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EETE APR 2014

OPTOELECTRONICS See-through OLED display goes eye-interactive By Dr. Uwe Vogel Micro-displays based on organic light-emitting diodes (OLEDs) achieve high optical performance with excellent contrast ratio and large dynamic range at low power consumption. Their direct light emission enables small-footprint and lightweight devices without additional backlight, making them suitable for mobile near-to-eye (NTE) applications such as electronic viewfinders or head-mounted displays (HMD). Fig. 1: Cross-section of OLED-on-CMOS setup in bi-directional OLED microdisplay and functional demo. In state-of-the-art applications the micro-display typically acts as a purely unidirectional output device. With the integration of an additional image sensor, the functionality of the microdisplay can be extended to a bidirectional optical input/output device. The major aim is the implementation of eye-tracking capabilities in see-through HMD applications to achieve gazebased human-display-interaction. While today’s mobile information systems such as smartphones and tablets are usually touch-controlled, micro-displays with state-of-the art pixel count but significantly decreased geometrical size, have found their way into consumer electronic products in the shape of electronic view finders in digital cameras. Micro-displays based on Organic Light Emitting Diodes (OLED) could have a very promising future for video and data display, especially if they can double up as an input channel. Now, OLED technology offers the possibility to integrate highly efficient light sources with photo detectors on a CMOS backplane. This enables fully integrated opto-electronic and smart applications based on silicon chips. One can realize micro-scale optical emitters and receivers on the same chip, e.g., in an array-type organization as “bidirectional OLED microdisplay”, thus performing a device that presents and captures images in the same place and even at the same time. This can be the foundation for a complete new class of devices for personalized information management: presenting information to the user while optically recognizing the user’s interaction. Implemented as augmented reality glasses that carry bidirectional micro-displays, such devices could feed visual information deliberately or unconsciously adapted to the context of operation, controlled through eye movement alone. Bi-directional OLED microdisplay and optics To achieve high-performance OLED characteristics on a standard CMOS process, a modification of the top-metal layer is necessary. The common requirements of an OLED-compatible top-metal layer are high reflectivity in the visible range of light, a smooth surface to prevent shorts in the OLED stack, and avoiding oxidation. A top emitting p-i-n OLED has a reflective bottom electrode and a transparent top electrode. Between these electrodes, an OLED stack with doped transport layers together with a triplet emitter system makes up a highly efficient and low voltage light emitter. The modified top-metal layer is used as bottom electrode and defines the shape and size of an OLED pixel. Below the bottom electrode there is space to integrate further drive circuitry. The photodetector device is realized by n-well diffusion in a p-substrate. By using this structure it is possible to realize a light emitting and a photo detecting device Dr. Uwe Vogel is Business Unit Manager for OLED Microdisplays and Sensors at the Center for Organic Materials & Electronic Devices Dresden (COMEDD) - He can be reached at uwe.vogel@ipms.fraunhofer.de Fig. 2: Optical setup of bi-directional near-to-eye optics. 36 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
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