William B. Heuer

Oligothiophene as photonic/electronic property modulator

Several different series of conjugated oligomers bearing various π-centers such as dithienothiophene (DTT), fluorene and terthiophene moieties, attaching electron donor and/or electron acceptor units through conjugation were synthesized and assessed for their nonlinear optical, two-photon absorption and redox properties. Discussion is made on the property modulation role of the π-centers, particularly by DTT oligothiophene, which displays a unique and efficient electronic mediation.

New Dicarboxylic Acid Bipyridine Ligand for Ruthenium Polypyridyl Sensitization of TiO2

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.

Bipolar behavior revealed by D-π-D chromophores bearing dithienothiophene (DTT) as π-center in redox- and LE properties

Abstract A series of D/D- and D/A pair conjugated chromophores based on dithienothiophene, DTT, as conjugated linker (π-center) were synthesized (D-π-D and D-π-A) and studied optical and electrochemical properties. Unusual bipolar behavior was observed with D-π-D, displaying the redox property similar to that of D-π-A. Single-layer light-emitting (LE) devices fabricated with such electronically dissimilar oligomers exhibited a close resemblance in their diode characteristics that are commonly observed with bipolar LE materials.

Safer salts for CdTe nanocrystal solution processed solar cells: the dual roles of ligand exchange and grain growth

Inorganic CdSe/CdTe nanocrystals for solid-state photovoltaic devices are typically sintered into a bulk-like material after annealing in the presence of solid cadmium chloride. As in commercial CdTe devices, this salt exposure is a key component to improve device performance by promoting grain growth. However, in contrast to vapor depositions, we demonstrate that the role of the salt treatment also involves crucial ligand removal reactions, which are a unique challenge facing nanocrystal ink depositions. After testing other salts such as CdF2, CdCl2, CdBr2, CdI2 and Cd(NO3)2 for oleate ligand removal as determined by FTIR, SEM imaging of CdTe grain growth revealed the largest grains were observed from reactions with CdCl2 (142 ± 26 nm) and, to a lesser extent, CdBr2 (131 ± 19 nm). These results were used to identify cadmium-free alternatives. Trimethylsilyl chloride (28.0 ± 5.1 nm), NH4Br (75.5 ± 31 nm) and NH4Cl (136 ± 39 nm) were also tested, demonstrating comparable ligand removal and grain growth to the cadmium halides. In order to validate these observations, heterojunction photovoltaic devices were fabricated from CdSe/CdTe nanocrystals treated with non-toxic NH4Cl in place of the conventional CdCl2. Under AM 1.5G illumination, open circuit voltages (Voc), short circuit currents (Jsc) and efficiencies (η) of solar cells processed with evaporated Au and commercial ITO were found to be Voc = 0.46 ± 0.02 V, Jsc = 9.27 ± 0.6 mA cm−2, and η = 1.73 ± 0.24 demonstrating minimal differences in film morphology and device performance compared to those fabricated using cadmium chloride. Specific properties of the salts (solubility, reactivity, melting point and the identity of both the cation and the anion) were found to have a profound impact on grain growth and consequently device performance, suggesting the need for further investigation of additional non-toxic metal halide salts for this reaction.

Influence of hemicyanine dye structures on spectral properties of their supramolecular complexes with amylose

Hemicyanine chromophores bearing long-chain alkyl substitutents on the donor and/or acceptor ends have been prepared, and absorption and emission spectra of their supramolecular complexes with amylose have been studied in order to relate them to their supramolecular structures.

Synthesis and characterization of nickel-group bis(dithiocroconate) complexes and dicyanomethylene-substituted analogues

Two series of nickel-group metal bis(dithiolene) complexes with ligands 4,5-disulfanylcyclopent-4-ene-1,2,3-trionate (L1) and 2-dicyanomethylene-4,5-disulfanylcyclopent-4-ene-1,3-dionate (L2) have been prepared and characterized: [NBu4n]2[M(Li)2](M = Ni, Pd or Pt; i= 1 or 2). Oxidation of the dianion complexes yielded paramagnetic monoanions with ESR spectra indicative of a delocalized b3g HOMO (highest occupied molecular orbital), like that previously found for comparable bis(dithiolene) complexes. The intense low-energy visible absorptions and multiple, reversible reductions exhibited by the dianions likewise suggest that the LUMO (lowest unoccupied molecular orbital) is a ligand-based au(π*) orbital, rather than the b1g(dxy) orbital as commonly found for such complexes. The stabilization of the ligand-based LUMO in this case is attributed to the strongly electron-withdrawing character of the ligand substituents. Iodination of [NBu4n]2[Pd(L2)2] in CH2Cl2 solution yielded the novel iodine inclusion compound [NBun4]2[Pd(L2)2]·I2, which crystallizes in space group P with Z= 1, a= 10.792(3), b= 13.995(5), c= 10.737(3)A, α= 105.48(1), β= 115.15(1) and γ= 76.51(2)° at 25 °C. The I2 molecules are associated with the complex anions through short [3.1696(9)A] S ⋯ I contacts; however, the observed I–I distance [2.7354(4)A] indicates that the degree of charge transfer associated with this interaction is small.

Electrical Measurement Under Atmospheric Conditions of PbSe Nanocrystal Thin Films Passivated by Remote Plasma Atomic Layer Deposition of Al

PbSe nanocrystal thin-film transistors (TFTs) were passivated using remote plasma atomic layer deposition (ALD) of a ~10 nm thick Al2O3 film at 150 °C. By using a highly reactive remote oxygen plasma source, the time for one complete ALD cycle was about 15 s with growth rates of ~1.1 Å/cycle. The effective mobilities measured under atmospheric condition from AlO-passivated PbSe nanocrystal TFTs were comparable to the values reported previously for air-free PbSe nanocrystal TFTs, demonstrating that ALD Al2O3 layers prevent oxidation and degradation of nanocrystal films from air exposure. The variation in the effective mobility of passivated devices was also found to be negligible under ambient conditions over a period of 30 days. The results show that remote plasma ALD processing of Al2O3 is capable of producing an effective passivation layer on air-sensitive nanocrystals with high deposition rates at reduced temperature.

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The syntheses of copper and bis(η-cyclopentadienyl)molybdenum complexes of a dithiolene ligand incorporating a trioxadithia-15-crown-5-unit [1,4,7-trioxa-10,13-dithiacyclopentadec-11-ene-11,12-dithiolate(2–)] are described. The electrochemistry of these complexes was investigated by cyclic voltammetry in acetonitrile. Upon addition of an excess of alkali-metal cations large shifts of the redox potentials of the compounds were observed.

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We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 °C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl(-) anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136±39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.