Flexible OLED for homogeneous light in the operating room

Large-area lighting applications with organic light-emitting diodes (OLEDs) on flexible substrates were the aim of the joint project LAOLA (funding code: 03INT509AF), which was funded by the BMBF and ended in December 2021. The focus was on ultra-thin glass, which offers advantages over plastic as a substrate due to its excellent barrier properties. At the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, the OLEDs were applied to the flexible glass using a roll-to-roll process. A surgical light designed using this process will be presented at LOPEC 2022 (March 23 and 24, 2022, Munich) at the joint stand of the project coordinator Organic Electronics Saxony e.V. (OES).

Experiments at the FEP Large-arealighting applications with organic light-emitting diodes (OLEDs) on flexible substrates were the goal of the joint project LAOLA, which was funded by the German BMBF and ended in December 2021. The focus was on ultra-thin glass, which offers advantages over plastic as a substrate due to its excellent barrier properties. At the Fraunhofer Institute FEP, the OLEDs were applied to flexible glass using a reel-to-reel process. An operating room light based on this technology will be presented at LOPEC 2022 (March 23 and 24, 2022, Munich) at the joint booth of the project coordinator Organic Electronics Saxony e.V. (OES).

The glare-free, homogeneous light of large-area organic light-emitting diodes (OLEDs) is perceived as very pleasant and offers many advantages for product design. In the recently completed LAOLA project, OLEDs were therefore developed as wide-area lighting for a variety of applications on flexible substrates. The focus was on flexible ultra-thin glass, which offers advantages over plastic as a substrate thanks to its excellent barrier properties. Some of the technologies were researched as part of the LAOLA-related internationalization project between Japan and Germany with cooperation partners from the Japanese partner cluster YUFIC at Yamagata University.

In particular, the establishment of flexible ultra-thin glass as a substrate was promoted here. In order to consider suitable applications in addition to technological developments, Wolfram Designers and Engineers (WDI) developed a specific area of application for OLEDs on ultra-thin glass, which resulted in the implementation of an operating light that combines large OLED light surfaces with LED spotlights in its design. The OLEDs are installed as wing elements and provide indirect, glare-free lighting, while the LED spotlights provide direct lighting.

German-Japanese collaboration on ultra-thin glass technologies

A number of other partners worked together to produce the actual OLEDs on ultra-thin glass. At the beginning of this value chain is Nippon Electric Glass Co, Ltd (NEG) as a manufacturer of ultra-thin glass rolls. A transparent conductive oxide (TCO) was deposited on the ultra-thin glass with a width of 300 mm for further processing at Yamagata University as an anode material for the OLED. However, the sheet resistance of ~30 Ω per square meter is not sufficient to homogeneously illuminate the entire luminous area of 206 by 95 mm. To solve this, thin reinforcing lines were printed. This was done on a roll-to-roll screen printing system at Yamagata University in collaboration with the company Seria Engineering Inc. (roll-to-roll screen printing process) and Fujikura Kasei Co, Ltd (printing paste manufacturer).

New technologies for evaporation, cutting and structuring processes

One of the OLED lighting elements for the operating lamp. The OLED elements provide the background lighting, while the spot lighting is provided by LEDs"Ensuring the long-term stability of the OLED components and the hygienic surface of the lamp played a key role in the selection of ultra-thin glass as a substrate," explains Dr. Jacqueline Hauptmann, scientist at FEP. "One focus of the project was the conversion of an existing roll-to-roll vacuum coating system at the institute in order to be able to wind, coat and encapsulate pure ultra-thin glass of 50 and 100 µm thickness with strip tensions in the order of 30 to 50 N without any problems. The conversion of the system was carried out by FHR Anlagenbau GmbH."

The metal evaporator was converted by project partner Creavac-Creative Vakuumbeschichtung GmbH for the deposition of thin metal layers using the roll-to-roll process for anode and cathode coating. This allowed calcium and silver to be evaporated simultaneously in order to achieve transparent layers of 8 nm thickness (calcium/silver ratio 1:7) over a width of 290 mm with a layer thickness variation of ~1 %.

The laser cutting and structuring processes required to separate and interconnect the OLEDs proved to be a further challenge. Together with the project partner Heliatek GmbH, an alternative structuring method was developed that has enormous potential for subsequently structuring finished components with a low particle content. To achieve this, the anode, which is covered with a printed passivation, is lasered through the ultra-thin glass. Furthermore, the use of thermally vaporized melamine was validated in the project and advanced with the project partners Creaphys GmbH and Heliatek GmbH. Both technologies have enormous potential for utilization in new fields of application in flexible organic electronics

Ready for technology transfer to industry

The final separation of the OLEDs was successfully developed within the project with the project partner 3D-Micromac AG. Using a laser with Bessel optics [1], it was possible to demonstrate the so-called filamentation of the ultra-thin glass on both sides of the substrate and encapsulation and the subsequent mechanical separation of the adhesive. Cutting speeds of 400 millimeters per second were achieved. Project partner tesa SE tested various adhesive tapes for encapsulation in thin glass laminates, including with water trap components, and tested the cut glass and glass-adhesive-glass laminates for strength. A flexible stainless steel film from Nippon Steel Chemical & Material CO, Ltd. was tested for the encapsulation of opaque OLED components. The 30 µm thin film can be processed very well in a roll-to-roll process and promises to be a promising alternative to ultra-thin glass encapsulation due to its more favorable temperature management. Mitsuboshi Diamond Industrial Co., Ltd (MDI) [2] carried out the separation of the glass-adhesive-stainless steel OLED.

In addition to coordinating the project, Organic Electronics Management GmbH prepared a market study for the lead applications developed by WDI as well as a production concept, thus paving the way for technology transfer by the partners. Dr. Jonas Jung, project manager at OES, says: "Through the use of innovative production technologies by all partners, a promising OLED demonstrator has been developed that opens up new application possibilities for flexible electronics. This great result of the LAOLA project underlines the innovative power of the long-standing German-Japanese cooperation."

The results obtained in the three-year LAOLA project (2018-2021) can be directly transferred to other existing roll-to-roll tape lines. The successful separation of the OLED modules from the bonded glass-glass composite, which was in a rolled-up state after processing, can also be easily transferred in the future. We would like to thank the German Federal Ministry of Education and Research (BMBF) for its support in the LAOLA project (Large-area OLED lighting applications on thin flexible substrates, funding reference 03INT509A) as well as all German and Japanese project partners. [1]

References and comments

[1] R. Liebers: White paper: Optimized Laser Cutting Processes and System Solutions for Separation of Ultra-Thin Glass for OLED Lighting and Display Applications
[2] T. Furukawa; J. Hauptmann et al.: Roll-to-Roll Fabrication for OLED Lighting Using Ultra-Thin Glass Substrate and Encapsulating Stainless Steel Foil, IDW'21, FLX5/FMC6-1 2021