Charge trapping model for investigating oil backflow mechanism in electrowetting display pixels

Electrowetting displays (EWDs) have shown great success owing to their advantages of quick response, wide color gamut, low power consumption, and high contrast. However, the presence of charge trapping effect in EWDs' hydrophobic insulating layer can give rise to phenomena such as oil backflow and contact angle saturation. The maximum aperture ratio of the pixel and the grayscale stability are limited by these phenomena. Therefore, the charge trapping behavior in a pixel was investigated. Firstly, an equivalent model of the pixel was constructed based on the principle of electrowetting-on-dielectric, and the quantitative correlation between the aperture of the pixel and the potential of trapped charges was investigated. Secondly, a method was developed to calculate the potential of trapped charges, the surface charge density in water phase, and the electrowetting force by measuring the aperture ratio of the pixel. Finally, the influence of trapped charges on the surface charge density and electrowetting force was analyzed to investigate the mechanism of oil backflow phenomenon. The experimental results indicated that the potential of trapped charges could be approximated as a linear function of the driving time during 120-second direct current (DC) driving. When a higher driving voltage was applied, the average growth rate of the potential of trapped charges was greater. As the driving voltage increased to 30 V, the average growth rate of the potential of trapped charges could reach 0.053 V/s.