Transflective liquid crystal display with adjusted dual-thickness liquid crystal layer and method of fabricating the same

• US 6,657,689 B2
• Filed: 05/08/2001
• Issued: 12/02/2003
• Est. Priority Date: 05/08/2000
• Status: Expired due to Term

• Current Assignee: LG Display Co., Ltd.
• Original Assignee: LG Philips LCD Company Limited (LG Display Co., Ltd.)
• Inventors: Baek, Heum-Il
• Primary Examiner(s): Ton, Toan
• Assistant Examiner(s): Rude, Timothy L
• Application Number:
  US09/850,186
• Publication Number:
  US 20020041351A1
• Time in Patent Office: 938 Days
• Field of Search: 349/114, 349/113
• US Class Current: 349/114
• CPC Class Codes: G02F 1/133371 Cells with varying thickness of the liquid crystal layer
• CPC Class Codes: G02F 1/13363 Birefringent elements, e.g. for optical compensation
• CPC Class Codes: G02F 1/133638 Waveplates, i.e. plates with a retardation value of lambda/n
• CPC Class Codes: G02F 1/1393 the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells G02F1/1396, G02F1/141 take precedence
• CPC Class Codes: G02F 2202/40 Materials having a particular birefringence, retardation
• CPC Class Codes: G02F 2203/09 transflective
• CPC Class Codes: G02F 2413/04 Number of plates greater than or equal to 4
• CPC Class Codes: G02F 2413/08 with a particular optical axis orientation

Abstract

The invention relates to a transflective liquid crystal display device that has a high contrast ratio. The transflective liquid crystal panel includes a homogeneous liquid crystal such that the transflective liquid crystal display device will have an optical retardation when the voltage is applied. Therefore, in order to compensate the optical retardation caused by this liquid crystal, a thickness of the liquid crystal layer is adjusted. Moreover, a thickness of the retardation film is also adjusted. Accordingly, the complete dark state and the high contrast ratio are achieved in the liquid crystal display.

Claims

1. A transflective liquid crystal display, comprising:

upper and lower substrates facing into and spaced apart from each other, wherein the upper and lower substrates include a plurality of pixel regions that display images;

a liquid crystal layer interposed between the upper and lower substrates, wherein the liquid crystal layer has a first adjusted thickness to compensate a residual optical retardation of incident light caused by anchored liquid crystals near an alignment layer when a maximum operation voltage is applied;

a first upper retardation film over the upper substrate;

a second upper retardation film between the first upper retardation film and the upper substrate, wherein the second upper retardation film has a second adjusted thickness for compensating an optical retardation caused by the liquid crystal layer;

an upper polarizer on the first upper retardation film;

a transparent common electrode on a surface of the upper substrate facing into the lower substrate;

a pixel electrode over the lower substrate, wherein the pixel electrode corresponds to each pixel region, and the pixel electrode is divided into transparent and reflective portions;

a second lower retardation film on the other surface of the lower substrate, wherein the second lower retardation film has a third adjusted thickness to compensate a residual optical retardation caused by the liquid crystal layer when a maximum operation voltage is applied, wherein a slow axis of the second lower retardation film is perpendicular to a slow axis of the second upper retardation film;

a first lower retardation film under the second lower retardation film, wherein a slow axis of the first lower retardation film is perpendicular to a slow axis of the first upper retardation film;

a lower polarizer under the first lower retardation film, wherein a transmissive axis of the lower polarizer is perpendicular to a transmissive axis of the upper polarizer; and

a backlight device arranged adjacent to the lower polarizer,

wherein the first adjusted thickness is d+d₁ in the reflective portion of the liquid crystal layer and 2d+d₂ in the transmissive portion of the liquid crystal layer, the first adjusted thickness is calculated from the following equation, $T={\mathrm{Sin}}^2<br><br>2<br><br>{\mathrm{\phi <br><br>Sin}}^2<br><br>\left[\frac{\mathrm{\pi <br><br>}·<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>n·<br><br>d_1}{\mathrm{\lambda <br><br>}}\right],$

wherein d is a thickness of the liquid crystal layer in a reflective portion for an optical retardation of λ

/4, 2d is a thickness of the liquid crystal layer in a transmissive portion in order for an optical retardation of λ

/2d, T is a value of transmittance when a maximum operation voltage is applied, φ

is an angle between an optical axis of the liquid crystal layer and a transmissive axis of the polarizer, Δ

n is a birefringence of the liquid crystal layer, and d* is a thickness of the liquid crystal layer, d* is d₁ or d₂, and d₁ and d₂ are calculated from the above equation, d₁ is a first auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is γ

in the reflective portion, d₂ is a second auxiliary thickness of the liquid crystal when the residual optical retardation value of the light is ω

in the transmissive portion, and a phase difference between the transmissive and reflective portions is δ

=ω

−

γ

.

2. The transflective liquid crystal display according to claim 1, wherein the transparent portion of the pixel electrode includes a transparent electrode being disposed on a surface of the lower substrate facing into the upper substrate.

3. The transflective liquid crystal display according to claim 1, further comprising a passivation layer on the transparent electrode, wherein the passivation layer has a transmitting hole in a central portion.

4. The transflective liquid crystal display according to claim 3, wherein the reflective portion of the pixel electrode includes a reflective electrode.

5. The transflective liquid crystal display according to claim 4, wherein the reflective electrode is disposed on the passivation layer and has the transmitting hole in a central portion.

6. The transflective liquid crystal display according to claim 1, wherein the second lower retardation film has the thickness of “

d₄−

d<sub>3(δ

)</sub>”

for compensating the optical retardation.

7. The transflective liquid crystal display according to claim 1, wherein the first upper and lower retardation films are half wave plates (HWPs).

8. The transflective liquid crystal display according to claim 1, wherein the second upper and lower retardation films are quarter wave plates (QWPs).

9. The transflective liquid crystal display according to claim 1, wherein the transmissive axis of the lower polarizer is perpendicular to that of the upper polarizer.

10. The transflective liquid crystal display according to claim 1, wherein the slow axis of the first upper retardation film is perpendicular to that of the first lower retardation film.

11. The transflective liquid crystal display according to claim 1, wherein the slow axis of the second upper retardation film is perpendicular to that of the second lower retardation film.

12. The transflective liquid crystal display according to claim 1, wherein the optical axis of the liquid crystal layer is parallel to the slow axis of the second lower retardation film.

13. The transflective liquid crystal display according to claim 1, wherein φ

is about 45 degrees.

14. The transflective liquid crystal display according to claim 1,

wherein the second upper retardation film has a thickness of d₄ and a transmittance of T,

wherein the second adjusted thickness of the second retardation film is calculated using the following equation, $T={\mathrm{Sin}}^2\; <li></li>2<br><br>{\mathrm{\phi <br><br>Sin}}^2<br><br>\left[\frac{\mathrm{\pi <br><br>}·<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>n·<br><br>d_2}{\mathrm{\lambda <br><br>}}\right],$

where, T is equals to the value of the transmittance, φ

is an angle between a slow axis of the retardation film and a transmissive axis of the polarizer, Δ

n is a birefringence of the retardation film, and d* is a thickness of the liquid crystal layer, wherein the d* is d₁, d₂ or d₃, wherein d₁, d₂ and d₃ are calculated from the above equation and the following equation d₂(ω

)=d₁(γ

)+d₃(δ

), where d₁ is a first auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is γ

in the reflective portion, d₂ is a second auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is ω

in the transmissive portion, and then the phase difference between the transmissive and reflective portions is δ

=ω

−

γ

, and wherein the second upper retardation film has the thickness of “

d₄+d₁(γ

)”

for compensating the optical retardation.

15. The transflective liquid crystal display according to claim 14, wherein φ

is about 45 degrees.

16. The transflective liquid ti display according to claim 14, wherein the transparent portion of the pixel electrode includes a transparent electrode being disposed on a surface of the lower substrate facing into the upper substrate.

17. The transflective liquid crystal display according to claim 16, further comprising a passivation layer on the transparent electrode, wherein the passivation layer has a transmitting hole in a central portion.

18. The transflective liquid crystal display according to claim 17, wherein the reflective portion of the pixel electrode includes a reflective electrode.

19. The transflective liquid crystal display according to claim 18, wherein the reflective electrode is disposed on the passivation layer and has the transmitting hole in a central portion.

20. The transflective liquid crystal display according to claim 14, wherein the second lower retardation film has the thickness of “

d₄−

d₃(δ

)”

for compensating the optical retardation.

21. The transflective liquid crystal display according to claim 14, wherein the first upper and lower retardation films are half wave plates (HWPs).

22. The transflective liquid crystal display according to claim 14, wherein the second upper and lower retardation films are quarter wave plates (QWPs).

23. The transflective liquid crystal display according to claim 14, wherein the transmissive axis of the lower polarizer is perpendicular to that of the upper polarizer.

24. The transflective liquid crystal display according to claim 14, wherein the slow axis of the first upper retardation film is perpendicular to that of the first lower retardation film.

25. The transflective liquid crystal display according to claim 14, wherein the slow axis of the second upper retardation film is perpendicular to that of the second lower retardation film.

26. The transflective liquid crystal display according to claim 14, wherein the optical axis of the liquid crystal layer is parallel to the slow axis of the second lower retardation film.

27. The transflective liquid crystal display according to claim 1, further comprising an optical retardation of the upper QWP is λ

/4+α

, α

ranges from zero to 100 nm, the slow axis of the lower QWP is parallel with an orientation direction of the liquid crystal display layer, an optical retardation of the lower QWP is λ

/4−

β

, β

ranges from zero to 100 nm.

28. The transflective liquid crystal display according to claim 27, wherein the optical retardation of the liquid crystal layer is different between transmissive and reflective portions, the optical retardation is λ

/4+α

in the reflective portion, and the optical retardation is λ

/2+α

+β

in the transmissive portion.

29. The transflective liquid crystal display according to claim 27, wherein the slow axis of the upper HWP forms an angle of θ

with the transmissive axis of the upper polarizer.

30. The transflective liquid crystal display according to claim 27, wherein the slow axis of the upper QWP forms an angle of 2θ

+45°

with the transmissive axis of the upper polarizer.

31. The transflective liquid crystal display according to claim 27, wherein an optimum value of α

ranges from zero to 50 nm for adjusting the optical retardation.

32. The transflective liquid crystal display according to claim 27, wherein an optimum value of β

ranges from zero to 50 nm for adjusting the optical retardation.