Liquid Crystal Display

During my introductory materials science course, I worked on a team of three to develop our own liquid crystal display from scratch.

Introduction

In 1888, Friedrich Reinitzer discovered liquid crystals when he was melting cholesterol benzoate crystal and observed two distinct liquid phases [4]. Liquid crystal molecules in the nematic phase are randomly distributed but are locally aligned along the same direction, or director. The director has no  intrinsic preferred orientation, meaning it can be influenced by many external perturbations [4]. Liquid Crystal Displays (LCD) technologies rely on this capability. Our team investigated these influences by synthesizing an LCD.

 


Typical Assembly of a Liquid Crystal Display

The construction of the LCD utilizes the nematic properties of the liquid crystal, and the capabilities of applying external energies to alter the orientation of the liquid crystal.  These are the critical properties that make it work [1], shown in the figure below:

1.       The cell walls have a perpendicular surface treatment that twists the nematic liquid crystal.

2.       The glass plates are coated with a conductive material, such as Indium Tin Oxide (ITO) to create electrodes.

3.       The glass plates are separated by thin film spacers (5-25 um) in order to create the required electric field with reasonable voltages.

4.       Polarizers on either side, aligned with the surface treatment, allow for the light to be absorbed and transmitted as desired.

Figure 1.  Typical structure of a liquid crystal display.

           

The Optics: How we control the light

Polarizing filters linearize light by only allowing light vibrating in the plane parallel to it to pass [5]. If the second polarizer, the analyzer, is oriented perpendicular to the polarizer, then the light passing through the polarizer is perpendicular to direction of the analyzer, and will not be transmitted[5].




Figure 2: a) light passing through a vertical polarizer. b) two parallel polarizers on top of each other. c) two crossed polarizers on top of each other.[5]

 

Liquid crystals are birefringent, and can therefore change the direction light is travelling. When linearized light hits the liquid crystals parallel or perpendicular to the director, it will continue its path, allowing little light to transmit through crossed polarizers. In other orientations, the linearized light will change direction, allowing light to transmit through.

An applied electric field can alter the orientation of the liquid crystals so that light is not transmitted, and a surface treatment can be used to orient the crystals such that light is transmitted.

 

A surface treatment where the polymers create a twist allows light to transmit through, while a surface treatment that allows the 5CB to stack parallel to the plate does not (show in figure below).


Figure 3. Parallel and Perpendicular surface treatments. The perpendicular allows light to transmit.

 

 Building the LCD

We built 3 LCD cells using the parameters given in the table. Slide 1 failed to react to an applied electric field. We hypothesized:

      The width of the cell might be too large

      The surface treatment was too strong, making it difficult for the crystals to re-orient.

We built slide 2, changing the cell width and experimenting with two surface treatments (as given in the table).



Slide 2 did not react to an applied electric field, but slide 3 did. When we applied a voltage to slide 3, we can visibly see that the light intensity decreases (shown in figure below). Although it does not uniformly align the director and go perfectly dark, the fact that the light intensity is reduced proves that the electric field is moving the director from its surface pinning.

 

Figure 4. Electrical response of LCD pixel at 0 V (left), 21.4 V (middle), and 75 V (right).

 

When the voltage is turned off, the liquid crystals return to the twisted nematic state. We first start seeing a response at 21.4 V, but with an increased electric field, the effects are more intense, widespread and uniform. By 75V we achieve the best response possible given our equipment. In order to narrow down why slide 3 worked and slide 2 did not, we decided to investigate the strength of the surface treatment using the DSC, and also determine the electric field needed to create a response.

 

Surface Pinning Strength

The Differential Scanning Calorimeter comparison showed an endothermic dip occurred at 49.0 C for both treatments, which may be indicative that both surface treatments pin the liquid crystal with similar strength. Further experimentation is necessary to confirm this hypothesis.

 

Figure 5: DSC of 5CB in polyamide tape and PVA treated pans. The sample masses for each experiment were 2.4 and 3.0 mg, respectively.

Both samples were cycled 6 times from 28 C to 52 C.

 

Orienting 5CB with Electric Field

We used a typical parallel-plate capacitor configuration to calculate the electric field across the liquid crystal, taking into account the dielectric constant of our polymer surface treatment. Because we have different mediums with different dielectrics, we sum the effects, to find the voltage. For this system we have:


 

where E_gap is  the electric field across the liquid crystal layer. Since the liquid crystal responded at the critical voltage of 75 V, E_gap for PVA is .395 V/um. Using this electric field, we calculate that the polyamide tape chamber (slide 2) would have needed 510.8 V. This implies that the spacing was very critical to the success of the LCD.

 

Conclusions

By varying electrical and surface parameters, we ultimately synthesized a working LCD pixel. We found that the crossed surface treatment is integral to creating a working pixel, as the twisted nematic is required for the transmittance of light through crossed polarizers. Also because a relatively high electrical energy, 1.185 V/um, is required to overcome the surface treatment, the space between the glass plates needs to be very small. The cell that only had a 30 um between the electrodes successfully created a pixel, but the cells with gaps > 100 um were too large to create an electric field that would be strong enough to change the crystal alignment, given the available equipment.

 

References

[1] Collings, P. 1990. Liquid Crystals: Nature’s Delicate Phase of Matter. Princeton, NJ: Princeton University [2] Chandrasekhar, S. 1992. Liquid Crystals (2nd ed.). Cambridge,UK: Cambridge University Press. [3] Chaikin, P.M, Lubensky, T.C. 1995. Principles of Condensed Matter Physics. New York, NY: Cambridge University Press.  [4] Daoud, M. and Williams, C.  1999. Soft Matter Physics. New York, NY: Springer-Verlag.  [5] Murphy, D. 2001. Fundamentals of Light Microscopy. New York, NY: John Wiley and Sons

 

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