Designing eye-interactive, transparent OLED display
Micro-displays based on organic light-emitting diodes (OLEDs) can attain 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).
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 micro-display 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 gaze-based human-display-interaction.
While todays 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 realise micro-scale optical emitters and receivers on the same chip, e.g., in an array-type organisation as bidirectional OLED micro-display, 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 personalised information management: presenting information to the user while optically recognising the users 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 realised by n-well diffusion in a p-substrate. By using this structure it is possible to realise a light emitting and a photo detecting devicewith integrated driving circuits on a single CMOS chip.
Figure 1: Cross-section of OLED-on-CMOS setup in bi-directional OLED microdisplay and functional demo.
The active area of the bi-directional microdisplay consists of nested display and image sensor (embedded camera) pixels surrounded by a second image sensor (frame camera) as well as driving and control circuitry (figure 1). The display and image sensor systems are electrically independent of one another, simply interacting via synchronisation signals. A potential issue of such a bi-directional micro-display is optical crosstalk between display and camera. This crosstalk can be suppressed by operating display and camera time-sequentially.
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