Manufacturing of OLEDs – challenges and solutions
For quite some time now, research activities have focused on the development of energy-saving and cost-effective lighting and display solutions. Within this sector, organic light-emitting diodes (OLEDs) are established as a state-of-the-art solution. Due to the benefits of OLEDs, new all-time records have been set repeatedly: TV displays are getting even thinner and lighter. In addition, OLEDs offer area lighting in a straight-forward way without additional aid such as diffusing screens or glass fibers. This is in stark contrast to conventional, point-shaped light sources like LEDs or light bulbs and enables new lighting concepts for architecture and interior design such as illuminating wallpapers and windows. Flexibility is another key feature of the OLED technology: Using flexible glass or even plastic foils as a substrate, OLEDs can be made in a bendable or rollable way (Figure 1). Innovative prototypes, seemingly from the mind of science fiction writers, have been demonstrating this: Bendable tablet PCs, semi-transparent TV displays and beverage cans with luminescent OLED-labeling promise an exciting, bright future of the OLED- technology.
Figure 1: Flexible Device: Prototype of an OLED, which was produced by solution deposition methods on a flexible substrate, a key milestone on the path to light-emitting foils for packaging.
The happy ending to the OLED story is, however, yet to be written. Scientists and engineers are still working on some major fields: High priced products dominate the OLED market due to the complex fabrication methods. Thus, one of the most important issues is the development of new, cost-effective technologies as well as suitable materials. Their application in commercial printing techniques will play an important role for upcoming large-scale productions.
Working principle of OLEDs
OLEDs are based on the principle of electroluminescence and achieved their breakthrough by using multilayered devices. Several thin, functional layers are sandwiched between the electrodes, of which at least one is transparent (Figure 2).
Figure 2: Layered architecture of an OLED: OLEDs consist of several thin layers. Amongst the electrodes, which are needed to contact the device and close the electrical circuit, they consist of conductive layers for the transport of the negative and positive charges as well as an emissive layer. There, the charges recombine and light is generated. For better resolution click here.
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 colors) 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 unfavorable 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 multilayer 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 crystallization tendency, which corresponds to a low lattice energy and a good solubility, or the immobilization 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 multilayer architecture by slow diffusion of one or more components through the stack, so-called interlayer-diffusion. The processing of multilayer 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.
Consequently further efforts are necessary to benefit the advantages of solution deposition techniques and to develop reliable, industrial processes. One of the pioneers in this respect is cynora. One of their main targets is the stabilization of the solution processed layers to prevent mixing. A modular emitter system has been developed and patented, through which the solubility of a whole spectrum of different solvents from polar, biocompatible compounds like ethanol to nonpolar ones such as pentane can be adjusted. In this way, one is able to produce stable multi-layered architectures.
Materials that can be used in OLEDs
It is still challenging to manufacture long-time-stable multistack architectures. Moreover, the search for less pricey and more abundant metals than iridium or platinum as emitting compounds is ongoing. This is a problem as iridium only exists in small amounts in the earth’s crust and has to be produced and purified with great effort. Hence, the ultimate aim is the production of efficient large-area devices by solution-processing methods.
Promising new emitter materials are based on copper, silver or gold. Out of this group luminescent copper complexes are most promising candidates due to their rich structural and photophysical properties. Copper is pretty common, readily available and cheap. From this cynora’s primary goal follows: not only to provide alternatives to the commonly used triplet emitters based on iridium and platinum, but also to replace them in the long run. Another benefit of copper compounds is the use of the so-called “singlet harvesting effect”. Special properties of this kind of materials ensure very desirable physical properties, such as a high theoretical efficiency of 100% (i.e. every charge carrier couple that is injected in the device results in the formation of light).
Moreover, it is possible to synthesize new copper complexes based on a basic system with variable functional units. Some features allow to easily tune the emission color (Figure 4). Thus, red and green emitting systems are readily available, while – even more important – also deep blue emitters, which are pretty problematic today using common materials, are possible. Other material features enable the modulation of the solubility of the complexes to the relevant requirements without affecting the emission properties. With such emitters, OLEDs can be manufactured using simple solutions without the need of energy and resource consuming technologies like vaporization methods.
Figure 4: OLED materials under UV light. The same basic physical processes like in an OLED-device cause the emission of visible light. The exact color can be fine-tuned by changing molecular properties of the materials. By smart modification of the molecules, almost every color, as well as white light, can be generated with OLEDs.
There is still work to do
Research and development will continue on every step of the production chain of OLEDs. However, engineers and scientists accepted the challenge: From new, innovative materials such as copper complexes, to advanced concepts for material and device stabilization like crosslinking, further steps towards the optimization of OLED-production are taken every day by interdisciplinary teams from both industry and academia, big players such as Merck or Samsung as well as small companies such as Novaled or cynora. Today, several classes of readily-available, affordable and efficient materials suitable for an OLED-fabrication with printing and coating methods exist. One of the key steps is to further advance these techniques in order to establish processes for the production of larger-area devices. Once the process is fast, reliable and cost-efficient enough, some of the visions like smart packaging or printed displays may soon come true.
About the authors:
All authors are associated with cynora GmbH, a German R&D company. Founded in 2003, cynora came under new management in 2008. The company focuses on the research and development of innovative organic semiconductors whose physical properties as illuminants destine them for organic light-emitting diode (OLEDs) applications. From designing new functional molecules for OLEDs and solar cells all the way to the fabrication of OLED test devices using such compounds on a lab scale as well as testing them for physical suitability with advanced measurement technology and methods, cynora covers the entire spectrum of material and component development.
Besides exploring new ways of achieving additional efficiency gains, the young and dynamic team behind cynora with at the moment 15 employees also works on improving key aspects of OLED applications, such as cost-effective processing and life-expectancy. Printing optoelectronic components requires new, intelligent materials and approaches. Resulting products can be used as displays, light sources, design objects and solar cells for energy generation. For further information: www.cynora.com
All images courtesy Cynora