Thin Film Encapsulated Transparent Organic Light Emitting Diodes
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Thin Film Encapsulated Transparent Organic Light Emitting Diodes Cristina Tanase1, Peter van de Weijer2, Herbert Lifka2, and Gerard Rietjens2 1 Philips Research, Eindhoven, 5656AE, Netherlands 2 Philips Research, Eindhoven, 5656 AE, Netherlands ABSTRACT Highly transparent OLEDs are very attractive for lighting and light beautification applications. While the transparent anode is based on transparent ITO, the transparent cathode is based on a 3-layer approach: (semi)transparent electron injection layer that is a low work function metal, electrically transparent conductor used in order to limit the voltage drop across the OLED area, and overcoat to tune the optical properties without influencing the electrical properties. Transparent encapsulation based on thin film technology is used in order to protect devices from ambient exposure. Using this approach large area (50 cm2) transparent organic light-emitting device having 75% transparency in the off state is demonstrated. The efficiency of the transparent OLED is comparable with that of bottom emission OLED. It is demonstrated that by tuning the thickness and optical properties of both the cathode and the encapsulation the amount of light emitted through the anode and the cathode can be varied while the total amount of light emitted by the OLED remains the same. Moreover device optimization based on optical thin film calculations has been performed such that no angular dependence of emitted light is present both on anode and cathode side. INTRODUCTION Conventional OLEDs are usually processed in bottom emission architecture, having transparent substrate, transparent bottom anode, and non-transparent metal cathode [1,2]. The light is emitted through the bottom surface (Figure 1a). Using this transparent structure, the same technology can also be used for inverted or non-inverted top-emitting structures (Figure 1b) in which one of the electrodes (the cathode in the case of non-inverted structure, and the anode in the case of inverted structure) becomes transparent. This device architecture is advantageous in the case of active matrix displays and with opaque substrates. The top emitting structure can improve the effective active area with more than 50% and the power consumption of the display by directing the emitted light away from the thin film transistor (TFT) back plane rather than through it. In the case of lighting devices, top emission OLEDs enable the usage of nontransparent (e.g. metallic foil, silicon wafers) and flexible substrates. The third OLED design is the transparent, dual-side emission OLED in which both the anode and the cathode are (semi)transparent (Figure 1c). The difference between these three designs is the ability of the anode, cathode, substrate and encapsulation to let the light pass through or not. For example, in the case of bottom emission the light is emitted through the anode but reflected by the cathode, while in the case of dual-side emission the light passes both the anode and the cathode and is emitted forward and backwar
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