Usually, there are two types of display devices, projection and direct- facing types. There are several different types of projections. There are CRT, DMD, LCD and laser (DMD refers to name of the chip developed by TL). And there are two different types in direct-facing, CRT and flat panel. Types we discuss are OLED which belongs to emitter type.
Condition called EL was already discovered during 1920's in weapons. Later on, in early 1953, light emission of OLED was observed. After confirming light emission in direct currents, full scale research on OLED has begun. In 1987, Eastman Kodak, an American company, has acquired numerous basic patents by developing low voltage running, highly bright OLED pixels. Afterwards, due to characteristics like high brightness, direct current low voltage, fast response rate, world research focus has shifted to OLED. Pioneer was the first company to succeed in developing OLED, and they used Northeast Pioneer company to do mass production of OLED display for FM text broadcasting receiver for cars.
1. Self- light emission
Major difference from LCD is self- light emission. It means it gives off light by itself, and even in dark places and incoming lights, it has good visibility.
2. Wide viewing angle
View angle means an extent which screen can be viewed, and unlike LCD, when viewed sideways, picture quality doesn’t change.
3. Fast response rate
When playing time-varying image, low and high response rate determines quality of the picture. OLED can play better quality time-varying images than LCD (1,000 times faster response rate than LCD).
4. Simple and inexpensive manufacturing process
OLED: about 55 steps LCD: about 62 steps
5. Ultra thin, and uses very little electricity
Because it doesn’t require backlights, it uses 50% less electricity than LCD, and can reduce thickness as much as 1/3 comparing to LCD.
SECTIONS OLED LCD PDP FED CRT
Response speed X O O O
Viewing angle X
backlight Not required required Not required Not required Not required
Electricity consumption O X X X
thickness O O O X
weight O O O X
Reduction in price O X X X
OLED has many advantages as mentioned above, and it can have many applications in various areas. It is emerging as the most competitive medium for small display usage in mobile phone, digital camera, camcorder, PDA, CNS( Car Navigation System) and audio, and some companies are underling OLED display screen as their main sales pitch in its mobile phones sales, and now OLED became more practical and becoming part of our daily life.
Meanwhile, according to Korea Electronics Technology Institute, due to changes and improvements, around year 2007, it is predicting that it will be used in notebook computer displays and in 2009; it will progress to wall hanging TV, and 2012, it will progress to scroll TV. All these predictions are possible because of OLED’s advantage which is high brightness and ultra-thinness.
1. Theory of OLED
According to most definitions given in scientific journals, OLED is defined as a self-emitting display using the red, green, blue organic chemical compounds on a glass surface. According to very first thesis concerning OLED, when voltage is applied to organic crystals, it noticed illumination. If we express this phenomenon in systematic fashion, it can be said that OLED emit light in the potential wall between electrical charges and organic materials.

Basically, OLED consist of positive and negative charges, organic compounds and the surface. When voltage is applied to OLED cell, the injected positive and negative charges recombine in the organic compound layer and when this happens it creates lights as an energy gap. Conversion of energy into light can be explained using law of energy conservation learned in chemistry class. It is strictly natural phenomenon it gives off what it takes in.
Depending on materials, organic compound layer is divided into two parts low and high molecule type, and generally, thickness is about 100 nm. If we look closely, organic compound layer consist of hole related layer, electron related layer and emissive layer. Hole related layer are divided into hole- injection layer and hole-transport layer and also, electron related layer are divided into injection and transport layer as well. When voltage is applied, At positive charge, hole is injected and transported. At the negative charge, electron is injected and transported, and in the emissive layer, interactions with photons create positive and negative polaron separately. When these polaron recombine, it creates Exciton.

This Exiton spreads and create lights and becomes a ground state. Created lights are discharged to ITO and glass surface. Theory and efficiencies relating to Exciton is complicated matter requiring further scientific discussions and because of it we would like to skip discussions on Excitons.
2. OLED display method: Passive/ Active
As we have discussed above, in order to put OLED theory into actual use, we need matrix methods, and there are two types passive and active matrix.
Passive matrix is placing pixels at the crossing point of positive and negative anodes. Response time for OLED pixel is fast. Because it doesn’t store information like voltage and electrical currents, when running pulse is removed, it is off right away, so in order to maintain display brightness for everyday use, light brightness must be very substantial in the “on” state. Passive matrix has characteristic of simultaneous display, and under these conditions, as panel resolution and size increases, electricity consumption is significantly increased.
Therefore, it is mainly used for displays less than five inches in size, and various researches are attempted to reduce electricity consumption by increasing efficiency of pixels and reducing running voltage, finding ways to reduce resistance in wiring and by developing low electricity consuming IC. Active matrix uses more than one transistor for every OLED pixel, and every pixel controls on/off itself, and uses storage capacity to store information therefore it uses less electricity compare to passive matrix. Also, compare to passive matrix, pixel forming process is simpler, and has advantage of producing more high resolution panels.
Transistors used in this process, generally uses mono or polysilicon TFT, and in order to produce large size, but at the same time cost effective, it mainly uses LTPS TFT (low temperature polysilicon TFT. OLED pixels have advantages in electrical current running methods, so it has different structures compare to LCD which utilizes voltage running methods. Running on electrical currents, requires more than two transistors and in order to compensate for problem of unevenness in LTPS TFT, more than three TFT are generally used. But in doing so, TFT and other accessories take up larger spaces, and it reduces area for light emission.
In order to compensate for this problem, increases in electrical currents are inevitable as a result. Therefore, some manufacturers like Sony has adapted Top Emission method which allows discharge of lights in the opposite side of glass surface to compensate for reduction in product life. In this case, transparent materials must be used as negative anode, and because transparent anode has difficulties in injection of electrons, researches to find solutions are urgently needed.
3. Meaning of OLED pixels and display methods
Now, we want to examine the most important matters pertaining to OLED production.

Basic patents pertaining OLED technology is owned by few companies such as Eastman Kodak, UDC in America and CDT of Great Britain, and recently, cooperating relationships are actively formed in the way of establishing joint companies and sharing patent technologies. Most important part of these cooperating relationships is deciding on OLED materials and display methods. Only then, it can decide on other manufacturing processes like Polysilicon TFT, voltage, and display circuit of electrical currents etc.

No technology has been standardized yet, and we have produced a chart explaining various competing technologies based on information given by the ministry of industry and resources.
※ The comparison to the OLED materials and the drive mechanism
Sections Advantages Disadvantages Company possessing original technology
Low molecule Early mass production
possible
making large screen
difficult
Kodak
High molecule High color possible Reliability of material is
insufficient
CDT
Passive Low price Simple
manufacturing process
Electricity consumption
high Complicated
manufacturing process
Pioneer
Active Large screen can be
made Electricity
consumption low
High price Sanyo
Sections: divided into light emitting materials (high, low molecule) and method in signal expression
              on the screen (passive, active)