Zn2SiO4:Mn2+) nanophosphors were synthesized by flame spray pyrolysis (FSP) with different liquid precursors. Luminescence and crystalline properties were investigated with different Zn-source materials in aqueous precursor. The as-prepared particles were annealed for transformation to zinc silicate crystalline structures at a low temperature of 1000°C for 1hr. The emission peak was found at 525 nm in the region of spectrum which excited by 266 nm wavelength photons. The influence of different experimental parameters such as Zn-source in the liquid precursor and annealing temperature on both crystallinity and luminescence properties of Zn2SiO4:Mn2+ nanophosphors were investigated.
6:00 PM - T5.21
Synthesis of Luminescent Rare-earth Ion Doped Core-shell Nanostructures for Energy Harvesting.
James Dorman 1 , John Hoang 1 , Yuanbing Mao 2 , Jane Chang 1
1 , University of California, Los Angeles, Los Angeles, California, United States, 2 , Washington State University, Spokane, Washington, United States
Show AbstractAs the need to develop a clean, renewable and independent energy source increases, researchers are incorporating a new breed of nanomaterials into devices to produce higher energy efficiencies. Specifically, luminescent materials are being incorporated into photovoltaic devices to convert the unabsorbed photons into a “usable” wavelength. Currently, work has been completed using trivalent Er and Er/Yb ions as dopants in a sodium yttrium fluoride host lattice to convert IR photons to low energy visible light via the upconversion mechanism where Si solar cells have a higher absorbance. Since the energy transfer mechanism between rare earth (RE) ions is sensitive to the local crystal environment and separation distance, the luminescence spectra can be tuned by spatially controlling the dopant position. This work focuses on the synthesis RE doped nanostructures through a combination of hydrothermal and atomic layer deposition (ALD) and their resulting upconversion luminescence spectra.Our research concentrates on the synthesis of Er and Er/Yb co-doped yttrium oxide nanotubes and their upconversion luminescence. Using high-energy synchrotron radiation, the hydrothermal growth and concurrent dehydration process were characterized through in situ extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray diffraction (XRD). The formation of hexagonal Y(OH)3 was observed shortly after the onset of heating, as verified by the diffraction peaks. However, the Y(OH)3 final nanotube crystal environment is first seen with EXAFS after heating at 120°C for 7 hrs. The dehydration of Y(OH)3 to the desired cubic Y2O3 was found to proceed through a YOOH intermediate that was stable between 275 through 350 C. The conversion process was confirmed with thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). Additionally, when doped and co-doped systems are excited using IR light, green and red luminescence were observed through the 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 between 525 to 575 nm and 640 to 690 nm, respectively. Using a proposed upconversion mechanism for both the bulk and nanocrystalline product, the energy transfer rate and population densities are extracted from the spectral data. Finally, deposition of various shell layers through ALD allows for increased luminescence via two different mechanisms. First, the shell layer increases the separation of surface quenching sites, such as –OH, CO32-, and NO3-. Secondly, by doping the shell layer(s) we can control the distance between the luminescent centers in the core and sensitizer ions promoting the occurrence of energy transfer events. Finally, the incorporation of the RE ion doped core-shell nanostuctures into photovoltaics will be discussed to assess the feasibility of a broad absorption device.
6:00 PM - T5.22
Synthesis and Characterization of ZnMgO Nanoparticles and the Performance of P3HT/ZnMgO Nanoparticle Bulk Heterojunction Photovoltaics.
Summer Ferreira 1 , Robert Davis 1 , Yun-ju Lee 1 , Bell Nelson 1 , Provencio Paula 1 , Jianyu Huang 1 , Ping Lu 1 , Hsu Julia 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractOrganic/inorganic hybrid photovoltaic devices use low-temperature, solution-based processing which can be carried out in air, making them an attractive class of solar cells due to their potential for low cost, large-scale production. ZnO has recently been of intense interest due to its viability in various technological applications, including hybrid photovoltaics due to less stringent processing requirements and improved device stability. By blending nanoparticle ZnO in a conducting polymer, i.e. P3HT, a larger fraction of the polymer is near an oxide interface, from which photoexcitation can lead to charge generation and contribute to photocurrent. Concurrently, while bulk ZnO has a band gap of 3.3 eV, the band gap of nanoparticle ZnO increases with decreasing size below ~ 5 nm due to quantum confinement. It is known that the ZnO band gap can be further increased by Mg incorporation. D. Olsen et. al [1] incorporated Mg into ZnO sol-gel films and demonstrated an increase in the Voc in P3HT bilayer devices over the maximum found using pure ZnO, improving the overall device efficiency. However, Mg doping has, to our knowledge, not been explored for nanostructured ZnO, but represents a promising next step in organic/inorganic blend solar cells. In this work, we explore solution-based synthesis of ZnO and ZnMgO nanoparticles. We study the effect of Mg on nanoparticle size as a function of reaction time and on crystal structure through transmission electron microscopy (TEM). We quantify Mg incorporation in ZnO nanoparticles via inductively coupled plasma (ICP) and its effect on band gap through UV-Vis spectroscopy. Using TEM we perform structural characterization of blend device in cross sections to study the incorporation of nanoparticles in the active layers. In addition, the effect of Mg concentration on the Voc of blend devices is measured through solar simulation and its effect on device efficiency is determined. Furthermore, we explore the effect of solution processing conditions on device performance, including the solvents used in both nanoparticle synthesis and spin casting of the blend layer in these devices.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.Reference:[1] Olson, D. C.; Shaheen, S. E.; White, M. S.; Mitchell, W. J.; van Hest, M.F.; Collins, R. T.; Ginley, D. S. Adv. Funct. Mater. 2007, 17, 264-269.
6:00 PM - T5.23
ZnO Nanosphere Fabrication Using the Functionalized Polystyrene Nanoparticles for Dye-sensitized Solar Cells.
Mi-Hee Jung 1 , Ho-Gyeong Yun 1 , Hunkyun Pak 1 , Mangu Kang* 1
1 , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractRecently, the zinc oxide (ZnO) has been explored as an alternative material in dye sensitized solar cells (DSSCs). The ZnO-based dye DSSCs have attracted considerable interest due to the similarity of the energy band gap that of TiO2 and the much higher electron mobility ~115-155 cm2V-1s-1 than that for anatase TiO2 which was reported to be ~10-5 cm2V-1s-1. The ZnO have mostly focused on the photoelectrode with one dimensional structure such as nanowires and nanotubes. However, the overall efficiency was limited by the insufficient surface area of the nanowires and nanotubes. Light scattering within the film is considered as one of the approaches to increase photon capture efficiency, as well as the optical absorption of the photoelectrode in DSSCs. However, the specific surface area was decreased with the introduction of larger-sized particle and thus an undesired reduction in the dye absortion would result. Therefore, it is desirable to introduce large, submicrometer-sized light scattering layer into nanocrystalline films without losing the necessary surface area for dye absorption.Herein, we describe solar cell consisting of submicrometer-sized ZnO nanosphere fabricated from functionalized polystyrene nanoparticles 100 ~ 300 nm in size. We can control the ZnO nanosphere 200 ~ 1000 nm diameter sizes from varying the polystyrene nanoparticle sizes and reaction time. ZnO nanosphere was formed by the ZnO precursor was piled up the polystyrene nanoparticles because sulfate functionalized polystyrene latexes have a higher ionic strength. Subsequent thermal decomposition to remove the organic template followed by impregnation with N719 dye results in excellent ZnO sphere photoelectrodes with a photo-conversion efficiency as high as 2.2% under air mass 1.5 illumination. The novel fabrication method of ZnO nanosphere structured photoelectrode developed in this study is expected to create new opportunities for further light enhancement by the scattering effect in photovoltanic and photoelectrochemical cells.
6:00 PM - T5.3
Narrowing Size Distribution of Nanoparticles by Pulsed Precursor Delivery in a Gas-phase.
Ruzica Djenadic 1 , Qing Cao 1 , Markus Winterer 1
1 Department of Engineering Sciences, and Center for NanoIntegration Duisburg-Essen, CeNIDE, University Duisburg-Essen, Duisburg Germany