The effect of fluorescence of perylene red dyes on the contrast ratio of LCD color filters
Introduction
Color filters are essential components of liquid crystal displays (LCDs) [1], [2], [3], [4], [5], [6], [7], [8]. The desirable properties of color filters are color purity, contrast ratio, and brightness [9], [10], [11], [12]. High-contrast color filters are the most important components in the manufacture of displays with superior optical properties [9], [10].
Contrast ratio is the proportion of maximum brightness to minimum brightness [6], [9], [10]. To obtain a high contrast ratio, light should not be transmitted when the polarizers are closed to produce a dark image (full-black state) [9]. In traditional pigment-based color filters, a small amount of light is transmitted in the full-black state due to light scattering [5], [6], [10]. The problem, which is caused by the large size of the pigments, can be resolved by replacing pigments with dyes of smaller particle size [2], [4], [10]. However, many dyes that are used as colorants in LCD color filters exhibit fluorescence, which significantly brightens the full-black state of the device due to their emission. Consequently, dye-based color filters may exhibit a lower contrast ratio than expected.
Perylene-based dyes are suitable colorants for LCD color filters due to their strong color strengths and high thermal stabilities [2], [3], [4], [10], [12]. However, perylene derivatives with bulky substituents commonly exhibit strong fluorescence, because these groups can prevent intermolecular interactions and irradiative energy transfers that are the most important mechanisms of fluorescence quenching [13], [14], [15].
This paper will discuss the effect of fluorescence on the contrast ratio of dye-based and pigment-dye hybrid color filters. Perylene-based red dyes were synthesized with bulky substituents at terminal- and bay-positions to improve their solubilities in industrial solvents [2], [10]. Color filters were prepared with low color content to minimize the effect of aggregation on optical properties for the obvious investigation about the influence of the fluorescence on the contrast ratio. The fluorescence of the synthesized dyes in solution and the fabricated dye-based and pigment-dye hybrid color filters were measured and compared. The maximum brightness and the contrast ratio of the color filters were measured, and the minimum brightness was calculated from the results. Finally, the correlation between fluorescence and optical properties of the color filters was analyzed.
Section snippets
Materials and instrumentations
Perylene-3,4,9,10-tetracarboxylic dianhydride, 4-tert-octylphenol, iodine, sulfuric acid, bromine and acetic acid purchased from Sigma–Aldrich, 2,6-diisopropylaniline, 2,4,6-trimethylphenol and 2-allylphenol purchased from TCI, K2CO3 anhydrous, methylene chloride and other chemical solvents purchased from SAMCHUN pure chemical were used without any additional purification. Transparent glass substrates were purchased from Paul Marienfeld GmbH & CO. KG.
1H NMR and 13C NMR spectra were recorded by
Design concept and synthesis of the dyes
Perylene-based dyes have many advantages as colorants in optical devices including high color strength and thermal stability [2], [4], [10], [12]. However, their applications are limited by low solubility in industrial solvents due to their planar molecular structures [10], [12].
The synthetic routes and molecular structures of the synthesized dyes are shown in Scheme 1 and Fig. 1. All dyes are designed to have increased solubility in organic solvents. Un-substituted perylene dyes are not
Conclusion
To investigate the effect of fluorescence on the contrast ratio of color filters, six perylene-based dyes were synthesized, and dye-based and pigment-dye hybrid color filters were prepared from them. The fluorescence properties of the synthesized dyes in solution and the fabricated color filters were measured and compared to one another. The total visible-range emission intensity of the synthesized dyes in solution and the prepared color filters showed the same trend regardless of their states.
Acknowledgments
This work was supported by the Advanced Technology Center Program (No.10039098, Development of pigment-dye hybrid type of color mill base with ultra-brightness for LED TV) funded by the Small and Medium Business Administration.
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