Flat panel display devices are increasingly gaining market acceptance for a variety of different applications. For example, active matrix liquid crystal displays (AMLCD's) have found widespread use as the video monitors in laptop computers, video cameras and avionic navigation modules, to name but a few devices. Other types of display devices such as electroluminescent (EL) and field emissive displays (FED's) are also used in a variety of industrial and consumer applications. The advantage of each of these types of devices resides in the fact that they are all substantially flat, particularly as compared to the cathode ray tube that has been in use for the past fifty years.
In the AMLCD, the elements which cause the device to effect a desired optical characteristic are typically sandwiched between a pair of thin glass plates. These elements include first and second patterned electrodes for applying an electrical field to liquid crystal (LC) material disposed therebetween. Each pair of oppositely disposed patterned electrodes define a single picture element or pixel. The liquid crystal material typically is a conventionally known liquid crystal material, such as cholesteric, polymer dispersed liquid crystal materials, twisted nematic (TN), supertwist nematic (STN), chiral smectic and others. The application of an electrical field to the LC material causes it to change its orientation from a first condition to a second condition, for example, transparent to opaque. However, in order to control the orientation of the liquid crystals, it is necessary to provide numerous other optical elements, such as at least a pair of polarizers, and a plurality of alignment layers. A conventional AMLCD is fully described in, for example, U.S. Pat. Nos. 4,666,252, 4,715,685 and 5,061,040 all to Yaniv, et al., the disclosures of which are incorporated herein by reference. An additional U.S. Pat. No. 4,961,630 to Yaniv and Baron, teach an AMLCD having three electrodes, the third electrode provided to increase device capacitance.
Unfortunately, the construction of conventional AMLCD's and STN based displays generally is that using a twisted mode configuration leads to numerous deficiencies and disadvantages. For example, the need to provide two polarizers for conditioning the optical output substantially lowers the transparency of the device. The result is a darker display or alternatively one requiring a larger, i.e., higher powered backlight Accordingly, the polarizers either result in the need for larger backlights, increasing cost or have poorer color intensity, resulting in diminished display performance. Additionally, at least one, and typically a pair of alignment layers are necessary for purposes of properly orienting the liquid crystal molecules upon the application or removal of the electrical field. However, these layers must be carefully applied in order to achieve perfect orientation. The steps involved in depositing and preparing the alignment layers are difficult, time consuming, and introduce numerous opportunities for defects in the devices. Accordingly, the alignment layer contributes to lower device manufacturing yields and increased device costs.
Additional limitations to conventional AMLCD's resides in the basic characteristics of the LC material. Specifically, upon application of an electric field, the LC molecules will align themselves according to the field, including any effect of the interposed alignment layers, providing a desired optical effect Removal of the electrical field allows the LC molecules to "relax" back to the original state of orientation. However, the speed of relaxation is considerably slower than the speed of orientation in response to the electric field. This phenomenon has severe consequences for high speed operation of AMLCD's, and in particular STN's.
The problems noted above are further exacerbated when the device reaches higher temperatures, as can happen upon prolonged exposure to high intensity backlights. The high temperature dependency also has substantial consequences in terms of the types of application in which such a display may be used. For example, poor high temperature performance eliminates reliable use of AMLCD's in automotive applications. Operation speed is likewise deleteriously effected by lower temperatures, which substantially slow both excitation and relaxation speeds.
An additional deficiency of conventional AMLCD's relates to the relatively poor viewing angles of the devices. By this it is meant that the display appearance, at angles substantially off 90 degrees to the surface of the display, is substantially degraded. This degradation is due to the inherent characteristics of the polarized light emitted from a twisted configuration (IN, STN, etc.) AMLCD in conjunction with the need to interpose polarizers and alignment layers on the device substrates.
Accordingly, there exists a need for a display device which provides the desired changes and control in optical characteristics, while avoiding the problems inherent in conventional LCD's. Such a device should be easier to fabricate, have fewer optical components, and superior optical and electrical efficiency. The devices should also be easily adaptable to conventional semiconductor fabrication techniques, or even better, to screen printing technology, which is both simpler and less costly. The improved device should reduce dependency on polarizers and alignment layers to effect the desired optical performance.