M.S. Weaver

Use of poly(phenyl quinoxaline) as an electron transport material in polymer light‐emitting diodes

We report the use of a poly(phenyl quinoxaline) (PPQ) as an electron transporting conjugated polymer for electroluminescence (EL) applications. Single‐layer PPQ devices with ITO anode and aluminium cathode show unipolar electron transport with current densities up to 60 mA/cm2 but no emission. Two‐layer structures combining PPQ as electron transport material with the hole transporting poly(2,5‐dialkoxy‐p‐phenylene vinylene) (PDAOPV) show strong emission from the PDAOPV with brightnesses up to 250 cd/m2. These two‐layer structures have a maximum EL quantum efficiency of 0.35% which is ten‐fold enhanced compared with the corresponding single‐layer PDAOPV devices.

Photoprocessed and micropatterned conjugated polymer LEDs

Abstract We present a detailed study of the effect of fabricating light-emitting diodes (LEDs) containing conjugated polymers, using photolithographic processing; a technique more conventionally used in inorganic semiconductor device manufacture. It is shown that for the specific poly (2,5-dialkoxy- p -phenylenevinylene) used here, the photoprocessing procedure chemically modifies the polymer, resulting in an increase in the degree of conjugation and a concomitant reduction in photoluminescence quantum efficiency to 35% compared to an unprocessed film. A similar reduction in electroluminescence quantum efficiency indicates that this is also the dominant effect in photoprocessed LEDs. LED device characteristics show an increase in threshold field for the photoprocessed devices suggestive of the formation of a barrier layer at the polymer/cathode interface, resulting in a further partial reduction in device power efficiency. There are however no catastrophic effects on device performance, showing that standard photoprocessing is a viable approach to fabrication of polymer LED structures. It is anticipated that optimization of the procedures will allow much less degradation in device performance. In addition it is shown that photoprocessing can be readily applied to the fabrication of arrays of micron-sized LEDs, demonstrating advanced applications of this combination of technologies.

Bulk limited conduction in electroluminescent polymer devices

The current–voltage (J–V) characteristics of ITO/polymer film/Al or Au structures of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer have been recorded for a range of different film thickness d and temperatures T. At high applied bias all the characteristics can be fitted over a given range to a power law J=KVm, where m increases with decreasing T, log(K) is proportional to m, and K is proportional to d−α m, where α∼2 (ITO/polymer film/Al devices) and ∼1 (ITO/polymer film/Au devices). Different single carrier space charge limited conduction theories have been used to try and explain this behavior. The analytical theory in which the carrier density is decreased by an exponential trap distribution lying below effectively isoelectronic transport states is in good agreement, but cannot explain the thickness dependence of the ITO/polymer film/Au devices and can be criticized as being physically unreasonable. A numerical analysis in which the mobility has the field and temperature dependence found f...

Organic electroluminescence devices fabricated with chemical vapour deposited polyazomethine films

Abstract We demonstrate the use of chemical vapour deposition polymerization as an effective method to fabricate conjugated polymer films as constituents in organic electroluminescence (EL) devices. This technique provides excellent compatibility with vacuum sublimation deposition of low molecular weight materials for construction of hybrid devices. The specific polyazomethine polymer studied here, namely, poly (1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne), is shown to be an addition to the range of polymer structures that provide electron transport functionality and, in combination with suitable emissive materials, provides the basis for new device structures. From measurements on a series of devices we propose an energy level structure for this material. Despite remaining isoelectronic with poly (1,4-phenylenevinylene) the energy levels of the polyazomethine are strongly shifted through the replacement of a vinylene CH with a N atom.

Electroluminescence from a conjugated polymer microcavity structure

We report the observation of electroluminescence and photoluminescence from microcavity structures containing poly(2,5‐dialkoxy‐p‐phenylene vinylene) (PDAOPV) conjugated polymer emitting regions. Strong spectral narrowing from 112 nm full width half‐maximum to 34 nm in electroluminescence and from 128 to 16 nm in photoluminescence and clear angular dependence of the peak emission wavelength are observed. These are characteristic signatures of the modification of spontaneous emission properties in microcavity structures.

Recent progress in polymers for electroluminescence: microcavity devices and electron transport polymers

Abstract In this paper we briefly review the status of polymer electroluminescence and then report recent work on microcavity polymer devices and on electron transport polymers with electron-deficient nitrogen-containing aromatic moieties. Results are presented for electroluminescence and photoluminescence from microcavity structures containing a poly (2,5-dialkoxy-p-phenylene vinylene) conjugated polymer. Strong spectral narrowing from 108 nm full width half maximum to 34 nm in electroluminescence and from 128 nm to 16 nm in photoluminescence, marked enhancement of the peak emission intensity and clear angular dependence of the peak emission wavelength are observed. These are all characteristic signatures of the modification of spontaneous emission properties in microcavity structures and open the way to studies of microcavity physics in polymer based devices. A non-ether poly(phenyl quinoxaline) and poly(2,6-pyridine vinylene-co-2,5-diheptoxy-p-phenylene vinylene) have been investigated as new electron transport polymers and we present results for their operation in both single and bilayer polymer devices. Additional target polymers for electron transport functionality are described.

Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films

Abstract We have measured current-voltage ( I-V ) characteristics of vapour-deposited films of various oligothiophenes in the direction parallel to the substrate using a two-point probe technique with symmetric contacts. For applied field strengths up to 2×10 4 V cm −1 , the I-V characteristics are linear. The conductivity (σ) of freshly prepared films is very low (below 10 −11 S cm −1 for EC6T, an oligothiophene with end-substituted 4,5,6,7-tetrahydrobenzo groups (‘end caps’) and with six thiophene units), but can be increased by four orders of magnitude on doping with oxygen (σ = 4×10 −7 S cm −1 ), the doping process being strongly promoted by light and/or applied current. Using capacitance-voltage ( C-V ) spectroscopy a charge carrier density of 10 17 cm −3 was measured. The temperature dependence of σ (140–320 K) can best be fitted by an exponential law (exp( αT )).

Control of photoluminescence emission from a conjugated polymer using an optimised microactivity structure

Abstract It is shown that by careful control of the position of a thin emissive polymer layer poly( p -phenylene vinylene) [PPV] within a microactivity structure, it is possible to strongly influence the forward photoluminescence (PL) emission intensity. In one structure, the PPV layer was placed at a confined photon-field antinode where the coupling strength between the emitting dipoles and the field is expected to be a maximum. This resulted in an enhancement of the PL emitted into the forward direction. Placing the PPV layer at a photon-field node resulted in strong suppression of PL. The ratio of the forward emission intensity between these two extreme cases was 55 ± 25. The measured emission intensities are compared to predictions from a transfer matrix model and are shown to be in very reasonable agreement.

Organic light-emitting diodes (LEDs) based on Langmuir-Blodgett films containing rare-earth complexes

Abstract In this paper we report the construction of an organic light-emitting diode (LED) containing a rare-earth molecular complex that was deposited as a Langmuir-Blodgett (LB) film. The complex is a donor-conjugated π-electron system acceptor (D-π-molecular cation coupled to a monovalent anion containing a trivalent rare-earth (Nd 3+ ) cation surrounded by four organic singly charged anionic ligands. Devices were fabricated by LB deposition of multilayer films onto an indium-tin oxide (ITO)-coated glasls substrate on top of which aluminium was evaporated to form the diode structure. Such devices are found to electroluminesce when subjected to a forward bias of a few volts. We present here the electrical and optical characteristics of the device.

Characterization of the emission from a conjugated polymer microcavity

We report the results of a study on microcavity structures containing the conjugated polymer poly(2,5-dialkoxy-p-phenylenevinylene). Strong spectral narrowing, from a full width at half-maximum (FWHM) of 128 to 16 nm for photoluminescence (PL) and 112 to 35 nm for electroluminescence (EL) is observed. Modification of the cavity path length is used to produce a shift in the PL peak emission wavelength by 50 nm. In addition, we show that the EL emission wavelength has a clear angular dependence that can be described using a transfer-matrix reflectivity (TMR) model. These are characteristic signatures of the modification of spontaneous emission properties through coupling to resonant cavity modes.