The seminar is about polymers that can emit light when a voltage is applied to it. The structure comprises of a thin film of semiconducting polymer sandwiched between two electrodes (cathode and anode).When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light .The band gap, ie. The energy difference between valence band and conduction band determines the wavelength (colour) of the emitted light.
They are usually made by ink jet printing process. In this method red green and blue polymer solutions are jetted into well defined areas on the substrate. This is because, PLEDs are soluble in common organic solvents like toluene and xylene .The film thickness uniformity is obtained by multi-passing (slow) is by heads with drive per nozzle technology .The pixels are controlled by using active or passive matrix.
The advantages include low cost, small size, no viewing angle restrictions, low power requirement, biodegradability etc. They are poised to replace LCDs used in laptops and CRTs used in desktop computers today.
Their future applications include flexible displays which can be folded, wearable displays with interactive features, camouflage etc.
Introduction-Imagine these scenarios
- After watching the breakfast news on TV, you roll up the set like a large handkerchief, and stuff it into your briefcase. On the bus or train journey to your office, you can pull it out and catch up with the latest stock market quotes on CNBC.
- Somewhere in the Kargil sector, a platoon commander of the Indian Army readies for the regular satellite updates that will give him the latest terrain pictures of the border in his sector. He unrolls a plastic-like map and hooks it to the unit's satellite telephone. In seconds, the map is refreshed with the latest high resolution camera images grabbed by an Indian satellite which passed over the region just minutes ago.
Don’t imagine these scenarios at least not for too long.The current 40 billion-dollar display market, dominated by LCDs (standard in laptops) and cathode ray tubes (CRTs, standard in televisions), is seeing the introduction of full-color LEP-driven displays that are more efficient, brighter, and easier to manufacture. It is possible that organic light-emitting materials will replace older display technologies much like compact discs have relegated cassette tapes to storage bins.
The origins of polymer OLED technology go back to the discovery of conducting polymers in 1977,which earned the co-discoverers- Alan J. Heeger , Alan G. MacDiarmid and Hideki Shirakawa - the 2000 Nobel prize in chemistry. Following this discovery , researchers at Cambridge University UK
2. LIGHT EMITTING POLYMER
It is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film of semiconducting polymer sandwiched between two electrodes (anode and cathode) as shown in fig.1. When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light that escapes through glass substrate. The bandgap, i.e. energy difference between valence band and conduction band of the semiconducting polymer determines the wavelength (colour) of the emitted light.
2.1 CONSTRUCTION
Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxides typically used for the anode and aluminum or calcium for the cathode. Fig.2.1(a) shows the structure of a simple single layer device with electrodes and an active layer.
Single-layer devices typically work only under a forward DC bias. Fig.2.1(b) shows a symmetrically configured alternating current light-emitting (SCALE) device that works under AC as well as forward and reverse DC bias.
In order to manufacture the polymer, a spin-coating machine is used that has a plate spinning at the speed of a few thousand rotations per minute. The robot pours the plastic over the rotating plate, which, in turn, evenly spreads the polymer on the plate. This results in an extremely fine layer of the polymer having a thickness of 100 nanometers. Once the polymer is evenly spread, it is baked in an oven to evaporate any remnant liquid. The same technology is used to coat the CDs.
2.1.1 INK JET PRINTING
Although inkjet printing is well established in printing graphic images, only now are applications emerging in printing electronics materials. Approximately a dozen companies have demonstrated the use of inkjet printing for PLED displays and this technique is now at the forefront of developments in digital electronic materials deposition. However, turning inkjet printing into a manufacturing process for PLED displays has required significant developments of the inkjet print head, the inks and the substrates (see Fig.2.1.1).Creating a full colour, inkjet printed display requires the precise metering of volumes in the order of pico liters. Red, green and blue polymer solutions are jetted into well defined areas with an angle of flight deviation of less than 5º. To ensure the displays have uniform emission, the film thickness has to be very uniform.
For some materials and display applications the film thickness uniformity may have to be better than ±2 per cent. A conventional inkjet head may have volume variations of up to ±20 per cent from the hundred or so nozzles that comprise the head and, in the worst case, a nozzle may be blocked. For graphic art this variation can be averaged out by multi-passing with the quality to the print dependent on the number of passes. Although multi-passing could be used for PLEDs the process would be unacceptably slow. Recently, Spectra, the world’s largest supplier of industrial inkjet heads, has started to manufacture heads where the drive conditions for each nozzle can be adjusted individually – so called drive-per-nozzle (DPN). Litrex in the USA
2.1.2 ACTIVE AND PASSIVE MATRIX
Many displays consist of a matrix of pixels, formed at the intersection of rows and columns deposited on a substrate. Each pixel is a light emitting diode such as a PLED, capable of emitting light by being turned on or off, or any state in between. Coloured displays are formed by positioning matrices of red, green and blue pixels very close together. To control the pixels, and so form the image required, either 'passive' or 'active' matrix driver methods are used.
Pixel displays can either by active or passive matrix. Fig. 2.1.2 shows the differences between the two matrix types, active displays have transistors so that when a particular pixel is turned on it remains on until it is turned off.
The matrix pixels are accessed sequentially. As a result passive displays are prone to flickering since each pixel only emits light for such a small length of time. Active displays are preferred, however it is technically challenging to incorporate so many transistors into such small a compact area.
In passive matrix systems, each row and each column of the display has its own driver, and to create an image, the matrix is rapidly scanned to enable every pixel to be switched on or off as required. As the current required to brighten a pixel increases (for higher brightness displays), and as the display gets larger, this process becomes more difficult since higher currents have to flow down the control lines. Also, the controlling current has to be present whenever the pixel is required to light up. As a result, passive matrix displays tend to be used mainly where cheap, simple displays are required.
Active matrix displays solve the problem of efficiently addressing each pixel by incorporating a transistor (TFT) in series with each pixel which provides control over the current and hence the brightness of individual pixels. Lower currents can now flow down the control wires since these have only to program the TFT driver, and the wires can be finer as a result. Also, the transistor is able to hold the current setting, keeping the pixel at the required brightness, until it receives another control signal. Future demands on displays will in part require larger area displays so the active matrix market segment will grow faster.
PLED devices are especially suitable for incorporating into active matrix displays, as they are processable in solution and can be manufactured using ink jet printing over larger areas.
2.2 BASIC PRINCIPLE AND TECHNOLOGY
Polymer properties are dominated by the covalent nature of carbon bonds making up the organic molecule’s backbone. The immobility of electrons that form the covalent bonds explain why plastics were classified almost exclusively insulators until the 1970’s.
A single carbon-carbon bond is composed of two electrons being shared in overlapping wave functions. For each carbon, the four electrons in the valence bond form tetrahedral oriented hybridized sp3 orbitals from the s & p orbitals described quantum mechanically as geometrical wave functions. The properties of the spherical s orbital and bimodal p orbitals combine into four equal , unsymmetrical , tetrahedral oriented hybridized sp3 orbitals. The bond formed by the overlap of these hybridized orbitals from two carbon atoms is referred to as a ‘sigma’ bond.
A conjugated ‘pi’ bond refers to a carbon chain or ring whose bonds alternate between single and double (or triple) bonds. The bonding system tend to form stronger bonds than might be first indicated by a structure with single bonds. The single bond formed between two double bonds inherits the characteristics of the double bonds since the single bond is formed by two sp2 hybrid orbitals. The p orbitals of the single bonded carbons form an effective ‘pi’ bond ultimately leading to the significant consequence of ‘pi’ electron de-localization.
Unlike the ‘sigma’ bond electrons, which are trapped between the carbons, the ‘pi’ bond electrons have relative mobility. All that is required to provide an effective conducting band is the oxidation or reduction of carbons in the backbone. Then the electrons have mobility, as do the holes generated by the absence of electrons through oxidation with a dopant like iodine.
2.2.1 LIGHT EMISSION
The production of photons from the energy gap of a material is very similar for organic and ceramic semiconductors. Hence a brief description of the process of electroluminescence is in order.
Electroluminescence is the process in which electromagnetic(EM) radiation is emitted from a material by passing an electrical current through it. The frequency of the EM radiation is directly related to the energy of separation between electrons in the conduction band and electrons in the valence band. These bands form the periodic arrangement of atoms in the crystal structure of the semiconductor. In a ceramic semiconductor like GaAs or ZnS, the energy is released when an electron from the conduction band falls into a hole in the valence band. The electronic device that accomplishes this electron-hole interaction is that of a diode, which consists of an n-type material (electron rich) interfaced with p-type material (hole rich). When the diode is forward biased (electrons across interface from n to p by an applied voltage) the electrons cross a neutralized zone at the interface to fill holes and thus emit energy.
The situation is very similar for organic semiconductors with two notable exceptions. The first exception stems from the nature of the conduction band in an organic system while the second exception is the recognition of how conduction occurs between two organic molecules.
With non-organic semiconductors there is a band gap associated with Brillouin zones that discrete electron energies based on the periodic order of the crystalline lattice. The free electron’s mobility from lattice site to lattice site is clearly sensitive to the long-term order of the material. This is not so for the organic semiconductor. The energy gap of the polymer is more a function of the individual backbone, and the mobility of electrons and holes are limited to the linear or branched directions of the molecule they statistically inhabit. The efficiency of electron/hole transport between polymer molecules is also unique to
polymers. Electron and hole mobility occurs as a ‘hopping’ mechanism which is significant to the practical development of organic emitting devices.
PPV has a fully conjugated backbone (figure 2.2.1), as a consequence the HOMO (exp link remember 6th form!) of the macromolecule stretches across the entire chain, this kind of situation is ideal for the transport of charge; in simple terms, electrons can simply "hop" from one π orbital to the next since they are all linked.
Figure 2.2.1 A demonstration of the full conjugation of π
electrons in PPV.The delocalized π electron clouds are coloured yellow.
PPV is a semiconductor. Semiconductors are so called because they have conductivity that is midway between that of a conductor and an insulator. While conductors such as copper conduct electricity with little to no energy (in this case potential difference or voltage) required to "kick-start" a current, insulators such as glass require huge amounts of energy to conduct a current. Semi-conductors require modest amounts of energy in order to carry a current, and are used in technologies such as transistors, microchips and LEDs.
Band theory is used to explain the semi-conductance of PPV, see figure 5. In a diatomic molecule, a molecular orbital (MO) diagram can be drawn showing a single HOMO and LUMO, corresponding to a low energy π orbital and a high energy π* orbital. This is simple enough, however, every time an atom is added to the molecule a further MO is added to the MO diagram. Thus for a PPV chain which consists of ~1300 atoms involved in conjugation, the LUMOs and HOMOs will be so numerous as to be effectively continuous, this results in two bands, a valence band (HOMOs, π orbitals) and a conduction band (LUMOs, π* orbitals). They are separated by a band gap which is typically 0-10eV (check) and depends on the type of material. PPV has a band gap of 2.2eV (exp eV). The valence band is filled with
all the π electrons in the chain, and thus is entirely filled, while the conduction band, being made up of empty π* orbitals (the LUMOs) is entirely empty).
In order for PPV to carry a charge, the charge carriers (e.g. electrons) must be given enough energy to "jump" this barrier - to proceed from the valence band to the conduction band where they are free to ride the PPV chain’s empty LUMOs.(Fig. 2.2.2)
• A diatomic molecule has a bonding and an anti-bonding orbital, two atomic orbitals gives two molecular orbitals. The electrons arrange themselves following, Auf Bau and the Pauli Principle.
• A single atom has one atomic obital
• A triatomic molecule has three molecular orbitals, as before one bonding, one anti-bonding, and in addition one non-bonding orbital.
• Four atomic orbitals give four molecular orbitals.
• Many atoms results in so many closely spaced orbitals that they are effectively continuous and non-quantum. The orbital sets are called bands. In this case the bands are separated by a band gap, and thus the substance is either an insulator or a semi-conductor.
It is already apparent that conduction in polymers is not similar to that of metals and inorganic conductors , however there is more to this story! First we need to imagine a conventional diode system, i.e. PPV sandwiched between an electron injector (or cathode), and an anode. The electron injector needs to inject electrons of sufficient energy to exceed the band gap, the anode operates by removing electrons from the polymer and consequently leaving regions of positive charge called holes. The anode is consequently referred to as the hole injector.
In this model, holes and electrons are referred to as charge carriers, both are free to traverse the PPV chains and as a result will come into contact. It is logical for an electron to fill a hole when the opportunity is presented and they are said to capture one another. The capture of oppositely charged carriers is referred to as recombination. When captured, an electron and a hole form neutral-bound excited states (termed excitons) that quickly decay and produce a photon up to 25% of the time, 75% of the time, decay produces only heat, this is due to the the possible multiplicities of the exciton. The frequency of the photon is tied to the band-gap of the polymer; PPV has a band-gap of 2.2eV, which corresponds to yellow-green light.
Not all conducting polymers fluoresce, polyacetylene, one of the first conducting-polymers to be discovered was found to fluoresce at extremely low levels of intensity. Excitons are still captured and still decay, however they mostly decay to release heat. This is what you may have expected since electrical resistance in most conductors causes the conductor to become hot.
Capture is essential for a current to be sustained. Without capture the charge densities of holes and electrons would build up, quickly preventing any injection of charge carriers. In effect no current would flow.
2.3 COMPARISON TABLE
This table compares the main electronic displays technologies.Each display type is described briefly, and the relative advantages and disadvantages are reviewed. Display Type | Acronym | Emissive or Reflective | Technology | Advantages | Disadvantages |
Cathode Ray Tube | CRT | Emissive | The CRT is a vacuum tube using a hot filament to generate thermo-electrons, electrostatic and/or magnetic fields to focus the electrons into a beam attracted to the high voltage anode which is the phosphor coated screen. Electrons colliding with the phosphor emit luminous radiation. Color CRTs typically use 3 electron sources (guns) to target red, green, and blue patterns on phosphor the screen. | Very bright Wide viewing angle No mask, so no pixel size limitation for mono Minimum pixel size 0.2mm (color) Low cost standard sizes Low cost high-res color Wide operating temperature range Moderate (20khrs+) life | High (5kV to 20kV+) drive voltages Not a flat panel (rare exceptions) Can be fragile, particularly neck-end Heavy Source of X-rays unless screened Affected by magnetic fields Difficult to recycle or dispose of |
Liquid Crystal Display | LCD | Reflective | An LCD uses the properties of liquid crystals in an electric field to guide light from oppositely polarized front and back display plates. The liquid crystal works as a helical director (when the driver presents the correct electric field) to guide the light through 90° from one plate | Small, static, mono panels can be very low cost Both mono and color panels widely available | Backlight adds cost, and often limits the useful life Requires AC drive waveform Fragile unless |
3.1 ADVANTAGES
• Require only 3.3 volts and have lifetime of more than 30,000 hours.
• Low power consumption.
• Self luminous.
• No viewing angle dependence.
• Display fast moving images with optimum clarity.
• Cost much less to manufacture and to run than CRTs because the active material is plastic.
• Can be scaled to any dimension.
• Fast switching speeds that are typical of LEDs.
• No environmental draw backs.
• No power in take when switched off.
• All colours of the visible spectrum are possible by appropriate choose of polymers.
• Simple to use technology than conventional solid state LEDs and lasers.
• Very slim flat panel.
3.2 DISADVANTAGES
• Vulnerable to shorts due to contamination of substrate surface by dust.
• Voltage drops.
• Mechanically fragile.
• Potential not yet realized.
4. APPLICATIONS
Polymer light-emitting diodes (PLED) can easily be processed into large-area thin films using simple and inexpensive technology. They also promise to challenge LCD's as the premiere display technology for wireless phones, pagers, and PDA's with brighter, thinner, lighter, and faster features than the current display.
4.1 PHOTOVOLTAICS
CDT’s PLED technology can be used in reverse, to convert light into electricity. Devices which convert light into electricity are called photovoltaic (PV) devices, and are at the heart of solar cells and light detectors. CDT has an active program to develop efficient solar cells and light detectors using its polymer semiconductor know-how and experience, and has filed several patents in the area.
4.2 POLY LED TV
Philips will demonstrate its first 13-inch PolyLED TV prototype based on polymer OLED (organic light-emitting diode) technology Taking as its reference application the wide-screen 30-inch diagonal display with WXGA (1365x768) resolution, Philips has produced a prototype 13-inch carve-out of this display (resolution 576x324) to demonstrate the feasibility of manufacturing large-screen polymer OLED displays using high-accuracy multi-nozzle, multi-head inkjet printers. The excellent and sparkling image quality of Philips' PolyLED TV prototype illustrates the great potential of this new display technology for TV applications. According to current predictions, a polymer OLED-based TV could be a reality in the next five years.
4.3 BABY MOBILE
This award winning baby mobile uses light weight organic light emitting diodes to realize images and sounds in response to gestures and speech of the infant.
4.4 MP3 PLAYER DISPLAY
Another product on the market taking advantage of a thin form-factor, light-emitting polymer display is the new, compact, MP3 audio player, marketed by GoDot Technology. The unit employs a polymeric light-emitting diode (pLED) display supplied by Delta Optoelectronics, Taiwan , which is made with green Lumation light-emitting polymers furnished by Dow Chemical Co., Midland , Mich.
5. FUTURE DEVELOPMENTS
Here's just a few ideas which build on the versatility of light emitting materials.
High efficiency displays running on low power and economical to manufacture will find many uses in the consumer electronics field. Bright, clear screens filled with information and entertainment data of all sorts may make our lives easier, happier and safer.
Demands for information on the move could drive the development of 'wearable' displays, with interactive features.
Eywith changing information cole woul give many brand ownerve edge
The ability of PLEDs to be fabricated on flexible substrates opens up fascinating possibilities for formable or even fully flexible displays e catching packaging intent at the point of sa d s a valuable competition
FEW MORE DEVELOPMENTS
• Because the plastics can be made in the form of thin films or sheets, they offer a huge range of applications. These include television or computer screens that can be rolled up and tossed in a briefcase, and cheap videophones.
• Clothes made of the polymer and powered by a small battery pack could provide their own cinema show.
• Camouflage, generating an image of its surroundings picked up by a camera would allow its wearer to blend perfectly into the background
• A fully integrated analytical chip that contains an integrated light source and detector could provide powerful point-of-care technology. This would greatly extend the tools available to a doctor and would allow on-the-spot quantitative analysis, eliminating the need for patients to make repeat visits. This would bring forward the start of treatment, lower treatment costs and free up clinician time.
The future is bright for products incorporating PLED displays. Ultra-light, ultra-thin displays, with low power consumption and excellent readability allow product designers a much freer rein. The environmentally conscious will warm to the absence of toxic substances and lower overall material requirements of PLEDs, and it would not be an exaggeration to say that all current display applications could benefit from the introduction of PLED technology. CDT sees PLED technology as being first applied to mobile communications, small and low information content instrumentation, and appliance displays. With the emergence of 3G telecommunications, high quality displays will be critical for handheld devices. PLEDs are ideal for the small display market as they offer vibrant, full-colour displays in a compact, lightweight and flexible form. Within the next few years, PLEDs are expected to make significant inroads into markets currently dominated by the cathode ray tube and LCD display technologies, such as televisions and computer monitors. PLEDs are anticipated as the technology of choice for new products including virtual reality headsets; a wide range of thin, technologies, such as televisions and computer monitors. PLEDs are anticipated as the technology of choice for new products including virtual reality headsets; a wide range of thin, lightweight, full colour portable computing; communications and information management products; and conformable or flexible displays.
6. CONCLUSION
Organic materials are poised as never before to trans form the world of display technology. Major electronic firms such as Philips and pioneer and smaller companies such as Cambridge Display Technology are betting that the future holds tremendous opportunity for low cost and surprisingly high performance offered by organic electronic and opto electronic devices. Using organic light emitting diodes, organic full colour displays may eventually replace LCDs in laptop and even desktop computers. Such displays can be deposited on flexible plastic coils, eliminating fragile and heavy glass substrate used in LCDs and can emit light without the directionality inherent in LCD viewing with efficiencies higher than that can be obtained with incandescent light bulbs.
Organic electronics are already entering commercial world. Multicolor automobile stereo displays are now available from Pioneer Corp., of Tokyo And Royal Philips Electronics, Amserdam is gearing up to produce PLED backlights to be used in LCDs and organic ICs.
The first products using organic displays are already in the market. And while it is always difficult to predict when and what future products will be introduced, many manufactures are working to introduce cell phoned and personal digital assistants with organic displays within the next few years. The ultimate goal of using high efficiency, phosphorescent
flexible organic displays in laptop computers and even for home video applications may be no more than a few years in to the future. The portable and light weight organic displays will soon cover our walls replacing the bulky and power hungry cathode ray tubes.
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