Search

Transparent conducting oxides (TCOs) are becoming a more critical element in thin-film photovoltaic devices. In the continued drive to increase efficiency and stability while reducing cost and optimizing performance, the optical, electrical, and materials properties of TCOs gain increasing importance. TCOs can perform a variety of important functions, including contacts, antireflection coatings, and chemical barriers. In this paper, we will review some of the current advances in the field of transparent conductors and, where possible, will relate these advances to thin-film photovoltaic devices. Highlights will be on the rapidly growing collection of new n- and p-type materials; the implications of these materials on PV have not been fully assessed.

New and optimized metal-oxide thin films for both transparent-conductor and antireflection coatings have the potential to significantly improve photovoltaic devices. One approach to this task of discovery and improvement is to use a parallel optimization, i.e., a combinatorial, approach. Using internal seed funds, we have developed the initial deposition, characterization and analysis tools necessary for implementing a combinatorial approach to thin-film metal oxides, with a special focus on transparent conducting oxides (TCOs). 1. Introduction Transparent conducting oxides [1] play a key role in a number of thin-film opto-electronic devices, including photovoltaics, flat panel displays, low-e windows, electrochromic devices, and anti-static coatings [2]. The bulk of these applications rely on the established n-type transparent conducting oxides (TCOs), such as SnO 2 :F, ZnO:Al, and indium-tin-oxide (ITO) [3]. For many of these technologies, next-generation devices would be significantly enhanced with improved or new TCOs [4]. This is leading to the investigation of new n-type materials such as Cd

Although Tl-based superconductors show higher critical temperatures than other compounds, they suffer some disadvantages, such as surface roughness, non-uniform chemical composition, and the need for excess Tl from a separate source during post-deposition annealing. This paper reports a post-deposition annealing process (in the absence of a separate Tl source) of sequentially deposited metallic layers of Tl:Ba:Ca:Cu with excess amount of thallium in the multilayer. The films show uniform chemical composition and smooth surfaces. Critical temperatures as high as 101 K and critical current densities as high as 1.5 × 10 4 A/ cm 2 at 77 K in zero magnetic field have been achieved.

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.

Far-infrared (IR) focal plane arrays are becoming increasingly important for terrestrial and space-based imaging applications. For noise reasons, it is desirable to have transresistance preamplifiers at the focal plane, and, therefore, they need to operate over wide temperature ranges. High-temperature superconducting (HTS) amplifiers that have a high gain, relatively low noise, and response times of >

We have developed an aqueous etchant suitable for producing feature sizes of 1 μm or less in YBa2Cu3O7 and TlBaCaCuO while not degrading high‐frequency surface resistance. The 10%–300% increase in surface resistance previously seen in aqueous solutions is largely due to the presence of dissolved CO2; decarbonation of the water by bubbling N2 before and during etching can produce a better than 50% reduction in the surface‐resistance degradation. Surface‐resistance degradation can be further reduced by selecting the appropriate solution pH for ethylenediaminetetraacetic acid (EDTA)‐based etchants. We have minimized increases in surface resistance to less than 5% following etching in decarbonated disodium EDTA solutions for both YBa2Cu3O7 and TlBaCaCuO superconductors. Similar improvements can be expected from decarbonation and pH control of other aqueous processing solutions.

We report studies of Tl2Ba2CuO6+δ for material where δ→0. This results in the highest Tc (near 93 K) yet achieved in system and a relatively large positive value of dTc/dP to be contrasted with material with δ∼0.2 where Tc is near 40 K dTc/ dP is large and negative.

Non-equilibrium state defines physical properties of materials in many technologies, including architectural, metallic, and semiconducting amorphous glasses. In contrast, crystalline electronic and energy materials, such as transparent conductive oxides (TCO), are conventionally thought to be in equilibrium. Here, we demonstrate that high electrical conductivity of crystalline Ga-doped ZnO TCO thin films occurs by virtue of metastable state of their defects. These results imply that such defect metastability may be important in other functional oxides. This finding emphasizes the need to understand and control non-equilibrium states of materials, in particular, their metastable defects, for the design of novel functional materials.

One way to create hysteretic Josephson junctions from the currently available nonhysteretic high temperature superconducting junctions is to artificially add capacitance. We have adapted a multilayer technique for artificial capacitance addition/grain boundary modification to produce TlCaBaCuO step‐edge junctions exhibiting large amounts of hysteresis at 77 K. A gaplike structure is present at 32–36 mV that has a temperature dependence that does not contradict that predicted by the Bardeen‐Cooper‐Schrieffer theory. In addition, the junctions show fast switching times, less than a fixture limited 20 ps, and Fraunhofer‐like dependence of critical current on magnetic field.

A new approach was developed for the synthesis of nearly monodisperse CdSe nanoparticles directly in a polymer-containing solution in the absence of any other surface capping molecules. The comparatively high synthesis temperature reaction produces good quality crystalline CdSe nanoparticles. Time-resolved microwave conductivity measurements show that photoinduced charge separation occurs at the interface between the CdSe quantum dots and the polymer. This method can be extended to the synthesis of other II-VI semiconductor nanomaterials directly in a polymer-containing solution.

A thermal thin-film high-temperature superconductor (HTS) switch that is easily inserted into high-performance HTS transmission lines is presented. A microwave switch in Tl-Ca-Ba-Cu-O has been developed based on driving a small bridge, embedded in a transmission line, normal via current in an external control line. In the on-state, insertion loss was less than 1 dB over the tested range of 0.5-8.5 GHz. Isolation in the off-state exceeded 30 dB over this frequency range. Response times are on the order of a microsecond, adequate for many microwave/millimeter-wave applications such as switched phase shifters. >

A quasioptical technique of measuring superconductor surface resistance using a confocal resonator has been developed. The method has advantages of nondestructive analysis, high sensitivity, easy extension to higher frequencies, convenient experimental setup, and flexibility in sample size. Tl‐Ca‐Ba‐Cu‐O high‐temperature superconducting films have been measured with this technique and the measured surface resistances were less than 0.01 Ω at 36.135 GHz and 77 K. The measurements have been performed from 29 to 39 GHz, and all films showed roughly a quadratic dependence of surface resistance with frequency.

We have measured enthalpies of formation of YBa2Cu3O7−x, Y2BaCuO5, YBa2Cu4O8, and BaCuO2 using high precision isothermal solution calorimetry. Thermodynamic analysis of the calorimetric data indicates that YBa2Cu3O7−x at the pure-phase composition (1:2:3 cation ratio) is metastable at ≍873 K and 1 atm Po2.

The use of thin-film superconductors for microwave applications is known to depend on the material parameters of surface resistance and penetration depth, which are generally difficult to measure. A measurement technique is presented using microstrip ring resonators and contactless probing that determines these critical parameters. Analysis accounting for field penetration into the conductors is also presented. Normal metal resonators are used to calibrate and verify the measurement. The surface resistance of a Tl-Ca-Ba-Cu-O superconducting thin film measured at 8.03 GHz and 77 K was found to be about 0.5 m Omega . >

Highly crystalline, textured thin films of LiCoxAl1-xO2 (x=0, 0.5) have been grown by pulsed laser deposition. Films of both stoichiometries were dense and uniaxially textured with Li, Co (or Co, A1) layers parallel to the substrate. It was found that crystal quality depended strongly on oxygen partial pressure, substrate temperature, and substrate material. The deposition of LiCo0.5Al0.5O2 is also highly dependent upon laser fluence, requiring at least 12.8 J/cm2 for high quality films. Chemical diffusion measurements were performed over a wide range of lithium contents using the potentiostatic intermittent titration technique. Maximum and minimum effective for LiCoO2 were 4.0 × 10−11 and 1.2 × 10−2 cm2/s, respectively, and for LiCo0.5A10.5O2, 2.2 × 10−12 and 8.0 × 10−17 cm2/s, respectively.

Thin films of crystalline and amorphous V2O5 were deposited by pulsed laser deposition (PLD) and the chemical diffusion coefficients, , were measured by the potentiostatic intermittent titration technique (PITT). In crystalline V2O5 films, the maximum and minimum were found to be 1.7 × 10−2 cm2/s and 5.8 × 10−15 cm2/s respectively, with a general trend for to rise in single-phase regions. The changes in correlated well to the known phases in LiV2O5. In amorphous V2O5 films, exhibited a smooth, continuous decrease as the Li concentration increased.

Our team has been investigating the use of particle-based contacts in both Si and CdTe solar cell technologies. First, in the area of contacts to Si, powders of Al and Ag prepared by an electroexplosion process have been characterized by transmission electron microscopy (TEM), TEM elemental determination X-ray spectroscopy (TEM-EDS), and TEM electron diffraction (TEM-ED). These Al and Ag particles were slurried and tested as contacts to p- and n-type silicon wafers, respectively. Linear current-voltage (I-V) was observed for Ag on n-type Si, indicative of an ohmic contact, whereas the Al on p-type Si sample was non-ideal. A wet-chemical surface treatment was performed on one Al sample and TEM-EDS indicated a substantial decrease in the O contaminant level. The treated Al on p-type Si films exhibited linear I-V after annealing. Second, in the area of contacts to CdTe, particles of Hg-Cu-Te and Sb-Te have been applied as contacts to CdTe/CdS/SnO2 heterostructures prepared by the standard NREL protocol. First, Hg-Cu-Te and Sb-Te were prepared by a metathesis reaction. After CdCl2 treatment and NP etch of the CdTe layer, particle contacts were applied. The Hg-Cu-Te contacted cells exhibited good electrical characteristics, with Voc > 810 mV and efficiencies > 11.5 % for most cells. Although Voc > 800 mV were observed for the Sb-Te contacted cells, efficiencies in these devices were limited to 9.1%, presumably by a large series resistance (>20 {Omega}) observed in all samples.

We have measured for the first time the temperature‐dependent anisotropic resistivity of several Tl2Ca2Ba2Cu3Ox single crystals. The resistivity ratio between the c axis and the a‐b plane orientations in the best crystals is approximately 60 at room temperature. Resistivity data indicate substantial differences in both Tc and the transition width of nominal Tl2Ca2Ba2Cu3Ox plates grown from two different melt compositions. A Tl‐O and CuO‐rich flux yielded approximately stoichiometric plates with sharp transitions beginning near 113 K, while a Tl‐O‐rich flux produced plates containing more Tl and less Ba with broad transitions starting near 103 K. These data demonstrate the extreme sensitivity of the superconductivity to cation site disorder in the Tl‐Ca‐Ba‐Cu‐O system.

Flux creep data are reported which clearly demonstrate a dramatic enhancement in the pinning potential for magnetic vortices (fluxoids) in a Tl2Ca2Ba2Cu3Oy single crystal upon irradiation with 4.5 MeV protons. The creep rate is markedly reduced between 5 and 80 K, and the effective pinning potential is quadrupled from ≊100 to ≊400 meV at 20 K in a field of 50 mT. This stronger pinning enhances the magnetization hysteresis and hence the inferred low‐field critical current density by a factor of 5 at 40 K.

Cu-In-Ga-Se nanoparticle colloids have been used as precursors in the spray deposition of photovoltaic films. Precursor colloid was prepared by reaction of the metal iodides in pyridine with sodium selenide in methanol at reduced temperature according to one of two routes: synthesis of each of the component binary selenides (Type I) followed by physical mixing of the isolated particles; or a one-pot synthesis with all the metal iodides reacting together in one flask to form a mixed-metal Cu-In-Ga-Se colloid (Type II). The constituent nanoparticles in these colloids were analyzed by TEM and XRD and were determined to be amorphous as-synthesized. Crystalline phase formation of these nanoparticles was observed by XRD after a thermal treatment. These precursor colloids were sprayed onto Mo-coated glass substrates at elevated temperatures. The nanoparticle-derived Cu-In-Ga-Se films were characterized by SEM and XRD prior to being finished into CIGS solar cell devices according to standard NREL protocol. I–V characterization of these CIGS solar cells showed these devices are limited by a large series resistance.

Cyclodextrins appear to act as bifunctional linkers when interacting with anatase TiO2 particles under UV light, resulting in super long TiO2-containing wires. These assemblies display mechanical flexibility, stable electronics, and rapid response/long lifetime under photoinduced current.

The authors report the performance of various planar metal–insulator–metal (MIM) tunneling diodes, which are being investigated for use in rectenna devices for energy harvesting applications. Six cathode materials (M2): Nb, Ag, Cu, Ni, Au, and Pt are studied in conjunction with Nb as the anode (M1) and Nb2O5 (I) as the dielectric. The cathode materials selections were based on results from a prior rapid-screening study that employed a bent-wire metal cathode point-contact method. Planar devices, to enable analysis using standard MIM diode models, were fabricated with the resultant current density–voltage data obtained at both room temperature and 77 K. The tunnel barrier heights and dielectric properties for these systems were extracted from the modeling results. Nb/Nb2O5/Pt MIM diodes showed the best performance with an asymmetry ratio greater than 7700, a nonlinearity value of 4.7, and a responsivity of 16.9, all at 0.5 V and 300 K. These results confirm prior rapid-screening efforts and further validate the Nb/Nb2O5/Pt system in particular as a promising MIM architecture due to the low barrier height of the junction.The authors report the performance of various planar metal–insulator–metal (MIM) tunneling diodes, which are being investigated for use in rectenna devices for energy harvesting applications. Six cathode materials (M2): Nb, Ag, Cu, Ni, Au, and Pt are studied in conjunction with Nb as the anode (M1) and Nb2O5 (I) as the dielectric. The cathode materials selections were based on results from a prior rapid-screening study that employed a bent-wire metal cathode point-contact method. Planar devices, to enable analysis using standard MIM diode models, were fabricated with the resultant current density–voltage data obtained at both room temperature and 77 K. The tunnel barrier heights and dielectric properties for these systems were extracted from the modeling results. Nb/Nb2O5/Pt MIM diodes showed the best performance with an asymmetry ratio greater than 7700, a nonlinearity value of 4.7, and a responsivity of 16.9, all at 0.5 V and 300 K. These results confirm prior rapid-screening efforts and further validat...

We report on synthesis, stability, electronic structure and optical properties of CuTaN2 with the delafossite crystal structure and its potential use as an absorber for solar energy conversion applications. According to theoretical first-principles calculations, the formation enthalpy of CuTaN2 is negative (−0.66 eV per atom), but this material is metastable with respect to decomposition into Cu, Ta3N5 and N2. Nevertheless, the experimental thermal stability limit of single phase CuTaN2 powders synthesized using an ion exchange method is 250 °C in ambient atmosphere, according to combined temperature-dependent X-ray diffraction and thermo-gravimetric analyses. Electronic structure of the CuTaN2 is different compared to that of CuAlO2, in particular the band gap of this nitride delafossite (1.3 eV) calculated using HSE06+G0W0 is much smaller than the band gap of the oxide delafossite. The onset of optical absorption onset of CuTaN2 at 1.5 eV determined from experimental diffuse reflectance measurements is consistent with the theoretical 1.4 eV optical band gap and large calculated absorption coefficient (>105 cm−1 above 1.5 eV) determined from time-dependent HSE06 calculations corrected by a scissors operator. The significance of our findings is that optical properties of CuTaN2 are nearly optimal for photovoltaic energy conversion.

This work expands on the recently reported protonation of the donor molecule 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(4-(5′-hexyl-[2,2′-bithiophen]-5-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine) (d-DTS(PTTh2)2) by the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayer to include an electrostatic picture of interfacial energetic states. Ultraviolet photoemission spectroscopy results initially suggested favorable band level alignment for hole extraction between d-DTS(PTTh2)2 and PEDOT:PSS. However photovoltaic device performance yields a low fill factor and photovoltage, indicative of poor hole-extraction at the hole-collecting interface, relative to the nickel oxide device. Further investigation into the interfacial composition via theory and X-ray photoelectron studies of both the interface and a control system of d-DTS(PTTh2)2 reacted with p-toluenesulfonic acid verify the presence of a chemically unique species at the interface arising from protonation reaction with the residual acidic protons present in PEDOT:PSS that was masked in the UPS experiment. From these results, the energy band diagram is re-interpreted to account for the interfacial chemical reaction and modified interfacial density of states. Additionally, the detrimental protonation reaction is avoided when the pyridyl[1,2,5]thiadiazole acceptor unit was replaced with a 5-fluorobenzo[c][1,2,5]thiadiazole acceptor unit, which shows no such reaction with the PEDOT:PSS substrate. These results indicate the necessity of using a large analytical toolkit to elucidate the energetics and mechanisms of buried interfaces that will impact dynamics of hole collection.

CdTe thin films were synthesized using CdTe nanoparticle precursors. Capped CdTe nanoparticles were prepared by injection of a mixture of Cd(CH3)2, (n‐C8H17)3PTe and (n‐C8H17)3P into (n‐C8H17)3PO at elevated temperatures and characterized by x‐ray diffraction, UV‐Vis spectroscopy, transmission electron microscopy, thermogravimetric analysis and energy dispersive x‐ray spectroscopy. CdTe thin film deposition was accomplished by forming a colloidal suspension of CdTe nanoparticles in a suitable organic solvent and then spraying the colloid onto SnO2‐coated glass substrates at variable susceptor temperatures (‘‘one‐step process’’). In addition, a two‐step process was performed where a low‐temperature spray deposition (T=150 °C) was followed by sintering in air, forming gas or argon at variable annealing temperatures. The one‐step process yielded smooth and dense CdTe thin films using growth temperatures approximately 200 °C less than conventional spray pyrolysis approaches. CdTe films were characterized by x...

Superlattice structures, consisting of SrCuO2, (Sr,Ca)CuO2, and BaCuO2 layers in the tetragonal, “infinite layer” crystal structure, have been grown by pulsed-laser deposition (PLD). Superlattice chemical modulation is observed for structures with component layers as thin as a single unit cell (~3.4 A), indicating that unit-cell control of (Sr,Ca)CuO2 growth is possible using conventional pulsed-laser deposition over a wide oxygen pressure regime. X-ray diffraction intensity oscillations, due to the finite thickness of the film, indicate that these films are extremely flat with a thickness variation of only ~20 A over a length scale of several thousand angstroms. Using the constraint of epitaxy to grow metastable cuprates in the infinite layer structure, novel high-temperature superconducting structural families have been formed. IN particular, epitaxially-stabilized SrCuO2/BaCuO2 superlattices, grown by sequentially depositing on lattice-matched (100) SrTiO3 from BaCuO2 and SrCuO2 ablation targets in a PLD system, show metallic conductivity and superconductivity at T c(onset) ~70 K. these results show that pulsed-laser deposition and epitaxial stabilization have been used to effectively “engineer” artificially-layered thin-film materials.

In this article we report the first nanoparticle-derived route to smooth, dense, phase-pure CdTe thin films. Capped CdTe nanoparticles were prepared by injection of a mixture of Cd(CH3)2, (n-C8H17)3 PTe and (n-C8H17)3P into (n-C8H17)3PO at elevated temperatures. The resultant nanoparticles 32-45 Å in diameter were characterized by x-ray diffraction, UV-Vis spectroscopy, transmission electron microscopy, thermogravimetric analysis and energy dispersive x-ray spectroscopy. CdTe thin film deposition was accomplished by dissolving CdTe nanoparticles in butanol and then spraying the solution onto SnO2-coated glass substrates at variable susceptor temperatures. Smooth and dense CdTe thin films were obtained using growth temperatures approximately 200 °C less than conventional spray pyrolysis approaches. CdTe films were characterized by x-ray diffraction, UV-Vis spectroscopy, atomic force microscopy, and Auger electron spectroscopy. An increase in crystallinity and average grain size as determined by x-ray diffraction was noted as growth temperature was increased from 240 to 300 °C. This temperature dependence of film grain size was further confirmed by atomic force microscopy with no remnant nanocrystalline morphological features detected. UV-Vis characterization of the CdTe thin films revealed a gradual decrease of the band gap (i.e., elimination of nanocrystalline CdTe phase) as the growth temperature was increased with bulk CdTe optical properties observed for films grown at 300 °C.

The utilization of radio-frequency (RF) superimposed direct-current (DC) magnetron sputtering deposition on the properties of gallium doped ZnO (GZO) based transparent conducting oxides has been examined. The GZO films were deposited using 76.2 mm diameter ZnO:Ga{sub 2}O{sub 3} (5 at. % Ga vs. Zn) ceramic oxide target on heated non-alkaline glass substrates by varying total power from 60 W to 120 W in steps of 20 W and at various power ratios of RF to DC changing from 0 to 1 in steps of 0.25. The GZO thin films grown with pure DC, mixed approach, and pure RF resulted in conductivities of 2200 {+-} 200 S/cm, 3920 {+-} 600 S/cm, and 3610 {+-} 400 S/cm, respectively. X-ray diffraction showed all films have wurtzite ZnO structure with the c-axis oriented perpendicular to the substrate. The films grown with increasing RF portion of the total power resulted in the improvement of crystallographic texture with smaller full-width half maximum in {chi} and broadening of optical gap with increased carrier concentration via more efficient doping. Independent of the total sputtering power, all films grown with 50% or higher RF power portion resulted in high mobility ({approx}28 {+-} 1 cm{sup 2}/Vs), consistent with observed improvementsmore » in crystallographic texture. All films showed optical transmittance of {approx}90% in the visible range.« less

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoOx) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air yield continuous thin films of MoOx. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoOx and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoOx thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo5+ gap states in the MoOx HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoOx HCL.

This critical review focuses on the solution deposition of transparent conductors with a particular focus on transparent conducting oxide (TCO) thin-films. TCOs play a critical role in many current and emerging opto-electronic devices due to their unique combination of electronic conductivity and transparency in the visible region of the spectrum. Atmospheric-pressure solution processing is an attractive alternative to conventional vacuum-based deposition methods due to its ease of fabrication, scalability, and potential to lower device manufacturing costs. An introduction into the applications of and material criteria for TCOs will be presented first, followed by a discussion of solution routes to these systems. Recent studies in the field will be reviewed according to their materials system. Finally, the challenges and opportunities for further enabling research will be discussed in terms of emerging oxide systems and non-oxide based transparent conductors (341 references).

Laser-induced breakdown spectroscopy (LIBS) is a recently developed locally destructive elemental analysis technique that can be used to analyze solid, liquid, and gaseous samples. In the system explored here, a neodymium-doped yttrium aluminum garnet laser ablates a small amount of the sample and spectral emission from the plume is analyzed using a set of synchronized spectrometers. We explore the use of LIBS to map the stoichiometry of compositionally graded amorphous indium zinc oxide thin-film libraries. After optimization of the experimental parameters (distance between lens and samples, spot size on the samples, etc.), the LIBS system was calibrated against inductively coupled plasma atomic emission spectroscopy which resulted in a very consistent LIBS-based elemental analysis. Various parameters that need to be watched closely in order to produce consistent results are discussed. We also compare LIBS and x-ray fluorescence as techniques for the compositional mapping of libraries.

Organic light emitting devices have been achieved with an indium-free transparent anode, Ga doped ZnO (GZO). A large area coating technique was used (RF magnetron sputtering) to deposit the GZO films onto glass. The respective organic light emitting devices exhibited an operational voltage of 3.7 V, an external quantum efficiency of 17%, and a power efficiency of 39 lm/W at a current density of 1 mA/cm2. These parameters are well within acceptable standards for blue OLEDs to generate a white light with high enough brightness for general lighting applications. It is expected that high-efficiency, long-lifetime, large area, and cost-effective white OLEDs can be made with these indium-free anode materials.

We have used 2.0-μsec microwave pulses at a frequency of 2.856 GHz to rapidly heat thin amorphous yttrium-barium-copper-oxide (YBCO) films deposited onto silicon substrates. The samples were irradiated inside a WR-284 waveguide by single-pass TE10 pulses in a traveling wave geometry. X-ray diffractometry studies show that an amorphous-to-crystalline phase transition occurs for incident pulse powers exceeding about 6 MW, in which case the amorphous YBCO layer is converted to Y2BaCuO5. Microscopy of the irradiated film reveals that the phase transition is brought about by melting of the YBCO precursor film and crystallization of the molten layer upon solidification. Time-resolved in situ experiments of the microwave reflectivity (R) and transmissivity (T) show that there is an abrupt change in R for microwave pulse powers exceeding the melt threshold, so that measurements of R and T can be used to monitor the onset of surface melting.

The structural, electrical and magnetic properties are compared for three superconducting Tl-Ca-Ba-Cu-O thin films prepared by different deposition and sintering protocols. One film containing a mixture of Tl2Ca2Ba2Cu3Ox and Tl2Ca2Ba2Cu3Oy structural phases has the best superconducting properties. Deposition of a Tl-free Ca-Ba-Cu-O precursor film followed by sintering in Tl-O vapor may be the preferred protocol to obtain a single structural phase.

The critical current density inferred from magnetization hysteresis for epitaxial Tl2Ca2Ba2Cu3O10 films on LaAlO3 exhibits a pronounced increase with film thickness in the high-field, high-current regime. This thickness dependence does not extend to comparatively “thick” single crystals, however, suggesting that the grain boundaries play a major role in collective vortex pinning for Tl-based superconducting thin films.

The superconducting flux flow transistor (SFFT) is a single film, active device generally made from high temperature superconducting (HTS) materials. The device is based on the magnetic control of flux motion, has gain and can operate to over 40 GHz with 3 µm feature size. Existing models are reviewed along with the latest equivalent circuit values derived from S-parameter measurements. Noise performance is compared to that predicted by a relatively simple flux noise model and recent amplifier performance is discussed.

This article follows our previous work on the synthesis and characterization of pi-conjugated dendrimers for use in organic solar cells. Here we discuss five new thiophene-based dendrimers that were synthesized in order to study the relationship between their chemical structures and electronic properties. Three of these dendrimers incorporate acetylene spacers, included to relieve steric strain, between the thiophene arms and phenyl cores used in previous studies. Only a small effect on the electronic properties is observed upon inclusion of the acetylene spacer in the three-arm dendrimer, 3G1-2S-Ac. In contrast, a decrease in the bandgap is observed for the four-arm dendrimer, 4G1-2S-Ac, due to a reduction of interactions between the arms in the more sterically congested 1,2,4,5-arrangement around the phenyl core, resulting in delocalization of the exciton through the phenyl core. Incorporation of electron-withdrawing cyano groups on the phenyl core of the three-arm dendrimer, 3G1-2S-CN, resulted in a very large (∼0.5 eV) decrease in the bandgap, due to stabilization of the lowest unoccupied molecular orbital, and the low energy absorption band in this material is attributed to a transition with significant intramolecular charge-transfer character. The electronic properties of three- and four-arm dendrimers with electron-donating dibutylaniline moieties attached to the end of the thiophene dendron, 3G1-2S-N and 4G1-2S-N respectively, are almost identical, indicating that they are dominated by the arms, with no through-core communication allowed, even for the para-linked arms of 4G1-2S-N. However, there is a significant increase in the molar absorptivity of these materials, concomitant with significant broadening of the absorption spectrum, which is an important attribute in light-harvesting applications.

This study presents clear and compelling experimental evidence for the significant beneficial effects of nitrogen-doping on the activity of Pt/C catalyst systems for the methanol oxidation reaction. This evidence is obtained through the deployment of geometrically well-defined model catalytic systems consisting of tunable assemblies of Pt catalyst nanoparticles deposited onto undoped, Ar-doped, and N-doped highly oriented pyrolytic graphite (HOPG) substrates. Both Ar- and N-doping were achieved via ion beam implantation, and Pt was electrodeposited from solutions of H2PtCl6 in aqueous HClO4. Morphology from scanning electron microscopy (SEM) and aqueous electrochemical analysis of catalytic activity was utilized to examine the effect of N-doping compared to the undoped and Ar-doped control samples. The results strongly support the theory that doping nitrogen into a graphite support significantly affects both the morphology and the behavior of the overlying Pt nanoparticles. In particular, nitrogen-doping was observed to cause a significant decrease in the average Pt nanoparticle size, an increase in the Pt nanoparticle dispersion, and a significant increase in catalytic activity and durability for methanol oxidation. The model catalytic systems demonstrated here represent a versatile platform to study catalyst-support interactions in electrocatalytically relevant nanoparticle systems.

For organic photovoltaics (OPVs) to become a truly viable technology, it is critical to correctly identify their degradation mechanism(s) and pave the way for strategies to improve device lifetime. Changes in the active organic component are often cited as the leading degradation pathway in OPVs. In this shelf life study, we monitor device performance after storage in the dark at ambient conditions to show that decay behavior is dominated by changes at the top metal electrode and/or the metal–organic interface. Cross-sectional TEM reveals extensive void formation to be the primary degradation mechanism for Ca/Al contacts. Kelvin probe measurements show the work function of Ag contacts increases over time, consistent with silver oxide formation detected by TOF-SIMS depth profiles. The evolution of the work function is found to be advantageous for Ag as a hole-extracting contact, but unsuitable for electron-extraction. The combination of Ag positive contacts (to collect holes) and ITO/ZnO negative contacts (to collect electrons) yields devices that are stable up to one year when stored in ambient conditions.

TlCaBaCuO films using e-beam evaporation with subsequent sintering and annealing that have critical current densities of at least several hundred kA/sq cm, critical temperatures over 100 K, and surface resistance better than that of gold at 77 K and 8 GHz. Processing techniques have been developed for making microwave passive elements and a four-terminal active device called the superconducting flux flow transistor (SFFT) for microwave applications. The techniques include contacting, dielectric, normal layer deposition, and controlled HTS film etching. The SFFT shows promise as a microwave amplifier, oscillator, and active impedance converter and may also have many other applications. The advantages of the device include high speed, potentially low noise, and, for some applications, useful impedance levels.

The role of the work function and interfacial chemistry on organic device performance was investigated by using a series of contact materials. The active layer was a standard blend of poly(3-hexylthiophene) and [6-6]-phenyl C61-butyric acid methyl ester. Over 100 devices were fabricated and measured to obtain good statistics. Ba∕Al and Ca∕Al electrodes performed best, with similar open-circuit voltages and power conversion efficiencies. Device stability studies showed devices with these two electrodes remained similar after six weeks with degradation of 11%–16% in net conversion efficiency observed. The incorporation of silver into the electrodes led to considerably more degradation than other electrode types.