Developing Chemicals to Light the Future

i s t r y he demand for fl at, lightweight television and computer displays has given rise to many different fl at-panel display technologies. Recently, much emphasis has been placed on organic light emitting diodes (OLEDs), chemical systems which have the potential to be not only smaller and lighter, but also more energy effi cient than other current display technologies. The most common form of television and computer displays now in use are cathode ray tubes, often referred to as CRTs. These devices accelerate electrons (cathode rays) through an electrical potential until they strike a certain area on a screen. Attached to this screen are phosphors (luminescent compounds) that emit a photon (or “particle”) of light when the electron strikes it. The color contrast and resolution from a CRT is very good, but the device is bulky and thus impractical for portable and lightweight applications. The current technology used in portable televisions and lap top computers is liquid crystal displays (LCDs). These displays utilize liquid crystals, compounds that align in certain conformations when an electric potential is placed across them, to create an image on a screen. Aligning the crystals with electricity is very energy effi cient, but LCDs require a great deal of energy to back-light the display in order to make the image visible. The high energy input required to backlight the display makes the LCD screen very ineffi cient, and consequently many portable devices that utilize this technology, like lap top computers, have short battery lives. OLEDs make use of a phenomenon known as electroluminescence in which electronically excited materials emit light. An OLED consists of three basic layers of different chemical compounds sandwiched between an anode and a cathode (Fig. 1). When an electric current passes through the diode (from cathode to anode), electrons and holes (units of positive charge) travel through their respective transport layers until they recombine in the light-emitting layer. This process is called light-emitting charge recombination. The light-emitting layer contains the electroluminescent material, which emits photons that produce the visible image on a screen. The materials that occupy the light-emitting layer of the OLED must luminesce a desired wavelength so that a full range of colors can be created on a display. These considerations