Light-emitting diodes (LEDs) are used in both displays and illumination applications because they are small, robust, and potentially very efficient. Organic light-emitting diodes (OLEDs) continue to gain attention from the scientific and industrial community. In contrast to their inorganic counterpart, OLEDs are flat and diffuse area light sources with the device thickness being in the range of 1¨C2mm. Thus far, OLED development has been triggered mainly by applications in the display segment, starting with applications for MP3 music players, mobile phones, and other portable devices. Recently, Sony brought to market the first OLED TV, which indicates that a more general penetration of the display market is close at hand.

OLEDs have not yet entered the lighting market, but that will probably change soon. Already most of the big players in the field are preparing for OLEDs to become ¡®the next big thing.¡¯ However, several critical problems need to be solved before widespread use for lighting becomes feasible. Specifically, the lifetimes, power efficiencies, reliability, and cost-effectiveness of white OLEDs must be able to compete with existing lighting technologies.

There are many different approaches to generating white light with OLEDs, some of which we will discuss in this article. In contrast to inorganic LEDs, light of different colors can be generated inside an OLED by mixing different organic emitter molecules in the emission zone. A basic scheme of an OLED device is shown in Figure 1. Glass is usually used as substrate at the moment, although in principle flexible substrates, like polymer or metal foils, could also be selected.

Figure 1. In an organic light-emitting diode (OLED), holes and electrons are injected into a stack of organic layers, which are usually sandwiched between a transparent indium tin oxide (ITO) electrode and a metal electrode. Electrons and holes recombine in the emission zone of the device, which contains organic emitter dyes. Here, excitons are formed by the combining charge carriers, which eventually decay and emit as visible light.

Directly generating multicolored light has several advantages compared to approaches in which blue or UV light is (partly) converted into light of longer wavelengths, as is the case in, for example, fluorescent tubes or white LEDs. Designs that depend on downconversion rely on the availability of suitable phosphors. In contrast, the spectra that can be achieved with OLEDs are free of this constraint, which allows a broader coverage of the visible spectrum, resulting in better light quality and a higher color-rendering index (CRI). Figure 2 shows a typical white OLED spectrum generated by three different emitters (blue, green, and red). More important, phosphor-based systems inherently lead to loss of energy as photons are converted to photons of longer wavelengths and lower energy. The principal physical mechanism behind light generation in OLEDs bypasses that energy loss.