Appendix A Flat Panel Displays In the last years, the technological advances enabled flat panel displays (FPDs) to be introduced in a wide range of applications and devices and have enabled applications otherwise impossible, which are limited only by the fantasy of the designer. A.1 Technologies and Classification Other than the LCD, the subject of this book, many other types of flat display technologies have been devised such as: PDP (Plasma Display Panel), LED (Light Emitting Diode) OLED (Organic LED), VFD (Vacuum Fluorescent Display), TFELD (Thin Film Electroluminescent Display), FED (Field Emission Display), DMD (Digital Micro-mirror Machine), PALC (Plasma Addressed LCD), Bistable Displays (including the Electrophoretic and the Nematic Bistable). According to the application and price-to-performance ratio, the most suitable technology has to be chosen. In the following we shall briefly discuss the display technologies that have reached a sufficient development and commercial application. In such a multitude of applications and display types, it is useful to operate a first classification in: Direct view displays and Projection displays. In the first case the name is self explaining, in the second one the image can be projected on a screen either for home/office application or car/airplane application. The image can be also projected on the human eye retina by microdisplays. Then, a second classification, related to the display technologies, must be performed between: Emissive displays and Nonemissive displays In the first case, the device emits light from each pixel. In the second case, the device modulates light (from a light source) by means of absorption, reflection, refraction, and/or scattering mechanisms. Examples of emissive displays are: PDPs, LEDs, OLEDs, FEDs, VFDs (and CRTs) . As light modulator display we mention: LCDs and the Bistable Displays. 268 Appendix A: Flat Panel Displays Generally speaking the choice between light modulator and emissive display depends upon the ambient light. The emissive displays have both wide viewing angle and high color saturation but, despite the high brightness, in case of portable equipments for outdoor application they could not compete against the 120k cd/m2 of the sun light, originating the so called “wash out” phenomena. A.2 Emissive Displays Emissive displays do not require backlighting, thereby allowing a substantial reduction of power consumption. The process of luminescence, responsible of the light emission, is due to the electron/hole recombination. Different mechanisms generating the electron/hole couple can be found in emissive displays. If the cause is an UV wavelength irradiating a phosphor layer, the phenomenon is called “photoluminescence” or “phosphor-luminescence”. If the electron/hole couple is generated by a p-n junction, it is called “electroluminescence”. Finally, it is called “cathode luminescence” if the generation is caused by an electron beam. A.2.1 LEDs Light-emitting diodes are made up of crystalline semiconductors. Since the dimension of a single LED pixel is quite large (several millimeters), LEDs are used for large displays for outdoor signaling (e.g., traffic signals) with up to 100-in. and millions of LED pixels. Other than for displays, LEDs are used to replace the cold cathode fluorescent lamps (CCFL) as backlight modules for LCDs and for general lighting. A.2.2 OLEDs Among the emissive displays, the OLED technology has been the most studied initially at university level, then in company research laboratories, and is finding now more and more applications. The electroluminescence of organic materials was discovered by Ching Tang in 1979, a researcher of Eastman Kodak, who was experimenting new low cost solar cells, and, irony of the risen ones, he got the opposite: his organic material was able to convert electricity in light emission with a quantum efficiency as high as never seen for an organic material. Important development steps have been: the invention of small molecules heterostructure OLED at Eastman Kodak [TV1987] and in 1990 the conjugated polymers OLED by J.H. Burroughes of CDT, Cambridge Display Technologies [BBB1990]. At the beginning many issues had to be solved. Among them, life of organic layers was very short (indeed the emission was drastically reduced as a function of time), high current consumption, not enough quantum efficiency. After many years of studies and starting from low-end applications (e.g. displays for electric shavers), the strong potentiality of this technology, mainly in terms of color saturation and viewing angle, is giving the expected results. Passive-matrix OLED displays were the first to be marketed. In 2005, Samsung demonstrated a 40-in. prototype OLED TV. Sony put in commerce the world’s first OLED TV, the XEL-1, in December 2007. In 2008, Samsung released the SDI Liquid Crystal Display Drivers 269 AMOLED (Active Matrix OLED, 31-in. diagonal with 1980×1080 pixel resolution) and Sony released an 11-in. OLED TV of 70 000 hours life. The introduction of Active Matrix is further boosting the performance in terms of image quality and power consumption reduction. There is a growing confidence that OLED Display developers are mastering the technology, and it is time to compete with LCD technology, by leveraging on creative design. OLED displays are expected to compete with the LCD in practically all current applications. See the related “Further Reading” section at the end of this appendix for more details. As far as the driving technique is concerned, the substantial difference between LCDs and OLEDs is that the former are voltage driven and the latter are current driven. Figure A.1 shows the simplest circuit diagram of an Active Matrix OLED pixel with the pass transistor M1 (enabled by the Gate line) and the current-source transistor M2 that drives the OLED with the appropriate current. The hold effect between two subsequent addressing is performed by capacitor CS that keeps the gate-source voltage of M2 constant. The main problem of this scheme is due to the threshold voltage mismatches in the current source transistors that cause brightness variations for the same source voltage. At this purpose, more efficient driving schemes have been developed, see for instance [LEE2002, YYX2003]. Source line VDD M1 M2 Gate line CS OLED Fig. A.1 Schematic of an AMOLED pixel with addressing circuit A.2.3 Plasma Displays The photoluminescence is obtained by high voltage drive applied on a small cavity, as small as a RGB sub-pixel dimension, containing a gas at low pressure (neon, xenon). The gas discharge generates UV irradiation exciting the sub-pixel phosphorus layer. Therefore, the sub-pixel cell cannot be too much small, and the glass making the cell too much thin, otherwise cannot withstand with atmospheric pressure. First market applications, display banking and military, were introduced in 1985 and in 1993 it was developed a three electrode with barrier rib isolation for color displays [SWN1993]. The first color PDP was introduced by Fujitsu in 1992 [UH2003]. The high cost of electronic drive (because of the high voltage) and a relatively large sub-pixel dimension, are characteristics compatible to large size panel for 270 Appendix A: Flat Panel Displays public display or home theater, where the diagonal is equal or larger than 42 in. In December 2004, Samsung demonstrated the first 102 in. PDP display with a 1920×1080 pixel resolution [S2005]. In 2008, Panasonic demonstrated a 150-in. PDP TV (4096×2160 pixel resolution). A.3 Nonemissive Displays Apart form LCDs the most important categories of light modulator displays are: - DMDs (high performance for projection display) - Bistable Displays (new developments also on plastic substrate) A.3.1 DMDs The DMD (digital micro mirror machine) was developed by Texas Instruments in 1987, even if the idea of such a MEM (microelectronic machine) was conceived since the beginning of the seventies. The principle is based on a metallic micro- mirror that, according to its position, can efficiently reflect (or not) the light. Projectors have been designed and sold on the market since 1996. In 2008 the company Microvision Inc. has showed a projector prototype using three lasers as light source with a MEM scanning mirror imager to achieve a WVGA resolution. A.3.2 Bistable Displays The multiplex driving of PMLCD has the disadvantage to decrease the contrast ratio because of the phenomena known as “frame response”. The AM technique is an artifact to transform a monostable display (the stable state is without electrical field applied) into a bistable display (“dark” and “bright” states are both stable, and electrical pulse is needed only to change the stable state). A bistable display has therefore the great advantages: - to give an optical stable image, without any flicker, cross talk and with contrast ratio not degraded by the frame response - to consume electrical power only when changing the state Several bistable technologies have been developed: - Electrophoretic [OOY1973], and its evolution to micro-encapsulation electrophoretic by E-Ink Corporation and NOK [DCA1998, NKK1998]. - Cholesteric Liquid Crystal [GWK1973, YWC1994]. - Surface Stabilized Ferroelectric LC (SSFLC), [CL1980]. - Bulk Bistable Twisted Nematic (360° BTN), [BH1980]. - Surface Nematic Bistable: G. Durand of Paris Orsay University (1988), a student of the Nobel Prize professor DeGennes. Further developed by G. Durand with R. Barbieri and M. Giocondo in 1991 and 1992, University of Catanzaro [BD1991, BGD1992]. - Surface Controlled Bistable Nematic (BiNem) by I. Dozov, M. Nobili, G. Durand, Martinot-Lagarde (1997) of Nemoptic [DND1997, DMP1997]. - Zenithal Bistable Display (ZBD): G.P. Bryan-Brown (1997) [BBJ1997, WBB2000]. Liquid Crystal Display Drivers 271 In the case of Cholesteric LCDs, the planar texture reflects strongly one of the circular polarizations (white state, Fig. A.2a), whilst focal conic texture (Fig A.2b), consisting of small domains separated by defect lines, transmits the light and gives a dark state due to the light absorption on the back plate LIGHT (a) (b) Fig.