LED and OLED display

Rahul N16-04-16

A complex optical device that renders an image, graphics andtext by electrically addressing small switching elements (pixels)

Serves as an interface between human being and machine

? What is display

display that modulates backlight (light shutter)

BacklightEmissive

Reflectivedisplay that generates itsown light

display that rejects/reflectsambient light

STN, FLCD

CRTs, FEDs, LEDs, plasmaEL, VPD cholesteric LC,

STNs, MEMs, FLCDs

Super-twisted nematic(STN); ferroelectric liquid crystal display(FLCD); cathode-ray-tube(CRT); lightemitting diode(LED); vacuum fluorescent display(VPD); field emitter display(FED); electroluminescent(EL); micro-electro-mechanical(MEM); liquid crystal(LC)

LED display

? An LED display is a flat panel display, which uses an array of light-emitting diodes as pixels for a video display.

An LED display panel can be either a small display or part of a larger display.

LED diodes are used in order to make up an LED display.

An LED panel consists of several LEDs, whereas an LED display consists of several LED panels.

The first true all-LED flat panel television screen was possibly developed, demonstrated and documented by James P. Mitchell in 1977.

LED ?

“LED = a tiny little bulb Small, extremely bright and uses little power.”

Do not have a filament but are illuminated by themovement of electrons in a semiconductor material. No filament to burn out, so they last very long. Do not get hot as light bulbs do. Efficient in terms of electricity (none is wasted on heat)

Diode ? A diode is a section of N-type material bonded to asection of P-type materialElectrons from the N-type material fill holes fromthe P-type material along the junction betweenthe layers, forming a depletion zone.When the negative end of the circuit is hooked up to N-typelayer and the positive end is hooked up to P-type layer,electrons and holes move and the depletion zone disappears.Free electrons moving across a diode fall into holes in the P-typelayer. This involves a drop from the conduction band to a lowerorbital, so the electrons release energy in the form of photons.

The wider the energy gap – the higher the spectral frequency of the emitted photon. (silicon has very small gap so very low frequency radiation is emitted – e.g. infra red).

Diodes in LEDs are housed in a plastic bulb that concentrates the light in a particular direction. Most of the light from the diode bounces off the sides of the bulb, traveling on through the rounded end.

A LED pixel module is made up of 4+ LEDs of RGB.LED displays are made up of many such modules.• Several wires run to each LED module, so there area lot of wires running behind the screen.• Turning on a jumbo screen can use a lot of power.

Diode ?

LED displays

Organic Led Displays (OLED)An electronic device made by placing organic thin films between two conductors (Anode & Cathode). When electrical current is applied, a bright light is emitted.

OLED/LED: • Both OLED and LED use the same

principle of electroluminescence- the optical and electrical phenomenon where certain materials emit light in response to an electric current passing through it.

• OLED– Lighter weight– Perform at lower efficiencies– Less power consumption– Organic based chemicals

How It Works• Composed of two substrate

layers on the outside• Emissive and conductive layer

lie between the cathode and the anode layers

• A current is applied across the LED, where electrons move from cathode to anode

• The cathode gives electrons to the emissive layer, where the anode withdraws these electrons from the conductive layer

• The emissive layer becomes rich in negative charge while the conductive layer becomes more positively charged

• The two charges recombine in the emissive layer, creating a drop in energy levels of the electrons

• The drop in energy levels results in radiation that is on the visible spectrum, emitting light

Types of OLEDs

• Passive-matrix

• Active-matrix

• Transparent

• Top-emitting

• Flexible

• White

PMOLED: Passive Matrix OLED

PMOLED stands for Passive-Matrix OLED, which relates to the way you control (or drive) the display. A PMOLED display uses a simple control scheme in which you control each row (or line) in the display sequentially (one at a time). PMOLED electronics do not contain a storage capacitor and so the pixels in each line are actually off most of the time. To compensate for this you need to use more voltage to make them brighter. If you have 10 lines, for example, you have to make the one line that is on 10 times as bright (the real number is less then 10, but that's the general idea).

AMOLED: Active Matrix OLED

An AMOLED uses a TFT that contains a storage capacitor which maintains the line pixels lit all the time (even though just one line is changed each time). AMOLEDs consume less power than PMOLEDs, have faster refresh rates and allows to build larger display with higher resolutions. AMOLEDs are also more complicated and expensive to fabricate.AMOLEDs today are being used as displays for smartphones, digital cameras - and even OLED TVs.

Transparent OLED

Transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate. When a transparent OLED display is turned on, it allows light to pass in both directions. A transparent OLED display can be either active- or passive-matrix.

Top-emitting OLED

Top-emitting OLEDs have a substrate that is either opaque or reflective. They are best suited to active-matrix design. Manufacturers may use top-emitting OLED displays in smart cards.

Flexible/Foldable OLED

Flexible OLEDs have substrates made of very flexible metallic foils or plastics. Flexible OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable OLED displays can be attached to fabrics to create "smart" clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.

White OLEDs emit white light that is brighter, more uniform and more energy efficient than that emitted by fluorescent lights. White OLEDs also have the true-color qualities of incandescent lighting. Because OLEDs can be made in large sheets, they can replace fluorescent lights that are currently used in homes and buildings. Their use could potentially reduce energy costs for lighting.

White OLED

OLED advantages/disadvantages• Advantages

– Brighter, clearer picture

– More efficient viewing angle

– Thinner, lighter in weight

– Low powered– Can be printed

on various surfaces

• Disadvantages

– Short term battery life– Expense– Doesn’t like water

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