CH705051B1 - An active matrix display. - Google Patents

Abstract

The display device (1) is an active matrix. It comprises a lower substrate (2), a transparent top substrate (3), a substrate sealing frame defining a closed cavity for a substance (7) whose optical properties change in the presence of an electric field. A single transparent electrode (4) is disposed on an inner face of the upper substrate, while an inner face of the lower substrate carries an electrode matrix (11). The electrodes of the matrix are arranged in rows and columns, each being controlled by a respective thin-film transistor (T). The drain (11) of each transistor is connected to the corresponding electrode. The gates (12) of the transistors are connected in each column of the matrix by a respective conductive control track, while the sources of the transistors are connected in each row of the matrix by a respective data conductive track. First and second alignment layers (5, 6) of particles of the substance are placed on the electrodes of the upper and lower substrates. Each thin film transistor is an organic transistor, which comprises an organic semiconductor layer (13) between drain (11) and source (10) layers, and an organic insulating layer (14) on the semiconductor layer organic. A conductive grid layer (12) is placed on the organic insulating layer. This organic insulating layer comprises a crosslinking agent rendering it insensitive to solvents for deposition of an alignment layer.

Description

The invention relates to an active matrix display device, for example of the LCD type. The device comprises a lower substrate, a transparent upper substrate and a substrate sealing frame defining a closed cavity in which there is a substance whose optical properties change in the presence of an electric field. An inner face of one of the substrates carries a single electrode, while an inner face of the other substrate carries an array of electrodes for defining pixels of the device. The electrodes of the matrix are arranged in rows and columns, each being controlled by a respective thin-film transistor. The drain of each transistor is connected to the corresponding electrode. The gates of the transistors are connected in each column of the matrix by a respective conductive control track, while the sources of the transistors are connected in each row of the matrix by a respective data conductive track. First and second particle alignment layers of the substance are placed on the electrodes of the upper and lower substrates.

In a passive matrix LCD type display device, the conductive tracks of the rows and columns are controlled outside the cavity by a control circuit. By row, the pixels of a column are addressed one after the other. The pixel is off between two refresh operations. It may therefore be advantageous to use a liquid crystal display device, which has an active matrix, which makes it possible to keep a charge (information) on each electrode in the absence of a supply voltage.

It is well known to provide such active matrix liquid crystal display devices, especially for large display devices.

Thin-film transistors for connecting the matrix electrodes to the conductive tracks of rows and columns are traditionally made with inorganic materials. A disadvantage of the realization of these types of transistors is that the deposition operations of the different layers of inorganic material are expensive, and performed slowly and relatively high temperature. Therefore, it is necessary to make the transistors of such display devices on rigid substrates supporting high temperature.

The invention therefore aims to overcome the disadvantages of the state of the art by providing an active matrix display device, which is easy, inexpensive and quick to manufacture with low temperature operations for the production of thin-film transistors on any type of substrate.

For this purpose, the invention relates to an active matrix display device cited above, which comprises the characteristics defined in the independent claim 1.

[0007] Particular embodiments of the active matrix display device are defined in the dependent claims 2 to 10.

An advantage of the active matrix display device according to the invention is that it comprises organic thin film transistors, which are easy to perform at low temperatures. These transistors comprise in particular an organic insulating layer, which may be made of poly (2-hydroxyethyl methacrylate) (PHEMA). This organic insulating layer may be crosslinked by means of a crosslinking agent, such as poly (melamine-co-formaldehyde) in order to be insensitive to the solvents used in the production of alignment layers, for example polyimide. This makes it possible to remain compatible with the traditional production method of a display device, for example a liquid crystal display.

Preferably above the organic semiconductor layer, the organic insulating layer comprises a first layer of polymer, for example poly (2-hydroxyethyl methacrylate) and a second layer of poly (2-hydroxyethyl methacrylate) crosslinked on the first layer. This makes it possible to protect this insulating layer of solvents used during the deposition of the polyimide or other alignment layers. After polymerization and crosslinking, the first insulating layer PHEMA has a very good insulating behavior for the transistor, while the second insulating layer xl-PHEMA provides the necessary protection against solvents for depositing the alignment layers.

Advantageously, the substrates of the display device may be insulating plastic and flexible as the realization of the transistors is made at low temperature. At least the upper substrate is transparent. Thus the realization of such organic transistors is compatible with the method of manufacturing a conventional LCD display device, without the use of acid as initiator.

Advantageously, the organic transistors can be printed on flexible inorganic or organic substrates. These transistors can thus be easily integrated into a display device, for example a liquid crystal display.

The objects, advantages and characteristics of the active matrix display device will appear better in the following description on the basis of at least one non-limiting embodiment illustrated by the drawings in which: <tb> fig. 1 <sep> schematically represents an arrangement of conductive tracks connected via thin-film transistors to an electrode array of the display device according to the invention, and <tb> fig. 2 <sep> represents a vertical partial section of the active matrix display device according to the invention at a thin-film transistor.

In the following description, all elements of the active matrix display device which are well known to those skilled in this technical field will be reported in a simplified manner. Reference is made primarily to an active matrix liquid crystal display device, the substrates of which may be rigid or flexible.

In FIG. 1, there is shown schematically an arrangement of electrodes on one of the substrates of the device for defining pixels of an active matrix of the liquid crystal display device. These electrodes 11, which may be transparent, are each connected by means of a respective organic thin-film transistor T (TFT) to conductive tracks of rows L1, L2, and of columns C1, C2, which may also be transparent. These electrodes 11 are controlled by these thin-film transistors to be biased at a determined voltage level with respect to a determined potential of a single electrode facing the other substrate. Depending on the voltage level applied to a portion of the matrix electrodes, information may be displayed on the display as a result of the orientation of the liquid crystals above these electrodes.

It should be noted that in order not to overload FIG. 1, each thin-film transistor is represented only by its gate 12, its source 10 and its drain 11. Thin-film transistors T each comprise an organic semiconductor layer disposed partly between drain-conducting layers 11 and 10 source, which can be arranged directly on the lower substrate. At least one organic insulating layer is disposed on the organic semiconductor layer, and a gate conductive layer 12 is disposed on the organic insulating layer. The drain conductor layer 11 of each transistor is an integral part of the corresponding electrode of the large matrix with respect to the source and gate conductive layers.

All the electrodes of the matrix are preferably arranged in rows and columns. The gates 12 of the organic thin film transistors T are connected in each column of the matrix to a respective control conductor track C1, C2. The sources 10 of these transistors are instead connected in each row of the matrix to a respective data conductive track L1, L2. The set of conductive tracks of parallel columns crosses in isolation the set of conductive tracks of parallel rows. All conductive tracks are traditionally connected outside the liquid crystal cavity of the display device to a control circuit for controlling the display of information.

In FIG. 2, there is shown in greater detail a partial vertical section of the active matrix display device 1 according to the invention at an organic thin-film transistor. In known manner, the display device 1 comprises a transparent upper substrate 3, a lower substrate 2 and a not shown sealing frame defining a closed cavity in which there is a liquid crystal substance 7, for example of the TN or STN type. The substrates used may advantageously be flexible plastic substrates, since the production of the transistors of the active matrix is made at low temperature. The plastic material may be polyester (PES) or polycarbonate (PC).

The transparent and insulating upper substrate carries on an inner face a transparent electrode 4 for example indium oxide / tin, which extends over the entire inner face. A first layer of polyimide (PI) for alignment or orientation of the liquid crystals covers the transparent electrode 4 entirely.

As indicated above, the source 10 and drain 11 conductive layers may be made directly on an inner face of the lower substrate 2. The organic semiconductor layer 13 may be disposed on the inner face of the lower substrate between the layers drain and source and partly on the drain and source layers. The organic insulating layer 14 may be disposed on the organic semiconductor layer so as to cover it preferably completely. This insulating layer 14 makes it possible to guarantee a better barrier against the O2 and H2O compounds in order to protect the organic semiconductor layer (P3HT). This insulating layer 14 may also serve to serve as mechanical protection during the "friction" for the mechanical structuring of the second alignment layer. Finally, the gate conductive layer 12 of each thin-film transistor is disposed on the organic insulating layer partly over a space separating the drain layer and the source layer.

The drain layers 11, which constitute the electrodes of the matrix, and the source layers 10 of the thin-film transistors may be made of indium / tin oxide (ITO), or even of indium / zinc oxide. (IZO) or other transparent conductors. The organic semiconductor layer may be made of poly (3-hexylthiophene) designated P3HT. The organic insulating layer 14 may be made of either poly (methylmethacrylate) designated PMMA, or preferably poly (2-hydroxyethyl methacrylate) designated PHEMA. The gate layer 12 comprises a conductive material, such as silver and / or aluminum. This grid layer can be easily obtained by a heliographic printing technique or intaglio (gravure printing in English terminology) by means of a silver ink for example.

The organic insulating layer may also be deposited before the gate layer by means of the same heliographic printing technique with PMMA ink or poly (vinylpyrrolidone) (PVP) or poly (vinylalcohole). (PVA) or poly (2-hydroxyethyl methacrylate) (PHEMA). However, after this step, it is necessary to carry out a polymerization and / or crosslinking operation of said layer preferably at a low temperature so as not to destroy the other materials.

The assembly, which comprises the electrodes 11 of the matrix, the organic thin film transistors T, and the conductive tracks of rows and columns, is covered by a second polyimide alignment layer 6. realization of this second polyimide alignment layer, it is used solvents capable of dissolving the organic insulating layer and / or the organic semiconductor layer of each thin-film transistor. To protect the organic insulating layer 14 and / or the organic semiconductor layer 13 of each transistor, a solvent-insensitive crosslinking agent of the polyimide layer is added.

A solvent used for producing the polyimide alignment layers of the liquid crystal of the display device may be N-methylpyrrolidone (NMP) with butylcellosolve (trade name). This solvent may also be dimethylformamide (DMF) or dimethylacetamide.

The organic insulating layer 14 is preferably composed of two insulating layers. A first organic insulating layer is composed of a polymer, such as poly (2-hydroxyethyl methacrylate) designated PHEMA. This first organic layer is disposed on the semiconductor layer 13 having a good insulating behavior. A second organic layer is composed of cross-linked poly (2-hydroxyethyl methacrylate) designated x1-PHEMA. During the low temperature polymerization, the second organic layer is crosslinked by a crosslinking agent, which may be poly (melamine-co-formaldehyde) (PMF). However other crosslinking agents can be imagined. This second organic layer is placed on the first organic layer to protect it from the solvent (s) used during the deposition of the second polyimide alignment layer 6.

For the realization of some organic layers of thin film transistors, it may be envisaged to print these layers by a heliographic technique, which is faster than an inkjet technique. In the case where the organic insulating layer is composed of crosslinked poly (2-hydroxyethyl methacrylate), this can make it possible to guarantee a good mechanical behavior for a recessed impression by means of a conductive ink of the gate layer of each transistor.

The display device may be a twisted nematic type liquid crystal device (TN or STN), the cholesteric texture type (CT), a liquid crystal bistable display device (ZBD, Nemoptic or other devices). ), an electrolytic display, a bistable electrochromic display, an electrophoretic display, an electroweak display, a dispersed polymer display (PDLC), a display In-plane switching (IPS), an interference modulation display (IMOD), an organic light-emitting diode (OLED) display, or other display devices.

From the description that has just been made, several variants of the active matrix display device can be designed by those skilled in the art without departing from the scope of the invention defined by the claims. The arrangement of the layers of each portion of the thin film transistors can be realized differently. It can be provided for each transistor of the active matrix, to make the gate, the drain and the source below or above the semiconductor layer, or to make the gate below the semiconductor layer. and the drain and the source above the semiconductor layer. It may be provided to place the transparent electrode array with the thin-film transistors on the inner face of the transparent top substrate. Each substrate may also be glass. The operation by crosslinking the organic insulating layer can be carried out by ultraviolet treatment or by a heat treatment.

Claims (10)

An active matrix display device (1), the device comprising a lower substrate (2), a transparent upper substrate (3), a substrate sealing frame defining a closed cavity in which there is a substance (7) whose optical properties change in the presence of an electric field, an inner face of one of the substrates carrying a single electrode (4), while an inner face of the other substrate carries an electrode matrix (11) for defining pixels of the device, the electrode or electrodes on the upper substrate being transparent, the electrodes of the matrix being arranged partly in rows and in columns, each electrode of the matrix being controlled by a respective thin-film transistor (T), whose drain (11) is connected to the corresponding electrode, the gates (12) of the transistors being connected in each column of the matrix by a respective conductive control track ( C1, C2), while the sources of the transistors are connected in each row of the array by a respective data conductive track, a first alignment layer (5) of particles of the substance being placed on the substrate electrode (s). upper and a second alignment layer (6) of particles of the substance being placed on the entire surface of the electrode or electrodes of the lower substrate, characterized in that each thin-film transistor is an organic transistor, which comprises a semi-layer organic conductive element (13) arranged in part between drain (11) and source (10) conductive layers, and at least one organic insulating layer (14) arranged in contact with one side of the organic semiconductor layer, the gate conductive layer (12) being disposed on one side of the organic insulating layer opposite to the contact face of the organic semiconductor layer, and in that the organic insulating layer c includes a crosslinking agent rendering it insensitive to solvents for deposition of the second alignment layer. An active matrix display device (1) according to claim 1, characterized in that the electrode matrix and the thin-film transistors (T) are arranged on the inner face of the lower substrate (2), which is insulation, and in that the drain conductor layer of each transistor is an integral part of the corresponding electrode of the matrix. Active matrix display device (1) according to claim 2, characterized in that the drain (11) and source (10) conductive layers of each thin-film transistor are arranged directly on the inside of the substrate. lower, in that the organic semiconductor layer (13) is disposed on the inner face of the lower substrate between the drain and source conductive layers and partly on the drain and source conductive layers, in that the organic insulating layer (14) is arranged on the organic semiconductor layer, and in that the gate conductive layer (12) of each thin-film transistor is arranged on the organic insulating layer partly over a space. separating the conductive drain layer and the source conductive layer. 4. Active matrix display device (1) according to one of the preceding claims, characterized in that the drain and source conductive layers of each thin film transistor are made of indium / tin oxide, in that that the organic semiconductor layer is made of poly (3-hexylthiophene), in that the organic insulating layer is made of poly (2-hydroxyethyl methacrylate), and in that the gate conductive layer comprises a conductive material, such as only silver and / or aluminum. An active matrix display device (1) according to claim 4, characterized in that the gate conductive layer is a layer printed with a conductive ink comprising silver. An active matrix display device (1) according to claim 4, characterized in that the organic insulating layer completely covers the organic semiconductor layer of each thin-film transistor to act as a barrier against oxygen and some water. An active matrix display device (1) according to one of the preceding claims, characterized in that the organic insulating layer (14) comprises a first polymer insulating layer and a second poly (2-hydroxyethyl methacrylate) layer. crosslinked on the first layer, to be protected from solvents for depositing the second alignment layer, in particular polyimide, on the second layer of said organic insulating layer. 8. Device of said active matrix display layer (1) according to claim 7, characterized in that the first insulating layer of polymer comprises a layer of poly (2-hydroxyethyl methacrylate). An active matrix display device (1) according to claim 7, characterized in that the crosslinking agent is poly (melamine-co-formaldehyde). 10. Active matrix display device (1) according to one of the preceding claims, characterized in that the substrates (2, 3) are flexible plastic substrates.