A representative OLED panel consists of a substrate (typically glass), to which very thin layers of different organic materials are added. The first layer (typically indium tin oxide or ITO) is the anode – or positively charged – layer, which is transparent to allow the generated light (upon application of a DC current) to be emitted from the panel. On top is the hole transport layer (HTL) and then the electron transport layer (ETL). In between these two layers, there are other organic materials called dopants—that help with light emission and tune the properties of the emitted light. The top layer is the cathode, which is typically a layer of aluminum foil or similar negatively charged emitting material to complete the electrical connection. Holes and electrons recombine to create energetic Alq3 molecules which then emit light.
KEY SCIENTIFIC DEVELOPMENTS
OLEDs have come a long way since their discovery, and scientists continue to focus on the development of new organic materials to advance this technology. Some of the most significant breakthroughs are:
Controlling Emission Color
In early OLEDs the Electron Transport Layer (ETL) also served as the emitting material. Soon after, it was discovered that device efficiency could be enhanced by using a separate emitting material known as the “dopant”. Moreover, the emission color could be altered by using different dopant molecules. Dopant molecules are imbedded in a matrix known as the “host”, together they form the emitting layer which is the most critical layer in an OLED. The other layers in the OLED play the role of facilitating charges, both electrons and holes, reaching the emitting layer where they recombine and emit light.
An OLED may contain multiple emitting layers; furthermore, a single emitting layer may contain more than one dopant. Today’s white OLEDs generally contain three dopants--red, green and blue--that span the entire visible spectrum. By choosing different dopants and their relative weight, a host of emission attributes such as color and color rendering can be optimized.
Doping OLEDs with different color emitters resulted in a great leap forward in both device efficiency and versatility. The dopants used initially were fluorescent materials. In the late 90’s, researchers in the laboratories of Steve Forrest at Princeton University and Mark Thompson at the University of Southern California started using phosphorescent emitters – specifically organometalic compounds of platinum and iridium that could be up to four times as efficient as the fluorescent emitters.
Today, the most efficient OLEDs are doped with phosphorescent emitters. Red and green phosphorescent OLEDs are stable enough for most display and lighting products. The lifetime of blue phosphorescent devices remains an active area of research.
Transparent and Top-Emission OLEDs
Early OLEDs used a magnesium:silver alloy as the cathode, whereas aluminum in combination with an Electron Injection Layer (EIL) is more common today. Both aluminum and silver are reflective metals that ensure that light is emitted through the transparent anode and substrate. The organic layers in the OLED are thin and transparent; therefore, if only the cathode can be made transparent while maintaining enough conductivity, one would have a transparent OLED. This is indeed what’s done in transparent OLEDs. These devices are transparent in the off-state and emit light from both sides in the on-state. The basic principle may be technically straightforward but the combination of transparency and light emission has aroused endless fascination in all who have experienced it.
A closely related device is the top-emitting OLED which features a transparent cathode and a reflective anode. This structure allows the use of opaque substrates such as stainless steel or aluminum foils down to tens of microns thick.