Display technology has gradually but profoundly shaped the lifestyle of human beings. Liquid crystal display (LCD) and organic light-emitting diode (OLED) display are two mainstream displays in the market due to their high-quality performance, which are further being developed to be an ultra-thin, transparent, flexible and even foldable display. An alignment film is an indispensable part in LCDs as well as recently emerging transparent display, and deeply influence their contrast ratio (CR) and light efficiency. Alignment thin film used for LC molecules orientation should be characterized by high aligning order, non-contaminated film and robust photo-tolerance. The employment of an ultra-thin polarizing film is also significant in rigid or flexible OLED display to suppress the...[ Read more ]

Display technology has gradually but profoundly shaped the lifestyle of human beings. Liquid crystal display (LCD) and organic light-emitting diode (OLED) display are two mainstream displays in the market due to their high-quality performance, which are further being developed to be an ultra-thin, transparent, flexible and even foldable display. An alignment film is an indispensable part in LCDs as well as recently emerging transparent display, and deeply influence their contrast ratio (CR) and light efficiency. Alignment thin film used for LC molecules orientation should be characterized by high aligning order, non-contaminated film and robust photo-tolerance. The employment of an ultra-thin polarizing film is also significant in rigid or flexible OLED display to suppress the reflection from cathode layer so as to improve the CR and outdoor readability. Therefore, development on high-performance thin films either for LC alignment or flexible OLED display are highly demanded.

Firstly, polyimide (PI) is a dominating material for LC alignment and is widely used in display devices, one of which is smart window. In general, smart windows are developed either by optimizing the scattering composites or by designing an appropriate electrode pattern, while little investigation has been carried out on the alignment layer. Here, an inhomogeneous vertical alignment thin film with micro-domain structure was proposed and employed on a normally transparent smart window. This inhomogeneity morphology on the thin film can considerably enhance the power-on haziness while provide an ultra-transparent power-off state. In addition, the performance on off-axis viewing, response speed and power consumption are much better than commercialized polymer-dispersed liquid crystal (PDLC) smart window. The proposed smart window can serve as color transparent display using edge-light wave-guiding.

In addition to employing vertical polyimide thin film, we can use photoalignment material, azo-dye SD1, to develop a transmittance tunable smart window based on two axis-variable polarizers. Azo-dye SD1 is a very promising material for LC alignment, which gives high order and strong anchoring to LC after absorb a small amount of light dosage (i.e., 150 mJ/cm²). Making use of this advantage, continuously axis-variable alignment thin film was achieved, on top of which a liquid crystal polymer/dichroic dye composited was coated and then photo-polymerized to form a rigid axis-variable polarizer. By horizontally shifting one polarizer relative to the other, varying transmittance with good uniformity is obtained. The tunability on transmittance can be realized by a simple roller instead of conventional electrical-driving.

Indeed, the photo-sensitive azo-dye, SD1, does benefit vast varieties of devices, but this azo-dye has to be photo-stabilized when it is used in display panels, such as smartphone. A long-term exposure to the ambient light and the backlight will inevitably destroy the alignment quality so as to deteriorate the contrast ratio. Therefore, a method to stabilize azo-dye while maintain its original alignment performance is needed. Here, we introduced a polymer network into the azo-dye thin film. By thermally cross-linking the polymer network, the azo-dye thin film showed strong robustness against white light and heat. The electro-optical performance was even better than that of pure azo-dye thin film. A high CR (i.e., 2200) and an ultra-low pretilt angle (i.e., 0.025°) achieved by the stabilized film is extremely suitable for in-plane switching (IPS) display, a display mode currently widely used in smartphone.

What’s more, photoalignment thin film can even be developed as a polarizer. Another azo-dye AD-1, due to its ultra-high dichroism (i.e., 70~100) and ultra-thin profile (i.e., 150~300nm), is quite prospective as a thin-film polarizer for flexible display. Also, this azo-dye needs to be stabilized. The AD-1 film after stabilization showed quite fascinating performance: the film not only showed strong resistance to intense white light but also processed an extended absorbance over the entire visible region. A dichroic ratio up to 34 and corresponding polarization efficiency up to 99.5% was achieved by this 200nm-thick azo-dye polarizer. When combined with λ/4 waveplate, the reflection of OLED panel can be greatly suppressed and less than 1%.