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Illumination & Displays

Novel single-cell-gap transflective LCDs with photoalignment

A single display mode employs different pretilt angles to deliver matched voltage driving for both transmission and reflection.
24 November 2009, SPIE Newsroom. DOI: 10.1117/2.1200911.1849

Transflective LCDs have extensive application in mobile devices such as cell phones, palmtop computers, and camcorders because of their good readability in both indoor and outdoor environments. In these displays, each pixel is divided into two regions (subpixels). One region is reflective (R) and operates in intense ambient light, and the other is transmissive (T) and functions under dark illumination. The idea is that the two subpixels should exhibit matched electro-optical properties, thus requiring only one driving circuit to generate identical gray levels.

The electro-optical properties of an LCD are governed by the phase retardation of an LC pixel, δ=Δn×d, where Δn and d are the birefringence (double refraction) and thickness of the pixel, respectively. When the phase-retardation ratio of the T and R subpixels is 2:1, the single pixel that contains them will exhibit optimized electro-optical properties, and the regions are said to achieve matched phase retardations. In a ‘single-cell-gap’ transflective LCD, same-thickness T and R require different birefringence properties to achieve the ideal ratio. To address this problem, previous efforts1–5 have adopted different subpixel display modes—for example, vertically/hybrid or homogeneously/hybrid aligned—but the results are not consistent for all driving voltages. Moreover, mixing display modes leads to discrepancies between the voltage-dependent transmission and reflection curves, requiring complicated driving circuits and thus additional expense.

Figure 1. Schematic diagram of the experimental setup for fabricating a single-cell-gap transflective LCD. ITO: Indium tin oxide. LC: Liquid crystal. PI: Polyimide. Arrow (→): Rubbing direction. T: Transmissive. R: Reflective.

We recently reported a simple photoalignment method in which both the T and R regions use a single display mode but have different pretilt angles to give different birefringence modes.6 Our approach consists of exposing an LC cell doped with photocurable monomers (lauryl acrylate and biphenyl diacrylate) to UV irradiation (with a light intensity of 16mw/cm2 and centered at a wavelength of 365nm) through a photomask and subjecting it to an AC voltage of 9V (see Figure 1). The specific steps are as follows. In the sample cell, the UV light was blocked in the T and transmitted in the R region for 10min. After removing the photomask, the entire sample was exposed to UV light for 35min. The process formed polymer layers on the inside surfaces of the glass substrates to support the surface tilt of the LC layer on terminating application of the AC voltage. Alternating pretilt angles of 54 and 65° for T and R were produced, yielding optimal phase retardations of half-wavelength (180°)and quarter-wavelength (90°), respectively (see Figure 2).

Figure 2. Effect of UV exposure time on the pretilt angle of the LC layer and the induced phase retardation.

Figure 3. T and R regions showing different transmitted intensities in bright (0V) and dark (10V) states (inset) and their respective phase retardation as a function of driving voltage. The measurement wavelength was 655nm.

Figure 3 shows optical photographs of the bright (at 0V) and dark states (at 10V) in the transflective LCDs with backlight illumination. The different transmitted-light intensities for T and R indicate their different phase retardations. Those measured at 655nm are a function of the driving voltage (see Figure 3). Both regions achieve their respective optimal phase retardations of 180 and 90° at zero bias and exhibit the desired ratio of 2:1 at all driving voltages.

In summary, we have described a simple photoalignment method for obtaining unidirectional LC alignment with a controllable pretilt angle. We used this technique to construct a novel single-cell-gap transflective LCD in which both the T and R pixels are operated in the same electrically controlled birefringence modes with different pretilt angles of 54 and 65°, respectively. This type of transflective LCD exhibits optimal half- and quarter-wavelength phase retardations at zero bias and excellent phase match between the T and R pixels at all driving voltages, consequently requiring only one thin-film transistor. The alignment technique enables a variety of LC-layer pretilt angles on which many new applications depend. We will further develop new LC devices (e.g., twisted optically compensated bend mode, bistable cell) with pretilt in the range of 20–60°.

The author would like to thank the National Science Council of Taiwan for financial support.

Tien-Jung Chen
Department of Electro-Optical Engineering
National Taipei University of Technology
Taipei, Taiwan