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We report on studies of device degradation in organic photovoltaic devices based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Since delamination, oxidation, and chemical interactions at the metal electrode/organic interface have long been posited as degradation pathways in organic electronic devices, we first investigated the stability of a variety of electrodes for devices stored in an inert, dark environment. Second, a set of experiments was designed to separate the effects at the metal/organic interface from the degradation of the active layer or the hole extraction interface. To do this, Ca/Al electrodes were deposited to complete half of a substrate's devices, and samples were left both under constant illumination and 10% illumination (10% duty cycle of 1 sun illumination) in a glovebox environment. After more than 200 h of measurement, additional electrodes were deposited and device performance of each set was compared. Third, to assess the degree of degradation caused by photo-induced processes, device stability in an inert atmosphere under constant illumination, and 10% illumination conditions was also investigated. Last, various degradation mitigation strategies in air under constant illumination were explored. The results showed that the active layer itself is not inherently unstable on the timescales studied here. Choosing the appropriate electrode (Ca/Al) reduced interfacial degradation, storing the active layer in an inert, dark environment did not cause significant degradation, and storing the active layer under constant illumination caused only a limited reduction in performance. Our results indicate that the metal/organic interface can be a significant source of degradation in the devices, and we discuss approaches that could reduce this instability.

Proper contact and band alignment across the heterojunction interface is critical to the performance of hybrid pi-conjugated polymer-nanostructured metal oxide photovoltaic devices. We investigated the effect of ZnO surface processing on ZnO/poly(3-hexylthiolphene) device performance. Devices in which the ZnO films/nanorods were heated at 150 °C in air exhibited an open circuit voltage ∼200 mV higher than devices with ZnO treated by UV/ozone. We conclude that the differences observed in the two types of devices predominantly arise from changes in interfacial dipoles, and intermediate materials processing can be critical to solar cell performance.

The direct conversion of solar energy to electricity by photovoltaic cells or thermal energy in concentrated solar power systems is emerging as a leading contender for next-generation green power production. The photovoltaics (PV) area is rapidly evolving based on new materials and deposition approaches. At present, PV is predominately based on crystalline and polycrystalline Si and is growing at >40% per year with production rapidly approaching 3 gigawatts/year with PV installations supplying

Nb doped TiO 2 (Nb:TiO 2 ) is a promising indium-free transparent conducting oxide. We have examined the growth of Nb:TiO 2 thin films by pulsed laser deposition (PLD) on SrTiO 3 , LaAlO 3 , and fused silica. For oriented anatase Nb:TiO 2 films grown on SrTiO 3 by PLD at 550 °C, the conductivity can be as high as 2500 S/cm. A nearly thickness independent conductivity for Nb:TiO 2 demonstrates that the conductivity is a bulk property and not a substrate interface effect. In addition, Nb:TiO 2 films deposited at room temperature were annealed at temperatures up to 750 °C in either vacuum or 1.3 × 10 − 3 Pa O 2 . For these films, conductivities as high as 3300 S/cm on SrTiO 3 and 85 S/cm on LaAlO 3 substrates were obtained for the highest temperature vacuum anneals, albeit with some loss in transparency.

The photoconductivity of solution-cast Zn1–xMgxO (x=0-0.4) and poly(3-hexylthiophene) (P3HT) thin films, and Zn1-xMgxO/P3HT bilayers is investigated using Time-Resolved Microwave Conductivity (TRMC) with the aim of determining the locus of free charge carrier generation in the bilayer system. The photoconductivity of Zn1–xMgxO thin films, under illumination with 300 nm laser pulses, is limited by the formation of stable excitons and by scattering of the carriers at grain boundaries. The electron mobility in Zn1–xMgxO films decreases exponentially with Mg concentration, up to x=0.4. In agreement with previous work, free carriers are observed in the P3HT film under illumination with 500 nm pulses in the absence of an acceptor. Under illumination with 500 nm pulses, where only the polymer absorbs, the TRMC signal for the Zn1–xMgxO/P3HT bilayers for x≥0.2 is the same as that of pure P3HT, indicating that free carrier generation in these bilayers occurs predominately by exciton dissociation in the polymer bulk, and not at the interface between the polymer and the solution-cast oxide. At lower Mg concentrations (x

We present detailed investigations of the fabrication and characterization of photovoltaic devices consisting of poly(3-hexylthiophene) (P3HT) intercalated into a mesoporous structure of ZnO nanofibers. ZnO nanofibers were grown via a low-temperature hydrothermal route from a solution of zinc nitrate precursor. P3HT was spin-coated on top of the structure, and intercalation into the voids between the nanofibers was induced with annealing. A silver electrode was used as the top contact. Spin-coating, storage, and testing of the device were performed in air. We discuss the effects of atmosphere and ZnO nanofiber morphology on device performance. Optimized nanofiber devices exhibited a 4-fold increase in the short circuit current (2.17 mA/ cm 2 ) as compared to that of a planar ZnO-P3HT bilayer device (0.52 mA/cm 2 ) as a result of the increased donor-acceptor interfacial area. The efficiency of the nanofiber based device under 1 sun-simulated solar illumination was 0.53% and was found to increase at higher incident light intensities, reaching a value of 0.61% at 2.5 suns. Additionally, we found that for these devices fabrication in and exposure to air is required to obtain good diode characteristics. We also show that the morphology of the ZnO nanostructures in the nanocomposite directly impacts device performance. Treatment of the ZnO surface using surfactants increased the open circuit voltage at the expense of the short circuit current; however, there was little effect on overall device efficiency. Because of the inverted geometry of this device that allowed for the use of a silver top contact, the device was not susceptible to oxidative degradation when stored in the dark.

Effective infiltration of the polymer into the nanostructured oxide is critical for optimizing the performance of hybrid π-conjugated polymer/nanostructured metal oxide semiconductor photovoltaic devices. We investigated the effect of polymer processing parameters, solvent selection, and thermal annealing on poly(3-hexylthiophene) (P3HT)/ZnO nanorod photovoltaic devices and found that these play an important role in the degree of polymer infiltration and the subsequent device performance. We demonstrate that using dichlorobenzene as a solvent produced better performance devices than using chloroform. In addition, the infiltration of P3HT into the ZnO nanorod array has been improved through annealing and subsequent slow cooling. Time-resolved microwave conductivity studies reveal an increase in the photoconductivity of the composite devices with annealing, resulting from changes in both the polymer and ZnO. The device performance was shown to increase with enhanced infiltration, and the devices that had be...

We report on the incorporation of a low band gap copolymer based on thiophene and benzothiadiazole with a band gap of 1.65 eV into a bulk heterojunction device with the structure ITO/PEDOT:PSS/polymer:PCBM/Al. We have investigated the effects of process variable, including choice of solvent, polymer concentration and annealing temperature, on the photovoltaic device performance. The devices show spectral response down to the absorption edge of 1.65 eV and exhibit an efficiency of 1% under AM1.5 illumination and a peak external quantum efficiency of 18% at 600 nm.

Nanoporous titania (TiO2) or titania nanotubes could provide a continuous nanostructured electron-conducting anode for organic photovoltaics. In this work, nanoporous titania was formed by anodizing thin films of titanium on both glass and transparent conducting oxide (TCO) substrates. Titanium thin films (500–700 nm) were deposited by radio frequency (RF) magnetron sputtering. Films were anodized in acidic electrolytes containing small amounts of hydrofluoric acid (HF) at constant voltages ranging from 7 to 15 V. Scanning electron microscope (SEM) analysis revealed a nanoporous structure. Nanoporous titania structures were grown on glass in an electrolyte containing sulfuric acid, trisodium citrate, and potassium fluoride, with pore diameters around 50 nm. Analyzing the films at different anodization times, the stages of nanopore formation were elucidated. Additionally, nanoporous titania was formed on a TCO substrate by anodizing in an electrolyte containing acetic acid and hydrofluoric acid. While not completely transparent, the nanoporous titania is promising for use in organic photovoltaics.

Composite transparent contacts (TCs) based on metal nanowires and metal oxide matrix materials hold great promise for high performance transparent contacts for photovoltaics and opto-electronic technologies with the potential of all-atmospheric pressure processing. The metal nanowire mesh can provide both electrical conductivity and mechanical robustness against bending while the matrix material can both control the electrical interface and protect the metal nanowires. Here, we demonstrate all atmospheric pressure processed Ag-NW/ZnO composite TCs that are 90% transparent in the visible with sheet resistance Rs ≈ 10 Ohms/sq. In addition, the composite TCs have higher infrared transmission than conventional TCO films with the same sheet resistance.

The front contact metallization of Si solar cells begins with printing a mixture of an Ag powder, glass frit (mixture of metal oxides such as PbO, SiO2, B2O3, and Bi2O3) and an organic binder over the antireflection coating that is subsequently fired up to about 825 °C. It is known that the frit allows the paste to react with and burn through the anti-reflective coating such that the metal can react with the underlying c-Si during firing. However, the precise phase transformations between Ag, Si, SiNx, and the frit constituents, which happen within a few seconds during rapid thermal processing (RTP), giving rise to an Ag-Si contact, are not well understood in the absence of in-situ characterization under the actual processing conditions. We have carried out in-situ x-ray diffraction studies on sample mixtures of different component powders (Ag, sinks, PbO-fruit and Si) under realistic processing conditions using an in-situ rapid thermal processing setup. We track the phase progression and reaction pathways at a time resolution of 100 milliseconds. We show the direct evidence of SiNx oxidation by PbO between 600–750 °C. On subsequent heating to higher temperature, up to 825 °C, Ag dissolves into the frit etches the c-Si surface and is deposited on etch pits forming intimate electrical contacts.

NREL and DOE recently held a workshop to assess the challenges, opportunities and potential impacts for improved transparent contacts (TCs) to positively impact current and emerging photovoltaic conversion technologies. Here, we report on the workshop outcomes based on the collective input and participation from industry, academia, national laboratories and DOE. A primary conclusion is that new emerging materials can have significant impacts on the overall performance, reliability and cost for commercial scale PV. One key observation is that TC's should no longer be thought of as a single-layer single-purpose material but as an integrated contact layer stack that includes a charge selective interface layer, a conducting layer and sometimes layers for reliability or light management. In addition, the long standing goal of developing of high performance atmospheric-pressure-processed TCs has finally been met by the rapidly improving Ag nanowire based composites with better than ITO performance from an all ink based process. Even the more conventional metal oxide materials are improving rapidly by introducing solution processed buffer layers, and even conductor layers. This rapid evolution has opened the way to high-throughput low-cost TC processing. Key desired metrics for TCs and approaches to achieving them are discussed.

Both structure-property and composition-structure relationships are exploited to design and realize novel wurtzite-structure Mn1-xZnxO alloys. A proof of principle is provided that corroborates, in particular, the predicted favorable hole-transport properties of these alloys.

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

The emergence of methyl-ammonium lead halide (MAPbX3) perovskites motivates the identification of unique properties giving rise to exceptional bulk transport properties, and identifying future materials with similar properties. Here, we propose that this "defect tolerance" emerges from fundamental electronic structure properties, including the orbital character of the conduction and valence band extrema, the effective masses, and the static dielectric constant. We use MaterialsProject.org searches and detailed electronic-structure calculations to demonstrate these properties in other materials than MAPbX3. This framework of materials discovery may be applied more broadly, to accelerate discovery of new semiconductors based on emerging understanding of recent successes., Comment: 4 Figures, 1 Table. Appears in MRS Communications (2015), 1-11

We propose a new approach to growing photovoltaic-quality crystal silicon films on glass. Other approaches to film Si focus on increasing grain size in order to reduce the deleterious effects of grain boundaries. Instead, we propose aligning the silicon grains biaxially (both in and out of plane) so that (1) grain boundaries are ‘low-angle’ and have less effect on the electronic properties of the material and (2) subsequent epitaxial thickening is simplified. They key to our approach is the use of a foreign template layer that can be grown with biaxial texture directly on glass by a technique such as ion-beam-assisted deposition or inclined substrate deposition. After deposition of the template layer, silicon is then grown aligned to the template and subsequently thickened. Here, we outline this new approach to silicon on glass, describe initial experimental results and discuss challenges that must be overcome.

Organic semiconductor-based photovoltaic devices offer the promise of a low-cost photovoltaic technology that could be manufactured via large-scale, roll-to-roll printing techniques. Existing organic photovoltaic devices have currently achieved solar power conversion efficiencies greater than 3%. Although encouraging, the reasons higher efficiencies have not been achieved are poor overlap between the absorption spectrum of the organic chromophores and the solar spectrum, non-ideal band alignment between the donor and acceptor species, and low charge carrier mobilities resulting from the disordered nature of organic semiconductors. To address the latter issues, we are investigating the development of nanostructured oxide/conjugated polymer composite photovoltaic (PV) devices. These composites can take advantage of the high electron mobilities attainable in oxide semiconductors and can be fabricated using low-temperature solution-based growth techniques. Additionally, the morphology of the composite can be controlled in a systematic way through control of the nanostructured oxide growth. ZnO nanostructures that are vertically aligned with respect to the substrate have been grown. Here we discuss the fabrication of such nanostructures and present results from ZnO nanofiber/poly(3-hexylthiophene) (P3HT) composite PV devices. The best performance with this cell structure produced an open circuit voltage (Voc) of 440 mV, a short circuit current density (Jsc) of 2.2 mA/cm2, a fill factor (FF) of 0.56, and a conversion efficiency (η) of 0.53%. Incorporation of a blend of P3HT and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) into the ZnO nanofibers produced enhanced performance with a Voc of 475 mV, Jsc of 10.0 mA/cm2, FF of 0.43, and η of 2.03%. The power efficiency is limited in these devices by the large fiber spacing and the reduced Voc.

The interface of planar TiO(2)/polymer photovoltaic cells was modified with two carboxylated polythiophenes having different densities of carboxylic acid groups. Both of the interface modifiers increase the photocurrent of the cells but lower the open-circuit voltage. The work function of the TiO(2), measured using a Kelvin probe, increases with increasing density of carboxylic acid groups due to the formation of interfacial dipoles pointing toward the TiO(2) surface. The formation of interfacial dipoles results in a shift in the band offset at the TiO(2)/polymer interface, which explains the decrease in the open-circuit voltage. This work demonstrates that care must be taken when using carboxylic acid side groups to attach polymers to titania surfaces in photovoltaic cells. If the density of attachment groups is just enough to attach the polymer, then the benefits of the interface modifier can be realized without substantially decreasing the open-circuit voltage.

Harvesting energy directly from sunlight using photovoltaic technology is a way to address growing global energy needs with a renewable resource while minimizing detrimental effects on the environment by reducing atmospheric emissions. This issue of MRS Bulletin on “Organic-Based Photovoltaics” looks at a new generation of solar cells that have the potential to be produced inexpensively. Recent advances in solar power conversion efficiencies have propelled organic-based photovoltaics out of the realm of strictly fundamental research at the university level and into the industrial laboratory setting. Fabricated from organic materials—polymers and molecules—these devices are potentially easier to manufacture than current technologies based on silicon or other materials. In this introductory article, we describe the motivation for pursuing research in this field and provide an overview of the various technical approaches that have been developed to date.We conclude by discussing the challenges that need to be overcome in order for organic photovoltaics to realize their potential as an economically viable path to harvesting energy from sunlight.

Indium–zinc-oxide (IZO) compositional libraries were deposited with dc magnetron sputtering onto glass substrates at 100 °C and analysed with high throughput, combinatorial techniques. The composition range from 4 to 95 at% In for Zn was explored. A peak in conductivity with σ > 3000 (Ω cm)−1 was observed at an indium content of ~70%. The mobility exceeded 30 cm2 (V s)−1 and the carrier concentrations were greater than 8 × 1020 cm−3. Crystalline phases were observed for In concentrations less than 45% and greater than 80% with an intermediate amorphous region. The low indium content films have a zinc oxide type structure with a ZnO (002) spacing ranging from ~2.61 to 2.85 A for 4% In and 45% In, respectively. For indium contents between 82% and 95%, the In2O3 (222) spacing varied from 2.98 to 2.99 A. Regardless of the composition or the degree of crystallinity, all films showed high optical transparency with the transmission >80% across the visible spectrum.

Pulsed laser vaporization has been used to produce nanooctahedra of MoS2 and MoSe2. The nanooctahedra primarily form in two- or three-layer nested octahedra, although nesting up to five layers has been observed. Tilting the TEM sample stage and mapping how the images of single particles transformed provided the evidence to verify their octahedral geometry. Analysis of 30 two- and three-layered octahedra showed that their outer edge lengths clustered at ∼3.8 nm and ∼5.1 nm, respectively. This discreet sizing and the high symmetry of these closed nanooctahedra represent the closest inorganic analogy yet to the carbon fullerenes. The geometrical implications for forming octahedra from these layered compounds are investigated by considering different atomic arrangements assuming either trigonal prismatic or octahedral coordination around the Mo atom and yields two possible configurations for the actual structure of the nanooctahedra. A preliminary survey of pulsed laser vaporization of other layered metal cha...

Compositionally graded, combinatorial libraries of indium-doped zinc oxide (IZO) were grown by DC-magnetron sputtering on glass at 100°C. The In content ranged from 5 to 50 cation-%. The highest conductivity, σ=2 000 Ω -1 .cm -1 with μ = 25 cm 2 . V -1 . s -1 , was found at 50% in content with a visible transmittance greater than 80%. The (001) layer spacing increased from 2.6 to 2.85 A as the In content increased from 5 to 50%.

Transparent conducting oxides represent a key component in a number of important opto-electronic technologies. The demand for improved materials is increasing. It is increasingly important to develop improved materials and process techniques. We report on the PLD growth of both Mo doped In-O, an n-type material with enhanced mobilities, and p-type Ca–CuInO2 films with improved conductivity. In both cases, very high quality films have been obtained with improved opto-electronic properties. We also report on a solution based route to p-type Cu2SrO2 which has produced phase pure films.

Nanorods of manganese (IV) oxide of varying dimensions were synthesized by combining sol–gel chemistry and template-based methods, and have been characterized using XRD, SEM, TEM, EDX, and ICP analysis. Diffraction studies and ICP analysis indicate that the product formed is an α-MnO2 polymorph which crystallizes with hollandite-type structure. Preliminary electrochemical studies on 100 nm diameter nanorods show that these materials are promising candidates for positive electrodes in Li-ion batteries.

We report on the development of new combinatorial capabilities for both the synthesis of new solid state opto-electronic materials and optimization of existing materials for renewable energy applications especially photovoltaic devices. Some of the important materials for these renewable energy applications include semiconductors (such as for absorber layers), transparent conductors, energy storage materials and more. In this paper, we focus on the application of a combinatorial approach to two specific material areas, transparent conducting oxides and thin film Si. In the former case, libraries are generated by sputtering and in the latter by the hot-wire chemical vapor deposition. The application of a combinatorial approach to these materials areas can greatly accelerate the rate of discovery and optimization of new materials and the optimization of devices. We report on the development of tools for the production and characterization of libraries and on initial important results in both of these areas.

Silver nanowire (AgNW)-based transparent contacts are promising alternatives to transparent conducting oxides for solar cells owing to their promising optoelectronic properties. However, there is a need to improve issues such as adhesion to substrate, surface roughness, thermal instability, and loose contact between wires. If these issues could be resolved, it should be possible to replace the conducting oxide layers by AgNWs. In the present work, we report electrical, optical, and surface properties of silver nanowires with amorphous zinc tin oxide (ZTO) as overcoat. Two different sizes of silver nanowires have been employed for this study with average diameters of 35 nm (length = 10 µm) and 90 nm (length = 25 µm). Various concentrations of the solution and spin-coating speeds have been used to optimize transmittance and conductivity of bare AgNW films. After deposition, it is possible to improve conductivity by fusing wires through various processes such as thermal annealing. It is observed that thermal annealing at 200°C or less shows improved conductivity. Sheet resistance of 9.8 Ω/sq and transmittance (at 550 nm) of 91% are obtained for 90 nm diameter wires at an annealing temperature of 200°C. AgNW networks of varying sheet resistance and transmittance were over-coated with sputtered ZTO films of 20-, 50-, or 200-nm thickness. Microscope analysis of scratch-tested samples shows that 50-nm-thick ZTO film is sufficient to give robustness to the 90-nm AgNW films with minor changes to conductivity and transmittance.

We show in this article that the position of semiconductor band edges relative to the water reduction and oxidation levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compounds, we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining density functional theory and many-body GW calculations. From the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on average a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.

We have found that by varying only the substrate temperature and oxygen pressure five different crystallographic orientations of V2O5 thin films can be grown, ranging from amorphous to highly textured crystalline. Dense, phase-pure V2O5 thin films were grown on SnO2/glass substrates and amorphous quartz substrates by pulsed laser deposition over a wide range of temperatures and oxygen pressures. The films' microstructure, crystallinity, and texturing were characterized by electron microscopy, x-ray diffraction, and Raman spectroscopy. Temperature and oxygen pressure appeared to play more significant roles in the resulting crystallographic texture than did the choice of substrate. A growth map summarizes the results and delineates the temperature and O2 pressure window for growing dense, uniform, phase-pure V2O5 films.

In the interim between the conception of this issue of MRS Bulletin on transparent conducting oxides (TCOs) and its publication, the remarkable applications dependent on these materials have continued to make sweeping strides. These include the advent of larger flat-screen high-definition televisions (HDTVs), larger and higher-resolution screens on portable computers, the increasing importance of low emissivity (“low-e”) and electrochromic windows, a significant increase in the manufacturing of thin-film photovoltaics (PV), and a plethora of new hand-held and smart devices, all with smart displays.1-7 Coupled with the increased importance of TCO materials to these application technologies has been a renaissance over the last two years in the science of these materials. This has included new n-type materials, the synthesis of true p-type materials, and the theoretical prediction and subsequent confirmation of the applicability of codoping to produce p-type ZnO. Considering that over the last 20 years much of the work on TCOs was empirical and focused on ZnO and variants of InxSn1-xO2, it is quite remarkable how this field has exploded. This may be a function of not only the need to achieve higher performance levels for these devices, but also of the increasing importance of transition-metal-based oxides in electro-optical devices. This issue of MRS Bulletin is thus well timed to provide an overview of this rapidly expanding area. Included are articles that cover the industrial perspective, new n-type materials, new p-type materials, novel deposition methods, and approaches to developing both an improved basic understanding of the materials themselves as well as models capable of predicting performance limits.

Thin films of V 2 O 5 and LiCoO 2 were deposited by pulsed laser deposition (PLD) and the chemical diffusion coefficients, D were measured by potentiostatic intermittent titration technique (PITT). The PLD-grown V 2 O 5 and LiCoO 2 films are electrochemically similar to bulk powders and thin films produced by other techniques. In crystalline V 2 O 5 , the maximum and minimum D were found to be 1.7×10 −12 cm 2 s −1 and 5.8×10 −15 cm 2 s −1 respectively, with a general trend for D to rise in single-phase regions. In amorphous V 2 O 5 films, D was initially 5×10 −13 cm 2 s −1 and decreased steadily to 1.2×10 −13 cm 2 s −1 at Li 0.4 V 2 O 5 . The decrease in D then became more gradual with a final value of 5.52×10 −14 cm 2 s −1 at Li 1.5 V 2 O 5 . The chemical diffusion coefficient of Li in LiCoO 2 films ranged from 1×10 −12 –4×10 −11 cm 2 s −1 with a pronounced minimum at Li 0.7 CoO 2 . Thin films of LiCo 0.5 Al 0.5 O 2 also deposited by PLD exhibited limited cycling capabilities and an upper bound of D =9×10 −13 cm 2 s −1 .

Thick-film processing techniques such as electrodeposition, electrophoresis, spray pyrolysis, and ink spraying offer promising and economic (

The lithium ion charging capacity and cycling stability of vanadium oxide thin-films prepared by pulsed laser deposition (PLD) are reported. PLD films were prepared at various temperatures and atmospheres from a V{sub 6}O{sub 13} target. The charging capacity of these films depended strongly on the substrate deposition temperature and atmosphere. The best crystalline films were grown at 200 C. Crystalline oriented V{sub 2}O{sub 5} films were prepared by PLD at 200 C in an oxygen environment. These films can be cycled in the voltage range between 4.1 and 2.0 V for more than 100 cycles with very little capacity loss. The specific charge capacity of the films was 340 Ah/kg when the discharge current density was 0.1 mA/cm{sup 2}, which corresponds to 1.2 lithium atoms per vanadium atom. The capacity increased to 1.5 lithium atoms per vanadium atom when cycled at a current density of 0.02 mA/cm{sup 2}. Amorphous vanadium oxide films with similar specific capacities were prepared by PLD in vacuum at 200 C. The capacity loss in these films was less than 2% after 100 cycles. Although thermally evaporated vanadium oxide films had similar initial capacities under the same charging conditions, they lost more than 17% of theirmore » charging capacity after 100 cycles. The improved cycle stability in the amorphous vanadium oxide films may be partially attributed to the improved surface morphology of the PLD films.« less

The formation of CdTe thin films by spray deposition using nanoparticle colloids has been investigated. Employing a metathesis approach, cadmium iodide is reacted with sodium telluride in methanol solvent, resulting in the formation of soluble NaI and insoluble CdTe nanoparticles. After appropriate chemical workup, methanol-capped CdTe colloids were isolated. CdTe colloids prepared by this method exhibit a dependence of the nanoparticle diameter upon reaction temperature as determined by UV−visible spectroscopy (UV−vis), X-ray diffraction (XRD), and transmission electron microscopy (TEM). CdTe thin-film formation was performed by spray depositing the 25−75 A diameter nanoparticle colloids according to a one- or two-step method. Films derived from a one-step approach were sprayed onto substrates at elevated temperatures (Tdep = 280−440 °C) with no further thermal treatment. Two-step films were sprayed at lower temperatures (Tdep = 25−125 °C) and were subjected to subsequent thermal treatments (Tanneal = 25...

The Seebeck coefficient is a key indicator of the majority carrier type (electrons or holes) in a material. The recent trend toward the development of combinatorial materials research methods has necessitated the development of a new high-throughput approach to measuring the Seebeck coefficient at spatially distinct points across any sample. The overall strategy of the high-throughput experiments is to quickly identify the region of interest on the sample at some expense of accuracy, and then study this region by more conventional techniques. The instrument for spatially resolved Seebeck coefficient measurements reported here relies on establishing a temperature difference across the entire compositionally graded thin-film and consecutive mapping of the resulting voltage as a function of position, which facilitates the temperature-dependent measurements up to 400 °C. The results of the designed instrument are verified at ambient temperature to be repeatable over 10 identical samples and accurate to within 10% versus conventional Seebeck coefficient measurements over the −100 to +150 μV/K range using both n-type and p-type conductive oxides as test cases. The developed instrument was used to determine the sign of electrical carriers of compositionally graded Zn–Co–O and Ni–Co–O libraries prepared by combinatorial sputtering. As a result of this study, both cobalt-based materials were determined to have p-type conduction over a broad single-phase region of chemical compositions and small variation of the Seebeck coefficient over the entire investigated range of compositions and temperature.

Copper nitride (Cu3N) is a potential next-generation Earth abundant thin film solar cell absorber material. We performed reduction of Cu3N combinatorial optical and structural vector data to scalar metrics, and subsequent correlation of these metrics with other scalar quantities, such as isotropic bulk electrical conductivity or target-substrate distance during the growth. Insensitivity of the Cu3N conductivity to the structural and optical metrics is promising for large-scale fabrication of thin film solar cells where precise process control over the growth parameters may be challenging.

The influence of semiconductor layer morphology on the performance of solution-processed ZnO/Cu2O photovoltaics has been examined. ZnO films were prepared using three highly scalable, cost-effective methods: electrodeposition, zinc acetate decomposition, and diethyl zinc decomposition. To optimize device performance, it is found that a low density of nano-scale pores in the ZnO layer and large grains in the Cu2O is necessary. Through optimizing the ZnO morphology, one of the highest fill factors observed to date (up to 54%) in solution-processed ZnO/Cu2O was achieved. This value is comparable with the fill factor of the record-efficiency ZnO/Cu2O device, which was prepared with much larger energy inputs.

Discovery of new materials is important for all fields of chemistry. Yet, existing compilations of all known ternary inorganic solids still miss many possible combinations. Here, we present an example of accelerated discovery of the missing materials using the inverse design approach, which couples predictive first-principles theoretical calculations with combinatorial and traditional experimental synthesis and characterization. The compounds in focus belong to the equiatomic (1:1:1) ABX family of ternary materials with 18 valence electrons per formula unit. Of the 45 possible V-IX-IV compounds, 29 are missing. Theoretical screening of their thermodynamic stability revealed eight new stable 1:1:1 compounds, including TaCoSn. Experimental synthesis of TaCoSn, the first ternary in the Ta-Co-Sn system, confirmed its predicted zincblende-derived crystal structure. These results demonstrate how discovery of new materials can be accelerated by the combination of high-throughput theoretical and experimental methods. Despite being made of three metallic elements, TaCoSn is predicted and explained to be a semiconductor. The band gap of this material is difficult to measure experimentally, probably due to a high concentration of interstitial cobalt defects.

The work under the proposed CRADA will be a joint effort by BP Solar and NREL to develop new types of high performance inks for high quality contacts to silicon solar cells. NREL will develop inks that have electronic properties that will allow the formation of high quality ohmic contacts to n- and p-type crystalline silicon, and BP Solar will evaluate these contacts in test contact structures.

Historically, the point-contact metal-insulator-metal (MIM) architecture constituted the first approach for making MIM diodes for high-frequency rectification applications. Point-contact MIM rectifiers have been shown to operate at frequencies as high as 150 THz. In the last 3 decades, point-contact architectures have given way to more stable planar MIM architectures that are lithographically patterned to yield micro- and nanoscale device areas. But point-contact MIM architectures are still highly useful, for example, in facilitating high-throughput material screening studies. Such screening studies are of critical importance for developing material design rules to help identify optimal MIM materials for various rectification applications. In this chapter, modified approaches to the point-contact MIM architecture are presented. These architectures are employed to study the influence of material properties on rectification performance. This enables the correlation of performance to the work function values of the two metals, the electron affinity of the insulator and the thermodynamic stability of the metal/insulator interface. Material selection criteria are proposed based on these results. From these criteria a two-dimensional MIM material space map is constructed that can help identify and visualize application-specific champion MIM devices based on their material properties such as work function and electron affinity.

Conduction and rectification in nanoantenna-coupled NbOx- and NiO-based metal-insulator-metal (MIM) diodes (“nanorectennas”) are studied by comparing new theoretical predictions with the measured response of nanorectenna arrays. A new quantum mechanical model is reported and agrees with measurements of current–voltage (I–V) curves, over 10 orders of magnitude in current density, from [NbOx(native)-Nb2O5]- and NiO-based samples with oxide thicknesses in the range of 5–36 nm. The model, which introduces new physics and features, including temperature, electron effective mass, and image potential effects using the pseudobarrier technique, improves upon widely used earlier models, calculates the MIM diode's I–V curve, and predicts quantitatively the rectification responsivity of high frequency voltages generated in a coupled nanoantenna array by visible/near-infrared light. The model applies both at the higher frequencies, when high-energy photons are incident, and at lower frequencies, when the formula for classical rectification, involving derivatives of the I–V curve, may be used. The rectified low-frequency direct current is well-predicted in this work's model, but not by fitting the experimentally measured I–V curve with a polynomial or by using the older Simmons model (as shown herein). By fitting the measured I–V curves with our model, the barrier heights in Nb-(NbOx(native)-Nb2O5)-Pt and Ni-NiO-Ti/Ag diodes are found to be 0.41/0.77 and 0.38/0.39 eV, respectively, similar to literature reports, but with effective mass much lower than the free space value. The NbOx (native)-Nb2O5 dielectric properties improve, and the effective Pt-Nb2O5 barrier height increases as the oxide thickness increases. An observation of direct current of ∼4 nA for normally incident, focused 514 nm continuous wave laser beams are reported, similar in magnitude to recent reports. This measured direct current is compared to the prediction for rectified direct current, given by the rectification responsivity, calculated from the I–V curve times input power.

9th International Symposium on Transparent Oxide and Related Materials for Electronics and Optics (TOEO9) Preface

A spray pyrolysis route to superconducting Pb-, Sr-, and Ag-substituted Tl-Ba-Ca-Cu-O thick films has been developed. First, a precursor powder with the stoichiometry Pb/sub 0.46/Ba/sub 0.40/Sr/sub 1.52/Ca/sub 1.86/Cu/sub 3.00/O/sub x/(Ag/sub /0/sub .37/) (PBSCCO) was synthesized with the particle size of this powder being subsequently reduced to 4-6 /spl mu/m using a ball mill. Next, this powder was mixed with ethyl cellulose binder and sprayed with an airbrush using an N/sub 2/ carrier gas, polycrystalline Y/sub 2/O/sub 3/:ZrO/sub 2/ (YSZ) as single-crystal MgO, SrTiO/sub 3/, NdGaO/sub 3/, and LaAlO/sub 3/ substrates were mounted on a hot plate with typical growth temperatures of 80-100 /spl deg/C. Growth times of /spl sim/10 min provided films which were 5-20 /spl mu/m in thickness. After an intermediate O/sub 2/ anneal to remove the organic binder, these films were subjected to a flowing 2-zone thallination process. Very well c-axis oriented Tl/sub 0.70/Pb/sub 0.36/Ba/sub 0.39/Sr/sub 1.58/Ca/sub 1.98/Cu/sub 3/O/sub x/ (TP-1223) phase material was obtained for films grown on single-crystal LaAlO/sub 3/. Films grown on all other substrates surveyed in the study gave rise to impurity phase formation and/or incomplete reaction. The PBSCCO and TP-1223 films were characterized by /spl theta//2/spl theta/ and /spl omega/-rocking curve X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), variable temperature magnetic measurement, and inductively coupled plasma atomic emission spectroscopy (ICPAES). >

A spray pyrolysis growth system has been employed for the formation of superconducting YBa 2 Cu 3 O x (YBCO) thick films. YBCO films on polycrystalline Ni/NiO, Inconel 600, and Inconel X substrates ranging in thickness from 80–100 μm have been developed by a three-step process with observed T c zero values up to 85 K. In the first step, the metallic substrates were examined with and without a preoxidation anneal to form a native oxide layer. Second, superconducting YBCO powder (0.2 μm particle size) was mixed with ethanol and sprayed onto the substrates. Third, the films were sintered at high temperature and oxygenated to form the superconducting phase. The effect of a native oxide buffer layer upon superconducting properties was evaluated as a function of the duration of the preoxidation anneal and the sintering anneal. X-ray diffraction (XRD) θ/2θ, variable temperature four-probe resistance, and Auger line scan characterization of these films will be discussed.

Normally we think of the glassy state as a single phase and therefore crystallization from chemically identical amorphous precursors should be identical. Here we show that the local structure of an amorphous precursor is distinct depending on the initial deposition conditions, resulting in significant differences in the final state material. Using grazing incidence total x-ray scattering, we have determined the local structure in amorphous thin films of vanadium oxide grown under different conditions using pulsed laser deposition (PLD). Here we show that the subsequent crystallization of films deposited using different initial PLD conditions result in the formation of different polymorphs of VO2. This suggests the possibility of controlling the formation of metastable polymorphs by tuning the initial amorphous structure to different formation pathways.

World-wide research activities on ZnO and related transparent conductive oxides (TCO) in thin film, nanostructured, and multilayered forms are driven by the vast potential of these materials for optoelectronic, microelectronic, and photovoltaic applications. Renewed interest in ZnO applications is partly stimulated by cost reduction in material processing and device development. One of the most important issues is doping and alloying with Al, Ga, In, Sn, etc. in order to tune properties. When highly doped, these materials are used as transparent-conducting contacts on solar cells, as well as in catalytic, spintronic, and surface acoustic wave devices. Film growth conditions, including substrate type and orientation, growth temperature, deposition rate, and ambient atmosphere, all play important roles in determining structural, electrical, magnetic, and optical properties.