Figure 16: Basic structure of OLED
The latest innovative organic light emitting diodes (OLED) operates on the principle of electro-luminescence, converting electrical energy into light. Organic Light Emitting Diode technology, can produces full color, full-motion flat panel displays with a level of brightness and sharpness not possible with the use of other technologies. OLED is traditionally pioneered and patented by Kodak/Sanyo.
Unlike traditional LCDs, OLEDs are self-luminescent - they glow when an electrical field is applied to them - they don’t require any backlighting or reflective light source, diffusers, polarizers, or any of the other baggage that goes with liquid crystal displays. These allow them to be thinner, lighter, and more efficient than LCDs. The various formats include Flexible OLEDs (FOLED), Stacked, High-Resolution OLEDs (SOLED), and even Transparent OLEDs (TOLED).
Essentially, the OLED consists of two charged electrodes sandwiched on top of some organic light emitting material. This eliminates the need for bulky and environmentally undesirable mercury lamps and yields a thinner, more versatile and more compact display. Their low power consumption provides for maximum efficiency and helps minimize heat and electric interference in electronic devices. Armed with this combination of features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space.
Figure 17: Structure of OLED
The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers comprise a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. When an appropriate voltage (typically between 2 and 10 volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electro luminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device.
There are two forms of OLED displays: Passive-matrix and Active-matrix.
Figure 18: Structure of passive-matrix OLED display
The passive-matrix OLED display has a simple structure and is well suited for low-cost and low-information content applications such as alphanumeric displays. It is formed by providing an array of OLED pixels connected by intersecting anode and cathode conductors.
Organic materials and cathode metal are deposited into a “rib” structure (base and pillar), in which the rib structure automatically produces an OLED display panel with the desired electrical isolation for the cathode lines. A major advantage of this method is that all patterning steps are conventional, so the entire panel fabrication process can easily be adapted to large-area, high-throughput manufacturing.
To get a passive-matrix OLED to work, electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An external controller circuit provides the necessary input power, video data signal and multiplex switches. Data signal is generally supplied to the column lines and synchronized to the scanning of the row lines. When a particular row is selected, the column and row data lines determine which pixels are lit. A video output is thus displayed on the panel by scanning through all the rows successively in a frame time, which is typically 1/60 of a second.
In contrast to the passive-matrix OLED display, active-matrix OLED has an integrated electronic back plane as its substrate and lends itself to high-resolution, high-information content applications including videos and graphics. This form of display is made possible by the development of polysilicon technology, which, because of its high carrier mobility, provides thin-film-transistors (TFT) with high current carrying capability and high switching speed.
In an active-matrix OLED display, each individual pixel can be addressed independently via the associated TFTs and capacitors in the electronic back plane. That is, each pixel element can be selected to stay “on” during the entire frame time, or duration of the video. Since OLED is an emissive device, the display aperture factor is not critical, unlike LCD displays where light must pass through aperture.
Therefore, there are no intrinsic limitations to the pixel count, resolution, or size of an active-matrix OLED display, leaving the possibilities for commercial use open to our imaginations. Also, because of the TFTs in the active-matrix design, a defective pixel produces only a dark effect, which is considered to be much less objectionable than a bright point defect, like found in LCDs.
Table 3: Life span of each colour.
The OLED technology faces a bright future in the display market, as the ever-changing market environment appears to be a global race to achieve new success. However, the OLED forms of display still have many obstacles to overcome before its popularity. Still the most crucial factor which hinders the rate of progress is its reliability, whether it will be up to par with the standards expected by picky consumers. Although the technology presents itself as a major player in the field of displays, overcoming these obstacles will prove to be a difficult task.
Eventually, the technology could be used to
make screens large enough for laptop and desktop computers. Because
production is more akin to chemical processing than semiconductor
manufacturing, OLED materials could someday be applied to plastic and other
materials to create wall-size video panels, roll-up screens for laptops, and
even head wearable displays.
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