Alignment layer
{{Short description|Component of LCDs}}
Alignment layers, or alignment films,{{Cite journal |last=Xi |first=Xiaochuan |last2=Yan |first2=Cenqi |last3=Shen |first3=Larry Zhongxin |last4=Wang |first4=Yinghan |last5=Cheng |first5=Pei |date=2023-12-01 |title=Liquid crystal photoalignment technique: Basics, developments, and flexible/stretchable device applications |url=https://www.sciencedirect.com/science/article/pii/S2772949423000451 |journal=Materials Today Electronics |volume=6 |pages=100069 |doi=10.1016/j.mtelec.2023.100069 |issn=2772-9494|doi-access=free }} are thin films which are a crucial component of liquid crystal displays (LCDs). They are applied to the surfaces of the glass substrates that contain the liquid crystals. The primary function of these layers is to control the orientation of the liquid crystal molecules, which is essential for the proper operation of the display.{{Cite journal |last=Hoogboom |first=Johan |last2=Rasing |first2=Theo |last3=Rowan |first3=Alan E. |last4=Nolte |first4=Roeland J. M. |date=2006-03-24 |title=LCD alignment layers. Controlling nematic domain properties |url=https://pubs.rsc.org/en/content/articlelanding/2006/jm/b510579j |journal=Journal of Materials Chemistry |language=en |volume=16 |issue=14 |pages=1305–1314 |doi=10.1039/B510579J |issn=1364-5501|hdl=2066/35718 |hdl-access=free }} The alignment layer controls the alignment of the liquid crystal immediately adjacent to itself, and long-range interactions force that alignment to extend significantly into the crystal itself.
Alignment layers ensure that liquid crystal molecules are aligned in a specific direction when no electric field is applied. This is critical for the display's function; for example, in twisted nematic (TN) displays, the alignment layers on the two glass substrates are oriented at right angles to each other, creating a 90-degree twist in the liquid crystal molecules, allowing display to modulate light effectively when an electric field is applied.{{Citation needed|date=February 2025}} By applying a voltage across the liquid crystal layer, however, the orientation of the molecules can be altered, affecting the passage of light through the display, and enabling the control of brightness and contrast.{{Citation needed|date=February 2025}}
As of 2025, the two main techniques used to produce alignment layers are rubbing and photo-alignment.{{Cite journal |last=Yu |first=Xinmin |last2=Li |first2=Tangwu |last3=Sang |first3=Jingxin |last4=Xiao |first4=Ming |last5=Shang |first5=Jianhua |last6=Sun |first6=Jiatong |date=2025-01-01 |title=Orientation stabilization of light-controlling layers using liquid crystal polymers |url=https://www.sciencedirect.com/science/article/pii/S0141938224002579#b0015 |journal=Displays |volume=86 |pages=102893 |doi=10.1016/j.displa.2024.102893 |issn=0141-9382|url-access=subscription }} The rubbing method is a process that is nearly 100 years old—by rubbing spin-coated polymers with velvet cloth. As this process presents difficulties when used to make larger displays, other options have been investigated. The rubbing method can result in static electricity issues, dust deposition, and scratches.
Techniques
Traditionally, liquid crystals are aligned by rubbing electrodes on polymer covered glass substrates. Rubbing techniques are widely used in mass production of liquid crystal displays and small laboratories as well. Due to the mechanical contact during rubbing, often debris are formed resulting in impurities and damaged products. Also, static charge is generated by rubbing which can damage sensitive and increasingly miniature electronics in displays.{{Cite journal |last=Seki |first=Takahiro |date=2014-08-13 |title=New strategies and implications for the photoalignment of liquid crystalline polymers |journal=Polymer Journal |language=En |volume=46 |issue=11 |pages=751–768 |doi=10.1038/pj.2014.68 |issn=0032-3896 |doi-access=free}}
Photoalignment is a technique for orienting liquid crystals to desired alignments by exposure to polarized light and a photo-reactive alignment chemical.{{Cite journal |last1=Yaroshchuk |first1=Oleg |last2=Reznikov |first2=Yuriy |date=2012 |title=Photoalignment of liquid crystals: basics and current trends |journal=J. Mater. Chem. |language=en |volume=22 |issue=2 |pages=286–300 |doi=10.1039/c1jm13485j |issn=0959-9428}} It is usually performed by exposing the alignment chemical ('command surface') to polarized light with desired orientation which then aligns the liquid crystal cells or domains to the exposed orientation. The advantages of photoalignment technique over conventional methods are non-contact high quality alignment, reversible alignment and micro-patterning of liquid crystal phases.{{Citation needed|date=March 2025}}
= Advantages of photoalignment =
Many of the problems of rubbing can be addressed by photoalignment:
- Photoalignment is by definition a non-contact process. This allows alignment of liquid crystals even in mechanically inaccessible areas. This has immense implications in use of liquid crystals in telecommunications and organic electronics.{{Cite book |last1=Chigrinov |first1=Vladimir G. |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9780470751800 |title=Photoalignment of Liquid Crystalline Materials |last2=Kozenkov |first2=Vladimir M. |last3=Kwok |first3=Hoi-Sing |date=2008-06-06 |publisher=John Wiley & Sons, Ltd |isbn=978-0-470-75180-0 |series=Wiley-SID Series in Display Technology |location=Chichester, UK |language=en |doi=10.1002/9780470751800}}
- By optical imaging, very small domains can be aligned which results in extremely high quality alignments.{{Citation needed|date=March 2025}}
- By varying the orientation of liquid crystal alignment on a microscopic scale, thin film optical devices can be created like lens, polarizer, optical vortex generator, etc.{{Cite journal |last1=Pan |first1=Su |last2=Ho |first2=Jacob Y. |last3=Chigrinov |first3=Vladimir G. |last4=Kwok |first4=Hoi Sing |date=2018-02-14 |title=Novel Photoalignment Method Based on Low-Molecular-Weight Azobenzene Dyes and Its Application for High-Dichroic-Ratio Polarizers |journal=ACS Applied Materials & Interfaces |language=en |volume=10 |issue=10 |pages=9032–9037 |doi=10.1021/acsami.8b00104 |issn=1944-8244 |pmid=29442496}}{{Cite journal |last1=Ji |first1=Wei |last2=Wei |first2=Bing-Yan |last3=Chen |first3=Peng |last4=Hu |first4=Wei |last5=Lu |first5=Yan-Qing |date=2017-02-11 |title=Optical field control via liquid crystal photoalignment |journal=Molecular Crystals and Liquid Crystals |language=en |volume=644 |issue=1 |pages=3–11 |doi=10.1080/15421406.2016.1277314 |issn=1542-1406 |s2cid=100118998}}
History
The first technique used to produce an alignment layer on LCD devices was simply rubbing the surface of the glass with paper or leather. The practice of adding a rubbed polymer layer to the glass was adopted later, for increased reliability. Large, expensive machines were then invented to precisely rub the substrate.
= Photoalignment =
Photoalignment was first demonstrated in 1988 by K. Ichimura on Quartz substrates with an azobenzene compound acting as the command surface.{{Cite journal |last1=Ichimura |first1=Kunihiro |last2=Suzuki |first2=Yasuzo |last3=Seki |first3=Takahiro |last4=Hosoki |first4=Akira |last5=Aoki |first5=Koso |date=September 1988 |title=Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer |journal=Langmuir |language=EN |volume=4 |issue=5 |pages=1214–1216 |doi=10.1021/la00083a030 |issn=0743-7463}} Shortly after publication of Ichimura’s results, the groups from the USA (Gibbons et al.{{Cite journal |last1=Gibbons |first1=Wayne M. |last2=Shannon |first2=Paul J. |last3=Sun |first3=Shao-Tang |last4=Swetlin |first4=Brian J. |date=1991 |title=Surface-mediated alignment of nematic liquid crystals with polarized laser light |url=https://www.nature.com/articles/351049a0 |journal=Nature |language=en |volume=351 |issue=6321 |pages=49–50 |doi=10.1038/351049a0 |issn=1476-4687 |s2cid=4267126|url-access=subscription }}), Russia/Switzerland (Schadt et al.{{Cite journal |last1=Schadt |first1=Martin |last2=Schmitt |first2=Klaus |last3=Kozinkov |first3=Vladimir |last4=Chigrinov |first4=Vladimir |date=1992-07-01 |title=Surface-Induced Parallel Alignment of Liquid Crystals by Linearly Polymerized Photopolymers |url=https://iopscience.iop.org/article/10.1143/JJAP.31.2155/meta |journal=Japanese Journal of Applied Physics |language=en |volume=31 |issue=7R |pages=2155 |doi=10.1143/JJAP.31.2155 |issn=1347-4065 |s2cid=123181249 |doi-access=free}} and Ukraine (Dyadyusha et al.Dyadyusha, A.G., Kozenkov, V.M., Marusiy, T.Y., Reznikov, Y.A., Reshetnyak, V.Y. and Khizhnyak, A.I., 1991. Light-induced planar alignment of nematic liquid-crystal by the anisotropic surface without mechanical texture. Ukrainskii Fizicheskii Zhurnal, 36(7), pp.1059-1062.{{Cite journal |last1=Dyadyusha |first1=A. G. |last2=Marusii |first2=T. Ya. |last3=Reznikov |first3=Yu. A. |last4=Khizhnyak |first4=A. I. |last5=Reshetnyak |first5=V. Yu. |date=1992-07-01 |title=Orientational effect due to a change in the anisotropy of the interaction between a liquid crystal and a bounding surface |url=http://adsabs.harvard.edu/abs/1992JETPL..56...17D |journal=Soviet Journal of Experimental and Theoretical Physics Letters |volume=56 |pages=17 |bibcode=1992JETPL..56...17D |issn=0021-3640}}) almost simultaneously demonstrated LC photoalignment in an azimuthal plane of the aligning substrate. The latter results have been particularly important because they provided a real alternative to the rubbing technology.{{Cite patent|country=EP|number=0525478|title=Liquid crystal display cell|pubdate=1993-02-03|inventor1-last=Chigrinov|inventor2-last=Kozenkov|inventor3-last=Novoseletsky|inventor4=Victor Yurievich Reshetnyak;Yuriy Alexandrovich Reznikov;Martin Schadt;Klaus Schmitt|inventor1-first=Vladimir Grigorievich|inventor2-first=Vladimir Marcovich|inventor3-first=Nicolic Vasilievich|assign1=Hoffmann La Roche|assign2=Niopic Moscow Research & Production Association}}{{Cite patent|country=US|number=5389698|title=Process for making photopolymers having varying molecular orientation using light to orient and polymerize|pubdate=1995-02-14|inventor1-last=Chigrinov|inventor2-last=Kozenkov|inventor3-last=Novoseletsky|inventor4=Victor Yurievich Reshetnyak;Yuriy Alexandrovich Reznikov;Martin Schadt;Klaus Schmitt|inventor1-first=Vladimir Grigorievich|inventor2-first=Vladimir Marcovich|inventor3-first=Nicolic Vasilievich|assign1=Hoffmann La Roche|assign2=Niopic Moscow Research & Production Association}} Since then several chemical combinations have been demonstrated for photoalignment and applied in production of liquid crystal devices like modern displays.{{Cite journal |last1=Murata |first1=Mitsuhiro |last2=Yokoyama |first2=Ryoichi |last3=Tanaka |first3=Yoshiki |last4=Hosokawa |first4=Toshihiko |last5=Ogura |first5=Kenji |last6=Yanagihara |first6=Yasuhiro |last7=Kusafuka |first7=Kaoru |last8=Matsumoto |first8=Takuya |date=May 2018 |title=81-1: High Transmittance and High Contrast LCD for 3D Head-Up Displays |journal=SID Symposium Digest of Technical Papers |language=en |volume=49 |issue=1 |pages=1088–1091 |doi=10.1002/sdtp.12126 |issn=0097-966X}}
References
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