image: Figure 1 | Schematic illustrations of indirect photopatterning method of OLED EMLs based on RGB luminophores in single phase network structure. a, Process flow for the indirect photopatterning of the first EML pattern in the SPN structure. The first EML are patterned by i) forming a sacrificial photoresist (PR) pattern, ii) spin-coating the EML film, iii) forming the SPN structure by thermal annealing the EML at a low temperature (i.e. 110 °C), and iv) stripping the pre-formed PR pattern. Importantly, the SPN structure provides chemical resistance against solvents used in patterning steps (e.g., a PR solvent, a stripping solvent, and the subsequent primary color emissive material solution) b, Achievement of crosslinked RGB EMLs through repeated indirect photopatterning of the second and third EMLs in the SPN structure. c, Schematic of SPN structure formation using crosslinkable host and dopant molecules. The schematic demonstrates the SPN structure with host and dopant molecules being crosslinked.
Credit: Seunghan Lee et al.
High-resolution micro-organic light-emitting diodes (micro-OLEDs) have gained significant attention for futuristic microdisplays, particularly in virtual/augmented reality (VR/AR) applications. Recently, these devices have been successfully integrated into a commercial product, utilizing the OLEDoS technology, where a white-luminescent-OLED backlight is incorporated with red-green-blue (RGB) color filters (CF). CF-based displays, however, inherently suffer from insufficient brightness, as CF absorption reduces luminance. Consequently, there is a strong need in the field to develop micro-OLEDs based on direct electroluminescence driving scheme from a side-by-side patterned RGB subpixels. However, conventional patterning methods of an emissive layer (EML) based on evaporation process through fine metal masks (FMMs) can only produce patterns with critical dimensions larger than tens of micrometers due to the geometric constraint of the FMM. Moreover, the fabrication of FMM itself demands a high cost. Consequently, alternative methods capable of producing micrometer-scale EML patterns with exceptional reliability at a low production cost are highly sought to meet industrial standards.
In a new paper published in Light: Science & Applications, a team of scientists, led by a collaborative research team Professor Moon Sung Kang from Sogang University, and Professor BongSoo Kim from Ulsan National Institute of Science and Technology (UNIST), has developed an indirect photopatterning approach of solution-processed OLED EMLs in the single phase network (SPN) structure comprising host and dopant molecules, which leads to produce micrometer-scale RGB pixel patterns. Based on these strategies, they successfully fabricated organic EML patterns with a 3 µm scale. By repeating the patterning method with R/G/B color luminophores, pixel density larger than 3000 sets of RGB patterns in an inch (>3000 ppi) were successfully formed. The full color patterns at this resolution level have not been achievable previously through evaporation-based patterning methods.
The research team named their approach an indirect method, as EML can be patterned without direct exposure to UV irradiation or harsh etching processes. Additionally, the as-formed underlying EML patterns could be protected from direct exposure to chemicals used to form the next color EML patterns by the PR layer, preventing undesired degradation or contamination during the multicolor EML patterning process. Therefore, they highlight that our indirect photopatterning enables forming high resolution EML patterns, with avoiding possible destructive factors to the organic EML.
Professor Kang commented “In principle, the entire process can be conducted using a conventional photolithography setup already available in the industry, and the achievable pixel density is determined by the dimensions attainable from commercial PR. Therefore, we believe that this approach suggests a practical route to form micrometer-scale patterns of the EML for high-resolution full-color OLEDs.”
Journal
Light Science & Applications
Article Title
Micrometer-scale Indirect Photopatterning of RGB OLED Emissive Layers in Single Phase Network Structure