Thread: Current Technologies

Mobile electronics is presently dominated by LCDs. The bulky cathode ray tubes of TV sets and PCs too are gradually being replaced with LCDs. Organic light-emitting diodes (OLEDs) another competitor of LCDs. A brief overview of these two devices is given below.

LCDs.shows the cross-section of an LCD. LCDs consist of two polarised glass sheets with a liquid crystal solution sandwiched between them. The liquid crystals are very specific in that these act as shutters that open or shut out entry of light as the need arises depending upon the flow of electric current. The current passing through the liquid crystals is regulated by a voltage applied 'between the glass sheets through the transparent indium-tin oxide (ITO) electrodes that form a grid - having rows on one side of the panel and columns on the other - representing the pixels or picture elements.

But what exactly are these liquid crystals? Matter can be found in three states - solid, liquid and gas. Nevertheless, there are substances that exist in a state that is neither liquid nor solid at any point of time. The behaviour of their molecules is also peculiar. They tend to maintain their orientation, like the molecules in a solid, yet shuffle around taking up various positions, like the molecules in a liquid. Thus these liquid crystals can be termed neither solid nor liquid, though these are closer to a liquid state.

There are various types of liquid crystals. Those used in LCD panels are termed as nematic-phase liquid crystals. shows the pixel electronic circuit for the line-at-a-time scanning imaging method of conventional LCDs. The circuit consists of one thin-film transistor (TFT), which is connected in parallel with a storage capacitor (CST), and an electrode which applies the voltage to the liquid crystal layer (LC).




To draw a line of image on a row of pixels in an LCD panel, the data driver sends the pixel data through data lines (D) on the same row and the gate driver triggers the TFTs by sending a signal through a gate line (G). Then data is transferred to the storage capacitors through TFTs and the stored data in the storage capacitors provides a voltage to the liquid crystal layer during one frame period. In the figure, the circuit within the dotted box is pixel electronic circuit and the square block on the right side is.an equivalent circuit for the pixel electronic circuit.

OLEDs. Organic light-emitting diodes (OLED) have attracted a great deal of attention as a light source for display and lighting applications. As most of the organic,materlals are poor conductor (with tow carrier mobility of 10-3 to 10-7 cm2/Vs), the organic layer structure is relatively simple.

OLEDs are composed of hole and electron transport layers sandwiched between two metallic electrodes. Researchers are trying to enhance the device efficiency and stability by embedding an additional organic layer between the hole and electron transport layers which acts as either an emitting layer or a carriertransport controlling layer. OLED uses the idea of maximising the recombination of electrons and holes by forcing recombination to occur at the interface between two separate types of organic semiconductor layers, i.e., hole transport layer and electron transport layer. The organic semiconductor layers are deposited as thin amorphous films by means of vacuum evaporation.