(Invited) Advanced Transparent Conductive Top Electrode for Transparent and Top-Emitting Organic Light-Emitting Diodes
Authors: Lee Jeong-Ik , Lee Hyunkoo , Lim Jong Tae , Park Sunghee , Jin Won-Yong , Kwon Byoung-Hwa , Byun Chunwon , Cho Nam Sung , Kang Jae-Wook , Yoo Seunghyup 
Published: February 2020

Organic light-emitting diodes (OLEDs) have successfully entered mobile-phone display market and large-size OLED TVs have also been available by overcomming many technical obstacles. However, novel and innovative technologies are still required for flexible, transparent, and high-resolution displays. For example, augmented reaility (AR) and virtual reality (VR) have recently received peculiar attention and they need transparent and high-resolution displays with thousands of pixels per inch (PPI) to realize immersive displays. Top-emitting OLEDs (TEOLEDs) have an advantage for high-resolution displays due to their large aperture ratio. Top electrode is one of crutial factors to decide the performance of TEOLEDs as well as transparent OLEDs (TOLEDs) and should have high transmittance, low reflectance, and high conductivity.
There are many candidate materials for transparent conductive electrodes but the top electrode choices are limited, because top electrode materials should not damage the organic layers of OLEDs during the deposition process. Most TEOLEDs and TOLEDs that have been reported so far utilize semitransparent thin metal top electrode owing to their compatibility with OLED fabrication process, but thin metal top electrodes without sophisticated optical design degrade the efficiency and color quality [1]. Indium-tin-oxide (ITO) has optically good properties, but it needs sputtering and high temperature precess which leads to damage of organic layers in OLEDs [2].
Carbon-based materials suchs as carbon nanotubes (CNT), graphene, and conducting polymers are lately used as transparent conductive top electrodes in OLEDs and polymer solar cells because of their high trasmittance and low reflectance [3-6]. Most devices show optically good but electrical poor properties due to low conductivity of carbon-based non-metal transparent conductive materials. To apply carbon-based electrodes to large-size devices, their electrical conductivity should be improved.
In this work, we make carbon-based transparent conductive electrode films and enhace the conductivity of the film by embedding silver mesh electrodes. The film showed high transmittance and low reflectance in visible spectral region in spite of existence of metal auxiliary wire. We have fabricated small molecule-based transparent OLEDs with these electrode films as top electrodes using a dry lamination technique and inverted structure which has cathodes on the substrate is employed because the conducting polymer has high work function that is suitable for an anode instead of a cathode. The device exhibited low driving voltage with high transmittance. In addition, we have demonstrated a 4 cm x 4 cm transparent segment panel with silver mesh embedded conducting polymer top electrodes. We also investigated color shift of white TEOLEDs with conducting polymer top electrodes depending on viewing angle.
Acknowledgment
This work was supported by Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) (B0101-16-0133, The core technology development of light and space adaptable energy-saving I/O platform for future advertising service)
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