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) |
