Chapter 2 Overview of SSFLC Devices
2.5 Summary
(a) (b)
Fig.2.12. A uni-direction layer is formed since the Ps and director are fixed. (a) When the Ps is aligned into the page, a left-leaning layer is obtained. (b) When the Ps is aligned out the
page, a right-leaning layer is obtained.
2.5 Summary
We have introduced and discussed the chevron structure related to the SSFLC device.
Then, C1, C2 structures were reviewed and the reason for zigzag defect formation and the causes of horizontal chevron defect were also described. After that, different approaches to achieving defect free SSFLC were also reviewed and discussed. Finally, we proposed a novel hybrid cell structure to align the smectic layer without application of electric field.
Chapter 3
Measurement Systems
3.1 Introduction
In this chapter, the measurement setups used in the experiments were described in the following sections. The surface morphology was inspected by AFM. After making an empty cell, the cell gap was measured by interferometric method, which was described in detail in section 3.2.2. We assembled the laser optics system to measure the electro-optical response of the filled SSFLC cell, and the ConoScope was used to evaluate the optical performance, such as contrast ratio and viewing angle.
3.2 Atomic Force Microscope (AFM)
AFM consists of a scanning sharp tip at the end of a flexible cantilever across a sample surface while maintaining a small, constant force. The tips typically have an end radius of 2 nm to 20 nm, depending on tip type. The scanning motion is conducted by a piezoelectric tube scanner which scans the tip in a raster pattern with respect to the sample (or scans to the sample with respect to the tip). The tip-sample interaction is monitored by reflecting a laser off the back of the cantilever into a split photodiode detector. By detecting the difference in the photodetector output voltages, changes in the cantilever deflection or oscillation amplitude are determined. A schematic diagram of this mechanism is depicted in Fig. 3.1.
Fig. 3.1. Concept of AFM and the optical lever.
The two most commonly used modes of operation are contact mode AFM and TappingModeTM AFM, which are conducted in air or liquid environments. Contact mode AFM consists of scanning the probe across a sample surface while monitoring the change in cantilever deflection with the split photodiode detector. A feedback loop maintains a constant cantilever deflection by vertically moving the scanner to maintain a constant photodetector difference signal. The distance the scanner moves vertically at each x, y data point is stored by the computer to form the topographic image of the sample surface. This feedback loop maintains a constant force during imaging, which typically ranges between 0.1 to 100nN.
TappingMode AFM consists of oscillating the cantilever at its resonance frequency (typically ~300kHz ) and lightly “tapping” on the surface during scanning. The laser deflection method is used to detect the root-mean-square (RMS) amplitude of cantilever oscillation. A feedback loop maintains a constant oscillation amplitude by moving the scanner vertically at every x, y data point. Recording this movement forms the topographical image.
The advantage of TappingMode over contact mode is that it eliminates the lateral, shear forces present in contact mode, enabling TappingMode to image soft, fragile, and adhesive surfaces without damaging them, which can be a drawback of contact mode AFM.
3.3 Cell Gap Measurement System
For liquid crystal display, the thickness of cell gap usually affects the optical performance. Especially for SSFLC, the suppressing of helical structure needs a very small cell gap and, in addition, the gap between the two alignment layers would also influence the orientation of FLC. Thus, every time before the infection of FLC we need to measure the empty cell gap, and interferometric method [32] is what we use. The measurement instrument used is UV/Vis spectrometer LAMBDA 650 from Perkin Elmer, and the principle of this method is introduced as below.
The basic concept of the measurement method is based on the interference of light reflected by the two reflecting surfaces. The illustration is as Fig.3.2. R1, a coefficient of reflection, is defined as ratio of the light reflected by surface 1 to the incident light. R2 is the reflection coefficient of surface 2.
Fig.3.2. Two reflecting surfaces separated by a layer causing a light interference. The dotted line indicates the first internal reflection [32].
If the total incident light is I =cosωt and we assume there is no any absorption of light in surface 1and 2, then we can write the total reflected light R as
( ) (
o)
the wavelength, dgap is the thickness of the layer, ngap is the refractive index of the layer.
The cosine factor in Eq. (3-1) for k>1 are caused by internal reflections. Since R1<1 and R2<1, the magnitude of the cosine factors for k>1 is much smaller than for k=1. Therefore the internal reflection is chosen to be neglected, so
( )
⎟⎟Thus the reflected spectrum is
( )
λ[ (1 ) ]
2 (
1)
2 cos(
4π /λ)
The periodic term in Eq.(3-3) causes an interference pattern. The periodicity of the reflected interference spectrum determined the optical thickness of the cell gap, ngapdgap.
If λ1 and λ2 are the two wavelengths showing extrema in Eq.(3-3), then Where x is a natural number.
Based on Eqs.(3-4), (3-5), and (3-6), we can write and λ . It is better to choose the distance x between the two extrema as large as possible for 2 improving the accuracy of the calculation ofngapdgap. The sample data was shown in Fig.3.3 for a 5.0 μm cell.
Fig.3.3. The reflection as a function of wavelength using a air gap of 5.0μm.
3.4 Laser Optics Systems
Our laser optics system is as in Fig.3.4. This optical system is responsible for the measurement of electro-optical properties, such as V-T characteristics and response time. First of all, we have to reduce the intensity of laser source within the acceptable range of the photo detector by using a 10% ND filter. Next, the moderate unpolarized light becomes a polarized light after passing through the polarizer and then enters the LC cell. The LC cell, acted as a phase modulator, changes the phase of the incident polarized light by retardation Δn⋅d (Δn is the birefringence of LC, and d is the thickness of LC). Then the modulated light passes through the analyzer and the light output is received by the photo detector. The driving waveform of FLC is written by ourselves and sent by a waveform generator WFG500 (from FLC Electronics AB). The optical output received by the photo detector can be observed with the oscilloscope (from Tektronix) and quantified data can be read by a multimeter (from Keithley).
We use a bipolar square wave with various frequencies, applying on the cell, to measure the response time of the test cell. The V-T curve is measured by using a 0.1 Hz bipolar triangular wave.
Sending data
Fig.3.4. A schematic diagram of the laser optics system.
3.5 ConoScope
Fig.3.5.The schematic diagram of ConoScope.
The ConoScope (autronic-MELCHERS GmbH) can be used for visual performance evaluation, such as luminance, contrast ratio, color shift, gray scale and many characteristics.
The basic operating principle of the ConoScope is described as follows. A typical scanning device (so-called “gonioscopic system”) would scan the half cone above the display to detect the variations of luminance and color for each specific direction. Plotting each azimuthal angle on a circle with the radius from the center additionally indicates the polar angle results in the so-called polar coordinate system. Using such a polar plot to mark for each direction luminance and /or color will result in (in the case of color) a colored figure.
An arrangement of several lenses (here represented by single lens) modifies the light propagation directions in a way, that all beams emerging from the sample in the same direction will meet in one spot in the focal plane (or within the “conoscopic figure”) corresponds to one specific direction of the viewing cone. The conoscopic figure directly shows color and luminance as they would have been plotted in a polar coordinate system as described under Fig.3.6. In our measurement, we used the collimated illumination as the backlight for the transmissive mode to analyze the luminance, contrast ratio, and light distribution of the FLCD.
Fig.3.6. The function of the ConoScopic lens system.
Chapter 4
Experiments and Results
4.1 Introduction
For the fast sub-millisecond switching FLCD, the major problem degrading the performance is the alignment defects that cause light leakage in the dark state and result in low contrast. In this thesis, three alignment issues that directly relate to the orientation of SSFLC were discussed. One of the issues is the sign of the surface polarity which could probably results in Ps attraction [33] or repulsion; another one is surface roughness which affects the effective surface pretilt [25][34]; the other one is the surface rubbing conditions, that is, rubbing direction and rubbing strength.
As mentioned in the section 2.2.3, the formation of horizontal chevron defect results from the random orientation of Ps which means that layer structures in opposite directions show up randomly. In the following sections, a new hybrid cell structure was applied to effectively orient the Ps and to obtain a uniform uni-direction layer. On the other hand, to achieve a zigzag free orientation the C2 uniform structure is preferred because the alignment is relatively simple and it is much easier to avoid twist state in C2 than in C1 [8]. In C2 state small surface roughness is an essential criteria, thus two methods were tried to improve the surface roughness.
4.1.1 Cell Fabrication Process
The flow chart shown as below is the fabrication process of our liquid crystal cell:
A. Each piece of glass is rinsed with detergent and rubbed by hands carefully. Then, wash the glass with DI water until the water flowing along the surface smoothly. After that, put the glass loaded with a holder into hot DI water and shake it with ultra sonic for 30 minutes.
B. Blow the surface of glass with nitrogen gas to rip off the water, and then bake for 1 hour at 110℃. Next, use UV-ozone to treat surface for 10 minutes (that is for better adhesion of alignment layer).
C. Put the clean ITO glass onto the spin coater. Drop the solution of alignment material (make sure the solution to cover the whole glass), then spin. The procedure of spin-coating is as follows:
2.5wt%PVA Polyimide
Speed Time Speed Time
1st spin 500rpm 10s 1st spin 500rpm 10s
2nd spin 1500rpm 20s 2nd spin 5000rpm 20s
After spin-coating, we need to bake the samples for full adhesion for a short time, which is 30 minutes at 110℃ for PVA and 30 minutes at 200℃ for polyimide. PVA (Mw Cleaning of
ITO glass
Coating of
alignment film Baking Rubbing
Spacer
13,000-23,000, 98% hydroldyzed) is from Sigma-Aldrich, and polyimide (PIA-X201-G01) is from Chisso. The film thickness, which is measured by AFM (from Digital Instruments), of 2.5wt% PVA is 1000Å, and the thickness of 50wt% polyimide is about 350Å.
D. Rub the samples with rubbing machine. The rubbing strength is different depending on the LC material, alignment film, and pretilt angle. The rubbing condition used is as below:
E. Drop a small amount of UV glue (NOA-65, from Norland) mixing with 1.6um spacer at the four corner of the bottom plate. Cover the top plate and press. Next, place the cell under a UV lamp for 5 minutes to fully cure the glue, and then an empty cell is done.
F. Measure the cell gap of an empty cell by spectrometer UV-Vis 650 (from Perkin Elmer) using interferometric method. The details will be explained in section 3.2.4.
G. Heat up the cell and LC material to the temperature above the clearing point. Inject the LC material from the edge of the cell until the LC is full of the gap.
H. Anneal the cell carefully with programmable hot plate from high temperature to room temperature. The annealing process depends on the phase sequence of the LC material.
I. Solder the wire at the ITO contact, and then the test cell is available for a mount of electro-optical measurements.
4.1.2 Measurement of Sign of Surface Polarity
Since we proposed the hybrid cell structure to align Ps in section 2.4, the sign of surface polarity played an important role. In order to check the sign of the surface polarity of
PVA Polyimide
Pile impression 0.23mm Pile impression 0.2mm Rotation speed 300rpm Rotation speed 300rpm Advancing speed 7.3mm/s Advancing speed 7.3mm/s
alignment layer, two crossed-rubbing cells were made. One of the cells has PVA coating on both substrates, as in Fig.4.1 (a), and the other one has PI coating on both substrates, as in Fig..4.1 (b). After filling the FLC material with positive Ps into the cross-rubbing cell, a twisted structure was formed. According to the sign of surface polarity, the Ps of molecules at the top and bottom surface directed inward or outward the cell. In addition, based on the switching nature of FLC, a specific layer direction should be observed in the cross-rubbing cell and layer normal of FLC in PVA cell would be perpendicular to one in PI cell. The FLC material used is R3206, with Ps~18 nC/cm2. After injecting the FLC and cooling down the cells to room temperature, we found that the layer was formed and the layer normal direction of PVA cell was perpendicular to PI cell, as shown in Fig.4.2. The results matched the assumption in Fig.4.1. According to the results, sign of the surface polarity was confirmed.
Fig.4.1. The illustration of twisted structure in crossed-rubbing (a) PVA cell and (b) PI cell.
R,S represent rubbing direction and sign of surface polarity.
(a)
(b)
Fig.4.2. Pictures of the cross-rubbing cells taken by POM. Pictures at the right are the magnified layer structures. (a) Layer structure in PVA cell. (b)Layer structure in PI cell.
4.1.3 Observation of CDR Type FLC Cells
Since the relationship between Ps and the sign of the surface polarity was revealed, the uni-direction layer model could be applied first on the test cell with CDR type FLC. Two FLC materials would be used─R2301 (from Clariant) and R3206 (from AZ Electronic Materials).
R2301 has phase sequence, I 86.8-84.8 N* 64.7 SmC* and R3206 has phase sequence, I 107-105.4 N* 78.4 SmC* -12(-6) Cr. The filling process was as below: First, the FLC material was injected to the hybrid cell by capillary force in isotropic state. Then the test cell was annealed in N* phase for 5 minutes and annealed in SmC* phase for 30 minutes. Finally, the test cell was cooled down to the room temperature.
OFF STATE
ON STATE
Fig.4.3. (a)(c) is the OFF and ON state of R2301 in hybrid cell, and (b)(d) are the OFF and ON state of R3206 in hybrid cell.
After the filling of the two FLC materials, the test cells were observed under polarizing optical microscope (POM) and it was found that a uni-direction layer structure was formed both with R2301 and R3206, as shown in Fig. 4.3. Fig. 4.3 (a)(b) showed the OFF states between cross-polarizer and Fig. 4.3 (c)(d) showed the ON states with application of 100 Hz square wave. As compared with Fig.4.3 (a), the microdomain structure in Fig.4.3 (b) was more obvious, although uni-direction layer was obtained in both cells.
Contrast and viewing angle
To see how much the contrast can be raised by the hybrid cell structure, we have measured the symmetrical cell 71 with R3206, hybrid cell 97 with R3206, symmetrical cell 59 with R2301 and hybrid cell 98 with R2301, as shown in Fig.4.4-Fig.4.7. The symmetrical cell 59 and 71 with horizontal chevron defect showed poor contrast (CR~29 for cell59;CR~18 for cell71). On the other hand, an improved contrast (CR~86) was shown in cell 97 with R3206, and the highest contrast (CR~144) was performed in cell 98 with R2301. As for viewing angle, because of the quasi in-plane switching and small surface pretilt the viewing angle could reach over ±70°.
Besides, the switching mechanism could be observed in conoscopic pictures. In Fig.4.5 and Fig.4.7, we saw the asymmetric light distribution of the ON state which was resulted from a tilted uni-layer structure, as shown in Fig.2.12 (a)(b), leading to an asymmetric switching cone. On the contrary, as shown in Fig.4.4 and Fig.4.6, the light distribution of the ON state showed symmetry which was considered as the averaged switching of two different layers in horizontal chevron. Thus, according to the conoscopic pictures, the uni-layer structure was also confirmed.
Fig.4.4. CR v.s. viewing direction of symmetrical cell 71 with R3206.
Fig.4.5. CR v.s. viewing direction of hybrid cell 97 with R3206.
Fig.4.6. CR v.s. viewing direction of symmetrical cell 59 with R2301.
Fig.4.7. CR v.s. viewing direction of hybrid cell 98 with R2301.
4.1.4 Observation of FGLC Series Cells
The self-made FGLC mixtures [37] show wide smectic C temperature range and good alignment ability. The 2%FGLC-1 mixture shows V-shaped switching and has SmA phase in the phase sequence. We filled 2%FGLC-1 mixture into hybrid (PVA/PI) cell, and the orientation of uni-direction layer was observed under POM as shown in Fig.4.8. Picture at the right is the ON state which is driven by a 100Hz 17V square wave, and the picture at the left is the OFF state under crossed polarizer. Although hybrid cell structure seemed successful on 2%FGLC-1 mixture, we found that after driving for a long time some stripe defect appeared, as shown in Fig.4.9, and the higher the driving voltage was applied the faster the stripe defect showed up. The stripe defect usually grows from the boundary between the two different domains, but sometimes grows from the center of the well-aligned domain. The growing direction of defects is parallel to the rubbing direction. The reason of defect formation will be discussed in section 4.4.
Fig.4.8. The alignment of 2%FGLC-1 in 1.78um hybrid cell. (a) Dark state. (b) Bright state.
Fig.4.9. (a) Before driving. (b) After driving, the stripe defect grows along the rubbing direction during driving.
4.1.5 EO Properties of FGLC Series Cells
To see if the hybrid cell structure affect the electro-optical properties of SSFLC or not, the response time of 2%FGLC-1 in 2um hybrid cell has been measured using various frequencies, as shown in Fig.4.11. Also the response time of 2%FGLC-1 in symmetric cell was listed in Table.4.2. During the measurement of response time, two conditions, as shown in Fig.4.10, were tried: 1) The positive field direction of applied square wave was “parallel”
to the Ps direction. 2) The positive field direction of applied square wave was “opposite” to the Ps direction.
Fig.4.10. Response time measurement conditions: (a) The positive field direction of applied square wave is parallel to Ps (b) The positive field direction of applied square wave is
opposite to Ps
The response time of 2%FGLC-1 was found faster in hybrid cell than in symmetric premade cell. And it was found that when the positive field of applied square wave is opposite to the direction of Ps, the fall time would be faster. We supposed that after removing the voltage the director returned to the initial state more quickly because the asymmetric polarity in hybrid cell provided a restoring force. In addition, if the positive field of applied square wave is opposite to the direction of Ps, the rise time would be slower, comparing to cell with positive field of applied square wave parallel to the direction of Ps. It was inferred that because of the asymmetric polarity of the cell, a dragging force slowed down the switching when field opposite to the direction of Ps was applied.
Table.4.1. Response time of 2%FGLC-1 in 2μm premade cell driven by 1kHz 30V square wave. Both of the two alignment films in premade cell are the same.
Response time Material
Rise time(τ10-90) Fall time(τ90-10)
2%FGLC-1 680us 1.1ms
(a) (b)
0
Fig.4.11. Response times for the (a) rising process and the (b) falling process of 2%FGLC-1 in 2μm hybrid cell driven by a 30V bipolar square wave with various frequency. Blue dot is the data measured when the direction of positive electric field is opposite to the direction of
Fig.4.11. Response times for the (a) rising process and the (b) falling process of 2%FGLC-1 in 2μm hybrid cell driven by a 30V bipolar square wave with various frequency. Blue dot is the data measured when the direction of positive electric field is opposite to the direction of