News Article
Merck and University of Cologne develop direct lithography OLED
The first high-resolution, full-color organic light-emitting diode (OLED) display based on a direct photolithographic process was presented by the research group of Professor Klaus Meerholz (University of Cologne, Germany) at this year's Society for Information Display (SID) symposium.
The first high-resolution, full-color organic light-emitting diode (OLED) display based on a direct photolithographic process was presented by the research group of Professor Klaus Meerholz (University of Cologne, Germany) at this year's Society for Information Display (SID) symposium.
The direct photolithography technique avoids most of the issues involved with the prominent ink-jet-printing technique. The new process was developed in collaboration with Merck OLED Materials GmbH (formerly Covion), a Merck KGaA affiliate based in Frankfurt, Germany. Since the infrastructure in existing liquid crystal display (LCD) production facilities can be adapted easily for the new technology, the German researchers are convinced the new technique will allow for very cost-effective display manufacture.
OLED technology is among the most promising possibilities for the next generation of flat-panel displays. The intense research and development investments during the last decade have lead to tremendous advances in performance and stability. However, it still remains a challenge to pixelate the emissive layer of an OLED display as there are three competing requirements: (1) Being compatible with large substrates to satisfy the customer's demand for increasingly larger displays, (2) defining extremely fine pixel structures to meet the resolution requirements for the new generation of HDTV- and micro-displays, and (3) significantly reducing production costs compared to existing display technologies.
With the newly developed direct lithography technique, all of these requirements seem to be within reach for the first time. Unlike other approaches such as ink-jet printing, this technique does not require the development of an entirely new process technology but rather relies on the chemical modification of the organic material.
"By adding oxetane side groups our emissive polymers gain the properties of photo-resists. Therefore, thin films of these smart resists can be patterned simply by exposure to ultraviolet light," Professor Meerholz said. He and his research group have now used this method to fabricate fully operational true-color matrix displays. These first prototypes can display simple pictures and support videos while consuming less power than most conventional displays. The displays also feature excellent color saturation for the three primary colors.
Professor Meerholz is convinced the new technique is suitable for OLED display manufacture. "Although our labs are not especially equipped for display fabrication, it took us only two months to develop the first working prototype. I believe this nicely demonstrates how easily this technique can be implemented," he said.
Since most processing steps are solution based, the whole fabrication process is simple and cost-effective. Spin coating is used to deposit each polymer onto a transparent substrate. The polymer film is then irradiated with ultraviolet light through a shadow mask causing the polymer to cross link and to form an insoluble material. The non-cross-linked material in the non-illuminated areas of the film is then washed away with solvent. The two other polymers are subsequently deposited in the same way to fabricate the pixilated device with three individually addressable colors.
Compared to the prominent ink-jet printing approach, the direct lithography technique can easily achieve high resolution and does not struggle with film thickness variations. Whereas ink-jet printing generally requires a substrate pre-treatment even to achieve a resolution in the range of 100 ppi (pixels per inch), a super-high resolution of 2 µm (equivalent to much more than 1000 ppi) was demonstrated on standard commercial and untreated display mother glass. Even more importantly, there is no need to separate the pixels by special ink confinement structures. Direct photo-lithography, therefore, allows for extremely small pixel spacings and thus for high-fill factors. As a consequence, the current density during display operation can be decreased, which reduces the power consumption and has a positive effect on the device lifetime.
The thermal evaporation of small molecules is another competing technique for OLED display manufacture. However, the high vacuum required during the evaporation process renders this technique more expensive – especially if large displays are produced. Additionally, the shadow masks required to define the fine pixel structure is known to cause defective pixels by contact damage or particle formation. The direct lithography technique is essentially non-contact so that the thin polymer films are not damaged during the pixilation process.
The direct photolithography technique avoids most of the issues involved with the prominent ink-jet-printing technique. The new process was developed in collaboration with Merck OLED Materials GmbH (formerly Covion), a Merck KGaA affiliate based in Frankfurt, Germany. Since the infrastructure in existing liquid crystal display (LCD) production facilities can be adapted easily for the new technology, the German researchers are convinced the new technique will allow for very cost-effective display manufacture.
OLED technology is among the most promising possibilities for the next generation of flat-panel displays. The intense research and development investments during the last decade have lead to tremendous advances in performance and stability. However, it still remains a challenge to pixelate the emissive layer of an OLED display as there are three competing requirements: (1) Being compatible with large substrates to satisfy the customer's demand for increasingly larger displays, (2) defining extremely fine pixel structures to meet the resolution requirements for the new generation of HDTV- and micro-displays, and (3) significantly reducing production costs compared to existing display technologies.
With the newly developed direct lithography technique, all of these requirements seem to be within reach for the first time. Unlike other approaches such as ink-jet printing, this technique does not require the development of an entirely new process technology but rather relies on the chemical modification of the organic material.
"By adding oxetane side groups our emissive polymers gain the properties of photo-resists. Therefore, thin films of these smart resists can be patterned simply by exposure to ultraviolet light," Professor Meerholz said. He and his research group have now used this method to fabricate fully operational true-color matrix displays. These first prototypes can display simple pictures and support videos while consuming less power than most conventional displays. The displays also feature excellent color saturation for the three primary colors.
Professor Meerholz is convinced the new technique is suitable for OLED display manufacture. "Although our labs are not especially equipped for display fabrication, it took us only two months to develop the first working prototype. I believe this nicely demonstrates how easily this technique can be implemented," he said.
Since most processing steps are solution based, the whole fabrication process is simple and cost-effective. Spin coating is used to deposit each polymer onto a transparent substrate. The polymer film is then irradiated with ultraviolet light through a shadow mask causing the polymer to cross link and to form an insoluble material. The non-cross-linked material in the non-illuminated areas of the film is then washed away with solvent. The two other polymers are subsequently deposited in the same way to fabricate the pixilated device with three individually addressable colors.
Compared to the prominent ink-jet printing approach, the direct lithography technique can easily achieve high resolution and does not struggle with film thickness variations. Whereas ink-jet printing generally requires a substrate pre-treatment even to achieve a resolution in the range of 100 ppi (pixels per inch), a super-high resolution of 2 µm (equivalent to much more than 1000 ppi) was demonstrated on standard commercial and untreated display mother glass. Even more importantly, there is no need to separate the pixels by special ink confinement structures. Direct photo-lithography, therefore, allows for extremely small pixel spacings and thus for high-fill factors. As a consequence, the current density during display operation can be decreased, which reduces the power consumption and has a positive effect on the device lifetime.
The thermal evaporation of small molecules is another competing technique for OLED display manufacture. However, the high vacuum required during the evaporation process renders this technique more expensive – especially if large displays are produced. Additionally, the shadow masks required to define the fine pixel structure is known to cause defective pixels by contact damage or particle formation. The direct lithography technique is essentially non-contact so that the thin polymer films are not damaged during the pixilation process.
