The history of OLEDs is short but rich. Learn the science behind this technology and how it has evolved so rapidly into a viable light source for today. 


OLED stands for Organic Light-Emitting Diodes. They are “organic” in that the key functional layers are composed of organic compounds – complex carbon containing molecules. OLEDs as we know them today were invented in the 1980’s by Ching W. Tang and Steven Van Slyke at Kodak Research Laboratories.
There are, quite literally, millions of different organic compounds. In the early stages of OLED development it was customary to classify OLEDs by the molecular size of the materials used: either small-molecule or polymer (a long-chain molecule). The main difference lies in possible fabrication methods: small-molecule materials can be deposited by vacuum thermal evaporation (VTE), whereas polymers must be deposited via a solution process. VTE is the dominant fabrication method for commercial OLED products today. However, solution process holds the promise of low-cost manufacturing. As solution deposition methods for small-molecule materials are being developed, the distinction between the two is getting blurred. The first OLEDs, developed by Tang and Van Slyke, were of the small-molecule variety. Polymer OLEDs, also called PLEDs (polymer light emitting-diodes), were first developed in the laboratory of Richard Friend at Cambridge University in the early 90’s.


The image to the right is a diagram illustrating a simple Organic Hetero-junction. An organic material is made of carbon molecules – the fundamental building blocks of an OLED. ALQ3 is known as tris(8-hydroxyquinoline aluminum), and NPB is known as (Bis[N-(1-naphthyl)-N-phenyl]Benzidine).
OLED is similar to LED in that these are both semiconductor light sources. When this device is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. 

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.


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.
(Image Courtesy: OLED Info - LG Chem)
Flexible OLEDs 
The total thickness of the active layers of an OLED, from the anode to the cathode, is less than 1 micron – that’s 1/100 the thickness of a human hair. When a flexible substrate such as a plastic foil is used to support the device stack, the entire device can be made light weight and flexible. OLEDs on both plastic and thin stainless steel (with top-emission OLEDs) have been demonstrated. In the future, we may very well see rolled-up “light curtains” or lighting panels that are curved and bendable. 
Conductivity Doping
Doping as a means to increase conductivity is a well established practice for inorganic semiconductors such as silicon and nitrides. In the early 2000’s, researchers in the laboratory of Karl Leo at Technical University Dresden started applying conductivity doping (not to be confused with emitter doping) to OLEDs. Since then, both n and p type dopants have been developed. Conductivity doping is instrumental in reducing the operating voltage of OLEDs which increases their efficacy (lumens per watt).

Tandem Devices 
In 2003, Junji Kido and co-workers from Yamagata University, Japan started making OLEDs with multiple devices stacked on top of each other without any internal electrical contacts. The main advantage of such an arrangement is to increase the intensity of emission per unit area which significantly prolongs device lifetime. This approach was termed “multi photon emission” by Kido et al. The term “tandem devices” generally refers to a stack of two units on top of each other. OLED panel luminance is expected to be in the range of 2,000-5,000 cd/m2 for direct view sources. Since there is no upper limit on the number of units in the stack, a large number of stacked units could easily achieve luminance of 15,000-20,000 cd/m2 or more, which in conjunction with external intensity shaping optics could put certain outdoor applications within OLEDs’ reach.
OLED Products
Up to the end of the first decade of the 21st century, the main impetus behind the development of OLEDs had been its application in displays, especially for mobile devices like cellphones and mp3 players, where battery life is of paramount importance. Today, OLED display is increasing, being seen as a product differentiator in smart phones and tablets.