Advanced Blue Phase Liquid Crystal Displays

Thin-film transistor (TFT) liquid crystal displays (LCDs) have become indispensable in our daily lives. Their widespread applications range from smartphones, laptops, TVs to navigational devices, data projectors and wearable displays. Over past decades, massive efforts have been invested in device development, material characterization and manufacturing technology. As a result, the performance of LCDs, such as viewing angle, contrast ratio, color gamut and resolution, have been improved significantly. Nonetheless, there are still urgent needs for fast response time and low power consumption. Fast response time helps reduce motion image blurs and enable color sequential displays. The latter is particularly attractive since it eliminates spatial color filters, which in turn triples optical efficiency and resolution density. The power consumption can be reduced greatly by using color sequential displays, but liquid crystals with submillisecond response time are required to minimize color breakup. The state-of-the-art gray-to-gray response time of nematic LCDs is about 5ms, which is too slow to meet this requirement. With the urgent needs for submillisecond response time, polymer-stabilized blue phase liquid crystal is emerging as a strong candidate for achieving this goal. Compared to conventional nematic LCDs, blue phase LCDs exhibit several revolutionary features: submillisecond gray-to-gray response time, no need for alignment layer, optically isotropic voltage-off state, and large cell gap tolerance. However, some bottlenecks such as high operation voltage, low optical transmittance, noticeable hysteresis and slow TFT charging remain to be overcome before their widespread applications can be realized. This dissertation is dedicated to addressing these challenges from material development and device design viewpoints. First, we started to investigate the device physics of blue phase LCDs. We have built a numerical model based on the refraction effect for simulating the electro-optics of blue phase devices. The model well agrees with experimental data. Based on this model, we explored approaches from device and material viewpoints to achieve low operation voltage. On the device side, with protrusion and etched electrodes, we can reduce the operating voltage to below 10V and enhance the transmittance to over 80%. On the material side, high Kerr constant is indeed helpful for lowering the operation voltage, but we also need to pay attention to the individual ?n and ?? values of liquid crystal host according to the device structures employed. High-?? LC hosts help enhance Kerr constant, leading to a reduced operation voltage; but they may be subject to serious capacitance charging issues due to the huge dielectric anisotropy. Our model provides important guidelines for future device design and material development. To further enhance transmittance and reduce voltage, we have proposed a Z-shaped electrode structure. By optimizing the device structure, we have successfully reduced the operating voltage to ~8V and enhanced optical transmittance to \u3e 95% based on a lower-?? LC host not subjecting to charging issues, showing comparable or even better performance than the mainstream LCDs. This is the first approach to achieve such a high transmittance in blue phase devices without using a directional backlight. By using zigzag structure, the color shift and gray inversion are in unnoticeable range. In addition, hysteresis affects the accuracy of grayscale control and should be suppressed. We have proposed a double exponential model to analyze the electric field effects of blue phase, and found that electrostriction effect is the root cause for hysteresis under strong electric field. To suppress the electrostriction effect in blue phase, a method to stabilize the blue phase lattice via linear photo-polymerization is demonstrated for the first time. By illuminating the mono-functional and the di-functional monomers with a linearly polarized UV beam, we can form anisotropic polymer networks, which in turn lead to anisotropic electrostrictions. In experiments, we found that when the polarization of UV light is perpendicular to the stripe electrodes, the electrostriction effect can be strongly suppressed. The resulting hysteresis is reduced from 6.95% to 0.36% and response time is improved by a factor of two. We foresee this approach will guide future manufacturing process. The approaches and studies presented in this dissertation are expected to advance the blue phase LCDs to a new level and accelerate their emergence as next-generation display technology. It is foreseeable that the widespread application of blue phase LCDs is around the corner