H. Chayet

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.

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.

Space-charge limited current and electroluminescence in conjugated polymer-based LEDs

Abstract We have studied the current versus voltage (I–V) and the luminescence versus voltage (EL-V) characteristics in PPV-based LED devices for different film thickness, d. For voltages below the EL threshold voltage, the I–V curves exhibit a linear regime (I~V) at low voltages and quadratic (I~V2) regime for higher voltages. The later is attributed to the space charge limited current mechanism assuming a single shallow trap level. The fit of the model to the experimental results is very good with a single fitting parameter, namely, the effective mobility. The extracted mobilities are compared to those measured independently by time-of-flight (TOF) photoconductivity. The SCLC model gives a reasonable explanation for the voltage threshold in EL

Blue to red electroluminescence from self-assembled films

Abstract We present photoluminescence (PL) and electroluminescence (EL) studies of multilayer heterostructures containing alternate layers of conjugated copolymers and non-conjugated polymers (spacers). The self-assembled films are prepared either by electrostatic interactions between oppositely charged polyelectrolytes or using hydrogen bonding between molecules. We can tune both the PL and EL emission from blue to red region by: (a) using different copolymers with different emission properties; (b) controlling the thickness of the spacer layers in the assemblies; and to some degree (c) using different pH values of the polyelectrolyte solution

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

Transient Electroluminescence from PPV under Strong Voltage Pulses

The authors have studied the transient electroluminescence (EL) from poly(phenylenevinylene), PPV, as a function of electric field under strong electric pulses up to fields of E {approx} 10{sup 9} V/m with emphasis on (a) the time delay, {tau}, between the electric pulse and the onset of the EL emission pulse and (b) the EL intensity as a function of the field. A monotonic decrease of {tau} with increasing E is explained by an increase of the carriers mobility according to Frenkel-Poole model. The EL intensity at high fields is proportional to E{sup 3} suggesting that the contacts at the polymer-metal electrode interfaces are practically ohmic. The authors demonstrate significantly improved brightness, peak power and lifetimes for polymer-based light emitting diodes working under such a pulsed mode.