Abstract
As a new display technology, electrowetting display (EWD) has many excellent display characteristics, such as paper-like, low power consumption, quick response and full color. These characteristics make EWD devices very suitable for portable devices. However, the gray-scale distortion caused by the contact angle hysteresis of EWDs seriously affects the accuracy of gray-scale display. To improve this phenomenon, the hysteresis curve of an EWD panel was studied according to the motion characteristics of advancing contact angle and receding contact angle of oil in a pixel. Then, a driving scheme for EWDs using alternating current (AC) voltage instead of direct current (DC) voltage was proposed in this paper. And the advantages and disadvantages of the driving scheme at different AC frequencies from 90 to 2,700 Hz were analyzed through experiments. According to the stability of aperture ratio in EWDs, a 470 Hz AC driving scheme was determined. Experimental results showed that the aperture ratio distortion of EWDs could be reduced from 35.82 to 5.97%, which significantly improved the display performance of pixel units.
Highlights
Electrowetting display (EWD) is a new type of electronic paper display technology with excellent display characteristics: (a) reflective display, paper-like reading experience [1]; (b) response speed of several milliseconds which can meet the needs of video playback [2]; (c) paper-like reflective display mode, with low power consumption [3]; (d) high reflectivity and true color display can be implemented [4]; (e) pixel structure can be attached to flexible backplanes [5]; (f) high degree of overlap with the existing liquid crystal display (LCD) manufacturing process, which means a low manufacturing cost [6]
The influence of contact angle hysteresis on the gray-scale was analyzed when the driving voltage was turned on and off, and an alternating current (AC) voltage driving scheme was proposed, the influence of contact angle hysteresis on display performance could be significantly reduced by the proposed AC driving scheme compared with the traditional direct current (DC) voltage driving scheme
In order to reduce the distortion of aperture ratio and achieve precise control of gray-scales, an AC driving scheme was proposed for improving the performance of EWDs
Summary
Electrowetting display (EWD) is a new type of electronic paper display technology with excellent display characteristics: (a) reflective display, paper-like reading experience [1]; (b) response speed of several milliseconds which can meet the needs of video playback [2]; (c) paper-like reflective display mode, with low power consumption [3]; (d) high reflectivity and true color display can be implemented [4]; (e) pixel structure can be attached to flexible backplanes [5]; (f) high degree of overlap with the existing liquid crystal display (LCD) manufacturing process, which means a low manufacturing cost [6]. Lippmann-Young equation was first proposed by Gabriel Lippmann in 1898, and it is regarded as the theoretical foundation of electrowetting This equation describes the changing process of the contact angle of droplets on the solid surface with an applied voltage. The display is realized by an optical stack It consists of a white substrate, a hydrophobic insulation layer, colored oil, water and an ITO (indium tin oxide) glass. When the voltage is turned off, the force among the colored oil, the hydrophobic insulation layer and the water is balanced At this time, γeo > γso, and contact angle θ < 90. With a same driving voltage value, the difference between the advancing angle and the receding angle is the contact angle hysteresis in EWDs. The aperture ratio is a proportion of the exposed white substrate area in a whole pixel. This transparent grid structure has a small area which can be ignored in calculating the aperture ratio
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