Jiang Chengdian, Tang Biao, Xu Bojian, Groenewold Jan, Zhou Guofu
Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.
Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, China.
Micromachines (Basel). 2020 Jul 20;11(7):702. doi: 10.3390/mi11070702.
A pixel in an electrowetting display (EWD) can be viewed as a confined water/oil two-phase microfluidic system that can be manipulated by applying an electric field. The phenomenon of charge trapping in the protective dielectric and conductivity of the oil phase reduce the effective electric field that is required to keep the three-phase contact line (TCL) in place. This probably leads to an oil-backflow effect which deteriorates the electro-optical performance of EWD devices. In order to investigate charge trapping and conduction effects on the device electro-optical response, an EWD device was studied, which was fabricated with a black oil, aiming for a high-contrast ratio and color-filter display. For comparison, we also prepared a device containing a purple oil, which had a lower electrical conductivity. As anticipated, the black-oil device showed faster backflow than the purple-oil device. A simple model was proposed to explain the role of oil conductivity in the backflow effect. In addition, the rebound and reopening effects were also observed after the voltage was switched to zero. The above observations were strongly dependent on polarity. By combining observations of the polarity dependence of the oil conductivity and assuming that negative charges trap more strongly in the dielectric than positive charges, our experimental results on rebound and reopening can be explained. In the AC optical response, the pixel closing speed decreased in time for intermediate frequencies. This is likely related to the phenomenon of charge trapping. It was also found that the periodic driving method could not suppress the backflow effect when the driving frequency was above ~10 kHz. Our findings confirm the significance of the above charge-related effects of EWD devices, which need to be investigated further for better understanding in order to properly design/use materials and driving schemes to suppress them.
电润湿显示器（EWD）中的一个像素可被视为一个受限的水/油两相微流体系统，该系统可通过施加电场进行操控。保护电介质中的电荷俘获现象以及油相的导电性会降低保持三相接触线（TCL）位置所需的有效电场。这可能会导致油回流效应，从而使EWD器件的电光性能恶化。为了研究电荷俘获和传导对器件电光响应的影响，对一个采用黑油制造的EWD器件进行了研究，其目标是实现高对比度和彩色滤光片显示。为作比较，我们还制备了一个包含紫色油的器件，该紫色油的电导率较低。正如预期的那样，黑油器件显示出比紫色油器件更快的回流速度。提出了一个简单模型来解释油的电导率在回流效应中的作用。此外，在电压切换为零后还观察到了反弹和重新开启效应。上述观察结果强烈依赖于极性。通过结合对油电导率极性依赖性的观察，并假设负电荷在电介质中的俘获比正电荷更强，我们关于反弹和重新开启的实验结果可以得到解释。在交流光学响应中，对于中频，像素关闭速度随时间下降。这可能与电荷俘获现象有关。还发现当驱动频率高于约10 kHz时，周期性驱动方法无法抑制回流效应。我们的研究结果证实了EWD器件上述与电荷相关效应的重要性，为了更好地理解这些效应以便正确设计/使用材料和驱动方案来抑制它们，还需要进一步研究。
