Louis M. Leung

Organic polymer thick film light emitting diodes (PTF-OLED)

A guest–host approach was used to fabricate a one-layer organic light emitting diode (OLED). The thick film ink approach allows the two-dimensional OLED to be processed using traditional methods such as silk-screen printing. The I–V–L characteristics of the polymer thick film (PTF)-OLED were studied as a function of the device chemical compositions and physical configurations. Different polymers, hole and electron transporters, and emitters at different weight ratios were studied for its composition dependence. Device configuration also plays a significant role on its overall performance. Dependence on film thickness, electrode type, and the usage of additional charge injection layers were also investigated. The simplified one-layer device allows a straightforward interpretation for the charge-transport and recombination phenomena which shed light for its future improvement.

Phase-transfer catalysed synthesis of disubstituted poly(phenylene vinylene)

Abstract An attempt to synthesize a series of phenyl ring disubstituted poly(o-, m- and p-phenylene vinylene) is reported. In this ‘one-pot’ reaction, a phase-transfer catalyst was used to promote the extraction of strong base deprotonated bischloromethyl aromatic anions from the aqueous to the organic phase to sustain a nucleophilic substitution reaction with the remaining bischloromethyl monomers. The polymers obtained were characterized for their chemical compositions and physical properties by elemental, i.r. and u.v. spectroscopic, and thermal analyses. The o-substituted polymers were found to have low melting temperatures, good solubilities and high molecular weights, but low yields. The m-substituted monomers, however, produced only non-conjugated poly-m-xylylenes. The p-substituted polymers offered the best combination of yield and physical properties. Films cast from soluble poly(2,5-dibutoxy-p-phenylene vinylene) have electrical volume resistivity in the range 300–500 ohm cm when doped with concentrated sulfuric acid.

Electrical properties of anionically synthesized conducting block copolymer from the precursor polystyrene-block-poly(phenyl vinyl sulfoxide)

Abstract A series of styrene-phenyl vinyl sulfoxide diblock copolymers was prepared using the ‘living’ anionic polymerization technique. The thermally labile phenyl vinyl sulfoxides undergo an elimination process to yield acetylene moieties at 150°C. The weight and volume of the iodine-doped conducting fractions were adjusted by changing the length of the styrene block. A sigmoidal composition-dependent conductivity and a maximum in the dielectric constant measurement near the threshold composition suggested a ‘percolation-like’ conducting process. In addition to electrical measurement, the block copolymers were also characterized by infra-red spectroscopy, thermal analysis, 1 H n.m.r., and size exclusion chromatography.

High-Mobility Hole-Transporting Polymers for Electroluminescence Applications

A series of electroluminescent and conductive phenyl ring-substituted poly(N-phenyl-N-styryl-1-naphthylamine) (P-XNPA, where X=-H (P01), -OCH3 (P02) and -F (P03)) polymers were prepared. The polymers exhibit a high hole mobility, an excellent solubility and a good thermal stability. Their emission color, ionization potential (IP), quantum efficiency, glass transition temperature (Tg), efficiency and stability were all found to be related to the properties of the phenyl substituted side group. As an example, the hole mobility of poly(4-MeONPA) (P02) is 10-5 cm2V-1s-1 at room temperature, compared tothat of poly(N-vinylcarbazole) (PVK) being 10-7 cm2V-1s-1. P02 emits blue light at 450 nm and has a high thermal stability at an onset decomposition temperature above 420°C and Tg of 132°C. A simple organic light-emitting diode (OLED) device fabricated from the same polymer has a maximum luminance of 588 cd/m2 at 175 mA/cm2.

Studies on miscibility of poly(phenylene oxide)-based ionomer/polystyrene-based ionomer blends

The phase behavior of series of blends obtained from mixing carboxylated poly(phenylene oxide) with sulfonated polystyrene and their respective neutralized ionomers was studied by differential scanning calorimetry. A substantially broader range of miscibility was observed when both blend components were functionalized, compared with blends in which only one of the components contained an acid group or was an ionomer. The properties of metallic cations which were used to neutralize the acid groups in the blends were found to have an effect on the miscibility. The miscibility of the two acid polymers or their ionomers depended on the difference of their functionalization level instead of on the absolute percentage of functional groups on each polymer. It was found that the two acid polymers or their ionomers remained miscible as long as they had a similar percentage of functional groups.

Short poly(phenylene vinylene) chains grafted poly(organophosphazene)

Abstract Brominated poly(bis(4-methylphenoxyphosphazene) was allowed to react with 1,4-bischloromethylbenzene or 1,4-bischloromethyl-2,5-dimethoxybenzene in solution using phase transfer catalyst or potassium t -butoxide. Poly( p -phenylene vinylene) or poly(2,5-methoxy-1,4-phenylene vinylene) grafted organophosphazene copolymers were obtained. The UV–Vis absorption, photoluminescent, and thermal properties of the copolymers were measured. The copolymers are complete soluble in common organic solvents and fluoresce in the blue color range. The copolymers were used to build a series of organic light emitting diode (OLED). Only weak to nominated intensities with emission color from blue to red were obtained. The photoluminescent and electroluminescent (EL) spectra indicated there is a distribution in the PPV conjugated length. The compositions of the copolymers before and after the graft reaction were analyzed using NMR.

Electrical and optical properties of polyacetylene copolymers

Abstract Poly(phenyl vinyl sulfoxide) (PVSO) is a soluble precursor to polyacetylene (PA). Upon thermal elimination, the thermally labile PVSO can be converted to PA by undergoing a ‘zipper-like’ 5-members cyclic sigmatropic elimination process. Both PVSO and its copolymers can be synthesized via anionic polymerization in which stereospecific polymers with controlled molecular weight, narrow polydispersity, and known chemical composition distribution can be prepared. The degree of elimination and thus the mean conjugation length can be controlled by partially oxidizing the thermally labile sulfoxide moieties into thermally stable sulfone moieties. The optical properties of the PVSO copolymers in the UV–vis range were studied as a function of its thermolysis conditions both in solid-state and in solution forms. The eliminated PVSO can be doped (by p- or n-type dopants) to achieve conductivity to a range of 10 S/cm. The electrical properties of the PVSO copolymers and a series of blends were analyzed as a function of its chemical composition. In addition, the anionic synthesis method allows specific end groups to be inserted to the polymer chain ends. The synthesis of a series of carboxylic acid terminated PVSO polymers and their properties was also reported.

Effects of additives in polymer thick film-organic light emitting diodes (PTF-OLED)

The optimum composition and film thickness determined for a single-layer organic light emitting diodes (120–150 cd/m2 at 30 mA/cm2) has been determined in one of our previous publication [Display 21 (2001) 199]. In this following study, a series of additives was included in the original composition for the purposes of color tuning, enhancement of the device efficiency as well as the stability of the polymer thick film-organic light emitting diodes (PTF-OLED). The additives attempted including three dyes (rubrene, perylene and DCM), three sensitizers (naphthalene, anthracene and p-terphenyl), several stabilizers (antioxidant and photo-stabilizer) and a number of organic soluble salts known as phase transfer catalyst. Rubrene was found being able to improve the OLED efficiency by 3-fold and its durability by at least 60 times at a concentration of 5–10 pph (in part per hundred of Alq3 replaced). The color of the OLED can also be tuned according to the additional dye used. The sensitizer naphtahlene can also improved the efficiency of the OLED by 2-fold while the effect of the other additives was less significant. In addition, the lowering of the turn-on voltage and thus a higher current efficiency resulted by the addition of a charge injection layer (lithium fluoride or calcium fluoride) in between the cathode and the organic layer. The performance of the doped PTF-OLED is within an order of magnitude compared to a heterojunction small molecule-based OLED.

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 μm. The results are useful for optical evaluation of these polymers.

Anionic synthesis of telechelic polyacetylene

Abstract Poly(phenyl vinyl sulfoxide) which is a soluble precursor to polyacetylene can be prepared using the ‘living’ anionic synthesis method. The thermal labile sulfoxide moieties can be readily converted to acetylenic units upon heating. In this report, a series of monodisperse end-functionalized soluble precursors were prepared. Details on the synthesis and characterization procedures were described. The end groups were either Li, Na, or Cs salts of carboxylic acid and were the result of using the respective electron-transfer di-functional metal naphthalide initiator. The IR spectra for the resulting telechelic polyacetylene are also given.