E.Z. Faraggi

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.

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.

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.

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...

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.

Transient and AC Electroluminescence in Pyridylene Copolymers of PPV

We present transient and AC electroluminescence (EL) measurements on thin films of a newly synthesized copolymer of PPV where pyridine units were incorporated into the polymer backbone (co(PyPV)), and compare it to the EL of thin films of PPV prepared by the method of self-assembly and by spin casting. Transient EL under voltage pulses of co(PyPV) indicates that positive charge carriers have lower mobility than in PPV. Under pulses and AC square wave voltage we have observed a new type of prompt transient EL at the switch-off of the voltage. The transient EL effects are explained by the presence of non-bonding electrons in the polymer chain of co(PyPV). Co(PyPV) based LEDs have improved EL yield compared to PPV. Operations with sine AC voltage mode up to 10 kHz show better stability as compared to DC, without loss in efficiency.

PPV-based copolymers for light emitting diodes

Abstract PPV pyridine based copolymers were studied. The copolymers were synthesized via the conventional precursor polymer route, conversion time dependence was investigated. PL emission was determined and depended on excitation wavelength