Novel Liquid Crystal Displays Based on Highly Polarized Photoluminescent Polymer Films
- PDF / 869,699 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 81 Downloads / 188 Views
ABSTRACT Since the early 90's, much research has focused on the photoluminescence (PL) and electroluminescence (EL) properties of conjugated polymers, because of their potential application as emitting layer in EL devices. The introduction of uniaxial molecular orientation into films of luminescent polymers was naturally found to yield structures that emit polarized light. Rather surprisingly, the photoluminescence properties of oriented, conjugated polymers have attracted substantially less attention, especially from an application point of view. In this paper we report the fabrication of highly-polarized photoluminescent polymer films based on poly(2,5dialkoxy-p-phenyleneethynylene)s (PPE), and their use in a new family of liquid crystal displays (LCDs). As one relevant example, a back-lit twisted-nematic configuration of an LCD was built, in which one of the absorbing polarizers was replaced by a polarized PL film, characterized by a dichroic ratio in excess of 70. Such devices can exhibit a substantial improvement in brightness, contrast and viewing angle, since the polarized photoluminescent films can combine two separate features, i.e. the functions of a polarizer and an efficient color filter. INTRODUCTION Liquid crystal displays, which currently represent the dominant flat panel display technology, exhibit severe limitations in brightness and energy efficiency caused by the use of absorbing polarizers and color filters which convert a major amount of the incident light into thermal energy.' The use of photoluminescent materials which act as 'active' color filters and, therefore, might enhance the visual performance of LCDs, was previously suggested. 24 Several principal possibilities exist to incorporate PL materials into LCDs, including the use of2 fluorescent liquid crystals or the dispersion of luminescent molecules in a conventional LC layer, the application of PL plates 3 or front-face screens. 4 However, the proposed devices suffer from a number of drawbacks, related to limited stability, the difficulty to produce pixilated devices, 3 depolarization effects, or the required thickness and large area of the luminescent layer. Here, we introduce a new concept for the design of photoluminescent LCDs which are based on photoluminescent polarizers. The latter were recently demonstrated to combine two separate5 features: the linear polarization of light and the efficient generation of bright color. Photoluminescent polarizers rely on the fact that macroscopically uniaxially oriented photoluminescent matter usually also exhibits anisotropic, i.e. linearly polarized, absorption and emission. This phenomenon has been known for inorganic crystals for more than a century 6 and7 recently was also reported for uniaxially oriented films of conjugated, luminescent polymers. We suggest that the use of such polarized PL films in liquid-crystal-based photoluminescent display devices may lead not only to a simplification in device design, but also to a substantial increase in device brightness and efficiency, and, in add
Data Loading...
