S. A. Carter

Polymeric anodes for improved polymer light-emitting diode performance

We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes [σ>2(Ωcm)−1], diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.

Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion

[19] Since the mixing ratio of the anions in the solution from which the crystals were grown was revealed to be almost identical to the stoichiometry determined by EPMA of the obtained crystal, the mixing ratio of the anions in the solution was adopted in the chemical formula of the crystal. The results of EPMA of k-(BETS)2FexGa1‐xBr1.0Cl3.0 are: for x = 0.50, Fe/Ga/Br/Cl = 0.51:0.49:1.13:2.85 (0.50:0.50:1.00:3.00); for x = 0.40, Fe/Ga/Br/Cl = 0.40:0.60:0.96:2.87 (0.40:0.60:1.00:3.00); for x = 0.30, Fe/Ga/Br/Cl = 0.32:0.68:1.11:3.07 (0.30:0.70:1.00:3.00); for x = 0.20, Fe/Ga/Br/Cl = 0.24:0.76:1.11:3.16 (0.20:0.80:1.00:3.00); for x = 0.10, Fe/Ga/Br/Cl = 0.12:0.88:1.04:3.22 (0.10:0.90:1.00:3.00). [20] Recent susceptibility measurements revealed that the angle between the easy axis of the antiferromagnetic structure and the c axis is about 35 in k-(BETS)2FeCl4 (E. Ojima, T. Sasaki, private communication). While the easy axis of k-(BETS)2FeBr0.6Cl3.4 is approximately parallel to the b* axis.

TEMPERATURE- AND FIELD-DEPENDENT ELECTRON AND HOLE MOBILITIES IN POLYMER LIGHT-EMITTING DIODES

We have studied the transport properties of electron- and hole-dominated MEH-PPV, poly(2-methoxy,5-(2′-ethyl-hexoxy)-p-phenylene vinylene), devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (μ=μ0eγ√E) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility μ0 and electric-field coefficient γ are temperature dependent with an activation energy of the hole and electron mobility of 0.38±0.02 and 0.34±0.02 eV, respectively. At 300 K, we find a zero-field mobility μ0 on the order of 1±0.5×10−7 cm2/V s and an electric-field coefficient γ of 4.8±0.3×10−4 (m/V)1/2 for holes. For electrons, we find a μ0 an order of magnitude below that for holes but a larger γ of 7.8±0.5×10−4 (m/V)1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.

A General Model for Amyloid Fibril Assembly Based on Morphological Studies Using Atomic Force Microscopy

Based on atomic force microscopy analysis of the morphology of fibrillar species formed during fibrillation of alpha-synuclein, insulin, and the B1 domain of protein G, a previously described model for the assembly of amyloid fibrils of immunoglobulin light-chain variable domains is proposed as a general model for the assembly of protein fibrils. For all of the proteins studied, we observed two or three fibrillar species that vary in diameter. The smallest, protofilaments, have a uniform height, whereas the larger species, protofibrils and fibrils, have morphologies that are indicative of multiple protofilaments intertwining. In all cases, protofilaments intertwine to form protofibrils, and protofibrils intertwine to form fibrils. We propose that the hierarchical assembly model describes a general mechanism of assembly for all amyloid fibrils.

Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles

We study the effect of blended and layered titanium dioxide (TiO2) nanoparticles on charge transfer processes in conjugated polymer photovoltaics. A two order of magnitude increase in photoconductivity and sharp saturation is observed for layered versus blended structures, independent of the cathode work function. Using electrodes with similar work functions, we observe low dark currents and open circuit voltages of 0.7 V when a TiO2 nanoparticle layer is self-assembled onto the indium–tin–oxide electrode. Our results for the layered morphologies are consistent with charge collection by exciton diffusion and dissociation at the TiO2 interface.

Transient and steady-state behavior of space charges in multilayer organic light-emitting diodes

A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq3 layer and that the electroluminescence delay time is determined by electrons passing through Alq3 to the internal interface are self-consistently supported by ...

Enhanced luminance in polymer composite light emitting devices

We demonstrate that mixing insulating oxide nanoparticles into electroluminescent polymer materials results in increased current densities, radiances, and power efficiencies in polymer light emitting diode devices. For low driving voltages, an order of magnitude increase in current density and light output is achieved with minimal loss in device lifetime. At 5 V, we achieve radiances of 10 000 cd/m2 with external quantum efficiencies ∼1% for nanoparticle/MEH–PPV composite films.

Zinc-indium-oxide: A high conductivity transparent conducting oxide

We report the fabrication and characterization of zinc‐indium‐oxide films with similar electrical conductivity and better transparency in both the visible and infrared compared with indium–tin–oxide, a widely used transparent conductor in many technological applications. Dramatically superior transmission properties in the 1–1.5 μm range in particular make zinc–indium–oxide attractive for use in infrared devices, where transparent electrodes are required. Resisitivities as low as 400 μΩ cm result from doping with small quantities of Sn; Al, Ga, and Ge are also effective dopants. Deposition on glass and quartz substrates as amorphous films by pulsed laser deposition and dc reactive sputtering is described.

Polymeric anodes for organic light-emitting diodes

Abstract Polymer light-emitting diodes based on PPV, for example MEH-PPV, are known to be susceptible to photo-oxidative degradation. The formation of the carbonyl species in the polymer results in quenching of the luminescence. In addition the oxidation process reduces the conjugation of the polymer, leading to lower charge carrier mobilities and consequently higher operating voltages. Previous in situ FTIR studies revealed that even in a dry inert atmosphere polymer oxidation occurs, and that ITO can act as the source of oxygen. In order to explore further the nature of the oxidation mechanism and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with conducting polymer anodes. When a layer of polyaniline is present between the ITO and the MEH-PPV the device characteristics improve dramatically: the injection voltage drops, the luminous efficiency increases and, most significantly, the rate of decay of the luminance decreases by up to two orders of magnitude. These data not only confirm that ITO is a source of oxygen, but also imply that the oxidation mechanism is due to direct interfacial reaction. We compare several different forms of polyaniline, with different dopants, as well as a derivative of a polythiophene.

Semiconducting polymers and quantum dots in luminescent solar concentrators for solar energy harvesting

We compare the performance of luminescent solar concentrators (LSCs) fabricated with polymers and quantum dots to the behavior of laser dye LSCs. Previous research, centered around the use of small molecule laser dyes, was hindered by the lack of materials with small absorption/emission band overlap and longer lifetime. Materials such as semiconducting polymers and quantum dots present qualities that are desirable in LSCs, for example, smaller absorption/emission band overlap, tunable absorption, and longer lifetimes. In this study, the efficiency of LSCs consisting of liquid solutions of semiconducting polymers encased in glass was measured and compared to the efficiency of LSCs based on small molecule dyes and on quantum dots. Factors affecting the optical efficiency of the system such as the luminescing properties of the organic materials were examined. The experimental results were compared to Monte Carlo simulations. Our results suggest that commercially available quantum dots cannot serve as viable ...