Recent developments in organic light-emitting diode (OLED) technology are enabling greater deployment of OLED displays in a number of applications, including home appliances, consumer electronics, and even medical equipment. One factor behind this movement is the development of OLED materials with longer life. For 2007 production, some OLED displays will be capable of supporting lifetimes of 100,000 hours, bringing the technology on par with the best LCD and VFD lifetimes.

A number of emissive-type displays are used in today’s home appliances, including segmented LEDs to show time or numbers, LEDs for simple status indicators, and vacuum fluorescent displays (VFDs). Emissive displays are favored for their ease of readability from any angle and high legibility. OLEDs have the same advantages of other emissive display technologies while also being capable of delivering higher information density and better image quality.

Longer life and larger sizes make OLED displays suitable for more applications.

Home appliances typically require screen diagonals of 2 in. to 5 in. When OLEDs first came onto the market, most of the offerings were in the 1-in. class designed to support small devices like mobile phone sub-displays and MP3 players. Earlier this year, 2-in. to 3-in. OLEDs were offered for production and in 2007 4-in. to 5-in. OLEDs will be offered in standard industry formats. The combination of longer lifetimes with larger screen sizes offered by OLED display manufacturers puts OLED screens into alignment with the graphic-user-interface requirements of home appliance manufacturers.

The demand for OLEDs and higher information content displays in the home will be driven by more advanced features and functionalities that require more information be provided to the user. For example, air and water purification devices show information on when to change the filter, and some refrigerators now display the temperature in each compartment. More information requires more displays. This trend to provide more information to the user will inevitably continue and could someday include RFID functions that show when items in the refrigerator have passed their expiration dates, or offer suggestions for recipes, and help with ordering the required ingredients for automated delivery to the home.

How OLEDs work

OLED displays are available in a number of brilliant display colors.

OLEDs rely on organic materials (polymers or small molecules) that emit light when energized by an electrical current in a three-step process:

1. Electrons are injected from a cathode.
2. Holes are injected from an anode.
3. Recombination of electron hole pairs at the emissive polymer occurs.

Enlarge this picture Fig. 1. Structure of an Osram polymer OLED device.

This process causes the activated pixel to emit light. A circular polarizer is usually used on the front surface to prevent any ambient light present from reflecting back from the metalized traces on the cathode structure. The effect of the circular polarizer is to create a pure-black background in the off-pixel area that greatly enhances the contrast ratio. The combination of text shown with bright on-pixels and pure-black off pixels enables a display with excellent legibility. (See Fig. 1.)

Comparing displays

Enlarge this picture Table 1. Comparison of display technologies.

OLEDs can be used in applications where clear readability from any angle is desired. OLEDs have a dark ambient contrast ratio of 2,000:1, compared with 5:1 for a comparable LCD display. And where LCDs must be specified with a specific viewing direction of optimum contrast such as 6:00 or 12:00, OLEDs are Lambertian emitters and display information with the same level of contrast in all directions. The rapid response time of OLEDs offers advantages in applications that require information to be smoothly scrolled across the screen. (See Table 1.)

OLEDs offer a distinct advantage in image quality over VFDs. OLEDs can be produced with very fine pixel density so that information can be displayed with very smooth lines, minus the pixilation seen in VFDs. OLEDs also offer smaller package sizes with compact width, height and depth dimensions, compared with VFD or LCD displays. And, OLED lifetimes are now competitive with lifetimes for LCDs and VFDs. OLEDs with lifetimes exceeding 50,000 hours are currently in production, and next year, 100,000-hour OLED display modules in dot formats of 128 x 64 and greater will be commercially available. While OLEDs currently produced typically do not have as wide of a temperature range as VFDs, OLEDs with an operating temperature range of -40 DegC to 85 DegC will be offered next year.

Design considerations

Fig. 2. Examples of text shown in 4-bit (left) and 1-bit versions (right).

1-bit or 4-bit: Many standard OLED displays offer 4-bit grayscale, which supports up to 16 gray levels and enables anti-aliasing fonts that provide a smooth, rounded appearance in the user interface for better legibility and a premium user experience. Grayscale-enabled OLED displays also can produce shading effects on icons for emphasis or to show selected vs. non-selected icons. Attractive animations or video clips are also feasible with 4-bit OLEDs. Where no gray scale effects are required, and where cost is the prime consideration, designers can select 1-bit OLEDs, where all activated pixels are shown at the same luminance. (See Fig. 2.)

Optimized GUI design: OLED power consumption can be minimized by designing the GUI with a dark background and bright illuminated text pixels. The black, off pixels, which can comprise up to 90 percent of the total pixels in a typical text pattern, do not draw any power. By contrast, LCD backlight text illumination requires the entire surface area of the display to be powered, even when only one line of text is shown. An OLED with a 10 percent on typical text pattern illuminated against a dark background will consume only about 10 percent of the power of a similar LCD driven at the same luminance. The low power consumption attributes of OLED displays can contribute to appliance energy efficiency.

Color choices: OLEDs are capable of producing images in a wide range of colors and full-color images, but device performance will vary according to the color choice. For devices requiring longest lifetime, the OSRAM Elegance Yellow is a good choice as it offers 55,000 hours to half-luminance point (over six years of continuous operation), and this is expected to be extended towards 100,000 hours in 2007.

For applications requiring sustained, high-temperature operation, the OSRAM Spring Green OLED supports an operation temperature range of –40 DegC to 85 DegC. The Spring Green OLED material also has excellent high-ambient-light readability, even in direct sunlight. Blue, white and full-color OLEDs offer shorter lifetimes of 10,000 hours or less and are more suitable for applications requiring fewer lifetime hours, such and portable consumer devices and handheld remote controls.

Screen saver: The implementation of a simple screen saver is recommended in patterns that are constantly or repeatedly represented for long periods of time. This is commonly done through a simple 2 x 2 or 3 x 3 pixel shift that can be timed to be imperceptible to the user by timing the shift to occur once every hour or similar increment. Screen savers not only eliminate burn-in in heavily used pattern areas, but can also significantly extend the product’s life.

Integrating OLED displays

Integrating OLEDs into applications is relatively easy for design engineers familiar with interfacing with LCDs. As with LCDs, a standard 128 x 64 OLED can be interfaced either though a serial interface such as SPI, or 8-bit parallel interface compatible with either 6800 or 8080 series standards. OLEDs typically require 3 V for logic and 12 V for the display illumination. In the case of LCDs, this illumination power is running through the backlight instead of the main controller-driver interconnect as it does in the case of OLED displays.

To enable design engineers to quickly get up and running with OLED displays, OSRAM offers an OLED Reference Design Kit that supports both 2.7-in. and 1.6-in. 128 x 64 dot OLED displays. It allows users to quickly upload patterns and evaluate the technology in their application, and also contains useful hardware and software tools that speed the design and development process, which saves valuable engineering time.

Advances in OLED technology, including improvements in the physical size of standard display offerings and lifetimes that meet or exceed those of alternative display technologies, allows OLED technology to take its place as a viable technology for home appliances. In the future, OLED displays in the home will be found in the areas of lighting control, central audio control, security systems, universal remote controls, kitchen appliances, vacuum cleaners, and air purification systems.

For more information email: John.Cramer@osram-os.com