An attractive approach to full color OLED displays is based on white emitting copolymers and color filters. In this paper the special impact of broadband emitting copolymers based on polyspirobifluorene structures is discussed. The EL spectra of broadband emitters in PLEDs are strongly influenced by interference effects as well as by the driving conditions. Experimental results could be confirmed by modelling. Adjustment of emission spectra to the color filter characteristics lead to improved efficiency.
Organic light-emitting diodes (OLEDs) have recently attracted much interest among researchers as well as engineers as promising high quality self-emissive displays for all kinds of portable devices such as cellular phones, personal organisers, etc. While monochrome operation is sufficient for some applications, ultimately multi-color devices such as signs or even RGB (red, green, blue) matrix displays will be requested by the customer in the future. So far, this goal has been achieved with small-molecule devices fabricated by vacuum deposition. In contrast, electroluminescent (EL) polymers, which are commonly deposited by solution processing, seemed to be only poorly suited for this purpose owing to the lack of high-resolution patterning processes. Recent attempts, therefore, focus on the adaptation of common printing techniques such as screen printing and ink jetting, both having severe technical difficulties and drawbacks, such as limited resolution in the former and wetting issues in the latter case requiring extensive pre-treatment of the substrates. We demonstrate the use of a new class of EL polymers, which can be applied similar to a standard photoresist. Soluble polymers with oxetane sidegroups were crosslinked photochemically to yield insoluble polymer networks in the desired areas. The resolution of the process is sufficient to fabricate common pixelated matrix displays. Consecutive deposition of the three colors yielded a RGB device with efficiencies comparable to state-of-the-art EL polymers, even slightly reduced onset voltages, and improved efficiencies at high luminance levels. The improved thermal and morphological stability promises better performance in passive-matrix displays requiring high drive currents. The new method potentially allows efficient manufacturing of high-resolution multi-color polymer-based displays on large area using common lithography techniques.
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).
Although polyaniline (PAni) has been proposed for use as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and polymeric light emitting diodes (PLEDs) from very early on, the material does not seem to have found widespread use on a (pre)commercial scale. Recent results will be presented showing that PAni can be efficiently used as HIL, and that it even has some advantages over the often preferred poly[ethylenedioxy-thiophene] (PEDT). Intensive investigations on the influence of conductivity, morphology and especially the work function onto device performance have led to a commercially available water-borne PAni dispersion. The stable, nanoscaled system for HILs has a particle size of about 35 nm and a lateral conductivity (when deposited and dried) of around 10-6 S/cm. Using PAni dispersions for the generation of HILs the final device performance in OLEDs and PLEDs could be significantly improved. Depending on the used light emitting polymer (LEP), luminescence data were up to 30% more efficient compared to devices made with the widespread used PEDT.
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 year's 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.
In the last few years industrial research into materials fulfilling the needs of the fledgling OLED display industry have intensified considerably. At Covion we have developed a range of polymers based on phenyl-PPV derivatives which are now being commercially exploited in the first polymer LED applications. These materials have been developed systematically with the demanding requirements of the devices (e.g., high efficiency and lifetime) and the industrial applicability (e.g. processibility, reproducibility and reliability of supply) in mind. However due to market forces, such as the introduction of 3rd generation mobile communication technology, there will be an immediate demand for materials for full color OLED displays. In this paper we will report on progress in the development of Red, Green and Blue (RGB) materials at Covion. The requirements for the different colors vary depending on band gap (amongst others) and therefore the challenges for each color are different. The experience gained in understanding the important structure-property relationships in the phenyl-PPVs has been used to develop these new RGB materials.
The temperature stability of white and blue OLEDs was investigated by observing the I-V, EL-V and the spectral characteristics of various devices stored at elevated temperature (up to 130 degrees Celsius). Blue multilayer organic light emitting diodes (OLEDs) containing PEDT (polyethylenedioxythiophene) or PANI (polyaniline) derivatives as the hole injection and puffer layer, aromatic diamines like Spiro-TAD (2,2',7,7'- tetrakis(diphenylamino)spiro-9,9'-bifluorene) as a hole transport material (HTM), Spiro-DPVBi (2,2',7,7'- tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene) as an emitting material (EM) and of Alq3 (tris(8-hydroxy- quinoline)aluminum) as the electron-injection and electron- transport layer (ETL) were fabricated. White OLEDs were prepared, containing an additional DCM (dicyanmethylene-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran) doped Alq3 layer between the Spiro-DPVBi and Alq3 layer. Use of Spiro-TAD as a hole transport material (HTM) and of Spiro- DPVBi as an emitting material (EM) resulted in dramatically improved temperature stability: for the white and blue OLED no significant deterioration up to 130 degrees Celsius were found. Devices consisting of non spiro components like NPB and/or DPVBi already started to degrade at much lower temperatures.
Precision Nd:YAG laser cladding was investigated to achieve laser clads with specific pattern and roughness on very thin metal sheets (0.1 - 0.15 mm) to meet industrial requirements. Characterization of pulsed laser cladding on thin sheet substrate, side nozzle, coaxial nozzle and development, high speed video visualization of powder flow for both nozzles, microstructure evolution, vacuum clamping, chiller cooling were presented. Sound laser clad pattern with height/width ration 0.88 or 0.50 and identical quality was obtained. The present work demonstrates that precision laser cladding and/or direct laser manufacturing on thin sheet is another attracting field on laser cladding.