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Copper (Cu) incorporation is a key process for fabricating efficient CdTe-based thin-film solar cells and has been used in CdTe-based solar cell module manufacturing. Here, we investigate the effects of different Cu precursors on the performance of CdTe-based thin-film solar cells by incorporating Cu using a metallic Cu source (evaporated Cu) and ionic Cu sources (solution-processed cuprous chloride (CuCl) and copper chloride (CuCl2)). We find that ionic Cu precursors offer much better control in Cu diffusion than the metallic Cu precursor, producing better front junction quality, lower back-barrier heights, and better bulk defect property. Finally, outperforming power conversion efficiencies of 17.2 and 17.5% are obtained for devices with cadmium sulfide and zinc magnesium oxide as the front window layers, respectively, which are among the highest reported CdTe solar cells efficiencies. Our results suggest that an ionic Cu precursor is preferred as the dopant to fabricate efficient CdTe thin-film solar cells and modules.

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. Here we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 μs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination. Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

Because bifacial solar cells increase the power generated per area, their market share is projected to increase over the next decade. While silicon technologies have implemented bifacial technology...

Here, we report the solution-based synthesis of CuGaOX thin film, and its optimization for application as a back buffer layer in CdS/CdTe solar cells. The CuGaOX composition was optimized for best results by varying copper source concentration and annealing time post spin-coating. Minority carrier lifetimes and device parameters were recorded. For glass-side illumination, the best cell exhibited an improved efficiency of 14.6% mainly due to a significant increase in V OC (V OC = 863 mV, FF = 74.4%, J SC = 22.7 mA/cm2) compared to the standard device (Cu/Au back contact) best cell efficiency of 13.0% (V OC = 818 mV, FF = 75.9%, J SC = 21.0 mA/cm2).

The annual energy yields for metal halide perovskite/copper indium gallium diselenide (CIGS) tandem photovoltaics have been calculated for 16 different bandgap combinations, using both 2 terminal and 4 terminal device designs, with fixed-tilt mounting as well as 1- and 2-axis tracking. Measured complex index of refraction data were used for the materials comprising the devices, and hourly irradiance data were extracted from Version 3 of the National Solar Radiation Database. Simulations were performed for Toledo OH, Golden CO, Phoenix AZ, and New Orleans LA, and the effect of local temperature variation was also considered. The combination of irradiance and temperature variations throughout the year cause different devices to be optimal at different times of the year. Interestingly, devices constructed to maximize AM1.5 photoconversion efficiency do not necessarily maximize the annual energy yield. A detailed analysis of the monthly energy yields at the different locations reveals the interplay between the changing light and temperature conditions. Over the course of the year these effects average to some degree so that annual energy yields that are close to the maximum possible value can be achieved by several different tandem device designs. The conclusions are valid for devices made with relatively thin perovskite layers, such as those used in champion efficiency devices. When the perovskite layers are thicker, however, the device is less tolerant to variation. Our results show that close matching of bandgap pairs is not essential for the fabrication of high-performance tandems. These findings should allow manufacturing efforts to proceed without the need for precise compositional control during formation of the absorber layers.

Copper (Cu) doping is a crucial process to improve the hole concentration in CdTe absorber layer and reduce the back-barrier height for efficient CdTe solar cells. However, excess Cu creates detrimental defects and deteriorates device stability due to its high diffusivity. Here, we introduce a wet chemical method to incorporate Cu in CdTe using a CuCl solution. This approach allows a better control of Cu doping than the conventional method using evaporated Cu. Our investigation shows that the CuCl treatment can significantly improve the overall device performances due to an increased carrier concentration in the CuCl treated CdTe absorber layer. Furthermore, the CuCl treated devices show a longer minority carrier lifetime than the conventional Cu treated devices.

Sputtering is an efficient way to fabricate CdTe solar cells. However, the open-circuit voltage (Voc) of a sputtered device is low compared to close space sublimation or vapor deposited CdTe devices. Here, we report an outstanding Voc > 840 mV and fill factor (FF) > 71.0% for all-sputtered CdS/CdTe solar cells. These results were achieved by solution-based copper (Cu) doping of CdTe by copper (II) chloride (CuCl 2 ) in deionized water. Compared with evaporated Cu, solution-based doping increased the Voc and FF, and improved the optoelectronic properties of the sputtered CdTe thin films. Capacitance-voltage measurements indicated that the free hole concentration increased by two-fold for CuCl 2 treated devices.

Doping control of the emitter is critically important to high performance thin film photovoltaic devices. Here, we deposit magnesium zinc oxide (MZO) thin films for use as the emitter of CdTe-based devices through cosputtering of ZnO and MgO or MgF. The bandgap and sheet resistance of the MZO films are measured before and after the films are thermally processed under vacuum or in He environments. Through annealing of the films, they become conductive, but little bandgap or sheet resistance differences can be determined between the films deposited using the MgO target or the MgF target. Devices will be completed to determine if the addition of fluorine to the MZO film affects the emitter doping during device processing.

Cu doping of CdTe has enabled reasonable device performance, but has some significant drawbacks, including limited achievable carrier concentration and high mobility in the CdTe films. Group V elements, on the other hand, are expected to facilitate higher carrier concentration and expected to have low mobility. Consequently, incorporate of As in CdTe is of great interest to the CdTe community. Here, we deposited CdTe from the CdTe:As feedstock to complete devices. We demonstrate that the As diffuses through the CdTe absorber layer and accumulates at the front and back interfaces with Cl activation. Current density-voltage and capacitance-voltage measurements indicate that the devices have low carrier concentration, even though the concentration of As is high. These results indicate that the process employed here was insufficient to activate the As doping.

The potential release of toxic metals from damaged emerging photovoltaic (PV) cells has raised concerns about the safe use of these new types of PVs. In this study, this concern was addressed by analysing the life cycle toxicity of metals (cadmium, copper, lead, nickel, tin and zinc) that are commonly used in emerging PVs. In estimating the potential metal release, a new model that incorporates field conditions (crack size, time, and glass thickness) and physiochemical properties (diffusion coefficient and solubility product) was introduced. The results show that the use phase toxicity of copper and lead can be higher than the extraction phase toxicity. Thus, precautionary loss limits to manage toxic impacts from the use phase were proposed. Also, the toxicity from different layers of perovskite, copper zinc tin sulphide (CZTS), and quantum dot (QD) type of solar cells was compared. It was found that cadmium sulphide (compared to zinc oxide and tin oxide) and lead (II) sulphide (compared to lead (II) iodine and CZTS) were less toxic alternatives for the electron selective layer and light absorber, respectively. Finally, in comparing the toxic metal releases of the PVs to today's coal power plants, it was seen that the metal emissions from PVs are expected to be several times less than the emissions from coal.

Control over grain size and crystallinity is important for preparation of methylammonium lead iodide (MAPbI3) solar cells. We explore the effects of using small concentrations of Cd2+ and unusually high concentrations of methylammonium iodide during the growth of MAPbI3 in the two-step solution process. In addition to improved crystallinity and an enhancement in the size of the grains, time-resolved photoluminescence measurements indicated a dramatic increase in the carrier lifetime. As a result, devices constructed with the Cd-modified perovskites showed nearly a factor of 2 improvement in the power conversion efficiency (PCE) relative to similar devices prepared without Cd addition. The grains also showed a higher degree of orientation in the ⟨110⟩ direction, indicating a change in the growth mechanism, and the films were compact and smooth. We propose a Cd-modified film growth mechanism that invokes a critical role for low-dimensional Cd perovskites to explain the experimental observations.

Perovskite/silicon tandem solar cells with high power conversion efficiencies have the potential to become a commercially viable photovoltaic option in the near future. However, device design and optimization is challenging because conventional characterization methods do not give clear feedback on the localized chemical and physical factors that limit performance within individual subcells, especially when stability and degradation is a concern. In this study, we use light beam induced current (LBIC) to probe photocurrent collection nonuniformities in the individual subcells of perovskite/silicon tandems. The choices of lasers and light biasing conditions allow efficiency-limiting effects relating to processing defects, optical interference within the individual cells, and the evolution of water-induced device degradation to be spatially resolved. The results reveal several types of microscopic defects and demonstrate that eliminating these and managing the optical properties within the multilayer structures will be important for future optimization of perovskite/silicon tandem solar cells.

Optically transparent p-type materials play a critical role in transparent electronics including photovoltaic (PV) devices. P-type sulfide materials offer an alternative to oxides for PV application due to improved hole transport properties. Here, we report the solution-based synthesis of earth-abundant p-type transparent conducting barium copper sulfide (α-BaCu4S3, BCS) thin films. These films were characterized using scanning electron microscopy, X-ray diffraction, UV–Vis–NIR spectrophotometry, Raman spectroscopy, and spectroscopic ellipsometry. BCS films of ~100 nm thickness transmit >70% of visible light. We report on tests of the hole transport properties of these BCS films for cadmium telluride (CdTe) photovoltaics, finding that the BCS deposition process forms a beneficial tellurium (Te) rich surface on CdTe by selectively removing Cd from the surface. Based on our study, the BCS interface layer plays dual functions for CdTe PV devices as a hole transport material and as an etchant, enhancing the resulting device performance. We observed a significant increase in open-circuit voltage of CdTe solar cells with the BCS buffer layer. Additionally, we discuss semitransparent CdTe solar cells with BCS as a hole transport layer and indium tin oxide as a finishing electrode. Semitransparent CdTe solar cells shows 13.3% conversion efficiency for the front side illumination and 1.2% efficiency for back side illumination, indicating high recombination of charge carriers generated close to the rear CdTe/BCS/ITO contact.

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%.

Spectroscopic ellipsometry (SE) was performed on CuIn Se 2 (CIS) thin films and solar cells with a goal toward optimizing this low bandgap absorber for tandem applications. The CIS thin films and the absorbers in devices were deposited by one-stage thermal co-evaporation on silicon and on Mo-coated soda-lime glass substrates in a deposition system that has yielded CuIn 1-x Ga x Se 2 (CIGS) cells with > 17% efficiency using standard thickness (2.0 μm)x = 0.3 absorbers and > 13% using 0.7 μm low-Ga absorbers. In this study, a mapping capability for CIS Cu stoichiometry y = [Cu]/[In] over the film area was established based on a y-dependent parametric dielectric function (e 1 , e 2 ) with bandgap critical point E g decreasing linearly from 1.030 eV for y = 0.7 to 1.016 eV for y = 1.1. In addition, a full set of (e 1 , e 2 ) spectra measured for the CIS cell components enables analysis of SE data in terms of an accurate structural model for the device. With this model, spectra in the external quantum efficiency can be predicted, and deviations from this prediction can be attributed to incomplete collection of photogenerated electrons and holes as simulated with a carrier collection profile.

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1-xGax)₂Se₃ (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function e = e₁ - ie₂, spectra. Here, RTSE has been used to obtain the (e₁, e₂) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (e₁, e₂) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (e₁, e₂) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (e₁, e₂) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

The invention discloses nanocrystalline (NC) FeS 2 thin films as the back contact for CdTe solar cells. In one example, the FeS 2 NC layer is prepared from a solution directly on the CdTe surface using spin-casting and chemical treatment at ambient temperature and pressure, without a thermal treatment step. Solar cells prepared by applying the NC FeS 2 back contact onto CdTe yield efficiencies of about 95% to 100% that of standard Cu/Au back contact devices. In another example, FeS2 is interposed between Cu and Au to form a Cu/ FeS 2 NC/Au back contact configuration yielding an efficiency improvement of 5 to 9 percent higher than standard Cu/Au devices.

Formation of a Te-rich surface on the CdTe absorber is beneficial to the formation of a low barrier contact at the back junction in CdTe solar cells. Regardless of the CdTe fabrication method, surface etching processes are widely used to form Te-rich surfaces. Here, we show that methylammonium iodide (CH3NH31, MAl) can simply and controllably react with the CdTe surface to produce elemental tellurium. Both X-ray diffraction and Raman spectroscopy confirmed formation of a Te layer. Devices constructed with the MAl-treated CdTe absorber exhibited higher photovoltaic parameters of open circuit voltage $(\mathrm{V}_{\mathrm{o}\mathrm{c}})$ and fill factor (FF) relative to similar devices prepared without the MAI treatment. This indicates a reduction in the Schottky barrier at the back contact due to the MAI surface treatment. CdTe samples that were treated with 125 mM MAI solution and heated to 150 °C for 10 minutes showed the best power conversion efficiency (PCE) of 13.5% while without treating the standard device efficiency was 12.7%.

Single-walled carbon nanotube (SWNT) radical anions will react with tetrahydrofuran and generate ethylene, enolates, and a partially hydrogenated nanotube backbone. The experimental evidence suggests that there are sp3 C-H binding interactions. The total gravimetric content of hydrogen on a sample averages from 3.5% to 3.9% w/w, about four times the total amount observed for nanotubes hydrogenated via traditional Birch reduction reactions. Furthermore, the hydrogen desorbs at temperatures up to 400 °C less than those observed for the hydrogenated SWNTs formed after the Birch reduction. Finally, the first room temperature electron spin resonance spectrum of a nanotube radical ion is also reported.

We studied the electrocatalytic activity of an [FeFe]-hydrogenase from Clostridium acetobutylicum (CaH2ase) immobilized on single-wall carbon nanotube (SWNT) networks. SWNT networks were prepared on carbon cloth by ultrasonic spraying of suspensions with predetermined ratios of metallic and semiconducting nanotubes. Current densities for both proton reduction and hydrogen oxidation electrocatalytic activities were at least 1 order of magnitude higher when hydrogenase was immobilized onto SWNT networks with high metallic tube (m-SWNT) content in comparison to hydrogenase supported on networks with low metallic tube content or when SWNTs were absent. We conclude that the increase in electrocatalytic activities in the presence of SWNTs was mainly due to the m-SWNT fraction and can be attributed to (i) substantial increases in the active electrode surface area, and (ii) improved electronic coupling between CaH2ase redox-active sites and the electrode surface.

We report the results of a study using temperature programmed desorption (TPD) and a volumetric sorption technique to measure the hydrogen storage capacity of the Ti–6Al–4V alloy. Samples of various sizes and surface treatments were studied to obtain a statistically meaningful value for the maximum hydrogen storage capacity, as well as to understand the effect of sample size, sample oxidation, and hydrogen charging conditions on the measured capacity. We find a maximum reversible hydrogen storage capacity of ∼3.76 wt% with hydrogen exposures near ambient temperature and pressure. This value is higher than any reported in the literature previously, possibly due to the utilization of very small particles and rapid hydrogen exposures, which allow for equilibration times of approximately 1 h. Comparison of a variety of samples indicates that the measured hydrogen capacity is affected by surface oxidation. Samples generated in a strongly oxidizing environment exhibit decreased hydrogen uptake. The implications of these results are discussed with regards to previously reported capacity values in the literature.

Transition-metal (TM) boride and carboride nanostructures are studied as model organometallic materials for hydrogen storage. The dispersed TM atoms function as H2 sorption centers on the surface of the boron or carbon-boron substrate. The flexibility offered in the variety of possible structures permits the study of the effect of the TM-TM distance on the storage capacity. When the TMs are too close to one another, TM-TM bonding reduces the capacity. Even when separated by distances larger than the normal TM-TM bond length, delocalization of TM valence electrons can still lower the hydrogen capacity. An optimal TM-TM distance for the structural motifs studied here is approximately 6 A. Our study also permitted the evaluation of new TM boride nanostructures. We predict a low-energy single-walled scandium triboride (ScB3) nanotube that can bind approximately 6.1 wt % hydrogen with a binding energy of 22 approximately 26 kJ/mol.

Theoretical studies have predicted that scandium can bind to the twelve five-membered rings in C 60 . It is then possible to stabilize four dihydrogen ligands (H 2 ) on each Sc atom with a binding energy of ∼30 kJ/mol, ideal for vehicular hydrogen storage. The resulting C 60 [ScH 2 (H 2 ) 4 ] 12 complex is predicted to be a minimum energy structure with ∼7.0 wt% reversible hydrogen capacity. However, wet chemical synthesis of the calculated η 5 -coordinated fullerene complex is unprecedented. The chemistry of C 60 is generally olefinic (i.e., η 2 -coordination, in which the metal is coordinated to two carbon atoms contributing two electrons to the bonding). Furthermore, stabilization of multiple dihydrogen ligands on a single transition metal has not been demonstrated. Recently we have probed new synthesis techniques in order to coordinate C 60 with either Fe, Sc, Cr, Co or Li. The new compounds were characterized with solid-state nuclear magnetic resonance, and structures have been proposed. All of the structures were found to have unique binding sites for hydrogen employing the technique of temperature programmed desorption. Furthermore, some of the structures were shown to have significant hydrogen capacities with volumetric measurements.

We report a first-principle study of the tunable hydrogenation of endohedral metallofullerenes M@C60 and M2@C60, where M = Li, Be, Mg, Ca, Al, and Sc. The interaction between the encapsulated metal...

We have performed steady-state photoluminescence, time-correlated single photon counting, and Raman spectroscopy measurements on single-wall carbon nanotubes from 4 to 293 K. We observe novel photoluminescence spectra that cannot be attributed to vibronic transitions and verify the existence and energy levels of weakly emissive excitonic states. By combining photoluminescence intensity and lifetime data, we determine how nonradiative and radiative excitonic decay rates change as a function of temperature and contrast this with theoretical predictions. The results suggest that recombination kinetics are influenced by multiple excitonic states, including a dark lower state.

The Author(s) 2015. This article is published with open access at Springerlink.com Abstract This study investigates ultra-thin transparent conducting oxides (TCO) of indium tin oxide (ITO), alu- minum-doped zinc oxide (AZO) and zinc oxide (ZnO) to determine their viability as candidate materials for use in plasmonic-enhanced thin-film amorphous silicon solar photovoltaic (PV) devices. First a sensitivity analysis of the optical absorption for the intrinsic layer of a nano-disk patterned thin-film amorphous silicon-based solar cell as a function of TCO thickness (10-50 nm) was performed by simulation. These simulation results were then used to guide the design of the experimental work which investi- gated both optical and electrical properties of ultra-thin (10 nm on average) films simultaneously deposited on both glass and silicon substrates using conventional rf sputter- ing. The effects of deposition and post-processing param- eters on material properties of ITO, AZO and ZnO ultra- thin TCOs were probed and the suitability of TCOs for integration into plasmonic-enhanced thin-film solar PV devices was assessed. The results show that ultra-thin TCOs present a number of challenges for use as thin top contacts on plasmonic-enhanced PV devices: (1) optical and electrical parameters differ greatly from those of thicker (bulk) films deposited under the same conditions, (2) the films are delicate due to their thickness, requiring very long annealing times to prevent cracking, and (3) reactive gases require careful monitoring to maintain sto- ichiometry. The results presented here found a trade-off between conductivity and transparency of the deposited films. Although the sub 50 nm TCO films investigated exhibited desirable optical properties (transmittance greater than 80 %), their resistivity was too high to be considered as materials for the top contact of conventional PV devices. Future work is necessary to improve thin TCO properties, or alternative materials, and geometries are needed in plasmonic-based amorphous silicon solar cells. The sta- bility of ultra-thin TCO films also needs to be experi- mentally investigated under normal device operating conditions.

We thank Mark Davis for assistance with NMR experiments and Mowafak Al-Jassim for facilitating EELS measurements. This work was funded by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Hydrogen Program and by the Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering under Subcontract DE-AC36-99GO10337 to NREL.

Hot-wire chemical vapor deposition (HWCVD) has been demonstrated as a simple economically scalable technique for the synthesis of a variety of nano-materials in an environmentally friendly manner. For example we have employed HWCVD for the continuous production of both carbon single- and multi-wall nanotubes (SWNTs and MWNTs). Unanticipated hydrogen storage on HWCVD-generated MWNTs has led insight into the adsorption mechanism of hydrogen on metal/carbon composites at near ambient temperatures that could be useful for developing a vehicular hydrogen storage system. Recent efforts have been focused on growing MWNT arrays on thin nickel films with a simple HWCVD process. New data suggests that these MWNT arrays could replace the gold black coatings currently used in pyroelectric detectors to accurately measure laser power. Finally, we have very recently employed HWCVD for the production of crystalline molybdenum and tungsten oxide nanotubes and nanorods. These metal oxide nanorods and nanotubes could have applications in catalysis, batteries and electrochromic windows or as gas sensors. A summary of the techniques for growing these novel materials and their various potential applications is provided.

The Raman D-band feature (1350 cm 1 ) is examined with 2.54 eV excitation for pure bulk carbon single-wall nanotube samples before and after treatments that increase defect densities. Upon employing mass-transport-limited oxidation to introduce defects, the D-band intensity increases approximately linearly with reaction time. A relatively constant ratio of the D-band intensity and the major tangential G-band intensity (D/G) is observed for the purified samples examined at 2.54 and 1.96 eV suggesting a characteristic number of defects is introduced for given synthesis and purification processes. The D/G ratio is 1/190 and 1/40 for excitation at 2.54 and 1.96 eV, respectively. � 2004 Elsevier B.V. All rights reserved.

In the near future, the portable power market will demand greater specific energy and power from lithium battery technology. These requirements cannot be met by conventional batteries or through extrapolation of the capabilities of conventional systems. New materials and systems must be developed to meet these stringent future requirements. Nanomaterials offer a new exciting alternative to the standard materials traditionally used for fabrication of batteries. The work described herein, deals with a novel approach to the use of nanomaterials in the electrodes of lithium-ion batteries. We have synthesized and chemically modified carbon nanotubes and subsequently tested these modified nanotubes as electrodes in small lithium batteries. This paper describes electrochemical characterization of the novel electrodes as well as determination of the specific energy of simple one-cell batteries containing these novel electrodes. We have been able to demonstrate a laboratory cell with a specific exceeding 600 Wh/kg and pulse power exceeding 3 kW/kg. © 2004 Elsevier B.V. All rights reserved.

Hot wire chemical vapor deposition (HWCVD) has been adapted to be a continuous growth process for high-density carbon multiwall nanotubes (MWNTs). MWNT growth is optimized in 1:5 CH4:Ar at 150 Torr with reactor temperatures of 400 and 550 °C for static and flowing gases, respectively. Ferrocene is employed to provide a gas-phase catalyst. Highly graphitic nanotubes can be continuously deposited with iron content as low as 15 wt % and carbon impurities below thermal gravimetric analysis detection limits. The MWNTs are simply purified to 99.5 wt % with minimal structural damage and with a 75 wt % yield. Carbon multiwall nanotubes (MWNTs) are promising for multiple applications including strong composite materials, field emission displays, and adsorbents for gas separation or storage. A continuous low-cost production method for easily purified MWNTs is therefore desired. MWNTs were discovered while vaporizing carbon in an electric arc 1 and were then produced at higher yield by increasing the pressure of the helium gas atmosphere. 2 Chemical vapor deposition (CVD) techniques employing benzene pyrolosis, 3 and the decomposition of ethylene 4 and acetylene 5 on supported metal catalysts were also demonstrated as viable large-scale production methods. Recently, MWNTs have been grown on supported metal particles or films via CVD, 6-10 plasma enhanced CVD, 11-21 hot wire chemical vapor deposition (HWCVD), 22,23 and plasma enhanced HWCVD methods. 24,25

Hot-wire chemical vapor deposition (HWCVD) has been employed for the continuous gas-phase generation of both carbon multi-wall and single-wall nanotube (MWNT and SWNT) materials. Graphitic MWNTs were produced at a very high density at a synthesis temperature of ∼600 °C. SWNTs were deposited at a much lower density on a glass substrate held at 450 °C. SWNTs are typically observed in large bundles that are stabilized by tube–tube van der Waals’ interactions. However, transmission electron microscopy analyses revealed only the presence of isolated SWNTs in these HWCVD-generated materials.

Carbon single-wall nanotubes (SWNTs) grow efficiently from methane on alumina-supported metal catalysts within a fairly narrow temperature window from 680 to 850 °C. An abrupt onset in SWNT growth occurs at the low temperature side of the window, and SWNTs produced at these lower temperatures appear to be relatively free of amorphous or nanocrystalline carbon impurities. Raman spectroscopy shows that SWNT yield drops dramatically at the high-temperature side of the window where most previous chemical vapor decomposition (CVD) studies have been performed. The turn-on at the low-temperature side appears to be controlled by the thermodynamics of SWNT growth, while the turn-off at high temperatures is associated with competitive deposition of amorphous and nanocrystalline carbon. The existence of a temperature window for SWNT growth has not been reported elsewhere, and has important general consequences for CVD growth of SWNTs.

Copper diffusion from the back contact degrades the performance of CdTe solar cells over time and increases the levelized cost of electricity production from CdTe photovoltaics. Recently, carbon single-wall nanotubes (SWNTs) were shown to be a Cu-free, stable alternative that preserves the device efficiency (Phillips et al., Nano Letter, 2013). Large diameter tube samples containing a mixture of semiconducting (s-SWNT) and metallic (m-SWNT) species were used in the previous work, and the mechanisms leading to a low back barrier for majority carrier flow were not clear. The good performance of the back contact was ascribed to the interaction between the s-SWNTs in the film and the polycrystalline facets of the CdTe surfaces. In that case, the s-SWNT species had small bandgaps (∼0.6–0.8 eV). Here, in an attempt to develop a more detailed understanding of the SWNT/CdTe back contact, we employed SWNT samples that are predominantly semiconducting (95%) and of larger bandgap (∼1.1–1.3 eV). The power conversion efficiency of these unoptimized devices was 11.5 % with a s-SWNT back contact, as compared to 11.2% with a standard Cu/Au back contact.

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H2, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials.

We report inelastic neutron scattering results on hydrogen adsorbed onto samples containing single-wall carbon nanotubes. These materials have attracted considerable interest recently due to reports of high density hydrogen storage at room temperature. Inelastic neutron scattering clearly shows the ortho‐para conversion of physisorbed hydrogen in a nanotube containing soot loaded with hydrogen. From the rotational Ja 0! 1 transition, no indication of a significant barrier to quantum rotation is seen. ” 2000 Elsevier Science B.V. All rights reserved.

We demonstrate that laser peak pulse power can be employed to tune carbon single wall nanotube (SWNT) diameters. The production of SWNTs was investigated at room temperature and at 1200°C. The diameters were shifted to smaller sizes in both cases as the pulse power was increased. SWNT size distributions and yields were studied with Raman spectroscopy and transmission electron microscopy. The evolution of the material quality with laser energy parameters offers insight in to SWNT formation mechanisms. These studies should aid in the development of methods for the rational control of SWNT growth.

We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.

A novel, rapid and accurate calibration procedure as a means for quantitative gas desorption measurement by temperature programmed desorption (TPD) spectroscopy is presented. Quantitative measurement beyond the linear regime of the instrument is achieved by associating an instantaneous calibrated molar flow rate of gas to the detector response. This technique is based on fundamental methods, and is independently verified by comparison to the hydrogen desorption capacity of a known standard metal hydride with known stoichiometry. The TPD calibration procedure described here may be used for any pure gas, and the accuracy is demonstrated for the specific case of hydrogen.

Carbon single wall nanotube (SWNT) films were applied as electrodes to replace the p-layer in hydrogenated amorphous silicon (a-Si:H) solar cells. Devices were fabricated by transferring vacuum-filtered SWNT films of varying thickness onto a-Si:H layers grown by plasma enhance chemical vapor deposition on Pilkington TEC 15 glass substrates. Cells incorporating SWNTs were illuminated from each side (glass / SWNT). A cell illuminated through a 25 nm thick SWNT film yielded short circuit current density, open circuit voltage, and efficiency of 5.47 mA/cm2, 0.793 V, and 1.46%, respectively. Maximum quantum efficiency of 48% was measured at 475 nm for the same device.

Hot wire chemical vapor deposition (HWCVD) has been employed for the continuous generation of carbon single-walled nanotube (SWNT) materials. Interestingly, transmission electron microscopy analyses revealed only the presence of isolated SWNTs, rather than nanotubes existing in bundles. An analysis of the growth mechanism explaining the production of isolated SWNTs is provided. Also, the Raman radial breathing modes (RBMs) of the isolated HWCVD-generated nanotubes are compared to the RBMs of small bundles of nanotubes deposited by a conventional CVD technique having a similar diameter distribution.

Since their discovery in 1993, carbon single-wall nanotubes (SWNTs) have been of great interest because of their expected novel electronic, mechanical, and gas adsorption properties. Difficulties arise in measuring these various properties because the SWNTs currently available for analyses have very small bundle sizes (10--20 nm) and are typically in random orientations. A vital step for the industrial application of SWNTs is to align the individual tubes into bundles of a size that allows facile measurement of the various physical and chemical properties. This paper is the first to illustrate a set of procedures for the preparation and isolation of aligned SWNT superbundles.

Experimental wet chemical approaches have been demonstrated in the synthesis of a new chainlike (C60-Fe-C60-Fe)n complex. This structure has been proposed based on C solidstate nuclear magnetic resonance, high-resolution transmission electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. Furthermore, this structure has been shown to have unique binding sites for dihydrogen molecules with the technique of temperature-programmed desorption. The new adsorption sites have binding energies that are stronger than that observed for hydrogen physisorbed on planar graphite, but significantly weaker than a chemical C-H bond. Volumetric measurements at 77 K and 2 bar show a hydrogen adsorption capacity of 0.5 wt%. Interestingly, the BET surface area is ~31 m/g after degassing, which is more than an order of magnitude less than expected given the measured experimental hydrogen capacity. Nitrogen and hydrogen isotherms performed at 75 K show a marked selectivity for hydrogen over nitrogen for this complex, indicating hidden surface area for hydrogen adsorption. Various LixC60 compounds have also been synthesized, inspired by theoretical predictions of an optimized Li12C60 compound with substantial hydrogen capacity. Unfortunately, the theoretical structure was not experimentally achieved, nor was the predicted hydrogen capacity reached.

We present a comprehensive study of the effects of doping and temperature on the conductivity of single-walled carbon nanotube (SWNT) networks. We investigated nearly type-pure networks as well as networks comprising precisely tuned mixtures of metallic and semiconducting tubes. Networks were studied in their as-produced state and after treatments with nitric acid, thionyl chloride, and hydrazine to explore the effects of both intentional and adventitious doping. For intentionally and adventitiously doped networks, the sheet resistance (R(s)) exhibits an irreversible increase with temperature above approximately 350 K. Dopant desorption is shown to be the main cause of this increase and the observed hysteresis in the temperature-dependent resistivity. Both thermal and chemical dedoping produced networks free of hysteresis. Temperature-programmed desorption data showed that dopants are most strongly bound to the metallic tubes and that networks consisting of metallic tubes exhibit the best thermal stability. At temperatures below the dopant desorption threshold, conductivity in the networks is primarily controlled by thermally assisted tunneling through barriers at the intertube or interbundle junctions.

Single-walled carbon nanotube (SWCNT) fibers were engineered to become a scaffold for the storage of hydrogen. Carbon nanotube fibers were swollen in oleum (fuming sulfuric acid), and organic spacer groups were covalently linked between the nanotubes using diazonium functionalization chemistry to provide 3-dimensional (3-D) frameworks for the adsorption of hydrogen molecules. These 3-D nanoengineered fibers physisorb twice as much hydrogen per unit surface area as do typical macroporous carbon materials. These fiber-based systems can have high density, and combined with the outstanding thermal conductivity of carbon nanotubes, this points a way toward solving the volumetric and heat-transfer constraints that limit some other hydrogen-storage supports.

We report a Raman spectroscopy study of charge transfer interactions in complexes formed by single-walled carbon nanotubes (SWNTs) and [FeFe] hydrogenase I (CaHydI) from Clostridium acetobutylicum. The choice of Raman excitation wavelength and sample preparation conditions allows differences to be observed for complexes involving metallic (m) and semiconducting (s) species. Adsorbed CaHydI can reversibly inject electronic charge into the LUMOs of s-SWNTs, while charge can be injected and removed from m-SWNTs at lower potentials just above the Fermi energy. Time-dependent enzymatic assays demonstrated that the reduced and oxidized forms of CaHydI are deactivated by oxygen, but at rates that varied by an order of magnitude. The time evolution of the oxidative decay of the CaHydI activity reveals different time constants when complexed with m-SWNTs and s-SWNTs. The correlation of enzymatic assays with time-dependent Raman spectroscopy provides a novel method by which the charge transfer interactions may be investigated in the various SWNT-CaHydI complexes. Surprisingly, an oxidized form of CaHydI is apparently more resistant to oxygen deactivation when complexed to m-SWNTs rather than s-SWNTs.

Cointercalation of graphite with lithium and organic molecules, such as benzene and tetrahydrofuran (THF), is studied using first-principles calculations. The molecules play an important role in expanding the interlayer graphene distance to $\ensuremath{\sim}7.7\text{ }\text{\AA{}}$. The increased space permits multiple ${\text{H}}_{2}$ species to be bound to Li cations with a binding energy of 10\char21{}22 kJ/mol. Furthermore, in the interstitial area free of Li cations, the negative charge in the graphene sheets enhances the ${\text{H}}_{2}$ binding energy to $\ensuremath{\sim}9\text{ }\text{kJ}/\text{mol}$ through electrostatic attraction. In order to restrain nucleation of lithium hydrides, the densest Li array is determined to be a ${\text{Li}}_{4}(\text{THF}){\text{C}}_{72}$ structure, which absorbs $3.4\text{ }\text{wt}\text{ }%$ hydrogen molecules reversibly. Cointercalation offers an experimentally accessible approach to designing optimized hydrogen storage materials that have not been investigated previously.

Systematic investigation of the ligand exchange reactions between manganese(II) acetate and benzoic acid under solvothermal conditions led to the isolation of crystalline complexes {Mn5(OC(O)CH3)6(OC(O)C6H5)4}(infinity) ( 1) and {Mn5(OC(O)CH3)4(OC(O)C6H5)6}}(infinity) ( 2) in high (i.e.,90%) yields. The complexes are characterized structurally as 2-D honeycomb-like sheets comprised of edge-shared Mn 12 loops with some noteworthy differences as follows. First, buckling of the 2-D sheet in 1 is not observed for 2, presumably as a consequence of additional intersheet phenyl groups in the latter. Second, complex 1 is comprised of only six-coordinate MnII, while 2 has both pseudo-octahedral and distorted trigonal bipyramidal coordinate metal ions. Third, while complex 2 exhibits pi-stacking interactions with intersheet phenyl-phenyl contacts of 3.285 and 3.369 A, 1 exhibits no such bonding. Antiferromagnetic exchange is observed with Weiss constants (theta) of -28 and -56 K and Neel temperatures of 2.2 and 8.2 K for complexes 1 and 2, respectively. The paramagnetic transition at higher temperatures for complex 2 may be attributed to pi-pi exchange through phenyl groups in adjacent layers. Preliminary gas sorption studies (76 K) indidate preferential adsorption of H2 versus N2 for complex 1 only.

Carbon single-walled nanotubes (SWNTs) have been studied extensively as hydrogen storage materials. Herein, a novel hydrogen sorbtion behavior was observed for alkali metal reduced SWNTs and the mechanism of hydrogen binding in these materials has now been elucidated. SWNTs prepared by laser vaporization and purified by oxidation were reduced with Na in combination with naphthalene in tetrahydrofuran (THF) solution. The product, initially formulated as (Na+)xSWNTx-, was dark colored and insoluble in all common solvents examined. Temperature programmed desorption studies showed that hydrogen amounting to 3.5-4.2% w/w was released between 200 and 500°C from the Na-reduced material. This is consistent with hydrogenation of the reduced nanotubes to form C-H bonds with a C2H empirical formula. It appears that SWNT radical anions produced by reaction with sodium deprotonate THF to form hydrogenated nanotubes and the THF cleavage products ethylene and sodium enolate, as confirmed by isotope labeling. A structure consisting of pairs of lines of C-H units that spiral about the long tube axis with a coverage of 50% of the tube carbons is proposed.