5.2 Experiments and Simulations
5.2.3 Asymmetry Pretilt Angle Combination Simulations
By varying surfaces pretilt angle on both top and bottom surfaces for asymmetrical cells, the Voltage vs. Transmittance (V-T) curves simulated by DIMOS program suggested that the bend state configuration can also be achieved by surface pretilt angle of top/bottom substrates at 8°/47° and 10°/45° in a 4 μm cell. The V-T curves of the two combinations due to the Gibbs energy of bend state configuration ( 6.6 μJ/m2) lower than spray state are investigated in Fig. 5-5 and 5-6. As the results, the simulation result suggested that a transition-free Pi-cell can be prepared by asymmetrical cell with one regular 8° pretilt angle and the other surface greater than 47° when its cell gap smaller than 4 μm.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0 2 4 6 8
Applied voltage (V)
Transmittance (a.u.)
8°/47°‐R 8°/47°‐G 8°/47°‐B 10°/45°‐R 10°/45°‐G 10°/45°‐B
Cell Gap~4μm
Fig. 5-5 The simulations of V-T curves of two pretilt angle combinations.
7.158 7.169
6.789
6.603 6.594 6.5
6.6 6.7 6.8 6.9 7 7.1 7.2 7.3
8°/8° 8°/35° 8°/45° 10°/45° 8°/47°
Pretilt Angle Combination (degree)
Gibbs Energy
2 (μJ/m)
Transition Free
Fig. 5-6 The Gibbs energies of different pretilt angle combinations.
5.3 The Investigation of RM films’ Retardations
The critical voltages to hold bend state measured at λ= 633nm were 0.6, 0.2 and 0 V for RM’s thickness of 80, 110 and 134 nm in 5 μm RMM-Pi-cells, respectively.
On the contrary, the state transition was not found even in the thin RM layer for all 3 μm cells. Above mentioned results were shown in Tab. 5-2. The experimental results confirmed the DIMOS simulation. Further investigation was carried out to determine the RM’s surface pretilt angle. RMM-19B’s ΔnRM (at λ=452nm), 0.1449, was obtained from a 1.58 μm homogenous planar cell. Based on the measured result (ΔnRM
=0.1449), the retardation of a 200 nm homogenous RM thin film should be in 28.98 nm. The thin RM film’s retardations, however, were observed in 0.17 and 0.09 nm for the thickness of 237 and 120 nm, respectively. The discrepancy suggested that the liquid crystalline molecular orientation in RM film was highly tilted. In order to
simplify the liquid crystal’s pretilt angle calculation, we assumed a uniform molecular tilt orientation for RM film. In the uniaxial RM case, the index ellipsoid of the RM, as illustrated in Fig. 5-7, is represented by the Eq. (5-3). In the Fig. 5-7, the cross section of the RM cut by a plane, which is perpendicular to the wave vector, shows an ellipse whose long axis gives the effective extraordinary index of refraction ne-RM(θ ) and short axis gives the effective ordinary index of refraction no-RM(θ), which is equal to the ordinary index of refraction no-RM, for corresponding wave vector. Substituting the ne-RM(θ), which was calculated from the RM’s retardation value, into Eq. (5-3) [20][32], the average tilt angles θ of RM layers were calculated at 85° to 86° with assuming no-RM = 1.5, according to various film thickness. The results were summarized in Tab. 5-3. The deviation of tilt angle was around ±0.02° when RM’s no-RM increased or decreased by 10 %.
Fig. 5-7 The index ellipsoid of the RM in diagonal frame.
The calculated average RM tilt angle is much greater than theoretical critical pretilt angle required in an asymmetrical Pi-cell. As the results, the LC molecules in a
RMM-Pi-cell favored in bend orientation at its initial state without applying voltage [73]. To further confirm the bend state orientation, the retardation profiles of conventional Pi-cell and RMM-Pi-cell under different driven voltage from 0 to 6V with respect to different viewing angles were investigated in Fig. 5-8 (a) and (b). The conventional Pi-cells have larger retardation curves compared with the RMM-Pi-cells, when the applying voltage is higher than critical voltage. As the Fig. 5-8 (b), the retardation profiles of a RMM-Pi-cell driven from 0 to 6V were similar to the conventional ones’ driven at the voltages larger than critical voltage (from 2 to 6V).
The results confirmed that the LC molecules were arranged in the bend orientation at its initial state without applying any voltage in RMM-Pi-cells.
Tab. 5-2 The 4wt% RMM-Pi-cells and conventional Pi-cell.
Cell name Pi-cell RMM-1 RMM-2 RMM-3 RMM-4 RMM-5 RMM-6 Cell Gap
(μm) ~4.0 4.7±0.05 3.3±0.05
RM
thickness None 80nm 110nm 134nm 80nm 100nm 110nm
Critical
voltage 1.6V 0.6V 0.2V 0V 0V 0V 0V
CR 26 123 224 273 138 211 288
Dark State
(8V) 3.79 0.82 0.45 0.37 0.73 0.47 0.35
*Without using compensation films
Tab. 5-3 The pretilt angle calculations of the 4wt% RM films.
Coater Speed (rpm) 2000 4000 6000
Thickness (nm) 237 120 100
RMM Retardation (nm) 0.17 0.09 ≦0.09
Effective pretilt angle θ (degree) 85.74 85.48 ~85 ΔnRM-from a homogenous RMM-Cell (cell gap ~1.58um)
RMM material -no-RM (assumption)
0.1449 1.5
0
Fig. 5-8 Retardations of (a) the conventional Pi-cell, and (b) RMM-Pi-cell with different driving voltage from 0V to 6V in different viewing angles (Measuring wavelength=632.8nm).
5.4 The Optical Properties of Transition-Free RMM-Pi-cells
The normalized V-T curves of RMM-Pi-cell shifting to the left without critical voltage confirmed that it was a transition-free cell as illustrated in Fig. 5-9 (a).
Furthermore, the un-normalized R, G and B V-T curves indicated the transmittance in bright state was not seriously compromised in a RMM-Pi-cell as shown in Fig. 5-9 (b).
The residual retardation from RM’s splay orientation may have contributed to non-compromised transmittance in RMM-Pi-cell comparing to the high tilted conventional Pi-cell. The light leakage after 4V was largely suppressed in the proposed cell structure. The retardations of the RMM-Pi-cell corresponding to different viewing angles were lower than the conventional Pi-cell, shown in Fig. 5-8 (b). The results suggested that the LC molecules of the RMM-Pi-cells can be arranged vertically, much closer to the cell boundaries at the dark state. The light leakage, therefore, was reduced in RMM-Pi-cell. Without utilizing compensation films, the contrast ratio of the RMM-Pi-cell can be improved by a factor of 11 than the conventional Pi-cell. The improved contrast ratios of RMM-Pi-cell cells in 4.7 and 3.3 μm were shown in Fig. 5-10. The proposed device possessed high contrast ratio, therefore, the viewing angle of RMM-Pi-cell was also wider than conventional one’s.
As shown in Fig. 5-11, the Viewing-Angle (V.A.) ranges, defined by CR>10, of a Pi-cell and a RMM-Pi-cell, whose cell gaps were ~around 4 μm, were 0°/40°/0°/50°
and 40°/50°/60°/50° for the corresponding viewing orientations 0°/90°/180°/270°
(right/ up/ left/ down orientations) measured by green LED, respectively. However, the central contrast ratio of conventional Pi-cell cell was smaller than 10 in 10° V.A.
ranges. The RMM-Pi-cell’s driving voltage can be reduced to 0 ~ 7 V without applying high voltage pulse. The advantages of the transition-free RMM-Pi-cell are not only higher in contrast ratio, but also lower in power consumption than the
conventional Pi-cell.
Fig. 5-9 (a) The normalized RGB’s V-T curve, and (b) Un-normalized RGB’s V-T curve of RMM-Pi-cell and conventional Pi-cell without compensation films.
0
0 50 100 150 200 250 300
Contrast Ratio
RM Thickness
4.7μm 26.4 123.0 224.0 272.6
3.3μm 26.4 137.5 211.4 287.9
Conv. Pi‐
cell 80 nm 110 nm 134 nm
Fig. 5-10 The improved contrast ratios of RMM-Pi-cell cells in 4.7 and 3.3 μm (Max.
improved factor is 11).
(a) (b)
Fig. 5-11 The Iso-Contrast Contour Diagrams of (a) Pi-cell and (b) RMM-Pi-cell. The V.A. ranges are defined by CR>10 (Measuring light source is green LED.).
5.5 Discussions
The photos of the RMM-Pi-cell compared with conventional Pi-cell are shown in Fig. 5-12. The Figs. 5-12 (a), (b), (c) and (d) represent the different driving voltages were applied on a RMM-Pi-cell, respectively. The Figs. 5-12 (e), (f), (g) and (h) represent a conventional Pi-cell applied the different driving voltages, respectively. To compare Figs. 5-12 (a) and (e), the photo (a) showed the real bright state but photo (e) showed bluish color; in other words, the result meant the RMM-Pi-cell was in bend state without applying voltage and the conventional Pi-cell was in splay state until the driving voltage over 2V, shown in photo (f). As the results, the transition-free RMM-Pi-cell is achieved and the operation range is from 0 to 5V, smaller than the range of a conventional Pi-cell from 2 to 7V in real display applications.
Fig. 5-12 (a) ~ (d) were the photos of a RMM-Pi-cell and (e) ~ (f) were the photos of a Pi-cell (cell gap ~ 4μm) under different driving voltages, respectively.
The proposed transition-free RMM-Pi-cell can be realized, nevertheless, there is
(a) 0V‐bend (b) 2V (c) 4V (d) 7V
(e) 0V‐splay (f) 2V‐bend (g) 4V (h) 7V
RMM-Pi-cell (CR~288)
Conv. Pi-cell (CR~26)
suspected response time degradation. In the experiments, the average response time was not always less than 3ms; sometimes the response time would be degraded to the level of a TN-cell. One possibility is the anchoring energy maybe becomes weaker between LC molecules and the RM film during the fabrications. We think the RM film’s quality affects the anchoring force between the RM film and LC directors.
Therefore, both the anchoring energy and the fabrication conditions of the RM films need to be investigated in the future. The other possibility is the intrinsic properties of the RM film which are not stable enough for thin film (200~300nm) fabrications. In other words, the material properties of the RM film also have to be considered and further investigated. How to improve the anchoring force and find the optimized fabrications are important for keeping stable fast response property in Pi-cell. Anyway, the issue needs to be confirmed in the future work.
5.6 Summary
In summary, the RMM-Pi-cell can be driven without a state transition. The tilt angle of the RMM-Pi-cell has been confirmed theoretically and experimentally. The residual retardation of splay type thin film with highly tilt surface angle maintained the bright state’s transmittance and reduced the dark state’s light leakage. The contrast ratio was improved up to a factor of 11. The proposed device possessed high contrast ratio, therefore, the viewing angle was also wider than conventional one’s. The Viewing-Angle (V.A.) range, defined by CR>10, of RMM-Pi-cell was 40°/50°/60°/50°
for the corresponding viewing orientations 0°/90°/180°/270° (right/ up/ left/ down orientations) without using compensation films, respectively. Therefore, the proposed RMM-Pi-cell is suitable for low power consumption, for a better image quality in large size TFT-LCD applications.
Chapter 6
Conclusion and the Future Work
The themes of advanced TFT-LCD technology, shown in Fig. 6-1, are motion blur improvement, high brightness and high contrast ratio. In order to enable the Pi-cell to be more appealing for high-end display applications, we explored the fundamental LC features of a Pi-cell. There are two approaches reported here to improve the transition rate and eliminate the splay-to-bend transition of a conventional Pi-cell by the proposed NE-Pi-cell and RMM-Pi-cell. Our studies were focused on fast response Pi-cell development for resolving the motion blur issue and realizing non-color break up (CBU) FSC-LCDs in the future.
Fig. 6-1 The research topics explored and concluded in this dissertation.
Transition Free FSC‐LCD
Fast Response
Pi‐cell
Transition rate up Improvement
Motion Blur Improvement
High Brightness High
Contrast
NE‐Pi‐cell RMM‐Pi‐cell
High
Optical Throughput
High Image Quality
6.1 Conclusion
6.1.1 Investigation of Transition Effect in a NE-Pi-cell
Pi-cell, one of the fast response solutions in LC-cells, has been reported to have five intrinsic liquid state transitions which have led to several issues in operation. In LCD applications, the most popular technology to resolve slow and non-uniform transition processes is the high pulse transverse driving method which promotes transition, but the method reduces the aperture ratio of a pixel and increases the loadings of the panel and driver IC. Therefore, we proposed NE-Pi-cell which demonstrated a uniformly fast transition cell structure. The novel cell structure and nanostructure fabrication results are summarized in Fig. 6-2.
Fig. 6-2 The designed nano-particle treated cell structure and the density increase of nano-particles without aggregation.
The proposed method was successful to reduce transition time to almost 0 (reduction rate is over 99.9%). In addition, we also explored the fabrication method to
90nm 0.2wt%
Density=6.78%
Method 1=DI
90nm 0.2wt%
Density=11.6%
Method 2=EG:PGMEA
Transition time reduction rate
> 99.9%
Glass ITO Polyimide Nano-particle
(solvent: EG mixed with PGMEA)
Density up 70%
Without aggregation!
Manufacturing
overcome nano-particle aggregations with high nano-particle density. The nano-particle density is increased from 6.78% to 11.6% (improvement rate is over 70%).
The photographs of the nucleation processes proved the transition speed of a NE-Pi-cell was faster than the conventional one with same driving voltage, investigated in Fig.6-3. Figs. 6-3 (a) and (c) are the transition processes of a NE-Pi-cell; Figs. 6-3 (b) and (d) are the transition processes of a conventional Pi-cell.
Comparing Figs. 6-3 (a) with (b), the spherical cluster radius of NE-Pi-cell is larger than conventional ones. In other words, the nucleation proceeded and clusters grew faster in a NE-Pi-cell. The photographs (c) and (d) also show the same results.
According to our investigation, the transition rate is very positive in relation with the protrusion density, which determines the quantities of potential nucleation sites and limited by response time degradation. The results match with the transition rate equation in the nucleation theory.
Fig. 6-3 The photographs show the transition processes of (a) a NE-Pi-cell, and (b) a conventional Pi-cell from Ha to bend state, respectively; moreover, the (c) and (d) are with respect to the transition from twist to splay state of a NE-Pi-cell and a conventional Pi-cell.
Transition rate up: Larger cluster radius and faster clusters growing rate!
(c) Twist→Splay
Twist (a) Ha→Bend
Bend
Twist→Splay Bend
Splay Ha→Bend
6.1.2 Splay-to-Bend Transition-Free RMM-Pi-cell
Even if proposed NE-Pi-cell can improve the transition speed to almost invisible and proceed to nucleation uniformly, the compromised transmittance of R, G and B caused by splay-to-bend transition still degrades the optical performance of a LCD.
Moreover, the other drawback is the low contrast ratio (CR) caused by the light leakage in the dark state in which LC molecules cannot be vertically arranged under the strong boundary condition. The novel RMM-Pi-cell can achieve a transition-free Pi-cell. The design concept and the V-T curves of the RMM-Pi-cell are shown in Fig.
6-4. As the V-T curves of R, G, and B, we successfully eliminate the critical voltage and the maximum transmittance is driven at 0V. In other words, the transmittance of R and G are no longer compromised by critical voltage blue (~2V) and increased by 25%. Moreover, the contrast ratio can also be improved by a factor of 11.
Fig. 6-4 The improvements of the transmittance and the contrast ratio in a novel transition-free RMM-Pi-cell. Blue--Critical voltage (~2V)
Degraded TR
The comparison of various transition-free Pi-cells and RMM-Pi-cell is shown in Tab. 6-1. The RMM-Pi-cell is certainly a better solution for a transition-free request.
However, the suspected response time degradation should be further investigated in future works. The blue phase is also an interesting topic for us recently. We also compare it with the proposed RMM-Pi-cell in Tab. 6-1. The blue phase possesses wide viewing angles, high optical performance, fast response (~1ms) and easy fabrication.
However, the LC material of blue phase needs to face many issues, such as narrow temperature range (~60°K), lower induced optical birefringence and very high driving voltage (60~100V) [74]. Maybe, the blue phase will be a better choice in future display applications, but the issues lead to many difficulties in display applications.
Now we still want to propose the fast response, transition-free and easy commercialized Pi-cells for LCD image quality improvement and the realization of non-CBU FSC-LCDs.
Tab. 6-1 Comparison of different fast response LC-cells.
○
◎: Excellent ○: Good △: Acceptable X: Poor
6.2 Future Works
The proposed future works focus on three aspects — the investigation of the suspected response degradation in RMM-Pi-cell, the fabrication optimization of RM film for anchoring force enhancement and film uniformity, and the implementation of the RMM-Pi-cell. Due to suspected response degradation, one possibility is that anchoring energy between LC molecules and RM film becomes weaker. If the fabrication condition of RM film can be optimized and investigated more thoroughly, the response time issue can be clarified and resolved.
6.2.1 Anchoring Effect Investigation between RM Film and LC
In chapter 5, we proposed RMM-Pi-cell to resolve the transition and recovery issues in conventional Pi-cell. Even though the RMM-Pi-cell is transition-free with better optical properties compared with prior arts, the response time degradation is still suspected. The response time of an LC cell is influenced by the LC cell structure and the interaction between alignment layers and LC molecules, well-known as the anchoring effect. Due to the novel cell structure of RMM-Pi-cell, extra RM film was treated on the alignment layer (PI), thus, the anchoring force would be changed by RM film. Based on the anchoring effect, the anchoring force leads the LC directors to return to the easy axes which determined by the rubbing directions and pretilt angles of alignment layers at the both cell boundaries. If the anchoring force between LC molecules and alignment layers is not strong enough, the LC directors can not return rapidly when the driving voltage is released. In other words, the response time of an LC cell will become slower.
Therefore, the anchoring force between RM film and LC molecules needs to be investigated. How to precisely measure the anchoring energy of the cell boundaries is
important for clarifying the cause of response degradation in proposed RMM-Pi-cell.
Previous research has exposed the anchoring energy measurement, such as electrical field method [75-76]. However, these methods were proposed for general homogeneous or vertical alignment cells, not for asymmetric Pi-cells. To find a method for asymmetric Pi-cell and improve the measuring accuracy are essential. In the future, the anchoring effect of a RMM-Pi-cell should be studied in detail to find out the solution for the response time degradation.
6.2.2 Fabrication Conditions Optimization of RM Films
Based on aforementioned inference, the anchoring effect may be the key factor which results in the increase of response time. In addition to the anchoring energy investigation of a RMM-Pi-cell, the fabrication optimization of RM films also affects the interaction between LCs and RM film. Moreover, the RM film is very thin and easy to be broken during fabrication. It is necessary to find the optimized formula of RM and solvent for improving the coating uniformity and ductility. Simultaneously, the UV dose of RM films also need to be concerned for the anchoring force with LCs.
Besides, we hope that the pretilt angles of the RM films can be controlled by process conditions. The optimized pretilt angle should be controlled just higher than critical pretilt angle to prevent the transmittance degradation. The RM film’s quality evaluation is suggested by using the anchoring energy of the cell to be the index factor.
Accordingly, we can further illustrate the response mechanism and prevent the degradation of response time in RMM-Pi-cell.
6.2.3 Manufacturing Implementation of RMM-Pi-cell into a real LCD
Since the RMM-Pi-cell needs RM film treatment after PI printing process, the standard fabrication process of the LC cell has to add extra coating and baking steps.The inject-printing method is suggested to build in the available process for RM film coating, shown as Fig. 6-5.
Reactive monomer (RM) solution should be prepared in advance. The RM solution can be coated on PI buffed substrates by the ink-jet printing method; RM film can be also deposited by a spin coater. Since the RM film is very thin and broken easily, the RM treated substrates should be exposed under the UV light with a simultaneous baking process. In other words, the reactive monomers are reacted by UV light as the solvent is evaporated in RM film to prevent the fast shrinking and breaking of RM films. Then, the following processes are the same with standard LC cell fabrications. With the extra steps, the RMM-Pi-cell can be adopted into mass-production line.
Fig.6-5 Manufacturing Implementation of RMM-Pi-cell into an actual LCD.
PI coating
textile
ITO electrode PI
GLASS LCs
Roller direction roller
Baking PI rubbing
PI
APR
injector
injector