Phillip J. Brock

Degradation and failure of MEH‐PPV light‐emitting diodes

Light‐emitting diodes made with poly(2‐methoxy‐5(2′‐ethyl)hexoxy‐phenylenevinylene) (MEH‐ PPV) using indium‐tin‐oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self‐induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH‐PPV devices occurs when the regions of damaged cathode start to coalesce.

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.

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.

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.

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.

Photovoltaic measurement of the built-in potential in organic light emitting diodes and photodiodes

We measure the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.

Chemical vapor deposition of copper from 1,5‐cyclooctadiene copper(I) hexafluoroacetylacetonate

We have studied the chemical vapor deposition of copper from 1,5‐cyclooctadiene Cu(I) hexafluoroacetylacetonate, a moderately volatile yellow cystalline solid. It yields pure copper by pyrolytic decomposition at 150–250 °C, produces copper films with near bulk resistivity, and has the advantage of being air stable at room temperature.

Self‐Assembled Nanocomposite Polymer Light‐Emitting Diodes with Improved Efficiency and Luminance

Hybrid organic and nanoparticle-based systems have been recently studied as prospective materials for optoelectronics applications because they combine the advantages of organic polymers with those of inorganic clusters. For photonic applications, nanoparticles enable a wider variation of the dielectric constant (refractive index) and charge transport properties than polymers. The mesoscale character of nanoparticle dimensions, corresponding to the wavelength of visible light, allows for the assembly of superlattice structures with possible optical band gap properties or with microcavity effects for polymer lasing. Highly efficient photovoltaics can be made through organic dye sensitization of TiO2 and related nanoparticles. [4,5] More recently, dielectric oxide nanoparticles have been shown to modify the charge transport in polymer light-emitting diodes, resulting in an increase in both the current density and light emission. In this paper, we show that nanoparticles assembled at a semiconducting polymer/electrode interface can also affect charge injection into polymer lightemitting diodes. For the case of negatively charged dielectric SiO2 monolayers assembled at the anode interface, external electroluminescence quantum efficiencies approaching theoretical limits for radiative singlet decay can be achieved. Moreover, nanoparticle monolayers result in enhanced luminances at lower drive voltages, similar to what has been achieved with conducting polymer layers. These results indicate that interfacial nanoparticle layers offer a general method for enhancing the local electric field across the polymer/anode interface, providing versatility for improving performance of polymer optoelectronic devices. Since the discovery of electroluminescence in polymers, charge transport and injection in semiconducting polymers has been actively studied with the goal of achieving bright and highly efficient polymer light-emitting diodes operating at low electric fields. A key requirement for high electroluminescent efficiencies is balanced injection of holes and electrons at the anode and cathode interfaces, respectively. Such balance can be dramatically affected by controlling injection through modification of the interface between the semiconducting polymer and the electrodes. Although both electrode surfaces can be modified in principle, the modification of the indium tin oxide (ITO) anode is more common due to the atmospheric stability of the ITO surface that enables aweto preparation stages (cleaning, chemical modification, and spinning) before the vacuum stages such as evaporation of a metal cathode and protective coatings. For materials limited by holeinjection, the device efficiency can be improved by inserting a hole transporting layer that enables smaller tunneling barriers, or Ohmic injection, from the anode into the highest occupied molecular orbital (HOMO) of the polymer. 11] For electron-limited materials, the quantum efficiency can be improved by inserting a layer that effectively blocks electrons from reaching the anode. In this work, modification of the ITO transparent anode was achieved using self-assembled monolayers and electrostatically assembled SiO2 nanoparticles. This modification was performed in two stages. First, the 3-aminopropyltriethoxysilane molecules were attached to the ITO surface via chemo-adsorption from an ethanol solution. This procedure is similar to modification of Si described before, allowing NH3 functionalization of the ITO surface. The thickness of a self-assembled organic layer was estimated to be 0.9 nm from atomic force microscopy (AFM) measurements. In the second step of modification, nanoparticles were attached to the surface of ITO via electrostatic physical adsorption from water solution. Adjustment of the pH conditions on the surface of the SAM resulted in protonation of NH2 groups to NH + 3 charged groups and the formation of a complete monolayer of negatively charged nanoparticles consisting of SiO2 (Nissan Chemicals Co., 20 nm diameter) or polystyrene latexes spheres (Interfacial Dynamics Corp., 30 nm diameter) as demonstrated by AFM. The amine functionality also enables attachment of metallic gold nanoparticles (BBI International Co., 40 nm) through chemical tethering to the surface. For comparison, devices were made with bare ITO surfaces cleaned in H2O/ isopropanol bath and with polyaniline-PSS (PAni) conducting polymer layer. Table 1 describes the device structures, electrode modificaton and acronyms contained in the figures. The semiconducting polymers used in these study were poly(2-methoxy-5-(2¢-ethyl-hexoxy)-p-phenylene vinylene)

Pore size distributions in nanoporous methyl silsesquioxane films as determined by small angle x-ray scattering

Small angle x-ray scattering (SAXS) measurements were performed on nanoporous methyl silsesquioxane films that were generated by the incorporation of a sacrificial polymeric component into the matrix and subsequently removed by thermolysis. The average pore radii ranged from 1 to 5 nm over a porosity range of ∼5–50%. The distribution in pore size was relatively broad and increases in breadth with porosity. The values and observations obtained by SAXS are in good agreement with field emission scanning electron microscopy.

Hole limited recombination in polymer light-emitting diodes

By comparing the quantum efficiencies of light emission in a series of poly[2-methoxy-5(2′ethyl)hexoxy-phenylenevinylene] diodes with calcium cathodes and various anode metals, we show that, in all cases electrons are the majority carrier and recombination is limited by hole injection. These conclusions are confirmed by the examination of a second series of samples in which alkanethiol barrier layers of varying thickness, are deposited on a gold anode. The highest external quantum efficiency was achieved in these experiments using a clean, semitransparent gold anode. We suggest that electron and hole injection rates play the primary role in determining current balance and that mobilities play a minor role.