AMOLED(Active-Matrix Organic Light Emitting Diode)

Submitted By,Pratham Kumar

(0702931075) EC(4th year)

Introduction

What is AMOLED? Active-matrix OLED (Active-matrix organic light-emitting diode or AMOLED) is a display technology for use in mobile devices and televisions. OLED describes a specific type of thin display technology which doesn't require a backlight, and Active-Matrix refers to the technology behind the addressing of pixels. AMOLED technology continues to make progress towards low-power and low-cost large size (e.g. 40-inch) for applications such as TV.

What is OLED? An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds that emits light when an electric current passes through it. This layer of organic semiconductor material is formed between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and PDAs, watches, advertising, information and indication; they can also be used in light sources for general space illumination and in large-area light-emitting elements. OLED displays can use either passive-matrix or active-matrix addressing schemes.

Difference between AMOLED & PMOLED There are two types of OLEDs used in displays - PMOLED and AMOLED. The difference is in the driving electronics - it can be either Passive Matrix (PM) or Active Matrix (AM).

With Passive-Matrix OLEDs, the display is controlled by switching on rows and columns. When you turn on row number x and column number y, the pixel at the intersection is lit - and emits light. Each time you can choose just one pixel to light. So you have to turn these on and off very quickly. You do so in a certain sequence, and create the desired image.

PMOLEDs are very easy and cheap to build, but they are limited to small sizes. The image displaying is a bit complicated. Also the power consumption is not as good as AMOLEDs.

AMOLEDs have a different driver electronics - each pixel is controlled directly. AMOLEDs are more expensive, and much more difficult to

create, but can be used for larger displays (current prototypes are up to 40") and are very power efficient.

The first OLED products in the market used PMOLEDs - these were MP3 players, sub-displays on cellphones and radio decks for automobiles. The displays were small and usually with just one or two colors. When AMOLED panels started to emerge in 2007 and 2008 we have seen these larger displays in mobile video players, digital cameras, mobile phones main displays and even OLED TVs.

OLED Components

Like an LED, an OLED is a solid-state semiconductor device that is 100 to 500 nanometers thick or about 200 times smaller than a human hair. OLEDs can have either two layers or three layers of organic material; in the latter design, the third layer helps transport electrons from the cathode to the emissive layer. In this article, we'll be focusing on the two-layer design.

An OLED consists of the following parts:

Substrate (clear plastic, glass, foil) - The substrate supports the OLED.

Anode (transparent) - The anode removes electrons when a current flows through the device.

Organic layers - These layers are made of organic molecules or polymers.

Conducting layer - This layer is made of organic plastic molecules that

transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline.

Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. One polymer used in the emissive layer is polyfluorene.

Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device.

How OLED works? OLEDs emit light in a similar manner to LEDs, through a process called electro-phosphorescence.The process is as follows:

1. The battery or power supply of the device containing the OLED applies a voltage across the OLED.

2. An electrical current flows from the cathode to the anode through the organic layers (an electrical current is a flow of electrons).

The cathode gives electrons to the emissive layer of organic molecules.

The anode removes electrons from the conductive layer of organic molecules. (This is the equivalent to giving

electron holes to the conductive layer.)

3. At the boundary between the emissive and the conductive layers, electrons find electron holes.

When an electron finds an electron hole, the electron fills the hole (it falls into an energy level of the atom that's missing an electron).

When this happens, the electron gives up energy in the form of a photon of light .

4. The OLED emits light.5. The color of the light depends on the

type of organic molecule in the emissive layer. Manufacturers place several types of organic films on the same OLED to make color displays.

6. The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.

Working Principle A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. These materials have conductivity levels ranging from insulators to conductors, and therefore are considered organic semiconductors. The highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors.During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into

the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO. As electrons and holes are fermions with half integer spin, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have

been combined. Statistically three triplet excitons will be formed for each singlet exciton.

Decay from triplet states (phosphorescence) is spin forbidden, increasing the timescale of the transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic light-emitting diodes make use of spin–orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency.

Material technologies Molecules commonly used in OLEDs include

organometallic Chelates, fluorescent and phosphorescent dyes and conjugated dendrimers.

Triphenylamine and derivatives are commonly used as materials for hole transport layers.

The production of small molecule devices and displays usually involves thermal evaporation in a vacuum.

Working of PMOLED PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current. PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitry. PMOLEDs are most efficient for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as those you find in cell phones, PDAs and MP3 players.

Working of AMOLED AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image.

AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards.

Advantages

The different manufacturing process of AMOLEDs lends itself to several advantages over flat-panel displays made with LCD technology.

+ Low power+ Low cost+ Thin, lightweight and rugged+ Superior image quality+ Wide-viewing angle+ Rollable Display

Problems

Lifespan: The biggest technical problem for OLEDs was the limited lifetime of the organic materials.

Water damage: Water can damage the organic materials of the displays.

Screen burn-in: Unlike displays with a common light source, the brightness of each OLED pixel fades depending on the content displayed. The varied lifespan of the organic dyes can cause a discrepancy between red, green, and blue intensity. This leads to image persistence, also known as burn-in.

Color balance issues: Additionally, as the OLED material used to produce blue light degrades significantly more rapidly than the materials that produce other colors, blue light output will decrease relative to the other colors of light.

Manufacturing Cost: Quite high.

References

http://en.wikipedia.org/wiki/Active-matrix_OLED

www.google.com http://www.oled-info.com/oled-

technology http://electronics.howstuffworks.com/

oled1.htm http://www.oled-display.net/what-is-

amoled http://en.wikipedia.org/wiki/

Organic_LED http://www.gadgets-reviews.com/store/

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