G. Leising

Efficient white light-emitting diodes realized with new processable blends of conjugated polymers

An alternative method for producing efficient white light-emitting polymer diodes based on a blend of two polymers is reported. The white light emission is composed of a broad blue emission of laddertype (polyparaphenylene) (m-LPPP) and a red-orange emission of a new polymer, poly(perylene-co-diethynylbenzene) (PPDB). The red-orange electroluminescence emission is promoted by an excitation energy and charge transfer from m-LPPP to the PPDB. A concentration of 0.05% PPDB in the polymer blend is required in order to obtain white light emission. By inserting an insulating material in the blend, so that a maximum external quantum efficiency of 1.2% is obtained.

Highly efficient electroluminescence of new wide band gap ladder‐type poly(para‐phenylenes)

We present the optical and electronic properties of a polymeric semiconductor, a new soluble stable ladder‐type poly(para‐phenylene) (LPPP, band gap=2.69 eV) and demonstrate its excellent applicability in blue‐green electroluminescence (EL) devices. The high intrachain order and the low defect concentration of the polymer lead to a remarkably low absorption coefficient in the subgap region (αmin<12 cm−1 at 1.5 eV) and also to a high photoluminescence quantum efficiency (30% in the solid state). The current/electric field characteristics of the high efficiency (up to 0.04 photons/electron) EL devices reveal a polymer film thickness dependence on the required onset field due to ordering effects in the active polymer layer. The shape of the recorded EL spectra can be controlled by the applied electric field in the blue‐green spectral range.

Blue electroluminescent device based on a conjugated polymer

Abstract Polyparaphenylene (PPP) thin (≈0.5 μm) layers were investigated in order to realize a blue-light-emitting tunneling structure (LETS). The LETS consists of PPP confined between an indium/tin oxide (ITO) and an Al contact. Blue-light emission at room temperature occurred if a negative voltage exceeding 10 V was applied to the Al electrode. It is demonstrated that the device function is based on a field induced, tunneling assisted, radiative recombination process. Blue LETS are of considerable physical and technical interest due to the lack of a satisfactory material for such a device realization up to now.

Quantum efficiencies of electroluminescent poly(p-phenylenes)

Summary form only given. Poly(paraphenylene) (PPP) and some derivatives (ladder polymers) have proved to be suitable materials for electroluminescence devices. Photoluminescence and electroluminescence measurements require the preparation of homogenous thin films. They are obtained by spincoating of soluble material (precursor PPP and ladder-type PPP) or evaporation of low molecular weight oligophenyls. We present recent results on quantum efficiency and the electronic structure of these materials used as the active layer in electroluminescence devices. The influence of the molecular geometry and intermolecular effects on the emission- and excitation- spectrum as well as on the quantum efficiency (photo- and electroluminescence) is pointed out. The financial support of this research project by the Volkswagen Stiftung is gratefully acknowledged.

Structural properties of trans- and cis-(CH)x

Abstract The availability of highly oriented films of trans -(CH)x and cis -(CH)x permits to obtain reasonably resolved X-ray fibre patterns. We present an analysis of the intensity distribution of such patterns for trans -(CH)x and deduce the setting angle Φ, the dimerization distortion Δz, the geometry of the carbon chain, the space group, and the anistropy of the Debye-Waller factor. In addition, some preliminary data for the structure of cis -(CH)x are reported.

Optimisation of polyfluorenes for light emitting applications

The properties of polyfluorenes for use in luminescent devices have been improved by a variety of approaches. Their conjugation length may be extended by using more planar indenofluorene units. Formation of long wavelength emitting aggregates is suppressed by attachment of bulky dendron groups to give stable blue emission. The emission colour can be tuned across the visible spectrum by incorporation of perylene dye units on the main chain or as substituents. Rod-coil copolymers have been made in order to achieve control of the supramolecular order. Polyfluorenone prepared by a precursor route shows improved electron injection and transport properties.

High-mobility pentacene organic field-effect transistors with a high-dielectric-constant fluorinated polymer film gate dielectric

High-performance pentacene organic thin-film transistors with double layers of the terpolymer electret poly(vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene) and the polymer poly(vinyl cinnamate) as a gate dielectric are reported. The electret is a high-k dielectric polymer with a static dielectric constant of e=14. The transistors show an intrinsic field-effect mobility in the range of μi=1cm2∕Vs and an on- to off-current ratio of about 105. High-k polymer gate dielectrics seem promising for organic nonvolatile memory and sensor field-effect transistors.

Excitation energy migration in highly emissive semiconducting polymers

Abstract The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications, is probed by doping the blue light emitting methyl-substituted ladder-type poly( para -phenylene) with small concentrations of a highly fluorescent π-conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence is modeled and discussed by means of a two-step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Forster-type mechanism alone cannot account for the experimental facts in such a guest host system.

Blue green stimulated emission from a high gain conjugated polymer

An optically pumped waveguide structure has been fabricated with methyl substituted conjugated laddertype poly(paraphenylene) as the active material. The choice of the device parameters allows one to observe a high directionality, a small beam divergence, a complete linear polarization, and a linewidth with an upper limit of 1.7 nm at a considerable low threshold of 3 kW/cm2 at room temperature.

Electroluminescence of wide-bandgap chemically tunable cyclic conjugated polymers

Abstract In this paper we report on poly(paraphenylene) and its derivatives as active layers in light-emitting Schottky-like devices. The bandgap and therefore the colour of the electroluminescence emission is chemically tuneable over a wide spectral range. Electroluminescence spectra of poly(paraphenylene) and a soluble poly(paraphenylene) type ladderpolymer are presented. The possibility of tailoring the bandgap is demonstrated by examples of the phenylene ladderpolymer and alkyl-substituted polyphenylene copolymers. The absorption spectra and photoluminescence emission spectra of those polymers are presented.