Yair Avny

MICROFABRICATION OF AN ELECTROLUMINESCENT POLYMER LIGHT EMITTING DIODE PIXEL ARRAY

We describe a method to microfabricate a light emitting diode array with pixels based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. This method, based on direct photoablation with the 193 nm emission of an excimer laser, maintains the properties of these unique polymers. The technique as described here has already achieved an array of 20 μm×20 μm pixels with enhanced electroluminescence (EL) from these pixels and possible spectral tuning of the EL by the application of varying external field. This method can be extended to achieve nanometer dimensionalities using near‐field nanolithography.

Degradation of nonencapsulated polymer-based light-emitting diodes: Noise and morphology

We show that the degradation of a nonencapsulated polymer-based light-emitting diode (LED) is accompanied by the appearance of strong fluctuations,—that is noise both in the radiance and in the film resistance. We demonstrate a correlation between the morphological changes which occur during the degradation process and the noise, suggesting that the sampling of noise during LED operation can be used as a very efficient tool to predict the approaching failure of LEDs in real-life applications. The morphological changes in LED degradation are essentially a two-stage process. First, there is formation of “bubbles” at the metal–polymer interface due to delamination of the polymer film from the metal surface. Second, carbonized areas in the form of “black spots” are formed. Accumulation of carbonized areas leads to short and/or open circuits and final LED failure.

Newly synthesized conjugated copolymers for light emitting diodes

Copolymers derived from PPV in which naphthalene and 2,3,5,6-tetrafluorobenzene were incorporated into the PPV backbone were synthesized. The dark DC conductivity, electroluminescence (EL) and photoluminescence (PL) of the copolymers are reported.

NEUTRON AND X-RAY REFLECTIVITY STUDIES OF SELF-ASSEMBLED HETEROSTRUCTURES BASED ON CONJUGATED POLYMERS

Neutron and x-ray reflectivity studies of modulated heterostructures consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterostructures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fitting procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/spacer interface. This roughness parame...

Microfabrication of an electroluminescent polymer light emitting diode pixel array

Abstract We describe a method to miorofabricate a light emitting diode (LED) pixel array based on conjugated electroluminescent polymers sandwiched between ITO and aluminum. The method, based on direct photoablation using a 193 nm excimer laser, maintains intact the properties of the polymers. The technique as described here has already achieved array of 20μm × 20 μm pixels with enhanced electroluminescence (EL) from pixels. The method can be extended to achieve nanometer sizes using near-field nanolithography. The microfabrication of the LED array requires also the patterning of the ITO and the aluminum electrode. For better performance of the device it is important to map the conductivity of the patterned electrodes, For that purpose we have used a novel mm-wave conductivity microscope which is capable to measure the local conductivity of the patterned film with a spatial resolution of ~10–30 p.m.

Monomers for non-bond crosslinking of vinyl polymers

Abstract Monomers having on the one hand a polymerizable double bond and on the other hand a large macrocyclic ring (n≥27–28) through which a growing chain may be threaded are shown to be a new type of crosslinking agents which lead to non-bond crosslinking of vinyl polymers. This is demonstrated by cyclic octaethylene glycol fumarate, a 29-membered ring obtained in the reaction of fumaryl chloride with octaethylene glycol under high dilution conditions, which upon copolymerization with styrene or methylmethacrylate led to crosslinked polymers.

Possible evidence for quantum‐size effects in self‐assembled ultrathin films containing conjugated copolymers

We present photoluminescence (PL), UV absorption, electroluminescence and x‐ray reflectivity studies of self‐assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be ‘‘tuned’’ by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self‐assembled ultrathin films containing thin (∼7 A) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1/d2 where d is the thickness of the assembly, as expected for confined photogenerated electron‐hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (∼40 A) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on s...

Poly(phenylenevinylene) analogs with ring substituted polar side chains and their use in the formation of hydrogen bonding based self-assembled multilayers

Poly(arylenevinylene) homopolymers and copolymers with polar hydroxy and carboxy moieties attached to the aromatic phenyl ring were prepared. The copolymers and the related homopolymers are copoly[5-(2-hydroxyethoxy)-2-methoxy-1,4-phenylenevinylene/1,4-phenylenevinylene], co(PHydroxyV-PV), and copoly(5-carboxymethoxy-2-methoxy-1,4-phenylenevinylene/1,4-phenylenevinylene), co(PCarboxyV-PV). For co(PHydroxyV-PV) the photoluminescence and electroluminescence spectra can be adjusted over a range of 100 nm as a function of the percentage of the hydroxy substituted phenyl ring. For co(PCarboxyV-PV) the observed spectral features are a function of the pH from which the final conjugated polymer was prepared. The optical density and photoluminescence of co(PCarboxyV-PV) films prepared from solutions at pH=12 were significantly blue-shifted compared to polymers prepared from pH=2 solutions at up to 30% substitution at the phenyl ring. The presence of the polar side chain was used to form self-assembled multilayer films, poly(ethyleneimine)/poly(styrenesulfonate)/co(PHydroxyV-PV), based on hydrogen bonding interactions rather than electrostatic forces. The luminescence spectra in the layered systems were blue-shifted compared to the spin coated films.

Blue luminescence induced by confinement in self-assembled films

Abstract The fabrication and characterization by means of photoluminescence (PL), UV-vis absorption, electro-luminescence (EL) and X-ray reflectivity of multilayer heterostructures consisting of alternate layers of conjugated and non-conjugated polymers have been studied. The heterostructures are prepared by the layer-by-layer self-assembly technique, using two types of polyelectrolytes. The first are precursors of conjugated polymers such as poly(phenylenevinylene) (PPV) and other poly(arylenevinylene) polymers, and the second are non-conjugated polymers such as poly(styrene-4-sulfonate) (SPS), polyacrylic acid (PAA) and poly(allylamine hydrochloride) (PAH). The heterostructures consist of a repeated sequence of bilayers (layer pair) or multilayers, where the conjugated polymer is formed by heat treatment under vacuum. The thickness of each bilayer or multilayer was controlled by changing the non-conjugated polymer layer. Most importantly, we have found that the PL and EL spectral emissions can be ‘tuned’ by a proper ‘design’ of the heterostructure. Particularly, heterostructures in which the bilayer thickness is rather small and the electroluminescent layers are practically in contact show a blue shift upon decreasing the thickness of the assembly for ultrathin assemblies. In contrast, for assemblies where the electroluminescent layers are well separated by one or several non-conjugated layers (polyelectrolyte spacers), the emission is in the blue and independent of the assembly thickness (number of bilayers). We interpret the results as being due to confinement effects. Using this assembly technique, we were able to fabricate light emitting diodes (LEDs) which emit in the blue region.

Monomers for non-bond crosslinking of vinyl polymers: II. Cyclic octaethylene glycol 5-methacrylamido-isophthalate

The new concept of non-bond crosslinking of vinyl polymers is demonstrated with cyclic octaethylene glycol 5-methacrylamido-isophthalate, which was synthesized and copolymerized with methyl methacrylate to yield insoluble copolymers with a high swelling capacity, threading of polymer chains through the rings being the cause of the crosslinking. Evidence for the threading was obtained from 2D nuclear magnetic resonance.