DISPLAY CELL

Abstract: A display cell having two parallel transparent substrate plates which are interconnected at their periphery and which are each provided with an electrode and an orientation layer, and comprising a liquid crystalline compound in the cell space between the plates, the orientation layer being a monolayer of an alkyl mercaptan.

Description: The invention relates to a display cell comprising two parallel, closely spaced transparent substrate plates which are interconnected at the periphery by means of a seal, the plates being provided with an electrode at the facing surfaces and with an orientation layer which is applied to the electrode and which imposes a homeotropic or tilted orientation on a liquid crystalline compound which is provided in the space bounded by the plates and the seal. Various types of liquid crystalline displays (display cells) require a homeotropic orientation of the liquid crystalline (LC) compound relative to the substrate plate carrying the electrode. In the case of a homeotropic orientation the molecules of the LC compound have a preferred direction which is perpendicular to the surface of the substrate plate. A homeotropic orientation is necessary, inter alia , for LC displays having dynamic scattering, ECB (electrically controlled birefringence) and cholesteric-nematic phase transitions. In European Patent Application No. 0173.369 (PHN 11.102) in name of Applicants it is described that an orientation layer of certain silane compounds brings about a homeotropic orientation. Another important object in accordance with the invention is a display cell the orientation layer of which has a multilayer structure which brings about a tilted orientation of the LC compounds. For a proper understanding it is to be noted that an orientation layer does not have to be oriented itself. Such a layer imposes (induces) an orientation on molecules of a liquid crystalline compound. In the case of a tilted orientation, the molecules of the LC compound are oriented at an orientation angle (tilt angle) to the surface of the orientation layer. The orientation angle is calculated relative to the surface of the orientation layer. A tilt angle of, in particular, 15-30° is of substantial importance for the so-called STN displays (supertwist nematic) in which the liquid crystalline cell medium has a twisted configuration of 180-360°. Twist is to be understood to mean herein the rotation of the direction (director) of the longitudinal axis of the molecules of the LC compound across the cell thickness. An interesting display having a twist of 270° is known as supertwist birefringence effect cell (SBE). It is known from, for example, J.Appl. Phys. 56 (2), 15 July 1974, pp. 251-261, that in order to obtain a tilted orientation, an orientation layer of SiO is applied by means of vapour deposition at an angle of 80-86° relative to the normal to the substrate surface. In other words, an oblique vapour deposition. This process is very laborious and has the disadvantage that the process parameters are difficult to control. The tilt angle depends upon the vapour-deposition conditions. A precise and uniform adjustment of the tilt angle is impossible. The display cell in accordance with the invention does not have this disadvantage. In particular, the invention provides the possibility of precisely and uniformly adjusting any desired tilt angle. The objects mentioned hereinbefore are achieved in accordance with the invention by means of a display cell of the type mentioned in the opening paragraph, which is characterized in that the orientation layer comprises a monolayer of an alkylmercaptan. The monolayer imposes a homeotropic orientation on the LC molecules. The quality of the orientation (the directional force) is enhanced when the length of the alkyl group increases. Preferably, an alkyl mercaptan is used the alkyl group of which contains at least 12 carbon atoms, in particular of from 12-20 carbon atoms. A suitable orientation layer contains a monolayer of octadecylmercaptan. The orientation layer is bonded to the surface of the electrode via the mercaptan group. The long-chain alkyl group of the mercaptan compound extends perpendicularly to the surface of the electrode. In a suitable embodiment of the display cell in accordance with the invention, an orientation layer is used which contains a metal layer and a monolayer of alkyl mercaptan provided thereon. The mercaptan compound bonds very well to a metal surface. A metal layer which is preferably used is a layer of Ag or Au. By means of an electroless process such as a vapour deposition process or a sputtering process, the metal layer is provided in a very thin layer having a thickness in the nanometer range. In a very special embodiment of the display cell in accordance with the invention, an orientation layer is used which has a multilayer structure and which, starting from the electrode, consists of a layer of SiO x , wherein x = 1-2, a discontinuous metal layer and a monolayer of an alkyl mercaptan provided on the metal layer. By means of said special embodiment a tilted orientation with an adjustable tilt angle can be obtained. As is customary, the electrode used is transparent and is made of, for example, tin oxide or ITO (indium-tin-oxide). A layer of SiO x such as a layer of SiO is provided on the electrode by means of vapour deposition. The layer thickness is small and amounts to, for example, 10-30 nm. The obtained SiO x layer itself brings about an orientation of the LC compounds which is parallel to the surface. A discontinuous metal layer is provided on the SiO x surface. A suitable metal layer is, in particular, a layer of Ag or Au. The metal layer is deposited by means of an electroless process, in particular, a vapour deposition process or sputtering process. Vapour deposition or sputtering is carried out such that not the entire surface of the SiO x layer is covered with the metal layer. Such a discontinuous, interrupted, metal layer can for example be applied by using a mask which is provided with apertures and which covers parts of the surface of the SiO x layer. A suitable choice of the mask permits the metal layer to be provided in any desired pattern. Preferably, the discontinuous metal layer is a vapour-deposited layer in the insular stage which is evenly distributed and which has a thickness of no more than, approximately, 10 nm. Such a discontinuous metal layer can simply be obtained by stopping the vapour deposition process before the surface to be metallized is covered with a continuous layer. In this initial stage of the vapour deposition process the metal layer has an insular structure of separate areas which are not connected to one another. In a further preferred embodiment, the metal layer is an Ag layer or an Au layer. The discontinuous metal layer is provided with the above-said monolayer of alkyl mercaptan. To this end, the substrate containing the discontinuous metal layer is treated with a solution of the mercaptan in a suitable organic solvent such as an aromatic or aliphatic hydrocarbon, for example hexane. The treatment can be carried out by immersing the substrate in the mercaptan solution and removing the excess by rinsing. This solution may alternatively be provided on the substrate (with the metal layer) by mist spraying, spraying, pouring etc. The obtained tilt angle of the liquid crystalline compound depends on and is determined by the ratio between the surface of the discontinuous metal layer and the surface of the SiO x layer which is not covered with metal. If this ratio is high, a large tilt angle is obtained. The size of the tilt angle increases as the coverage of the SiO x layer with metal areas increases. If the SiO x layer is hardly provided with metal, the tilt angle is small and is close to 0° so that the orientation is substantially planar. As soon as the SiO x layer is substantially completely covered with metal a maximum tilt angle of almost 90° is obtained and, hence, the orientation is substantially homeotropic. The length of the alkyl chain of the alkyl mercaptan and the type of liquid crystalline compound also influence the tilt angle. The invention will now be explained in more detail by means of the drawing in which the Figure is a cross-sectional view of a display cell in accordance with the invention. In the drawing, the reference numerals 1 and 2 denote two parallel substrate plates which are made of glass and which are located at some distance from one another. The plates are interconnected at the periphery by a seal 3 which is made of, for example, an elastomer. At the facing surfaces, the plates 1 and 2 are each provided with a transparent electrode 4, 5, respectively, which is made of, for example, indium-tin-oxide. The electrodes are provided with a layer 6, 7, respectively, of SiO which is provided by means of vapour deposition at an angle of 40° to the normal to the substrate surface. The layer thickness is 20 nm. An SiO layer which is provided by means of vapour deposition at an angle of from 30-70° to the normal brings about a planar orientation of liquid crystalline compounds. A gold film having a thickness of 0.5 nm is provided by means of vapour deposition on the SiO layer 6, 7, respectively. The gold film of this thickness is in the so-called insular stage, in which separate areas 8, 9, respectively, of gold are deposited on the underlying SiO surface. The areas 8, 9, respectively, are provided with a monolayer of octadecylmercaptan which is not shown. To this end, the substrate plates 1 and 2 are immersed in a 0.7% solution of octadecylmercaptan in hexane for 3 minutes, after which they are rinsed in pure hexane. A liquid crystalline compound 10 is present in the cell space surrounded by the substrate plates 1, 2 and the seal 3. The tilt angle of the liquid crystalline compound is measured magnetically and amounts to 55°.

Claims: A display cell comprising two parallel, closely spaced transparent substrate plates which are interconnected at the periphery by means of a seal, the plates being provided with an electrode at the facing surfaces and with an orientation layer which is applied to the electrode and which imposes a homeotropic or tilted orientation on a liquid crystalline compound which is provided in the space bounded by the plates and the seal, characterized in that the orientation layer comprises a monolayer of an alkyl mercaptan. A display cell as claimed in Claim 1, characterized in that the orientation layer contains a metal layer and a monolayer of alkyl mercaptan provided thereon. A display cell as claimed in Claim 2, characterized in that the orientation layer has a multilayer structure and, starting from the electrode, consists of a layer of SiO x , wherein x = 1 or 2, a discontinuous metal layer and a monolayer of an alkyl mercaptan provided on the metal layer. A display cell as claimed in Claim 3, characterized in that the discontinuous metal layer is a vapour-deposited layer in the insular stage with a thickness up to 10 nm. A display cell as claimed in Claim 2, 3 or 4, characterized in that the metal layer is an Ag layer or Au layer. A display cell as claimed in Claim 1, characterized in that a monolayer of octadecylmercaptan is used.

Inventors: PONJEE, JOHANNES JACOBUS, VOERMANS, ANTONIUS BERNARD
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G09F9