The human eye places remarkably stringent requirements on the devices we use to illuminate objects or generate images. Exceedingly small deviations in color or contrast from what we consider natural are easily judged by the brain to be fake. Such cognition drives consumer practice, so great efforts have been made for over a century to synthesize emissive materials that match the response functions associated with the human perception of color. This is an extremely difficult task, given the diverse range of considerations, some of which include whether (1) the display is viewed under artificial light or natural sunlight, (2) the images are stationary or moving, and (3) the rendering of depth in a two-dimensional image is believable.
Established technologies including cathode-ray tubes (CRTs), vacuum fluorescent displays (VFDs), lamps, and x-ray phosphors have made possible a wide variety of display and imaging devices. However, continued advances are required to increase brightness, contrast, color purity, resolution, lifetime, and viewing angle while still lessening the cost, weight, volume, and power consumption. Mature or emerging technologies that address these issues include thin-film electroluminescent (TFEL) displays, liquid-crystal displays (LCDs),8 field-emission displays (FEDs),9 and plasma displays (PDs).10-12 Each of these technologies uses luminescent materials consisting typically of an activator from which light is emitted and a host for low concentrations of the activator (typically >1% activator). The requirements of the host and activator are discussed in a later section. The luminescent material can exhibit either a narrow emission spectrum, useful for color displays, or a broadband emission, which can extend into multiple colors. In addition, with multiple activator/host combinations, a luminescent material can emit several colors and even white light. While LCDs are light valves, which may be used in a reflective mode and therefore do not require a luminescent material, low-light situations require a backlight generated by a luminescent material. Many of the most versatile, efficient activators are rare-earth (RE) elements, for reasons that will be discussed. The ability of RE ions to emit red, green, and blue light make them well suited for application in visible-display technologies. This article reviews dopant and host material systems, excitation mechanisms, and the factors that limit the achievable luminescent intensity and efficiency. Device configurations for modern displays are discussed, as are materials and structures for next-generation technologies. Since each display technology has different performance and operational requirements, only the basic characteristics will be discussed here to enable an appreciation of emission from RE activators. References to the literature are supplied to further direct the reader to more in-depth discussions.