05-03-2011, 11:44 AM
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1.1 INTRODUCTION
Light emitting polymers (LEPs) or polymer-based light-emitting diodes discovered by Friend et al in 1990 have been found better than other displays like liquid crystal displays (LCDs), vacuum fluorescence displays, and electroluminescence displays. Though not commercialized yet, these have proved to be a milestone in the field of Flat Panel Displays (FPDs). Research on LEP is underway in Cambridge Display Technology Ltd, CDT, Cambridge, UK. The Cathode Ray Tube (CRT), invented by German physicist Karl Ferdinand Braun in 1897, remained the ubiquitous display in the last half of the 20th century. But the CRT’s long heritage in an environment where product life cycles are measured in months rather than years doesn’t mean that it is an ideal display solution. It is bulky, power hungry and expensive to manufacture.
The fact is that researchers haven’t come up with a better solution. Liquid Crystal Display (LCD) was pitched as the savior of the display industry. Its creators claimed that a slim profile would quickly make it the display of choice. But today, LCDs are far more pervasive. These offer a little bit benefit over their predecessor, the CRT. The cost of a LCD as well as a CRT monitor one-third of the total price of a computer. Says David Mentley, Vice President and display industry analyst at Stanford Resources, California, USA, “Although LCD is a highly successful technical achievement, the manufacturing archetype must change if flat panel displays are to compete directly across all applications.” In the last decade, several other contenders, such as Plasma and field emission displays were hailed as the solution to the pervasive display. Like LCD, they suited certain niche applications, but failed to meet the broad demands of the computer industry.
What if a new type of display could combine the characteristics of a CRT with the performance of an LCD and the added design benefits of formability and low power? Cambridge Display Technology Ltd (CDT) [1] is developing a display medium with exactly these characteristics. The technology uses a light emitting polymer (LEP)[2] that costs much less to manufacture and run than CRTs[3] because the active material is plastic.
1.2 Light Emitting Polymer (LEP)
It is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film semiconducting polymer sandwiched between two electrodes (anode and cathode). 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. the energy difference between valence band and conduction band, of the semiconducting polymer determines the wavelength (colour) of the emitted light.
The first polymer LEPs used poly phinylene vinylene (PPV) as the emitting layer. Since 1990, a number of polymers have been shown to emit light under the application of an electric field; the property is called the electroluminescence (EL).Efforts are on to improve the efficiency of polymer devices by modifying their configuration.
1.3 CHEMISTRY
LEPs are constructed from a special class of polymers called conjugated polymers. Plastic materials with metallic and semiconductor characteristics are called conjugated polymers. These polymers possess delocalized[4] pi electrons along the backbone, whose mobility shows properties of semiconductors. Also this gives it the ability to support positive and negative charge carriers with high mobility along the polymer chain. The charge transport mechanism in conjugated polymers is different from traditional inorganic semiconductors. The amorphous chain morphology results in inhomogeneous broadening[5] of the energies of the chain segments and leads to hopping type transport. Conjugated polymers have already found applications in battery electrodes, transparent conductive coatings, capacitor electrolytes and through hole platting in PCBs[6]. There are fast displaying traditional materials such as natural polymers etc owing to better physical and mechanical properties and amenability to various processes.
2.1 BASIC STRUCTURE
Like the CRT, LEP emits light as a function of its electrical operation. An LEP display solely consists of the polymer material manufactured on a substrate of glass or plastic and doesn’t require additional elements like the backlights, filters, and polarisers that are typical of LCDs. Fig. shows the structure of an LEP device. The indium-tin oxide (ITO) coated glass is coated with a polymer. On the top of it, there is a metal electrode of Al, Li, Mg, or Ag. When a bias voltage is applied, holes and electrons move into the polymer. These moving holes and electrons combine together to form hole-electron pairs known as ‘excitons’. These excitons are in excited state and go back to their initial state by emitting energy. When this energy drop occurs, light comes out from the device.
2.2 WORKING
Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxide is typically used for the anode and aluminium or calcium for the cathode. Fig.3 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. Also it shows a symmetrically configured alternating current light-emitting (SCALE) device that works under AC as well as forward and reverse DC bias..
2.2.1 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 4).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.
1.1 INTRODUCTION
Light emitting polymers (LEPs) or polymer-based light-emitting diodes discovered by Friend et al in 1990 have been found better than other displays like liquid crystal displays (LCDs), vacuum fluorescence displays, and electroluminescence displays. Though not commercialized yet, these have proved to be a milestone in the field of Flat Panel Displays (FPDs). Research on LEP is underway in Cambridge Display Technology Ltd, CDT, Cambridge, UK. The Cathode Ray Tube (CRT), invented by German physicist Karl Ferdinand Braun in 1897, remained the ubiquitous display in the last half of the 20th century. But the CRT’s long heritage in an environment where product life cycles are measured in months rather than years doesn’t mean that it is an ideal display solution. It is bulky, power hungry and expensive to manufacture.
The fact is that researchers haven’t come up with a better solution. Liquid Crystal Display (LCD) was pitched as the savior of the display industry. Its creators claimed that a slim profile would quickly make it the display of choice. But today, LCDs are far more pervasive. These offer a little bit benefit over their predecessor, the CRT. The cost of a LCD as well as a CRT monitor one-third of the total price of a computer. Says David Mentley, Vice President and display industry analyst at Stanford Resources, California, USA, “Although LCD is a highly successful technical achievement, the manufacturing archetype must change if flat panel displays are to compete directly across all applications.” In the last decade, several other contenders, such as Plasma and field emission displays were hailed as the solution to the pervasive display. Like LCD, they suited certain niche applications, but failed to meet the broad demands of the computer industry.
What if a new type of display could combine the characteristics of a CRT with the performance of an LCD and the added design benefits of formability and low power? Cambridge Display Technology Ltd (CDT) [1] is developing a display medium with exactly these characteristics. The technology uses a light emitting polymer (LEP)[2] that costs much less to manufacture and run than CRTs[3] because the active material is plastic.
1.2 Light Emitting Polymer (LEP)
It is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film semiconducting polymer sandwiched between two electrodes (anode and cathode). 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. the energy difference between valence band and conduction band, of the semiconducting polymer determines the wavelength (colour) of the emitted light.
The first polymer LEPs used poly phinylene vinylene (PPV) as the emitting layer. Since 1990, a number of polymers have been shown to emit light under the application of an electric field; the property is called the electroluminescence (EL).Efforts are on to improve the efficiency of polymer devices by modifying their configuration.
1.3 CHEMISTRY
LEPs are constructed from a special class of polymers called conjugated polymers. Plastic materials with metallic and semiconductor characteristics are called conjugated polymers. These polymers possess delocalized[4] pi electrons along the backbone, whose mobility shows properties of semiconductors. Also this gives it the ability to support positive and negative charge carriers with high mobility along the polymer chain. The charge transport mechanism in conjugated polymers is different from traditional inorganic semiconductors. The amorphous chain morphology results in inhomogeneous broadening[5] of the energies of the chain segments and leads to hopping type transport. Conjugated polymers have already found applications in battery electrodes, transparent conductive coatings, capacitor electrolytes and through hole platting in PCBs[6]. There are fast displaying traditional materials such as natural polymers etc owing to better physical and mechanical properties and amenability to various processes.
2.1 BASIC STRUCTURE
Like the CRT, LEP emits light as a function of its electrical operation. An LEP display solely consists of the polymer material manufactured on a substrate of glass or plastic and doesn’t require additional elements like the backlights, filters, and polarisers that are typical of LCDs. Fig. shows the structure of an LEP device. The indium-tin oxide (ITO) coated glass is coated with a polymer. On the top of it, there is a metal electrode of Al, Li, Mg, or Ag. When a bias voltage is applied, holes and electrons move into the polymer. These moving holes and electrons combine together to form hole-electron pairs known as ‘excitons’. These excitons are in excited state and go back to their initial state by emitting energy. When this energy drop occurs, light comes out from the device.
2.2 WORKING
Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxide is typically used for the anode and aluminium or calcium for the cathode. Fig.3 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. Also it shows a symmetrically configured alternating current light-emitting (SCALE) device that works under AC as well as forward and reverse DC bias..
2.2.1 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 4).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.
