Organization of this thesis

This thesis is organized as follows: in chapter 2, basic principles and theories used in this thesis will be introduced. The principles of mechanical design on the LC cells including the continuum theory and anchoring effect are described, and the principles of optical design on the LC cell including the extended Jones matrix and optical compensation methodologies are also presented. In chapter 3, the device fabrication and measurement will be reported. In chapter 4, a polymerization

technique for preventing the OCB cell from the undesirable recoveries is proposed, where the merit of this method is that the optical contrast will not be suppressed after the stabilization. In chapter 5, the Hs state in a pi-cell is investigated in depth. We verify the transient profile of LC director with the newly proposed synchronized illumination method. According to the measured profile, a method for extending the lifetime of Hs state is proposed and simulated. In chapter 6, the realization of embedding an OLED device directly into an LCD cell is demonstrated. Additionally, a volumetric scattering layer is introduced to further increase its sunlight visibility.

Finally, the conclusions and future works are presented in chapter 7.

Chapter 2

Theory and Principle

The mechanical and optical designs of an LCD are introduced in this chapter. In the principle of mechanical design, the continuum theory is used to calculate the Gibbs’s energy of a certain state, and Rapini-Papoular approach is used to calculate the alignment effect on the orientation of LC director. In the principle of optical design, the extended Jones matrix method is used to calculate the polarization state transformation between the birefringent media, and Poincare sphere is used to visualize the optical compensation mechanism.

2.1 Principle of mechanical design on LC cells

The switching performance of an LC device relies on its mechanical design. To design the response time, flow direction, and equilibrium LC profile, continuum theory is well-used by the LCD researchers. A basic continuum theory considers the elastic relaxation of the LC molecules. Taking more factors into consideration, such as the electric field, the magnetic field, and the anchoring effect makes the equation more appropriate to the actual LCD cases. The detailed description of continuum theory is explained as follows.

2.1.1 Continuum theory

Frank-Oseen continuum theory is well-used in calculating the equilibrium profile of LC directors in a cell [20-22]. This theory can be explained by equation (2-1), where n is the vector of the LC director, q0 is 2π/p (p: Pitch of liquid crystal helix), and K11, K22, and K33 are the elastic constants of splay, twist, and bend motions, respectively (as depicted in Fig. 2-1). These motions and the external electric field (E)

induce strain energy to the system, and thus the free energy of the system changes.

Fig. 2-1 The LC cell strains induced by the (a) splay, (b) twist, and (c) bend motions [23].

where F is an arbitrary function.

The elastic energy can be expressed as:

Ee=

{ [ ]

33

[

^{2 2 2 2}

] }

Furthermore, the electro-static energy can be considered as follows:

Es=− Dv•Ev

where

ε

// _and

ε

_⊥ denote the permittivities along and perpendicular to the long axis

of the LC director, respectively.

Since the LC directors are positioned randomly at (θ,φ) angles, the reference axis has to be rotated about the y axis by θ and about the z axis by φ.

The external electric field E can be substituted as follows:

D=εε₀E Substituting this electric field term into Eq. 2-2 so that the electro-static energy can be obtained.

To solve the continuum equation, the large pixel approximation is conventionally used, where the x and y directional variations are assumed to be very tiny. Therefore, the free energy density “g” can be simplified to be a function of nx(z), ny(z), nz(z),

Integrating the free energy density throughout the whole volume,

( ) ( )

no energy change at the surfaces)

i

Calculating Eq. (2-2) and Eq. (2-10) by energy relaxation method described in Eq.

(2-11), the minimum of free energy can be obtained, and the preferred LC director profile can be acquired.

2.1.2 Anchoring effect

The LC alignment affects the equilibrium state, switching property, and director configuration of an LC cell. This alignment is generally achieved by the anchoring effect with a pretreated layer. Most preferably, polyimide is used owing to its low cost, high stability, and easy process. Two major methods are used to pre-treat the alignment material: mechanical rubbing and photo-alignment. The mechanical

rubbing method is executed by a roller covered with woolen texture. By brushing in the same direction, the alignment layer is strained, and this strain aligns the LC director in a certain direction. The other method of pre-treating the alignment material is to use polarized UV light to orient the molecules of alignment layer in a certain direction. This method can be used along with a photomask to make multi-directional alignments in one substrate, but the uniformity is still an issue.

By varying the alignment directions of the upper and lower substrates, the cells can be stabilized in different LC profiles. As shown in Fig. 2-2, the 90°, anti-parallel, and parallel alignments can produce the twist nematic (TN), electrically controlled birefringence (ECB), and pi cells, respectively [23].

Fig. 2-2 Different alignment arrangements of upper and lower substrates in (a) TN, (b) ECB, and (C) pi cells.

This stabilization mechanism can be expressed by Rapini-Papoular approach [24-26] as follows:

where θ and ψ denote the polar and azimuthal angles, Fs denotes the total free

energy resulting from the anchoring effect, F^θ denotes the free energy component in terms of the polar angle, F^ψ denotes the free energy component in terms of the azimuthal angle. W^θ and W^ψ are the constants whose physical meanings are the interactions between the substrates and the LC directors. θo and ψo are the equilibrium angles in the polar and azimuthal dimensions, respectively.

By considering the anchoring effect, the overall continuum equation is then modified as follows. This equation can be used to describe the stabilized LC director profile of an LC cell.

2.1.3 Mechanical design of pi-cells

According to the continuum theory mentioned above, the pi-cell can be designed to operate in the specific state. By calculating the Gibbs’s free energy in equation 2-15, the stable state corresponding to the applied voltage can be obtained. As shown in Fig.

2-3, without applying a voltage signal, this cell is stabilized in the splay state. While applying a voltage larger than the critical voltage, the bend state becomes more stable than the splay state [27-29].

Fig. 2-3 Gibbs’s free energy of bend and splay states as a function of applied voltage.

Taking the pretilt into consideration, the Gibbs’s free energy diagram can be used to determine whether the bend state is possible to be obtained or not. As shown in Fig.

2-4 (a), if the pretilt is too low, the Gibbs’s free energy of bend state is too high, so the bend state can not be stabilized (no matter how much voltage signal is applied). On the other hand, if the pretilt is higher than the critical pretilt (the intersection in Fig.

2-4 (a)), the bend state can be stabilized without an external field. While in the intermediate case, the bend state can be initiated with nucleation by an applied voltage as shown in Fig. 2-4 (b) [35-39].

Fig. 2-4 Phase transition in a pi-cell, where (a) is the pretilt effect and (b) is the nucleation mechanism in a phase transition process.

In addition to the operational state, the switching speed is also an important factor related to the mechanical design of an LC cell. The pi-cell is noted for its fast switching due to its symmetric LC director profile and parallel backflow. As shown in Fig. 2-5, the central director of the pi-cell is fixed during the operation. Hence, this cell can be regarded as two halves of Fréedericksz cell. Because the equivalent cell gap is reduced to half, the response time is increased by a factor of 4. Moreover, the parallel backflow in pi-cell makes the relaxation faster, whereas in the case of TN mode, as shown in Fig 2-6, the anti-parallel backflows make the relaxation slower.

Experimentally, the intensity drop while turning off the voltage in the case of TN is slower than that of pi-cell, as shown in Fig. 2-7. Thus, while designing the pi-cell operations, the twist motion or asymmetric flow has to be avoided.

Fig. 2-5 Schematic diagram of two equivalent halves of Fréedericksz cell.

Fig. 2-6 Backflows in the OCB and TN cells.

Fig. 2-7 Comparison of response times in (a) TN and (b) OCB cells [23].

2.2 Principle of optical design on LC cells

In the optical design of an LCD device, the most important factors to be considered are: contrast ratio, viewing angle, brightness, and color accuracy. In this section, we will discuss the principles for optimizing these factors.

2.2.1 Operational principle of an LCD device

The general LCD device includes an LC cell, a pair of crossed polarizers, a color filter, and possibly some phase retarders. As shown in Fig. 2-8, the light impinges on the first polarizer (to purify the state of incident light to be linearly polarized), passes through the LC cell (to alter its polarization state), and then be attenuated by the second polarizer to exhibit the grey scale. After that, the light will be filtered by a color filter to show the specific color. A further phase retarder can be adopted in the position before the light passing through the second polarizer to optimize the viewing angle and contrast ratio performances of the LC cell.

Fig. 2-8 Optical configuration of a general LCD device [7].

2.2.2 Extended Jones matrix for light field distribution calculation

The gray scale is shown by modulating the intensity of incident light passing

through the LCD device. To accurately control the gray scale for rendering the high quality images, the changes of polarization state between the components in the LCD device have to be precisely designed. The principle of this intensity modulation can be expressed by Jones matrix, which was first derived by Jones in 1941 and widely used to study the transmission characteristic of an optical system consisting of birefringent materials. This method; however, is limited to the case of normal incidence. To generalize the case from the normal incidence to the oblique incidence, Prof. P. Yeh introduced an extended Jones matrix method in 1982, which can be used to calculate the transmission of off-axis light in a birefringent network [30,31]. The method is described as follows: referring to Fig. 2-9, the relations between the electric field of incident, reflected, and refracted lights can be written as:

Incident:E=(A_sS+A_pP)exp[i(ωt−k•r), (2-16) Reflected:E=(B_sS+B_pP)exp[i(ωt−k•r), (2-17)

Refracted:E=(C₀oe^−ikr+C_eoe^−ikr)exp(iωt), (2-18)

where the suffixes “e” and “o” denote the extra-ordinary and ordinary ray, respectively. In addition, the suffix “s” is a unit vector parallel to the incident plane (plane y-z) and “p” is a unit vector perpendicular to the incident plane. Hence, the relations between p and s, o and e can be written as

k

(a)

(b)

Fig. 2-9 Schematic diagrams for describing the interaction between the incident light and media. (a) the incident wave-vector k lies on the yz plane and the c axis

of the birefringent medium lies on the xy plane; (b) reflection and refraction at the interface between the uni-axial and iso-tropic mediums.

Because the tangential components of electric and magnetic fields must be continuous at the boundary, we can thus obtain four equations. By using several mathematical operations, the transmission coefficients of tso, tpo, tse, and tpe can be obtained. Moreover, the expression of Jones matrix describing a light passing through a uniaxial plate can be further simplified as

⎟ ⎟

This is the general form of extended Jones matrix used for calculating the transmission characteristic of uniaxial medium such as liquid crystal layers, polarizers, or compensation films. With this calculation, the contrast ratio, viewing angle, and color accuracy can be optimized.

2.2.3 Principles of optical compensation

Most optical design for an LCD device assumes that the observer is in the normal direction. However, this assumption makes the images distorted in the oblique observing angle. To resolve this issue, there are two kinds of optical compensation:

one is to compensate the oblique deviation of polarizers; the other is to compensate the inaccurate phase retardation of LC director.

Before discussing the optical compensation methods, it is necessary to introduce the Stokes parameter and Poincare sphere first, since they are very elegant methods for describing the concept of optical compensation.

The Stokes parameters were defined by G. G. Stokes in 1852, which have been used for defining the polarization states of electro-magnetic wave. Generally, the electro-magnetic wave can be described as^E=A(t)exp

[

^i(wt−kr)

]

. In this case, the polarization states can be further defined by four parameters listed below:

Table 2-1 The Stokes parameters and their physical definitions.

S = − 0° or 180° linearly polarized component S2 S₂ =2E_x E_y cosδ ±45° linearly polarized component

S3 S₃ =2E_x E_y sinδ Circularly polarized component

To visualize the polarization conversion, Poincare sphere was introduced. As shown in Fig. 2-10 (a) [30], the equator represents the linear polarization states, where the polarization state of axis S1 is orthogonal to that of –S1. In addition, the axis S3 denotes a circularly polarized state, and the meridians represent the ellipsoidal polarization states. Each polarization state on Poincare sphere can be described by two factors (ε,θ), whereεis the ellipticity defined as ⎟

⎠

2-10 (b), andθis the rotation angle between the long axis of the ellipse and the x axis.

As a result, the Stokes parameters have correlations between the (ε,θ) as follows:

)

Fig. 2-10 (a) the configuration of Poincare sphere and (b) its corresponding ellipticity.

In the case of polarized light passing through a wave-plate, there are two factors shall be considered: one is the relative phase retardation Γ (which is correlated to the relative angle between the incident ray and the optical axis of the wave-plate); the other is the absolute phase retardation Φ (which is correlated to the birefringence and the thickness of the wave-plate)

d

Where λ denotes the wavelength of the incident ray, d denotes the thickness of the wave-plate, and ns and nf denote the refractive indexes of the slow and fast axes, respectively.

With the calculation of Γ and Φ, the corresponding polarization state variation can be also obtained on the Poincare sphere. With this calculated result, we can easily visualize the outcome after compensation.

2.2.3.1 Optical compensation on polarizer

The crossed polarizers can accurately control the light intensity passing through the LC cell when they are exactly orthogonal to each other (i.e. the polarization states of the incident and exit rays are on the opposite sides of Poincare sphere). As shown in Fig. 2-11 (a), in the ideal case, as the first and second polarizers are perfectly orthogonal to each other, the first polarizer is used for refining the polarization state of the incident light, and the analyzer is used to block certain amount of the light to exhibit the specific gray scale of the image. However, as shown in Fig. 2.11 (b), as the observation is at the oblique direction, the crossed polarizers are not exactly orthogonal in this case. This deviation makes the dark state incomplete.

To resolve this issue, a stacking of A- and C- plates was proposed [32-36]. This

method can be explained by Fig. 2-12. When the deviation occurs as shown in Fig.

2-12 (a), the polarization state of P1 is not orthogonal to A1; therefore, it is needed to transfer the polarization state of P1 to E. One of the methods proposed by P. J. Bos was to use an A-plate and a C-plate to rotate the polarization state, as shown in Fig.

2-12 (b). Note that using bi-axial film is also capable of having the same effect.

(a) (b)

Fig. 2-11 Projection of polarizer and analyzer at (a) normal and (b) oblique directions.

(a) (b)

Fig. 2-12 The compensation mechanism with wave-plates. (a) the deviation of crossed polarizers and (b) the mechanism of compensation using A- and C-plates.

2.2.3.2 Optical compensation on liquid crystal

Owing to the viewing angle dependence of phase retardation, the effective phase retardation differs in different viewing directions; therefore, the optical compensation on the LC director is indispensable for achieving high image quality. There are some issues mainly affecting the degradation of the optical contrast apart from the normal observing direction [37]. As shown in Fig. 2-13 (a), the asymmetric orientation of the LC director makes the phase retardations observed from right and left directions different. In addition, the projected phase retardation at small viewing angle is different from that at large viewing angle. This difference results in the grey scale inversion (Fig. 2-13 (b)). These issues can be resolved by a combination of wave-plates. As shown in Fig. 2-14, the compensation films made of hybrid-aligned wave-plate makes the effective phase retardation to be zero at any viewing direction.

(a) (b)

Fig. 2-13 (a) The asymmetric contrast degradation and (b) grey-scale inversion phenomenon [29].

Fig. 2-14 The configuration of optical compensation foils.

2.2.4 Optical compensation on pi-cells

Eliminating the residual phase retardation was proposed to improve the dark-state of pi-cell [38, 39]. As shown in Fig. 2-15, while applying a high voltage, the LC director is aligned along the vertical direction, resulted in a dark state. However, because of the pretilt, the LC director adjacent to the substrates is slightly inclined.

This inclination results in the light leakage of dark state. To resolve this issue, Prof. T.

Uchida proposed using a combination of wave-plates to compensate the remained phase retardation caused by the LC director adjacent to the substrates, as shown in Fig.

2-16. By using the same concept, we will be able to compensate the relaxed bend state stabilized pi-cell (discussed in chapter 4).

Fig. 2-15 Incomplete dark state under high electrical field in a pi-cell.

Fig. 2-16 Compensation mechanism of using wave-plates on the pi-cell.

Chapter 3

Device fabrication and measurements

The basic fabrication technologies including LCD and OLED processes are described in this chapter. These fabrications will be used to make our designed devices. After the devices being fabricated, the characterizations of the optical performance, LC director profile, and device cross-section are measured with the conoscopic system, the spectroscopic ellipsometry, and the focus ion beam assisted scanning electron microscopy (FIB/SEM), respectively.

3.1 Fabrication technologies 3.1.1 LCD device fabrication

An LCD device is made by stacking two substrates with intermediate LC material.

As shown in Fig. 3-1, one of the substrates is formed with thin-film-transistor (TFT) array, and the other one is coated with color-filter (CF). The array section of the LCD device is made with conventional TFT fabrication. Generally, for amorphous-Si TFT process, there are 5 sequences of masks for lithography. Each sequence includes UV light exposing, developing, removing, etching, depositing, and polishing. The CF section is made with black matrix, color filter resin, and Indium-Tin-Oxide (ITO).

After the substrates being prepared, the polyimide (PI) is then coated on the substrates to align the LC molecules. Subsequently, the roller with woolen texture is used to rub on the PI coated substrate. After that, the spacers are sprayed on the substrate to sustain the cell gap, and the top and bottom substrates are assembled. Afterwards, the assembled substrates is filled with LC material and attached with polarizers. Finally, the external circuits are connected to the LCD device [40].

Fig. 3-1 Standard LCD fabrication.

3.1.2 OLED device fabrication

A conventional OLED can be made by stacking the organic semiconductor between an anode and a cathode. This fabrication can be carried out by chemical vapor deposition (CVD) and physical vapor deposition (PVD). As shown in Fig. 3-2, the OLED fabrication process is executed as follows: cleaning the prepared ITO glass,

shaping the ITO anodes, depositing the organic layers, forming the cathodes, and finally encapsulating with a glass lid [41-44].

The performances of an OLED can be improved by modifying the device structure.

For example, to increase the light efficiency, the carrier injection and transportation

For example, to increase the light efficiency, the carrier injection and transportation

在文檔中 快速響應、高對比、超輕薄之向列型液晶顯示元件 (頁 24-0)