Heinrich Becker

Multi-colour organic light-emitting displays by solution processing.

Organic light-emitting diodes (OLEDs) show promise for applications as high-quality self-emissive displays for portable devices such as cellular phones and personal organizers. Although monochrome operation is sufficient for some applications, the extension to multi-colour devices—such as RGB (red, green, blue) matrix displays—could greatly enhance their technological impact. Multi-colour OLEDs have been successfully fabricated by vacuum deposition of small electroluminescent molecules, but solution processing of larger molecules (electroluminescent polymers) would result in a cheaper and simpler manufacturing process. However, it has proved difficult to combine the solution processing approach with the high-resolution patterning techniques required to produce a pixelated display. Recent attempts have focused on the modification of standard printing techniques, such as screen printing and ink jetting, but those still have technical drawbacks. Here we report a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials—soluble polymers with oxetane sidegroups that can be crosslinked photochemically to produce insoluble polymer networks in desired areas. The resolution of the process is sufficient to fabricate pixelated matrix displays. Consecutive deposition of polymers that are luminescent in each of the three RGB colours yielded a device with efficiencies comparable to state-of-the-art OLEDs and even slightly reduced onset voltages.

Highly efficient solution-processed phosphorescent multilayer organic light-emitting diodes based on small-molecule hosts

The authors report on highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on a low-molecular weight electron-conducting, bis-spirobifluorene host doped with a soluble derivative of the green emitter fac-tris(2-phenylpyridine) iridium (III) [Ir(ppy)3]. All organic layers were spin coated and a strong improvement of performance was achieved by introduction of a hole-transporting double layer based on cross-linkable low-molecular weight molecules. The devices combine the easy fabrication procedure known from polymer-based OLEDs with the higher efficiency of small molecules. Maximum luminous and power efficiencies of 59cd∕A and 58lm∕W, respectively, are obtained, combined with a low driving voltage and high efficiencies even at high brightnesses. At 1000cd∕m2 the efficiencies are as high as 55cd∕A and 49lm∕W.

Development of high performance PPVs: implications of the polymer-microstructure

Poly(p-phenylene vinylene)s (PPVs) are promising materials for optoelectronic applications, especially for displays based on polymer light emitting diodes (PLEDs). We report here our findings concerning defect structures in this class of materials and the implications of the discovered irregularities on important material properties, that is, the operational life in a PLED. Recent improvements, which were deduced from this findings, are presented: optimized PPVs with a lower amount of defects result in a strong increase of operational lifetime.

5.1: Purity of OLED-Materials and the Implication on DevicePerformance

The purity of OLED materials is examined by various analytical tools down to ppm level. Especially halogenated impurities are found to have a significant effect on device performance. HPLC MS coupling methods are needed to identify those harmful impurities. Three case studies are described, but the results are likely to be representative for most classes of OLED materials.

21.2: Materials and Inks for Full Color PLED-Displays

Progress in new materials for full color displays are presented. New polymers based on the “Spiro-Principle” show encouraging properties in electroluminescence performance and lifetime. The spiro-polymers can be tailor made to fit the RGB color requirements of a full color display. They are readily soluble, show excellent thermal stability and their synthesis is scalable to an industrially viable scale.

Influence of conductivity and work function of polyaniline-based HIL on PLED device performance

Polyaniline (PAni) dispersions can be efficiently used as hole injection layers (HIL) for passive and active matrix display applications. In earlier work the influence of conductivity and work function of HILs spin coated from water based PAni/PSS dispersions on device performance had already been presented. Recent investigations on hole transport mechanism in polyaniline systems now show the necessity of a minimum conductivity and an optimum work function for hole injection. Electrochemical Impedance Spectroscopy measurements combined with luminescence investigations showed that the lateral conductivity in the PAni films must be >10-6 S/cm. Otherwise, a decrease in maximum efficiency and an increase in driving voltage in dependence on coating thickness occurs. Work function investigations on water-free, highly conductive polyaniline dispersions emphasize the theory of an optimum range for hole injection from the anode into the light emitting polymer. The work function of highly conductive, non-aqueous PAni dispersion (0.1-5 S/cm) was determined by Scanning Kelvin Probe method to be 4.5 - 4.7 eV, which is outside of the optimum range at about 4.95 - 5.05 eV for polymeric light emitting diodes, resulting in poor efficiency values (max. 30 - 50% compared to PAni/PSS standard).

Light-emitting polymer materials for full-color displays

In the last few years, industrial research into materials fulfilling the needs of the maturing OLED display industry has intensified considerably. A first generation of polymers (phenyl-PPVs) is now being commercially exploited in first monochrome polymer LED displays. Nevertheless, due to market interest, there is a huge demand for materials for full-color OLED displays. After giving some initial results on our work in this field at last years SPIE, we will report on the progress in the development of polymers for red, green, and blue emission. Our main focus here lies on the improvement of the properties of various polymers derived from the spiro-bifluorene core. Depending on the color, the main issues vary strongly: Whereas e.g. for BLUE materials, efficiency, color coordinates, and processibility fulfill already commercial demands, operational lifetime still needs to be improved strongly. For RED materials, in contrast, the operational lifetime is already excellent, whereas the efficiency and the driving current still need to be improved. For GREEN acquiring saturated emission, whilst maintaining the other properties (high efficiency, long operational lifetime), is still challenging. Also, we will report on advances in full-color patterning, especially techniques based on Ink-Jet Printing.

22.2: Novel Triplet Host Materials: High Performance Made Easy

Novel materials are the key to high performance phosphorescent devices. New host materials show significant advantages over the currently used class of carbazole based materials. A significantly simpler device structure can be used applying the new material classes. Compared to the classical triplet device structure (employing electron transport layers and hole-blocking layers) an even better EL performance in terms of efficiency and lifetime is observed.

Influence of carrier conductivity and injection on efficiency and chromaticity in small-molecule white organic light-emitting diodes based on 4,4'-bis(2,2'-diphenylvinyl)-1,1'-spirobiphenyl and rubrene

Organic light-emitting devices (OLEDs) employing yellow-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-spirobiphenyl are optimized using a vacuum thermal evaporator. The influence of various hole injection/hole transport stacks and electron transport materials on the device performance and the electroluminescence spectra are discussed. Device characteristics are explained by the charge carrier distribution among the organic layers. OLEDs with warm-white emission with color coordinates of x=0.43 and y=0.42 were produced with power and current efficiencies of 5lm∕W and 10.9cd∕A, respectively, at a luminance of 1000cd∕m2. The maximum external quantum efficiency at a current density of 20mA∕cm2 was 4.6%.

45.1: Full‐Color Polymer‐LEDs by Solution Processing

We demonstrate the synthesis and use of a new class of EL polymers, which can be applied similar to a standard photoresist. Soluble poly-spiros with oxetane sidegroups were crosslinked photochemically to yield insoluble polymer networks in the desired areas with μm resolution. Consecutive deposition of the three colors yielded an RGB device with efficiencies and lifetimes comparable to state-of-the-art EL polymers.