I. OLED device structure and operation
OLED is a multi-layered device. Emissive layer and conductive layer are the most important layers among them. Both layers contain HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) band. They are analogous to inorganic semiconductors' valence and conduction band. Emissive layer is connected to cathode and conductive layer is connected to transparent anode. The transparent anode is made up of Indium Tin Oxide (ITO). Electrons are pumped from cathode to emissive layer's LUMO region. Likewise holes are pumped into conductive layer's HOMO region by anode. It reaches emissive layer's HOMO region. As the electron find hole in the emissive layer, it plunges into the hole leaving the excess energy as photon (light particle) [1]. This phenomenon is called electroluminance. Photon colour depends upon the polymer used in the emissive layer. For example, PPV (polyphenylene vinylene) emits yellow-green light at 510 nm and Poly florene emits blue color. Above the cathode opaque substrate is placed. Below the transparent anode transparent glass is placed. Through the glass generated light passes and reach the viewers. Few more layers are added to improve the efficiency of the OLED. Discussing them is beyond the scope of this article. Human hair is 200 times bigger than OLED structure. So, one can imagine the size of OLED.
OLED is a multi-layered device. Emissive layer and conductive layer are the most important layers among them. Both layers contain HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) band. They are analogous to inorganic semiconductors' valence and conduction band. Emissive layer is connected to cathode and conductive layer is connected to transparent anode. The transparent anode is made up of Indium Tin Oxide (ITO). Electrons are pumped from cathode to emissive layer's LUMO region. Likewise holes are pumped into conductive layer's HOMO region by anode. It reaches emissive layer's HOMO region. As the electron find hole in the emissive layer, it plunges into the hole leaving the excess energy as photon (light particle) [1]. This phenomenon is called electroluminance. Photon colour depends upon the polymer used in the emissive layer. For example, PPV (polyphenylene vinylene) emits yellow-green light at 510 nm and Poly florene emits blue color. Above the cathode opaque substrate is placed. Below the transparent anode transparent glass is placed. Through the glass generated light passes and reach the viewers. Few more layers are added to improve the efficiency of the OLED. Discussing them is beyond the scope of this article. Human hair is 200 times bigger than OLED structure. So, one can imagine the size of OLED.
II. OLED Types
OLED can be broadly classified into two categories. One is AMOLED (Active Matrix OLED) and another is PMOLED (Passive Matrix OLED). If faster refresh rates are required as in Television or Smart phone AMOLED is used. Display devices like 16 character display and low end cell phone displays go for PMOLED. As usual they are cheaper to fabricate but their life is limited. PMOLED has stripes of anode and cathode that criss-cross to form a matrix as in Fig 1.(a). In the AMOLED common cathode is used as in Fig. 1. (b). Anode is connected to TFT (Thin Film Transistor) layer. This TFT contains array of Transistors and capacitors. Transistor acts as on-and-off switch and capacitor stores the energy to supply the anode. Refreshing circuitry is used to refresh TFT in regular intervals. Thus each pixel in AMOLED is quickly and accurately energized. Fabrication of TFT into OLED structure is complex but it provides good quality display. Fig 1. (b) is not actual structure. It is author's rendition after studying text related to AMOLED.
It is possible to have transparent cathode instead of transparent anode. Having transparent cathode helps to overcome the restriction posed by TFT structure on the transparent anode. Thus more light is emitted and improvement in pixel brightness and higher resolution is achieved. This technique is called top emission adaptive current drive technology [1].
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| Fig 1. (a) Structure of PMOLED (b) Structure of AMOLED (author's rendition) |
III. OLED Manufacturing
OLED can be manufactured by three ways. Dr. Ching W. Tang et al. developed efficient OLED using Alq3 molecules which belong to organometallic chelates. As they have less molecules, OLED developed by this method is called Small Molecule (SM) OLED devices. Production of SM-OLED involves thermal evaporation in a vacuum. This makes the production process expensive. At the same time it offers high control over formation of multi-layer structures. In the second method long chains of polymers are used to make OLED devices. They are manufactured using Organic Vapor Phase Deposition (OVPD) technique. In the third method Light Emitting Polymers (LEP) devices are made by dissolving the polymers in solutions and devices are fabricated using Inkjet printing. Thus last two techniques require lower capital cost and offers making of large scale structures.
Manufacturing large displays are still a nightmare. The manufacturing process has not matured enough to produce maximum yields. Thus cost of large size OLED TV is beyond the reach of common man.
IV. OLED Merits
- As seen already size of OLED is very small. This enables to manufacture OLED TV with 4 mm thickness.
- In LCD display system external light sources are filtered by Liquid Crystals and polarizers to produce an image. The light source may be fluorescent lamp or LED. If LED source is used then people call it LED TV. It is a misnomer. It is a LCD TV only. Unlike LCD, OLED is an light source by itself.
- Power consumption is very low.
- Ratio between the brightest pixel to darkest pixel is called contrast ratio. OLED is light source by itself. So, high brightness is guaranteed. Next, individual pixel can be switched off to produce black pixel. Thus Contrast ratio is high for OLED. As Liquid Crystals cannot block 100% of light, producing a black pixel is not possible. Next light passes through Liquid Crystals shed some brightness. Thus LCD contrast ratio is low.
- Viewing angle is 170 degrees. Thus most suited for Television.
- Transparent OLED displays are possible. So, when the TV is switched off just a glass is hanging on the wall. By comparison an 80 inch LCD TV hanging in the centre of living room (offers a black screen at switched off condition) grabs unnecessary attention and spoils the aesthetics of the room.
- OLED offers a scope for flexible display devices. Thing of a situation where a 80" screen can be rolled up when not required. Looks like science fiction. But, it is possible to do.
- OLED opens a possibility to have flexible or Roll-to-Roll manufacturing due to the presence of organic polymers.
V. OLED demerits
- OLED devices are very sensitive to contamination. If oxygen or water molecules creep into OLED device then life span comes down drastically.
- The blue OLED degrades faster than red and green OLED. Thus size of the blue light is doubled compared to red and green so as to increase life span display system.
- The transparent anode is made up Indium Tin Oxide. Indium is a rare earth material. So, availability is very limited.
Reference
[1] Overview of OLED Display Technology | http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf (File Size, 900 KB)
[1] Overview of OLED Display Technology | http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf (File Size, 900 KB)
Note
A good article should cite sufficient references which has a good academic standing. I got bit and pieces of information from the Internet and prepared an article. In Internet articles on OLED, is either very generic or highly scholarly. So, I am not in a position to give proper citation. This article will provide “quick overview” about OLED and suited for beginners or dummies. Content is not accurate.
