ITO-free large-area top-emission organic light-emitting diode by blade coating Hao-Wen Chang a , Hsin-Fei Meng b, *, Sheng-Fu Horng a , Hsiao-Wen Zan c a Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan b Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan c Department of Photonics and the Institute of Electro-Optical Engineering, Hsinchu 300, Taiwan A R T I C L E I N F O Article history: Received 22 October 2015 Received in revised form 23 November 2015 Accepted 24 November 2015 Available online xxx Keywords: Organic light-emitting diode ITO-free Top emission Large area Solution process Blade coating A B S T R A C T Large-area top-emitting organic light-emitting diodes (TEOLEDs) with multi-layer structure are successfully demonstrated using the solution-processable blade coating on ITO-free substrate. The semitransparent cathode of TEOLED is composed of lithium fluoride (LiF), aluminum (Al) and silver (Ag). The composition of 3 nm Al and 10 nm Ag has a transmittance of 56% and a sheet resistance of 11 V/&. It is applied to the green phosphorescence device with an emissive area of 2 cm by 2.5 cm. The maximum current efficiency is 25.2 cd/A with high light-emission uniformity within 10% variation. The large-area TEOLEDs show comparable current efficiency as the small-area devices with an emissive area of 2 mm by 2 mm (having the same device structure) and better efficiency than traditional large-area bottom- emitting devices. Cesium fluoride (CsF) and n-doped electron transport layer are applied to improve electron injection. At 6 V, the luminance is raised from 141 cd/m 2 to 502 cd/m 2 and 304 cd/m 2 , respectively. In n-doped device, a simple Al/Ag cathode is used without LiF. ã 2015 Elsevier B.V. All rights reserved. 1. Introduction Organic light-emitting diodes (OLEDs) have been actively studied in the past several years due to their potential for solid- state lighting and flat panel display applications [1–7]. Indium tin oxide (ITO) is commonly used as the transparent electrode for OLEDs due to its high transmission in visible wavelength range, good electrical conductivity, and high work function. However, since indium is a scarce material, it leads to the high cost of ITO. In addition, during OLEDs’ operation, indium migrates from ITO into the emission layer and this reduces OLEDs’ lifetime [8,9]. On the other hand, top-emitting OLEDs (TEOLEDs) of conventional architecture consists of a reflective anode and a semitransparent cathode, where light emits through the semitransparent cathode. The reflective anode is usually of high reflectance, excellent electrical conductivity, and relatively cheap metals, e.g., Al, Ag, etc. Thus, using these metals to replace ITO can help to save costs, and potentially raise lifetime. They could also be fabricated on opaque and flexible substrates, such as metal foils and plastics. For TEOLEDs, the deposition for organic layers is mainly vacuum evaporation in small-area devices [10–19]. However, the material utilization of vacuum evaporation is low that limits OLED products from wide development. Furthermore, for OLED lighting, large- area device with uniform luminance is necessary. Metal grid is applied for large-area OLED by vacuum process [20], but the process is complex and the grid may cause uniformity constraint. It is highly desired to realize a large-area TEOLED in a simple sandwich structure by solution process on an ITO-free substrate. The solution-processable blade coating method is developed for fabricating multi-layer organic semiconductors. Blade coating has the advantage of depositing multiple layers with minimal dissolution due to its rapid drying process. Compared with 10% material utilization to spin coating, blade coating is almost 100% that saves considerable costs. Furthermore, the method can be easily scaled up to large area with high throughput. From previous studies, multi-layer bottom-emitting OLEDs fabricated by blade coating showed good performance [21–25]. In this paper, we present large-area top-emitting OLEDs fabricated via blade coating. The semitransparent cathode is composed of LiF/Al/Ag. Two main factors affecting the light- emitting efficiency are the transmittance and sheet resistance of the semitransparent cathode. Therefore, three different Ag thicknesses are evaporated to acquire their transmittances and sheet resistances. In order to demonstrate the potential of large- area fabrication, the light-emitting areas are set to 2 cm by 2.5 cm. The light emits uniformly from the 2 cm by 2.5 cm areas, with peak * Corresponding author. E-mail address: meng@mail.nctu.edu.tw (H.-F. Meng). http://dx.doi.org/10.1016/j.synthmet.2015.11.025 0379-6779/ ã 2015 Elsevier B.V. All rights reserved. Synthetic Metals 212 (2016) 19–24 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/sy nmet