1	Introduction to Optoelectronics Displays (2) Katsuaki Sato
2	Flat panel displays Liquid crystal display (LCD) Plasma display panel (PDP) Field emission display (FED, SED) Electroluminescence display (ELD) Organic light emitting diode display (OLED)
3	Liquid Crystal Displays What is “liquid crystal”? How can liquid crystals control light? Structure of LCD panel.
4	What is liquid crystal? ●LC is an intermediate substance between liquid and solid Application of LC to displays is first proposed by Williams (RCA Corp) in 1963年, who found electrical change of transparency in LC. Heilmeyer et al. (RCA Corp) invented a display device using LC. At that moment LC is unstable and not suited consumer use. In 1973, Sharp Corp first applied LCD to an electronic calculator (EL-805) In 1976, Prof. Gray（Hull Univ.） discovered stable liquid crystal.
5	History In 1888, an Austrian botanist named Friedrich Reinitzer, interested in the biological function of cholesterol in plants, was looking at the melting behavior of an organic substance related to cholesterol. In 1922 in Paris, France, Georges Freidel suggested the classification scheme which is used today with different phases of liquid crystals called nematic, smectic and cholesteric.
6	Liquid crystal display (LCD) LC works as optical switch Transmission of polarized light through LC Orientation of LC director by electric field Selection of pixels by ＴＦＴ(thin film transistor)：a-Si:H to polycrystalline Si Merit:thin, low power, high definition, no flicker Demerit: Field of view, Backlight, Difficulty in larger size
7	Anchoring and alignment of LC molecules Alignment of LC molecules Coat with PVA and apply “rubbing” to align LC molecules to one direction Control of orientation direction by applied electric filed LC molecule is an electric dipole
8	Structure of LCD Color LCD takes a layered structure 1－Polarizer controls incident light polarization 2－Glass substrate prevents leakage of current to undesired portion 3－Transparent electrode applies necessary electric field for switching molecular direction 4－Alignment layer is necessary to align molecules along a certain direction 5－As LC, twisted nematic liquid crystal is used 6－Spacer is necessary to give an appropriate spacing between glass plates
9	Active matrix X electrodes switch (on/off) active elements associated with each pixel An active elements of “on” state keeps the voltage to connect Y electrode with pixel LC Application of voltage on Y-electrode can switch the LC to light
10	TFT (thin film transistor) Color and brightness of each pixel (picture element) is controlled by applied voltage using thin film transistor attached to the pixel. Conventionally amorphous silicon has been used as TFT material. Recently polycrystalline silicon with higher switching speed becomes available.
11	Plasma display panel Gas discharge®UV®Excite RGB phosphors (Photoluminescence): the same mechanism as fluorescent lamps Continuity of discharge ®memory property Merit: Larger size(32 to 60”), Thin(1/10 of CRT), light in weight (1/6 of CRT), wide field of view (>160°), good color reproduction (self emission) Demerit: High cost, large power consumption, Complex circuits necessary for high voltage operation
12	Structure of PDP PDP is an ensemble of small fluorescent lamps of RGB in color Each discharge cell corresponds to a dot. By increasing the number of cell larger size display is obtained The plasma produced by the discharge emits UV light which excite phosphors to emit visible light of R,G or B.
13	Electroluminescence panel Applying high electric field to semiconductors doped with luminescent centers electrons are injected as “hot electrons” which cause an impact ionization or impact excitation of the emission center. Long history in ZnS:Cu polycrystalline ELP Double insulating layer ZnS:Mn EL (Sharp Corp) was developed 30 years ago Problems in blue emission Advantage: Low power consumption, thin, without vacuum Disadvantage: High voltage (200V)
14	Structure of Inorganic EL Electrons are emitted from insulator/ZnS:Mn interface They move as “hot electrons” by accelerating voltage Hot electrons collide with emission centers Electron system of the center is excited Radiative emission at the center Long lifetime
15	Luminescence in ZnS:Mn (Crystal field transition) ZnS:Mn emission is produced by electronic transition in 3d⁵ many electron system of Mn²⁺ion (From ⁴T₁ to ⁶A₁states) Such transitions are called crystal field transition The transition cannot be explained in the framework of one electron band picture
16	Inorganic EL display of 35 inch in size DNP-Sanyo,iFire jointly developed 35” inorganic EL TDK developed very bright EL display with 200 Cd/m2 with lifetime more than 30000 hours
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20	Organic LED Electrons and holes are injected to active layer of OLED Electron transport and hole transport layers are combined Problems in lifetime
21	Organic LED materials
22	Electronic structure of OLED Carriers move by hopping and recombine each other Organic layer with several tens and hundreds nm in thickness Hole injection efficiency is improved using buffer layers Electron injection efficiency is improved by an addition of Ca to electrode
23	Hetero structure oganic LED Organic layer consists of three layers, the energy diagram under application of electric field is shown in Figure. MEH-PPV which connects positive hole-injecting anode and PPV layers called HTL (hole transporting layer), while CN-PPV which connects electron-injecting cathode and PPV is called ETL (electron transporting layer). Light emission occurs in the PPV layer in which carriers are confined and recombination occurs is called EML (emission layer).
24	Field emission displays(FED) Small CRT is aligned Cathodoluminescence
25	FED Structure Vacuum chamber is formed between two glass sheets, of which cathode emits electrons by field emission. Electrons are generated by potential difference between cathode and gate electrodes Electrons emitted into vacuum travels towards anode direction, followed by collision with phosphors to emit visible light. The visible light emitted from a set of three phosphors (RGB) forms one pixel on the display. Conventional electron emitter in FED was a microtip. A few thousand microtips are necessary for one pixel. ®Carbon nanotubes are favorable for this purpose.
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