Organic light emitting diodes, or OLEDs, are a type of material capable of generating light when an electric current is passed through it. Having only reached the mainstream consumer market within the past 5 years, OLEDs currently have very limited commercial use. Their most common use is currently as display screens for modern handheld electronics, such as cell phones and digital cameras. They are also starting to be used as television screens, with OLED TVs currently entering the market. Despite these seemingly limited uses, OLEDs are a new, young technology that has great potential to revolutionize modern life. While OLEDs have some great advantages over other display technologies, such as being lightweight and thin, their greatest promise lies in their flexibility. Unlike LCD or LED displays which are rigid and stiff, OLEDs can be produced to be extremely flexible, to the point where one could wrap it completely around a cylindrical object without damaging the display. The other exciting aspect of OLEDs is the fact that they can be made transparent. These factors allow OLEDs to potentially be used in hundreds of novel uses. Imagine being able to change the wallpaper on your house daily with the click of a button, or being able to carry around an entire television in a backpack. With OLEDs, these scenarios may actually be possible.
The OLED contains three basic layers: the cathode, anode and organic layer. In early OLEDs, the organic layer consisted of an emissive layer and a conductive layer. When a current is passed through the material, electrons are generated at the cathode, and “holes” are generated at the anode. The electrons and holes travel into the emissive layer and conductive layer, respectively, and combine at the interface of the two layers. This combination moves an electron from the lowest unoccupied molecular orbital (LUMO, analogous to the conduction band in LEDs) of the emissive layer to the highest unoccupied molecular orbital (HOMO, analogous to the valence band in LEDs) of the conduction layer. This transfer results in the release of energy in the form of light. However, more modern OLEDs employ an organic layer containing a hole transport layer (HTL), an electron transport layer (ETL), and an emissive layer. In this setup, the HTL and ETL transport electrons and holes to the emissive layer. Once localized on the emissive layer, the holes and electrons eventually combine and release light. Using three organic layers instead of two has been shown to increase OLED efficiency.
OLEDs are currently divided up into two classes: small molecule OLEDs (SMOLEDs), which use small organic molecules in their organic layer and polymer OLEDs (POLEDs), which use organic polymers. SMOLEDs as a class can be further broken down into two sub-classes based on if the light is generated using fluorescence or phosphorescence.
In SMOLEDs, the organic layers are comprised of a diverse range of different molecules. Materials commonly used in the HTL include TPD (N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine) and NPB (1,4-bis(1-naphthylphenylamino)biphenyl), while those commonly used in ETL include PBD (2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole), Alq3 (tris(8-hydroxyquinoline) aluminium), and TPBI (1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene). The type of emissive layer used determines whether the SMOLED is fluorescent or phosphorescent. Commonly used fluorescent materials include DPVBi (4, 4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl) for blue colored light, Alq3 doped with DMQA (N, N′-dimethylquinacridone) for green colored light, Alq3 doped with DCJTB (4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) for red colored light, and SAlq (bis(2-methyl-8-quinolato)-(triphenylsiloxy)aluminium(III)) doped with DPVBi for white colored light. Phosphorescent compounds are usually comprised of PVK (poly(n-vinylcarbazole)) doped with various organometallic iridium complexes. Several examples include (DFPPM)2Ir(CN)(PPh3) for blue light, (PPM)2Ir(acac) for green light, (DPQ)2Ir(acac) for red light, and (MPPZ)2Ir(acac) for yellow light.. The anode is usually ITO (indium tin oxide), a material whose transparency allows for generated light to leave the OLED while the cathode is usually made of barium or calcium.
The structure of POLEDs is very similar to SMOLEDs. The major difference is that the HTL and emissive layer are made from organic polymer materials instead of small molecules. The HTL is usually comprised of either PEDT:PSS (polyethylene dioxythiophene polystyrene sulphonate) or PEDOT (poly-(3,4-ethylenedioxythiophene)). POLED emissive layers is an area currently undergoing research. Polymers currently being studied include derivatives of PF (poly(fluorine)), PPV (poly(p-phenylenevinylene)), PT (poly(thiophene)), and PPP (poly(p-phenylene)).
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