Backside Contacting for Uniform Luminance in Large-Area OLED
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Backside Contacting for Uniform Luminance in Large-Area OLED
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P. Pfeiffer , X. D. Zhang , D. Stümmler , S. Sanders , M. Weingarten , M. Heuken , 1 1 A. Vescan and H. Kalisch 1
GaN Device Technology, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany 2 AIXTRON SE, Dornkaulstr. 2, 52134 Herzogenrath, Germany ABSTRACT We have investigated organic light emitting diode (OLED) backside contacting for the enhancement of luminance uniformity as a superior alternative to gridlines. In this approach, the low-conductivity OLED anode is supported by a high-conductivity auxiliary electrode and vertically contacted through via holes. Electrical simulations of large-area OLEDs have predicted that this method allows comparable luminance uniformity while sacrificing significantly less active area compared to the common gridline approach. The method for fabricating backside contacts is comprised of five steps: (1) Thin-film encapsulation of the OLED, (2) Patterning of the OLED surface with lithography (resist mask defining via hole positions), (3) Via hole formation to the bottom anode by a plasma etching process, (4) Organic residues removal and sidewall insulation. (5) Contacting of the anode with a high-conductivity auxiliary electrode. Backside-contacted OLEDs processed by organic vapor phase deposition show high luminance uniformity. Scanning electron microscopy pictures and electrical breakthrough measurements confirm efficient sidewall insulation. INTRODUCTION Large-area organic light emitting diodes (OLEDs) up to 320 mm × 320 mm in size are commercially available (LG Display) but are still suffering from the requirement for gridlines in combination with stacking to achieve high luminance uniformity. Especially when OLED sizes are growing further, gridlines limit the active area and are not aesthetical. For larger OLED sizes, even broader gridlines will be required to achieve the same luminance homogeneity [1]. To overcome the limitations of gridline technology, OLED backside contacting [2] as a superior alternative is investigated. In this method, the low-conductivity anode (e.g. ITO) of a conventional bottom-emitting OLED with thin-film encapsulation (TFE) on the top cathode is supported by a high-conductivity auxiliary electrode layer (e.g. Al) atop the TFE. This auxiliary electrode is insulated from the Al cathode by the TFE and connected to the buried anode by etched and metallized via holes distributed over the active OLED area. Unlike in gridline technology, in which the lateral gridlines entirely reduce the active area of the OLED, in backside contact technology only the via holes cross-sections are visible. The current is distributed in the high-conductivity auxiliary electrode layer on top of the TFE and has no observable negative influence.
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