Alain Hilberer

Indium contamination from the indium-tin-oxide electrode in polymer light-emitting diodes

We have found that polymer light‐emitting diodes (LEDs) contain high concentrations of metal impurities prior to operation. Narrow peaks in the electroluminescence spectrum unambiguously demonstrate the presence of atomic indium and aluminum. Rutherford backscattering spectroscopy (RBS) and x‐ray photoelectron spectroscopy (XPS) depth profiling data corroborate this result. An average indium concentration of 5×1019atoms/cm3 originating from the indium–tin–oxide (ITO) electrode has been found in the polymer layer.

Blue superradiance from neat semiconducting alternating copolymer films

Blue superradiant emission in thin polymer films with a low energy threshold, as reported here, offers hope for the possible development of solid-state electrically pumped polymer lasers The absorbance, emission, and fluorescence spectra of the blue-light emitting copolymer poly[dimethylsilylene-p-phenylene-vinylene-(2,5-di-n-octyl-p-phenylene)-vinytene-p-phenylene] are presented and discussed. It is suggested that lasing in the superradiant regime occurs as a result of the combination of optically induced net gain and waveguiding in the thin film.

Novel high efficiency copolymer laser dye in the blue wavelength region

The lasing performance and wavelength tunability of a novel high efficiency copolymer poly[(2, 5, 2‘, 5‘‐tetraoctyl)‐p‐terphenyl‐4,4‘‐ylene vinylene‐p‐phenylene vinylene], TOP‐PPV, pumped with third harmonic radiation of a Nd:YAG laser, was studied in various organic solvents. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP‐PPV copolymer in hexane exceeds that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm (Δλ=38 nm).

Conjugated block copolymers for light-emitting diodes

In this communication we report results obtained in our laboratory on the design, the synthesis, and the characterization of new light-emitting copolymers. We are interested in block copolymers containing well defined conjugated sequences, which give rise to luminescence. Those chromophores, which are (pi) -conjugated oligomers like oligo(phenylenevinylene)s, are linked to each other by non (pi) -conjugated oligomers, for example oligo(silanylene)s, or directly in a non-coplanar way. In such systems color tuning of the luminescence is achieved through precise control of the length of the conjugated blocks and by changing the chemical structure of those blocks. Variations of the chemical structures of both the luminescent and interrupting blocks also give control over properties like flexibility and solubility of the polymers, which are relevant for device fabrication and optimization.

Model compounds for light-emitting PPV's: Optical and structural data of substituted oligomers

Phenylene-vinylene oligomers are investigated as model compounds for the luminescent moiety of PPV-type homopolymers or copolymers having oligo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have been substituted at various positions on the vinylene linkages. For several compounds single crystals have been obtained and their structure has been resolved, which allows the spectral response to be related to molecular conformation and packing. Thin film spectra demonstrate that cyano substitution results in a considerable red shift of the emission: a more planar conformation, π-stacking and, possibly, dipolar interactions cooperate with the electronic effect of the cyano group. The use of a cyano-substituted five-ring compound in a multilayer LED was found to strongly increase the luminescence efficiency.

LEDs based on conjugated PPV block copolymers

A way to control the bandgap in semi-conducting polymers is by preparing polymers with a partially conjugated backbone. In our laboratory, three conjugated copolymers containing PPV trimers as light emitting chromophores have been synthesized, which emit in the blue, green and orange wavelength region. The copolymers have a well defined conjugated backbone consisting of regularly alternating terphenylene and p-phenylene-vinylene blocks. The desired control of conjugation length is achieved through steric interaction induced by the side-chains in the terphenyl blocks of the copolymer. In this paper we evaluate the electrical and optical properties of LED devices based on these conjugated PPV block copolymers.

Blue Light Emitting Diodes based on a partially conjugated Si-containing PPV-copolymer in a multilayer configuration

Efficient blue Light Emitting Diodes (LEDs) based on a novel partially conjugated co-polymer (SiPPV) have been realized by a combination of techniques known to enhance the quantum efficiency of organic devices. The copolymer is homogeneously blended in a PVK-matrix to reduce the number of non-radiative decay paths. In single-layer devices the high barrier at the Al top electrode limits efficient electron-injection into the device. In double-layer structures with an evaporated PBD-film at the cathodic side, electron and hole currents are much more balanced, resulting in an internal quantum efficiency of 3.2%. The average turn-on field of a double-layer device is reduced compared to a single-layer LED.