Stefan Tasch

Blue electroluminescence with ladder-type poly(para-phenylene) and para-hexaphenyl

Abstract Conjugated polymers and oligomers are very interesting materials, with a number of possible electronic, optoelectronic and photonic applications. New synthesis techniques allow the preparation of extremely pure soluble ladder-type poly(para-phenylenes) (LPPPs), showing a steepness at the band edge comparable to conventional semiconductors. One consequence of this high purity is an improved photoluminescence quantum yield, around 30% for thin films. Oligomers of poly(poly-phenylene), like para-hexaphenyl (PHP) can be produced with extremely high purity, either as homogeneous disordered layers or well-ordered thin films, which show a high photoluminescence quantum yield in excess of 30%. All these materials are applied as the active layer in light-emitting devices (LEDs), giving bright blue light electroluminescence (EL) emission, and we report on their performance and efficiencies. For the LEd device with LPPP as the active layer, we observe the maximum of the EL emission around 460 nm, whereas for the PHP-based EL devices the emission maximum is located between 400 and 450 nm. The light emission of these EL devices is very high and exceeds a luminance of 2000 cd/m2 for a typical PHP multi-heterolayer EL device. For the overall EL quantum efficiency we obtain values up to 4% in homolayer EL devices. Ordering effects in thin films of conjugated materials, polymers and oligomers, strongly influence their optical properties. This ordering occurs in the active layers of EL devices and determines the threshold field for EL mission. Finally, we demonstrate that the emission colour of our EL devices can be controlled by the driving electric field.

The Effect of the Phosphor Particle Sizes on the Angular Homogeneity of Phosphor-Converted High-Power White LED Light Sources

Based on optical ray tracing, we discuss the effect of the phosphor particle sizes on the angular homogeneity of the light emitted from phosphor-converted LEDs. Since the blue LED and the yellow-converted light have rather different emission characteristics, which have to be harmonized to one another by the scattering processes within the color conversion element, the phosphor particle size turns out to be an essential parameter in order to attain angular homogeneity. This can be attributed, on the one hand, to the number of scattering processes within a specific unit volume for a given phosphor concentration, and on the other hand, to the specific scattering functions, both of which depend on the phosphor particle diameter.

Efficient full-colour electroluminescence and stimulated emission with polyphenylenes

Abstract Soluble and stable polyparaphenylene-type ladder polymers (LPPPs) with an extraordinarily high degree of intrachain order and exceptionally low concentration of defects belong to the class of best-defined conjugated low-dimensional organic semiconductors currently available. Parahexaphenyl, the highly stable oligomer of polyparaphenylene, can be synthesized in the form of single crystals, disordered thin films and highly ordered epitaxially grown thin films. We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light-emitting devices with these electroactive materials. These RGB devices are fabricated based on a new technique, which allows the realization of full-colour flat-panel displays. Using this new technique, we are able to produce devices having any desired emission colour in the visible and near-infrared spectrum. An efficient white-light emission is generated by an internal excitation energy (Forster-type) transfer from the blue LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present an optically pumped methyl-substituted LPPP waveguide, laser structure, which shows a spectrally very narrow and highly directional blue-green light output.

The Impact of Inhomogeneities in the Phosphor Distribution on the Device Performance of Phosphor-Converted High-Power White LED Light Sources

We present a study by optical ray-tracing in order to determine the impact of an inhomogeneous phosphor distribution in the color conversion elements (CCE) of phosphor-conversion- based white LED light sources. It turns out that in particular the color temperature and its angular variation, but also the flux-output are highly sensitive towards phosphor distribution variations.

Blue electroluminescence devices based on parahexaphenyl

Abstract Homogeneous thin films of the organic oligomer parahexaphenyl (PHP) allow the application of this material as a stable emitting layer in blue electroluminescence (EL) devices. We used oligoazomethine (OAM) and diaminoctofluorobiphenyl (DOB) as charge transport layers, which allowed us to achieve excellent EL performance with enhanced EL quantum efficiencies above 1%, improved brightness up to 2000 cd/m2 Luminance and increased rectification ratio in excess of 103 compared to single layer devices.

Optical anisotropy in thin films of a blue electroluminescent conjugated polymer

We determined the anisotropic optical constants of thin films of a conjugated polymer, the methyl-substituted ladder-type poly(paraphenylene) (m-LPPP), over a large spectral range between 1.55 and 3.5 eV. In the subgap region the anisotropic refractive indices were obtained via waveguiding experiments at 1.55 eV and 1.95 eV. Propagation loss measurements revealed that the absorption coefficient parallel to the polymer film surface is very small (<12.6 cm−1) in this energy region, due to the high purity of the polymer. In the strongly absorbing energy region of the π–π* electronic transition (2.6–3.5 eV) we performed spectroscopic ellipsometric measurements, in order to determine the anisotropic optical constants parallel (No) and perpendicular (Ne) to the polymer surface. The optical anisotropy in the m-LPPP films can be attributed to a preferential orientation of the polymer chains in the plane of the substrate.

Efficient Single-Layer Yellow-Light Emitting Diodes with Ladder-Type Poly(p-phenylene)/Poly(decyl-thiophene) Blends

We present the realisation of efficient polymer light emitting diodes (PLEDs) with yellow emission in a single-layer configuration. We found that the external electroluminescence-quantum efficiencies of PLEDs made of a blend of a blue emitting laddertype poly(p-phenylene) (m-LPPP) and small amounts of an orange emitting poly(decyl-thiophene) (PDT) are significantly improved to 4.2% compared with PLEDs fabricated of pure m-LPPP (2%). In this paper, electroluminescence and photoluminescence properties of different PLEDs consisting of m-LPPP and different amounts of PDT are compared. The results are discussed in terms of excitation energy transfer of Forster type and charge transfer.

Light-emitting electrochemical cells and light-emitting diodes based on ionic conductive poly(phenylene vinylene): a new chemical sensor system

Abstract Red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene] also named poly[2,5-bis (triethoxy-methoxy)-1,4-phenylene vinylene], (BTEM-PPV) are fabricated and characterized. BTEM-PPV combines good electronic conductivity with ionic conductivity in the oxidized and reduced state due to its conjugated backbone and oligo(ethylene oxide) attached as side chains. When applying this polymer in LECs one obtains devices of moderate brightness and with fast response times. This high performance is achieved without blending an additional ionic conductive polymer into the film. The response time of such devices driven with square waveform voltage pulses was determined to be 480 μs. The turn-on voltage for electroluminescence occurs at 2 V and at 3 V a brightness of about 35 cd/m 2 was obtained. Due to the covalent linkage of glyme like side chains to the PPV backbone, BTEM-PPV, complexed with metal ions, shows an ionochromic effect in the absorption spectra and also in the electroluminescence spectra, which can be a new approach to chemical sensors.

Photophysics of excitation energy transfer in highly fluorescent polymers

Abstract We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. The blue emitter laddertype poly(para-phenylene) (LPPP) is blended as a host with the red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We use transient and steady-state photoluminescence as well as near-steady-state photoinduced absorption to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. In addition, we describe analytical relations to determine the lifetime of these species from photomodulation spectroscopy.

Red-green-blue emission of parahexaphenyl devices with color-converting media

The realization of efficient red, green, blue (RGB) light emission can be achieved by covering a blue light emitter with color-converting dye/matrix layers. We present theoretical calculations and experimental data of an electroluminescence device based on parahexaphenyl, an organic semiconducting material, which emits bright blue light when used as an active layer in a thin film device. The waveguiding effects in the various layers making up the RGB device are investigated in order to maximize the brightness, and optimize the angular light intensity distribution of the external emitted light. Quantum efficiencies (QE) for green color conversion of 48%, and red conversion of 13% from blue emission were determined by calculating over all emission angles and confirmed by experimental QE measurements. The efficiency can be drastically improved when the device layers are directly coupled to each other by refractive index matching, where color-conversion efficiencies of 90% for blue to green and 80% for blue t...