WO2002073306A1

WO2002073306A1 - Liquid crystal display panel

Info

Publication number: WO2002073306A1
Application number: PCT/IT2002/000105
Authority: WO
Inventors: Romeo Beccherelli
Original assignee: Universita' Degli Studi Di Roma 'la Sapienza'
Priority date: 2001-03-12
Filing date: 2002-02-21
Publication date: 2002-09-19
Also published as: ITRM20010124A1; EP1368696A1; ITRM20010124A0

Abstract

Display panel based on the use of a liquid crystal (8) which modulates a light emitted by a backlighting light source (electrodes 2, 4 and emitting layer 3) based on organic electroluminescent materials and directly fabricated onto one of the sublayers (1) thereof. Preferably, the electroluminescent material consits of liquid crystalline polymers emitting polarized light.

Description

LIQUID CRYSTAL DISPLAY PANEL

DESCRIPTION The present invention relates to an LCD display panel apt to modulate light emitted from a light source incorporated therein. Polymeric or organic light-emitting diodes (LEDs) used in image representation systems are known. These diodes are used to fabricate flat panel displays in which the intersections of rows and columns define picture elements (pixels) which are controlled individually by means of a matrix of switching devices, or scanned by means of a matrix of passive electrodes, thus taking advantage of the intrinsically nonlinear relationships among applied voltage current and photon flux generated by the emitting diode.

These diodes are also used as light source for display panels based on the modulation effects of liquid crystals on transmitted light.

However, a direct control is viable merely with a very small number of picture elements, due to the complexity and to the costs deriving from the increasing number of controlling circuits and related connections. The control by a matrix having switching devices is restricted by the areas on which such switching devices can be cost-effectively fabricated. The control by the intrinsic nonlinearities entails the flowing of quite a large current during a short row addressing time. This limits the maximum brightness of the panel and the lifetime thereof, as well as introducing flicker problems irksome to the user.

It is also known, from US 5,965,907 in the name of Rong-Ting Huang et al., the use as backlight source of overlapped panels, each corresponding to a single organic semiconductor LED of one of the three primary colors (red, green, blue) as backlight source for an LCD panel. Such a hybrid liquid crystal display panel can be provided with suitable laminated polarizers and it is apt to independently modulate the light sequentially emitted from each of the three emitting panels. By doing so, either colored filters or the increase of the number of physical elements (subpixels) constituting the pixels can be avoided. However, this solution requires the overlapping of several sublayers in order to implement the trichromatic light source, with the entailed increase in weight and bulk, ill-tolerable in applications in which the dimensions and the weight of the display panel are of primary importance.

Object of the present invention is to solve the abovementioned problems of the known art providing an LCD display panel, comprising: - one or more supporting layers;

- two or more conductive layers, each thereof being apt to serve as electrode; - one emitting layer, apt to implement a light source; and

- one liquid crystal light-modulating layer, characterized in that said light source is incorporated therein and it is of the electroluminescent semiconductor type. The main advantage of the panel according to the present invention lies in the use of a backlight source consisting of organic light-emitting semiconductor diodes, and not resulting in a package of greater weight and bulkiness.

This source is based on preferably organic electroluminescent materials and it is fabricated directly onto one of the substrates of the same panel. This fabricating technique determines a structure which is much thinner, lighter and more compact compared to that of the traditional flat LCD panels in which the backlighting is provided by a discrete and much bulkier package, usually consisting of one or more fluorescent tubes, light guides and diffusers.

A second advantage of the present invention lies in that the display panel is sturdier and it consists of a reduced number of parts and components, with the entailed simplification of the production processes, which therefore are also more cost-effective.

The LCD display panel according to the present invention is apt to be used in all those applications in which thickness, weight and power consumption are crucial. Exemplary fields and marketed products in which said panel is advantageously employable are: second- (GSM) and third- (UMTS) generation cell phones with connectivity to the Internet , global positioning systems (GPS), viewfinders, portable

TV sets, e-newspapers and e-books, portable computers and calculators, palm-top digital organizers, viewfinders for cameras and electronic videocameras and other portable display systems powered by batteries or storage cells.

Moreover, the reduction in the number of the parts and components also entails a reduction in the production, assembly and maintenance costs which are advantageous also for display devices in systems powered by main, or by other power supplies which are not particularly limited, desktop computer and TV monitors, programmable windows and decorative walls, navigation devices for motor vehicles, aircrafts and ships, devices for projecting virtual images on motor vehicle, aircraft and ship screens.

Moreover, the incorporation of a light source and of a modulator can be advantageous in order to implement a particularly effective optical processing element.

Further advantages, features and modes of employ of the present invention will be made apparent in the following detailed description of preferred embodiments thereof, given by way of example and without limitative purposes, making reference to the figures of the attached drawings, wherein: figure 1 is a sectional perspective view of a first embodiment of the panel according to the present invention; figure 2 is a sectional perspective view of a second embodiment of the panel according to the present invention; figure 3 is an exploded perspective view of a display panel according to the present invention; and figure 4 is an examplary timing of the control signals of a panel according to the present invention.

With initial reference to figure 1, a sectional perspective view of a first embodiment of the panel according to the present invention is illustrated.

On a first supporting layer 1, preferably made of materials like plastics, glass, quartz or metal, and of a thickness typically ranging from 100 μm to 1 mm, a first conductive layer 2, is deposited or grown. The layer 2 is itself made as a reflective metal layer . The layer 2 has a low work function, (e.g. Aluminum, Calcium, Barium, Lithium, Magnesium or Magnesium/Silver alloys). Said layer 2 can be made or treated to limite mirror reflection and enhance light diffusion.

The layer 2 also serves as a cathode for an emitting layer 3 made of a material emitting in the visible wavelengths of light (390-789 nm) which is deposited by spin coating, electrochemically, by inkjet printing or vacuum techniques, The emitting material can be of inorganic type, like e.g. ZnS, ZnSe, CdS, etc., or of organic polymer type, like e.g. the poly(para-phenylenevinylene), the poly(para-phenylene), the poly(alkylfluorene), the polythiofene, the poly(alkylthiofene), the poly(N- phenylcarbaxole), the polyphenylacetylene, the polyaniline and several variants and derivatives thereof. It can also be an olygopolymer formed by the same monomers constituent of the above polymers. Moreover, it may be a "low molecular weight" material, like e.g. the anthracene, the naphtalene, the 8-hydroxylquinoline

Aluminium (known as Alq3). In the case of an organic material, it can preferably be sandwiched between other two or more separation layers of material which facilitate electron and hole transport.

To increase the efficiency, dopants apt to accelerate the phosphorescence- enhancing radiative emission processes can be added as well.

A second conductive layer 4 is overlapped to the layer 3. The layer 4 is of the transparent type and it also serves as anode, being characterized by a high work function.

The constituent materials of said second conductive layer 4 may be of inorganic (e.g., Indium and/or Tin oxides in vacuum or from a liquid dispersion) or of organic type (polyaniline, deposited electrochemically or by spin coating).

The anodic and the cathodic functions are also swappable therebetween. A transparent cathode can be made of a suitably doped polyaniline. A reflecting anode can be made, e.g., with a noble metal like gold. In this configuration, anodes can still be made with transparent materials. However should be overlapped to a further reflective layer. This further reflective layer can be a low cost reflecting metal like alluminium or silver or dielectric mirror. The light emitted by the emitting layer 3 can be monochromatic, polychromatic or white, unpolarized or partially or totally polarized.

One dielectric layer 5, made of an either organic- or inorganic-type material, is overlapped to the emitting layer 3, in order to electrically insulate and chemically protect the latter from the subsequent overlapped layers.

Alternatively, the layer 5 can be fabricated with a thin layer of prelaminated material, like e.g. the mylar.

Onto the layer 5 it is deposited a third conductive layer 6, transparent and made, with lithography techniques, of an organic- or inorganic-type material, apt to define display panel rows.

Alternatively, in the case of the latter can be deposited with inkjet-printing techniques directly to form the row electrodes.

The panel according to the present invention further comprises one or more alignment layers 7, 9 for liquid crystals deposited according to techniques known to the art, like e.g. the spin-coating for organic materials (polyimide, polyamide, polyvinyl alcohol), and aligned by rubbing, or irradiation with (optionally polarized) light (visible or UV).

Moreover, said alignment layers can be fabricated by sputtering or thermal evaporation of, e.g., Silicon oxides with a suitable incidence angle.

Onto a protective layer 11, optionally also serving as support, preferably made of plastics or vitreous material, it is deposited a fourth conductive layer 10 onto which the panel column electrodes are lithographically defined. Onto said conductive layer

10 the second alignment layer 9 is deposited.

The supporting layer 1 and the protective layer 11, with the layers deposited thereon, are assembled to form a panel as in the known LC art. A liquid crystal light- modulating layer 8 (modulator 8), sandwiched between the layers 1 and 11, can be injected from a slit in its isotropic phase and left to cool until it reaches a more ordered phase. Alternatively, it is possible to spread (eg. by screen printing techniques) the liquid crystal layer 8 processed onto the as abovedescribed layers 1 and/or 11 and then to carry out the assembling.

Furthermore, the positions and/or the functions of the row and of the column electrodes are easily swappable therebetween, as it is apparent to those skilled in the art. In order to ensure a tight seal, the panel is sealed.

Onto the outer side of the layer 11 a first polarizing layer 12 is laminated. Moreover, also further layers, apt to improve the angle of sight and not shown in the figure, can be used.

For single-polarizer applications the liquid crystal can be of the guest-host type. A panel as the one hereto described can function in a transmissive as well as in a reflective mode. In the first case (transmissive mode) the light is generated by the layer 3 when a sufficient voltage is applied onto the electrodes 2 and 4. Then the light is modulated by the liquid crystal layer 8 under the action of the commands (controls) applied onto the electrodes 6 and 10. In the second case (reflective mode) no electric field is applied between the electrodes 2 and 4, rather the environment light is modulated via the polarizing layer 12. In this case, the modulation effect can be the guest-host one.

The various operation modes of the liquid crystal, the selection of the relative directions according to which the alignment layers 7 and 9 should be rubbed and the absorption axis of the polarizing layer 12 be positioned, as well as the selection of the dielectric anisotropy and optical dichroism signs are known, hence they will not be detailed hereinafter.

In general, light extinction will be attained when the absorption axis of the dichroic dye lies among the layers and in a direction perpendicular to that of the polarizer absorption. Instead, a partial light transmission will be attained when the absorption axis of the dichroic dye does not lie along said direction.

Next, figure 2 is a sectional perspective view of a second embodiment of the panel according to the present invention.

In order to use a liquid crystal technology operating under polarized light, between the layers 4, 5 a second polarizing layer can be sandwiched. This polarizing layer can be fabricated overlapping a layer 13 of polymerized cholesteric liquid crystal to a quarter- wave foil 14. The dielectric layer 5 can be present as in the figure, precede the layer 5, or be absent. The liquid crystal (e.g., a monoacrylate) preferably in a nematic phase is mixed to a chiral dopant (e.g., a diacrylate) and to a photoinitiator. hi the presence of an UV radiation the liquid crystal polymerizes stabilizing the planar helical cholesteric structure.

The helical structure configuration behaves as a Bragg reflector and it reflects at a wavelength X_R = n.p (where n=refractive index and p=pitch) a single circular polarization, letting the other one pass. The range of wavelenghts at which the reflection occurs can be approximated to Δλ = p.Δn (where .Δn= dielectric anisotropy of the cholesteric mixture). For applications under a substantially white light, said range Δλ can be widened by modulating the pitch of the helical structure. This may be carried out by continuously varying the concentration of the chiral dopant. The component of the circular or helical polarization reflected by the Bragg reflector is diffused by the metallic cathode, fabricated so as to be diffusive to the utmost, and the luminous intensity is depolarized and again reflected towards the Bragg reflector, where once more one of the circular or helical polarizations is transmitted and the other one thereof is reflected.

After several reflections, a satisfactory quantity of light crosses the Bragg reflector with a circular (or a helical) polarization, and, transiting the quarter-wave foil 14, it is turned into a linear polarization.

The operation principle and the Bragg reflector theory are well known to those skilled in the art, and hence will not be detailed hereinafter.

Small losses can occur by absorption into the emitting layer 3 and a fraction of the absorbed light may be reemitted (as photoluminescence). According to a third embodiment of the panel according to the present invention, the light is generated by the emitting layer already in a polarized form, in order to be able to use a liquid crystal technology operating under polarized light.

This may be effected conferring to the emitting conjugated polymer a directional order, typical of nematic phases, and optionally also a positional one, typical of smectic or hexatic phases. This order can be conferred by rubbing the emitting conjugated polymer or a related underlying layer (e.g. the poly(para- phenylenevinylene) or derivatives thereof) like a hole-transport layer or another dedicated layer. In this latter case, useful polymers are the polyamides, the polyimides, the polyvinylalcohol, the polyester, the polyaniline aligned with various rubbing techniques or the polytetrafluoroethylene hot-deposited by friction, as well as the photoalignable polymers.

In order to facilitate the charge transport and to reduce the threshold voltages for the light emission, the alignment layer can be doped with charge transfer complexes like, e.g., the tetrathiafulvalene /7,7,8,8-tetracyanoquinodimethane complexes. Likewise, the same emitting materials can be aligned (can be photoalignable conjugated polymers).

Finally, the emitting layer can be already polymerized prior to the deposition or it can be polymerized in situ starting from mesogens.

A fourth embodiment of the panel according to the present invention is provided in order to be able to use a liquid crystal technology operating under polarized light. To this end, the light emitted by the emitting layer (optionally already partially polarized) passes, before reaching the liquid crystal light-modulating layer, through a further layer fabricated with thin film techniques (spin-coating, spray or roll techniques) and serving as polarizer.

This further layer comprises molecules (optionally polymerized) of a dichroic dye. Preferably, said molecules have self-assembling properties and are, optionally or additionally, alignable, e.g. by rubbing techniques.

The same technique for fabricating a thin film polarizer can be used to fabricate the polarizing layer 12. The latter, besides being overlapped to the protective layer 11, can also be located at an intermediate position sandwiched between said layer 11 and the liquid crystal light-modulating layer 8.

The availability of polarized light as obtained in the two abovedescribed embodiments enables to use a very wide range of modulation effects by the top liquid crystal layer. To this end, advantageously the guest-host effects can still be used, or "S" or "B" effects of the nematic liquid crystals, or twisted nematics (TN), supertwisted nematics (STN), hybrid alignment nematics (HAN) or in-plane switching nematics, as it is known in the art of the liquid nematic crystals, included those having surface or volume memory with the entailed bi- or multistability.

Alternatively, liquid smectic-phase crystals like SmA*, SmC*, SmCA*, SmI*, SmLA* or the various subphases comprised thereamong can advantageously be used. In case a bistable operation liquid crystal technology is used, the production of several gradations for each color proves particularly cumbersome. A viable solution is to spatially subdivide the pixels, defined by the intersection of rows and columns, into subpixels related thereamong as the powers of two. Alternatively, the refresh period (frame) can be subdivided into several time subintervals having durations related thereamong as the powers of two. It is also possible to jointly use spatial and temporal subdivisions, adopting suitable subdivision ratios. However, the spatial subdivision complicates and increases the costs related to the connections and to the control circuits, and the temporal subdivision requires shorter times, and therefore higher control voltages, which are unattainable. The panel according to the present invention simply solves the temporal subdivision problem, as the response of the emitting layer is fast and the light can also be amplitude modulated. Hence, it is possible to write «fields» all alike in duration and therefore to modulate the intensity of the light emitted from the various fields according to ratio related thereamong as the powers of two. The frame reduction factor for N=2ⁿ levels is [(N-l)/n].

Preferably, the light source can be controlled only after a "field" has been written. Alternatively, by subdividing the source in the direction of the rows, a greater panel luminosity, and lesser stresses in terms of current and of peak voltage can be provided thereto.

Thus, a portion of the panel can be written whereas the others are lightened. Satisfactory results are obtained with a number of subdivisions ranging between two and a quarter of the number of rows, in order to limit the number of circuits apt to control the light source.

Next, figure 3 is a schematic perspective view of the panel according to the present invention.

In particular, there are highlighted the layers serving as electrodes (anode and cathode) for the light source and the row and column electrodes of the liquid crystal modulator.

Figure 4 shows an exemplary timing of the selection signals in the case of a panel in which the liquid crystal modulator consists of 64 rows (and of a number of columns which is irrelevant to the ends of the description) and the light source is subdivided into eight strips.

The lighting onto the first strip is activated during the scanning and writing steps of all of the rows except from those comprised between the first and the eighth. For sake of simplicity, it is assumed that said steps be concomitantly carried out, though different cases may be envisaged. For an improved operation, the emitting layer can be apt to emit solely at the pixel defined by the intersection of the row and column electrodes of the liquid crystal modulator. This can easily be carried out, e.g. by lithographic definition of one of the layers serving as electrode, preferably the layer indicated with 2 in the figure.

Thus, the equivalent of a "black mask" is implemented, with the corresponding absence of emission in the intra-pixel separation areas. This is particularly advantageous jointly to the use of bistable liquid crystals which would be in an indefinite modulation state in the intra-pixel separation areas.

A panel having the above characteristics could have various operation modes: - with memory and reflecting without its own lighting, with the advantage of a virtually nil consumption. A typical application can be that of a display panel of a portable phone displaying the name of the service provider or of an electronic notebook displaying a list of meetings; - with memory and transmitting with its own lighting, wherein the consumption is due to the sole inner light source, e.g. when any one key of a portable phone be pressed;

- with gradation in reflection, with the liquid crystal modulating light coming from the environment;

- with gradation in transmission, with the liquid crystal modulating light from an inside source.

In order to provide a color image, the light source is advantageously fabricated overlapping several emitting layers, sequentially modulated, each emitting at a primary color wavelength of one of the primary colors. Therefor, further layers fabricated according to the techniques described in the present invention, yet not explicitly shown in the embodiments of figures 1 and 2, are required.

The liquid crystal can modulate continuously, or a modulation technique analogous to the abovedescribed one can be adopted. Alternatively, in order to provide a color image starting from the white source, color filters, preferably in a number equal to three, e.g. defining stripes parallel the columns of the liquid crystal modulator, can be used. Said strips can be located above the emitting layer or onto the opposite layer 11. The pixel of the modulator can be subdivided in correspondence of the emitting stripes. hi order to provide a color image starting from a blue source it is advantageous to use two photoluminescent layers re-emitting preferably in the remaining two primary colors (green and red).

Alternatively, in order to provide a color image starting from an UV source, it is advantageous to use three photoluminescent layers re-emitting preferably in the three primary colors (blue, green and red).

In these two latter cases as well, the solution of the stripes parallel to the columns deposited on the side of the layer 11 of the liquid crystal modulator is advantageous.

In order to ensure an elevated image quality, there may advantageously be used a matrix having a switching device at each liquid crystal subpixel. Each switching device can be a three-terminal device like a thin film transistor, or a two-terminal device like a back to back diode. The material used to fabricate the switching devices can be amorphous Silicon, polycrystalline Silicon, or single-crystal Silicon, or a chalcogenide, or an organic semiconductor. Preferably the liquid crystal used is of nematic type, however it can also be of twisted nematic type, vertical alignment nematic, hybrid alignment nematic, plane commutation nematic, flexoelectric nematic, Smectic A*, Smectic C*, surface-stabilized in a "chevron", "bookshelf, or "quasi-bookshelf configuration, ferroelectric surface-stabilized in a twisted structure, or deformed-helix-ferroelectric, antiferroelectric with or without threshold.

Of course, the abovedescribed possible variants can be used combined thereamong. E.g., it will be possible to use a polymer emitting a partially polarized light, a thin film polarizer, fabricating onto the top sublayer a thin film transistor matrix in amorphous Silicon or in polycrystalline Silicon.

Or, it will be possible to use light sources by overlapping the sources emitting light in the three primary colors to a thin film transistor matrix.

The present invention has hereto been described according to preferred embodiments thereof, given by way of a non-limiting example. It is understood that other embodiments may be provided, all to be construed as falling within the protective scope thereof, as defined by the appended claims.

Claims

I. An LCD display panel, comprising: one or more supporting layers (1);

- two or more conductive layers (2, 4, 6, 10), each thereof being apt to serve as electrode; one emitting layer (3), apt to implement a light source; and one liquid crystal light-modulating layer (8), characterized in that said light source is incorporated therein and it is of the electroluminescent semiconductor type. 2. The panel according to claim 1, further comprising a protective layer (11).

3. The panel according to claim 1 or 2, wherein at least one of said two or more conductive layers is apt to define panel rows.

4. The panel according to any one of the preceding claims, wherein at least one of said two or more conductive layers, is apt to define panel columns. 5. The panel according to any one of the preceding claims, further comprising a dielectric layer (5).

6. The panel according to any one of the preceding claims, further comprising one or more alignment layers (7, 9).

7. Panel according to claim 6, wherein one of said one or more alignment layers is ofthe Langmuir-Blodgett type.

8. The panel according to claim 6 or 7, wherein one of said one or more alignment layers is of polymeric type.

9. The panel according to any one of the claims 6 to 8, wherein one of said one or more polymer alignment layers is of the photoalignable type. 10. The panel according to any one of the claims 6 to 9, wherein one of said one or more polymeric alignment layers is of the rubbed type.

II. The panel according to any one of the claims 6 to 10, wherein one of said one or more polymeric alignment layers is deposited by hot friction.

12. The panel according to any one of the claims 6 to 11, wherein one of said one or more polymeric alignment layers is of conductive type.

13. The panel according to any one of the preceding claims, further comprising a first polarizing layer (12).

14. The panel according to any one of the preceding claims, wherein said light source comprises a layer of conjugated polymeric material. 15. The panel according to any one of the preceding claims, wherein said light source comprises a layer of conjugated oligomeric material.

16. The panel according to any one of the preceding claims, wherein said light source comprises a layer of molecular organic electroluminescent material.

17. The panel according to any one of the preceding claims, wherein said light source comprises a layer of inorganic electroluminescent material. 18. The panel according to any one of the claims 14 to 16, wherein said light source is characterized by a liquid-crystal phase with symmetry in the nematic phase and/or in the smectic phase and/or in the exatic phase and/or in the smectic phase, and/or in the discotic phase.

19. The panel according to claim 18, wherein said liquid-crystal phase is aligned by photoalignment techniques.

20. The panel according to claim 18, wherein said liquid-crystal phase is aligned by rubbing.

21. The panel according to claim 18, wherein said liquid-crystal phase is aligned by applying an electrical field. 22. The panel according to claim 18, wherein said liquid-crystal phase is aligned by applying a magnetic field.

23. The panel according to claim 18, wherein said liquid-crystal phase is aligned by overlapping it to one of said alignment layers.

24. The panel according to any one of the claims 14 to 17, wherein said material implementing said light source is doped with an element apt to enhance the radiative phosphorescence..

25. The panel according to any one of the preceding claims, further comprising one or more separation layers, located between said emitting layer (3) and one of said two or more conductive layers (2), said separation layers being fabricated with organic material and apt to facilitate the electron transport.

26. The panel according to any one of the claims 1 to 16 or 18 to 25, further comprising one or more separation layers, located between said emitting layer (3) and one of said two or more conductive layers (4), said separation layers being fabricated with organic material and apt to facilitate the hole transport. 27. The panel according to any one of the claims 1 to 16 or 18 to 26, wherein said light source is apt to emit polarized light.

28. The panel according to any one of the preceding claims, further comprising a second polarizing layer, sandwiched between said emitting layer (3) and said light- modulating layer (8). 29. The panel according to claim 28, wherein said second polarizing layer is fabricated with thin film techniques.

30. The panel according to claim 29, wherein said second polarizing layer is a circular polarizer.

31. The panel according to claim 30, wherein said circular polarizer is fabricated overlapping a Bragg reflector to a quarter-wave foil.

32. The panel according to any one of the claims 29 to 31, wherein said circular polarizer comprises one layer of polymerized cholesteric liquid crystal.

33. The panel according to any one of the preceding claims, further comprising one or more color filters.

34. The panel according to any one of the preceding claims, further comprising one or more photoluminescent filters. 35. The panel according to any one of the preceding claims, further comprising electronic circuits for the control thereof.

36. The panel according to claim 35, comprising a matrix having switching devices , controlled by said electronic circuits.

37. The panel according to claim 36, wherein each of said switching devices is a two-terminal device.

38. The panel according to claim 36, wherein each of said switching devices is a three-terminal device.

39. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with amorphous Silicon. 40. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with polycrystalline Silicon.

41. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with single-crystal Silicon.

42. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with chalcogenides.

43. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with conjugated polymers.

44. The panel according to any one of the claims 36 to 38, wherein each of said switching devices is fabricated with conjugated organic molecules. 45. The panel according to any one of the preceding claims, wherein the liquid crystal modulating layer (8) is of the bistable type.

46. The panel according to claim 45, wherein the emitting layer (3) is apt to emit light solely in correspondence of pixels defined by the intersection of rows and columns. 47. The panel according to any one of the preceding claims, wherein the emitting layer (3) is subdivided into strips, each of said strip corresponding to a set of rows of the modulator.

48. The panel according to any one of the preceding claims, wherein the emitting layer (3) is fabricated overlapping two or more emitting layers, each of said emitting layers being apt to emit light in a predetermined wavelength range.

49. The panel according to any one of the claims 45 to 48, wherein fields are sequentially written onto the modulator (8) and the emission of the emitting layer (3) is weighed so as to attain intermediate light and/or color gradations.