Showing papers on "Thin film published in 2012"

Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%

Abstract: We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH3NH3)PbI3 as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI2 and deposited onto a submicron-thick mesoscopic TiO2 film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (JSC) exceeding 17 mA/cm2, an open circuit photovoltage (VOC) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH3NH3)PbI3 NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO2 film. The use of a solid hole conductor dramatically improved the device stability compared to (CH3NH3)PbI3 -sensitized liquid junction cells.

Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors

Abstract: We present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of 2400 m2 g–1 with a high in-plane electrical conductivity of 5880 S m–1. This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor using these carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent series resistance on the order of 0.1 ohm, and a high-power delivery of about 500 kW kg–1. While higher frequency and power values for graphene materials have been reported, these are the highest values achieved while simultaneously maintaining excellent specific capacitances and energy densities of 120 F g–1 and 26 W h kg–1, respectively. In addition, these free-standing thin films provide a route to simplify the electrode-manufacturing process by eliminating...

Fabrication of flexible MoS2 thin-film transistor arrays for practical gas-sensing applications.

Abstract: By combining two kinds of solution-processable two-dimensional materials, a flexible transistor array is fabricated in which MoS2 thin film is used as the active channel and reduced graphene oxide (rGO) film is used as the drain and source electrodes. The simple device configuration and the 1.5 mm-long MoS2 channel ensure highly reproducible device fabrication and operation. This flexible transistor array can be used as a highly sensitive gas sensor with excellent reproducibility. Compared to using rGO thin film as the active channel, this new gas sensor exhibits much higher sensitivity. Moreover, functionalization of the MoS2 thin film with Pt nanoparticles further increases the sensitivity by up to ∼3 times. The successful incorporation of a MoS2 thin-film into the electronic sensor promises its potential application in various electronic devices.

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

Abstract: Hematite (α-Fe2O3) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical–chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. T...

Graphene and boron nitride lateral heterostructures for atomically thin circuitry

Abstract: Precise spatial control over the electrical properties of thin films is the key capability enabling the production of modern integrated circuitry. Although recent advances in chemical vapour deposition methods have enabled the large-scale production of both intrinsic and doped graphene, as well as hexagonal boron nitride (h-BN), controlled fabrication of lateral heterostructures in these truly atomically thin systems has not been achieved. Graphene/h-BN interfaces are of particular interest, because it is known that areas of different atomic compositions may coexist within continuous atomically thin films and that, with proper control, the bandgap and magnetic properties can be precisely engineered. However, previously reported approaches for controlling these interfaces have fundamental limitations and cannot be easily integrated with conventional lithography. Here we report a versatile and scalable process, which we call 'patterned regrowth', that allows for the spatially controlled synthesis of lateral junctions between electrically conductive graphene and insulating h-BN, as well as between intrinsic and substitutionally doped graphene. We demonstrate that the resulting films form mechanically continuous sheets across these heterojunctions. Conductance measurements confirm laterally insulating behaviour for h-BN regions, while the electrical behaviour of both doped and undoped graphene sheets maintain excellent properties, with low sheet resistances and high carrier mobilities. Our results represent an important step towards developing atomically thin integrated circuitry and enable the fabrication of electrically isolated active and passive elements embedded in continuous, one-atom-thick sheets, which could be manipulated and stacked to form complex devices at the ultimate thickness limit.

Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride

Abstract: The complex refractive index components, n and k, have been studied for thin films of several common dielectric materials with a low to medium refractive index as functions of wavelength and stoichiometry for mid-infrared (MIR) wavelengths within the range 1.54–14.29 μm (700–6500 cm−1). The materials silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and titanium oxide are prepared using room temperature reactive sputter deposition and are characterized using MIR variable angle spectroscopic ellipsometry. The investigation shows how sensitive the refractive index functions are to the O2 and N2 flow rates, and for which growth conditions the materials deposit homogeneously. It also allows conclusions to be drawn on the degree of amorphousness and roughness. To facilitate comparison of the materials deposited in this work with others, the index of refraction was also determined and provided for the near-IR and visible ranges of the spectrum. The results presented here should serve as a useful information base for designing optical coatings for the MIR part of the electromagnetic spectrum. The results are parameterized to allow them to be easily used for coating design.

Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

Abstract: The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

Incipient Ferroelectricity in Al-Doped HfO2 Thin Films

Abstract: Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric-to-ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non-centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with "Co−Pi"-Coated Hematite Electrodes

Abstract: Uniform thin films of hematite (α-Fe(2)O(3)) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, "Co-Pi," were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co-Pi thickness were performed in order to clarify the mechanism by which Co-Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co-Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co-Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co-Pi catalyst coupled with semiconductor electrodes for water-splitting applications.

Wafer-Scale MoS2 Thin Layers Prepared by MoO3 Sulfurization

Abstract: Atomically thin molybdenum disulfide (MoS2) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS2 atomic thin layers is still lacking. Here we report that wafer-scale MoS2 thin layers can be obtained using MoO3 thin films as a starting material followed by a two-step thermal process, reduction of MoO3 at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS2 films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.

Structure, physical properties, and applications of SrRuO3 thin films

Abstract: SrRuO3 is endowed with three remarkable features. First, it is a moderately correlated material that exhibits several novel physical properties; second, it permits the epitaxial growth of essentially single-crystal films; and third, because it is a good conductor, it has attracted interest as a conducting layer in epitaxial heterostructures with a variety of functional oxides. In this review, the present state of knowledge of SrRuO3 thin films is summarized. Their role as a model system for studying magnetism and electron transport characterized by intermediate electron correlation and large magnetocrystalline anisotropy is demonstrated. The materials science of SrRuO3 thin film growth is reviewed, and its relationship to electronic, magnetic, and other physical properties is discussed. Finally, it is argued that, despite all that has been learned, a comprehensive understanding of SrRuO3 is still lacking and challenges remain.

Lithium niobate on insulator (LNOI) for micro-photonic devices

Abstract: The state-of-the-art of high-refractive-index-contrast single-crystalline thin lithium niobate (LiNbO3) films as a new platform for high-density integrated optics is reviewed. Sub-micrometer thin LiNbO3 films are obtained by “ion-slicing”. They can be bonded by two different techniques to a low-index substrate to obtain “lithium niobate on insulator” (LNOI) even as wafer of 3” diameter. Different micro- and nano-structuring techniques have been used to successfully develop micro-photonic devices. To be specific, the fabrication and characterization of LNOI photonic wires with cross-section < 1 µm2, periodically poled LNOI photonic wires for second harmonic generation, electro-optically tunable microring resonators, free standing microrings for hybrid integration, and photonic crystal structures are described.

Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy

Abstract: Hydrogen generation through photoelectrochemical (PEC) water splitting using solar light as an energy resource is believed to be a clean and efficient way to overcome the global energy and environmental problems. Extensive research effort has been focused on n-type metal oxide semiconductors as photoanodes, whereas studies of p-type metal oxide semiconductors as photocathodes where hydrogen is generated are scarce. In this paper, highly efficient and stable copper oxide composite photocathode materials were successfully fabricated by a facile two-step electrochemical strategy, which consists of electrodeposition of a Cu film on an ITO glass substrate followed by anodization of the Cu film under a suitable current density and then calcination to form a Cu2O/CuO composite. The synthesized Cu2O/CuO composite was composed of a thin layer of Cu2O with a thin film of CuO on its top as a protecting coating. The rational control of chemical composition and crystalline orientation of the composite materials was easily achieved by varying the electrochemical parameters, including electrodeposition potential and anodization current density, to achieve an enhanced PEC performance. The best photocathode material among all materials prepared was the Cu2O/CuO composite with Cu2O in (220) orientation, which showed a highly stable photocurrent of −1.54 mA cm−2 at a potential of 0 V vs reversible hydrogen electrode at a mild pH under illumination of AM 1.5G. This photocurrent density was more than 2 times that generated by the bare Cu2O electrode (−0.65 mAcm−2) and the stability was considerably enhanced to 74.4% from 30.1% on the bare Cu2O electrode. The results of this study showed that the top layer of CuO in the Cu2O/CuO composite not only minimized the Cu2O photocorrosion but also served as a recombination inhibitor for the photogenerated electrons and holes from Cu2O, which collectively explained much enhanced stability and PEC activity of the Cu2O/CuO composite. Thus, the electrochemical strategy proposed in this study for the synthesis of the Cu2O/CuO composite opens a new way to use copper oxides as photocathode materials in PEC cells for a highly stable and effective water splitting.

Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor

Abstract: Different nanostructures (Ns), such as nanobelts, nanobricks and nanosheets, of polypyrrole (PPy) were successfully fabricated on stainless steel substrates by simply varying the scan rate of deposition in the potentiodynamic mode. These PPy Ns were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and surface area measurement. The XRD analysis showed the formation of amorphous PPy thin films, and the FTIR studies confirmed characteristic chemical bonding in the PPy materials. SEM images depicted that a high scan rate of deposition can form multilayer nanosheets with high porosity leading to a system with excellent processability. The PPy nanosheets possess a higher Brunauer-Emmett-Teller (BET) surface area of 37.1 m 2 g -1 than PPy nanobelts and nanobricks. The supercapacitive performances of different PPy Ns were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge techniques in 0.5 M H 2SO 4. A maximum specific capacitance of 586 F g -1 was obtained for multilayer nanosheets at a scan rate of 2 mV s -1. In addition, impedance measurements of the different Ns of PPy electrodes were performed suggesting that the PPy electrodes with multilayer nanosheets are promising materials for the next generation high performance electrochemical supercapacitors.

Metamaterial-Plasmonic Absorber Structure for High Efficiency Amorphous Silicon Solar Cells

Abstract: We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.

Polymeric multilayer capsules for drug delivery.

Abstract: The advent of Layer-by-Layer (LbL) assembly to fabricate polymeric as well as hybrid multilayer thin films has opened exciting avenues for the design of multifunctional drug carriers with extreme control over their physico-chemical properties. These polymeric multilayer capsules (PMLC) are typically fabricated by sequential adsorption of polymers onto a spherical substrate with dimensions varying from 10 nm to several microns and larger. In this critical review, we give an overview of the recent advances in the field of PMLC with respect to drug delivery and point out how sophisticated capsule engineering can lead to well-defined drug carriers with unique properties (139 references).

Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition

Abstract: Atomic layer deposition (ALD) is a technique capable of producing ultrathin conformal films with atomic level control over thickness. A major drawback of ALD is its low deposition rate, making ALD less attractive for applications that require high throughput processing. An approach to overcome this drawback is spatial ALD, i.e., an ALD mode where the half-reactions are separated spatially instead of through the use of purge steps. This allows for high deposition rate and high throughput ALD without compromising the typical ALD assets. This paper gives a perspective of past and current developments in spatial ALD. The technology is discussed and the main players are identified. Furthermore, this overview highlights current as well as new applications for spatial ALD, with a focus on photovoltaics and flexible electronics. 2012 American Vacuum Society.

Efficient light trapping in inverted nanopyramid thin crystalline silicon membranes for solar cell applications.

Abstract: Thin-film crystalline silicon (c-Si) solar cells with light-trapping structures can enhance light absorption within the semiconductor absorber layer and reduce material usage. Here we demonstrate that an inverted nanopyramid light-trapping scheme for c-Si thin films, fabricated at wafer scale via a low-cost wet etching process, significantly enhances absorption within the c-Si layer. A broadband enhancement in absorptance that approaches the Yablonovitch limit (Yablonovitch, E. J. Opt. Soc. Am.1987, 72, 899–907 ) is achieved with minimal angle dependence. We also show that c-Si films less than 10 μm in thickness can achieve absorptance values comparable to that of planar c-Si wafers thicker than 300 μm, amounting to an over 30-fold reduction in material usage. Furthermore the surface area increases by a factor of only 1.7, which limits surface recombination losses in comparison with other nanostructured light-trapping schemes. These structures will not only significantly curtail both the material and proc...

High-efficiency solution-processed Cu2ZnSn(S,Se)4 thin-film solar cells prepared from binary and ternary nanoparticles.

Abstract: A new solution-based method to fabricate Cu2ZnSn(S,Se)4 (CZTSSe) thin films is presented. Binary and ternary chalcogenide nanoparticles were synthesized and used as precursors to form CZTSSe thin films. The composition of the CZTSSe films can be easily controlled by adjusting the ratio of the nanoparticles used. The effect of compositional adjustment on device performance is illustrated. Laboratory-scale photovoltaic cells with 8.5% total-area efficiency (or 9.6% active-area efficiency) were demonstrated without anti-reflective coatings. Material characterization data revealed the formation of a bilayer microstructure during thermal processing and suggested a path forward on device improvement.

Synthesis, Characterization, and Transistor and Solar Cell Applications of a Naphthobisthiadiazole-Based Semiconducting Polymer

Abstract: We report the synthesis and characterization of a novel donor–acceptor semiconducting polymer bearing naphthobisthiadiazole (NTz), a doubly benzothiadiazole (BTz)-fused ring, and its applications to organic field-effect transistors and bulk heterojunction solar cells. With NTz’s highly π-extended structure and strong electron affinity, the NTz-based polymer (PNTz4T) affords a smaller bandgap and a deeper HOMO level than the BTz-based polymer (PBTz4T). PNTz4T exhibits not only high field-effect mobilities of ∼0.56 cm2/(V s) but also high photovoltaic properties with power conversion efficiencies of ∼6.3%, both of which are significantly high compared to those for PBTz4T. This is most likely due to the more suitable electronic properties and, importantly, the more highly ordered structure of PNTz4T in the thin film than that of PBTz4T, which might originate in the different symmetry between the cores. NTz, with centrosymmetry, can lead to a more linear backbone in the present polymer system than BTz with ax...

Applications of magnetoelectrics

Abstract: A short review of the development of magnetoelectrics is presented, primarily of recent work (2009–2012). Emphasis is upon the different applications for different generic types: for example, single-phase thin films for magnetic tunnel junctions but thick sandwich-structure multilayers and nano-composites for weak magnetic field detectors.

Thickness-Independent Transport Channels in Topological Insulator Bi 2 Se 3 Thin Films

Abstract: With high quality topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to $\ensuremath{\sim}8\text{ }\text{ }\mathrm{QL}$ (where QL refers to quintuple layer, $1\text{ }\text{ }\mathrm{QL}\ensuremath{\approx}1\text{ }\text{ }\mathrm{nm}$). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at $\ensuremath{\sim}3.0\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ only down to $\ensuremath{\sim}8\text{ }\text{ }\mathrm{QL}$. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.

Active MnOx Electrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions

Abstract: The ability to deposit conformal catalytic thin films enables opportunities to achieve complex nanostructured designs for catalysis. Atomic layer deposition (ALD) is capable of creating conformal thin films over complex substrates. Here, ALD-MnOx on glassy carbon is investigated as a catalyst for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), two reactions that are of growing interest due to their many applications in alternative energy technologies. The films are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, ellipsometry, and cyclic voltammetry. The as-deposited films consist of Mn(II)O, which is shown to be a poor catalyst for the ORR, but highly active for the OER. By controllably annealing the samples, Mn2O3 catalysts with good activity for both the ORR and OER are synthesized. Hypotheses are presented to explain the large difference in the activity between the MnO and Mn2O3 catalysts for the ORR, but similar activity for the OER, including the effects of surface oxidation under experimental conditions. These catalysts synthesized though ALD compare favorably to the best MnOx catalysts in the literature, demonstrating a viable way to produce highly active, conformal thin films from earth-abundant materials for the ORR and the OER.