Electronic Design

You Can See Clearly Now With Compact P-OLED Microdisplay

This high-resolution, full-color device brings moving, TV-quality, large virtual displays to portable applications.

I guess you could call it eye candy. Say hello to what's claimed as the first—and only—polymer organic light-emitting diode (POLED) microdisplay. Developed by Scotland-based MicroEmissive Displays (MED), the "eyescreen" ME3204 is a full-color, high-resolution microdisplay on a chip.

The ME3204 delivers ultra-low power dissipation and high-level image quality (in terms of contrast and pixelation). Neither backlighting nor a driver chip is required. Polarization can be dismissed as well, since it uses an emissive technology. And there's no need for complex optics.

The unit provides QVGA resolution of 320 by 240 pixels in a compact 6-mm (0.24-in.) diagonal array with 230,000 dots. It also features a 4:3 aspect ratio. A fill factor (the proportion of the area of each pixel that lights up) of about 80% frees moving color video images from the distracting flicker and blurring that often are experienced with other microdisplays. Video frame rates ranging from 50 to 120 frames/s are possible.

The ME3204 measures a mere 14 by 10 mm with an integrated 50-mm flexible ribbon cable (Fig. 1). It operates from a 2.5-V supply and dissipates 50 mW—about one-fourth the power dissipated by other competitive LCD microdisplays. That equates to a theoretical life of 30 hours from a single alkaline AA battery, which is crucial in the all-important power-consumption category for microdisplay design. The ME3204 weighs less than other microdisplays, too.

The product is rated to operate from -20°C to 60°C. It's fully scalable to widescreen QVGA (WQVGA) resolution with a widescreen 16:9 aspect ratio, which is something MED plans to implement in the future (see "Color P-OLEDs Scale Up To 21-in. TVs,"). Its present aspect ratio is currently the standard for video glasses for TV on mobile and personal media players.

The ME3204 integrates the drive electronics and color filters (Fig. 2). It also contains a serial two-wire RGB interface and a parallel video interface that's compatible with the BT.656 standard, enabling a digital signal path. As such, product design engineers can focus their efforts on software development.

The digital signal path removes the need for an analog-to-digital converter (ADC) and eliminates loss in the dynamic range of composite video signals associated with analog-to-digital conversion. All that's needed, if the application dictates it (e.g., projection displays), are magnifying optics.

The ME3204 uses a patented structure of P-OLED material layered on a silicon chip. The device is based on a fabless design. The manufacturing process starts by taking a mirrored and fully processed CMOS wafer that's supplied by an outside source in Asia. This layer acts as the active-matrix driver backplane.

Next, a series of nanoscale layers is deposited on the wafer. One of these layers consists of the P-OLED material, which generates white light when a small current passes through it. The cathode is then deposited as a thin transparent metallic layer.

After that, a protective inorganic thin-film encapsulation layer is deposited over the entire structure for protection from external elements.

The next step is to laminate the silicon wafer to a glass wafer containing the patterned color filters, which filter the white light to provide the RGB subpixels that make up the color display. All of these steps are performed in MED's manufacturing facility in Dresden, Germany. The final step is to assemble and integrate the flexible cable, a process that's performed in a plant in Asia.

Ian Underwood, cofounder of MED and the company's strategic marketing officer, sees the ME3204 competing with transmissive thin-film-transistor (TFT) LCDs and reflective liquid-crystal-on-silicon (LCOS) displays. He believes TFT LCDs don't scale as well to small pixel sizes, where a lower overall optical efficiency and a lower fill factor (less than 25%) make pixelation artifacts more visible.

Moreover, only low levels of integration are possible with TFT LCDs, since they require more support chips. Underwood also points out that reflective LCOS units suffer from complicated optics and the system complication of sequential color.

So where's the "eyescreen" ME3204 headed? Targeted areas include head-mounted displays and electronic viewfinders (Fig. 3). In fact, it already has found a home in a variety of "near-to-eye" video imaging products, including digital still cameras and night-vision systems. Consumer applications for video glasses and headsets enabled by the ME3204 include DVD and video-clip viewing via personal media players, handheld electronic gaming, and watching TV and surfing the net via mobile phones.

"The confluence of mobile video broadcasting, video iPods, and advances in microdisplays and optics now means the technology and application are much more in line for a significant market opportunity in consumer video eyeware to finally arrive," says Chris Chinnock, founder and president of Insight Media, a market research firm specializing in displays.

The product is being manufactured under license from Cambridge Display Technology (CDT), a developer of P-OLED technology based in Cambridge, the U.K., and Japan's Sumitomo Chemical. Known as Sumation, the joint venture allows MED to manufacture the ME3204 using CDT's proprietary P-OLED Technology and Sumitomo's materials and high-quality manufacturing expertise.

The "eyescreen" ME3204 costs $15.75 each in quantities of 1000.

MicroEmissive Displays

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