Brilliant Plastics

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BRILLIANT PLASTICS(PLED) K.SANTOSH REDDY

D.ADITHYA

(03611A0346) (03911A0302) (PRRMEC) (VJIT) E:MAIL:[email protected]

Svits (jkc) Abstract Polymer Light Emitting Displays are a new type of thin emissive displays predicted to possess superior properties to existing display techniques like Liquid Crystal Display (LCD). The main advantages of Polymer light emitting displays are low power consumption and a thin display structure. The active layer of these LEDs is prepared by simple coating-procedure, so these devices have the potential to provide an innovative low cost technology for backlighting, illumination, and display applications. This report contains an explanation of the emissive PLED (Polymer Light Emitting Diode) display technology, its functionality, physics of the Polymer layer structure in an OLED described with respect to the two classes of Polymer materials used in displays, small molecules and conjugated polymers, its applications. The information is derived from a study of literature. Recent advances include high-performance with the external quantum efficiency of over 5% and the power efficiencies are easily exceeding 10 lm/W. In this paper we also review the current status of this PLED technology with the best performance data as well as the potential of this class of light source for display applications. A conclusion with the advantages and drawbacks with the PLED technology summarises the report together with a short analysis of the future for PLEDs.

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Introduction In recent years there is an explosion of TFT (Thin Film Transistor), color LCD panels of ever-increasing size into laptops and flat screen monitors, PDP (plasma display panels) for high definition TV CRT replacement. Despite this, however, remains high production and commercial expenses that have never come down enough to successfully mass market these displays, leaving the technology vulnerable to new innovations. With the imaging appliance revolution underway, the need for more advanced handheld devices that will combine the attributes of a computer, PDA, and cell phone is increasing and the flat-panel mobile display industry is searching for a display technology that will revolutionize the industry. The need for new lightweight, low-power, wide viewing angled, handheld portable communication devices have pushed the display industry to revisit the current flat-panel digital display technology used for mobile applications. Struggling to meet the needs of demanding applications, the flat panel industry is now looking at new displays known as Polymer Light Emitting Diode (PLED) displays. What is a Polymer light emitting diode (PLED)? If the emissive layer material of an Light Emitting Diode (LED) is an organic compound, it is known as an Organic Light Emitting Diode (OLED). To function as a semiconductor, the organic emissive material must have conjugated pi bonds. The emissive material can be a small organic molecule in a crystalline phase, or a polymer. Polymer materials can be flexible; such LEDs are known as Polymer light emitting diodes (PLEDs) or FLEDs. Although the technology behind these Polymer LED displays is pure chemistry, the applications are much more everyday - mobile telephone and television screens, laptop and stereo displays, car navigation systems, or even billboards. This PLED technology is based on a revolutionary discovery that light-emitting, fast switching diodes that could be made from organic macromolecules.

3 Basic Principle and Technology of PLED The basic principle of a polymer light emitting diode can be explained by a simplified scheme of energy levels. Through a metallic electrode, electrons are injected into the conduction band and holes are injected into the valence band of the polymeric semiconductor. The injected electrons and holes can diffuse towards each other and finally recombine. By this process, neutral excitations are created. These neutral excitations are bound states of an electron-hole pair (excitons), which can move along a polymer chain. Once these exited states decay into their ground state, a characteristic fluorescence may be generated. However, the excitons may be either in a singlet-state that spontaneously generates fluorescence, or in a triplet-state that due to spin selection has a very long lifetime and typically decays with very low quantum yield for light generation.

figure 1: Polymer LED The probability of a preferred singlet-exciton to appear by the recombination of an electron-hole pair can be deduced from a simple spin-statistics argument. The two electron spins may recombine into either one singlet state or three triplet states. According to this argument singlet states will be generated with a probability of 1/(1+3) i.e. with a 25% probability. The ground state into which the exciton has to decay in order to generate a photon is a singlet state. An intersystem crossing from the triplet- to singletstate can be neglected in the polymers due to a low transition probability. Therefore the upper limit for the internal quantum yield of electroluminescence is estimated to be 25% of the fluorescence quantum yield of the polymer.

4 In conjugated polymers there are approximately 50% singlet and 50% triplet excited states. Instead of emitting light the triplet state emit heat that contributes to the degradation of the device. For small molecules there are about 25% singlet state and the small molecule materials therefore inhabit theoretically lower power efficiency compared to conjugated polymers. To enhance the efficiency for small molecules doping with a phosphorescent-material can be used. In real devices the internal quantum is efficiency much lower than the theoretical value but doping of the materials does enhance their performance. The reduction of the internal efficiency is mainly due to the absorption of the emitted light due to Stokes shift. Currently, laboratories are finding that the most efficient approach is to use small molecules, which are sometimes capable of both optoelectric excitation states: fluorescence and phosphorescence. In the past, polymer OLEDs have used only what scientists call the "singlet state." This state arises when the voltage pumps energy into the polymer's electrons, which then release this energy as visible radiation when they return to the ground state – the phenomenon of fluorescence. At the same time, electrons are excited to the "triplet state," which occurs three times as often but has less energy. When these electrons fall back to the ground state, they also give off radiation, but it is usually invisible; this is phosphorescence. Techniques like the use of certain doping agents can be used to activate the triplet state and incorporate it into the emission, which could increase the efficiency of polymer OLEDs by a factor of up to four. Efficiency of Polymer LED devices Efficiency of Organic El Devices is given by:

ηext= ηintηp= γηrφfηp ηext = external quantum efficiency

= Internal quantum efficiency ηp = Light out-coupling effect ηint

γ = charge carrier balance factor (e/h) ηr = efficiency of exciton production φf = Internal quantum efficiency luminescence Maximum external quantum efficiency is ~ 5%

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Building a Polymer LED-Device By using selected light-emitting polymers, PLED devices can be fabricated by the Piezoelectric Ink Jet printing method. Starting from the bottom a transparent glass or polymer substrate, e.g. polyethylene-terephthalate (PET), covered with a transparent indium tin oxide (ITO) electrode. A thin film of the semiconducting polymer is printed on the ITO substrate from a solution. The second electrode is then deposited onto the polymer via vacuum evaporation of a metal (e.g. Calcium) and the device is encapsulated against oxygen and moisture.

Figure 2: Layers in OLED Upon application of a comparatively low DC voltage, light is generated and passes through the transparent electrode. The big advantage of the manufacturing process is its simplicity and therefore its potential for low cost; only a very limited number of process steps are needed. This procedure requires fewer manufacturing steps than the manufacturing of LCDs, and, more importantly, fewer materials are used. In fact, the whole display can be built on one sheet of glass or plastic, so it should be cheaper to manufacture. Semiconducting Polymers used in PLEDs Through chemical variation of the side-chains of substituted poly (p-phenylene vinylenes) (PPVs), the energy gap was tuned resulting in orange, yellow and green light emission. In order to obtain blue color, a PPV cannot be used due to the low energy gap. Instead of this, Poly (p-phenylenes) (PPPs) are preferred for blue emission.

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Figure 3: Structure and synthesis of PPV An excellent control over the color, solubility, morphology, and other important features can be gained with the Spiro concept invented by Hoechst. The Spiro concept covers the use of orthogonal-linked, short polymer units (oligomers) as e.g. the spiro-6paraphenylene with the advantage of the high thermal stability.

Figure 4: Emission spectra of PLEDs

Polymer Light Emitting Diode Display Technology Polymer Light Emitting Diode technology enables full color, full-motion flat panel displays with a level of brightness and sharpness not possible with other technologies. Unlike traditional LCD’s, PLEDs are self-luminous and do not require backlighting, diffusers, polarizers, or any of the other baggage that goes with liquid crystal displays. Essentially, the PLED consists of two charged electrodes sandwiched on top of some organic light emitting material. This eliminates the need for bulky and environmentally undesirable mercury lamps and yields a thinner, more versatile and more compact display. Their low power consumption provides for maximum efficiency and helps minimize heat and electric interference in electronic devices. Armed with this

7 combination of features, PLED displays communicate more information in a more engaging way while adding less weight and taking up less space. There are two forms of PLED displays: Passive-matrix and Active-matrix. Passive-matrix: The passive-matrix PLED display has a simple structure and is well suited for low-cost and low-information content applications such as alphanumeric displays. It is formed by providing an array of PLED pixels connected by intersecting anode and cathode conductors.

Figure 5: Passive matrix OLED Organic materials and cathode metal are deposited into a “rib” structure (base and pillar), in which the rib structure automatically produces a PLED display panel with the desired electrical isolation for the cathode lines. A major advantage of this method is that all patterning steps are conventional, so the entire panel fabrication process can easily be adapted to large-area, high-throughput manufacturing. To get a passive-matrix PLED display to work, electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An external controller circuit provides the necessary input power, video data signal and multiplex switches. Data signal is generally supplied to the column lines and synchronized to the scanning of the row lines. When a particular row is selected, the column and row data lines determine which pixels are lit. A video output is thus displayed on the panel by scanning through all the rows successively in a frame time, which is typically 1/60 of a second. Active-matrix: In contrast to the passive-matrix PLED display, active-matrix PLED has an integrated electronic back plane as its substrate and lends itself to high-resolution, high-information content applications including videos and graphics. This form of

8 display is made possible by the development of polysilicon technology, which, because of its high carrier mobility, provides thin-film-transistors (TFT) with high current carrying capability and high switching speed. In an active-matrix PLED display, each individual pixel can be addressed independently via the associated TFT’s and capacitors in the electronic back plane. That is, each pixel element can be selected to stay “on” during the entire frame time, or duration of the video. Since OLED is an emissive device, the display aperture factor is not critical, unlike LCD displays where light must pass through aperture. Therefore, there are no intrinsic limitations to the pixel count, resolution, or size of an active-matrix PLED display, leaving the possibilities for commercial use open to our imaginations. Also, because of the TFT’s in the active-matrix design, a defective pixel produces only a dark effect, which is considered to be much less objectionable than a bright point defect, like found in LCD’s. Starting at a screen diameter of eight inches, which translates to more than one million pixels, individual pixels can no longer be activated passively. Instead, they have to be activated directly. In these active matrix displays, each pixel always radiates precisely the amount of light into the environment that is needed for the desired color impression. As a result, it is not only possible to make larger surfaces; but also to significantly lengthen the service life of display screens. This is because the individual light spots don't have to be as bright as their counterparts in passive matrix displays. Polymer LEDs shift into high gear than Organic LEDs Polymer LEDs have lagged behind other organic light emitters because of the problem of preparing films with the right electronic properties. Normally, high performance in LEDs is achieved in mulilayered films, which are easier to prepare with non-polymer organic materials. Organic light-emitting films are usually made by spin coating a material onto indium tin oxide (ITO), a transparent conductor. The reason non-polymer organics have been more successful than polymers is that thinfilm formation involves important chemical action, such as purification. With polymers,

9 the material must be prepared in its finished form before deposition, creating one essential problem: the subsequent chemical reactions between polymer layers. In addition, the electronic properties of the layers in a device have to be carefully tailored. For example, the lowest layer must be very good at capturing and transmitting electrons injected from the ITO electrode. New materials have allowed chemists to address those issues. In particular, the fluorenebased polymers have turned out to be easy to modify electronically. PLEDs are built by layering the fluorene-equipped polymers with non-active monomer layers. That strategy makes it possible to carefully tailor the electronic properties of the devices. Blue and yellow-light emitting devices are built that are based on both electron and hole transport. A new approach to building efficient electron-transporting organic light emitters uses the versatility of polymers to add side chains to the main backbone. In this case adding side chains that are deficient in electrons in order to boost the ability of the initial layer to capture injected electrons. The addition produced an order-of-magnitude increase in the light-emitting efficiency of the polymers.

Figure 6: semiconducting plastics are cheap, processable, lightweight and flexible. The advantages of PLED displays relative to conventional liquid crystal display (LCD) monitors: •

Brilliant, highly luminous colors, an unimpeded viewing angle of nearly 180 °, and an extremely thin structure. Although it will still be a while before the new

10 technology has matured to the LCD component level, the enormous potential for simpler and less expensive system solutions is already obvious •

Simpler structure. The additional light source previously required for backlighting is no longer necessary, nor are the color filters necessary for variegated LCD displays. Polymers form a solid cover after the coating process. By contrast, LCD crystals are liquid, and must be held in tiny cells. PLEDs are tough enough to use in portable devices such as cellular phones, digital video cameras, DVD players, car audio equipment and PDA’s.

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The new technology does not function on the basis of complex reorientation effects (liquid crystals only alter the transmission of light), but on the selfluminescent properties of the organic material.

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High Resolution – High information applications including videos and graphics, active-matrix PLED provides the solution. Each pixel can be turned on or off independently to create multiple colors in a fluid and smooth edged display.

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Power Usage – Takes less power to run (2 to 10 volts).

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All this is possible without adversely affecting the energy balance

Disadvantages: •

Engineering Hurdles – PLEDs are still in the development phases of production. Although they have been introduced commercially for alphanumeric devices like cellular phones and car audio equipment, they still face many obstacles before production.

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Color – The reliability of the PLED is still not up to par. After a month of use, the screen becomes non-uniform. Reds, and blues die first, leaving a very green display. 100,000 hours for red, 30,000 for green and 1,000 for blue. Good enough for cell phones, but not laptop or desktop displays.

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Overcoming LCD’s – LCD’s have predominately been the preferred form of display for the last few decades. Tapping into the multi-billion dollar industry will require a great product and continually innovative research and development.

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Polymers are highly sensitive to contaminants. Organic molecules decompose when they come into contact with water or oxygen. Absolutely clean conditions are therefore essential during material preparation.

Future Outlook The PLED technology faces a bright future in the display market, as the ever-changing market environment appears to be a global race to achieve new success. Eventually, the technology could be used to make screens large enough for laptop and desktop computers. Because production is more akin to chemical processing than semiconductor manufacturing, PLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, and even head wearable displays. However, the complicated processes necessary to make PLED components don't exactly make large-scale production easy. After all, the components must be manufactured in clean rooms, sometimes even in hermetically sealed glass cabins containing an inert gas atmosphere or a vacuum. Each end product must be encapsulated absolutely airtight before it leaves the PLED production line. The current PLED performance is adequate for many display applications, and a few monochrome PLED displays are already available commercially. Today's PLED performance is not yet adequate for general lighting, however other key challenges that PLED technology must meet to enable general illumination applications are: •

Producing high-quality white light.

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Achieving low manufacturing costs.

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Increasing efficiency and lifetime at high brightness.

12 With a continuous "roll-to-roll" manufacturing method, large sheets of PLEDs could be prepared at a very low cost. These sheets could replace wallpapers or coat ceilings and provide very pleasant uniform lighting in any home or work area. The same material could be woven into a fabric and used as a curtain in daylight or a light source at night. With further innovations in device designs and new materials, PLEDs might replace some fluorescent lights in the next 10 --20 years. Conclusions Polymer LEDs are only the tip of the iceberg as far as this new semiconductor technology is concerned. These brilliant Plastics will soon enable us not only to make innovative light sources, but also their optoelectronic counterpart—the solar cell. Optical memories, transistors, even microprocessors made out of the revolutionary synthetic material are conceivable. The Polymer Light Emitting Diode forms of display still have many obstacles to overcome before it’s popularity and even more importantly, its reliability are up to par with standards expected by consumers. Although the technology presents itself as a major player in the field of displays, overcoming the obstacles will prove to be a difficult task. Polymer LEDs are much more promising than previously thought. “Organic semiconductors will not replace established silicon technology; but they will offer inexpensive solutions for simple applications”. For all we know and can hope for.... PLEDs could change the ways in which we see things.

Bibliography [1]Nu Yu, Heinrich Becker Covion Organic Semiconductors GmbH, 65926 Frankfurt am Main, Germany(2002) [2]”ELECTRONIC PAPER”: Organic Light Emitting Diodes [3]Clint DeBoer www.audioholics.com/techtips/ specsformats/organicOLEDsdisplays.php (2004)