전자정보소재공학특론(Liquid Crystal 3)

10.01 , 2014

Sung-Kyu Hong, Ph.D

Chemical & Biochemical EngineeringDongguk University

History of Liquid Crystal Display

• 1973 년 액정계산기가 발매된 이래 , 액정 디스플레이 시장은 성장을 계속하고 있다 . 최초의 액정계산기는 DSM(Dynamic Scattering Mode) 이었으나 곧 저소비전력 TN(Twisted Ne-matic)LCD 로 교체되어 시계 , 계산기 , 자동차재료용으로 보급되었다 .

• 1980 년 LCD 시장은 500 억 엔이 되어 , 액정 디지털 표시회사에도 인지되었으나 , 그 후 게임 등에 일시적으로 활황 하였을 뿐 , 표시성능의 문제로 시장은 주춤하였다 .

• 1986 년 STN(Super Twisted Nematic)LCD 가 개발 , 워드프로세서 등이 보급되고 시장이 확대되어 1990 년에는 3,300 억 엔이 되었다 . 그 후에 이동통신기기와 노트북으로 용도가 확대 되었으나 , 시야각과 응답속도 등의 문제가 생겼다 .

• 1990 년대 후반이 되어 , TFT(Thin Film Transistor) 를 이용한 AM(Active Matrix)LCD 의 대형설비투자에 힘입어 고품질 TFT-LCD 가 저가격화 되어 , 노트북 시장을 비약적으로 확대 되었다 . 더욱이 모니터에도 확대되어 , LCD 시장은 2000 년에 2 조 6,000 억 엔에 도달하였다 . TV 의 급속한 보급 초기 , TFT-LCD 의 표시모드는 TN 부터 시야각이 뛰어난 VA(Vertical Alignment) 와 IPS(In Plane Switching) 로 교체되어 오고 있다 .

• 향후 기술로 PDP(Plasma Display Panel) 와 LED(Light Emitting Diode) 와 경합을 하고 있으나 , 액정 TV 의 확대판매에 LCD 시장의 확대는 되고 있다 .

Driving Principle of Liquid Crystal DisplayDriving Principle of Liquid Crystal Display• For expressing the image information in LCD, the voltage should be applied to each

pixel using thin film transistor (TFT) established on each pixel. One TFT exists at each pixel and is insulated from another pixel.

• TFT has the role of switch to input needed image information from to data line to pixel. When the larger voltage than the threshold voltage is applied to gate line, the electric current can flow from data line to pixel electrode. The data line is estab-lished vertically with respect to LCD panel and get the voltage from data driving IC.

• Gate line is horizontally positioned in panel, which is corresponding to pixels being vertical resolution.

• For color TFT-LCD, three times pixels of horizontal resolution exist at each line. • The voltage applied to each pixel induce the change of LC alignment and control

the white light intensity irradiated from BLU. • And then three types of color, red, green and blue is represented through passing

the color filter layer.

The LCD Types by Driving Method The LCD Types by Driving Method

• LCD is classified with TN (Twisted Nematic) mode, VA (Vertical

Alignment) mode, IPS (In-Plane Switching) mode according to

LC alignment without electric field..

Liquid Crystal Display (LCD) Mode Liquid Crystal Display (LCD) Mode

• For TN mode, LC molecules is surrounded between two transmissive electrodes, which are existed as 90 degree twisted state from bottom electrode to upper one due to each substrate coated with rubbed alignment layer.

• In the no applied electric field, LC directors are continuously twisted as 90 de-gree from bottom substrate to upper one. On the other hand, in the presence of electric field, vertically aligned against two substrates.

• There are two types of TN mode such as a normally white type and a normally black type. For the Normally white type, LC is surrounded by two polarizers which is perpendicularly located each other, for no applied electric field, white image is displayed, in contrast to, in the presence of electric field, black image displayed.

• On the other hand, for the normally black mode, LC is surrounded by two polar-izers which is parallelly located each other, for no applied electric field, black image is displayed, in contrast to, in the presence of electric field, white image displayed.

Twisted Nematic (TN) Technology

LCD Mode (TN) LCD Mode (TN)

• Transmittance of TN mode is obtained on the basis of Gooch & Terry equation in normally black type as follows when incident light is monochromic.

• Where T is transmittance of monochromic light and u is variables being dependent of wavelength of in-cident light (λ) and birefringence (Δn).

• In the Normally black type TN mode, when u = , optimum cell gap, d satisfying the darkest black state black become above equation at first min-imum condition.

• On the other hand, for normally white type TN mode, the brightest state is presented at the cell gap which is first maximum condition.

LCD Mode (TN) LCD Mode (TN) Transmittance of Twisted Nematic (TN) Mode

TN & STN ModeTN & STN Mode

TN & STN ModeTN & STN Mode

• In VA mode, LC molecules (having Δε<0) are vertically aligned be-tween two polarizer attached glass plates and black state is dis-played without electric field.

• When the electric field is applied, LC parallelly align to two glass plates, white state is displayed, since birefringence of LC is induced due to tilt of LC by electric field.

LCD Mode (VA)LCD Mode (VA)

Vertical Alignment (VA) Mode

Transmittance of Vertical Alignment (VA) mode

• For VA mode, when electric field is not applied black state is displayed. As the results VA mode shows the lower black image than those of IPS and TN mode.

• The transmittance of VA mode can be expressed by

• Where Δn(E) is birefringence depending on the applied electric field, d is thickness of LC layer and λ is wavelength of incident light.

• For no applied electric field, LC molecules vertically align to both sub-strates, and then Δn(E)=0 and become black state not transmitting through VA cell.

• On the other hand, in the presence of electric field, LC molecules hori-zontally align to the both substrates, and then Δn(E) become maximum transmittance of light.

• To achieve a optimum cell gap (d) having maximum transmittance, next equation is used.

LCD Mode (VA)LCD Mode (VA)

• IPS mode which LC(having Δε>0) is horizontally moved between the date and common electrodes established same plane on TFT array substrate.

• Therefore viewing angle of IPS mode is wider than those of VA and TN mode. • The transmittance of IPS mode is expressed by

• Where ϕ is azimuth angle of LC on axis of substrate, Δn is birefringence of LC, d is LC layer thickness and λ is wavelength of incident light.

• Above equation, πΔnd/λ is not dependent upon the applied electric field and be-come criteria to determine optimum cell gap to present maximum transmission as same as VA mode.

• For IPS mode, Transmittance is changed by which azimuth angle ϕ is changed with applied electric field.

• For example, T is minimum when LC is aligned to 0 〬 without electric field, whereas T is maximum when LC is aligned to 45 〬 in the presence of electric field.

LCD Mode (IPS)LCD Mode (IPS) In Plane Switching (IPS) Mode

Single domain 과 multi domain

Single Domain Two Domain

1 1 21 2

4-Domain V/A Structure

1

2

3

4

8-Domain V/A Structure

2

45

1

3

6

8

7

• Multi-domain technology is defined as technology improving viewing angle by multi-divide of pixel.

• In the case of single domain without pixel divide, LC molecular alignment state is changed with main viewing angle since is LC molecules align to one direction. As a result different brightness image is observed ac-cording to observing direction.

• In contrast to, in the case of two domain pixel structure uniform brightness image is observed since two horizontally divided domains having different brightness is ob-served as average brightness be-tween two domains.

• In addition, the wider viewing angle LCD is achieved by applying 4-do-main divided by four directions or 8-domain by eight directions LCD.

Multi-domain Technology

Wide Viewing Technology of LCDWide Viewing Technology of LCD

Comparison MVA mode and PVA mode

• VA modes are classified by MVA(Multi-domain VA) and PVA(Patterned VA) in accordance with pixel-divided type.

• For MVA mode, each pixel is divided by protrusion. Also PVA mode is divided by patterning of ITO electrode.

• In general, MVA and PVA mode exhibit a pixel divided by four domain.

• Recently S-PVA mode that is each pixel divided by eight do-mains is used for wider viewing angle.

Wide Viewing Technology for Vertical Alignment (VA)

Wide Viewing Technology of LCDWide Viewing Technology of LCD

IPS Cell 구조OFF ON

전극이 Rubbing방향과 약 15° 기울어짐

S-IPS Cell 구조

OFF ON

0° 방향 Rubbing(Chevron 형태

전극 )IPS 와 S-IPS 비교

• IPS modes are classified with IPS mode having stripe type pixel and S-IPS mode having chevron type pixel.

• S-IPS mode has two domain chevron structure in pixel and common elec-trode. As a result LC molecules align to two direction such as left rotation or right rotation by applied electric field from 0 degree in azimuth angle without electric field, the viewing angle and color shift of S-IPS become better than those of IPS.

Wide Viewing Technology for IPS Mode

Wide Viewing Technology of LCDWide Viewing Technology of LCD

Comparison between IPS mode and FFS mode

• IPS mode has disadvantages of slow response time and low transmittance since both pixel and common electrode is established on one side of LCD.

• Recently, IPS-pro(or FFS : Fringe Field Switching) mode is proposed to over-come these disadvantages.

• FFS mode has the similar one side electrode structure with IPS mode, but insulating layer is arranged between pixel and common electrode on same substrate. Therefore, for FFS mode horizontal and vertical electric field is applied to LC at the same time, where as for IPS mode only horizontal elec-tric field is applied to LC.

• As a result, FFS mode has the higher transmittance due to alignment of LC molecules near patterned pixel electrode and the faster response time LC due to reduction of spacing between patterned electrodes by applied elec-tric field

Fringe Field Switching (FFS) Mode

Wide Viewing Technology of LCDWide Viewing Technology of LCD

• In general, LCD black state is which LC vertically align between two substrates which top and bottom polarizers are perpendicularly at-tached each other except IPS mode.

• When ellipsoid of refractive index for LC become sphere on the front view the birefingence of LC is eliminated. However light leakage due to birefringence of LC is generated on the left or right side view.

• LC compensate film is a role of elimination of change in contrast ra-tio on the side view since retardation of compensate is contrary to that of LC. The compensate film is positioned on the front of LCD panel.

Wide Viewing Technology of LCDWide Viewing Technology of LCD Wide Viewing Film

• Retardation change of LC according to viewing angle can be compensated by nega-

tive C-plate on the off axis since LC has positive C-plate at black state.

• However LC viewing is not sufficiently improved by only the compensation on off

axis, additional compensation on axis is required using A-plate.

• The A-plate only has function to compensate thickness retardation on axis but to

improve leakage due to deviation of axis between two crossed polarizer with change

of viewing angle.

• Two biaxial plates can be used instead of a negative C plate and a A plate.

Wide Viewing Technology of LCDWide Viewing Technology of LCD

• Wide viewing film is optical compensation and protecting film that is at-

tached on top and bottom substrate in LCD.

• There are two types of compensate film for LCD. One is the film which align-

ment layer is coated on optically isotropic TAC(Tri-Acetyl Cellulose)film, then

LC is coated on the film. The other one which optically isotropic TAC film

combined with or added by high refractive index component is stretched

undirectionally or bi-directionally.

• Where on axis retardation, Re=(nx-ny)⨉d and off-axis retardation , Rth =

{(nx+ny)/2-nz}⨉d and d is a film thickness. Recently, transparent cyclic olefin

type polymer (COP: Cyclic Olefin Polymer) film which is stretched to uni-axis

or bi-axis is used compensation film in stead of TAC film.

Wide Viewing Technology of LCDWide Viewing Technology of LCD

• Compensate films are classified as follows.

① O plate : film which distribution of refractive in-dex is continuesly changed. ② A plate : film with nx > ny = nz on rectangular coordinate system③ Negative C plate : film with nx = ny > nz

④ Positive C plate : film with nx = ny < nz

⑤ Biaxial plate : film with nx > ny > nz

• For TN mode, O-Plate which disk-like or sylinder-like LC mole-

cules having tilt alignment is coated on TAC film. It is called

• WV(Wide View Film) film.

TN Mode Wide Viewing Technology TN Mode Wide Viewing Technology TN Mode Wide Viewing Film

21/33

Polarizing film

WV film

LCD cell

WV film

Polarizing film

Absorption axis

Optical direc-tion

Rubbingdirection

TN Mode Wide Viewing Film

TN Mode Wide Viewing Technology TN Mode Wide Viewing Technology

content WV-A WV-SA WV-EA

Viewing angle

(Left/Right/

Up/Down)

90/120 120/140 160/160

Optimum

retardation360nm 400nm 400nm

Discotic

LC tilt anglelow middle high

TAC

PVA Polarizer

TAC

TAC

PVA Polarizer

TAC + compensate film

TAC

PVA Polarizer

TAC

Compensate film

TAC

PVA Polarizer

TAC

TAC + compensate film

PVA Polarizer

TAC

TAC + compensate film

PVA Polarizer

TAC

TFT-LCD Panel TFT-LCD Panel

TFT-LCD Panel

• For VA mode, A-plate 와 negative-C plate or biaxial plate is used to

compensate vertical aligned LC molecules.

Wide viewing film for VA mode

VA Mode Wide Viewing Technology VA Mode Wide Viewing Technology

• 보상 필름을 이용해서 VA 모드 LCD 패널의 측면 빛샘 및 칼라시프트를 최소화하기 위해서는 아래 그림에 나타낸 바와 같이 LCD 패널의 액정의 두께방향 위상차와 보상필름의 두께방향 위상차의 파장분산성 ( 입사광 파장에 따르는 굴절률 변화 ) 이 유사해야 가장 이상적이다 .

• 또한 LCD 패널의 액정의 면내위상차와 보상필름의 면내위상차의 파장분산성이 서로 반대가 되어야 측면 color shift 를 최소화 할 수 있다 .

VA 모드 광시야각 필름 Color Shift

VA Mode Wide Viewing Technology VA Mode Wide Viewing Technology

TAC

PVA Polarizer

Z- TAC

TAC

PVA Polarizer

Z-TAC

Arton Z

PVA Polarizer

TAC

Z-TAC

PVA Polarizer

TAC

TFT-LCD Panel TFT-LCD Panel

IPS mode compensate film

IPS Mode Wide Viewing Technology IPS Mode Wide Viewing Technology

• For IPS mode, positive C-plate is used to compensate horizontal

aligned LC molecules.

• Recently, Z-TAC has been used for improvement of leakage and

color shift on side view.

액정은 계면을 이루는 고체 ( 고분자 ) 표면과의 분자간 상호작용에 의해 여러가지 배향상태가 얻어진다 .

( γlc < γsolid : 수평 배향 , γlc =γsolid : 경사 배향 γlc >γsolid : 수직 배향 ,)

고체 표면과의 분자간

상호작용에 의해 초기

배향이 결정되며 , 이에

따라 액정 mode 가 결정

된다 .

LC Mode of LCDLC Mode of LCD

Random Orientation of LC molecules

p

A

or

I = I0 sin22θsin2(πΔn(E)d/λ), θ=π/4

Imax = πΔn(E)d/λ= π/2, λ=550nm

Imax = Δn(E)d= λ/2 = 275nm

흡수축 (0 도 , 90 도 휘도 최소 )

액정 배향 방향

( π/4, 3π/4, 5π/4, 7π/4 휘도 최대 )

편광판 vs 광

액정 vs 광

편광판 , 액정 vs 광

The Operation Principles of LCDThe Operation Principles of LCD

TAC: Triacetyl Cellulose Cellulose TriacetatePVA: Polyvinyl AlcoholPSA: Pressure Sensitive Adhesive

PolarizerPolarizer

Δ n= ne - no

ne

no

LC mol-cules

Polarizer Analyzer

Δ n= ne - no

ne

no

LC mol-cules

Polarizer Analyzer

Δ nd ( 위상차 )大

Δ nd ( 위상차 )小

The Operation Principles of LCDThe Operation Principles of LCD