M. Halim

An efficient electron-transporting polymer for light-emitting diodes

The efficient operation of polymer light-emitting diodes (LEDs) requires balanced injection and transport of electrons and holes. This has stimulated much research into suitable electron-injecting and transporting materials. We report the use of polypyridine as an efficient electron-transporting polymer. We have achieved much-improved LED performance by incorporating polypyridine as an electron-transporting layer in a poly(p-phenylene vinylene) (PPV) LED and optimizing layer thicknesses to balance transport of electrons and holes. The external quantum efficiency of these LEDs is 0.25%, 60 times greater than similar devices without the electron-transporting layer.

Conjugated dendrimers for LEDs: Control of colour

The use of conjugated dendrimers as a novel molecular architecture for organic LEDs is investigated. Control of colour is demonstrated using materials based on a porphyrin, anthracene and distyrylbenzene core with stilbene dendrons. The resulting dendrimers are solution processible and give photoluminescence and electroluminescence in the red, green and blue regions of the spectrum.

Photo- and electroluminescence of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) Langmuir-Blodgett films

Abstract Langmuir-Blodgett (LB) layers of poly(2-methoxy,5-(2′-ethylhexyloxy)- p -phenylene vinylene) (MEH-PPV) have been built up on quartz and indium-tin oxide (ITO) substrates. The optical properties of the films were characterized using time-correlated single-photon counting, and absorption and steady-state fluorescence spectroscopy. Electroluminescence (orange-yellow light) from ITO/MEH-PPV LB/A1 devices under forward bias was observed. The dependence of the electrical and electroluminescent characteristics on LB film thickness is reported. The quantum efficiency was 7 × 10 −3 % for a device containing 64 LB layers.

Measurement of time dependent quantum yield of poly(p-pyridine) and poly(p-phenylenevinylene)

In this paper we report the absolute photoluminescence quantum yield of poly(p-pyridine) (PPy), which is a promising electroluminescent polymer. The influence of vacuum and air on the quantum yield of the PPy is investigated. We find a significant decay of the efficiency under vacuum whereas it is stable in air. We compare the results with those obtained on poly(p-phenylenevinylene)(PPV).

Protonation effects on the photophysical properties of poly(2,5-pyridine diyl)

We have recently shown that poly(2,5-pyridine diyl) (PPY) can be synthesized to yield a polymer with high photoluminescence quantum yield (PLQY) in the solid state, and that it is an excellent electron transport material. To explore the photophysical properties of PPY further, we have used a range of acidic “dopants” to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general, we find that sulphonic acids have the greatest effect, causing perturbations to both the ground-state and excited-state properties of the PPY. These changes occur with only moderate reduction of the PLQY, whereas nonsulphonic acids cause a larger reduction in PLQY without significantly affecting the ground- or excited-state energy levels. These aspects of the photophysics of PPY can be described using a simple ring torsion argument. This model can also account for the observed shifts between solution state and solid-state emission wav...

Control of colour and charge injection in conjugated dendrimer/polypyridine bilayer LEDs

Conjugated dendrimers are an interesting class of materials for light emitting diodes, and can be used either as light-emitting or charge-transporting layers. We report a study of the use of a solution-processible dendrimer with distyrylbenzene core and stilbene dendrons. We find that its use in bilayer LEDs with polypyridine gives efficiencies ten times greater than for single layer polypyridine LEDs. The light emission can be either from the polypyridine or the dendrimer layer, depending on the layer thicknesses. This offers a way of tuning the colour of bilayer LEDs.

Structure-property relationships in conjugated molecules

Conjugated dendrimers are ideal materials to study the structure-property relationships in conjugated molecules. We have developed a family of dendrimers that contain t-butyl surface groups, stilbene dendrons, and luminescent cores. For porphyrin and tris(distyrylbenzenyl) amine cored dendrimers cyclic voltammetry (CV) showed that the redox processes occurred at the core. Combining the results from the CV experiments with the device characteristics it was determined that for the dendrimers the cores were held further away from each other with increasing generation. For the amine cored dendrimers it was found that the hole mobility decreased with generation, which is consistent with the cores being further apart. We also found for the amine cored dendrimers that the decrease in hole mobility was matched with increase in device efficiency

The effect of dendrimer generation on LED efficiency

The use of phenylenevinylene based dendrimers for organic LEDs was investigated. We have found when a family of distyrylbenzene centred dendrimers were used as the light-emitting layers in LEDs that the efficiency was dependent on the dendrimer generation number. A maximum efficiency of 0.09% was observed in an ITO/ second generation dendrimer/Ca device.

Synthetic routes to phenylene vinylene dendrimers

Abstract Dendrimers offer the opportunity to vary the electronic properties of an electroactive material without changing the processing procedure. Poly(1,4-phenylenevinylene) and its derivatives have been widely studied for use in light-emitting diodes. We have investigated two different iterative routes for the formation of phenylenevinylene based dendrimers to compare their properties with the linear counterparts. Our approach involves palladium-catalysed coupling of styryl derivatives to halo-benzenes which has enabled the rapid construction of a family of dendrimers

Photophysical properties of protonated poly(p-pyridine)

Abstract To probe the role nitrogen lone pair electrons play in the photophysics of poly( p -pyridine) (PPY) we have used a range of acidic ‘dopants’ to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general we find that sulphonic acids have the greatest effect, causing perturbations to both the ground and excited state properties of the PPY. These changes occur with only moderate reduction of the photoluminescence quantum yield (PLQY), whereas non-sulphonic acids cause a larger reduction in PLQY without significantly effecting the ground or excited state energy levels. These aspects of the photophysics of PPY can be described using a simple inter-ring torsion argument