Traffic Accident Automatic Detection Remote Alarm Device

Transparent Display Technology

Transparent Display Technology

1. History OF OLED

The first observations of electroluminescence in organic materials were in the early 1950s by A. Bernanose and co-workers at the Nancy-Université, France. They applied high-voltage alternating current (AC) fields in air to materials such as acridine orange, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons.

In 1960, Martin Pope and co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals. They further described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current (DC) electroluminescence under vacuum on a pure single crystal of anthracene and on anthracene crystals doped with tetracene in 1963using a small area silver electrode at 400V. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.

Pope's group reported in 1965 that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in 1965, W. Helfrich and W. G. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,the forerunner of modern double injection devices. In the same year, Dow Chemical researchers patented a method of preparing electroluminescent cells using high voltage (500–1500 V) AC-driven (100–3000 Hz) electrically-insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder, tetracene, and graphite powder.^[ Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.

Device performance was limited by the poor electrical conductivity of contemporary organic materials. This was overcome by the discovery and development of highly conductive polymers.For more on the history of such materials, see conductive polymers.

Electroluminescence from polymer films was first observed by Roger Partridge at the National Physical Laboratory in the United Kingdom. The device consisted of a film of poly(n-vinylcarbazole) up to 2.2 micrometres thick located between two charge injecting electrodes. The results of the project were patented in 1975 and published in 1983.

The first diode device was reported at Eastman Kodak by Ching W. Tang and Steven Van Slyke in 1987.This device used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in a reduction in operating voltage and improvements in efficiency and led to the current era of OLED research and device production.

Research into polymer electroluminescence culminated in 1990 with J. H. Burroughes et al. at the Cavendish Laboratory in Cambridge reporting a high efficiency green light-emitting polymer based device using 100 nm thick films of poly(p-phenylene vinylene).

2. Organic light-emitting diode (OLDE)

An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent.     

               

OLEDs are used in television set screens, computer monitors, small, portable system screens such as mobile phones and PDAs , watches, advertising, information, and indication. OLEDs are also used in light sources for space illumination and in largearea light-emitting elements. Due to their early stage of development, theytypically emit less light per unit area than inorganic solid-state based LED point-light sources.

                     

         An OLED display functions without a backlight. Thus, it can display deep black levels and can be thinner and lighter than liquid crystal displays. In low ambient light conditions such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCD using either cold cathode fluorescent lamps or the more recently developed LED backlight.

There are two main families of OLEDs: those based upon small molecules and those employing polymers. Adding mobile ions to an OLED creates a Light-emitting Electrochemical Cell or LEC, which has a slightly different mode of operation.

OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, and can make higher resolution and larger size displays possible.

3. Architecture of OLEDs

3.1Substrate (clear plastic, glass, foil)

The substrate supports the OLED.

3.2Anode (transparent)

The anode removes electrons (adds electron "holes") when a current flows through the device.

                        

3.3 Organic layer:

3.3.1Conducting layer

This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline.

       

3.3.2  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.

3.4 Cathode (may or may not be transparent depending on the type of OLED)

The cathode injects electrons when a current flows through the device. 

4. AMOLED

Active-matrix OLED (active-matrix organic light-emitting diode )

AMOLED is a display technology for use in mobile devices and televisions. Oled scribes a specific type of thin film display technology in which organic compounds  form the electroluminescent material, and active matrix refers to the technology behind the addressing of pixels.  

 

   

Active matrix (AM) OLED displays stack cathode, organic, and anode layers on top of another layer – or substrate – that contains circuitry. The pixels are defined 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. AM OLED pixels turn on and off more than three times faster than the speed of conventional motion picture film – making these displays ideal for fluid, full-motion video.

5. Technical of AMOLED

            Two primary TFT backplane technologies, poly-Silicon (poly-Si) and amorphous-Silicon (a-Si) are used today in AMOLEDs.

                  Passive-Matrix Structure                                              Active Matrix Structure
  

AMOLED is a display technology for use in mobile devices and televisions. Oled scribes a specific type of thin film display technology in which organic compounds  form the electroluminescent material, and active matrix refers to the technology behind the addressing of pixels.

 

TFT backplane  technology is crucial in the fabrication of AMOLED displays.

Two primary TFT backplane technologies, namely polycrystalline silicon  (poly-Si) and amorphous silicon  (a-Si), are used today in AMOLEDs.

These technologies offer the potential for fabricating the active matrix backplanes at low temperatures (below 150°C) directly onto flexible plastic substrates for producing flexible  AMOLED displays.

 

6. Advantages of AMOLED

6.1 Lower cost in the future:

OLEDs can be printed onto any suitable substrate by an inkjetprinter or even by screen printing, theoretically making them cheaper to produce than LCD or plasma displays. However, fabrication of the OLED substrate is more costly than that of a TFT LCD, until mass production methods lower cost through scalability. Roll-roll vapour-deposition methods for organic devices do allow mass production of thousands of devices per minute for minimal cost, although this technique also induces problems in that multi-layer devices can be challenging to make.

6.2 Light weight & flexible plastic substrates:

                  

OLED displays can be fabricated on flexible plastic substrates leading to the possibility of flexible organic light-emitting diodes being fabricated or other new applications such as roll-up displays embedded in fabrics or clothing. As the substrate used can be flexible such as PET., the displays may be produced inexpensively.

6.3 Wider viewing angles & brightness: improved

             

OLEDs can enable a greater artificial contrast ratio (both dynamic range pixel colours appear correct and unshifted, even as the viewing angle approaches 90° from normal.and static, measured in purely dark conditions) and viewing angle compared to LCDs because OLED pixels directly emit light. OLED

6.4 Better power efficiency:

LCDs filter the light emitted from a backlight, allowing a small fraction of light through so they cannot show true black, while an inactive OLED element does not produce light or consume power.

6.5 Response time:

OLEDs can also have a faster response time thanstandardLCD screens. Whereas LCD displays are capable of between 2 and 8 ms response time offering a frame rate of +/-200 Hz, an OLED can theoretically have less than 0.01 ms response time enabling 100,000 Hz refresh rates.

6.6 High Perceived Luminance

Perceived luminance is 1.5 times higher than that of conventional lcd display

       

6.7 True Colors

High color gamut and no color shift by viewing angle and/or gray scales

6.8 Fast Response

            More vivid and dynamic image quality is realized in moving pictures

7. Disadvantages of AMOLED

7.1 Current costs:

OLED manufacture currently requires process steps that make it extremely expensive. Specifically, it requires the use of Low-Temperature Polysilicon backplanes; LTPS backplanes in turn require laser annealing from an amorphous silicon start, so this part of the manufacturing process for AMOLEDs starts with the process costs of standard LCD, and then adds an expensive, time-consuming process that cannot currently be used on large-area glass substrates.

7.2 Lifespan:

The biggest technical problem for OLEDs was the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours to half original brightness (five years at 8 hours a day) when used for flat-panel displays. This is lower than the typical lifetime of LCD, LED or PDP technology—each currently rated for about 25,000 – 40,000 hours to half brightness, depending on manufacturer and model. However, some manufacturers' displays aim to increase the lifespan of OLED displays, pushing their expected life past that of LCD displays by improving light out coupling, thus achieving the same brightness at a lower drive current. In 2007, experimental OLEDs were created which can sustain 400 cd/m² of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs.

7.3 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. This differential color output change will change the color balance of the display and is much more noticeable than a decrease in overall luminance.This can be partially avoided by adjusting colour balance but this may require advanced control circuits and interaction with the user, which is unacceptable for some users. In order to delay the problem, manufacturers bias the colour balance towards blue so that the display initially .

7.4 Efficiency of blue OLEDs:

Improvements to the efficiency and lifetime of blue OLEDs is vital to the success of OLEDs as replacements for LCD technology. Considerable research has been invested in developing blue OLEDs with high external quantum efficiency as well as a deeper blue color. External quantum efficiency values of 20% and 19% have been reported for red (625 nm) and green (530 nm) diodes, respectively.However, blue diodes (430 nm) have only been able to achieve maximum external quantum efficiencies in the range between 4% to 6%.

7.5 Water damage:

Water can damage the organic materials of the displays. Therefore, improved sealing processes are important for practical manufacturing. Water damage may especially limit the longevity of more flexible displays.

7.6 Outdoor performance:

            As an emissive display technology, OLEDs rely completely upon convertingelectricity to light, unlike most LCDs which are to some extent reflective; e-ink leads the way in efficiency with ~ 33% ambient light reflectivity, enabling the display to be used without any internal light source. The metallic cathode in an OLED acts as a mirror, with reflectance approaching 80%, leading to poor readability in bright ambient light such as outdoors. However, with the proper application of a circular polarizer and anti-reflective coatings, the diffuse reflectance can be reduced to less than 0.1%. With 10,000 fc incident illumination (typical test condition for simulating outdoor illumination), that yields an approximate photopic contrast of 5:1.

7.7 Power consumption:

While an OLED will consume around 40% of the power of an LCD displaying an image which is primarily black, for the majority of images it will consume 60–80% of the power of an LCD – however it can use over three times as much power to display an image with a white background such as a document or website. This can lead to reduced real-world battery life in mobile devices.

7.8 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.

7.9 UV sensitivity:

OLED displays can be damaged by prolonged exposure to UV light. The most pronounced example of this can be seen with a near UV laser (such as a Bluray pointer) and can damage the display almost instantly with more than 20 mW leading to dim or dead spots where the beam is focused. This is usually avoided by installing a UV blocking filter over the panel and this can easily be seen as a clear plastic layer on the glass. Removal of this filter can lead to severe damage and an unusable display after only a few months of room light exposure.

         

       

8. Applications of AMOLEDs

1.      TVs

2.      Cell Phone screens

3.      Computer Screens

4.      Keyboards (Optimus Maximus)

5.      Lights

6.      Portable Divice displays

8.1 AMOLED Televisions

Sony                                                       

·         Released XEL-1 in February 2009. 

·         First OLED TV sold in stores.

·         11'' screen, 3mm thin

·         $2,500 MSRP

·         Weighs approximately 1.9 kg

·         Wide 178 degree viewing angle

·         1,000,000:1 Contrast ratio

8.2 Optimum Maximums Keyboard

                           

•         Small OLED screen on every key

•         113 OLED screens total

•         Each key can be programmed to preform a series of functions

•         Keys can be linked to applications

•         Display notes, numerals, special symbols, HTML codes, etc...

•         SD card slot for

•          storing settings

        

9. Future Uses for AMOLED

9.1 Lightin

•         Flexible / bendable lighting

•         Wallpaper lighting defining new ways to light a space

•         Transparent lighting doubles as a window  

9.2 Cell Phones

•         Nokia 888 

9.3Transparent Car Navigation System  on Windshield

•         Using Samsungs' transparent  AMOLED technology

•         Heads up display 

•         GPS system    

                   

9.4  Scroll Laptop

•         Nokia concept AMOLED Laptop