COLLEGE OF ENGINEERING

KIDANGOOR Under Co-operative Academy of Professional Education (CAPE)

Est. by the Govt. of Kerala

KIDANGOOR SOUTH P O, KOTTAYAM – 686 583

DEPARTMENT OF ELECTRONICS & INSTRUMENTATION ENGINEERING

SEMINAR REPORT

ON

<ACTIVE MATRIX ORGANIC LIGHT EMITTING DIODE>

SUBMITTED BY

<RINKU MARIA MATHEW>

Department of Electronics and Instrumentation Engineering September, 2010

COLLEGE OF ENGINEERING

KIDANGOORUnder Co-operative Academy of Professional Education (CAPE)

Est. by the Govt. of KeralaKIDANGOOR SOUTH P O, KOTTAYAM – 686 583

DEPARTMENT OF ELECTRONICS & INSTRUMENTATION ENGINEERING

CERTIFICATEDate :13-9-

2010

Certified that this seminar titled <ACTIVE MATRIX ORGANIC LIGHT EMITTING DIODE>

is the bonafied record of the work done by <RINKU MARIA MATHEW>

of seventh semesterB.Tech in Electronics and Instrumentation Engineering

towards the partial fulfillment of the requirementfor the award of the Degree of Bachelor of Technology, by the

Cochin University of Science And Technology.

Seminar Co-ordinator Head of the Department

ACKNOWLEDGEMENT

Bowing heads to the one and only one, THE ALMIGHTY, and my beloved

parents, nothing else other than their loving prayers would have taken us to a successful

stage like this.

I am grateful to our respected principal, Dr. Praseeda Lekshmi V and

respected head of the department Mr. Pradeep T S for being our source of inspiration

and the torch of all our endeavors.

I express my heartfelt gratitude towards our seminar coordinators Mrs.

Chinchu M (Lecturer, Dept of ECE), Mr. Vishnu Mohan and Mrs. Sheffy Thomas

(Lecturers, Dept of EIE) for their valuable support and guidance. I specially mention my

gratitude to lab assistants, computer system administrator and other non – teaching staffs

who helped me to collect information regarding this seminar.

ABSTRACT

AMOLED (active- matrix organic light-emitting diode) is an emerging display technology for use in mobile devices like mobile phones. An active matrix OLED (AMOLED) display consists of OLED pixels that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that illuminate light upon electrical activation, which functions as a series of switches to control the current flowing to each of the pixels. They are very thin and light weight and have greatly minimized propensity for breakage. They have been extensively studied owing to their promising features like thin thickness, self emission, and lower driving voltage, a wide viewing angle, fast response time, high brightness and flexible characteristics.

CONTENTS

PAGE NO

1 INTRODUCTION 1 2 ORGANIC LIGHT EMITTING DIODE 2

3 WORKING PRINCIPLE 3

4 MATERIAL TECHNOLOGIES 5

5 OLED STRUCTURE 6

6 ADVANTAGES AND DISADVANTAGES 7 7 COMMERCIAL USES 8

8 ACTIVE MATRIX OLED 9

9 PASSIVE AND ACTIVE MATRIX 11 10 AMOLED DISPLAY PANEL 12

11 FACTORS AFFECTING BRIGHTNESS 15

12 NOVEL LTPS-TFT PIXEL CIRCUIT 17

13 ADVANTAGES 19

14 DISADVANTAGES 21

15 COMMERCIAL USE 21

16 CONCLUSION 23

17 REFERENCES 24

INTRODUCTION

AMOLED (active-matrix organic light-emitting diode) is an emerging display technology for use in mobile devices like mobile phones. OLED describes a specific type of ultra thin, ultra bright 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. Active matrix (AM) OLED displays stack cathode, organic, and anode layers on top of another layer – or substrate- that contains circuitry. The pixels are defied by the deposition of the organic material in a continuous, discrete “dot” pattern. Each pixel is activated directly. A corresponding circuit delivers voltage to the cathode and anode materials, stimulating the middle organic layer. AMOLED pixels turn on and off more than three times faster than speed of conventional motion picture film- making these displays ideal for fluid, full motion video.

ORGANIC LIGHT-EMITTING DIODE

An organic light emitting diode (OLED), also light emitting polymer (LEP) and organic electro luminescence (OEL), is a light emitting diode (LED) whose emissive electroluminescent layer is composed of a film of organic compounds. This layer of organic semiconductor material is formed between two electrodes. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple “printing” process. The resulting matrix of pixels can emit light of different colors.

Such systems can be used in television screens, computer monitors, small, portable system screens such as cell phones and PDAs, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light emitting elements. OLEDs typically emit less light per area than inorganic solid state based LEDs which are usually designed for use in point light sources.

A significant benefit of OLED displays over traditional liquid crystal displays is that OLEDs do not require a backlight to function. Thus they draw far less power and when powered from a battery, can operate longer on the same charge. Because there is no need for a backlight OLED display can be much thinner than LCD panel.

The electroluminescence in organic materials were first produced in 1950s by applying high voltage alternating current fields in air to acridine orange either deposited on or dissolved in cellulose or cellophane thin films. In 1960s, martin pope discovered ohmic injecting electrode contacts to organic crystals. And the injecting hole and electron injecting electrode contacts are the basis of all current OLED devices, molecular and polymeric. In 1965 he refined his experiment and showed that in the absence of an external electric field, the electroluminescence in anthracene single crystal was caused by thermalized electrons and holes.

The first diode device was invented at Eastman Kodak in the 1980s. this diode giving rise to the term OLED used a novel two layer structure with separate hole transporting and electron transporting layers such that recombination and emission occurred in the middle of the organic layer. This resulted in the reduction in operating voltage and improvements in efficiency, and started the current era of OLED research and device production.

WORKING PRINCIPLE

A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of organic molecules that conduct electricity the layers have conductivity levels ranging from insulators to conductors, so OLEDs are considered organic semiconductors.

The first most basic OLEDs consisted of a single organic layer, for example the first light-emitting polymer device involved a single layer of poly (p-phenylene vinylene). Multilayer OLEDs can have more than two layers of to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted.

Schematic of a 2-layer OLED: 1. Cathode (-), 2. Emissive layer, 3.emission of radiation, 4.conductive layer, 5.anode

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraw electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer.

Soon, the emissive layer becomes negatively charged holes. Electrostatic forces bring the electrons and the holes towards each other and they recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons. The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the visible region. That is why this layer is called emissive.

The device does not work when the anode is put at a negative potential with respect to the cathode. In this condition, holes move to the anode and electrons to the cathode, so they are moving from each other and do not recombine.

Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer. Metals such as aluminium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the polymer layer.

OLEDs can be categorized into passive-matrix and active-matrix displays. Active matrix OLEDs require a thin film transistor backplane to switch the individual pixel on or off, and can make higher resolution and larger size displays possible.

MATERIAL TECHNOLOGIES

Small molecules

OLED technology using small molecules was first developed at Eastman Kodak Company. The production of small molecule displays involves vaccum deposition, which makes the production process more expensive than other processing techniques. Since this is typically carried out on glass substrates, these displays are also not flexible, though this limitation is not inherent to small molecule organic materials. The term OLED traditionally refers to this type of device, though some are using the term SM-OLED.

Molecules commonly used in OLEDs include organo- metallic chelates, for example Alq3, used in the first organic light emitting device and conjugated dendrimers.

Contrary to polymers, small molecules can be evaporated and therefore very complex multilayer structures can be constructed. This high flexibility in layer design is the main responsible for the high efficiencies in the SM OLEDs. Recently hybrid light emitting layer has been developed that uses nonconductive polymers doped with light emitting conducting molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longetivity that they have in the SMOLEDs.

Polymer light emitting diodes

Polymer light emitting diodes, also light emitting polymers, involve an electroluminescent conductive polymer that emits light when connected to an external voltage source. They are used as a thin film for full spectrum

color displays and require a relatively small amount of power for the light produced. No vaccum is required and the emissive materials can be applied on the substrate by a technique derived from commercial printing. The substrate used can be flexible. And this flexible polymer light emitting diode displays, also called flexible OLED may be produced inexpensively. Typical polymers used in polymer light emitting diode displays include derivatives of poly (p-phenylene vinylene) and polyfluorene.Phosphorescent materials

Phosphorescent OLED (POLED)uses the principle of electro phosphorescence to convert electrical energy in an OLED into light in highly efficient manner.

Patterning technologies

Patternable OLED, uses a light or heat activated electro active layer. A latent material is included in this layer that upon activation, becomes highly efficient as a hole injection layer. Using this process, light emitting devices with arbitrary patterns can be prepared.

OLED structures

Bottom emission/top emission

Bottom emission uses a transparent or semitransparent bottom electrode to get the light through a transparent substrate. Top emission uses a transparent or semi transparent top electrode to get the light through the counter substrate.

Transparent OLED

Transparent organic light emitting device uses a proprietary transparent contact to create displays that can be made to be top –only emitting, bottom only emitting, or both top and bottom emitting(transparent). TransparentOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.

Stacked OLEDs

Stacked OLEDs uses pixel architecture that stacks the red, green and blue sub pixels on top of one another, instead of next to one another. This leads to substantial increase in gamut and color depth, and greatly reducing pixel gap. At the moment all display technologies have the RGB pixels mapped next to each other.

ADVANTAGES AND DISADVANTAGES

The radically different manufacturing process of OLEDs lends itself to many advantages over flat panel displays made with LCD technology. They can have a significantly lower cost than LCDs or plasma displays. OLEDs enable a great range of colors, gamut, brightness, contrast and viewing angle than LCDs because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an off OLED element produces no light and consumes no power.

Energy is also wasted in LCDs because they require polarizers that filter out about half of the emitted by the backlight.

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. The intrusion is the limited lifetime of the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

COMMERCIAL USES

OLED technology is used in commercial applications such as small screens for mobile phones and portable digital audio players, car radios, digital cameras and high resolution micro displays for head mounted displays. Such portable applications favor the high light output of OLEDs for readability in sunlight, and their low power drains. Portable displays are also used intermittently, so the lower life span of OLEDs is less important here. Prototypes have been made of flexible and rollable displays which use OLEDS unique characteristics. OLEDs have been used in most Motorola and Samsung color cell phones as well as some LG and Sony Ericsson phones, notably the Z610i and some models of the Sony walkman.

ACTIVE MATRIX OLED

An active-matrix OLED (AMOLED) display consists of OLED pixels that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that illuminate light upon electrical activation, which functions as a series of switches to control the current flowing to each of the pixels. The TFT array continuously controls the current that flows to the pixels, signaling to each pixel how brightly to shine.

Typically, this continuous current flow is controlled by at least two TFTs at each pixel, one to start and stop the charging of a storage capacitor and the second to provide a voltage source at the level needed to create a constant current to the pixel and eliminating the need for very high currents required for passive OLED matrix operation.

AMOLED screens have four separate layers to control the picture.

An anode layer A middle organic layer A cathode layer A bottom layer which contains circuitry

The major feature of the AMOLED display is the use of a thin film transistor(TFT) technique to drive the organic light emitting diode, and the driving integrated circuit(IC) is installed on the panel directly, so as to be small in volume and low in cost. The digital display is characterized by a display screen composed of multiple pixels in a matrix arrangement. In order to control individual pixels, a specific pixel is commonly selected via a scanning line and data line, and an appropriate operating voltage is also provided, so as to display information corresponding to this pixel.

The active matrix organic light emitting diode (AMOLED) display technology is a newly developed technology, and will be mainstream for display devices accompanying liquid crystal displays (LCDs) in the future. The major feature of the AMOLED display is the use of a thin film transistor technique to drive the organic light emitting diode, and the driving integrated circuit is installed on the panel directly, so as to be small in volume and low in cost. The AMOLED display can be applied on a medium or small sized panel in a cellular phone, PDA, digital camera and palm game player, portable DVD player and automobile global positioning system.

The digital display is characterized by a display screen composed of multiple pixels in a matrix arrangement. In order to control individual pixels, a specific pixel is commonly selected via a scanning line and a data line and an appropriate operating voltage is also provided, so as to display information corresponding to this pixel.

In order to create an AMOLED display, a TFT substrate and organic light emitting diode (OLED) films are incorporated into the AMOLED display pixels. When the TFT and OLED degrades, the entire display degrades as well. One approach that the design of the pixels must be geared towards compensating for the degradation of the TFT. Judging from the current technology, however the brightness of the OLED cannot be maintained, even if the electric currents provided by the TFT are kept constant. This is because the efficiency of the OLED itself declines with time, and it declines faster than the TFT. Therefore according to conventional techniques, even when electric currents are kept steady by the TFT, the brightness of the AMOLED display still decays.

PASSIVE AND ACTIVE MATRIX

Passive matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. It starts with two glass layers called substrates. One substrate is given columns and the other is given rows made from a transparent conductive material.This is usually indium tin oxide. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched between two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel the integrated circuits sends a charge down the correct column of one substrate and a ground activated on the correct row of the other. The row and column intersect at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that pixel.

But it has significant drawbacks, notably slow response time and imprecise voltage control. Response time refers to the LCDs ability to refresh the image displayed. The precise voltage control hinders the passive matrix’s ability to influence only one pixel at a time. When voltage is applied to untwist one pixel, the pixels around it also partially untwist, which makes image appear fuzzy and lacking in contrast.

Active matrix LCDs depend on thin film transistors (TFT). Basically TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all the other rows the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through.

AMOLED DISPLAY PANEL

Active matrix element: TFT backplane technology

A thin film transistor is a special kind of field effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays. This differs from the conventional transistor where the semiconductor material typically is the substrate such as silicon wafer. TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of silicon based TFT depend on the crystalline state. That is the semiconductor layer can be either amorphous silicon, microcrystalline silicon or it can be annealed into polysilicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide and metal oxides such as zinc oxide. TFTs have also been made using organic materials. By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent. Because the substrate cannot withstand the high annealing temperature, the deposition process has to be completed under relatively low temperature. Chemical vapor deposition, physical vapor deposition are applied.

TFT backplane technology is a crucial enabler for the fabrication of flexible AM OLED displays.

The conventional glass substrate based TFT process cannot be used with the flexible plastic substrates, primarily because of the low temperature process constraint.

Two primary TFT backplane technologies poly- silicon (poly-si) and amorphous-silicon (a-si) are used today in AMOLEDs these technologies offer the potential for fabricating the required active matrix backplanes at low temperatures(<150 degree Celsius) directly on the flexible plastic substrate for producing flexible AMOLED displays.

In a typical AMOLED display panel, the TFT device circuits are formed on a TFT back panel of the display panel. The TFT devices, which generally include a polycrystalline silicon film as a semiconductor layer, may be a bottom gate type or a top gate type, such as low temperature polysilicon thin film transistor. The polycrystalline silicon film requires high electron mobility in order for the TFT device to function optimally. In general the polycrystalline silicon film is formed from an amorphous silicon film. One way to form the polycrystalline silicon film from the amorphous silicon film is to crystallize the amorphous silicon film by irradiating it with laser light, such as a high power excimer laser. An excimer laser is a pulsed laser having KrF, ArF, or XeCL as a light source. The amorphous silicon film is generally crystallized over its entire surface by irradiating the substrate from one end to the other with excimer laser light that has been processed to have a linear shape. The linear shaped laser beam generally spans a portion or the whole length of a TFT back panel and is scanned in a lateral direction.

AMOLED CIRCUIT LAYOUT

Illustrated in FIG is a 4*4 pixel array portion of a conventional AMOLEDs TFT back panel 200. As illustrated, pixel region 210 comprises a TFT circuit

portion 212 and an OLED circuit portion 214. The amorphous silicon film layer is initially deposited over the entire TFT back panel 200 and crystallized into polycrystalline form using the excimer laser annealing process. A linear –shaped excimer laser beam 220 is scanned over the entire surface of the TFT back panel 200 by irradiating a portion of the TFT back panel 200 at a time. Since the size of the laser beam is limited, many pulses of the laser beam are required to cover the entire TFT back panel 200. After the amorphous silicon film is laser annealed into polycrystalline film, subsequent photolithographic process remove unnecessary portions of the polycrystalline film except for the polycrystalline islands that are required for the source, drain, and channel regions of the TFT devices in the TFT circuit portion 212.

Although the polycrystalline silicon film is subsequently removed from the OLED portion 221, this often results in undesirable line Mura defects in the finished AMOLED display panel. Mura defects are defects that exhibit as non-uniform contrast regions on an LCD or an OLED display panel and are attributed to pulse-to-pulse variations in the laser beam energy that is used to crystallize the amorphous silicon film. These defects are more pronounced when a constant gray value image or patter is displayed. In AMOLED display panels, the laser anneal irradiation of the non-TFT regions such as the ILED circuit portion 221, on the TFT back panel often results in line shaped Mura defects. The non uniform laser beam energy caused by pulse to pulse variations in the laser beam energy results in non-uniform performance of polycrystalline silicon. And because the TFT characteristics is sensitive to the performance of the polycrystalline silicon and the TFT devices drive the OLED devices, the non uniform characteristics results in non-uniformity in OLEDs brightness.

FACTORS AFFECTING BRIGHTNESS

Active matrix organic light-emitting diodes(AMOLEDs) have been extensively studied owing to their promising features such as thin thickness, self emission, lower driving voltage, a wide viewing angle, fast response time, high efficiency, high brightness and flexible characteristics. Two programming methods exist- the passive matrix (PM) and active matrix (AM) driving methods (fig 1). The PM driving method has some benefits, such as a simple structure and low cost. However the PM method cannot emit light continuously and power consumption is markedly higher then that of the AM method when applied to large displays.

Passive matrix active matrix

fig1. Schematic of PMOLED (PM) and AMOLED (AM)

The difference between PMOLEDs and AMOLEDs is that the pixel in an AMOLED includes a capacitor for storing data and eliminating light continuously. Therefore, AMOLED is applicable to large-high-resolution displays and highly promising for future flexible displays. However the structure of AMOLEDs is mare complex than that of PMOLEDs additionally, the following factors affect the brightness uniformity in AMOLED.

1. Threshold voltage variations:

The OLED current is determined by the driving thin-film transistor (TFT), operated in the saturated region. Therefore, Vth variations are due to process differences and long term operational result for OLED brightness non-uniformity. The OLED current formula is IOLED =1/2k (VGS-VTH) 2, Where Vth represents the threshold voltage of a driving TFT.

2. OLED degradation and brightness efficiency:The anode potential of the OLEDs must be considered as OLED threshold voltage degrades at 0.2mV/h during operation. In most OLED pixel circuits, the source node of an n-type drive TFT connects the OLED anode. Therefore, an n-type pixel circuit must accommodate the threshold voltage degradation of OLEDs as OLED current is determined by VGS of the driving TFT.

3. Process consideration.The AMOLED pixel driver can be fabricated as a low-temperature polysilicon (LTPS) and amorphous silicon (a-si). An n-type TFT can be fabricated by LTPS and a-Si processes. However p-type TFT can only be fabricated using the LTPS process. Furthermore, although including several advantages like low cost, mature manufacturability and high stability an a-Si process technology has a serious threshold voltage shift over long-term operation.

Due to these problems associated with AMOLEDs, numerous mechanisms where developed to maintain display brightness uniformity. However these studies focused on TFT threshold voltage variation; the effect of OLED degradation on brightness is seldom discussed. To overcome the brightness degradation, some compensating techniques have been developed, such as the optical feedback and ac driving methods. However the disadvantage of optical feedback is a strong wavelength dependence on photo efficiency and sensitivity to ambient light. Additionally the ac driving methods does not account for VTH variations completely. Therefore a work is presented where a novel five TFT pixel circuit with a feedback structure that detects OLED ageing phenomena and produces additional current to compensate for OLED degradation, thereby preventing luminescence drops.

NOVEL LTPS-TFT PIXEL CIRCUIT

(a) Circuit schematic (b) Control signal timing diagram

Fig. 3. Circuit schematic and control signal timing diagram for the proposed driving method

To reduce the non uniform brightness problem mentioned above, this work proposes a novel circuit comprising of 3 n-type TFTs, 2 p-type TFTs, a storage capacitor, and an additional control signal. Fig.3 schematically depicts the proposed pixel circuit and its control signal timing diagram. First, TFT1 determines the OLED current by analyzing the storage capacitor voltage of C1, utilized to store the driving voltage during one frame, and the other TFTs are used for a switching function. The operational scheme and compensation principle of the proposed circuit are described as follows.

1. Initialization period: during the first period, SCAN1 goes To low voltage and SCAN2 is high voltage; TFT2, TFT4 and TFT5 are turned on and TFT3 is turned off. The data voltage (V data) is applied to node A (VA) through TFT5 and the gate voltage

of TFT1 (VB) is charged approaching to the voltage difference between VDATA and VB, that is, the initialization period.

2. compensation period: SCAN2 signal goes to low voltage,Turning off TFT2, and TFT3remains turned off. The VB is discharged through TFT4 and TFT1 until TFT1 is turned off. This node voltage settles to VOLED_0 +VTH_T1, where VTH_T1 is the threshold voltage of TFT1, and VOLED_0 is the threshold voltage of OLED when no current is flowing through OLED. The storage capacitor (C1) is discharged to VOLED_0 +VTH_T1-Vdata.

3. emission period: both TFT2 and TFT3 are turned on: VDD is connected to the drain node of TFT1 through TFT2. Node A is connected to a source node of TFT1, such that VA becomes VOLED_1. the C1 continuous to sustain the voltage (VOLED_0 +VTH_T1 - V data) until the next initialization period; consequently, VB becomes VOLED_0 + VTH_T1 +VOLED_1 -V data, where VOLED_1 is the voltage of OLED when the OLED is emitting light. The corresponding OLED current is determined based on the VGS of TFT1. At this time, the drain- current of TFT1 (IOLED) is given as

IOLED=1/2k (VGS-VTH_TFT1)2

=1/2k (VOLED_0+VTH_TFT1+VOLED_1-V data-VOLED_1-VTH_TFT1)2 =1/2k (VOLED_0-VDATA)2

ADVANTAGES

Very thin and light weight Greatly minimized propensity for breakage.

High perceived luminescence: Perceived luminescence is 1.5 times higher than thet of conventional lcd display

Contrast ratio the contrast of an AMOLED is unbelivable it offers clear images and readability in any environment.

o Wide viewing angle

True colorsHigh color gamut and no color shift by viewing angle/or gray scales.

Fast response

More vivid and dynamic image quality is realized in moving pictures.

Lower power, highly rugged with superior image quality and low cost compared to the current LCD display.

Due to their inherent ruggedness, allow a unique form factor of conformability and roll ability during use, transportation and storage.

DISADVANTAGES

AMOLED displays are prone to material degradation. However technology has been invented to circumvent this problem.

COMMERCIAL USES AMOLED displays are used in mobile phones, PDA, digital camera and palm game player, portable DVD player and automobile global positioning system.

In April 2009, Samsung brought to the United States the first phone using an AMOLED display.

In July 2009, Samsung electronics launched a 3.5inch WVGA AMOLED 3g-full touch screen phone called “HAPTIC AMOLED”.

Samsung NV24HD

NOKIA N85 has a 2.6 inch display with support for up to 16 million colors. The colors are bright and evenly lit with no side or backlighting. The display is viewable for virtually any angle and looks great in the dark and in direct sunlight.

The world AMOLED market will grow to US$ 4.6 billion by 2014, representing a compound annual growth rate of 83.3 percent up from $67 million in 2007

CONCLUSION

Active matrix organic light emitting diode (AMOLED) displays have been considered a potential candidate for the next generation of flat panel displays due to the ability of AMOLED in providing wider viewing angle, larger color gamut, and lower fabrication cost. It can be efficiently utilizes in mobile displays. AMOLED continues to make progress towards low power and low cost applications.

REFERENCES

www.google.com

http://research.ncku.edu.tw/re/articles/e/20071102/3.html

www.patentstorm.us/patents/7352345/description.html

www.wikipedia.org