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Symposium T: Photovoltaics and Optoelectronics from Nanoparticles

* Invited paper
 

8:30 AM *T1.1
Synthesis and Surface Modification of Silicon Nanocrystals for Photovoltaics. Mark T. Swihart^1,2, Folarin A. Erogbogbo^1,2, Chen-An Tien², Sung Jin Kim^1,3 and Alexander N. Cartwright^1,3; ¹Institute for Lasers, Photonics, and Biophotonics, The University at Buffalo (SUNY), Buffalo, New York; ²Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York; ³Electrical Engineering, The University at Buffalo (SUNY), Buffalo, New York.

Solution-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.


9:00 AM T1.2
Silicon Quantum Dots Composites for Photovoltaic Applications. Xavier Paquez¹, Yann Leconte¹, Olivier Sublemontier¹, Philippe Thony², Nathalie Herlin-Boime¹ and Cecile Reynaud¹; ¹DSM/IRAMIS/SPAM, CEA, Gif sur Yvette, France; ²INES, CEA, Le Bourget du Lac, France.

The 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.


9:15 AM T1.3
Gas Phase Synthesis of Highly Specific Silicon Nanoparticles on the Pilot Plant Scale for Optoelectronic Applications.Tim Huelser¹, Christoph Rier², Sophie M. Schnurre¹, Dieter Jaeger^2,4, Christof Schulz^3,4 and Hartmut Wiggers^3,4; ¹Nano-Energy & Nano Particle Synthesis, Institute of Energy and Environmental Technology (IUTA), 47229 Duisburg, Germany; ²Center for Semiconductor Technology and Optoelectronics (ZHO), University of Duisburg-Essen, 47057 Duisburg, Germany; ³Institute for Combustion and Gas Dynamics (IVG), University of Duisburg-Essen, 47057 Duisburg, Germany; ⁴Center for NanoIntegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, 47057 Duisburg, Germany.

We 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.


9:30 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, Zachary Holman, Rebecca Anthony, Ryan Gresback and Uwe Kortshagen; Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota.

Theoretical 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.


9:45 AM T1.5
Chemical Vapor Functionalization of ZnO Nanocrystals. Moazzam Ali¹, Marty Donakowski² and Markus Winterer¹; ¹Nanoparticle Process Technology, University Duisburg-Essen, Duisburg 47057, Germany; ²Department of Chemistry, University of Minnesota, Minneapolis, Minnesota.

In 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.


 

10:30 AM *T2.1
High Performance Solution-processed Indium Zinc Oxide and Indium Gallium Zinc Oxide Thin Film Transistors. Rebecca Peterson¹, Kulbinder Banger¹, Yoshi Yamashita², Kiyotaka Mori² and Henning Sirringhaus¹; ¹University of Cambridge, Cambridge, United Kingdom; ²Panasonic, Osaka, Japan.

Transparent 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.


11:00 AM T2.2
TCO Thin Films from Nanoparticles for Optoelectronic Devices. Roland Schmechel¹, Gabi Schierning¹, Ralf Theissmann¹, Simon Bubel¹, Norman Mechau² and Anna Prodi-Schwab³; ¹Faculty of Engineering, University Duisburg-Essen and CeNIDE, Duisburg, Germany; ²Institute of Nanotechnology, Forschungszentrum Karlsruhe, Eggenstein-Lepoldshafen, Germany; ³Creavis Technologies and Innovation, Evonik Degussa GmbH, Marl, Germany.

TCO 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.


11:15 AM T2.3
Inkjet Printed, Electrochemically-gated Field-effect Transistors With ITO Nanoparticles as Active Layer. Subho Dasgupta, Norman Mechau, Jooyoung Lee, Robert Kruk and Horst Hahn; Institute of Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Baden-Würtemberg, Germany.

Indium 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


11:30 AM T2.4
An in-situ Dynamic ZnO Electrophoretic Deposition Technique. Chunwei Wu, Sandip Mitra and Michael Zachariah; Department of Mechanical Engineering, University of Maryland, College Park, Maryland.

Electrophoretic 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.


11:45 AM T2.5
Ink-jet Printing of ZnO Nanoink. Ahmed Khalil, Moazzam Ali and Markus Winterer; NPPT& CeNIDE, Duisburg-Essen University, Duisburg, Germany.

For 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.


 

1:30 PM *T3.1
Optical Properties of Impurity Doped Si Nanocrystals. Minoru Fujii, Department of Electrical & Electronic Engineering, Kobe University, Kobe, Japan.

Optical 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.


2:00 PM T3.2
Silicon Nanoparticles for Photovoltaic and Optoelectronic Applications. Axel Lorke¹, Andreas Gondorf¹, Matthias Offer¹, Jens Theis¹, Nadine van der Schoot¹, Cedrik Meier² and Hartmut Wiggers³; ¹Physics Dept. and CeNIDE, University of Duisburg-Essen, Duisburg, Germany; ²Physics Department and CeOPP, University of Paderborn, Paderborn, Germany; ³Institute for Combustion and Gas Dynamics and CeNIDE, University of Duisburg-Essen, Duisburg, Germany.

Silicon 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


2:15 PM T3.3
Temperature Dependent Properties of Gallium, Indium, and Tin Doped CdSe Quantum Dots.Christopher J. Tuinenga and Viktor Chikan; Chemistry, Kansas State University, Manhatan, Kansas.

The 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.


2:30 PM T3.4
Effect of Air Exposure on Carrier Relaxation Dynamics in Colloidal Quantum Dots. Milan Sykora, Alexey Y. Koposov, John A. McGuire, Roland K. Schulze and Victor I. Klimov; LANL, Los Alamos, New Mexico.

Understanding 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.


2:45 PM T3.5
Biexciton Quantum Yield of Single Semiconductor Nanocrystals. Jing Zhao, Gautham Nair, Tara Sarathi and Moungi G. Bawendi; Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts.

The 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.


 

3:30 PM *T4.1
Nanocrystal and Nanowire Hybrid Organic Semiconductor Photovoltaics. Cherie Kagan, University of Pennsylvania, Philadelphia, Pennsylvania.

Hybrid 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.


4:00 PM T4.2
Novel 3D Composite Photoanode for Enhanced Efficiency in Photovoltaics. Nicolas Tetreault¹, Jeremie Brillet¹, Geoffrey A. Ozin² and Michael Graetzel¹; ¹SB ISIC LPI, EPFL - École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; ²Chemistry Department, University of Toronto, Toronto, Ontario, Canada.

We 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.


4:15 PM T4.3
Inorganic Nanocrystal Three-dimensional TiO2/PbS Solar Cells. Tong Ju¹, Qiaoer Zhou², Lily Yang¹, Glenn B. Alers², Alison Breeze³ and Sue A. Carter¹; ¹Physics, University of California, Santa Cruz, Santa Cruz, California; ²Electrical Engineering, Unversity of California,Santa Cruz, Santa Cruz, California; ³Solexant Inc, San Jose, California.

In 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.


4:30 PM T4.4
Infrared Solar Cells Based on a Colloidal Quantum Dots/Organic Bilayer Structure. Ni Zhao¹, Tim Osedach¹, Liang-Yi Chang², Maddalena T. Binda³, Scott Geyer², Darcy Wanger², Moungi G. Bawendi² and Vladimir Bulovic¹; ¹Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts; ²Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts; ³Electronics and Information Technology, Politecnico of Milan, Milan, Italy.

The 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 (V_OC). 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 V_OC, 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 V_OC 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


4:45 PM T4.5
Germanium Nanocrystal Solar Cells. Zachary Holman and Uwe Kortshagen; Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota.

Semiconductor 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.


 

T5.1
Nonlinear Optical Absorption and Scattering of Lead Chalcogenide Nanocrystals. Daniel J. Asunskis, Igor L. Bolotin, Ali M. Jawaid, Frank D. Pleticha, Preston T. Snee and Luke Hanley; Chemistry, University of Illinois at Chicago, Chicago, Illinois.

The 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.


T5.2
A Novel Way of Improving Light Harvesting in Dye Sensitized Solar Cells - Electrodeposition of Titania.Shih-Yuan Lu and Tsung-Yu Tsai; Chemical Engineering, National Tsing-Hua University, Hsinchu, Taiwan.

Light 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.


T5.3
Narrowing Size Distribution of Nanoparticles by Pulsed Precursor Delivery in a Gas-phase. Ruzica Djenadic, Qing Cao and Markus Winterer; Department of Engineering Sciences, and Center for NanoIntegration Duisburg-Essen, CeNIDE, University Duisburg-Essen, Duisburg, Germany.

The product quality and application characteristics of nanostructured materials depend strongly on the powder characteristics. Powders of small grain size, narrow size distribution, low agglomeration and high purity are required in a wide range of processes and applications. Gas- phase processes allow the generation of particles with unique combination of properties. Relative to solution routes to powders, gas-phase routes have the advantage that their particles are formed at high temperatures, which allows the formation of highly crystalline materials. On the other hand, nanoparticles produced in the gas-phase form agglomerated particles with broad size distribution which can be a limiting factor during processing of powders into bulk materials and films. We developed a gas-phase synthesis method (chemical vapor synthesis, CVS) in which precursor(s) are delivered in pulses. This pulsed precursor delivery leads to a decrease of particle number concentration and subsequently a narrower particle size distribution compared to continuous precursor delivery. Different pulse lengths and duty cycles have been investigated.


T5.4
High-density CIGS Thin-films from Aerosol Deposition of Nanoparticle Precursors. Jesse Williams¹, Naoki Ohashi¹, Jun Akedo² and Jae-Hyuk Park²; ¹National Institute for Materials Science, Tsukuba, Ibaraki, Japan; ²National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan.

Thin-films formed by spray depositing nanoparticles are normally porous and rough. Using the aerosol deposition technique, we spray deposit thin-films from CIGS nanoparticles that are characterized by highly density and low roughness. The CIGS nanoparticle precursors are synthesized using thermal refluxing, and the size of the nanoparticles is 500 nm to 1 µm; both the size and shape are critical to aerosol deposition. We characterize the nanoparticle precursors and the deposited thin-films with SEM, TEM, XRD, and NIR-vis-UV spectrometry. The as-deposited thin films have very small grain size, on the order of 10 nm, so the films are annealed to reduce the density of grain boundaries and point defects. Carrier concentration, carrier mobility, and I-V measurements are made on the as-deposited and annealed thin-films.


T5.5
Microfluidic Synthesis and Functional Patterning for Advanced Nano-technology. Kyung M. Choi, University of California, Irvine, California.

The ability to fabricate small patterns on flexible substrates has received considerable attentions due to potential applications to develop low coat plastic/organic/molecular electronics. We demonstrate microfluidic synthesis and microfabrications of functional polymers to bring innovations in nanotechnology. We employed a microfluidic approach to synthesize molecularly imprinted polymer (MIP) particles, which is a highly cross-linked macroporous thermoset with both high internal surface areas and specific molecular recognition sites for fabricating bio-chemical sensors. In order to achieve high sensitive, we provide micro-sized MIPs’ particles, which have only high affinity receptor sites since particle sizes of MIP polymers are directly related to their affinity functions of specific molecular recognition. We also carried out the microfabrications of MIP through MIMIC process using photomasks. This study presents fluorescence microscopic images of MIPs’ system by rebinding synthesized fluorescent templates. We also present designs of new materials for functional fabrications.


T5.6
Synthesis and Characterization of PbSe and Pb1-xCoxSe Nanoparticles. D. Srikala and S. Patnaik; School of Physical Sciences, Jawaharlal Nehru University, New Delhi, Delhi, India.

We present structural and magnetic characterization of PbSe, and Pb1-xCoxSe (0 ≤ x ≤ 0.03) nanoparticles. Colloidal solution of PbSe nanoparticles were synthesized by chemical route from a reaction mixture of lead oxide and TOPSe (TOP: tri-n-octylphosphine) in the presence of surfactants. Transition metal Co, with smaller ionic radius, was successfully doped in place of Pb. The synthesis was carried out at 260 °C for different reaction times: 10, 30, and 45 minutes. High temperature synthesis was carried out to enhance the doping level of Co2+ ions. The effect of reaction time on particles diameter and doping were studied. Structural, surface and magnetic properties were studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and superconducting quantum interference device (SQUID). Cubic rock salt structure of nanoparticles was confirmed. The size and shape of the nanoparticles depended on the reaction time and from TEM and XRD measurements it was observed that for longer reaction times, the nanoparticles showed Ostwald ripening characteristics. Structural evidence for doping was obtained from the powder X-ray diffraction that exhibited a lattice compression with Co2+ doping and it was observed that smaller reaction times favoured the doping. Chelating bidentate character of oleate groups on PbSe nanoparticles revealed the successful capping by oleic acid. PbSe and Co2+ doped PbSe nanoparticles showed curious size dependent diamagnetic properties. The SQUID investigations confirmed that the Co ions were successfully incorporated into the PbSe nanoparticles. This opens a possibility of a diluted magnetic semiconductor in nano-scale.


T5.7
Photovoltaic Applications of C60/Silicon Nanocrystals Self-assembled Nanostructures Induced by Nanosecond Laser Fragmentation in Water. Vladimir Svrcek¹, Davide Marrioti², Yosei Shibata¹ and Michio Kondo¹; ¹Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,, Japan; ²Nanotechnology and Integrated BioEngineering Centre, University of Ulster at Jordanstown, Ulster, Ireland.

The stringent requirements for today energy harvesting technology are pushing the photovoltaic (PV) industry towards the development of advanced materials capable of increased conversion efficiency. On the other hand, Si-based materials keep playing an important role for low-cost PV technology development. Silicon nanocrystals (Si-ncs), due to carrier multiplication, low-toxicity and the established bulk silicon PV industry, represent an attractive and advanced photovoltaic material. Among other challenges, the success of a PV technology based on Si-ncs depends on our ability to collect the photogenerated current, and electronic transport through closely packed Si-ncs can be an effective solution.. In order to achieve such structures of closely packed Si-ncs, self-assembly can be exploited to produce complex Si-ncs photoconductive nanoarchitectures. The fabrication approach based on self-assembly strongly relies on surface interactions and it is therefore important to have accurate control over the Si-dioxide interface layer on Si-ncs. In addition, Si-dioxide is, and most likely will remain, a fundamental interface for several Si-ncs functionalities so that a laser processing technique capable of direct fabrication of the silicon dioxide layer on Si-ncs in liquid media would be advantageous. Recently we have introduced an efficient method for the self-assembly of networks of photosensitive Si-ncs micrograin produced by nanosecond laser fragmentation and oxidation in water [Chem. Phys. Lett .(2009) 478 224.].In this contribution we will demonstrate that the fragmentation process in ethanol is more efficient, however only the fragmentation in water leads to self-assembled nanoarchitectures. Dewetting effects and surface non-uniformity on the micrograin fragmentation in water enhance a dipole-dipole interactions contributing to Si-ncs self-assembly. Electrical conductivity through the self-assembled and luminescent Si-ncs based networks has been recorded. The laser process in water forms stable nanoarchitectures compatible with subsequent plasma and heat treatments. Specifically, surfactant free nanoarchitectures are sufficiently stable for deposition of fullerenes (C60). Therefore, we have investigated bulk-heterojunction formed by C60 and self-assembled Si-ncs nanoarchitectures. The results indicate that the difference in electron affinity and ionization potential between the nanocrystals and C60 provide a driving force for exciton dissociation which result in photoconductivity. After exciton dissociation, electrons are swept into the C60 and the holes are trapped in the Si-ncs; charge accumulation will then promote hole tunnelling between Si-ncs. Finally, micro-printing processes have been also investigated as possible routes to implement self-assembled Si-ncs nanoarchitectures in specific low-cost photovoltaic devices. These results will also be discussed in details.


T5.8
Low-temperature Wet Chemical Deposition of Ultra-thin ZnS/ZnO Bilayers on Plastic Substrates for Applications of Photovoltaic Devices. Rong-Fuh Louh, William Wu, Jean Liu and Irene Tsai; Materials Sci. & Engr., Feng Chia University, Taichung, Taiwan.

The ultra-thin II-VI semiconductor ZnS/ZnO bilayers (< 50 nm thickness for each layer) can be easily formed on the plastic substrates at 70~80oC for 20 min. by low temperature wet chemical synthesis techniques, namely chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). The specific microstructure of such ZnS/ZnO bilayers including film thickness, particle size and morphology, is also modified and obtained in accordance with processing parameters. Along with thin film quality and morphology, the transmittance and reflectance of ZnS/ZnO layers can be measured by field emission SEM and UV-Vis spectroscopy. Besides the bilayer of ZnS (~35 nm thick)/ZnO (~50 nm thick) film with uniform thickness was successfully deposited on the optical grade PET substrates, a well-dispersed layer of ZnO nanoparticles with ~100 nm size on the top of ZnS (35 nm thick) film was also attempted. The average transmittance of these bilayer samples can reach greater 85%. Our future goal is to employ such ZnS/ZnO bilayer structure on potential organic substrates to be associated with flexible photovoltaic devices to meet desired cost-effectiveness requirements.


T5.9
Thermal Annealing of Layer-by-layer Deposited Nanopaticles Composites for Photovoltaic Applications. Joe Briscoe², Diego Gallardo² and Steve Dunn¹; ¹Materials, Queen Mary, University of London, London, United Kingdom; ²Materials, Cranfield University, Cranfield, Beds, United Kingdom.

Self assembled layers of CdTe nanoparticles (CDNP) were made using a layer by layer method. Cleaned ITO and glass substrates were dipped alternately in solutions of thiol-capped water soluble CDNP and a polymer (poly(diallyldimethylammonium chloride), PDDA) solution in water. In between dipping the films were rinsed to remove unattached material, and dried in air. The process was repeated 20 times, so that 20 layers of nanoparticles (NP) and polymer formed on the surface of the substrates. These films were annealed, in air or under a vacuum of 1x10-7 mbar, to study the effect on the polymer and the NP at temperatures ranging from 210°C to 450°C for 1 hour. The films annealed in air changed from slightly red to a dark brown appearance between 210°C and 280°C. X-ray photoelectron spectroscopy (XPS) measurements showed a large increase in the height of the Te-O peak which is indicative of the CdTe being oxidised to compounds such as CdTeO3. In the samples annealed under vacuum there was no measured increase in the Te-O peak, indicating there was little chemical change in the CdTe during the vacuum annealing process at temperatures up to 450°C. Energy-dispersive x-ray spectroscopy (EDS) performed on vacuum annealed films demonstrate a large reduction in the carbon content to 11.4 at% in samples annealed at 350°C compared to 18.5 at% in unannealed samples. C fell below detectable limits in samples annealed at 450°C. Cd and Te were present in all samples. The main source of carbon comes from the PDDA suggesting vacuum annealing was selectively removing the polymer and leaving CdTe. This was supported by literature reports of thermogravimetric analysis of PDDA that show almost all PDDA was lost by 450°C. The measured band gap of unannealed particles was approximately 1.85 eV. This was shifted from the bulk value of 1.56 eV due to quantum confinement. Absorption measurements of the vacuum annealed samples indicated that as the annealing temperature was increased the band gap of the films shifted towards the bulk value; this suggests that the particles are gradually agglomerating/sintering with increased annealing temperature, reducing quantum confinement and moving the measured band gap towards the bulk value. These results are very significant, as they demonstrate a level of control over the properties of a self assembled film of NPs, whereby annealing can be used to remove the polymer component of the film and gradually shift the optical absorption onset.


T5.10
Effects of Gallium Doping on CdSe Quantum Dots. Christopher J. Tuinenga, Brett Vaughn and Viktor Chikan; Chemistry, Kansas State University, Manhatan, Kansas.

CdSe 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.


T5.11
Self-organized Formation of Ge Nanocrystals Out of (GeO_x-SiO₂) Superlattice Structures. Manuel Zschintzsch, Nicole M. Jeutter, Johannes von Borany and Arndt Muecklich; Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden, Sachsen, Germany.

Bandgap engineered Si and Ge nanocrystal solar cells are supposed to be a candidate for high effective 3^rd 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 GeO_x-SiO₂ multilayer structures based on the phase separation of GeO_x 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 GeO_x layer. The GeO_x-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 GeO_x 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 GeO_x 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


T5.12
ZnO Nanorods Functionalized With Supramolecular Porphyrin-Fullerene Complexes. Syed Mujtaba Shah¹, Aiko Kira², Hiroshi Imahori², Frederic Fages¹ and Joerg Ackermann¹; ¹Centre Interdisciplinaire de Nanoscience de Marseille, CINAM UPR-CNRS 3118, Marseille, France; ²Department of Molecular Engineering, Graduate School of Engineering, Engineering, Kyoto University, Kyoto, Japan.

Supramolecular 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


T5.13
Synthesis of Nano Porous CdS Thin Films for Hybrid Solar Cells. Bharath Reddy, Vignesh Gr and Rajeev Jindal; R&D, Moserbaer Photovoltaic Ltd, Greater NOIDA, Uttar Pradesh, India.

Organic 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.


T5.14
Silver Doping of Semiconductor Nanocrystals. Ayaskanta Sahu¹, Moon Sung Kang¹, Andrew Wills², C. Daniel Frisbie¹ and David J. Norris¹; ¹Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota; ²Department of Chemistry, University of Minnesota, Minneapolis, Minnesota.

Colloidal 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.


T5.15
Synthesis of Ultrabright Fluorescent Mesoporous Silica Particles. Igor Sokolov^1,2,3 and Dmytro Volkov¹; ¹Physics, Clarkson University, Potsdam, New York; ²Chemical and Biomolecular Science, Clarkson University, Potsdam, New York; ³Nanoengineering and Biotechnology Laboratories Center (NABLAB), Clarkson University, Potsdam, New York.

Here 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.


T5.16
High Luminance YAG:Ce Nanoparticles Fabricated by Soft-Chemical Route.Yi-Wen Kao and Kuo-Chuang Chiu; Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan.

High 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.


T5.17
Fabrication and Characterization of Si Nanocrystals Embedded in SiC Matrix by Magnetron Sputtering for Third Generation Solar Cell Applications. Arife G. Imer, Ilker Yildiz and Rasit Turan; Physics, Middle East Technical University, Ankara, Turkey.

SiC 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.


T5.18
Abstract Withdrawn


T5.19
Laser Welding of Nanocrystalline Titania and Transparent Conducting Oxide Electrodes for High-efficiency Solar Cells. Myeongkyu Lee, Jinsoo Kim and Jonghyun Kim; Dept. of Materials Science and Engineering, Yonsei University, Seoul, Korea, Republic of.

Since 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.


T5.20
Synthesis and Characterization of Zn₂SiO₄:Mn²⁺ Nanophosphors Prepared by Flame Spray Pyrolysis. Jae Seok Lee¹, Myoung Hwan Oh¹, Aniruddh Khanna¹, Purushottam Kumar¹, Madhav B. Ranade² and Rajiv K. Singh¹; ¹Materials Science & Engineering, University of Florida, Gainesville, Florida; ²Particle Engineering Research Center, Univsrsity of Florida, Gainesville, Florida.

Mn-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.


T5.21
Synthesis of Luminescent Rare-earth Ion Doped Core-shell Nanostructures for Energy Harvesting.James A. Dorman¹, John Hoang¹, Yuanbing Mao² and Jane P. Chang¹; ¹University of California, Los Angeles, Los Angeles, California; ²Washington State University, Spokane, Washington.

As 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.


T5.22
Synthesis and Characterization of ZnMgO Nanoparticles and the Performance of P3HT/ZnMgO Nanoparticle Bulk Heterojunction Photovoltaics. Summer R. Ferreira, Robert J. Davis, Yun-ju Lee, Bell S. Nelson, Provencio P. Paula, Jianyu Huang, Ping Lu and Hsu W. Julia; Sandia National Laboratories, Albuquerque, New Mexico.

Organic/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.


T5.23
ZnO Nanosphere Fabrication Using the Functionalized Polystyrene Nanoparticles for Dye-sensitized Solar Cells. Mi-Hee Jung, Ho-Gyeong Yun, Hunkyun Pak and Mangu Kang*; Electronics and Telecommunications Research Institute, Daejeon, Korea, Republic of.

Recently, 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.


 

8:30 AM *T6.1
Nanowire-Quantum-Dot Solar Cells. Kurtis S. Leschkies, Moon-Sung Kang, Alan G. Jacobs, Timothy J. Beatty, David J. Norris and Eray S. Aydil; Chem. Eng. & Mat. Sci., Univ. of Minnesota, Minneapolis, Minnesota.

We recently reported solar cells based on vertically oriented arrays of single-crystal ZnO nanowires that were sensitized with CdSe semiconductor nanocrystals (or quantum dots). However, these devices suffered from limited efficiencies and poor stability because they utilized a liquid electrolyte, which is known to cause corrosion of semiconductors. Moreover, CdSe nanocrystals do not absorb the near-infrared portion of the solar spectrum. Here we report approaches to improve the performance and understand the mechanism of nanocrystal-based solar cells. First, we studied devices based on planar heterojunctions between PbSe semiconductor nanocrystals and thin ZnO films. PbSe nanocrystals were chosen as they can absorb a larger portion of the solar spectrum than CdSe. We found that such cells generate large photocurrents and higher photovoltages compared to Schottky cells assembled from similar nanocrystal films. The photovoltage also depends on the nanocrystal size, increasing linearly with their effective band gap energy. Second, we examined solar cells in which a vertically oriented array of single-crystal ZnO nanowires was completely infiltrated with colloidal PbSe nanocrystals such that the liquid electrolyte could be avoided. We observed significant photocurrent and power conversion improvement with increasing nanowire length, which is consistent with higher exciton and charge collection efficiencies. When illuminated with 100 mW/cm2 of simulated solar light, these nanowire-quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with planar ZnO devices constructed with the same amount of nanocrystals.


9:00 AM T6.2
Self-assembled CdTe Nanoparticle Absorbers for ZnO Nanorod Solar Cells - The Influence of Annealing on Cell Performance.Joe Briscoe, Diego E. Gallardo and Steve C. Dunn; Microsystems and Nanotechnology Centre, Cranfield University, Cranfield, Bedfordshire, United Kingdom.

A development of the high surface area nanostructured solar cell is presented using nanoparticles as absorber material. ZnO nanorods grown by an aqueous chemical method are used to create a high surface area onto which a light absorbing material is deposited; specifically for this work CdTe nanoparticles were used. The use of nanoparticles in solar cells is new area that offers the potential for greater flexibility in cell design: quantum confinement of the absorber can shift the band gap depending on particle size. Therefore, the absorption onset can be varied to maximise efficiency. Previous cells using nanoparticle-based absorbers have suffered from insufficient light absorption because only very thin coatings of nanoparticles are used; in this work a layer by layer (LbL) method for deposition of the nanoparticles onto the ZnO nanorods was used so that layers of nanoparticles many tens of nm thick could be produced. The as-grown and coated nanorods were characterised using scanning electron microscopy (SEM), absorption spectroscopy and photoluminescence (PL). SEM shows the LbL method produces a uniform, conformal coating over the ZnO nanorods. By increasing the number of coated layers more incident light is absorbed. Additionally, PL shows an electronic interaction between the ZnO and CdTe; photogenerated electrons are transferred from CdTe to ZnO before radiatively recombining. To complete the device this is coated with a p-type semiconductor (copper thiocyanate). It is shown that thermal annealing of the CdTe-coated nanorod composites change the optoelectronic properties of the system. The main impact is a reduction in series resistance of the PV cell as the nanoparticles start to agglomerate and sinter, giving a better path to the ZnO. Increasing the annealing temperature leads to a red shift in absorption onset of the cell as the effective particle size increases towards the bulk properties. A cell annealed at 350°C shows a fill factor of 0.35, short circuit current density, Jsc, of 0.12 mA/cm^2 and an open circuit voltage, Voc, of 49 mV. This is a significant result, as it represents a possible paradigm shift in the manufacturing process of solar cells using nanostructured components. Additionally, using post-deposition annealing, the absorption onset of the cell could be shifted to the desired position.


9:15 AM T6.3
``Brick and Mortar” Strategy for the Formation of Highly Crystalline Mesoporous Titania Films from Nanocrystalline Building Blocks for Photovoltaic Applications. Johann M. Szeifert¹, Dina Fattakhova-Rohlfing¹, Johann M. Feckl¹, Vit Kalousek², Jiri Rathousky², Daibin Kuang³, Sophie Wenger³, Shaik M. Zakeeruddin³, Michael Graetzel³ and Thomas Bein¹; ¹Department Chemistry and Biochemistry and Center for NanoScience (CeNS), University of Munich (LMU), Munich, Germany; ²J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic; ³Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.

Nanostructured films of TiO₂ have enormous potential for applications in photovoltaics and energy storage. However, reaching this potential requires films that simultaneously feature both large and easily accessible surface area and highly crystalline pore walls. Crystalline titania layers are most commonly assembled from crystalline particles by sintering, but this approach is limited concerning the possibility to tune the structure and porosity. Templated sol-gel processes are used to overcome these shortcomings, but the crystallinity of the resulting TiO₂ frameworks is usually only moderate.in-situ 2D-GISAXS, WAXS and TEM measurements to monitor and visualize the seeding effect, crystal growth and mesostructure development during the calcination, respectively. The porous and crystalline nature of the films and their applicability as capacitors and for energy storage is shown by lithium insertion.< for by This diffusion with from are shows that to of as the in and which can be [1] M. crystalline employed films using < br> Johann Szeifert, Dina Fattakhova-Rohlfing, Dimitra Georgiadou, Vit Kalousek, Jiri Rathouský, Daibin Kuang, Sophie Wenger, Shaik Zakeeruddin, Michael Grätzel Thomas Bein,>Chem. Mater. 2009, 21 (7), 1260-1265.


9:30 AM T6.4
Functionalized Luminescent Silicon Quantum Dots. Vincent Groenewegen and Carola Kryschi; Dept. Chemistry and Pharmacy, Physical Chemistry I, Friedrich-Alexander University of Erlangen-Nuremberg, Erlangen, Germany.

One of the central challenges in fabrication of luminescent Si quantum dots (SiQDs) tailored for nano-optolelectronics is to functionalize their surfaces with electronically efficiently coupled molecules that mediate carrier injection into the bulk and allow optical control of charge separation as well as luminescence properties of the bulk. Successful surface grafting with suitable organic compounds requires the quantitative characterization of photo-excited carrier dynamics and therewith the identification of the different pathways for photo-induced carrier transfers between electronic surface and bulk states. A nanoscopic understanding of ultrafast carrier dynamics in SiQDs on the subpicosecond time scale may be obtained through the use of femtosecond laser spectroscopy. In this contribution we will show that the photoluminescence properties of alkenyl-passivated SiQDs may be tailored by both, core size and surface states that are efficiently coupled to resonant bulk states. Therefore, a two-step wet-chemistry synthesis route was developed which provides SiQDs with adjustable sizes and surface properties. While the energy gap of the Si core could be size-tuned by HF etching, resonant electronic surface states may be attained by hydosilylating the SiQD surface with suitable 1-ethynyl derivatives. The sizes, cristallinity and shapes as well as the surface structures of differently functionalized SiQDs were examined employing high-resolution transmission electron microscopy (HRTEM) and FTIR spectroscopy, respectively Stationary and time-resolved photoluminescence spectroscopy experiments provided essential information of luminescent surface and bulk states. The interplay between electronically excited molecular states and conduction band states was examined upon directly monitoring photo-excited carrier dynamics with femtosecond transient absorption spectroscopy. For instance, 3-vinylthiophene, 2- and 4-vinylpyridine ligands were found to act as surface-bound antennae that mediate ultrafast electron transfer across the SiQD interface.


9:45 AM T6.5
Optical Properties of Silicon Quantum Dots: Influence of Etching, Surface Oxidation and Surface Functionalization. Anoop Gupta¹, Sebastian Kluge¹, Christof Schulz^1,2 and Hartmut Wiggers^1,2; ¹Institut für Verbrennung und Gasdynamik, University of Duisburg-Essen, Duisburg, Germany; ²Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Duisburg, Germany.

The light emission from silicon quantum dots (Si-QDs) has sparked a great interest in their research due to the possibility of constructing optoelectronic devices, full color displays and optical sensors based on silicon. We investigated the properties of Si-QDs after surface etching, surface re-oxidation and surface functionalization process. Surface etching of as-synthesized Si-QDs with hydrofluoric acid causes a blue shift compared to their initial emission spectrum with increased intensity, indicating the etching induced decrease in nonradiative defects and a slight decrease in size of Si-QDs. A further size reduction of Si-QDs with a mixture of HF acid and HNO3 acid allowed us to tune the emission from red to green, supporting the origin of orange-to-green photoluminescence (PL) from the quantum confinement effects. Time dependent re-oxidation of orange emitting Si-QDs at 620 nm showed the emergence of blue emission at 450 nm. We observed that the main peak at 620 nm shows continuous blue shift in the spectrum with decreased PL intensity, while the peak position at 450 nm was not influenced by the oxidation. These results indicate that orange-to-green emission is associated with quantum size effects while the blue emission is assumed to be related to defect states. In order to stabilize Si-QDs against re-oxidation, the surface must be adequately passivated. Therefore, we terminated the surface of freshly etched Si-QDs with organic molecules by reacting them with alkenes using a thermally induced hydrosilylation process and examined their stability in air. We find that the surface functionalization using alkenes with ester group provide much better passivation against surface oxidation compared to n-alkenes.


 

10:30 AM *T7.1
Printable Particle Systems. Wolfgang Peukert, Particle Technology, University of Erlangen-Nuremberg, Erlangen, Bavaria, Germany.

Progress in various fields such as electronics, photonics, energy conversion requires the development of techniques for the production of thin films. Vacuum processes for thin film production are state-of-the-art and may provide excellent performance but are often too costly. Alternative approaches based on liquid phase processing use various printing techniques. Especially roll-to-roll processes promise a tremendous reduction in production costs thus opening completely new fields of applications. A major scientific challenge is the development of technologies for the reliable production of electronic devices from printable nanoparticulate pastes. The advantages of flexible production of polymers are thus combined with the advantages of classical silicon technology. A unique project has been implemented at the University of Erlangen where physicists, chemists, chemical engineers, materials scientists and electrical engineers are organized along the process chain in order to demonstrate printable electronics. These groups further co-operate with an industrial partner (EVONIK) in order to transfer the results from basic research directly into industrial practice. This approach opens new applications in the field of flexible opto-electronics, e.g. integrated circuits for consumer products, radio frequency tags or flexible displays. The process chain is set up for ZnO as well as ITO and consists of the following steps: particle synthesis, particle stabilization and dispersion, ink formulation, thin film formation and post-processing by laser sintering. This approach allows the optimization of each process step with respect of the performance of the functional product. The presentation outlines this process chain and highlights critical aspects along the chain, i.e. particle formation, particle stabilization, suspension rheology, thin film formation and characterization. We show how dispersion and particle stabilization on the one side and post-processing of the films by laser annealing on the other side influence the thin film morphology (homogeneity, roughness) and performance (transmittance, conductivity). As analytical tools we used SEM, TEM and AFM (structure, roughness), impedance spectroscopy and current-voltage measurements (conductivity, carrier mobility), UV-Vis, FTIR, ellipsometry (optical properties), and photoluminescence spectroscopy (defects). Ordering and structure formation of nanoparticles in the fabricated layers affect the conductivity as well as the optical properties of the layers and can be controlled by modifying the interactions between particles in the suspension. Guidelines for the formulation of printable inks will be defined. Acknowledgement: This work is supported by DFG by the Research Training Group “Disperse systems for electronics” (in cooperation with EVONIK).


11:00 AM T7.2
Deposition of Optoelectronic Precursor Nanomaterials by Inkjet Printing. Peter D. Angelo and Ramin R. Farnood; Chemical Engineering & Applied Chemistry, University of Toronto, Toronto, Ontario, Canada.

Recently, inkjet-printed optoelectronics have attracted attention as a promising alternative to conventionally prepared electronic devices. Because inkjet printers require nanoscale particle sizes in the ink pigments, printable optoelectronics require inks containing either suspended nanoparticles or emulsified conjugated polymers. The primary building blocks for most optoelectronic devices are conductive species to function as electrodes and semiconductive species to function as light-emitting or light-absorbing layers. In this study, the formulation and application by inkjet of three functional inks, comprising a conductive species, a semiconductive species, and a dielectric, was considered. The preparation of these inks provides an opportunity for the deposition low-cost emissive optoelectronics using a high-speed, efficient printing method under low-temperature, non-vacuum conditions. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), or PEDOT:PSS, a conductive polymer suspension, was incorporated into an inkjet-printable conductive ink. A proportion of single-walled carbon nanotubes (SWCNTs) was added to the ink to facilitate charge carrier transport across the PEDOT:PSS and thereby increase its conductivity. Time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS) provided high-resolution imaging of the distribution of the PEDOT:PSS on the substrate. Enhancement of charge carrier mobility by “smoothing” of the energy diagram was confirmed using cyclic voltammetry and measurement of the electronic band gap of the PEDOT:PSS, PEDOT:PSS/SWCNTs, and SWCNTs. Conductivity was measured on several substrates using a 2-point probe method. Copper- and manganese-doped zinc sulfide (ZnS:Cu and ZnS:Mn) nanoparticles, which are both photoluminescent and electroluminescent, were dispersed in a second ink, containing a UV-polymerizable monomer species. ZnS nanoparticles were synthesized using a completely aqueous method from precursor salts. The molar ratio in the precursor solution of dopant:Zn²⁺:S^2- was optimized to achieve the brightest photoluminescent emission from the nanoparticles. Crystalline structure was confirmed using X-ray diffraction (XRD) and TEM. The particle size was controlled during synthesis in order to provide particles with sufficiently small size for inkjet printing and confirmed using dynamic light scattering (DLS) and TEM. After printing, bright photoluminescence and electroluminescence of the nanoparticle/polymer printed films were observed. BaTiO₃ nanoparticles were dispersed in an alcohol/monomer solution to prepare insulating films. Again, DLS/TEM were used to establish the particle size and degree of dispersion of the nanoparticles. Uniformity and topography of the BaTiO₃/polymer films after printing/curing were observed using high-resolution scanning electron microscopy (HRSEM). Film topography and ink solids content were correlated to dielectric constant and breakdown strength of the film.


11:15 AM T7.3
Fabrication of Low-cost CuInS₂ Solar Cells by Air-stable Ink Rolling (AIR) Process.Stephen T. Connor, Benjamin Weil and Yi Cui; Stanford University, Stanford, California.

Solution-based deposition techniques are widely considered to be a route to low-cost and high-throughput device fabrication through the use of roll-to-roll processing. We have developed a new solution-phase deposition technique which we call “Air-stable Ink Rolling (AIR),” which can quickly form CuInS₂ absorber layers. Photovoltaic devices made with CuInS₂ absorbers deposited by "AIR" show comparable efficiencies to other nanoparticle-based solar cells. In this process, the precursor ink is deposited in a manner similar to paint rolling, in which a small volume of fluid is smoothly distributed across a substrate by application of a roller-bar. This eliminates some of the inconsistencies in similar deposition techniques, such as doctor-blading and Mayer-rod coating. Metal and sulfur precursors are reacted at room temperature in air to create the nanocrystal ink, and then deposited onto a metal coated substrate by our roller casting process. The nanocrystal ink leaves no residue upon decomposition, and the films created by this method are both smooth and dense. Finally, the films are annealed in a sulfurous atmosphere. "AIR" could also be applicable to the deposition of many other inorganic materials, including oxides, chalcogenides, and metal alloys for device applications. The integration of these layers into electronic devices will be discussed.


11:30 AM *T7.4
Development of Low Temperature Solution-processed Metal-oxide TFT Materials. Ralf Anselmann and Dieter Adam, Creavis, Evonik Industries, Marl, Germany.

We will present recent developments at Evonik in regard to material synthesis and processing techniques of nanomaterials for the use in TFT applications. The work on several systems like semiconductors and dielectrics will be presented. Inorganic oxides are one of the most promising candidates for the next generation of printed semiconductors. At the moment many high performing systems are available just for CVD and sputter processes. We have developed systems, based on nanomaterials, which are solution processable with common printing techniques like inkjet. Their advanced performance, compared to organic systems, make them an attractive candidate as n-type semiconducting materials in applications like RFID tags or display backplanes. The successful development and increasing performance of such systems and their performance in thin film transistors (TFTs) will be shown. Special focus is the film homogeneity, electric performance and the use of different TFT architectures (top or bottom gate). Our systems show almost the same stable performance in these different TFT designs due to their very homogeneous film forming properties and low surface roughness. We also developed stable ink formulations out of these materials, which can be processed complete under ambient conditions and show stable field effect mobilites of 5 cm2/Vs in standard bottom gate, bottom contact TFT architecture with tempering steps not higher than 250 °C. These high values were achieved without any surface treatment of the TFT substrate, which is usually needed with many organic systems. Parallel to the high performing semiconductor we also developed fitting dielectrics which can be processed with different printing techniques. These materials and their behavior in combination with our semiconductors will be presented.


 

1:30 PM *T8.1
Point Defects, Surfaces, and Loss Mechanisms in Nitrides. Chris G. Van de Walle, Materials Department, University of California, Santa Barbara, California.

The nitride semiconductors are extremely promising materials for light emitters as well as photovoltaics. Controlling their conductivity is essential, and both point defects and sur-face reconstructions can have a profound effect. First-principles calculations based on density functional theory, most recently combined with advanced methodologies such as many-body perturbation theory and hybrid functionals, are playing a key role in estab-lishing a fundamental understanding of these phenomena [1]. InN is the member of the nitride family that has been least thoroughly investigated to date. The material exhibits a high tendency for unintentional n-type conductivity, both in the bulk and on the surface. This tendency has been attributed to nitrogen vacancies, but our first-principles calculations show that this is highly unlikely [2]. Instead, attention should be focused on unintentional incorporation of impurities, in particular, hydrogen [2]. In addition to the bulk conductivity, accumulation of electrons has been almost univer-sally observed on InN surfaces. While donor impurities adsorbed on the surface could of course contribute to this conductivity, we have proposed that the accumulation layers are an intrinsic property of the material that can be attributed to the fact that on polar surfaces occupied surface states are located above the conduction-band minimum (CBM). Fermi-level pinning occurs due to occupied surface states above the CBM, for all In/N ratios, thus explaining the observed electron accumulation [3]. Interestingly, we have found an absence of electron accumulation on nonpolar surfaces of InN at moderate In/N ratios, a prediction that has been experimentally confirmed. In the course of this work, our detailed investigations of the band structure have revealed that the nitrides may exhibit loss mechanisms that differ from those in conventional semiconductors. First-principles evaluations of Auger recombination as well as free-carrier absorption will be discussed. This work was performed in collaboration with A. Janotti, E. Kioupakis, J. L. Lyons, J. Neugebauer, P. Rinke, C. Stampfl, Q. Yan, and D. Segev. [1] C. G. Van de Walle and J. Neugebauer, J. Appl. Phys. 95, 3891 (2004). [2] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 92, 032104 (2008). [3] D. Segev and C. G. Van de Walle, EuroPhys. Lett. 76, 306 (2006).


2:00 PM T8.2
Frequency-dependent Electron Spin Resonance Study of Doping and Surface States in Freestanding Silicon Nanocrystals. Rui N. Pereira¹, Andre R. Stegner², Hartmut Wiggers³, Martin Stutzmann² and Martin S. Brandt²; ¹Institute of Nanostructures, Nanomodeling and Nanofabrication, University of Aveiro, 3810-193 Aveiro, Portugal; ²Walter Schottky Institut, Technische Universität München, 85748 Garching, Munich, Germany; ³Institut für Verbrennung und Gasdynamik, Universität Duisburg-Essen, 47048 Duisburg, Germany.

Owing to their size-tunable optical and thermal properties, freestanding Si nanocrystals (Si-NCs) are attractive for many applications from printable solar cells and light emitters to thermoelectric power devices [1-3]. Gas-phase growth by decomposition of SiH4 in a plasma [4] allows the preparation of macroscopic amounts of Si-NCs with high yield and a narrow particle size distribution [1,5]. Due to the ubiquitous role of doping in electronic devices based on bulk semiconductors, it is expected that doping will also be crucial in Si-NCs [6]. In gas phase-grown Si-NCs, P doping is achieved by adding phosphine to the precursor gas [7]. X-band electron spin resonance (ESR) has recently proven to be very useful for the study of dopant properties in Si-NCs such as confinement [10] and interaction with surface states [11]. Due to a strong broadening of dopant-related lines with increasing temperature and overlapping with ESR resonances originating from surface states at X-band frequencies, these experiments have focused on low temperature properties of dopants (below 100 K). In this work, we show that Q-band ESR allows the investigation of P in Si-NCs even at room temperature, which is more relevant for their envisaged applications, and present the results of a detailed study of Si-NC samples in a wide range of mean particle sizes (3.5-45 nm) and doping levels. From the analysis of the broadening of the P-related lines as a function of doping level and NC size, as well as from a comparison with X-band observations, we are able to gain further insight into fundamental properties such as effective doping concentration and spin relaxation times at room temperature. The observation of unsaturated ESR lines, both for P and surface states, also enables the direct study of the interactions between these states leading to e.g. compensation. Moreover, a comparative discussion of our results and data reported for P in bulk Si will be given. [1] C.-Y. Liu, Z. C. Holman, U. R. Kortshagen, Nano Lett. 9, 449 (2009). [2] K. Nishigushi, X. Zhao, S. Oda, J. Appl. Phys. 92, 2748 (2002). [3] R. Lechner, H. Wiggers, A. Ebbers, J. Steiger, M. S. Brandt, M. Stutzmann, Phys. Status Solidi (RRL) 1, 262 (2007). [4] M. Otobe, T. Kanai, T. Ifuku, H. Yajima, S. Oda, J. Non-Cryst. Solids 198-200, 875 (1996). [5] A. Gupta, M. T. Swihart, H. Wiggers, Adv. Funct. Mater. 19, 696 (2009). [6] D. J. Norris, A. L. Efros, S. C. Erwin, Science 319, 1776 (2008). [7] A. R. Stegner, R. N. Pereira, K. Klein, R. Lechner, R. Dietmueller, M. S. Brandt, M. Stutzmann, H. Wiggers, Phys. Rev. Lett. 100, 026803 (2008). [8] R. N. Pereira, A. R. Stegner, T. Andlauer, K. Klein, H. Wiggers, M. S. Brandt, and M. Stutzmann, Phys. Rev. B 79, 161304R (2009). [9] A. R. Stegner, R. N. Pereira, R. Lechner, K. Klein, H. Wiggers, M. Stutzmann, M. S. Brandt, Phys. Rev. B 80, 165326 (2009).


2:15 PM *T8.3
Properties of Quantum-sized Nanosilicon as a Functional Photonic Material. Nobuyoshi Koshida¹, Bernard Gelloz¹, Romain Mentek¹, Hideo Yoshimura¹ and Yoshiyuki Hirano²; ¹Graduate School of Eng., Tokyo Univ. of A & T, Tokyo, Japan; ²Science & Technology Research Laboratories, NHK, Tokyo, Japan.

Quantum-sized nanosilicon layers prepared by either of wet- or dry-processing exhibit some specific features as a strongly confined material. The physical properties of single-crystalline silicon are totally modified associated with a band gap widening. Induced useful functions provide opportunities for the development of silicon devices in the forthcoming post-scaling era [1]. The band gap engineering confirmed in nanosilicon opens the door toward silicon optoelectronics in the region from near-IR to near-UV. Controllability of dielectric constant and refractive index in a wide range are available for photonic integration. Complete surface passivation is critical for the best use of confinement effects of nanosilicon. As an approach for enhancing and stabilizing PL and EL, high-pressure water vapor annealing (HWA) has been introduced into nanosilicon prepared by electrochemical anodization. Under the optimum HWA condition, the external quantum efficiency of the red PL reaches 23% at room temperature [1]. Electronic structure analyses indicate that HWA-treated nanosilicon surfaces are covered with a high-quality unstrained thin SiO2 tissue, and that excitons are strongly localized in nano-dots with little non-radiative interfacial defects. The EL operation is significantly stabilized by HWA without affect on the carrier injection into nano-dots. The appropriate surface termination also produces desirable effects on the operation of monolithic waveguide, and optical microcavity [2], including silicon nanowires. A combination of HWA with thermal oxidation can tune the emission band from red to blue, and then generate efficient blue phosphorescence with a lifetime of several seconds [3]. This is due to luminescence centers in nanosilicon network embedded within high-quality oxide. Observed extremely slow transitions via triplets suggest the appearance of a molecular-like nanostructure. Related possible application is optical energy transfer [4]. Controllable band gap of nanosilicon is, on the other hand, very attractive from a viewpoint of applications to photo-sensing and advanced photovoltaic conversion. Actually, the nanosilicon layer exhibits a highly sensitive photoconduction for blue-light incidence. Even an avalanche multiplication of photo-carriers has been observed [5]. Optical properties of nanosilicon with a confined band gap appear in various manners. The use of this multi-functionality should amplify silicon technology more than just a scaling. 1. N. Koshida (Ed.), Device Applications of Silicon Nanocrystals and Nanostructures (Springer, New York, 2009) 348p. 2. M. Ghulinyan et al, Appl. Phys. Lett. 93, 061113 (2008). 3. B. Gelloz and N. Koshida, Appl. Phys. Lett. 94, 201903 (2009). 4. A. Chouket et al, J. Luminescence, 129, 1332 (2009). 5. Y. Hirano, K. Okamoto, S. Yamazaki, and N. Koshida, Appl. Phys. Lett. 95, 063109 (2009).


2:45 PM T8.4
Enhanced Photoconductivity from Plasmonic Nanoparticle Arrays on Thin Film Photovoltaic Silicon Absorber Layers. Krista Langeland, Imogen Pryce, Vivian Ferry and Harry A. Atwater; California Institute of Technology, Pasadena, California.

Recent research has demonstrated the potential for plasmon nanoparticle arrays to enhance light trapping and absorption in thin film photovoltaic devices. (K.R. Catchpole, et al., Appl. Phys. Lett. 93, (2008); K. Nakayama, et al., Appl. Phys. Lett. 93, (2008)). Synthesis of particles with uniform size and spacing enables quantitative characterization of enhanced absorption. We have employed a template-based method for synthesizing regular arrays of silver nanoparticles and examined their effect on photoconductivity and spectral response in thin film silicon on insulator absorber layers. This approach enables fabrication of regular arrays of nanoparticles with sizes from 60-200nm and coverage from 13% to 53%, and we have examined the effect of these arrays on spectral response and photoconductivity in thin films of silicon. Using anodized aluminum oxide membranes, we are able to fabricate large area (greater than 1cm2) masks through which we can then evaporate regular arrays of metal nanoparticles. By varying the anodization and evaporation conditions, we are able to control the pore spacing and size. Beginning with SOI wafers with a 220nm thick device layer and boron-doped to 1x1015cm-3, we fabricated 1cm2 structures with aluminum contacts at each corner. The photoconductivity and spectral response of these structures were measured before and after metal nanoparticle deposition to examine the effect of these nanoparticles on light absorption in the silicon. Results show that the photoconductivity increases more than ten-fold following nanoparticle deposition for a particle size of 100nm and a spacing of 200nm. Further increases in photoconductivity may be achieved by varying nanoparticle shape, size, and spacing. We will discuss the relationship between light absorption and particle size and array density in tailored geometric arrays. We will demonstrate the effect of this geometry on the measured photoconductivity and spectral response in thin film silicon, and we will relate these measurements to recent results from full-field simulations of plasmonic nanoparticle arrays.


 

3:30 PM *T9.1
Operating Principles of Colloidal Quantum Dot LED Technologies. Vladimir Bulovic, MIT, Cambridge, Massachusetts.

We demonstrate that electroluminescence of thin film colloidal quantum dot LED can be generated by multitude of proceses including energy transfer from excitonic thin films neighboring QD lumophores, direct charge injection into the luminescent QDs, or field induced QD ionization. Quantitative analysis of device operation reveals their operating mechanism.


4:00 PM T9.2
All-inorganic Light Emitting Devices Based on Semiconducting Nanoparticles. Ekaterina Neshataeva¹, Tilmar Kuemmell¹, Andre Ebbers² and Gerd Bacher¹; ¹Electronic Materials and Nanostructures, University Duisburg-Essen, Duisburg, Germany; ²Creavis, Degussa Evonik GmbH, Marl, Germany.

Nanoparticles are very attractive candidates for future large-area light emitting applications that are both robust and cost-effective. However, light emission at low operation voltages is mostly achieved by organic layers which support injection and transport of the charge carriers into the vicinity of the nanoparticles. These organic layers are susceptible to atmospheric conditions, humidity, electrochemical and thermal degradation which limits the lifetime of such organic/inorganic hybrid light emitting devices. In this contribution, we demonstrate nanoparticle light emitting devices, realized without any organic support layers. The fabrication process is suitable for low-cost mass production and is principally compatible with printing techniques. We used commercially available ZnO nanoparticles, synthesized in the gas-phase and dispersed in butyl acetate. The nanoparticles were spin-coated on top of a fluorine-doped tin oxide (FTO) glass substrate, resulting in a tight homogeneous layer. A metallic contact acting as a cathode was thermally evaporated on top of the nanoparticle layer, acting as a cathode, whereby FTO acts as a transparent anode. The device shows non-linear I/V-characteristic, which is attributed to the space charge limited transport. At room temperature, the device operates at voltages above 3 V and shows a broad defect-related electroluminescence in the visible spectral range [1] and a pronounced near-band gap peak in the UV-range [2], indicating an efficient electrical carrier injection. The device structure was further modified by inserting an additional p-type Si nanoparticle layer between FTO and naturally n-type ZnO to balance the charge carrier concentrations within the device. First nanoparticle bi-layer devices were successfully manufactured and showed stable electroluminescence. We believe, our findings open a path towards all-inorganic large-area nanoparticle based luminescent devices. [1] E. Neshataeva, T. Kuemmell, A. Ebbers, und G. Bacher, “Electrically driven ZnO nanoparticle light emitting device,” Electronics Letters, vol. 44, 2008, S. 1485. [2] E. Neshataeva, T. Kuemmell, G. Bacher, und A. Ebbers, “All-inorganic light emitting device based on ZnO nanoparticles,” Applied Physics Letters, vol. 94, 2009, S. 091115.


4:15 PM T9.3
Field Driven Electroluminescence from Colloidally Synthesized Quantum Dots.Vanessa Wood¹, Matthew J. Panzer¹, Scott Geyer², Moungi G. Bawendi² and Vladimir Bulovic¹; ¹Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts; ²Chemistry, MIT, Cambridge, Massachusetts.

QD-LEDs are of interest for applications such as thin film displays with improved color saturation and white lighting with high color rendering index. To date, the most efficient visible-emitting QD-LEDs involve a monolayer of QDs sandwiched between organic charge transport layers; however, the use of molecular organic materials as charge transport layers introduces fabrication challenges similar to those facing organic LEDs (OLEDs), namely the need for packaging in order to prevent degradation due to atmospheric oxygen or water vapor exposure. We reported the first all-inorganic QD-LED with n- and p-type metal oxide charge transport layers [1]. This QD-LED had uniformly emitting pixels with a peak luminance of nearly 2000 Cd/m² and an extended shelf life, but displayed limited efficiency and only enabled excitation of red-emitting colloidal QDs. We recognize that unlike QD-LEDs containing organic charge transport layers, which benefit from the concomitant roles of both Förster energy transfer and direct charge injection, QD-LEDs with metal oxide charge transport layer operate solely via direct charge injection, requiring device designs to be based largely on energy band alignment considerations [2]. We therefore explore a novel type electrical excitation of colloidal QDs that highlights the possibility for a paradigm shift away from direct charge injection into QDs as a means for electroluminescence (EL) in inorganic-based QD-LED structures. Here we report the first observation of field driven QD-LED excitation, which we explain via electron extraction from the valence band of the QDs under high electric fields. To understand this field-driven excitation mechanism, we develop QD thin film electroluminescent (TFEL) structures, which sandwich QDs between two insulating metal oxide layers. Because no current is transported through the TFEL devices, the need for alignment of energy bands is eliminated, and electroluminescence from QDs with emission peaks at wavelengths from 450-1500 nm is achieved using the same device structure. We confirm that our electron extraction mechanism is responsible for EL by demonstrating the correlation between the QD band gap and the electric field required for electroluminescence. We also demonstrate that this field-driven mechanism enables electrical excitation of QDs embedded in insulating polymers. Polymer and QD composites preserve the high photoluminescent efficiency of QDs in a thin film and provide a longer device shelf life [3]. [1] J.-M. Caruge, J.E. Halpert, V. Wood, V. Bulović, and M.G. Bawendi. Nature Photonics 2 (2008). [2] V. Wood, M.J. Panzer, J.-M. Caruge, J.E. Halpert, M.G. Bawendi, and V. Bulović. ACS Nano, in press 2009. [3] V. Wood, M.J. Panzer, J. Long, M.S. Bradley, J.E. Halpert, M.G. Bawendi, V. Bulović. Advanced Materials 21 (2009).


4:30 PM T9.4
Ion-Gel-Gated Nanocrystal Thin-film Transistors.Moon Sung Kang, Ayaskanta Sahu, Jiyoul Lee, David J. Norris and C. Daniel Frisbie; Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota.

Thin-film transistors based on semiconductor nanocrystals (NCs) offer not only a platform to probe the electrical properties of NCs but also an approach to utilize NCs in optoelectronic applications such as light emitting transistors (LETs). Here, we report thin-film transistors combining NC films (CdSe or PbSe) with high-capacitance ion-gel gate dielectric layers. These devices feature accumulation carrier densities higher than 10¹⁴ carriers/cm² which results in high carrier mobilities at operation voltages below 2.5 V. In particular, from CdSe NC based transistors, electron motilities as high as 0.4 cm²/Vsec were achieved. Moreover, the electrochemical potential at the CdSe NC/ion gel interface in these transistors can be monitored using a silver wire reference electrode embedded into the ion gel layer. Correlation between the referenced turn-on voltage (V_t^ref) of the devices with the 1S_e energy level of the CdSe NCs was observed such that V_t^ref decreased with increasing particle size. This confirms that it is easier to inject electrons into the films with larger NCs as they have lower 1S_e energy levels. For PbSe NC based transistors, efficient transport of both electrons and holes with mobilities as high as 0.4 and 0.02 cm²/Vsec, respectively, was obtained by inducing more than 3 carriers per NC, which represents an important first step toward LETs based on PbSe NC films.


4:45 PM T9.5
Ultrasmall White-light CdSe Nanocrystal Photo- and Electroluminescence for Solid-state Lighting. Michael A. Schreuder¹, Jonathan D. Gosnell^2,3, Kai Xiao⁴, Ilia N. Ivanov⁴, Sharon M. Weiss^2,3 and Sandra J. Rosenthal^1,2,3; ¹Chemistry, Vanderbilt University, Nashville, Tennessee; ²Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, Tennessee; ³Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee; ⁴Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Replacing inefficient fluorescent, halogen, and incandescent lighting in the United States with solid-state lighting (SSL) by 2025 could reduce energy lighting needs by as much as 326 TWH and carbon emission by 42 megatons. While the economical and ecological ramifications of replacing these outdated light sources are significant, it is also important to note the quality of light provided by new SSL. Lighting based on the mixing of narrowband emitters may represent significant advances in efficiency; however, these sources have difficulty replicating the quality of light from the sun and incandescent light bulbs. With the discovery of ultrasmall CdSe nanocrystals (diameter < 2 nm), came the possibility of a singe-source white-light emitter. This trap-based white light can be tuned through modification of the nanocrystal surface ligands. We have fabricated photoluminescent devices making use of these nanophosphors by encapsulating them in a biphenyl perfluoro-cyclobutyl polymer and coating ultraviolet LEDs with a thin film of this mixture. This encapsulation allowed for ease of deposition, in addition to providing a mechanically stable environment with some protection against heating and photo-oxidation. The resultant LEDs had CIE chromaticity coordinates of (0.324, 0.322) and a high color-rendering index of 93. Furthermore, we have demonstrated white electroluminescence from these ultrasmall nanocrystals. A thin layer of nanocrystals was sandwiched between a hole-transport polymer and a silver cathode. This allowed for the injection of charge carriers directly into the nanocrystal trap state responsible for the white emission. LEDs fabricated in this manner had pure white CIE coordinates (0.333, 0.333), color-rendering indexes as high as 96.6, and correlated color temperatures from 5461 to 6007 K. The electroluminescence from these devices is from the smallest size known to date; a size it was previously believed could not be excited electrically.


 

T10.1
Electroluminescence from Silicon Nanoparticles Integrated in Semiconductor Heterostructures.Jens Theis^1,2, Cedrik Meier³, Axel Lorke^1,2 and Hartmut Wiggers^4,2; ¹Experimental Physics, University of Duisburg-Essen, Duisburg, Germany; ²CeNIDE - Center for Nanointegration Duisburg--Essen, University of Duisburg-Essen, Duisburg-Essen, Germany; ³Physics Department & CeOPP - Center for Optoelectronics & Photonics Paderborn, University of Paderborn, Paderborn, Germany; ⁴Institute for Combustion and Gas Dynamics, University of Duisburg-Essen, Duisburg-Essen, Germany.

We have fabricated electroluminescence devices with silicon nanoparticles as the optically active medium by using a micropatterned GaAs heterostructure as a template. The wavelength of the emitted light is dependent on the size of the embedded nanoparticles. By varying the mean size of the nanoparticles it is possible to tune the wavelength without changing the material system and the processing sequence. The Si nanoparticles have been synthesized from the gas phase in a low-pressure microwave plasma using SiH4 as a precursor. The nanoparticles have been dispersed onto the patterned GaAs sample from an aqueous solution. For carrier injection, the nanoparticle layer was integrated into a capacitor-like structure, where a transparent indium tin oxide layer served as the top-electrode and a doped GaAs layer as the back contact. When an AC-voltage is applied to the structure, electrons are accelerated from the top gate towards the back contact. As these hot electrons traverse the Si nanoparticle layer, they generate secondary electrons and holes by impact ionization and therefore induce electron-hole pairs in the nanoparticles. This result is supported by the fact that light is only emitted when a negative voltage is applied at the top gate electrode, so that the device is under reverse bias. We find that optical emission from both Si nanoparticles and GaAs is observed. Additionally, we can distinguish between the luminescence from silicon nanoparticles embedded into the sample structure and the luminescence from oxygen deficient defects in the silicon dioxide insulating layer. Photoluminescence measurements made on separate samples of the same nanoparticles used in the presented device by using a Nd:YAG laser supports the results that the emitted electroluminescene light arises from the nanoparticles. We study the influence of various parameters on the electroluminescence, such as waveform, frequency and amplitude. The I-V measurements shows a rectifying behavior of the device. Applying a positive voltage at the top gate results in high currents without light emission, whereas negative voltage results in lower currents and light emission from the device when reaching the required threshold voltage of 1.3 V. With increasing voltage a blueshift of the emission from the nanoparticles can be observed. In a wide range the light emission is independent from the frequency of the square waveform voltage. A rapid drop in the light emission can be observed for frequencies above 5 MHz.


T10.2
ZnO Nanoparticle Light Emitting Device on NiO Coated FTO Substrates. Patrick Felbier¹, Ekaterina Neshataeva¹, Tilmar Kuemmell¹, Andre Ebbers² and Gerd Bacher¹; ¹Electronic Materials and Nanostructures, University Duisburg-Essen, Duisburg, North Rhine-Westphalia, Germany; ²Science-to-Business Center, Evonik Degussa GmbH, Marl, North Rhine-Westphalia, Germany.

Full-inorganic light emitting devices (LEDs) based on semiconductor nanoparticles promise to combine the enhanced stability of anorganic materials with respect to air and humidity with the potential of large scale and cost-effective processing as e.g. well established for organic LEDs. Recently, such inorganic LEDs using ZnO nanoparticles as active centers embedded between TCO and Al have been presented, showing broad luminescence spectra with a pronounced UV peak as well as defect related emission in the visible spectral range [1]. However, research on these devices has just started and brightness and stability remain important issues. In order to optimize the carrier injection into the active layer, we therefore introduce a nickel oxide (NiO) layer, which is expected to serve as a charge carrier transport layer and has already been proven to stabilise current flow in LED devices based on CdSe nanoparticles as active elements [2]. We have built full-inorganic LEDs based on a glass substrate coated with fluorine doped tin oxide (FTO), which serves as an anode. On top of the FTO, NiO has been sputtered with layer thicknesses between 50 and 200 nm. The roughness of the layer was about 10 nm (root mean square), which is mainly controlled by the surface topography of the FTO surface. On top of the NiO layer, ZnO nanoparticles are deposited by spin coating at 4000 rpm. The samples have been annealed at 150°C for 30 minutes and finally, Al was evaporated on top as cathode. First devices show good electroluminescence in forward direction, while no significant emission can be observed in reverse direction. Electroluminescence appears at typical current densities in the order of about 0.1 A/cm2 and the emission spectrum exhibits both, defect related and near bandgap emission. [1] E. Neshataeva, T. Kummell, G. Bacher, and A. Ebbers, “All-inorganic light emitting device based on ZnO nanoparticles,” Applied Physics Letters, vol. 94, Mar. 2009, pp. 091115-3. [2] J.M. Caruge, J.E. Halpert, V. Wood, V. Bulovic, and M.G. Bawendi, “Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers,” Nat Photon, vol. 2, Apr. 2008, pp. 247-250.


T10.3
Phase-separation in Zn_xCd_1-xSe/C Core/shell Nanocrystals Studied With Cathodoluminescence Imaging and Spectroscopy. Daniel H. Rich¹, Y. Estrin¹, O. Moshe¹, Sayan Bhattacharyya² and A. Gedanken²; ¹Department of Physics, The Ilse Katz Institute for Nanoscience and Nanotechnology, Ben-Gurion University of the Negev, Beer-Sheva, Israel; ²Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan, Israel.

Zn_xCd_1-xSe/C core/shell nanocrystals with 31-39 nm diameter semiconductor cores and 11-25 nm-thick carbon shells were synthesized from solid state precursors in large scale amounts. Transmission electron microscopy (TEM) showed striations in the nanocrystals that are indicative of a composition modulation, and reveal a chemical phase separation and spinodal decomposition within the nanocrystals. The optical properties and carrier relaxation kinetics of the nanocrystals were examined with time-resolved cathodoluminescence (CL) and monochromatic CL imaging. We observed that groups of nanocrystals within regions of constant wavelength exhibit very similar local CL spectra, in which the highly focused electron beam remains fixed during the acquisition of a CL spectrum. The CL spectral lineshape was found to depend on the excitation level, temperature, and time-window during time-delayed spectroscopy. As the excitation level is increased, carrier filling in the phase-separated Cd-rich (x ≈ 0.41) regions leads to a partial saturation of states before the onset of carrier filling in the higher bandgap homogenous Zn_0.47Cd_0.53Se regions. A 2D band-filling model was used to examine the carrier filling in Cd-rich regions as a function of e-beam current. Variations in the CL spectral lineshape with temperature and excitation conditions were found to be consistent with carrier transport and thermalization between the phase-separated Cd-rich regions and the homogenous Zn_0.47Cd_0.53Se alloy regions in the nanocrystals. The activation energy for carrier emission and transfer between the two regions was determined. A wavelength dependence of the carrier lifetime is observed in which the lifetime increases as the wavelength increases. Time-delayed CL spectroscopy at different temperatures was used to construct snap shots of the spectral lineshape during the decay. These results illustrate that compositional phase separation on the scale of ~1-5 nm in II-VI nanocrystals can lead to potentially useful quantum effects and interesting optical properties.


T10.4
Electronic Properties of Hematite Nanoparticles and Films for Photoelectrochemical Solar Energy Conversion. Debajeet K. Bora^1,2, Selma Erat^1,5, Artur Braun¹, Edwin C. Constable², Thomas Graule¹, Olga Safonova⁴ and Ariffin Ahmad³; ¹Lab for high performance ceramics, EMPA-DUBENDORF, Dubendorf, Zurich, Switzerland; ²Department of Chemistry, Universitat Basel, Basel, Switzerland; ³BESSYII GmbH, Helmholtz Centre for Material and Energy, Berlin, Germany; ⁴Swiss Norwegian Beamline(BM01B), European Synchroton Radiaition Facility, Grenoble, France; ⁵Department of Non metallic Inorganic Materials, ETH Zurich, Zurich, Switzerland.

The use of solar energy to split water is making tremendous impact in the sustainable energy sector.The photoconverison of water to hydrogen and oxygen is seen as the preferable solution for getting greener energy. Hematite nanoparticles are of great technological importance for its use in photoe-lectrochmical water splitting reaction for the production of hydrogen. Hematite is the material of choice for PEC application due to inexpensive, suitable band gap (2.2 eV), valence band edge position and Earth abundant nature. In the present study, hematite nanoparticles were synthesized by a non aqueous soft chemistry route using fatty acid complex with different annealing temperature. XRD study revealed that at the lower annealing temperature nanoparticle comprises both the maghemite and hematite phase. As the temperature got increased upto maximum all the maghemite phase seemed to be changed to the hematite phase. TEM result showed that the nanoparticles were per-fectly crystalline with crystallographic plane oriented along particular direction. The particle size distribution of nanoparticles followed a log normal distribution and this gets increased with the in-crease in size of the particle. Finally, from the pre edge analysis of the Fe-K edge XANES spectra, it was proposed that the pre edge peak intensity increased with respect to the annealing temperature which might be due to change in oxidation state and site symmetry of the nanoparticle. To study further the functionality of the nanoparticle in PEC application, hematite film has been fabricated by dip coating & post annealing process at 500 degree centigrade over FTO substrate. Same precursor solution has been applied as in case of nanoparticle synthesis. The synthesized film was further characterized by XRD to determine the presence of hematite phase. From XRD result it was found that the diffraction peaks arise mainly from the dense SnO2 coating on the glass substrate. There is only one strong peak due to hematite, namely the (110) reflection (in hexagonal coordinates).FESEM further signifies the highly porous architecture of the films with a thickness of 630 nm. Also, from the dark and light CV measurement in a three electrode cell with 1 M NaOH, the photocurrent density was found out to be 400μA/cm2 which confirmed that the films deposited were photo active.In the study of electronic structure,it has been found that O K edge NEXAFS spectra of these photo electrochemically processed film showed an extra peak just below the pre edge due to the reduction of Fe (III) to Fe(II) upon electrochemical recycling. In other words, it is noteworthy to mention that the spectra follow the same pattern as that of Goethite.


T10.5
The Detailed Balance Efficiency Calculations of Multiple Exciton Generation Photovoltaic Devices Under Concentrated Light. Jongwon Lee and Christiana Honsberg; Electrical Engineering, Arizona State University, Tempe, Arizona.

The multiple exciton generation (MEG) photovoltaic device with nano-crystals(NC) is a promising technology to surpass Schockley Queisser (SQ) limit. It has been reported that the maximum efficiency and optimum bandgap are 43% and 0.76 eV under standard AM1.5G illumination in the detailed balance calculation. If both the light concentration and carrier multiplication effect are used at the MEG devices, the maximum efficiency will be enhanced and optimum bandgap for materials will be reduced. We present the efficiency of MEG with NCs from the detailed balance calculations under concentrated light condition and find the proper materials from optimized bandgap in AM1.5 solar spectrum.


T10.6
Structural and Optical Properties of SiOxNy Containing Silicon Nanoparticles Fabricated by Plasma Enhanced Chemical Vapour Deposition Technique. Ferblantier Gerald¹, Marzia Carrada¹, Florian Delachat¹, Marco Ficcadenti², Dominique Muller¹ and Abdelillah Slaoui¹; ¹Matériaux et Concepts pour le Photovoltaïque, CNRS - InESS - UdS, Strasbourg, France; ²Dipartimento di Fisica, Università degli Studi di Camerino, Camerino, Italy.

Silicon oxynitride (SiOxNy) layers can be of great interest for applications in microelectronics and in photovoltaic as a passivation layer, antireflection and photonic conversion. In this work, we investigated the structural and optical properties of deposited SiOxNy films in an Ecr-Pecvd reactor. These films have been deposited on silicon and quartz substrates heated from 30 to 500°C by using N2O and SiH4 as precursor gas. Nanoparticles have been obtained by annealing the films at high temperature after deposition. A comparative study of the layers composition and their physical properties are discussed according to the reactive gas flow. Spectroscopic ellipsometry, Fourier Transform Infrared Spectroscopy, micro-Raman Spectroscopy, Rutherford Backscaterring Spectroscopy and Transmission Electron Microscopy were employed in order to characterize the layers before and after annealing. According to these results, the SiOxNy layers composition can be varied gradually from SiO2 to SiN thanks to the variation of the N2O and SiH4 gas flow. However, the optical constant determined by ellipsometry measurements is increasing from 1.4 to 3, in agreement with the structural results. Moreover, the silicon nanoparticles formation will be study as a function of the silicon excess in the SiOxNy matrix. This is for interest for the elaboration of graded layers or stacked multilayers for photonic conversion used in solar cells.


T10.7
Zn Doped Nanocrystalline CuCl Thin Films for Optoelectronic Applications.Rajani K.Vijayaraghavan, Stephen Daniels, F. Olabanji Lucas, A. Cowley, M. M.Alam and P. J.McNally; School of Electronic Engineering, Dublin City University, Dublin, Ireland.

The development of materials for solid state lighting has an important role in the progress of cost-effective and environmentally friendly light sources. Here we present results on the performance of doped CuCl:Zn thin films, which appear to be an attractive candidate material system for optoelectronic applications. Zn doped nanocrystalline CuCl thin films are successfully deposited on glass and Si substrates by pulsed dc magnetron sputtering. Structural and morphological properties are investigated using X-ray diffraction studies and Scanning Electron Microscopy respectively. The average crystalline size of CuCl:Zn films varies from 30- 50 nm. The resistivity of the CuCl:Zn films are explored using the four point probe technique. The influence of the weight percentage of Zn in the sputtering target (0-5%) on the crystal size and the resistivity of the films are reported. The resistivity of the CuCl film is reduced by an order of magnitude by the inclusion of Zn. The lowest achieved resistivity is ~ 9 Ωcm for Zn wt% of 5, which compares to undoped CuCl resistivity of ~ 250 Ωcm. A decrease in the FWHM and an improvement in the crystallinity of CuCl are also noticed as a result of the doping with Zn. The doped CuCl films display strong room temperature and low temperature photoluminescence at λ~ 385nm and UV/VIS absorption excitonic studies are also presented and analysed.


T10.8
Enhanced Cu Dopant Fluorescence of ZnS:Cu/ZnS Core/Shell Nanocrystals. Carley J. Corrado^1,2, Jin Z. Zhang¹, Morgan Hawker¹, Grant Livingston¹ and Frank Bridges²; ¹Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, California; ²Physics, University of California, Santa Cruz, Santa Cruz, California.

ZnS:Cu,Cl/ZnS core/shell nanocrystals (NCs) have been synthesized via a facile aqueous synthesis. Upon growth of a ZnS shell around ZnS NCs doped with 0.2% Cu, the intensity of the fluorescence emission at 445 nm is shifted to 465 nm as well as increased by 33%. The fluorescence emission can be deconvolved into two peaks, one from the ZnS host and the other from the Cu dopant. The apparent shift in fluorescence is actually due to the increase in the Cu fluorescence emission, resulting from capping the Cu-doped ZnS NCs with a ZnS shell. The increase in Cu emission is evidence that Cu atoms occupying non-emissive surface sites on the doped ZnS NCs were encapsulated by the ZnS shell. The local structure around the Cu is analyzed using Extendend X-Ray Fine Structure (EXAFS). The shell growth of the NCs was observed via a redshift in the UV-Vis absorption spectra as the size of the NCs was increased.


T10.9
Quantum Dot Sensitized Solid-state Photovoltaic Device. Shawn Rosson¹, Kevin J. Emmitt^2,1, Michael A. Schreuder¹, James R. McBride¹ and Sandra J. Rosenthal¹; ¹Chemistry, Vanderbilt University, Nashville, Tennessee; ²Physics, Columbia University, New York, New York.

Fossil fuels provide approximately 80% of our country’s energy, and at our current rate of consumption, we will inevitably have shortages within the next few decades. Presently, there has been a drive to make renewable-energy producing photovoltaic devices that rival the cost of fossil fuels, while providing a cleaner energy source. We have been developing an all solid-state ordered-heterojunction quantum dot sensitized solar cell that will meet these demands. Quantum dot sensitized devices have promising theoretical thermodynamic efficiency limits because of the possibility of multi-exciton generation (MEG), which provides multiple charge carriers per photon of incident light. We have constructed and tested devices using an ordered titanium dioxide (TiO2) nanotube array sensitized with CdSe nanocrystals and pore-filled with N,N-Bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) hole conducting polymer. Titanium dioxide nanotubes are made by an anodization process of a titanium foil followed by pore-filling with nanocrystals and hole conducting material. This device structure allows for unidirectional transport of charge carriers to the electrodes. Entirely solid-state devices also provide more stability and less maintenance than electrolyte-based devices. Solid state structures such as ours will be more resilient to temperature changes, cracking, and decomposition. For this reason, we have also been moving toward an inorganic hole conducting material, namely indium-tin oxide, to fill the pores instead of organic polymers. Pore filling has been a fundamental issue in a device using closed-bottom nanotube arrays due to pore clogging at the surface of the tubes. Open pores would likely eliminate this aggregation of nanocrystals at the surface. Until recently, there was no way of getting free-standing, open-pore arrays without destroying the ordered structure. We have produced open-pore nanotubes using a primary anodization, annealing, then secondary anodization process. We have been conducting nanocrystal loading studies on these free-standing nanotubes to demonstrate the quantity of nanocrystals that can be deposited on the nanotubes. The nanocrystal sensitized free-standing array is then capable of being used in a device. We have evaporated aluminum as an electrode, giving better electron transport properties than titanium. With increased surface coverage and improved electron transport, the efficiency of the device should be significantly improved.


T10.10
Modification of Structural and Optical Properties of CdSe Nanoparticles by Exchange of Surface Capping Layer. Xiang Dong Luo², Nguyen Tam Nguyen Truong¹, Umme Farva^1,3 and Chinho Park¹; ¹Yeungnam University, Gyeongsan, Korea, Republic of; ²Nantong University, Nantong, China; ³Dongguk University, Gyeongju, Korea, Republic of.

TOPO-capped cubic CdSe nanoparticles with different diameters were synthesized, and pyridine-capped cubic CdSe quantum dots (QDs) were prepared by exchanging TOPO with pyridine using liquid-liquid extraction process. The TEM images showed that, when TOPO was substituted by pyridine, the size of CdSe was decreased with the decrease in diameter increased with the size of original TOPO-capped CdSe. All of the photoluminescence (PL) spectra of pyridine-capped CdSe QDs exhibited a blue-shift in peaks compared to those of corresponding TOPO-capped CdSe QDs. By analyzing the PL spectra and its size decrease of pyridine-capped CdSe QDs, it was found that the decrease of size made a blue-shift of 25 to 55 meV, which was caused by the quantum confinement effect. While, by analyzing the PL blue-shift of pyridine-capped CdSe QDs with respect to the corresponding TOPO-capped CdSe with approximately same diameter, re-distribution of surface electronic density of pyridine-capped CdSe upon exchanging TOPO with pyridine was conjectured to occur. Comparing the experimental results with theoretical curves, a 23 meV PL red-shift of pyridine-capped CdSe was indicated, compared with the TOPO-capped CdSe with the same QD size, which was the result of re-distribution of surface electronic density of CdSe by exchanging TOPO with pyridine. And our results suggested that a red- or blue-shift of PL spectra for CdSe quantum dots with the same size is affected by the properties of surface capping materials. Our results also suggested that, when the size of CdSe QDs is close to its Bohr diameter, a red-shift of PL spectra of CdSe will occur when the passivating material was exchanged from TOPO to pyridine.


T10.11
Abstract Withdrawn


T10.12
Fabrication and Characterization of CuInS2 Thin Film Solar Cells. Fengyan Zhang, Cyril Bajracharya, Chivin Sun, Joshua J. Pak and Rene G. Rodriguez; Chemistry Department, Idaho State University, Pocatello, Idaho.

Solar cells with structure of Glass/Mo/CIS/CdS/ZnO/ZnAlO/Al have been fabricated. The light absorbing CIS layer was applied using CuInS2 ink though printing method. The CdS buffer layer is deposited though CBD method using CdCl2.2.5H2O, NH4OH, NH4Cl, and Thiourea (CS (NH2)2). The ZnO and ZnAlO are deposited through DC sputtering process in which the ZnAlO layer has achieved resistivity of 2.57x10-3 ohm-cm and transparency at 90%. Detailed characterization of the solar cell’s performance and how it is affected by the CIS layer’s heat treatment, the thickness and heat treatment of CdS buffer layer and the properties of ZnO/ZnAlO double layer will be discussed.


T10.13
Abstract Withdrawn


T10.14
Electroluminescence in ZnS Nanoparticle Films. Benjamin Balaban, Carley Corrado and Chris France; UC Santa Cruz, Santa Cruz, California.

Thin-film electroluminescent phosphors such as ZnS:Cu,Cl,Mn offer robust, efficient, and low-cost possibilities for the future of solid state lighting. Undoped ZnS films will exhibit DC electroluminescence at very high voltages, but effective doping with CuS nanostructures has been shown to induce lower-electric-field AC thin film electroluminescence (ACTFEL) in the phosphor films. Nonetheless, operation voltages on the order of 100 Volts peak-to-peak make them unpractical for most applications. Since the particles within the device experience a local field directly proportional the applied voltage and inversely proportional to the width of the device, a primary goal of EL phosphor research is the creation of thinner devices in order to facilitate lower operating voltages while maintaining light output levels. Commercial grade ZnS:Cu,Cl,Mn phosphors have typical particle sizes of 80 μm or more. Our attempts to create smaller devices in the sub 10 μm range by using a micro-mill to grind commercial phosphors have been shown to be ineffective. Though thinner devices do show greater ACEL at a given applied voltage than thicker phosphors, the device efficiency of the ground particles is decreased when compared to devices made from unground powders. EXAFS analysis of size separated ground particles has shown that the structure of the CuS precipitates is changed by the grinding. A solution to this challenge is to design nanostructured devices from the bottom-up using colloidal nanoparticles. For example, conducting nanorods can be added to ZnS:Cu nanoparticles to simulate ACTFEL devices on the nanoscale. For this research, we synthesized organic soluble oleic acid capped ZnS nanoparticles synthesized at the oil/water interface of a normal microemulsion to maintain the photo and electroluminescent properties when spin cast into thin films. Using this technique, it is possible to create EL phosphor devices with thicknesses in the 100nm range, with none of the damaging effects of grinding. Single layer 200nm thick spun films have demonstrated photoluminescence in the 400-450nm range when exposed to a wide range of excitation light. We discuss the results from devices made from these ZnS nanoparticle films as a function of film thickness and doping with conducting nanowire materials, such as Cu2S.


T10.15
Abstract Withdrawn


T10.16
Absorption Coefficients of the Material Systems With Negligible Valence Band Offsets for Quantum Dot Intermediate Band Solar Cells. Som N. Dahal and Christiana Honsberg; Electrical,Computer and Energy Engineering, Arizona State University, Tempe, Arizona.

Heterostructures that include self-assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures, necessary for the formation of SAQDs, introduces strain throughout the structure, making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain, a materials search was performed for high efficiency QD solar cells among III-V binaries and ternaries with negligible valence band offsets. Based on the effective band gaps, some material combinations are recommended for the implementation of IB. The density of states and absorption coefficients of IB material system based on QDs is calculated and presented for the application on detailed balance efficiency calculations.


T10.17
Exciton Density-dependent Energy Transfer Rate in Mn-doped Semiconductor Nanocrystals. Hsiang-Yun Chen and Dong Hee Son; Chemistry, Texas A&M University, College Station, Texas.

The exciton density-dependent rate of energy transfer from exciton to doped Mn ions in colloidal Mn-doped CdS/ZnS nanocrystals was investigated employing pump-probe transient absorption spectroscopy. By the comparative analysis of the exciton relaxation rate of doped and undoped nanocrystals, time scale of the exciton-dopant energy transfer was obtained as a function of the initial photoexcited exciton density in the range of 1-20 excitons per particle. Energy transfer rate increased with the increase of photoexcited exciton density, effectively competing with Auger relaxation process responsible for nonradiative dissipation of the energy of multiple excitons in typical undoped semiconductor nanocrystals. The increased energy transfer rate is ascribed to the increased population of the excitons at higher energy states, which have a better wavefunction overlap with Mn ions. The higher energy transfer rate with increasing exciton density can be utilized to retain the energy of multiple excitons with less loss of energy as heat before they are extracted as photons or current.


T10.18
Electroluminescence of ZnO Nanorod-polymer Hybrid LED. Won Kook Choi, Young Jai Oh and Dong-Ick Son; Thin Film Material Research Center, Korea Institute of Science and Technology, Seoul, Korea, Republic of.

A hybrid polymer-nanocrystals (NC) light-emitting diode (LED) device with the hole-conducting polymer poly (9-vinyl carbazole) (PVK) and ZnO nanorods (NRs) composite structure of glass/indium-tin-oxide (ITO)/(PEDOT:PSS/PVK + ZnO nanorods)/Al is simply fabricated by a spin coating. In current-voltage characteristic curve measurement, negative differential resistance (NDR) phenomenon is observed where Al electrode is directly deposited on PVK instead of ZnO NRs. Such an inhomogenous deposition is caused by the agglomeration of ZnO NRs and their low probability of adsorption on PVK due to two-dimensional structural property. Carrier transport behavior in the LED device is well described by both ohmic and space-charge-limited-current (SCLC) processes. Broad blue electroluminescence (EL) consisting of two sub peaks centered at 441 nm and 495 nm is observed, which indicates that the ZnO nanorod plays as a recombination center of exciton. Red shift in EL position compared to photoluminescence can be well explained by band offsets happened at the heterojunction between PVK and ZnO NRs.


T10.19
Surface Plasmon Enhanced Photoconductance and Single Electron Effects in Gold Nanoparticles Embedded Mesoporous TiO2 Nanofibers. Minsoo Son¹, Kyung-Hwa Yoo^1,3, Ji Eun Im², Kang-Kyun Wang², Seung-Lim Oh² and Yong-Rok Kim²; ¹Physics, Yonsei University, Seoul, Korea, Republic of; ²Chemistry, Yonsei University, Seoul, Korea, Republic of; ³Nano Medical NCRC, Yonsei University, Seoul, Korea, Republic of.

We have synthesized mesoporous TiO2 nanofibers loaded with Au nanoparticles (MTNF-Au) and fabricated single nanofiber-based devices on SiO2/Si substrates. Upon illumination with visible light, the MTNF-Au device exhibited wavelength-dependent and reversible photoresponses, which were caused by the SPR absorption. In addition, we also investigated the temperature dependence of electronic transport properties for both the MTNF and MTNF-Au devices. These two devices differed in their temperature dependence at low temperatures. In particular, the single electron effects were demonstrated in the MTNF-Au device, suggesting that the variations in temperature difference were attributable to electronic tunneling among the Au nanoparticles embedded in the TiO2.


T10.20
Enhanced Power Conversion Efficiency of Textured Polycrystalline Silicon Solar Cells Utilizing Indium-Tin-Oxide Nano-whiskers. Chia-Hua Chang¹, Min-Hsiang Hsu¹, Peichen Yu¹, Wei-Lun Chang² and Wen-Ching Sun²; ¹Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan; ²Photovoltaics Technology Center, Industrial Technology Research Institute, Hsinchu, Taiwan.

The anti-reflective (AR) coating plays an important role in high-efficiency photovoltaic systems, which is in general realized by the deposition of multiple dielectric layers with gradient refractive indices. Over the past few years, versatile sub-wavelength structures (SWS), such as periodic nano-pyramids, and random nano-rods, have emerged as promising candidates for AR coatings due to excellent AR properties over a broad range of incident angles and wavelengths. However, the fabrication cost involving either electron beam lithography and/or dry etching techniques is significant. The resulting surface states could also degrade the device performance, making applications of SWS in commercial solar cells unrealistic. Recently, multiple studies have been made on indium-tin-oxide (ITO), titanium dioxide (TiO2), and silicon dioxide (SiO2) nanostructures employing oblique-angle deposition methods, where the refractive indices of the nano-porous materials can be engineered by adjusting the air volume ratio. Still, the materials require multiple layers to effectively suppress the Fresnel reflection. In this paper, we introduce a deposition technique to form ITO nano-whiskers on surface-textured silicon solar cells. The characteristic whisker formation is assisted by glancing-angle electron-beam deposition in a nitrogen ambient. We show that the growth mechanism involves catalyst-free vapor-liquid-solid phase transitions. UV-VIS spectroscopy show that the fabricated silicon cells with the nano-structured material exhibit broadband AR characteristics (R < 5%) for the 300 nm- 1200nm wavelength range, which are also better than those with conventional SiNx AR coatings. Current-voltage characteristics measured under the AM1.5G spectrum show that the conversion efficiency of a ITO nano-whisker cell is ~17.13% with a fill factor (FF) of 75.12, while a SiNx AR cell is ~16.08% with FF=73.44. Quantum efficiency analyses confirm that the nano-whisker AR coating enhances the optical absorption for the near-infrared wavelengths, and hence improves the conversion efficiency.


T10.21
Dye-sensitized Solar Cells Based on Sn-doped In₂O₃ Nanowire. Jun Hong Noh¹, Hyun Soo Han¹, Sangwook Lee¹, Hyun Suk Jung² and Kug Sun Hong¹; ¹Materials Science and Engineering, Seoul National University, Seoul, Korea, Republic of; ²Advanced Materials Engineering, Kookmin University, Seoul, Kosovo.

Sn-doped In₂O₃ (ITO) nanowire array on ITO film coated glass substrate was fabricated by a vapor transport method (VTM) using mixed powder of indium and tin for dye sensitized solar cells (DSSCs). Highly conductive ITO nanowire was not directly used as photoelectrode in DSSCs due to its low dye adsorption and large back electron leak from ITO to electrolyte. Therefore, core-shell structure was required to adsorb dye molecular and to prevent back electron leak. TiO₂ shell layers on ITO nanowire were coated by an atomic layer deposition and TiO₂ nanoparticles on ITO nanowire were grown using a TiCl₄ solution. Cell performances of the ITO core-TiO₂ shell nanowire array-DSSC discussed in terms of charge transport in comparison with those of TiO₂ nanoparticles-DSSC.


T10.22
Magnetic Phase Diagram of Transition Metal Doped ZnO from Density Functional Calculations and Monte Carlo Simulations. Sanjeev K. Nayak¹, Ruzica Djenadic², Markus Winterer² and Peter Entel¹; ¹Faculty of Physics and CeNIDE, University of Duisburg-Essen, Duisburg, Germany; ²Faculty of Engineering and CeNIDE, University of Duisburg-Essen, Duisburg, Germany.

Diluted magnetic semiconductors with zinc oxide (ZnO) as the host semiconductor are much in discussion. There is yet no detailed understanding on these systems owing to various types of results discussed both in experiment and theory with respect to their magnetic properties. We study transition metal (TM) doped ZnO as cluster and bulk material using density functional theory to understand the mechanism for magnetism. We have calculated the distance dependent exchange interactions J_ij(r) between cobalt spins in Zn_1-xCo_xO for various concentrations x of Co by the Korringa-Kohn-Rostoker method together with coherent potential approximation (KKR-CPA). The J_ij(r) so obtained are taken as input for Monte Carlo simulations to determine the Curie temperature. Thus we obtain the magnetic phase diagram of Zn_1-xCo_xO with respect to the concentration x. The phase diagram shows that Co doped ZnO exhibits ferromagnetic behavior for concentrations above the percolation threshold, which is 18% in case of wurtzite structure [1,2]. In the dilute limit the system sustains no ferromagnetism. Similar studies for other transition metals will be presented. We also study the morphology of ZnO clusters by relaxing the structures to minimize forces. The relaxation is done using the conjugate gradient method. Role of such structural relaxation on the magnetic properties of TM in ZnO clusters will be also discussed. [1] S. K. Nayak et al., Phys. Stat. Sol. (a), 1839 (2008), [2] S. K. Nayak et al., J. Phys.: Condens. Matter 21, 064238 (2009).


T10.23
Abstract Withdrawn


T10.24
Abstract Withdrawn


T10.25
Hybrid Heterojunction Solar Cell Based on CdSe Nanocrystal Quantum Dots. So-Myung Jeong¹, Kyung-Nam Kim¹, Seoung Hum Eom², Chang-Soo Han¹, Soo-Hyung Lee² and Sohee Jeong¹; ¹Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon, Korea, Republic of; ²School of Semiconductors and chemical engineering, Chonbuk National University, Jeonju, Korea, Republic of.

Semiconductor nanocrystal quantum dots (NQDs) have recently attracted considerable interest for use in photovoltaics. Band gaps of NQDs can be tuned over a considerable range by varying the particle size thereby allowing enhance absorption of solar spectrum. NQDs, synthesized using colloidal routes, are solution processable and promise for a large-area fabrication. Recent advancements in multiple-exciton generation in NQD solutions have afforded possible efficiency improvements. Various architectures have attempted to utilize the NQDs in photovoltaics, such as NQD-sensitized solar cell, NQD-bulk-heterojuction solar cell and etc. Here we have fabricated CdSe NQDs with the band gap of 1.8 eV to 2.1 eV on thin-layers of p-type organic crystallites (1.61 eV) to realize a donor-acceptor type heterojuction solar cell. Simple structure as it was, we could control the interface of electrode-p-layer, and n-p-layer and monitor the following efficiency changes. Specifically, surface molecules adsorbed on the NQDs were critical to enhance the carrier transfer among the n-layer where we could verify by measuring the photo-response from the NQD layers only. Further modifying the annealing temperature after the deposition of NQDs on p-layers allowed higher conversion efficiencies in the device.


T10.26
Abstract Withdrawn


T10.27
Raman Scattering Studies of CuIn1-xGaxSe2 Nanoparticles. Christine Kim¹, Ah-Rum Jeong¹, William Jo¹ and Yoon Seokhyun^1,2; ¹Physics, Ewha Womans University, Seoul, Korea, Republic of; ²Chemistry and Nano Sciences, Ewha Womans University, Seoul, Korea, Republic of.

We studied CuIn1-xGaxSe2 (CIGS) nanoparticles synthesized by pulsed laser ablation method. The nanoparticles were grown on three different substrates, glass, glass coated with indium tin oxide (ITO) and Si, and heat-treated at different temperatures. We performed micro-Raman scattering measurements on CIGS nanoparticles grown and treated under different conditions. We found the x-dependence of a phonon mode near 177 cm-1, which can serve as an indicator of relative Ga content in CIGS compounds. Additionally, large frequency shift of phonon modes were observed between nanoparticles grown on different substrates. We also observed a change in crystallinity when the heat-treating temperature is different. Our result may provide a useful mean to optimize growth conditions of CIGS nanoparticles, which can be applied to a cost-effective, high-efficiency solar cell.


T10.28
Semiconducting Nanomaterial Schottky Solar Cells. Joondong Kim¹, Ju-Hyung Yun², Chang Hyun Kim^1,3, Yun Chang Park⁴, Chang-Soo Han¹ and Jeunghee Park³; ¹Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Daejeon, Korea, Republic of; ²Electrical Engineering, University at Buffalo, State University of New York, Buffalo, New York; ³Chemistry, Korea University, Seoul, Korea, Republic of; ⁴Measurement and Analysis Division, National Nanofab Center, Daejeon, Korea, Republic of.

Solar energy is one of the promising renewable and sustainable energy sources. The remarkable demand of nanomaterials has been addressed for cost-effective solar cells. The nanostructure has a large photo-active surface at a fixed volume and a high potential to be adopted in large area applicable processes, such as such as a coating or a printing method. But the promising of the nanomaterial solar cells has not been achieved due to the difficult architecture of photo-active region in tiny scale nanostructures [1]. We present the feasible design scheme of nanomaterial Schottky solar cells. Two different metals were applied to obtain an Ohmic and a Schottky contact by the different work functions to the semiconducting nanomaterials. References [1] J. Kim, J.-H. Yun, C.-S. Han, Y. J. Cho, J. Park, Y. C. Park, Appl. Phys. Lett. 95, 143112 (2009).


T10.29
Si Quantum Flakes and Their Optical Properties. IlSoo Kim¹, Myoung-Ha Kim¹, Yong-Hee Park¹, Tae-Eon Park¹, Hong-Gyu Ahn², Seung-Han Park² and Heon-Jin Choi¹; ¹Materials Science and Engineering, Yonsei university, Seoul, Korea, Republic of; ²Department of Physics, Yonsei University, Seoul, Korea, Republic of.

The nanostructures having quantum confinement effect (QCE) have been interested due to their novel physical- and chemical properties as well as their potential toward nano-devices. Indeed, many nanostructures such as semiconductor quantum dots (QDs) or carbon nanotubes have shown the QCE and their potential toward nano devices. Meanwhile, silicon (Si) nanostructures have been the more interested due to their compatibility with conventional complementary metal oxide semiconductor (CMOS) technology. SiQDs have been developed in this regard and their QCE has observed. However, integration of SiQDs into device architectures is difficult. For example, SiQDs based light emitting diodes (LEDs) architecture is consisted of isolated SiQDs inside matrix and it makes current injection into SiQDs difficult. Thus, SiQDs-based LEDs generally require either very high voltage or very thin SiQDs layers that can limit the light output. Herein we report on the synthesis of novel Si nanostructures having QCE, Si quantum flakes (SiQFs), and their optical properties. The SiQFs were synthesized by conventional chemical vapor transport process. Transmission electron microscopy analysis indicated that the SiQFs are single crystalline with diameter of about 0.5 micrometers and thickness below 1 nm. The 1/3(422) reflections, which was forbidden pattern for bulk Si, indicates that the SiQFs are very flat and thin Si single crystals. Optical properties of SiQFs were characterized by photoluminescence (PL) measurements with 325 nm HeCd laser as excitation source. The SiQFs showed luminescence from 400 to 700 nm in wavelength depending on the growth conditions. It corresponds to the band gap from 1.77 to 3.1 eV. Our systematic investigation indicated that the PL could be explained by QCE in these Si nanostructures. Based on the experimental results, the optical properties as well as fundamental aspect of growth mechanism of these novel Si nanostructures will be discussed in this talk.


T10.30
Novel Nanohybrids of CdS Quantum Dot and Tungsten Oxide with Tunable Optical Property. Hyo Na Kim^1,2, Tae Woo Kim¹, In Young Kim^1,2 and Seong-Ju Hwang^1,2; ¹Center for Intelligent Nano-Bio Materials (CINBM), Ewha Womans University, Seoul, Korea, Republic of; ²Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul, Korea, Republic of.

Novel CdS quantum dots (QDs)-tungsten oxide nanohybrids were synthesized by electrostatic attraction between positively charged CdS QDs and negatively charged polyoxotungstate nanoclusters. As a precursor, amine-modified CdS QDs were prepared by one-pot soft-chemical synthetic method. UV-vis spectroscopy, zeta potential measurement and transmission electron microscopy clearly demonstrated the formation of monodisperse positive surface charged (+53.2 mV) CdS QDs showing the first excitation peak at 370 nm and the uniform particle size of ~2.5 nm. The reaction between the CdS QDs and polyoxotungstate nanoclusters produced the CdS-WOx nanohybrid showing the retention of the optical and structural properties of CdS. The crystal structure, morphology, and optical properties of the nanohybrids were systematically investigated. Of particular interest is that the present CdS-WOx nanohybrid shows immediate color change from yellow to green under the irradiation of visible light (λ > 420 nm), which contrasts with no color change of bare tungsten oxide. This finding can be regarded as clear evidence for electron transfer between quantum dots and tungsten oxides.


T10.31
The Effect of RF-sputtered TiO2 Passivating Layer on the Performance of Dye Sensitized Solar Cells.Young Sam Jin, Jong Min Kim, Jung Soo Hong, Kyung Hwan Kim and Hyung Wook Choi; Electrical Engineering, Kyungwon University, Seongnam, Gyeonggi-do, Korea, Republic of.

The aim of this work is to prevent back transfer of electrons due to direct contact between the electrolyte and the conductive substrate using TiO2 passivation. Thin TiO2 passivating layer was deposited on FTO glass by RF magnetron sputtering with different working pressures. Thickness and crystalline structure was adjusted by various working conditions. TiO2 passivating layer was grown rutile phase as the decrease of working pressure. Nanoporous TiO2 films were prepared by sol-gel method on the TiO2 passivating layer. The crystal structure and morphology were characterized by X-ray diffraction XRD, Scanning Electron Microscope (SEM). The transmittance and absorbance of TiO2 films were characterized by UV-vis. The TiO2 films were calcinated conventional method and Rapid Thermal Annealing system. The TiO2 films calcinated at low temperature were showed anatase phase, and they were grown rutile phase as the increase of calcination temperature. It was found that the conversion efficiency of DSSC was highly affected by crystalline structure of passivating layer.


T10.32
Annealing Condition Dependent Room Temperature PL of Si Nano-crystals Thin Film. Yi-Heng Tsai¹, Yi-Shian Lin², Tzu-Yueh Chang^1,2 and Po-Tsung Lee^1,2; ¹Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan; ²Department of Photonics & Display Institute, National Chiao Tung University, Hsinchu, Taiwan.

Si nano-crystals (NCs) embedded in dielectric matrix integrated with silicon-based solar cells is one of the proposed solar cell structures to achieve high energy conversion efficiency due to its flexibility in band-gap engineering of absorber layers. However, there are many defect states between Si NCs and dielectric matrix, which will affect the performance of Si NCs thin film. Besides, the effect of these defect states on the photovoltaic properties of the Si NCs thin film is still not well understood. In this study, we paid attention to the influence of annealing condition on room temperature photoluminescence (PL) properties of Si NCs thin film. Si NCs thin film was formed by silicon rich oxide (SRO) thin film deposited by radio-frequency co-sputtering of Si and SiO2, for the large flexibility in tuning the composition of the precursor thin film, then annealed by rapid thermal annealing (RTA) and/or furnace. Quantum confinement of Si NCs and localized states were found in room temperature PL spectra of Si NCs thin films with different SRO layer thicknesses. Moreover, interface states between Si NCs and SiO2 matrix were only found in room temperature PL spectra of Si NCs thin films after RTA. Interface states between Si NCs and SiO2 matrix could trap photon-excited excitons, which would then reduce photovoltaic performance of Si NCs thin films. As a result, annealing condition plays an important role on the properties of Si NCs thin films.


T10.33
Characterization of Solar Cells Fabricated by Using Boron-doped Si Nanocrystals. Seung Hui Hong¹, Min Choul Kim¹, Suk-Ho Choi¹, Jong Shick Jang² and Kyung Joong Kim²; ¹Kyung Hee University, Yongin, Korea, Republic of; ²Korea Research Institute of Standards and Science, Daejeon, Korea, Republic of.

Boron-doped SiO_x/SiO₂ multilayers have been prepared on n-type Si (100) wafer by ion beam sputtering and subsequently annealed to form p-type Si nanocrystals (NCs)/n-type Si wafer structures for solar cells. For the growth of the multilayers, the alternating SiO_x/SiO₂ layers were deposited by controlling the oxygen flow rate during sputtering. The boron doping of Si NCs was achieved by co-sputtering using a combination target of a heavily-doped p-type Si wafer attached with a small piece of boron nitride film. The growth rate/stoichiometry (x) of the SiO_x and SiO₂ layers were controlled and analyzed by in situ x-ray photoelectron spectroscopy, and the doping profiles of the NC layers were obtained by secondary ion mass spectroscopy. Transmission electron microscopy and photoluminescence spectroscopy proved that Si NCs were well formed within SiO₂ matrix by the annealing at 1100 ^oC. Al electrodes were evaporated on top of the deposited films and on rear side of the Si substrates for the electrical measurements. Current-voltage and photovoltaic characteristics were investigated as functions of x, layer thickness, and doping concentration, and discussed based on possible physical mechanisms


T10.34
The Exciton Emission of ZnSe QDs Taking into Account the QDs Interactions.Wallace C.H. Choy, Sha Xiong and YuXiu Sun; Department of Electrical & Electronic Engineering, The University of Hong Kong, Hong Kong, China.

Since ZnSe is less toxic than cadmium based materials, ZnSe quantum dots (QDs) is a potential candidate for optoelectronic and fluorescent labeling applications. In this report, colloidal ZnSe QDs are successfully synthesized from zinc stearate and elemental selenium in a paraffin hot-matrix. The method is environmental friendly and low cost as compared with the conventional methods which are generally toxic and expensive. The QD-QD interactions will significantly affect the optical properties and thus a detailed study is desirable. Here, we will comprehensively study the interaction through the van der Waals interaction energy, electrostatic energy, steric energy and depletion interaction energy. The corresponding photoluminescence and absorption spectra show obvious excitonic features. The QDs have been characterized by High-Resolution Transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and Energy-dispersive X-ray spectroscopic (EDX). The environmental friendly synthesis method is good for the biomedical application. We have used a conversional ligand exchange method by using thiolycolic acid which allows dispersal of the QDs in various organic solvents. In the n-hexane solution, the photoluminescence (PL) of the nanocrystals capping with stearate ions is dominated by the exciton emission with sharp and symmetrical peak. In the aqueous solution, it can be observed that the emission has an extended tail on the long wavelength direction. The tail emission can be explained by the contribution of surface defect which is due to the photocatalytic oxidation of the thiol ligands into disulfides and surface-binding species. A little red-shift of the PL may be caused by the congregation because the repulsive force between thiolycolic acid molecular is relatively weak.


T10.35
Effective Wavelength Conversion from Near-UV to Red for Photovoltaics: Potential of Transparent YVO₄:Bi³⁺,Eu³⁺ Nanophosphor Film.Satoru Takeshita¹, Kenji Nakayama¹, Tetsuhiko Isobe¹, Tomohiro Sawayama² and Seiji Niikura²; ¹Department of Applied Chemistry, Keio University, Yokohama, Japan; ²SINLOIHI Co., Ltd., Kamakura, Japan.

Widely used Si-based solar cells can convert only the photons of energy close to the band gap of Si to the electricity, and hence the sunlight with wavelength regions shorter than ∼ 400 nm and longer than ∼ 1100 nm are completely wasted. Spectral convertor is an idea for enhancing the solar cell efficiency by converting these near-UV and IR lights to the visible ones using a wavelength convertor with a high transparency in the visible region. Most of the previous works on such wavelength convertors from near-UV to visible light focus on transparent resins containing organic fluorochromes or luminescent metal complexes, although practical applications are limited due to their low stability. On the contrary, commercial inorganic phosphors of micrometers in size are not appropriate for such wavelength convertors because of their opacity. Inorganic nanophosphors are the most appropriate candidates from the aspects of their high transparency and high stability. YVO₄:Bi³⁺,Eu³⁺ emits red under the excitation of near-UV light through the charge transfer transition from Bi³⁺ to V⁵⁺, followed by the energy transfer to Eu³⁺. In this work, we produce the transparent near-UV to red wavelength conversion thick films containing YVO₄:Bi³⁺,Eu³⁺ nanoparticles (NPs) and characterize their optical properties in comparison to those of the films containing YVO₄:Bi³⁺,Eu³⁺ micron-sized particles (MPs). YVO₄:Bi³⁺,Eu³⁺ NPs of 10.8 ± 1.6 nm in diameter were synthesized by a wet chemical method in the presence of citrate ions. YVO₄:Bi³⁺,Eu³⁺ MPs of 1.12 ± 0.48 μm in size were synthesized by a conventional solid-state reaction. The thick film containing 38.8 wt% of NPs shows a high transparency in the visible region, where the transmittance at the emission peak of the phosphor, 619 nm, is ∼ 97% irrespective of the film thickness. In contrast, the film containing 30.0 wt% of MPs is completely opaque in the whole visible region. To evaluate the wavelength conversion efficiency, the emission intensity at 619 nm was measured by a detector on the front side of the film when the back side of the film was irradiated by 365 nm excitation light. The emission intensity of the film containing NPs increases with increasing the film thickness up to 400 μm, whereas that of the film containing MPs reaches the maximum at the film thickness of ∼ 40 μm. This is attributed to the lower light scattering loss of the NPs in comparison to the MPs. The NPs also shows sufficient stability for practical use over 5 years outdoors by light fastness test using a carbon arc fademeter. Therefore, we conclude that YVO₄:Bi³⁺,Eu³⁺ NPs could be applied for the spectral convertor of Si-based solar cells. S.T. thanks the JSPS for the doctoral fellowship (DC1).


T10.36
Characterization of an Ultra Thin, Dense Hafnium Oxide Compact Layer With Electrochemical Impedance Spectroscopy and Open-voltage Decay for Dye-sensitized Solar Cell Application.Braden Bills, Mariyappan Shanmugam, Mahdi Farrokh Baroughi, Danny Andrawis and David Galipeau; Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota.

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into the electrolyte solution. It was shown that contact between the electrolyte and transparent conducting oxide (TCO) can create an electrical short leading to poor photovoltaic performance [1]; sol-gel processed titanium oxide (TiO₂) compact layers were then introduced to prevent such electrical shorts [2]. However, electrolyte could still diffuse through the nanoparticles of the sol-gel processed compact layer, resulting in the need to use a thicker compact layer to further separate the TCO and electrolyte in order for the layer to still be effective. But a thick compact layer can interrupt the injection of electrons from the porous TiO₂ nanoparticles to TCO by acting as a resistive layer due to the formation of a Schottky barrier [3]. An atomic layer deposited (ALD) hafnium oxide (HfO₂) ultra thin compact layer was grown on the surface of the TCO and its effects on the performance of DSSCs were studied with dark and illuminated current-voltage, electrochemical impedance spectroscopy and open-circuit decay measurements. The surface topography of the layer was studied with atomic force microscope. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% (from 3.6% to 6.0%) compared to a DSSC with a conventional sol-gel processed TiO₂ compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, large band offset between the TCO and HfO₂ layer and the higher compactness obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO₂ compact layer. References [1] P. J. Cameron, L. M. Peter, J. Phys. Chem. B 109 (2005) 930. [2] A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, M. Grätzel, Nano Lett. 8, 4, (2008) 977. [3] S. Lee, J. H. Noh, H. S. Han, D. K. Yim, D. H. Kim, J. K. Lee, J. Y. Kim, H. S. Jung, K. S. Hong, J. Phys. Chem. C 113 (2009) 6878.


T10.37
TiO2/Dye/Electrolyte Interface Engineering by Atomic Layer Deposited Ultra Thin Al: SiO2 for Improved Dye Sensitized Solar Cell Performance. Mariyappan Shanmugam, Braden Bills, Mahdi Farrokh Baroughi and David Galipeau; Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota.

Mesoporous TiO2 based dye sensitized solar cell was introduced with 7.9-12% in 1991[1]. Further enhancement in the short circuit current density (JSC) and open circuit voltage (VOC) depends on efficient photoabsorption of dye used to sensitize the mesoporous TiO2, effective carrier injection from the LUMO of the dye to conduction band of TiO2 and carrier collection at the transparent conducting oxide electrode. However, defects present at the surface of the mesoporous TiO2 play a vital role on photo generated carrier loss. It was experimentally demonstrated that metal oxide layers can control the reverse electron recombination with the oxidized dye molecular species and the redox electrolyte [2, 3]. This paper reports the fabrication and characterization of DSSCs with and without SiO2 treated mesoporous TiO2 and study the effects of ultra thin SiO2 in the performance of DSSCs. DSSC fabrication procedures were already presented in our previous work [4]. SiO2 ultra thin layers of 5, 10 and 15 cycles were deposited on the photoelectrodes by ALD (Savannah 100, Cambridge NanoTech) method. Trimethylaluminum (Al2(CH3)6) and tris(tert-butoxy)silanol (ButO)3SiOH were used as silicon and oxygen precursor gases respectively and were applied sequentially into the deposition chamber to deposit SiO2 layer by layer at 200°C. Dark and illuminated current-voltage (I-V) characteristics of the fabricated DSSCs were measured using Agilent 4155C semiconductor parameter analyzer and a Xe arc lamp equipped with an AM1.5 filter calibrated at 100 mW/cm2 illumination power was used. Ultra thin SiO2 surface treatment controlled the dark current and enhanced overall DSSC performance. DSSC with 5 cycles of SiO2 treated mesoporous TiO2 showed 80% enhancement (from 4 to 7.2%) compared with the reference DSSC as shown in the Table 1. JSC and VOC of the SiO2 treated DSSC also were improved significantly compared to the reference DSSC. The significant enhancement of the DSSCs with SiO2 ultra thin layers suggests that carrier loss at the TiO2/dye/electrolyte interface has been reduced compared to the reference DSSC. Further, improved JSC and VOC addresses the activity surface defects on photoexcited electrons at mesoporous TiO2/dye/electrolyte interface was suppressed greatly by SiO2 surface treatment. References [1] B. O’Regan et. al., Nature 353 (1991) 737-739 [2] A. Kay et. al., Chem. Mater., 14 (2002) 2930-2935. [3] E. Palomares et. al., J. Am. Chem. Soc., 125 (2003) 475-482. [4] M. Shanmugam, et al., Thin Solid Films (2009), doi:10.1016/j.tsf.2009.08.033


T10.38
Improved Spectral Response of Quantum Dot Solar Cells Using InAs Multi-stack High Density Quantum Dot Molecules. Ong-arj Tangmettajittakul, Pornchai Changmuang, Supachok Thainoi, Songphol Kanjanachuchai, Somchai Rattanathammaphan and Somsak Panyakeow; Electrical Engineering, Chulalongkorn University, Bangkok, Bangkok, Thailand.

Quantum dot solar cells are considered as one of third generation solar cells having unique properties of their quantum nanostructures. Quantum dots (QDs) are grown by self-assembly method using lattice-mismatch systems like Si/Ge or InAs/GaAs. Multi-stack quantum dots are inserted into solar cell structure as an active layer in single junction. High density of uniform quantum dots are believed to be proper for high performance quantum dot solar cells due to creation of intermediate band in the band gap of conventional solar cells. However, multi-stacking of quantum dots is not easy in practical due to long processing steps and to increasing defects. Therefore, realization this idea is not well implemented. We propose to use high density quantum dot molecules (HD-QDMs) instead of conventional quantum dots. HD-QDMs are grown by modified MBE process called thin-capping-and-regrowth technique. The dot density of QDMs is more than one order of magnitude higher than that of QDs. Moreover, repeated growth cycles of QDMs give several uniform dot sizes leading to broader absorption band. In addition, HD-QDMs provide less stack number giving less defects in the QD solar cell structure. We report here the improvement of spectral response of quantum dot solar cells when InAs multi-stack HD-QDMs are used as an active layer of GaAs Schottky structure. 3-stack HD-QDMs sample is compared with 15-stack QD and without-QD control samples. The experimental results confirm that HD-QDMs give broader (910-985 nm) and 3 times more sensitive (comparing to QDs) spectral response beyond the band edge of GaAs, i.e. 870 nm. 30 % more photovoltaic power could be evident from extended spectral response curves measured at various light intensities. These preliminary results indicate that HD-QDMs are potential candidates for high performance QD solar cells.


T10.39
Photovoltaic Devices from Silicon Nanoparticles. Christoph Rier¹, G. Schierning^2,4, H. Wiggers^3,4, R. Schmechel^2,4 and D. Jaeger^1,4; ¹Center for Semiconductor Technology and Optoelectronics, University Duisburg-Essen, Duisburg, NRW, Germany; ²Faculty of Engineering, University Duisburg-Essen, Duisburg, NRW, Germany; ³Institute for Combustion and Gasdynamics, University Duisburg-Essen, Duisburg, NRW, Germany; ⁴Center for Nanointegration Duisburg-Essen (CeNIDE), University Duisburg-Essen, Duisburg, NRW, Germany.

In this paper the results of designing, fabricating and characterizing photovoltaic devices made from tailored silicon nanoparticles are shown as proof-of-principle to adopt this material into the photovoltaic sector. We report on the investigation of silicon nanoparticles as active material for direct separation of the light induced charge carriers. This means the nanoparticle layers are a combined absorbing and charge separating medium. Out of these considerations we have developed a concept including a homo pn-junction. The transition from p-type to n-type is formed directly during the fabrication process. This saves energy and material costs in contrast to the conventional overcompensation of the wafer doping in state-of-the-art photovoltaics, which is complex, costly and limiting the fabrication. A gas phase process based on a microwave plasma reactor is used to synthesize silicon nanoparticles. Crystalline particles of predefined size in a narrow distribution are obtained. N-type doping is realized by adding phosphine (PH₃), p-type doping by adding diborane (B₂H₆) to the precursor gas. The nominal dopand concentration is 5 x 10²⁰ cm^-3 for either type. For a first trial and to minimize the degree of freedom for possible error sources pellets from a p-doped silicon wafer with n-doped nanocrystals as well as pellets from n-doped silicon wafer with p-doped nonocrystals have been built. The sequence of layers in the investigated samples is obtained by pouring first nanoparticles, then a silicon wafer into a graphite crucible. The stack has been pre-compacted by hand. Spark plasma sintering was applied to electrically activate the nanoparticulate layer. Sintering temperature was 1050 °C with a hold time at that temperature of 2 minutes. The sintering procedure was carried out in argon with a uni-axial pressure during sintering of 16 MPa. The pellets have been polished, cleaned and provided with metal contacts. Subsequent electrical characterization by measuring dark and illuminated current-voltage-characteristics was conducted. From these measurements a reproducible short-circuit current of up to 30 μA was determined. Also a current density of 2.5 μA/cm² was realized which is nearly three times higher than that of devices with a lateral Si/SiO₂ quantum well structure [1]. This shows that the demonstrated concept may be promising for future devices made entirely from nanoparticles by direct implementation of the tailored doping profile using differently doped nanoparticles of silicon. [1] R. Rölver, B. Berghoff, D. L. Bätzner, B. Spangenberg, H. Kurz, Appl. Phys. Lett. 92, (2008), 212108


T10.40
CdSe and CdSe-ZnS Core-shell Structure Quantum Dots Sensitized Solar Cells on the ZnO Colloidal Nanocrystal Clusters. Mi-Hee Jung, Ho-Gyeong Yun, Hunkyun Pak and Man Gu Kang*; Next Generation Energy Technology Team, Advanced Solar Technology Research Division, Convergence Components & Materials Research Laboratory, Electronics and Telecommunications Research Institutes, Daejeon, Korea, Republic of.

Semiconductor quantum dots (QDs) sensitized solar cell offer significant advantages over dyes sensitized solar cells. QDs provided ability to match the solar spectrum better because their absorption spectrum can be tuned with particle size. In addition, it has been shown recently that the QDs can generate multiple exciton which could improve the efficiency of the device. We report on comparative experimental study of QDs sensitized solar cells: bare CdSe and core-shell(CdSe)ZnS dots into a ZnO colloidal nanocystal cluster(CNC) using the linker molucules mercaptopropionic acid. Core-shell(CdSe)ZnS dots into a ZnO CNC cells demonstrate more than 2 order of magnitude improvement in the overall device performance relative to identical structures based on bare CdSe dots-sensitized cells. The charge recombination between electrons injected in the conduction band of the semiconductor and the oxidized sensitizer is one of the major limiting factor to the photoelectrical conversion efficiency. The ZnS shell on CdSe QDs suppressed the surface trapping process of photoexcited carrier in the CdSe QDs while the photogenerated electrons in CdSe is quickly transferred to ZnO. Moreover, taking note of the energetic band of CdSe and ZnS, the higher conduction edge of ZnS than that of CdSe QDs prevent the leakage of current from CdSe to the elctrolyte, causing more electrons to be injected into the ZnO conduction band and consequently increasing both current density (Jsc), the open circuit voltage (Voc)


T10.41
Photoelectrochemical Solar Cells Made from SnO2/ZnO Composite Films Sensitized With an Indoline Dye.Boateng Onwona-Agyeman¹, Motoi Nakao² and Gamaralalage A. Kumara³; ¹Applied Science for Integrated System Engineering, Kyushu Institute of Technology, Kitakyushu, Japan; ²Applied Science for Integrated System Engineering, Kyushu Institute of Technology, Kitakyushu, Japan; ³Chemistry, University of Peradeniya, Peradeniya, Sri Lanka.

Recent success of ultrathin oxide coatings on nanoporous n-type semiconductors such as TiO2, ZnO and SnO2 with regard to the efficiency of dye-sensitized solar cell has received substantial attention. Nanocrystalline SnO2-based dye-sensitized (DS) photoelectrochemical cells (PECs) have very low open-circuit voltages and efficiencies of about 1%. However, on coating the SnO2 crystalline surface with a thin layer of ZnO an obvious enhancement in the cell parameters was noted. Recently, an indoline dye based DS PEC was found to yield an efficiency of about 8 % with a peak incident photon to photocurrent efficiency exceeding 80%. This dye strongly adsorbs into the SnO2 surface and found to be highly stable. Organic dyes are cheap, they degrade into non-toxic constituents and conventional synthesis procedures can be adopted to tailor the structure to suit requirements. Furthermore, organic dyes posses higher molecular extinction coefficients compared to inorganic sensitizers. An advantage of high molecular extinction coefficient happens to be the possibility of achieving a sufficient light absorption cross-section at a reduced film roughness factors. This is advantageous as the diffusive electron transport in the nanocrystalline semiconductor matrix sets an upper limit to the film thickness. In this work, we have used an indoline dye as a sensitizer for SnO2/ZnO composite film and describe the construction and characteristics of DS SnO2/ZnO PEC and explain how the ZnO layer on SnO2 could improve the cell parameters. Our SnO2/ZnO composite solar cells gave an overall energy conversion efficiency of 3.8% whilst the SnO2 and ZnO individual cells yielded 2.8% and 1.2% respectively under AM 1.5 standard solar simulated light (100 mW cm-2).


T10.42
Investigating Electrical Properties of ZnO Nanoparticles: Controlled ``Annealing” of Nanocrystals With Moisture. Sonja Hartner¹, Moazzam Ali², Markus Winterer^2,3 and Hartmut Wiggers^1,3; ¹Institute for Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg, NRW, Germany; ²Nanoparticle Process Technology, University of Duisburg Essen, Duisburg, Germany; ³CeNIDE- Center for Nanointegration Duisburg-Essen, University of Duisburg Essen, Duisburg, Germany.

Zinc oxide (ZnO) is one of the favored transparent conducting materials in electronics industry due to its availability and its low price. In comparison to conventional thin film preparation technologies by CVD or PVD processes, printing of ZnO nanoparticles enables for cost effective film formation under ambient conditions. As grain boundary resistances dominate the conductivity of ZnO thin films prepared from nanoparticles, we investigated their electrical properties during the “annealing” of as-prepared zinc oxide under humid conditions by means of impedance spectroscopy. ZnO nanoparticles from a hot wall reactor with a mean diameter of about 18 nm were pressed into pellets with a diameter of 5mm and a thickness around 0.09mm. Different concentrations of humidity were adjusted by a constant flow of argon trough a water bubbler at different temperatures and the humidity within the measurement setup was examined by commercial humidity sensor. The ZnO nanoparticles show typical semiconductor behavior and we observed an increase in conductivity with increasing moisture level. After removing any moisture from the setup a further increase in conductivity is observed until it reaches a final value, 5 orders of magnitude higher than the initial conductivity. We find that the zinc oxide particles with an initial size of about 18 nm grow depending on the level of humidity by a factor of two to three. The growth can be attributed to a steady state between hydroxyl groups and condensation to ZnO, resulting in a drastic reduction of number of grain boundaries. The usage of non-functionalized, as-prepared zinc oxide nanoparticles opens a way to transparent conducting layers that can be “annealed” under mild conditions even on temperature-sensitive substrates.


T10.43
Activation of TiO2 Photoelectrode and Its Influence on the Performance of Dye-sensitized Solar Cells. Yi-Fang Chiang and Tzung-Fang Guo; Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan.

This work elucidates a method that can be used to activate the TiO2 photoelectrode for the absorption of N719 dye in fabricating dye-sensitized solar cells (DSSC) and thereby improve the device performance. Firstly, the TiO2/fluorine doped tin oxide (FTO) substrate was immersed in N719 solution to absorb the dyes. Secondly, the N719 dyes on TiO2/FTO substrate were desorbed by dipping it in various concentrations of basic solutions, such as NaOH, CH3COONa and NH4OH aqueous solutions. Accidentally, it is found that the surface of TiO2/FTO substrate was activated by the repeative absorption and desorption process of the N719 dyes, which markedly increases the amount of N719 dyes absorbed on TiO2/FTO substrate when we immersed the substrate in N719 solution again for the second time of absorption. Accordingly, DSSCs made of the activated TiO2/FTO substrate presented a significant enhancement in the open-circuit voltage (Voc) (from 0.66 V ~ 0.74 V). The improvement of Voc is attributed to the higher adsorption of dye on TiO2 surface that forms a potential barrier at TiO2/dye/electrolyte interface leading to suppression of the dark current. The optimized device performance is of 5.5% power conversion efficiency (AM 1.5G, 100 mW/cm2), which is a 14% improvement as compared to that of the device without the activation process.


T10.44
EFM Study on Ge Island: Carrier Charge and Storage Effect. Zhen Lin¹, Pavel Brunkov², Fraud Bassani³ and Georges Bremond¹; ¹institute of nanotechnology in Lyon, National Institute of Applied Sciences in Lyon, Lyon, France; ²Ioffe Physico-Technical Institute RAS, Saint-Pétersbourg, Russian Federation; ³IMINP, LYON, France.

Nowadays, the semiconductor technology is facing great challenges to increase the device performance while reducing its dimension. This downscaling in microelectronics industry causes a drastic development of microscopy to reveal new physical characteristics at nanometer scale such as Coulomb blockade1-3, quantized charging effects4-7 and single electron transfers8-13. Atomic force microscopy (AFM) 14 is becoming a powerful tool for nanometer scale measurement. It can provide simultaneous topography and various physical feature images with some additional applications such as SCM, EFM, TUNA KPFM etc. Electrostatics force microscopy (EFM) is used specially for characterizing materials for accurate local and non-destructive electrical properties for a wide range of characterisations such as surface potential, charge distributions15-16, doping concentration17 and dielectric constant18. EFM is also able to easily and non-destructively inject and detect the localised charge in nanostructures, on or below the surface. This ability has been used to study the distribution of trapped charge in various types of samples such as individual quantum dots19-21. Such confined samples characteristics, individual conductive nanostructures deposited on an oxide layer, were few studied by EFM at room temperature. However, such nanostructures have a great interest because the injected charge carriers are constrained in their propagation, and interact in a finite geometry. In this paper, a series of individual Ge nanostructure on top of a silicon dioxide layer of thickness 5 nm prepared on an n+ type doped silicon (001) substrate have been studied. The charging ability of these individual Ge dots was evaluated by the EFM measurement. The nanostructure becomes an iso-potential and behaves as a conductive material after charging. We can consume that these charges were injected into the nano dots and were trapped homogenous in the Ge dots. From the comparison, negative charge bias is much easier to inject carriers to these Ge dots. In order to study the emission or dissipation of these trapped charges over the Ge dot, continuous EFM phase images were captured after charging by -7V for 3 minutes. The charges over Ge dot emitted and dissipated gradually to the surface around in almost 2 hours from our results. And the influence of different tip bias voltage was also studied.


T10.45
Comparative Study of the Hybrid Solar Cells from P3HT and Diamond, Silicon or CdSe Nanocrystals.Oleksandr Kutsay, Chao P. Liu, Hong E. Wang, Zhen H. Chen, Andrei S. Susha, Andrei I. Rogatch and Igor Bello; Physics and Material Sciense, City University of Hong Kong, Hong Kong, China.

O. Kutsay, C.P. Liu, H.E. Wang, Z.H. Chen, A.S. Susha, A.I. Rogach, I. Bello Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China We are reporting new concept of hybrid solar cells based on blends of the diamond, silicon or CdSe nanocrystal (NCs) and poly-3(hexylthiophene) (P3HT) polymer in which a percolating network of the nanocrystals acts as the electron-conducting phase. The properties of different composite NCs/P3HT devices made by spin-casting NCs and P3HT from a common solvent were studied as a function of NCs type and size and NCs/P3HT ratio. The open-circuit voltage and short-circuit current are observed to depend on the type and size of the nanocrystals due to changes in the bandgap and surface-area-to-volume ratio. Under simulated one-sun A.M. 1.5 direct illumination, the optimized devices made with Si NCs 5 nm in average size showed about 1% power conversion efficiency. UV-visible and photoluminescence spectroscopy, atomic force microscopy and transmission electron microscopy have been used to explore the properties and morphology of these blends. This work was supported by General Research Fund of RGC under grant number GRF CityU110209.


T10.46
Optical Characterization of Delta Doped InAs QDs on GaAsSb for the Intermediate Band Solar Cells.Keun-Yong Ban¹, Woong-Ki Hong² and Christiana B. Honsberg¹; ¹Electrical Engineering, Arizona State University, Tempe, Arizona; ²Material Science and Engineering, Gwangju Institute Science and Technology, Gwangju, Jeonnam, Korea, Republic of.

Intermediate band solar cells (IBSCs) are new approach to drastically increase the efficiency by adding additional quasi-Fermi level. The intermediate band introduced by quantum dots (QDs) or wells (QWs) can allow additional absorption spectra from the valence band to intermediate level and from the intermediate level to the conduction band. As one of promising candidates we have chosen InAs quantum dots on GaAsSb system calculated from 8 band k.p method. Its strength is that it only contains small valence band offset which can prevent energy loss due to phonon scattering as well as open circuit voltage loss. The structure has been grown by molecular beam epitaxy (MBE) with the optimum growth condition that we have got. The primary goal of this work is to control the occupancy of subband levels of conduction band offset (CBO) which is basically empty and investigate the optical properties in InAs/GaAsSb material system. Since subband levels in CBO may be corresponding to intermediate band in the solar cell device it should be carefully controlled so that the solar cell device would have the optimum performance. In addition it is fundamental to establish doping concentration comparable to the QD density to incorporate the appropriate number of electrons into each dot. Delta doping plane with different doping levels has been applied because it has several strengths, for instance, it can efficiently provide carriers without impurity scattering and severe band structure alteration. Therefore, we have investigated here delta doped InAs/GaAsSb material systems using optical characterization like time integrated and time resolved photoluminescence (TIPL & TRPL). We can take into account carrier behavior as thermal activation energy and full width at half maximum (FWHM) change as a function of delta doping level. Also, excitation power dependent PL can show such information as how many subband levels there are and how increased electron-hole pair affects FWHM. For in-depth study, TRPL at low temperature will be also performed to study more on carrier dynamics such as carrier relaxation time and lifeime from PL rise time and decay time, respectively. In conclusion, this study would be helpful in controlling transition rates among quasi-Fermi levels by understanding carrier dynamics.


T10.47
Abstract Withdrawn


T10.48
Multi-layered Panchromatic Dye-sensitized Solar Cells by Selective Positioning of Dyes. Se Woong Park, Doh-Kwon Lee, Min Jae Ko and Kyungkon Kim; Solar Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul, Korea, Republic of.

Dye-sensitized solar cells (DSCs) are considered one of promising candidates for a renewable energy. However, they show relatively low photocurrent densities in comparison with inorganic solar cells due to Gaussian shaped-absorption spectra of the dyes. To overcome this limitation, we investigated multi-layered DSCs using several dyes having different absorption wavelengths. Selective positioning of the dyes were achieved by controlling desorption and adsorption depths, which is confirmed EPMA analysis, using a pore filling polymer and a viscous desorbing solution. Incident photon to current conversion efficiency (IPCE) spectra of multi-layered DSCs showed sum of the each dye. Energy conversion efficiencies were also enhanced as the absorption spectra were broadened. We expect that panchromatic high efficient DSCs could be realized by this technology.


T10.49
Anisotropic Type II Nanocrystal Heterostructures. Hunter McDaniel and Moonsub Shim; MatSE, UIUC, Urbana, Illinois.

Type II nanocrystal heterostructures (NCHs) exhibit photo-induced charge separation, long fluorescence decay times and broad red-shifted absorption compared with their single phase counterparts. Due to properties that are strongly dependent on size and shape, and furthermore, due to the necessity for physically and chemically accessing both components of the heterostructure (which may be achieved via enhancing anisotropy), the key step towards functional multi-component systems is the ability to control not only the size and shape but also spatial orientation of each component with respect to each other. In order to better understand growth mechanism leading to enhanced anisotropy and charge separation in NCHs synthesized from monodisperse nanorod seeds, we have examined various factors that contribute to and the effects of structural diversification in the type II CdSe/CdTe system. Highly Stokes-shifted emission arises from heterointerfacial recombination and can be enhanced or suppressed via controlled positioning of CdTe on CdSe seeds. A careful examination of these NCHs using high resolution TEM and STEM techniques allow for a spatial mapping of the composition which is then correlated with electronic and optical features. A comparison of strained CdSe/CdTe (7.1% bulk mismatch) to lattice matched CdSe/ZnTe NCHs (also type II) yields information about how the interfacial strain is relieved in such systems and how it might be engineered to promote charge separation for photovoltaic applications.


T10.50
Blocking Materials for Solution-processed Nanocrystal Infrared Photodetectors. Galileo Sarasqueta, Kaushik Roy Choudhury and Franky So; Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida.

Inorganic nano-crystalline materials from II-VI and IV-VI groups have shown great promise as candidates for optoelectronic devices due to their simple colloidal synthesis, tunable band gaps, and inexpensive processing. There have been several attempts to fabricate devices for photo-detection employing these inorganic nano-crystals; however, the devices usually suffer from slow modulation response, lack of stability in ambient conditions, and exceedingly high dark currents. A common approach to reduce dark currents in these devices is improve the rectification through ligand exchange and solvent treatments. However, due to the limitation of small bandgap energy, dark currents are still too high and the detectivity of the devices does not compare to other inorganic counterparts. In this study we attempt to reduce the dark currents in PbSe nanocrystal photodetectors by employing wide band-gap materials as blocking layers at the anode and cathode contacts. The electrical characteristics of PbSe nanocrystal photodetector devices depend greatly on their energy alignment as well as the surface states of the quantum dots. Due to the small bandgap and trap states of PbSe quantum dots, leakage currents are large in these devices. To avoid this problem and to prevent high dark currents under operating conditions, we studied the effect of different wide band-gap materials as blocking layers, including poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), poly(N,N -diphenylbenzidine diphenylether) (poly-TPD) and bathocuproine (BCP). The spectral responsivity of the photo-detector devices was also measured and the quantum efficiency as well as the detectivity was estimated in relation to the dark current. The linearity of response of the photodetectors to varying incident light intensities was also determined. The blocking layers were found to greatly enhance the rectification of the photodetectors while significantly reducing the dark current in the devices. The detectivity of the devices was estimated to be in the order of 10E12 Jones due to the low dark currents, making them comparable to their silicon counterparts. Additionally, the stability of the devices in normal ambient conditions was greatly improved by using the blocking layers.


T10.51
Chromium-doped Zinc Selenide Nanoparticles for Electrically-pumped Middle-infrared Laser Sources. Sonal Singh, Parimal Bapat, Jonathan Williams, Renato P. Camata, Vladimir Fedorov and Sergey Mirov; Physics, University of Alabama at Birmingham, Birmingham, Alabama.

Electrically pumped middle-infrared (mid-IR) laser sources are in high demand for numerous applications including molecular spectroscopy, non-invasive medical diagnostics, environmental monitoring, and numerous defense related applications such as monitoring of munitions disposal and detection of explosion hazards. Mid-IR lasers based on transition-metal (TM) doped II-VI semiconductors (TM:II-V) are promising for such applications as they can deliver affordable, broadly tunable, compact sources operating over the “molecular fingerprint” 2-15 µm spectral range. One of the main challenges in the development of TM:II-VI sources is the low efficiency of energy transfer from the II-VI semiconductor host to the TM impurity responsible for IR emission. Quantum confinement effects in quantum dots and small nanoparticles are expected to enhance the efficiency of energy transfer from charge carriers to TM optical centers. In this study we have used nanoparticle beam pulsed laser deposition (NBPLD) to deposit chromium (Cr) doped ZnSe nanoparticles and investigated their optical properties and potential as gain media for electrically-pumped mid-IR sources. Nanoparticles of Cr2+:ZnSe of well-controlled size are deposited using NBPLD while gas-phase species can be used to deposit a distinct matrix material by conventional PLD. In NBPLD gas-suspended nanoparticles are generated by the ablation of a target prepared by mixing Cr and ZnSe powders in various concentrations in a glove box under inert atmosphere, followed by powder pressing and high-temperature annealing. Nanoparticles are generated by the ablation of these composite targets using a KrF excimer laser with fluence between 0.2 J/cm2 and 3 J/cm2 at pressures between 60 Torr and 760 Torr (Ar atmosphere). The nanoparticle aerosol formed in the ablation plume goes through an ionization zone where it acquires an equilibrium charge distribution. The charged particles are then sorted according to size based on their different migration velocities in an electric field across a particle-free laminar gas stream. Size-selected nanoparticles are extracted from the classification region within a desired window of sizes and delivered to the substrate as a nanoparticle beam. Transmission electron microscopy and atomic force microscopy show that this method leads to particles with 15% size control in the 1-20 nm range. Photoluminescence studies are carried out to determine to what extent Cr2+ incorporation can be confirmed in the ZnSe nanocrystals and for confirmation of mid-IR luminescence over 2-5 μm spectral range. Our approach permits the deposition of multiple layers of size selected nanoparticles of Cr2+:ZnCdSe with various diameters (1-10 nm) embedded in a matrix of slightly higher bandgap (ZnSSe) deposited by conventional PLD. This deposition process leads to structures featuring multiple doped nanoparticle layers as active light emitting material that may be suitable for electrical excitation.


 

8:30 AM *T11.1
Novel Functional Semiconductor Nanocrystal Quantum Dots and Nanowires for Applications Involving Energy Conversion. Jennifer Hollingsworth, Han Htoon, Javier Vela, Alfred Wooten, Yongfen Chen, Nickolaus Smith, Rawiwan Laocharoensuk, Victor Klimov, Floren Garcia-Santamaria, Donald Werder, Joanna Casson and Darrick Williams; Los Alamos National Lab, Los Alamos, New Mexico.

To advance the state-of-the-art in efforts to utilize nanoparticles for either optoelectronic or photovoltaic applications, both new semiconductor nanomaterials and new synthetic approaches are required. Here, we describe our efforts to address the former by (1) developing novel functional nanoscale “architectures” and (2) synthesizing underrepresented compositions, e.g., complex ternary and quaternary systems. We further address the latter by improving upon traditional solution-based synthesis routes to establish a new level of control that enables in situ nanoparticle heterostructuring and ordering. Specifically, we describe a functionally new class of nanocrystal quantum dot (NQD), the so-called “giant” NQD (g-NQD), which due to its unique physical and electronic nanoscale architecture exhibits new and useful photophysics for light-emission applications. Conventional NQD optical properties are sensitive to NQD surface chemistry and chemical environment. In contrast, we recently reported that key NQD optical properties—QY, photobleaching and blinking—can be rendered independent of NQD surface chemistry and chemical environment by growth of a very thick, defect-free inorganic shell (Chen et al. J. Am. Chem. Soc. 2008). We also altered NQD electronic structure from traditional NQD (core)shell systems. Together, these factors resulted in g-NQDs that do not photobleach, are insensitive to changes in surface chemistry, and show markedly suppressed blinking. Furthermore, ensemble PL dynamical studies on g-NQDs revealed direct evidence for strong suppression of non-radiative Auger recombination - including biexciton lifetimes that are 50 times longer than those obtained for conventional NQDs (Garcia-Santamaria et al. Nano Lett. 2009). Also, we performed low-T single-g-NQD PL studies that revealed unprecedented emission from multiexciton states. In this way, it is clear that the g-NQDs can afford new exciton→photon conversion pathways for enhanced efficiencies and stabilities for applications in optoelectronics. Secondly, we address the opposite technological need - nanoparticles that convert light into electric energy. Here, we describe the first solution-phase synthesis of high-quality CuInSe₂ nanowires (NWs) (Wooten et al. J. Am. Chem. Soc. ASAP 2009). We show that the “solution-liquid-solid” (SLS) growth method coupled with the use of a single-source chemical precursor is capable of yielding this complex ternary compound. This has implications for photovoltaics, as NWs in general are considered important alternatives to isotropic nanocrystals as “building blocks” for light-harvesting devices, with key candidates including the structurally complex CuInSe₂. Finally, we advance the state-of-the-art in SLS growth by establishing “flow-SLS” as an alternative solution-phase processing approach that provides unprecedented control over NW internal structure and ordering.


9:00 AM T11.2
Doped-CuInS₂ Semiconductors for Photovoltaic Applications: An Anomalous X-ray Diffraction Study. Sumohan Misra¹, Stephen T. Connor², Michael F. Toney¹ and Yi Cui²; ¹Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California; ²Department of Materials Science and Engineering, Stanford University, Stanford, California.

Ternary I-III-VI₂ semiconductors have garnered great interest due to their promise for photovoltaic applications. These materials have many benefits, including large optical absorption coefficients and good stability under solar radiation. Thin films of CuIn_xGa_1-xSe₂ (CIGS) with a band gap of 1.3 eV have been demonstrated to have a high power efficiency of ca. 20%, which is the highest among thin-film solar cell technologies. An analogous system, CuInS₂ (CISu), has a direct band gap of 1.5 eV, which is also well matched with the solar spectrum and is environmentally benign due to the absence of Se when compared with CIGS materials. It has been postulated that two of the leading reasons for high performance in CIGS solar cells are Cu diffusion during the chemical bath deposition process, creating a buried junction, and heterogeneous distribution of Ga. Furthermore, diffusion of dopants into the absorber from a buffer layer and the steel substrate, have been observed to alter CIGS solar cell performance. So, while many studies have been done to analyze the effect of dopants in CIGS, few studies have been done to understand the dopant locations (e.g., substitutional, interstitial, segregated) in either CIGS or CISu. We have studied the effect of various dopant elements in CISu nanoparticles including Ga, Fe, and Zn for 5-20 mole percent doping. To study the distribution of the dopant atoms in the crystal structure we have employed anomalous X-ray diffraction (AXRD) by collecting data both close to and away from the absorption edges of the dopant elements. This has allowed us to differentiate between Cu and the dopants elements and has helped us understand the structure-property relationship in these inorganic materials. These results show the incorporation of Zn & Fe into the host crystal structure but not that of Ga as suggested by our initial TEM and lab-based XRD results. Along with X-ray fluorescence measurements, these AXRD results show that though Ga is not included into CISu crystal structure, it is present on the surface of the nanoparticles and proves to be an essential tool in pinning down these dopants in these nanoparticle semiconductor materials.


9:15 AM T11.3
Preparation of Silicon Quantum Dots in Silicon Carbide. Matthias Kuenle¹, Philipp Loeper¹, Marcel Rothfelder¹, Stefan Janz¹, Klaus G. Nickel² and Oliver Eibl³; ¹Materials - Solar Cells and Technolgy, Fraunhofer Institute for Solar Energy, Freiburg, Germany; ²Institute for Geoscience, Applied Mineralogy, Eberhard-Karls-Universität, Tübingen, Tübingen, Germany; ³Institute for Applied Physics, Eberhard-Karls-Universität, Tübingen, Tübingen, Germany.

Periodically aligned silicon (Si) nanocrystals (NCs) in a dielectric silicon carbide (SiC) matrix is a promising material for the application as an upper cell of a Si based tandem solar cell to realize very high efficiency solar cells. In contrast to other dielectrics SiC has several advantages: it can be doped with B or P and its barrier height to Si is lower than the barrier height of silicon nitride (Si3N4) and silicon oxide (SiO2) to Si, providing a higher charge carrier mobility. The fabrication of an effective absorber material implies the accurate control of the Si NC size and density. A size control of Si NCs in a SiO2 matrix was achieved by a multilayer approach, where the Si NC size is restricted by a stoichiometric diffusion barrier and the Si NC density is controllable by the amount of excess Si in the Si-rich layers. However, the preparation of Si NCs in SiC is more challenging compared to Si NCs in SiO2, due to the small difference of crystallization temperatures of Si and SiC. In this paper the preparation of Si NCs in a SiC host matrix and their size control by a multilayer approach is investigated. Si NCs in a SiC host matrix are prepared by a multilayer approach, where 20 bilayers were deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). Every bilayer consists of a stoichiometric a-SiC:H and a-SixC1-x:H layer. Samples differ in the thickness of the a-SixC1-x:H layers and the amount of H incorporated during deposition. After deposition, a subsequent thermal annealing step, e.g. rapid thermal processing (RTP), is performed. The annealed multilayers are hydrogen (H) passivated in a Remote Plasma Hydrogen Passivation (RPHP) reactor to saturate dangling bonds for a lower defect density. The structural and optical properties of SixC1-x/SiC multilayers annealed at different temperatures will be discussed. During deposition a large amount of H is incorporated in the layers. When the mutlilayers are annealed at low temperatures, decomposition and H out-diffusion occurs. This was measured with Fourier Transformed Infrared Spectroscopy (FTIR). Glancing Incidence X-Ray Diffraction (GIXRD) showed, that multilayers are amorphous up to annealing temperatures of 800°C. Annealing at 900°C leads to the crystallization of both Si and SiC nanocrystals. However, the Transmission Electron Microscopy (TEM) analysis reveals a strong dependency of the crystallization temperature of SiC on the thickness of the a SixC1-x:H layers. The TEM analysis also showed that nucleation and crystallization of Si and SiC NCs is strongly influenced by interfaces. After annealing, the multilayers were H passivated to saturate dangling bonds within the dielectric matrix and at crystalline surfaces, which cause non radiative electron transitions. The unpassivated and passivated multilayers will be investigated by Photoluminescence (PL) spectroscopy.


9:30 AM *T11.4
Conformal Coating of ZnO-based Nanostructures With Oxide Heterostructures. Marius Grundmann and Ruediger Schmidt-Grund; Inst. f. Experimental Physics II, Universitaet Leipzig, Leipzig, Germany.

ZnO nanorods have been fabricated with various lateral density, size and aspect ratio. These structures have been conformally coated with two different kind of heterostructures: (i) (Mg,Zn)O/ZnO/(Mg,Zn)O quantum wells and (ii) oxide Bragg mirrors. The quantum wells have been grown in axial and radial direction. Grown on top of the wires, we find the nominal quantum well to consist of quantum dot like luminescence centers. The axial quantum well exhibits clear confinement effects. The conformal Bragg mirrors create a three-dimensional resonator. We investigate the mode structure as a function of core radius and temperature and find evidence for strong coupling of the optical modes and the ZnO excitons. This work was supported by Deutsche Forschungsgemeinschaft within the framework of Forschergruppe FOR522, the Graduate School "Leipzig School of Natural Sciences - BuildMoNa" as well as the European Social Fund. The work has been performed in cooperation with (alphabetically) B. Cao, C. Czekalla, H. Hilmer, A. Hinkel, M. Lange, M. Lorenz, R. Schmidt-Grund, C. Sturm, H. von Wenckstern, J. Zúñiga-Pérez.


 

10:30 AM *T12.1
Robust, Functional Nanocrystal Solids by Infilling with Atomic Layer Deposition. Matt Law, Department of Chemistry, University of California, Irvine, Irvine, California.

Colloidal semiconductor nanocrystals (NCs) are metatstable objects prone to thermal and oxidative degradation driven by their large surface-to-volume ratio. The fabrication of practical electronic devices based on NC solids hinges on developing methods to prevent oxidation, diffusion, sintering and other undesirable physical and chemical changes to which these materials are susceptible. First we describe systematic measurements of the room-temperature electron and hole field-effect mobilities of alkanedithiol-treated PbSe NC films as a function of NC size and the length of the alkane chain. These results establish a baseline for mobility trends in PbSe NC solids and have implications for fabricating high-mobility NC-based optoelectronic devices. Then we combine optical, electrical and photoelectron spectroscopy measurements to monitor the room-temperature oxidation of films of PbSe NCs that are treated in solutions of short-chain thiols or carboxylic acids to produce electronically-coupled NC solids. Air exposure causes a blueshift of the NC bandgap for all the chemical treatments studied. Two stages of oxidation are identified in the case of films treated in 1,2-ethanedithiol (EDT). We show that surface oxidation can be prevented by infilling/overcoating NC films with thin (5-10 nm) Al2O3 layers deposited by low-temperature atomic layer deposition (ALD). ALD treatment of complete PbSe NC field-effect transistors yields high-performance devices that operate indefinitely in air. ALD infilling of NC films is a promising route to the preparation of stable, all-inorganic NC solids with tunable electrical properties, and may prove an important breakthrough in the fabrication of stable, high-efficiency quantum dot solar cells.


11:00 AM T12.2
Statistical Model for the Effects of Dephasing on Transport Properties of Large Samples. Matias Zilly^1,2, Orsolya Ujsaghy³ and Dietrich E. Wolf^1,2; ¹Theoretical Physics, University of Duisburg-Essen, Duisburg, Germany; ²CeNIDE, Duisburg, Germany; ³Department of Theoretical Physics and Condensed Matter Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Budapest, Hungary.

We present a statistical model for the effects of dephasing on the transport properties of large devices. The physical picture is different from earlier models which assume that dephasing happens continuously throughout the sample, whereas we model the dephasing in a statistical sense, assuming a distribution of completely phase randomizing regions between which the transport is coherent and described by the nonequilibrium Green’s function method. Thus the sample is effectively divided into smaller parts making the numerical treatment more efficient. As a first application the conductances of ordered and disordered linear tight-binding chains are calculated and compared to the results of other phenomenological models in the literature.


11:15 AM T12.3
The Use of Thermally Decomposable Ligands for Conductive Films of Semiconductor Nanocrystals. Andrew Wills¹, Moon Sung Kang², Wayne L. Gladfelter¹ and David Norris²; ¹Chemistry, University of Minnesota, Minneapolis, Minnesota; ²Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota.

Poor conductivity is a bottleneck hindering the production of nanocrystal-based devices. In most nanocrystal syntheses, ligands with long alkyl chains are used to prepare monodisperse, crystalline particles. When these nanocrystals are incorporated into devices as films, the bulky ligands form an insulating layer that prevents charge transfer between particles. While annealing or post-deposition chemical treatments can be used to strip surface ligands, each of these approaches has disadvantages. Here we demonstrate the use of a novel family of ligands comprised of primary alkyl dithiocarbamates to stabilize CdSe and PbSe nanocrystals. Primary dithiocarbamates, which can bind to cadmium and lead, are known to decompose to the corresponding sulfides when heated under mild conditions. In our scheme, CdSe or PbSe nanocrystals are first synthesized with standard ligands. These ligands are then exchanged to short chain dithiocarbamates in solution. When a film is cast and annealed at low temperature, the dithiocarbamates are removed. Films prepared in this manner are smooth and crack-free, in contrast to films treated post-deposition with short chain ligands such as ethanedithiol. This dithiocarbamate treatment therefore avoids labor-intensive layer-by-layer dipcoating that is currently used to deposit continuous conductive films of nanocrystals. Alternatively, UV exposure of dithiocarbamate-treated films provides an additional route for passivation of the nanocrystal surface. We incorporate dithiocarbamate treated films into transistors and evaluate the device characteristics after heat and UV-treatment. Thus, primary dithiocarbamates offer a simple strategy for preparing nanocrystal films with enhanced electronic coupling.


11:30 AM T12.4
Electrical Properties of Functionalized Silicon Nanoparticles. Sonja Hartner¹, Anoop Gupta¹ and Hartmut Wiggers^1,2; ¹Institute for Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg, NRW, Germany; ²CeNIDE- Center for Nanointegration Duisburg-Essen, University Duisburg- Essen, Duisburg, Germany.

The utilization of silicon nanoparticles for printable electronics like solar cells, sensor devices and transistors requires highly stable, (semi)conducting materials. As silicon nanoparticles tend to form a native oxide when handled in air, a stable surface functionalization is required to make them applicable for semiconductor purposes. The electrical property of different alkene-stabilized silicon nanoparticles as well as their long-term stability when stored in air is investigated. We synthesized phosphorous-doped as well as undoped Si-NPs with a mean particle diameter of about 50 nm from a mixture of silane, phosphine, hydrogen and argon in a microwave plasma reactor. The natural oxide was removed by etching the particles with hydrofluoric acid. In order to prevent the particles from re-oxidation, the surface of freshly etched particles was functionalized with different alkenes (hexene (C6), decene (C10), dodecene (C12), tetradecene (C14), octadecene (C18)) via thermal alkylation. The electrical conductivity of as-prepared, freshly etched, and functionalized Si-NPs was measured using impedance spectroscopy in the temperature range between 50 and 400 °C. For all samples, the freshly etched Si-NPs showed a large increase in conductivity (about 4 orders of magnitude) compared to the respective as-prepared samples. Surprisingly it is found that doping with phosphorus shows almost no difference in conductivity compared to undoped samples. After surface functionalization, conductivity decreases depending on length of the alkene used for stabilization. While functionalization with alkenes from C6 to C12 resulted in higher conductivity compared to the as-prepared materials, samples functionalized with C14 and C18 showed very poor conductivity. Dodecene-terminated nanoparticles showed the highest conductivity; while FTIR-spectroscopy indicated that surface functionalization with C6-C10 is not very stable due to a creeping re-oxidation. Dodecene terminated SiNPs also showed good conductivity after storage under ambient conditions for half a year. We conclude that conductivity of silicon nanoparticle ensembles is dominated by inter-particle transport. Therefore, engineering of the particle surface is the most important challenge for a useful application of silicon nanoparticles in printable electronics.


11:45 AM T12.5
Excited State Charge Transfer in Dyads of ZnO Nanocrystals and Organic or Transition Metal Dyes. Wayne L. Gladfelter, Julia E. Saunders, Raghu Chitta, Adam Huss, Andrew Bierbaum, David A. Blank and Kent R. Mann; Chemistry, University of Minnesota, Minneapolis, Minnesota.

To better understand the specific charge transfer events that occur within a dye-sensitized solar cell (DSSC), we synthesized well-defined ZnO:dye dyads. The ZnO nanocrystals were synthesized following literature procedures from zinc acetate and a hydroxide source in ethanol. The absorption onset of the ZnO nanocrystals was observed using UV-Vis measurements, from which estimated nanocrystal diameters were determined. At room temperature, the synthesis yielded nanocrystals ranging in diameter from 2-4 nm. Dispersions of ZnO nanocrystals in ethanol were mixed with separate solutions containing [Ru(bpy)₃](PF₆)₂ , [Ru(bpy)₂(dcbpy)](PF₆)₂ , [Ru(bpy)₂(daeabpy)](PF₆)₄ , ZnPor-COOH , or Ph-3T-COOH where (bpy) = 2,2’-bipyridine, (dcbpy) = 4,4’-dicarboxy-2,2’-bipyridine, (daeabpy) = 4,4’-diaminoethylamide-2,2’-bipyridine, ZnPor-COOH = 5-(4-carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II), and Ph-3T-COOH = 5’’-phenyl-3’,4’-di(ⁿbutyl)-[2,2’:5’,2’’]terthiophene-5-carboxylic acid. Depending on the attached dye, some of the dyads were isolated from solution. Using FT-IR and fluorescence spectroscopy, it was verified that the dye molecules were adsorbed to the ZnO surface via their respective -CO₂^- or -NH₂ groups while the number of dye molecules adsorbed to the surface was quantified using a combination of techniques. The ability of ZnO nanocrystals to quench the emission of the dye due to a charge-transfer reaction was dependent on each dye’s excited-state reduction potential energy and was studied for dyads containing [Ru(bpy)₂(dcbpy)](PF₆)₂ , ZnPor-COOH , or Ph-3T-COOH. For the dyad containing Ph-3T-COOH, a thorough investigation was completed which included adsorption isotherms to probe dye surface coverage, determination of a dye binding constant, and a series of fluorescence quenching experiments. The charge-transfer dynamics of the system were elucidated using ultra-fast laser spectroscopy.


 

1:45 PM *T13.1
Conductivity, Doping and Carrier Mobility in Arrays of Semiconductor Nanocrystals. Jong-Soo Lee, Maksym V. Kovalenko, Jing Huang, Chengyang Jiang and Dmitri Talapin; Department of Chemistry, University of Chicago, Chicago, Illinois.

The development of applications ranging from displays, photovoltaic cells to thermoelectric, light-emitting devices and sensors could be accelerated by introducing lower cost alternatives to conventional single-crystal based semiconductor technology. Chemically synthesized semiconductor nanocrystals are considered promising candidates which allow inexpensive solution-based device fabrication with precise engineering of electronic structure due to quantum size- and shape effects. At the same time, several fundamental problems have to be solved before these materials will compare favorably to the competitive approaches, e.g. organic electronic materials. Many practical implementations of nanocrystals are hindered by the poor electronic coupling in close-packed nanocrystal films, caused by the presence of bulky insulating organic surface ligands. We developed a simple ligand-exchange procedure for complete replacement of original organic ligands by metal chalcogenide complexes such as SnS44-, Sn2S64- SnTe44-, In2Se42-, etc. These surface ligands can serve as “electronic glue” for colloidal nanocrystals. Nanocrystal solids prepared from this new class of colloids show a set of advantages such as all-inorganic design, small interparticle spacing and greatly improved transport properties. Combining semiconductor nanocrystals with other semiconducting materials, metals and magnets either through mixing different nanocrystals or by synthesizing multicomponent nanoscale heterostructures allows tailoring the materials properties.


2:15 PM T13.2
Temperature Dependence of Electrical Properties of PbS Quantum Dot Solar Cells. Rebekah L. Graham, Tong Ju, Yvonne Rodriguez, Sue A. Carter and Glenn B. Alers; Physics, University of California, Santa Cruz, Santa Cruz, California.

The PbS quantum dot materials have been proposed as one of the most promising materials for multiple exciton generation as well as for infrared-active solar cells. While several results have been recently published on PbS-based solar cells, temperature dependent studies have been limited. Here, we study the temperature dependence of solution-based PbS quantum dot solar cells between 100 K and 350 K to gain insight into their charge transport properties and power efficiency. The solar cells under investigation were based on solar devices comprised of ITO/TiO₂/PbS/Au and ITO/PbS/Al. The TiO₂ film was spin coated onto ITO coated substrates. The PbS film was deposited by a dip-coating method in an inert atmosphere, using ethanedithiol (EDT) as a ligand exchange. Electrodes were deposited by evaporation through a shadow mask. After this preparation procedure, each substrate had 6 solar cells, with each solar cell having a top contact area of 3 mm²; therefore, several devices could be tested per substrate to check for reproducibility. The solar cells were cooled in a vacuum less than 10^-2 Torr, and their current-voltage characteristics were measured. The dark current and light current were studied as a function of temperature, and the temperature dependence of the charge transport, open circuit voltage (V_OC), fill factor (ff), and short circuit current density (J_SC) were determined. Preliminary results show that V_OC remains largely temperature independent; however, a substantial increase in J_SC and power efficiency is observed for decreasing temperatures, reaching a maximum around 230 K. These results are in contrast to nanoparticle-polymer blend and all inorganic nanoparticles solar cells that show an increase in J_SC with increasing temperature (above 230 K), consistent with thermally-activated hopping. Consequently, the temperature dependence of PbS quantum dot devices shows transport characteristic of Bloch-like transport and thermal-activated hoping at high and low temperatures, respectively. This effect can be attributed to a crossover between quantum confinement dominating at low temperatures and charge tunneling dominating at high temperatures; therefore, PbS may be an ideal system for probing the balance between quantum confinement and change transport in quantum dot solar cells. Temperature dependent data on a variety of PbS nanoparticles with different sizes and capping, using both white and monochromatic light intensity, will be used to further probe this balance and to understand the possibility of multiple carrier extraction in devices.


2:30 PM T13.3
Temperature Dependent Studies of PbS Nanocrystal Films and Applications in Infrared Imaging. Scott Geyer, Liang-Yi Chang, Darcy D. Wanger and Moungi G. Bawendi; Chemistry, MIT, Cambridge, Massachusetts.

PbS nanocrystal films have emerged as promising materials for photovoltaics and infrared detectors. We present fundamental studies of carrier transport in nanocrystal films and discuss techniques for incorporation of nanocrystal films into focal plane imaging arrays. Temperature dependent field effect transistor measurements and photo response measurements such as time of flight are used to characterize the electronic properties of the nanocrystal film. These measurements allow for separation of the temperature dependence of the mobility and the carrier density. Of particular interest is the role which oxidation plays in altering the density of states within the band gap, since partial oxidation has been used to sensitize PbS NC based infrared photoconductive detectors and improve the open circuit voltage in NC solar cells. PbS NC photoconductive detectors based on lateral gold electrodes are attractive for incorporation into focal plane imaging arrays. Unlike top contact style devices, there is no danger of shorting through the NC film when the top contact is evaporated or sputtered. In addition, all of the clean room and vacuum processing can be completed prior to NC deposition. We characterize the detectivity of lateral PbS photo detector devices which exhibit high gain and correspondingly low bandwidth. We demonstrate that in the frequency range of application for video monitoring, the noise spectrum of the NC based photodetectors is dominated by 1/f noise. This results in decreasing noise at higher frequencies of operation which partially compensates for the low bandwidth of these devices, producing a slowly varying detectivity as a function of operation frequency.


2:45 PM T13.4
Capacitance-Voltage Characterization of Solar Cells With CdS Nanodipole in CdTe Matrix. Jorhan Ordosgoitti¹, Xiangxin Liu², Kristopher A. Wieland², Alvin D. Compaan² and Rashmi Jha¹; ¹Department of Electrical Engineering and Computer Science, University of Toledo, Toledo, Ohio; ²Department of Physics and Astronomy, University of Toledo, Toledo, Ohio.

Capacitance-Voltage (CV) measurement were performed on thin film solar cells fabricated with CdS nanodipole particles uniformly embedded in CdTe material, with TCO and Au as front and back electrodes, respectively. The rationale behind such device is that strongly polarized dipoles in CdS nanoparticle can produce fields comparable to a strong p-n junction, thus providing a route to fabricate simple and higher efficiency solar cells. In our experiment, these solar cells showed well behaved diode characteristics in dark and an efficiency of 8% under 1.5 AM illumination. CV profiling and Drive Level Capacitance Profiling (DLCP) measurements were performed to calculate the doping profile and depletion width. Both of these techniques indicated consistent results. The device was observed to be fully depleted at zero bias. The depletion width was calculated to be 2 um while the carrier density was calculated to be 3.9x1015 cm-3 in reverse bias using DLCP. In order to understand the charging and discharging effect in CdS nanoparticles, the devices were stressed in dark by applying -1.0 V and + 0.7 V for 7 minutes. After the voltage stressing, the CV curves were measured in dark automatically by sweeping the voltage from reverse to forward bias at a frequency of 100 KHz. The reverse bias capacitance was not affected by this stressing condition. The forward bias capacitance decreased by 11% with -1 V stress while increased by 63% with + 0.7 V stress at 1 V forward bias compared to the unstressed devices. These observations will be explained by understanding the fundamental behavior of CdS nanoparticles in CdTe matrix with voltage stressing, low temperature, and variable frequency CV measurement. In our knowledge, this is the first CV based electrical characterization of the nanodipole photovoltaic device composed of CdS nanodipole particle embedded in CdTe matrix. The overall observation indicated that the thin film solar cells fabricated with an optimum dimension and distribution of CdS nanodipole particles in CdTe material holds great potential for achieving cost effective and high efficiency solar cells.


 

3:30 PM T14.1
Effects of CdCl₂ Treatment and Sintering on Photovoltaic Performance, Charge Trapping, and Grain Size of ITO/CdTe/Al Solar Cells. Chris E. France¹, Rebekah L. Graham¹, Anna Bezryadina^2,1, Lily Yang¹, Jeremy D. Olson¹, Sue A. Carter^1,2 and Glenn B. Alers^1,2; ¹Physics Department, University of California-Santa Cruz, Santa Cruz, California; ²Advanced Studies Laboratory, NASA Ames Research Center, Moffett Field, California.

Sintered thin-film inorganic photovoltaic devices produced from nanoparticle solutions are an exciting technology because they have rapid near-atmospheric fabrication similar to organic materials with the potential for bulk-like electrical properties of high-vacuum vapor deposited inorganic films. For our work, nanoparticles suspended in organic solvents are spun-cast onto Indium Tin Oxide (ITO) patterned glass. Upon briefly annealing and sintering, the organic solvent is removed and nanoparticles form larger crystal grains with sharper band-edge absorption and more bulk-like transport properties. Using a single layer (~300nm) of spin-cast CdTe nanorods, and evaporated Al back electrodes we have achieved ITO/CdTe/Al Schottky-type solar cells with over 4.5% power conversion efficiency (PCE). A vital step to produce our best devices is a post-treatment of CdCl₂ and sintering at 400°C for 2-10min. As shown in Table 1, untreated and unsintered films show short-circuit current (J_sc) values 300 times less then the optimally sintered films, while over-sintered films produce high dark currents and no detectable photocurrent or open-circuit voltage (V_oc). Open-circuit voltage and fill factors (FF) are also both increased by greater than 45% during the sintering process. To better understand the device performance's sensitive dependence on sintering time we have measured trapped charge density with an assortment of methods including photothermal deflection spectroscopy (PDS) and charge extraction on devices with a series of sintering times from 0-10min. In both PDS and charge extraction methods we have found that the amount of trapped charge increases over several orders of magnitude as the devices range from unsintered, thru optimally sintered, and to over-sintered. The PDS and charge extraction results agree with capacitance versus voltage (CV) measurements that show an increase in device capacitance with an increase in sintering time.


3:45 PM T14.2
Hybrid Optoelectronics for Photovoltaic and Light Emitting Applications. Junis Rindermann, Soontorn Chanyawadee and Pavlos Lagoudakis; School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom.

The brightness, large absorption cross-section and flexibility of colloidal nanocrystal quantum-dots (NQDs) renders them promising new materials for light harvesting and light emitting applications. However, colloidal NDQs are plagued by low charge-transfer efficiency that limits the overall power conversion efficiency of these materials in photovoltaic devices (PVs) and light emitting diodes (LEDs) when compared to their epitaxial p-n junction based counterparts. A route to circumvent altogether issues associated to low charge-transfer in NQDs is to engineer devices that utilise alternative pumping schemes to electrical injection and transport while still benefiting from their large oscillator strength. In nature, funnelling of energy between different chromophores predominantly occurs through a non-radiative dipole-dipole coupling mechanism, first studied by Förster, that does not involve charge transfer or emission and absorption of photons between donor and acceptor and that can exceed the radiative energy transfer routinely used in phosphor light emitting devices. Here we will present recent advances in the field of hybrid optoelectronics where non-radiative energy transfer is used to combine the high carrier mobility of single crystal inorganic semiconductor heterostructures and the versatility offered by colloidal NQDs both in light harvesting and light emitting applications [1-5]. Recently we observed experimental evidence of the above mechanism in hybrid semiconductor heterostructures under optical excitation between carriers in a single semiconductor quantum well and a vicinal layer of NQDs or organic molecules [1, 2]. Towards a real world electrically interconnected device we also reported a novel design fabrication route for hybrid PVs that utilizes non-radiative energy transfer to extract carriers from NQDs and efficiently transfer them into a single crystal p-i-n structure, resulting in a strong enhancement of the measured photocurrent [3, 4]. In the reverse configuration we also reported on a novel method to integrate non-radiative energy transfer in colour conversion lighting by depositing bright NQDs on surface-patterned GaN-based LEDs [5]. Unlike in conventional colour conversion LEDs, a deep pattern on the surface brings NQDs (acceptors) into the close vicinity of the active layers (donors) and enhances the NQDs electroluminescence due to efficient non-radiative energy transfer, which is proved by time resolved spectroscopy. Finally we will discuss our most recent currently unpublished work on real world hybrid NQDs/LEDs and NQDs/PVs utilising non-radiative energy transfer and compare their properties with their bare counterparts. [1] Rohrmoser et al, Appl. Phys. Lett. 91, 092126 (2007) [2] Chanyawadee et al Phys. Rev. B 77, 193402 (2008) [3] Chanyawadee et al Phys. Rev. Lett. 102, 077402 (2009) [4] Chanyawadee et al Appl. Phys. Lett. 94, 233502 (2009) [5] Chanyawadee et al Adv. Mat. in print 10.1002/adma.200902262 (2009)


4:00 PM T14.3
Interface Passivation and the Role of Interface Defects of Silicon Nanocrystals for Future Photovoltaic Applications. Daniel Hiller¹, Mihaela Jivanescu², Andre Stesmans² and Margit Zacharias¹; ¹Nanotechnology group, University of Freiburg - IMTEK (Department of Microsystems Engineering), Freiburg, Germany; ²Department of Physics and Astronomy, Katholieke Universiteit Leuven, Leuven, Belgium.

Silicon nanocrystals (SiNC) in insulating matrix are a promising candidate for nanoparticle based photovoltaics in all-silicon tandem cells. In this concept a standard bulk silicon solar cell (bandgap 1.1 eV) is combined with a silicon nanocrystals based solar cell of around 1.8 eV bandgap increasing the overall conversion efficiency theoretically up to 42.5% [1]. In order to achieve quantum confined silicon based materials a precise size control of the SiNC is mandatory. A reliable growth process for this challenge is based on the SiO/SiO2 superlattice approach [2]. The fundamental research of SiNC properties and in particular the origin of the luminescence experienced recently a crucial step forward. In [3] the role of quantum confinement and Si-SiO2 interface defects was investigated and the implications on the luminescence were identified. In the study presented here we demonstrate the crucial role of a reliable interface passivation for quantum confinement. The role of the annealing atmosphere (nitrogen or argon) on the interface properties will be presented in detail based on size controlled nanocrystals. While argon stays absolutely inert even at high temperatures the nitrogen gains ability to react with Si-SiO2 material in particular at the nanocrystal-matrix interface. Clear evidence of the presence of nitrogen at the nanocrystal interface will be shown by means of time of flight secondary ion mass spectroscopy (TOF-SIMS), infrared spectroscopy (FTIR) and elastic recoil detection analysis (ERDA). The results indicate that nitrogen plays a considerable role for optical properties and interface defects as will be demonstrated by a combined study of electron spin resonance (ESR) cross linked to optical properties. Implications for photovoltaics will be discussed. [1] G. Conibeer et al, Thin Solid Films 516 (20), 6748-6756 (2008) [2] M. Zacharias et al., Appl. Phys. Lett. 80 (4), 661-663 (2002) [3] S. Godefroo et al., Nature Nanotechnology 3, 174-178 (2008)


4:15 PM T14.4
Utilizing Semiconducting Polymer Nanoparticles for Organic Solar Cells. David F. Kavulak¹, Jill E. Millstone¹, Claire Woo², Thomas Holcombe¹ and Jean Frechet^1,2; ¹Chemistry, University of California, Berkeley, Berkeley, California; ²Chemical Engineering, University of California Berkeley, Berkeley, California.

In the field of solution processed organic photovoltaics, a bulk heterojunction (BHJ) morphology is an efficient architecture for many donor-acceptor pairs, but requires the formation of a continuous, interpenetrating, nanoscale morphology. The kinetic trapping of what is often a non-thermodynamically favored morphology has led to many difficulties in the solution processing of efficient donor-acceptor BHJs. The ability to independently control the nanoscale size of the donor and acceptor domains before device fabrication would provide a universal route to bulk heterojunction fabrication with any donor-acceptor system. We demonstrate the surfactant-free synthesis and characterization of sub-30 nanometer, size-controlled, semi-crystalline polymer nanoparticles. By controlling the physical parameters of the initial polymer, we are able to tune both the average particle size and degree of crystallinity. The development of these materials allow us to separately control both the domain size and domain crystallinity of bulk heterojunction donor-acceptor pairs. Nanoparticle size and crystallinity will be examined and correlated to thin film transistor and solar cell device performance.


4:30 PM T14.5
Nanostructured Solar Cell Sensitized With CdSe Quantum Dots. Athavan Nadarajah, Robert C. Word and Rolf Koenenkamp; Physics, Portland State University, Portland, Oregon.

In recent years there have been significant investigations to create solar cells based on nanostructured materials. The surface enlargement in nanostructured materials presents significant advantages for light absorption and charge separation, two critical steps for solar-to-electric energy conversion. Solar cells that make use of nanostructured wide bandgap semiconductor films photosensitized with semiconducting quantum dots have emerged as a promising and potentially low-cost alternative to the traditional photovoltaic devices. Nanocrystalline CdSe has a bandgap of 1.7 eV with suitable valence and conduction band positions for photo-generated carriers transfer to n-type ZnO and organic p-type materials. As a result, CdSe quantum dots-sensitized ZnO nanowire solar cell could offer new and enhanced opportunities to harvest light energy in the whole visible region of solar light. In this work, we present a combined morphological, electrical and optical study of a solar cell based on ZnO nanowires and CdSe quantum dots. The growth of n-type ZnO nanowires involves electrodeposition at 80^oC on a compact ZnO thin film and doping with Al from the electrolyte solution. A wet chemical method was used for synthesizing the quantum dots. Spray pyrolysis was carried out for depositing the quantum dots on the nanowires as well as creating a compact ZnO thin film on the FTO/glass substrate to act as a seed layer for the growth of the nanowires. The deposition of a thin compact ZnO layer leads to a significant increase in solar cell performance by avoiding subsequent short-circuiting between front contact and p-type semiconductor layer. Microscopic studies show the conversion of CdSe quantum dots into an inter-connected and continuous polycrystalline thin film coating on the ZnO nanowires upon annealing in CdCl₂/ambient air. This structural change of the CdSe quantum dot layer provides excellent charge transfer between the absorber layer and the adjacent layers is observed. The back contacts are a spin/drop coated hole-conducting material and thermally evaporated Au layer. Several intermittent annealing steps at moderate temperatures are applied. The optimized quantum dots sensitized solar cell has exhibited an external quantum efficiency as high as 65% and an overall conversion efficiency of 2.1%.