EP0968538A1

EP0968538A1 - Polymer semiconductor device comprising at least a rectifying function and method for making same

Info

Publication number: EP0968538A1
Application number: EP98906977A
Authority: EP
Inventors: André Lorin; Céline Fiorini; Jean-Michel Nunzi; Carole Sentein
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1997-02-10
Filing date: 1998-02-05
Publication date: 2000-01-05
Also published as: US20030010973A1; FR2759495B1; US6545290B2; FR2759495A1; WO1998035393A1

Abstract

The invention concerns a polymer semiconductor device comprising a rectifying function formed by a polymer film (12) between a first (11) and a second (13) electrode, the polymer film constituting a host matrix for polar molecules, the polar molecules being electrically oriented in a direction perpendicular to the electrodes, the electric charges of the polar molecules having the same sign being directed towards the same electrode. The invention also concerns a method for making such a device.

Description

POLYMER SEMICONDUCTOR DEVICE COMPRISING

LESS RECTIFIER FUNCTION AND METHOD FOR

MANUFACTURE OF SUCH A DEVICE

The present invention relates to a polymer semiconductor device comprising at least one rectifier function. It relates in particular to the diodes and in particular the photovoltaic and light-emitting diodes. It also relates to devices such as transistors.

The use of polymer to produce semiconductor devices is technically quite interesting. In fact, the polymers can be formed by the wet method from a solution, which leads to techniques which are easy to implement, inexpensive and compatible with other techniques.

In order to produce semiconductor devices such as light-emitting diodes and photovoltaic cells, it is usually necessary to make a junction. It may be a Schottky junction obtained by bringing a doped semiconductor into contact with a metal having a rectifier contact with the semiconductor used. It can also be a pn junction obtained by the juxtaposition of a p-type semiconductor and an n-type semiconductor. The junction is said to be homojunction if the semiconductor material is the same for the entire junction. Otherwise, we speak of heterojunction.

It is known to use polymers to produce these two types of diode (Schottky and pn junction). Depending on the type of conductivity required p or n, it is preferable to chemically dope the polymer with electron acceptor or donor atoms (or molecules), respectively.

A disadvantage of Schottky diodes produced by the juxtaposition of a metal and a semiconductor polymer is their short lifespan. This is due to the diffusion of the metal in the polymer, at the level of the contact between the two materials, because of the difference in electrochemical potential existing between these two materials. A pn junction should allow a priori to correct this drawback. It is produced by juxtaposition of a p-type polymer and an n-type polymer. However, it is extremely difficult to produce such pn junctions because the choice of the different polymers is not always compatible with the requirements of implementation. In addition, n-type polymers are uncommon and often unstable to oxygen.

Another problem inherent in polymer diodes of these two types (Schottky and pn) is related to the small extension of the depletion zone at the junction and which is less than 20 nm. In order to avoid short circuits between the electrodes, the devices made up of thin layers of polymer are produced with a thickness greater than or equal to 100 nm. As the mobility μ of the charges in the organic components is a rapidly increasing function with the electric field E internal to the semiconductor according to empirical law

μ = μ ₀ .exp [(E / E ₀ ) P]

where the exponent p is close to 0.5 and since the internal field E is only important in the depletion zone, the electric charges circulate badly in semiconductor polymer devices according to known art. This results in a low efficiency of these devices.

The present invention overcomes these drawbacks. It consists in producing a rectifier homojunction gradient equivalent to a pn junction in a single thin layer of polymer which serves as host matrix for appropriately oriented polar molecules. This results in a significant increase in the mobility of the carriers throughout the thickness of the layer, which results in a depletion zone distributed uniformly. This principle significantly improves the performance of junction devices such as light-emitting diodes, solar cells and transistors made from polymers.

A first object of the present invention therefore consists of a semiconductor polymer device comprising at least one rectifying function, characterized in that the function is performed by a layer of polymer comprised between first and second means forming electrodes, the layer of polymer constituting a host matrix for polar molecules, the polar molecules being oriented electrically in a direction perpendicular to the first and second electrode means, the electric charges of the same sign of the polar molecules being directed towards the same electrode means. A second object of the present invention consists of a method for producing a polymer semiconductor device comprising at least one rectifying function, characterized in that it includes the following steps: - formation of a polymer-based layer comprising polar molecules, the polymer constituting a host matrix for the polar molecules, - orientation of the polar molecules in the host matrix so that the electric charges of the same sign of the polar molecules are directed d 'one side.

The invention will be better understood and other advantages and features will appear on reading the description which follows, given by way of nonlimiting example, accompanied by the appended figures among which:

FIG. 1 is a current-voltage diagram of a structure comprising a layer of polymer placed between two electrodes,

FIG. 2 is a diagram representing the variation in the height of the potential barrier between an electrode and the polymer layer of a structure according to the invention as a function of the orientation rate of the polar molecules,

FIG. 3 is a diagram representing the mobility of the electrons in a structure according to the invention before polarization and for two opposite values of the polarization field,

FIG. 4 represents a side view of a photocell according to the present invention,

- Figure 5 is a perspective view of a photocell device according to the present invention, - Figure 6 shows, side view, a light emitting diode according to the present invention.

The diagrams of FIGS. 1, 2 and 3 make it possible to illustrate the principle which is the basis of the present invention. They relate to a symmetrical diode made up of a thin film of semiconductor polymer located between two electrodes of the same kind and comprising polar organic molecules. The thin film is a 50/50 (MMA-DR1) copolymer which will be defined below, MMA playing the role of host matrix and DR1 constituting the polar molecules. The electrodes are made of aluminum.

Initially, the structure thus formed is perfectly symmetrical as shown by its current-voltage characteristic represented by curve 1 on the diagram in FIG. 1 and recorded at room temperature. The principle on which the present invention is based consists in inducing a pn type rectifier diode function by modifying the very nature of the polymer layer situated between the electrodes. The modification is carried out by orientation of the polar molecules in the host matrix. By applying a static electric field to the thin film and simultaneously bringing this thin film to a temperature close to its glass transition temperature Tg, the polar molecules are oriented quantitatively according to the electric field. This orientation is fixed while maintaining the static electric field during the cooling phase of the structure. It is possible to directly link the internal field induced in a polarized structure to the orientation rate of polar molecules.

The effect of the orientation of the polar molecules appears on the curves of the diagram I = f (V) of FIG. 1. Curve 1 showing the current-voltage characteristic before polarization of the structure, curve 2 shows the current characteristic- voltage of a similar structure obtained after polarization at 5 V, at a temperature of 130 ° C for 10 mm, curve 3 shows the current-voltage characteristic of a similar structure obtained after polarization at 10 V, at a temperature of 130 ° C for 10 mm, curve A shows the current-voltage characteristic of a similar structure obtained after polarization at 15 V, at a temperature of 130 ° C for 10 mm. The rectifier effect, after polarization at a temperature close to the glass transition temperature appears clearly on the diagram of FIG. 1.

By taking the classic model of the Schottky diode for semiconductors, we can make a theoretical adjustment of the current-voltage curves using usual parameters:

I = I _s [exp (qV / nkT) -l]

with I _s : saturation current

V: applied voltage n: ideality factor q: electron charge k: Boltzmann constant

T: absolute temperature. It is thus possible to estimate for each of the polarization values of the structure the saturation current. By analogy with mineral semiconductors, we can then calculate Φ _j - ,, metal-semiconductor potential barrier by the following expression:

with A: area of the diode

R: Richardson constant,

R ^* = 4πm _e qk ² / h ³ The value Φj-, is compared with orientation rate measurements made by generation of second harmonic. These orientation rate measurements can be carried out as indicated in the thesis of G. GADRET, entitled "Nonlinear optical properties of polarized polymers: study of the chromatic dispersion of their electro-optical coefficient by modulation of the reflectivity" and sustained at the University of Paris XI in 1993. The variation in the height of the metal / organic semiconductor potential barrier is thus linked to the orientation rate of the molecules in the polymer matrix.

FIG. 2 shows that, by orientation of the molecules, a function equivalent to a pn junction has been produced with an internal potential difference close to 0.2 eV. It is a homojunction gradient distributed over the entire thickness of the polymer film. It can also be seen in FIG. 1 that in addition to the rectifier effect (asymmetry of the characteristic), the conductivity of the diode in the on state (positively polarized diode) is significantly increased after orientation of the molecules. This effect is confirmed by measurement of flight time. This measurement makes it possible to determine the type of conduction of a material

(conduction n if the electrons are the majority carriers, or p if they are the holes) and the mobility of the charges. The diagram in FIG. 3 shows the results obtained for the mobility of the electrons before and after orientation of the molecules for two opposite values of applied polarization field at 130 ° C. Mobility in the diagram in Figure 3 is plotted against electric field E applied during the measurement. The mobility law μ = μ ₀ .exp [(E / E ₀ ) ^l / ² ]

is verified. The increase in mobility of electrons is asymmetrical. It goes in the direction of the induced rectifying effect.

In the diagram of FIG. 3, the mobility μ of the electrons is plotted on the ordinate axis and the electric field E on the abscissa axis. In the diagram of FIG. 3, the triangle-shaped sign represents the mobility of the electrons before polarization under an electric field, the square-shaped sign represents the mobility of the electrons after polarization resulting from the application of a negative voltage of 100 V, for 5 mm and at 130 ° C, the sign in the shape of a circle represents the mobility of the electrons after polarization resulting from the application of a positive voltage of 100 V, for 5 mm and at 130 ° C. The diagram in FIG. 3 shows that, by orientation of the molecules, the electronic mobility μ ₀ in the passing direction without an applied external field is increased by a factor of 4. This effect is fundamental for the performance of semiconductor devices of the photocell and diode type. electroluminescent.

It is advantageous to use, as polar molecules, active organic molecules derived from the field of quadratic nonlinear optics. They are of the "push-pull" type, that is to say that they have both an electron donor group (of the ammo type) and an electron acceptor group (of the nitro type), separated l 'from one another by one or more groups comprising conjugated π electron systems, therefore capable of moving over several atoms (diazobenzene in the case of DR1 molecule, for example). This type of molecule is widely described in "Molecular Nonlinear Optics: materials, physics and devices", edited by J. ZYSS at Académie Press, Inc. (1994), "Organic Nonlinear Optical Materials", Vol. 1, by Ch. BOSSHARD, K. SUTTER, Ph. PRETRE, J. HULLIGER, M. FLORSHEIMER, P. KAATZ, P. GUNTER, edited by Gordon and Breach Publishers, (1995) and in document FR-A-2 732 482 disclosing a "Method for manufacturing structures in organic polymers, transparent, self-organized for optical frequency conversion", by F. CHARRA, C. FIORINI, A. LORIN, JM. NUNZI, P. RAIMOND.

An example of a polar molecule is Disperse Red 1 (DR1):

4- [N- (2-hydroxyethyl) -N-ethyl] -amino-4 '-nitroazobenzene (Aldrich, recrystallized) which has the following chemical structure:

This molecule can be used in a solar cell as a photo-generator of charges.

Another example of a polar molecule is DCM: 4- (dicyanomethylene) -2-methyl-6- (p-dimethylaminostyryl) - 4H-pyrane (Exciton) which has the following chemical structure:

This molecule can be used in a light-emitting diode as a red light emitter at 620 nm.

The polymer which should be used is advantageously soluble. It must have significant carrier mobility after inclusion of the active molecules by doping or by grafting to the main chain. They may be semiconductor polymers such as those described in "Handbook of conductive g polymers", published in two volumes by SKOTHEIM in 1986 by Marcel Dekker. Soluble derivatives of polythiophene (PT), polyparaphenylene (PPP), and polyparaphenylenevmylene (PPV) are some examples. It can also be polymers which become semiconductor by doping, for example polymethylmethacrylate (PMMA), polyvmylcarbazole (PVK), polycarbonate (PC), polystyrene (PS), and polyvmylchloride (PVC).

For example, polymethylmethacrylate (PMMA) is used as an isotropic polymer matrix, optically mactive and transparent in the visible and near infrared. Its chemical formula is as follows:

The active molecule can be chemically attached (grafted) to the polymer. Thus, the DR1-MMA copolymer grafts at 50 ° into moles of chromophores (DR1-MMA 50/50) is obtained by radical polymerization from a solution of methylmethacrylate (MMA) and N-ethyl-N- (methacryloxyethyl) -4 '-ammo-4-nιtroazobenzene (derivative of DR1). This material has a glass transition temperature close to 132 ° C. Its formula is as follows:

The semiconductor device according to the present invention allows in particular the production of a solar cell (photovoltaic cell). A conventional embodiment of a solar cell is described in the article "Organic Solar Cells" by D. OHRLE and D. MEISSNER, published in Advanced Materials, Vol. 3 (1991), n ° 3, pages 129-138. The photocell according to the invention is shown diagrammatically in FIG. 4. It comprises a transparent substrate 10, for example made of glass, supporting a transparent electrode 11, for example made of mixed tin and indium oxide (ITO electrode). An organic semiconductor layer 12 consisting of a polymer including polar molecules is included between the transparent electrode 11 and a metal electrode 13, for example made of aluminum. The electrodes 11 and 13 are respectively connected to output terminals 14 and 15. In operation, the light, indicated by arrows in Figure 4, is absorbed by the transparent face of the photodiode. Electric charges are then photo-generated near the transparent electrode 11 - polymer layer 12 interface. The electric field internal to the structure, which is induced by the orientation of the polar molecules contained in the polymer host matrix, promotes separation of charges and the displacement of one of the carriers towards the aluminum electrode. A current is then generated.

For a more industrializable realization of the solar cell, it is possible to use a substrate constituted by a standard glass slide, covered with ITO (treatment Conduct 013 A from Balzers) with a resistance equal to 13 Ω / D, and a thickness equal to 1 mm. The conductive coating is then severe by laser attack according to the configuration illustrated in FIG. 5 or the reference 20 designates the glass slide. The laser ablation divides the conductive and transparent coating into three parts 21, 22 and 23. After cleaning with a conventional detergent (TDF4), the parts 21, 22 and 23 are rinsed with distilled water and then undergo several treatment cycles with ultrasound. The substrate formed by the glass slide and its conductive coating is then heated in an oven at 400 ° C. so as to remove any residual organic impurities by pyrolysis.

The organic film is obtained by centrifuging the polymer solution on the substrate (spinning deposit). The thickness of the layer and its homogeneity depend on the parameters of rotation of the spinner and on the viscosity of the solution. The device is set to double speed.

Films of 0.13 μm thickness are obtained from a 50/50 a copolymer solution (MMA-DRl) 30 g / 1 in trιchloro-1, 1, 2-ethane. The solution is filtered at the time of deposition through a 0.5 μm filter. The centπfugation parameters are as follows: Deposit phase: first acceleration:

200 rpm / s, first cycle: 110 rpm, for 10 s, Drying phase: second acceleration: 300 rpm / s, second cycle: 5000 rpm for 50 s. After deposition, the substrate covered with the thin film is annealed for 15 minutes at 80 ° C., which makes it possible to remove the residual solvent. The thickness is measured using a Veeco profilometer model Dektak 3ST.

The left part of the thin film corresponding to parts 22 and 23 is then partially scraped to expose these parts 22 and 23 which will serve as electrical contact pads. Similarly, the extreme right part of the thin film is also scraped to facilitate contact.

Then, as can be seen in FIG. 5, aluminum electrodes 24 and 25 are deposited on the organic thin film 29 by evaporation through a mask. Evaporation is carried out under vacuum at 1.33.10 ^-4 Pa (10 ^{~ 6} Torr). It simultaneously allows the deposition of the conductive strips 26 and 27 ensuring the electrical connection between the electrodes 24 and 25 and the pads 22 and 23 respectively. Two solar cells are obtained, the unit area of which is 38 mm ² and having a common electrode formed by the conductive part 21. The device obtained is then brought to a temperature of 130 ° C. and a voltage of 15 volts is applied to each diode for 10 minutes. Then the temperature regulation of the device is stopped while keeping the same applied bias voltage. The temperature of the device is allowed to drop to 25 ° C in 30 minutes. The applied static electric field is then cut off.

The effect of the DR1 dye is that the majority charge carriers in the 50/50 (MMA-DRl) copolymer are the electrons (n-type material). Their mobility is therefore increased by orienting the chromophores of DR1 with their electron donor group (ammo) on the side of the ITO coating and their electron acceptor group (nitro) on the side of the aluminum electrode. This is achieved by applying the positive value of the voltage V to the ITO coating during the polarization phase. More generally, for a polymer film of the same composition and of different thickness, the device will be polarized with a voltage V close to 100 V / μm.

The photocell thus described generates a current of electrons at the aluminum electrode when it is illuminated in white light on the transparent face. The semiconductor device according to the present invention also allows the production of a light-emitting diode. A conventional embodiment of light-emitting diode is described in the article "Blue light-emittmg diodes with doped polymers" by E. GAUTIER, J. -Mr. NUNZI, C. SENTEIN, A. LORIN and P. RAIMOND publishes in Synthetic Metals, Vol. 81, 1996, p. 197-200.

A light-emitting diode according to the present invention is shown diagrammatically in FIG. 6. Its structure is identical to that of FIG. 4. It comprises a transparent substrate 30, for example made of glass, supporting a transparent electrode 31, for example an ITO electrode. An organic semiconductor layer 32 consisting of a polymer including polar molecules is included between the transparent electrode 31 and a metal electrode 33, for example made of aluminum. The electrodes 31 and 33 are respectively connected to input terminals 34 and 35. In operation, light, indicated by arrows in FIG. 6, is emitted through the transparent face of the light-emitting diode.

When the diode is subjected to a positive voltage on the transparent electrode 31, charges are injected by the two electrodes 31 and 33 into the polymer 32. By recombining, these charges emit light through the transparent face of the device. The electric field internal to the structure, which is induced by the orientation of the polar molecules contained in the polymer host matrix, favors the injection of the charges and their recombination by +/- pairs in the polymer. Indeed, this internal field limits the direct transit, without recombination, of charges from one electrode to another.

Light-emitting diode structures can also be produced according to the arrangement shown in FIG. 5. The organic mmce film can be obtained from a mixture of the PMMA polymer at 30 g / 1 in trιchloro-1, 1, 2-ethane with the DCM molecule at 5 ° by mass of PMMA (1.5 g / 1 of solution). By following the procedure described above for the photocell in FIG. 5, organic films 0.1 μm thick can be obtained. The aluminum electrodes are also deposited as described above. The polarization of the device is also done in the manner indicated above. The light-emitting diode thus described emits red light at 620 nm from the transparent electrode when it is subjected to a voltage greater than 20 V (positive on the transparent electrode). Different variant embodiments can be envisaged in the context of the present invention. Thus, the active organic layer can be produced by mixing different polymers and different molecules. Likewise, these different species can be chemically linked (copolymer).

The transparent substrate can be ordinary glass, but any other transparent dielectric material is suitable, for example transparent plastics such as polycarbonate, PMMA. These plastic materials allow the production of flexible devices (in sheets, rolls, etc.).

In order to let the useful light through, at least one of the electrodes must be transparent. If, as is the case described so far, it is the substrate which is transparent, the electrode adjacent to this substrate may be made of medium oxide, of mixed oxide of etam and medium or may be made of a thin film of transparent conductive polymer in the useful light field, for example of the polyaniline (Pani, distributed by Monsanto) or PEDT type (distributed by Bayer), or else be made of a metallic thin film ( aluminum, gold, silver, ...) deposited by evaporation and with a thickness close to 10 to 50 nm. The upper electrode is advantageously a thin metallic film deposited by evaporation (aluminum, gold, silver, magnesium, etc.). Depending on the type of device to be produced, this electrode can be opaque or semi-transparent. There are different techniques for applying a static electric field to a thin film. We can polarize thanks to electrodes evaporated on the substrate then on the film. This is the method used above. This field can also be applied by depositing on the film, which is generally highly resistive, ions created by the Corona effect near a point or a metallic wire. This is the method that polarizes the cylinders of laser printers. Polarization can also be carried out cold, when it is optically assisted.

The technique of polarization by electrodes consists in applying an electric field by means of electrodes placed on either side of the film. The lower electrode may simply be a conductive substrate (doped silicon for example), a metal electrode evaporated on the substrate before the deposition of the organic film, or a transparent electrode (ITO) depending on the type of device to be produced. The upper electrode is a thin metallic film deposited by evaporation.

The direct and precise access to the value of the electric field applied to the film constitutes the essential advantage of this method. In addition, it is a method applicable to the device as it is produced. By bringing the material at the time of polarization to a temperature close to its glass transition temperature, the mechanical stresses imposed by the polymer chains constituting the matrix decrease and this leads to the orientation of the doping molecules. It is then necessary to maintain the static field during the cooling phase of the sample, before cutting it at room temperature to freeze the orientation of the molecules. For certain molecule / polymer pairs, it is not necessary to heat the organic layer to orient the molecules. This is the case of DR1 in the PVK. For other molecule / polymer pairs, orientation can be achieved without heating, by applying a light beam absorbed by the molecules, which beam produces an effect equivalent to heating.

As mentioned above, polarization can also be carried out by the Corona effect. The application of an electrical potential on a metal surface with a small radius of curvature (tip) results in the creation of a high surface density of charges at the convex interface. The electric flow is therefore concentrated near this point, leading to the ionization of a gas present near the interface. The appearance of ions in the gas increases its conductivity, and an electric discharge appears when the voltage applied to the gas exceeds the value of the dielectric breakdown voltage. Corona polarization consists of depositing these ions on the thin film to polarize it. The substrates to be used are the same as for the previous method. The metallic upper electrode will then be deposited, once the polarization has been achieved.

It is also possible to produce the devices described above with an inverted structure. In this case, the substrate and / or the lower electrode may be opaque. The upper electrode is then necessarily transparent or semi-transparent. It can be carried out by cathodic deposition of ITO or deposition of a metallic thin film of thickness close to 10 to 50 nm (aluminum, gold, silver, etc.). The advantage of such a type of device is that it can be produced by deposition on a film flexible aluminum type substrate Mylar® (distributed by Dupont de Nemours). We then gain a production step, that relating to the deposition of the lower electrode. It is also within the scope of the present invention to produce semiconductor devices other than light-emitting or photovoltaic diodes, for example field effect type transistors of which the active semiconductor layer is formed and / or produced according to the characteristics of the invention. In this case, the increase in the mobility of the charge carriers induced according to the proposed principle is of great interest because the characteristics of the transistors (contrast between the on state and the blocking state) strongly depend on this. The advantage of organic transistors is that they make it possible to produce sensors (detectors and dosimeters).

Claims

1. polymer semiconductor device comprising at least one rectifying function, characterized in that the function is performed by a layer of polymer (12,29,32) between the first (11,21,31) and the second (13, 24,25,32) means forming electrodes, the polymer layer constituting a host matrix for polar molecules, the polar molecules being oriented electrically in a direction perpendicular to the first and second means forming electrodes, the electric charges of the same sign of the polar molecules being directed to the same electrode means.

2. Device according to claim 1, characterized in that said polar molecules are molecules doping the host matrix.

3. Device according to claim 1, characterized in that said polar molecules are molecules grafted into the host matrix.

4. Device according to any one of claims 1 to 3, characterized in that said polar molecules have an electron acceptor group and an electron donor group separated by one or more groups comprising conjugated π electron systems.

5. Device according to any one of claims 1 to 4, characterized in that the host matrix is based on a polymer chosen from polythiophene, polyparaphenylene, polyparaphenylenevmylene, polymethylmethacrylate, polyvmylcarbazole, polycarbonate, polystyrene and poly (vinyl chloride).

6. Device according to any one of claims 1 to 5, characterized in that it comprises a substrate (10,20,30) serving as a support for the first electrode forming means (11,21,31), at least one of the elements consisting of either the substrate and the first electrode means or the second electrode forming means (13,24,25,33) being transparent.

7. Device according to claim 6, characterized in that the substrate (10,20,30) serving as support for the first electrode means is made of glass or plastic.

8. Device according to any one of claims 1 to 5, characterized in that it comprises a conductive substrate serving both as a support and as first means forming an electrode.

9. Device according to any one of claims 6 to 8, characterized in that, the device being a photocell, said polar molecules are molecules of 4 [N- (2-hydroxyethyl) -N- ethyl] -amino-4 '-nitrobenzene.

10. Device according to any one of Claims 6 to 8, characterized in that, the device being a light-emitting diode, said polar molecules are molecules of 4- (diciyanomethylene) -2-methyl-6- (p-dimethylammostyryl) - 4H-pyrane.

11. Method for producing a polymer semiconductor device comprising at least one rectifying function, characterized in that it includes the following steps:

- formation of a layer (12, 29, 32) based on polymer and comprising polar molecules, the polymer constituting a host matrix for the polar molecules,

- orientation of polar molecules in the host matrix so that electrical charges likewise sign polar molecules are directed on the same side.

12. Method according to claim 11, characterized in that the polar molecules are introduced as dopants in the host matrix.

13. Method according to claim 11, characterized in that the polar molecules are introduced by grafting into the host matrix.

14. Method according to any one of claims 11 to 13, characterized in that the orientation of the polar molecules in the host matrix is carried out:

- by applying, for a determined period, a static electric field to said layer (12,29,32), this layer being brought to a temperature close to its glass transition temperature,

- Then, maintaining the static electric field during the cooling of said layer.

15. Method according to any one of claims 11 to 13, characterized in that the orientation of the polar molecules in the host matrix is carried out by applying, for a determined period, a static electric field on said layer, this layer being subjected on impact of a light beam.

16. Method according to one of claims 14 or 15, characterized in that the electric orientation field is applied to said layer (12,29,32) by means of electrodes (11,13,21, 24,25,31,33) and intended to constitute electrodes for the device.

17. Method according to one of claims 14 or 15, characterized in that the electric orientation field is applied to said layer by Corona effect.