A. Bleyer

Polymer light emission : control of properties through chemical structure and morphology

Abstract We report on recent work concerning polymers for improved electron injection and transport, for enhanced photoluminescence quantum efficiency and for polarised emission. A new blue-emitting triazole containing co-polymer with high photoluminescence efficiency is described. This polymer acts to enhance the electroluminescence efficiency when used in two layer devices with poly(p-phenylene-vinylene) [PPV] as a hole transporting emitter. The two layer device has an external quantum efficiency of ≈ 0.08% at a luminance of 250 cd/m2 for a current density of 100 ma/cm2. Use of a meta-phenylene-vinylene conjugation interrupting unit as a means of enhancing photoluminescence quantum efficiency is also described. The resulting co-polymer allows fabrication of three layer devices that have an external efficiency of ≈ 1% at a luminance of 96 cd/m2 for a current density of 3.7 mA/cm2. Finally, we report on the orientation of a polyfluorene conjugated polymer via alignment of its liquid crystal phase on rubbed substrates. Thermotropic liquid crystal phases offer an approach to polarised electroluminescence emission and we discuss the merits of this and various other approaches.

Effective stimulated emission and excited-state absorption cross-section spectra of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and t,t′-didecycloxy-II-distyrylbenzene

Abstract Effective stimulated emission cross-section spectra, σ em, eff ( λ ), stimulated emission cross-section spectra, σ em ( λ ), and excited-state absorption cross-section spectra, σ ex ( λ ) = σ em, eff ( λ ), are determined in the fluorescence region for the conjugated polymer poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) in tetrahydrofuran, xylene, toluene, and for the model molecule t,t′-(didecycloxy)-II-distyrylbenzene. For theconjugated polymer σ em, eff ( λ ) is found to be small compared to σ em ( λ ) indicating that σ ex ( λ ) and σ em ( λ ) are of similar strength and spectral shape, while for the model compound σ ex ( λ ) is small compared to σ em ( λ ). In the polymer the laser excitation to the broad S 1 absorption band leads to the formation of discrete lattice relaxed exciton conformations extending on the average over four monomer units which are responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.

The photovoltaic effect in poly(p-phenylene-2,3'-bis(3,2'-diphenyl )-quinoxaline-7-7'-diyl)

Abstract We have investigated the photovoltaic properties of poly( p -phenylene-2,3′-bis(3,2′-diphenyl)-quinoxaline-7-7′-diyl) (PPQ) in sandwich structure devices. This polymer contains electron deficient quinoxaline moieties that are expected to give rise to a high electron affinity and to the possibility of good electron transport properties. The spectral response of the photocurrent shows qualitatively that the electron mobility is higher than the hole mobility in PPQ. The quantum efficiency of the short-circuit photocurrent is, however, small, namely only 1.7 × 10 −5 % under 10 mW illumination at 420 nm. Doping with tetra(1-dimethylamino-phenyl)-ethynylene (TDPE) increases the quantum efficiency of the short-circuit current to 4 × 10 -4 %, probably due to enhanced exciton dissociation. Even with the TDPE dopant, the limiting factor for the photocurrent appears to be charge transport to the contacts.

Electroabsorption spectroscopy of distyrylbenzene derivatives

Abstract We have studied the linear and nonlinear optical properties of model oligomers based on trans , trans ′ -distyrylbenzene to investigate the effects of chemical modification on the electronic structure of stilbenoid oligomers and polymers. Replacing the central para -phenylene ring with a meta -phenylene ring reduces the conjugation of the molecule and introduces disorder into the film, which significantly reduces Davydov splitting of the main absorption band. Alkoxy substitution of any of the phenylene rings red shifts the lowest energy absorption band and enhances the oscillator strength of a high-energy sideband. Absorption by aggregates, which is obscured by effects of scattering on the absorption spectrum, is clearly revealed by electroabsorption spectroscopy.

Efficient LEDs with a conjugated co-polymer as the emissive layer

Abstract We report on the use of a new highly luminescent conjugated polymer as an emissive layer in single and multi-layer electroluminescence devices. Poly( m -phenylenevinylene-co-2,5-dioctyloxy- p -phenylenevinylene) [P m PV-co-DOctOPV] was prepared via a Wittig synthesis reaction. The resulting polymer has a high photoluminescence quantum efficiency in the solid state with an emission spectrum peaking at 506 nm (2.45 eV) in the green. Three different electroluminescence devices were fabricated: (i) Single layer devices containing only PmPV-co-DOctOPV; (ii) Double layer devices with PmPV-co-DOctOPV and an evaporated film of 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl)phenylene [OXD-7] as an electron transport layer; (iii) Triple layer devices containing PmPV-co-DOctOPV, OXD-7 and in addition a polyvinylcarbazole [PVK] hole transport layer. All the devices utilised an ITO anode and a MgAg cathode. Electroluminescence external quantum efficiencies for these devices were found to be respectively up to 0.08%, 0.55% and 1% respectively, corresponding to luminous efficiencies of = 0.5, ≈ 3 and ≈ 6 lm/W and power efficiencies of 8.5 × 10 −5 , 5.9 × 10 −4 and 6.0 × 10 −4 W/W.

Effective stimulated emission and excited-state absorption cross-section spectra of luminescent polymers

The effective stimulated emission cross-section spectra of a para-phenylene-vinylene polymer and of two paraphenylene- ethynylene polymers are determined by a fluorescence amplification technique. For these luminescent polymers laser action has been achieved in solution. The effective stimulated emission cross-section spectra are found to be small compared to the stimulated emission cross-section spectra calculated from fluorescence quantum distribution and fluorescence lifetime measurements. The laser excitation of the polymer solutions seems to form lattice relaxed exciton conformations extending over a few monomer units which are thought to be responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.

Electroabsorption spectroscopy of poly(m-phenylenevinylene-co-2,5-dioctoxy-p- phenylenevinylene) and related materials

We report experimental studies of poly(m-phenylenevinylene- co-2,5-dioctoxy-p-phenylenevinylene) (PmPV-co-DOctOPV). Model oligomers based on this polymer have been synthesized and results on these materials are also presented. Electroabsorption measurements have been performed on spin coated films between 1.5 eV and 5 eV with electric fields of up to 200 KV/cm. Using Stark shift analysis the electroabsorption spectra are compared with the first derivative of the corresponding low temperature absorption spectra. We also investigate the effect of changes in chemical structure by studying the variation between the electroabsorption spectra of related materials. Samples of these polymers and oligomers that differ by the proportion of trans and cis vinylene bonds are also compared to see what effect the trans or cis bond has on the electronic structure. Increased cis content enhances emission efficiency but little alters the electroabsorption spectra. The results for the different materials are compared and several features occurring at the onset and at the minima of the electroabsorption spectra are considered. These features do not depend on the trans/cis ratio and we present evidence that suggests aggregation effects play an important role in these samples.