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Resolving OLED manufacturing challenges

Posted: 26 Jun 2013     Print Version  Bookmark and Share

Keywords:OLEDs  manufacturing  fabrication  singlet harvesting effect  electroluminescence 

The thickness of the whole functional OLED-stack is only in the order of 100 – 200 nm. While the layout of an OLED is quite different from the usual design of a LED on the first look, there are intersections: While all relevant physical processes (charge injection, charge transport, recombination and light-emission) are covered by one single material in the LED-case, there are several materials involved in the case of an OLED.

When applying a voltage, charge carriers (electrons and holes) are injected with the help of hole- and electron-transporting materials. They move through organic conductive layers towards their respective counter electrode. Once the electrons and holes encounter each other in the emissive layer, which consists of an organic, conductive material doped with emitter molecules, they recombine and form a so-called exciton. Thus, the emitter molecules are excited and subsequently emit light whose frequency is in the visible region. Depending on the emitter molecules it can be red, green, blue or white, as a combination (figure 3).

Figure 3: Emission spectra of a series of luminescent copper complexes. Different materials emit light with tunable frequencies and can be used to process red, green, blue or even white (as a combination of the mentioned colours) OLEDs.

Manufacturing of OLEDs
Two different approaches enable the manufacturing of OLEDs: by vacuum deposition techniques or by solution deposition methods. The vacuum deposition techniques are state-of-the-art and enable ultrapure and precise layer architectures. Small molecules are being evaporated in a vacuum chamber and deposited on a substrate. Nevertheless, these current methods are unfavourable due to high costs for the materials and the processing of large devices. One main drawback is the fact that the material deposition is neither specific nor efficient: Not only the substrate but the whole equipment is usually coated with the OLED-materials. Although the OLED manufacturing needs only tiny amounts of material, the vacuum deposition process is accompanied with huge material loss. Also, the need to clean the evaporation chambers after a small number of production cycles further complicates the process and prevents continuous production.

In contrast to vacuum deposition, OLEDs can be produced by solution deposition methods such as printing or coating which promise to be cost-effective and to allow high flow-rates. Furthermore, they are more suited to form large-area films. Modifications of commercial printing or coating techniques (spin coating, gravure printing, screen printing, inkjet, etc.) enable the application of soluble materials (mostly polymers, but also transition-metal compounds) on substrates. The manufacturing of first-grade and ultrathin devices on flexible and (semi-)transparent films becomes possible and opens a new application spectrum. Consequently, solution deposition methods could establish as prospective state-of-the-art technologies.

Challenges in solution processing of OLEDs
Still the processing of the materials remains a major challenge. Especially, the manufacturing of multi-layer devices by solution deposition methods still makes high demands on industrial practicability due to three main problems:

First of all, grave problems arise from the insolubility of many functional materials known from vacuum-deposited OLEDs in common organic solvents. The low solubility of one of the standard emitters used in high-performance OLEDs hinders the preparation of homogenous thin films with a suitable thickness for OLEDs.

The second problem arises out of the first one: Morphological defects like crystalline grains in functional layers act as charge traps while aggregation of small molecules causes emission quenching. The application of materials with a low crystallisation tendency, which corresponds to a low lattice energy and a good solubility, or the immobilisation of the relevant molecules, e.g. by attaching them to a polymeric backbone can avoid such morphological defects.

As third problem, the functional layers often mix when a solvent used for the deposition of further layers is able to dissolve already cast layers. This undesired blending leads to changing properties in the long term operation of the diode. Another factor is the gradually blending of a multi-layer architecture by slow diffusion of one or more components through the stack, so-called interlayer-diffusion. The processing of multi-layer devices by solution deposition methods still suffers from the requirement of orthogonal solvents so far. In turn, these solvents make great demands on the applied materials and require careful adjustments of the processing steps. A more attractive answer to this issue seems to hold the use of cross-linking techniques. The materials are processed in solution and afterwards cross-linked to form insoluble layers.

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