Fenglin Peng

High performance liquid crystal displays with a low dielectric constant material

We report high performance liquid crystal displays (LCDs), including fringe field switching (p-FFS) and in-plane switching (p-IPS), with a small average dielectric constant (e) but positive dielectric anisotropy material. Our low e based p-FFS and p-IPS LCDs offer several attractive properties, such as high transmittance, low operation voltage, fast response time (even at −20°C), which is particularly desirable for outdoor applications of mobile or wearable display devices, and suppressed flexoelectric effect. Combining these advantages with the inherent outstanding features, such as wide viewing angle, no grayscale inversion, negligible color shift, and pressure resistance, the low e LC based p-FFS and p-IPS are strong contenders for next-generation mobile displays, and high resolution and high frame rate TVs.

Fringe-Field Switching with a Negative Dielectric Anisotropy Liquid Crystal

We report a high performance negative dielectric anisotropy (Δε) liquid crystal for fringing field switching (n-FFS) display. We compare the electro-optic characteristics of FFS cells using positive and negative Δε LCs. With comparable driving voltage and response time, the n-FFS cell has advantages in higher transmittance, single gamma curve, less cell gap sensitivity and slightly wider viewing angle. LC director deformation distribution is analyzed to explain these performance differences.

Ultra-low viscosity liquid crystal materials

We report five ultra-low viscosity nematic liquid crystal mixtures with birefringence around 0.1, dielectric anisotropy in the range of 3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity mixtures is low activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm cell at −20°C is only 30 ms. Widespread application of these materials for display devices demanding a fast response time, especially at low temperatures, is foreseeable.

Electro-optic response of polymer-stabilized blue phase liquid crystals

The dynamic response of a polymer-stabilized blue phase liquid crystal (BPLC) is comprised of two distinct processes: Kerr effect-induced local reorientation and electrostriction-induced lattice distortion. A double exponential rise/decay model is proposed to analyze the underlying physical mechanisms. If the electric field is below a critical field (Ec), Kerr effect dominates and the response time is fast. However, when E > Ec electrostriction effect manifests, leading to an increased response time and a noticeable hysteresis. A higher polymer concentration helps suppress electrostriction, but the tradeoff is increased operation voltage. These results provide useful guidelines for future BPLC material and device optimizations.

Image sticking in liquid crystal displays with lateral electric fields

We propose a kinetic model to account for the nonuniform adsorption and desorption processes in fringe field switching (FFS) and in-plane-switching liquid crystal displays. An equation is proposed to describe the generation mechanism of residual DC voltage and good agreements with experiment are obtained. Based on this model, the mechanisms underlying the formation and relaxation processes of residual DC voltage as well as their dependences on offset DC voltage and temperature are investigated. Moreover, the residual DC voltages of FFS cells employing positive and negative dielectric anisotropy LCs are compared and the physics responsible for the observed difference is explained.

Fast-response liquid crystals for high image quality wearable displays

We demonstrate two ultra-low viscosity liquid crystal mixtures to enable field-sequential-color wearable displays for low temperature operation, while keeping a wide color gamut. Our mixtures offer ~4X faster response time than a commercial material at 20°C and ~8X faster at −20°C. Other major attractive features include: (1) submillisecond response time at room temperature and vivid color even at −20°C without a heating device, (2) high brightness and excellent ambient contrast ratio, and (3) suppressed color breakup with 360Hz frame rate.

Low loss liquid crystals for infrared applications

Seven fluorinated and chlorinated terphenyl compounds intended for mid-wave infrared (MWIR) applications are synthesised and two eutectic mixtures formulated, and their physical properties evaluated. In addition to low absorption, some desirable properties for MWIR applications include wide nematic range, high birefringence, large dielectric anisotropy and low viscosity. The fluorinated terphenyl mixture exhibits a relatively low absorption in the vicinities of λ ~ 3 μm, but a fairly strong overtone appears in the 4–5 μm region. To suppress these overtone absorptions, short-chain chlorinated terphenyl compounds are proven to be useful.

Fast-response infrared phase modulator based on polymer network liquid crystal

We report a high birefringence terphenyl liquid crystal mixture, designated as M3, for infrared phase modulation with special emphasis on mid-wave infrared (MWIR). In addition to high birefringence, M3 exhibits excellent UV stability, modest dielectric anisotropy, and a very broad nematic range. The high birefringence enables a thin cell gap to be used for achieving a 2π phase change while maintaining a high transmittance (T>98%) in the MWIR region. To achieve fast response time, we employed a polymer network liquid crystal using M3 with 2π phase change at λ = 4μm and 3.6-ms response time. This response time is about 100X faster than that of a nematic LC phase modulator.

Low temperature and high frequency effects on polymer-stabilized blue phase liquid crystals with large dielectric anisotropy

We report the low temperature and high frequency effects on polymer-stabilized blue phase liquid crystals (BPLCs) comprising of a large dielectric anisotropy nematic host. Debye dielectric relaxation sets a practical limit even when the device operation temperature is still within the blue phase range. To explain these phenomena, we propose a model to describe the temperature and frequency dependent Kerr constant and obtain excellent agreement with experiment. Doping a diluter compound to the BPLC host helps to reduce viscosity, which in turn boosts the dielectric relaxation frequency and extends the low temperature operation range.

Low voltage polymer network liquid crystal for infrared spatial light modulators.

We report a low-voltage and fast-response polymer network liquid crystal (PNLC) infrared phase modulator. To optimize device performance, we propose a physical model to understand the curing temperature effect on average domain size. Good agreement between model and experiment is obtained. By optimizing the UV curing temperature and employing a large dielectric anisotropy LC host, we have lowered the 2π phase change voltage to 22.8V at 1.55μm wavelength while keeping response time at about 1 ms. Widespread application of such a PNLC integrated into a high resolution liquid-crystal-on-silicon (LCoS) for infrared spatial light modulator is foreseeable.