Alexander Miedaner

Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P2PhN2Ph)2(CH3CN)](BF4)2, 3a (where P2PhN2Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H2 production rate to be determined. The turnover frequency of 350 s−1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P2CyN2Bz)2](BF4)2, 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (Keq = 190 atm−1 at 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A–4C, which have been identified by a combination of one- and two-dimensional 1H, 31P, and 15N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P2CyN2Bz)2](BF4), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.

Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

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.

Direct write processing for photovoltaic cells

Direct writing of solar cell components is an attractive processing approach. We have fabricated a 6.8% Si solar cell using silver ink based electrodes. Ohmic contact through the antireflection (AR) coating was obtained with pure Ag electrodes at 850/spl deg/C. We also report on highly conductive silver metallizations and initial results on direct-write TCO demonstrating a 100-micron spatial resolution produced by inkjet printing.

Spray and Inkjet Printing of Hybrid Nanoparticle-Metal-Organic Inks for Ag and Cu Metallizations

Metal-organic and hybrid metal-organic/metal nanoparticle inkswere evaluated for use in the inkjet printing of copper and silver conducting lines. Pure, smooth, dense, highly conductive coatings were produced by spray printing with (hexafluoroacetylacetonato)copper(I)-vinyltrimethylsilane Cu(hfa)·VTMS) and (hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene) (Ag(hfa)COD) metal-organic precursors on heated substrates. Good adhesion to the substrates tested, glass, Kapton tape and Si, has been achieved without use of adhesion promoters. The silver metal-organic ink has also beenused to print metal lines and patterns with a commercial inkjet printer. Hybrid inks comprised of metal nanoparticles mixed with the metal-organic complexes above have also been used to deposit Cu and Ag films by spray printing.This approach gives dense, adherent films that are much thicker than those obtained using the metal-organic inks alone. The conductivities of the silvercoatings obtained by both approaches are near that of bulk silver (2 μΩ·cm). The copper coatings had conductivities at least an order ofmagnitude less than bulk.

Direct write contacts for solar cells

Ag, Cu and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si and PCB have been printed at 100-200 /spl deg/C in air and N/sub 2/ respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850 /spl deg/C resulting in 8% cells. Next generation multicomponent inks (including etching agents) have also been developed with improved fire through contacts leading to higher cell efficiencies. PEDOT-PSS polymer based conductors were inkjet printed with conductivity as good or better than that of spin-coated films.

Direct inkjet printing of composite thin barium strontium titanate films

Composite Ba 0 . 6 Sr 0 . 4 TiO 3 /MgO thin films with 60% tuning and tan δ of 0.007 at 2 GHz were deposited using metal organic decomposition inks by spin coating on single crystal MgO substrates. The films with approximately 1 mol% MgO in Ba 0 . 6 Sr 0 . 4 TiO 3 had a better tuning/loss ratio than either the 0 or the 10 mol% MgO substituted films. Crystalline Ba 0 . 5 Sr 0 . 5 TiO 3 films were produced on both MgO and alumina substrates by inkjet printing of metalorganic precursors with subsequent thermal decomposition followed by annealing at 900 °C. Barium strontium titanate lines as narrow as 100 μm were printed on the alumina substrates. The inkjet-printed films were predominantly (100) oriented on MgO and (110) oriented on alumina. The crystalline quality of the inkjet-printed films was improved by annealing at 1100 °C for 3 h in oxygen. Both the printed and the spin-coated films had smooth surfaces (300 A root-mean-square roughness) as required for subsequent deposition of high-resolution metal electrodes. An inkjet-printed Ba 0 . 5 Sr 0 . 5 TiO 3 film (3500 A) on MgO annealed at 1100 °C had 20% tunability of the dielectric constant (∈) at 9. 1 V/μm direct current bias and tan 8 < 0.002 at 1 MHz.

Solution Synthesis and Characterization of Indium-Zinc Formate Precursors for Transparent Conducting Oxides

A series of In-Zn formate mixtures were investigated as potential precursors to amorphous In-Zn-oxide (IZO) for transparent conducting oxide (TCO) applications. These mixtures were prepared by neutralization from formic acid and characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction, and thermogravimetry-differential scanning calorimetry (TG-DSC) measurements. Thermal analysis revealed that a mixture of In and Zn formates reduced the overall decomposition temperature compared to the individual constituents and that OH-substitution enhanced the effect. In terms of precursor feasibility, it was demonstrated that the decomposition products of In-Zn formate could be directed toward oxidation or reduction by controlling the decomposition atmosphere or with solution acid additives. For TCO applications, amorphous IZO films were prepared by ultrasonic spray deposition from In-Zn formate solutions with annealing at 300-400 degrees C.

Conjugated polymer/nanostructured oxide semiconductor composite photovoltaic devices

Organic semiconductor-based photovoltaic devices offer the promise of low cost photovoltaic technology that can be manufactured via large-scale, roll-to-roll printing techniques. Existing organic photovoltaic devices are currently limited to solar power conversion efficiencies of 3-5%. The reasons for this include 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. For both ZnO and TiO/sub 2/, nanostructures that are vertically aligned with respect to the substrate can be grown. Here we discuss the fabrication of such nanostructures and present preliminary results from ZnO nanocarpet/poly(3-hexylthiophene) composite PV devices.

Solution deposition of amorphous IZO films by ultrasonic spray pyrolysis

Atmospheric-pressure solution deposition of the transparent conducting oxide (TCO), amorphous indium-zinc oxide (α-IZO), was investigated as an alternative to traditional vacuum-based physical vapor deposition techniques for photovoltaic applications. Solution processing is attractive due to its ease and potential to lower device manufacturing costs. Here we report on α-IZO films prepared by ultrasonic spray pyrolysis from solutions of an indium-zinc formate (IZF) precursor. Thin, crack-free, amorphous IZO films with good optical transmittance (≫75%) and conductivities of ∼34 S/cm were produced from an IZF-HNO3-methanol ink using joint RTP and Ar-H2 annealing.

Low cost copper indium gallium selenide by the FASST® process

Low cost manufacturing of Cu(In,Ga)Se2 (CIGS) films for high efficiency PV devices by the innovative Field-Assisted Simultaneous Synthesis and Transfer (FASST®) process is reported. The FASST® process is a two-stage reactive transfer printing method relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage these precursors are brought into intimate contact and rapidly reacted under pressure in the presence of an applied electrostatic field. The method utilizes physical mechanisms characteristic of anodic wafer bonding and rapid thermal annealing, effectively creating a sealed micro-reactor that insures high material utilization efficiency, direct control of reaction pressure, and low thermal budget. The use of two independent precursors provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the second stage FASST® synthesis of CIGS. High quality CIGS with large grains on the order of several microns are formed in just several minutes based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 12.2% have been achieved using this method.