Until fairly recently, the preferred, indeed the only means by which to display information in the electronic medium was to use a video monitor comprising a cathode ray tube ("CRT"). CRT technology has been well known for over 50 years, and has gained widespread commercial acceptance in applications ranging from desktop computer modules to home televisions and industrial applications. CRTs are essentially large vacuum tubes having one substantially planar surface upon which information is displayed. Coated on the inside of the CRT planar surface is a layer of phosphors which respond by emitting light when struck by electrons emitted from the electron gun of the CRT. The electron gun is disposed in an elongated portion which extends away from the inside of the CRT display surface.
While CRTs are widely used in numerous applications, there are several inherent limitations to the application of CRT technology. For example, CRTs are relatively large and consume a great deal of energy. Moreover, as they are fabricated of glass, the larger the display surface, the heavier the CRT. Given the need for the electron gun to be spacedly disposed from the phosphors surface of the display surface, CRTs have a substantial depth dimension. Accordingly, CRTs have little use in small and portable applications, such as handheld televisions, laptop computers, and other portable electronic applications which require the use of displays.
To answer the needs of the marketplace for smaller, lighter, more portable display devices, manufacturers have created numerous types of flat panel display devices. Examples of flat panel display devices include active matrix liquid crystal displays (AMLCD's), plasma displays, and electroluminescent displays. Each of these types of displays has use for a particular market application, though each is accompanied by various limitations which make them less than ideal for certain applications. Principal limitations inherent in devices such as AMLCD's relate to the fact that they are fabricated predominantly of inorganic semiconductor materials by semiconductor fabrication processes. These materials and processes are extremely expensive, and due to the complexity of the manufacturing process, cannot be reliably manufactured in high yields. Accordingly, the costs of these devices are very high with no promise of immediate cost reduction.
One preferred type of device which is currently receiving substantial research effort is the organic electroluminescent device. Organic electroluminescent devices ("OED") are generally composed of three layers of organic molecules sandwiched between transparent, conductive and/or metallic conductive electrodes. The three layers include an electron transporting layer, an emissive layer, and a hole transporting layer. Charge carriers specifically, electrons and holes, are generated in the electron and hole transporting region. Electrons are negatively charged atomic particles and holes are the positively charged counterparts. The charge carriers are injected into the emissive layer, where they combine, emitting light. OED's are attractive owing to their thin profile, light weight, and low driving voltage, i.e., less than about 20 volts. Hence, they have a potential application as full color flat emissive displays.
Small to medium size displays with diagonal viewing dimensions of less than about four inches are in great demand for portable applications such as video cameras, digital cameras, personal digital assistants, personal handy phones, cellular phones, and a host of other devices which require low power, lightweight, and multi-colored direction. In many such applications, icons, especially color icons, have been used along with a main display to indicate the miscellaneous information relative to the device, examples of which include operation mode, the signal carrier, power indicators, alarm codes, and others. For those applications where color icons are required in addition to dynamic or varying information, the practice has been to combine a standard AMLCD as the main display with light emitting diodes (LED's) as the color icons. Due to the limitations of viewing angle and night visibility with AMLCD's and the high cost and high power drain associated with LED's, this solution has been less than optimal. Moreover, the manufacturing difficulty of combining two or more types of display device results in significantly lower manufacturing yields and hence higher prices.
Accordingly, there exists a need for an emissive display which provides both color iconic information as well as dynamic, varying information in a traditional display format. Such a display device should have low power drain, wide viewing angle, good night visibility, and be both low cost and highly manufacturable.