Transflector

Figure 9.4 Schematic illustration of the first three major types of transflectors (a) opening transflector, (b) half-mirror metal transflector, and (c) multilayer dielectric film transflector. Zhu 2006. Reproduced with permission from IEEE.

Multilayer dielectric film is a well-developed technique in thin-film optics, but only very recently was it incorporated in transflective LCDs [16]. As illustrated in Figure 9.4(c), two dielectric inorganic materials with refractive indices and 2 are periodically deposited as thin films on the substrate. By controlling the refractive index and thickness of each thin layer, as well as the total number of layers, one can obtain the desired reflectivity and transmissivity. Similar to the half-mirror transflector, the transmittance/reflectance ratio of the multilayer dielectric film is sensitive to each layer s thickness. In addition, to produce several layers successively increases the manufacmring cost. Therefore, the multilayer dielectric film transflector is rarely used in commercial transflective LCDs. 

The orthogonal polarization transflector has a special characteristic that the reflected and the transmitted polarized lights from the transflector have mumally orthogonal polarization states. For instance, if a transflector reflects a horizontal linearly (or right-handed circularly) polarized... 

Figure 9.6(a) shows a transflective LCD stmcture using a half-mirror metallie transflector, two quarter-wave films, and nematic phase LC/dye mixtures [15]. In the figure, the upper and lower halves stand for the voltage-off and voltage-on states, respectively. 

In the abovementioned transflective guest-host LCD, the inner quarter-wave film is put between the transflector and the guest-host layer. There are two optional positions for the trans-flector. If the transflector is located inside the LC ceU, then the quarter-wave film should also be... 

LCDs [33]. Figure 9.7 shows the schematic structure and operating principles of the LC gel-based transflective LCD. The device is composed of an LC gel cell, two quarter-wave films, a transflector, a polarizer, and a backlight. The cell was filled with homogeneously aligned nematic LC and monomer mixture. After UV-induced polymerization, polymer networks are formed and the LC molecules are eonfined within the polymer networks. 

In the T-mode, the unpolarized backhght turns into a linearly polarized p-wave after passing the polarizer, the second quarter-wave film, the transflector, and the first quarter-wave film, successively. Thereafter, similar to the case of R-mode, the p-wave is scattered by the activated LC gels, resulting in a scattering translucent output. 

The 90° TN cell can be used not only in transmissive and reflective LCDs [38], but also in transflective LCDs [39]. Figure 9.11(a) shows the device configuration of a transflective TN LCD. A 90° TN LC cell, which satisfies the Gooch-Tarry minima conditions [40], is sandwiched between two crossed polarizers. In addition, a transflector is laminated at the outer side of the bottom polarizer and a backlight is intended for dark ambient. 

Compared to the conventional transmissive TN LCD, the above transflective TN LCD only requires one additional transflector between the bottom polarizer and the TN LC layer. Naturally, this transflective LCD device configuration can also be extended to an STN-based transflective LCD [42]. In contrast to the so-called polarization rotation effect in TN LCD, the STN LCD uses the birefringence effect of the super-twisted nematic LC layer [43]. Therefore, a larger twist angle (180 270°), a thicker LC cell gap, and a different polarizer/analyzer configuration are required. 

Figure 9.12 Schematic view of the cause of parallax phenomenon in the R-mode of a transflective LCD with polarizer and transflector laminated outside of the bottom substrate. Zhu 2006. Reproduced with permission from IEEE.

To overcome the parallax problem in transflective TN and STN LCDs, the bottom polarizer and transflector must be located inside the LC cell. A burgeoning in-cell polarizer technology, based on thin crystal film growth from aqueous lyotropic LC of supramolecules, has attracted a certain amount of attention in the transflective LCD industry [45]. By depositing both transflector and polarizer inside the cell, the abovementioned annoying parallax problem can be significantly reduced. 

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