WO2007107522A1

WO2007107522A1 - Devices with a pressure-sensitive layer comprising intrinsically pressure-sensitive organic material

Info

Publication number: WO2007107522A1
Application number: PCT/EP2007/052529
Authority: WO
Inventors: Michael Olk; Alexei Boulbitch; Aloyse Schoos
Original assignee: Iee International Electronics & Engineering S.A.
Priority date: 2006-03-17
Filing date: 2007-03-16
Publication date: 2007-09-27
Also published as: WO2007107523A1; WO2007107524A1; WO2007107525A1

Abstract

A switching element responsive to pressure comprises at least two electrodes, which are separate from one another and an intrinsically pressure-sensitive organic layer arranged between and in contact with the electrodes in such a way that, in response to a pressure being applied to the switching element, the intrinsic electrical conductivity of the pressure-sensitive organic layer increases between at least one pair of the electrodes and, consequently, the electrical resistance between the pair of electrodes decreases. At least one of the electrodes comprises a resistive layer where this electrode faces another one of the electrodes with respect to the intrinsically pressure-sensitive organic layer.

Description

DEVICES WITH A PRESSURE-SENSITIVE LAYER COMPRISING INTRINSICALLY PRESSURE-SENSITIVE ORGANIC MATERIAL

Technical field

[0001] The present invention generally relates to intrinsically pressure-sensitive organic materials and in particular to electrical devices, e.g. pressure-sensitive switching elements comprising a pressure-sensitive layer containing such intrinsically pressure-sensitive organic material.

Background Art

[0002] Force or pressure sensors whose operation is based on conductivity of a special material depending on the applied force or pressure are known in the art. For instance, Ruben (U.S. patent 2,375,178) and Costanzo (U.S. patent 3,386,067) disclose a sandwiched structure enclosing a fibrous or sponge-like material containing conducting particles.

[0003] Mitchell (US patent 3,806,471) describes a switch based on a pressure responsive semiconductive composite material containing particles of molybdenum disulphide connected by an elastic binder material.

[0004] Eventoff (US patents 4,315,238 and 4,314,227) teaches a pressure sensor based on an elastic membrane that is deflected under the action of a pressure or force so as to establish an electrical contact between two electrodes through a thin pressure-sensitive layer. According to Eventoff, the pressure-sensitive layer may consist of 1 to 10 μm semiconductive particles (such as molybdenum disulphide) mixed with a binder material such as a resin. Such materials are presently available on the market and are used to produce stress sensors (see also LaIIi, J. H et. al, Proc. SPIE 5758, 333 (2005)).

[0005] Another approach to pressure sensors has been disclosed by Anderson (U.S. patent 4,347,505). Anderson introduced a pressure-sensing mat containing a thin, resiliently deformable sheet of semiconductor material having a pressure-independent intrinsic electrical conductivity. The sheet of semiconductor material is sandwiched between a pair of conductive electrodes, e.g. metal sheets. The surface of the semiconductor material has microscopic ridges and depressions therein, which are deformed in the presence of applied pressure in such a way that the conductivity of the mat varies with pressure.

[0006] Force or pressure sensors according to Ruben, Costanzo, Mitchell and Eventoff rely on composite pressure-sensitive layers formed by an electrically insulating matrix material that contains conductive or semiconductive particles referred to as "pigment particles" in applications related to printing. The conductivity of such a composite pressure-sensitive layer increases as soon as two conditions are fulfilled. First, at least some of the conductive or semiconductive particles must be brought in a direct (i.e. geometrical) contact, thus forming electric contacts. Second, the particles in contact must form electric paths through the whole matrix material to electrically connect the electrodes flanking the composite pressure-sensitive layer. The first possibility is that the conductive paths through the whole matrix material exist without any pressure applied and that the number of such paths merely increases under applied pressure, thus leading to increased conductivity of the whole composite pressure-sensitive layer. The second possibility is that without any pressure applied, the electric paths do not exist and are only formed under pressure. The electrical conductivity is therefore related to the percolation phenomenon and the transition from the insulating to the conducting state takes place as soon as the percolation threshold of the composite material is achieved. Due to the well- known S-shaped characteristics of the dependence of the conductivity of the percolation medium on the concentration of conductive inclusions (see for example, Sahimi, M. Applications of Percolation Theory (Taylor&Francis Ltd., London, 1994)) the highest sensitivity with respect to pressure is exhibited in the vicinity of the threshold.

[0007] It is known, however, that a certain value of the percolation threshold is established as an average (with respect to an ensemble of samples) of the thresholds in different samples. The thresholds in any individual samples may significantly deviate from this average value. [0008] In addition, the threshold depends on the shapes of the pigment particles. For this reason the threshold value differs from sample to sample in systems in which the particles shapes are not well defined. At present, this is however, the case in all the industrial applications.

[0009] These reasons make it impossible to produce sensors with a precisely defined turn-on pressure based on the said composite pressure-sensitive layers. This represents a significant drawback peculiar to the approaches of Ruben, Costanzo, Mitchell and Eventoff.

[0010] There is another important drawback to the pressure sensors described above. In order to establish or to increase the conductivity through the composite pressure-sensitive layer sandwiched between two electrodes the particles must form conductive paths penetrating through the whole volume of the matrix and electrically connecting the said electrodes. This implies that multiple particles must inevitably stay in (or come into) a direct geometrical contact, such that their surfaces touch one another. Thus, multiple interfaces between the conducting particles and the insulating matrix participate in the conductivity. For this reason, the functioning of any device based on such a conducting/insulating composite is sensitive to the state of the said interfaces. Any corrosion or chemical modification of the surfaces of the particles as well as eventual splitting of the particles from the matrix of the composite gives rise to degradation of the device. This problem is referred to as the problem of "hidden surface".

[0011] In the following, the term "intrinsically pressure-sensitive organic layer" designates a layer comprising or consisting essentially of an organic material (e.g. a polymer, a blend of polymers or a non-polymeric organic material) that has an intrinsic (inherent) electrical conductivity that changes as a function of applied pressure. Such organic material is herein referred to as "intrinsically pressure-sensitive organic material". It is worthwhile noting that it is the intrinsic electric conductivity of the pressure-sensitive organic material that changes, i.e. the change in conductivity is due to changes on molecular level. This is in contrast to composite pressure-sensitive materials having electrically conductive particles embedded in an insulating matrix material. In the present context, a layer comprising or consisting essentially of intrinsically pressure- sensitive organic material designates a layer, whose electrical properties are significantly, respectively essentially determined by the intrinsically pressure- sensitive organic material. For the present disclosure, an intrinsically pressure- sensitive organic layer should not be limited a priori to a layer consisting exclusively of intrinsically pressure-sensitive organic material. In addition to intrinsically pressure-sensitive organic material, an intrinsically pressure- sensitive organic layer may comprise other substances or molecules, e.g. inorganic or organic materials in general, polymers, oligomers, monomers, non- polymeric substances, e.g. molecules of organic conductors (such as, for instance, pentacene, conductive organic salts, etc.) or other organic molecules (such as e.g. phenolphthaleine, etc.). Such additives may help to adjust the properties of the intrinsically pressure-sensitive organic layer. The intrinsically pressure-sensitive organic layer has an electrical conductivity depending upon mechanical stress applied to the layer surface. In most practical applications, such stress is applied normally to the surface of the intrinsically pressure- sensitive organic layer (i.e. pressure). For reasons of conciseness, stress applied to the surface of the intrinsically pressure-sensitive organic layer is herein generally referred to as "pressure". One should nevertheless keep in mind that components of the stress tensor other than the normal components may cause the same effect.

[0012] The intrinsically pressure-sensitive materials discussed herein may be grouped into two different categories, according to the behaviour of the materials in response to pressure. The materials of the first group exhibit an abrupt, step like increase of the conductivity as a function of applied pressure. The materials of the second group exhibit an essentially continuous increase as a function of applied pressure, in a relatively broad pressure range.

[0013] For the materials of the first group, the curve representing the dependence of the conductivity of the intrinsically pressure-sensitive organic layer upon the applied pressure is essentially s-shaped. At low pressure values, the intrinsically pressure-sensitive organic layer exhibits a low conductivity (typical for insulators) and at high pressure values it exhibits a high conductivity (typical for semiconductors or metals, depending on the intrinsically pressure- sensitive organic layer under consideration). The conductivity thus increases considerably within a certain, relatively narrow pressure interval (for polyphtalides, the increase in conductivity typically amounts to 6 to 9 orders of magnitude). It is convenient to characterize this pressure interval by two pressure values, which are herein referred to as "pressure thresholds". Below the lower or first pressure threshold, the intrinsically pressure-sensitive organic layer has the conductivity of an insulator; above the higher or second pressure threshold, the conductivity of the intrinsically pressure-sensitive organic layer is that of a semiconductor or a metal. Between the said first and second thresholds, the intrinsically pressure-sensitive organic layer undergoes a transition between the two conductivity states. Most of the intrinsically pressure- sensitive organic materials presently known (such as polydiphenilenephthalide) possess relatively narrow transition intervals, i.e. the difference between the second and the first pressure thresholds is smaller than the value of the first threshold. In this case, the intrinsically pressure-sensitive organic layer can be characterized by only one pressure threshold, P_cr, chosen in the middle of the transition interval. In the vicinity of this pressure threshold P_cr the conductivity of such pressure-sensitive layer varies considerably (by 6 to 9 orders of magnitude in the case of polyphthalides), so that the conductivity as a function of pressure essentially behaves like a step function.

[0014] The materials of the second group have a slower transition from the insulating state to the conductive state. The transition between low conductivity and high conductivity may take place continuously in a relatively broad transition region (e.g. covering the entire working pressure range).

[0015] Intrinsically pressure-sensitive organic materials suitable in the context of the present invention have been disclosed, for instance, by A. N. Lachinov and S. N. Salazkin in RU 2 256 967 C1 or WO 2005/076289 A1 , by Tsukagoshi et al. in US 2003/0178138 A1 , by Miyadera et al. in JP 2002-241591 and by Tomono et al. in JP 2004-013451. Examples of intrinsically pressure-sensitive organic materials are the so-called polyphthalides. Structural formulas of the chemical composition of these polyphthalides can be found in the references just mentioned.

[0016] An intrinsically pressure-sensitive organic material usable in the intrinsically pressure-sensitive organic layers of electrical devices is, for instance, poly(3,3'-phthalidylidene-4,4'-biphenylene), a polymer which, if a pressure greater than the pressure threshold P_cr is applied, exhibits high (metal- like) electrical conductivity in the direction of the compression (which is usually normal to the intrinsically pressure-sensitive organic layer), whereas the conductivity in the directions normal to the compression (i.e. usually parallel to the intrinsically pressure-sensitive organic layer) remains substantially lower (see e.g. Lachinov, et al. Synthetic Metals 57, 5046-51 (1993); Lachinov et al. Sov. Phys. JETP 75, 99-102 (1992); Lachinov et al. Synthetic Metals 41 , 805- 809 (1991 )).

[0017] Other (non-polymeric) intrinsically pressure-sensitive organic materials usable in the intrinsically pressure-sensitive organic layers of electrical devices are, for instance, pressure-sensitive organic salts or pressure-sensitive organic crystals such as those disclosed in Lyubovskii, R. B., Laukhina, E. E., Makova, M. K. & Yagubskii, E. B., Anomalous conductivity dependence of beta-(PT)₂h and beta-(PT)₂IBr₂ (PT = bis(propylenedithio)-tetrathiafulvalene) organic conductors under high pressure, Synth. Met. 40, 155-160 (1991 ).

[0018] Intrinsically pressure-sensitive organic materials whose electrical conductivity substantially differs for different spatial directions are herein referred to as "anisotropic intrinsically pressure-sensitive organic materials". These anisotropic materials include, for instance the above-mentioned polyphthalides, whose electrical conductivity increases in the direction of the compression, whereas the conductivity transversal to the direction of compression remains low. The anisotropic materials also include materials whose electrical conductivity increases only in a direction (or directions) transversal to the direction of the compression. In contrast, intrinsically pressure-sensitive organic materials whose electrical conductivity changes substantially isotropically are referred to as "isotropic intrinsically pressure- sensitive organic materials". If not otherwise explained in the context, the term "intrinsically pressure-sensitive organic material" designates both isotropic and anisotropic intrinsically pressure-sensitive organic materials. If a distinction between these two types of intrinsically pressure-sensitive organic materials is intended to be made, the specifications "isotropic" and "anisotropic" are used, respectively.

[0019] Any references cited hereinbefore are herewith incorporated herein by reference.

Object of the invention

[0020] It is an object of the present invention to provide an improved device responsive to pressure. The object is achieved by a switching element as claimed in claim 1.

General Description of the Invention

[0021] The invention proposes a switching element responsive to pressure, comprising at least two electrodes, which are separate from one another and an intrinsically pressure-sensitive organic layer arranged between and in contact with the electrodes in such a way that, in response to a pressure being applied to the switching element, the intrinsic electrical conductivity of the pressure- sensitive organic layer increases between at least one pair of the electrodes and, consequently, the electrical resistance between the pair of electrodes decreases. According to an important aspect of the invention, at least one of the electrodes comprises a resistive layer where this electrode faces another one of the electrodes with respect to the intrinsically pressure-sensitive organic layer. Those skilled will appreciate that this reduces the risk of a short circuit in the switching element. Indeed, during the production of the organic layer, formation of through-holes may occur, through which the electrodes may get into direct electric contact. Providing at least one of the electrodes with a resistive layer (the electrode could also consist of a resistive layer) at least in those areas, where it faces another one of the electrodes. Such configuration limits the electrical current that might flow in the region of a through-hole. [0022] According to a preferred variant of the invention, the intrinsically pressure-sensitive organic layer comprises or consists of an intrinsically pressure-sensitive polymer or a mixture of such polymers. It should be noted that such intrinsically pressure sensitive polymer (mixture) may comprise organic and/or non-organic non-polymeric additives.

[0023] According to another preferred variant of the invention, the intrinsically pressure-sensitive organic layer comprises an intrinsically pressure-sensitive organic salt or an intrinsically pressure-sensitive organic crystal.

[0024] According to a first embodiment of the invention, the switching element is configured and arranged as a so-called through-mode switching element. In this configuration, the electrodes include a first electrode and a second electrode, which sandwich the intrinsically pressure-sensitive organic layer and which are arranged in facing relationship with one another on opposite sides of the intrinsically pressure-sensitive organic layer. Preferably, each one of the first and second electrodes comprises a resistive layer where it faces the other one of the first and second electrodes with respect to the intrinsically pressure- sensitive organic layer. Advantageously, the pressure-sensitive switching element of this first embodiment comprises a first substrate carrying the first electrode and/or a second substrate carrying the second electrode. Such substrates may include, for instance, electrically insulating films, foils, plates, etc.

[0025] According to a second embodiment of the invention, the electrodes include a first electrode and a second electrode and are arranged adjacent to one another on the same side (hereinafter: the first side) of the intrinsically pressure-sensitive organic layer. Most preferably, the switching element includes at least one third electrode arranged in facing relationship with the first and second electrodes on a second side of the intrinsically pressure-sensitive organic layer, the second side being opposite the first side. In this configuration, when pressure is applied to the switching element, the intrinsic electrical conductivity of the pressure-sensitive organic layer increases between the first and the third electrodes as well as between the second and the third electrodes. This results in a decrease of resistance between the first and the second electrodes, which can be detected. The third electrodes, which are normally not connected (but could be), are referred to as shunt electrodes; such switching device is therefore also known as shunt-mode switching device.

[0026] In the second embodiment, it has proven to be advantageous to arrange and configure the first and second electrodes in such a way that they interdigitate. Preferably, the at least one of the electrodes that comprises a resistive layer includes the third electrode or electrodes.

[0027] Still referring to the second embodiment, the pressure-sensitive device advantageously comprises a common substrate (e.g. an electrically insulating film, foil, or plate) that carries the first and second electrodes.

[0028] With regard to the reduction of short-circuit probability, it is considered advantageous to give the electrodes an open structure, e.g. a comb-shaped layout, a grid-shaped layout, a lattice-shaped layout etc.

[0029] The intrinsically pressure-sensitive organic layer can be an isotropic intrinsically pressure-sensitive organic layer or an anisotropic intrinsically pressure-sensitive organic layer. As will become apparent to those skilled, in particular embodiments of the invention, one might have to choose one of these options. More details on this issue are given in the description of specific embodiments.

[0030] Most preferably, the switching element of the present invention comprises a protective layer.

[0031] According to a particular embodiment of the invention, the switching element comprises a membrane arranged substantially parallel to the intrinsically pressure-sensitive organic layer, the membrane being arranged and configured so as to deflect towards the intrinsically pressure-sensitive organic layer when pressure is applied to the device, thereby reducing the pressure acting on the intrinsically pressure-sensitive organic layer. The pressure is thus not applied directly to the pressure-sensitive layer but the reaction of the membrane compensates the pressure up to a certain threshold pressure. Above this threshold pressure, a part of the pressure is mechanically communicated to the intrinsically pressure-sensitive organic layer. In fact, the membrane serves to buffer the pressure. The membrane parameters can be dedicatedly chosen in order to achieve that the switching element switches at a desired pressure.

[0032] Advantageously, the pressure-sensitive switching element comprises a luminescent layer emitting light in response to a current flowing through the luminescent layer. As will be appreciated, the pressure-sensitive device can also be connected in an electric circuit comprising a light emitting diode in such a way as to influence the emission of light of the light-emitting diode.

[0033] The present invention could, in principle, be integrated into any kind of electric appliance, e.g. a household device, a computer, a cell phone, a radio device, etc., as a human-appliance interface.

[0034] Regarding manufacturing of switching elements as described herein, it should be noted that any suitable methods could be used. The intrinsically pressure-sensitive organic layer may, for instance, be produced using a spin- coating method, a printing method, such as e.g. screen-printing, offset printing, gravure printing, flexography printing and/or inkjet printing, or any other deposition method. Though any suitable method may be used, inkjet printing is preferred, since it allows forming layers with a well-defined thickness between about 500 nm and a 5-1 Oμm in a single print and with pixel sizes of about 10 μm. Greater thickness may be achieved by using multiple prints. At present, the achievable lateral resolution lies in the range of tens of μm (1 pixel may e.g. correspond to 75 μm). If the pressure-sensitive polymer disclosed in patent application WO 2005/076289 A1 is used as the intrinsically pressure-sensitive organic layer, the thickness of the polymer layer is preferentially comprised in the range between 1 and 30 μm. For any of the printing methods, any suitable ink based on the pressure-sensitive material may be used. Such ink could comprise 0.5 to 5 wt.% of an intrinsically pressure-sensitive polymer, such as e.g. poly(diphenylenephthalide), poly(terphenylenephthalide), polyfluoronylene- phthalide, polyarylenesulforphthalide, or any other suitable intrinsically pressure-sensitive polymer, 95-99 wt.% of a solvent, such as e.g. cyclohexanone, nitrobenzene, chloroform or any other suitable solvent, and 0.001-0.1 wt. % of additives. Blends of intrinsically pressure-sensitive polymers as well as blends of one or more intrinsically pressure-sensitive polymers and other polymers can be used in the ink.

[0035] As concerns any conductive electrodes, conductive layers or conductive patterns, they could be made of bulk material (e.g. bulk metal), but preferably they are formed using methods such as e.g. metal evaporation or chemical precipitation or a printing process (such as e.g. screen-printing, off-set printing, gravure printing, flexography printing and/or inkjet printing). The conductive electrodes and layers might e.g. be formed by screen printing using conductive inks, such as e.g. Acheson Electrodag PF-007, Acheson Electrodag PF-046 or DowCorning PI-2000, or by inkjet printing using silver inkjet inks, e.g. Cabot AG- IJ-150-FX, Cima Nanotech IJ 242/21 , Tetenal Silver Fluid UV, Harima Chem. NanoPaste or any other suitable inks.

[0036] As concerns resistive electrodes and resistive layers or resistive patterns, they may be formed of any suitable resistive bulk material or by using any suitable application process, e.g. screen-printing, offset printing, gravure printing, flexography printing and/or inkjet printing. Preferred resistive inks include, for instance, the Acheson Electrodag PF-407C, Nicomatic LtD C-100 or Nicomatic LtD C-200 inks for screen printing as well as, for instance, Tetenal UV Resistor LR, Tetenal UV Resistor MR, Tetenal UV Resistor HR carbon inks for inkjet printing.

[0037] The substrate or substrates that carry the electrodes or the switching element can be flexible, elastic or rigid. In case of printed electrodes and a printed intrinsically pressure-sensitive organic layer, the rigidity/flexibility of the substrate may determine to a large extend the rigidity/flexibility of the switching element as a whole.

Brief Description of the Drawings

[0038] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein: Figs. 1-5 are cross sectional views of several variants of though-mode pressure-sensitive switching elements;

Figs. 6-9 are cross sectional views of several variants of though-mode pressure-sensitive switching elements arranged on a substrate;

Fig. 10 is a perspective view of a though-mode pressure-sensitive switching element with comb-shaped electrodes;

Figs. 11-13, 15 are perspective views of different variants of shunt-mode pressure-sensitive switching elements;

Fig. 14 is a cross sectional view of the shunt-mode pressure-sensitive switching element of Fig. 13;

Fig. 16 is a cross sectional view of the shunt-mode pressure-sensitive switching element of Fig. 15;

Figs. 17-19 are top views of different variants of shunt-mode pressure- sensitive switching elements;

Fig. 20 is a cross sectional view of a shunt-mode pressure-sensitive switching element covered with an functionalizing or protective layer;

Fig. 21 is a cross sectional view of a pressure-sensitive switching element combined with an additional activation layer;

Fig. 22 is a cross sectional view of a variant of the pressure-sensitive switching element of Fig. 21 ;

Figs. 23-25 are block schematic diagrams illustrating how a pressure- sensitive switching element or a pressure sensor can be combined with a light emitting diode.

Description of Preferred Embodiments

[0039] Figs. 1-5 show cross-sectional views through very simple pressure- sensitive switching element 10a-10e. The switching elements 10a-10e comprise a first electrode 12 and a second electrode 14 arranged in facing relationship with one another. A layer 16 comprising intrinsically pressure-sensitive organic material is sandwiched between the first and second electrodes 12, 14.

[0040] A pressure P acting on the pressure-sensitive switching element 10a-10e is illustrated by the arrows 18. Below the pressure threshold, the intrinsically pressure-sensitive organic layer 16 behaves like an insulator so that the electrodes are essentially insulated one from the other. Above the pressure threshold, the intrinsically pressure-sensitive organic layer 16 behaves like a conductor, so that there is only little electrical resistance between the electrodes in those regions were the pressure actually is applied. In order to detect the pressure acting on the switching element 10a-10e, the electrodes 12, 14 are arranged in an electric measurement circuit (not shown in the drawings), by which a defined voltage or current is applied and the electrical resistance between the electrodes 12, 14 determined. For intrinsically pressure-sensitive organic layers with a narrow transition region, the resistance of the said layer allows to determine whether the pressure applied to the switching element is below or above the pressure threshold. In case of an intrinsically pressure- sensitive organic material with a wide transition region, the resistance allows to determine the amount of pressure applied to the element, if this pressure lies within the transition interval, i.e. above the first but below the second pressure threshold.

[0041] In Figs. 1-3, both electrodes 12, 14 are resistive electrodes, which means that they have each an electrically resistive layer 12.1 , 14.1 , e.g. made of a resistive material such as carbon, graphite or any other suitable resistive material, in contact with the intrinsically pressure-sensitive organic layer 16. The provision of resistive rather than conductive electrodes has the following advantage: during the application of the intrinsically pressure-sensitive organic layer formation of through-holes may occur, through which the first and second electrodes 12, 14 may be in direct electric contact. If both the first and second electrodes 12, 14 were made of conductive material, this would give rise to a short circuit. This scenario would be very probable in case that at least one of the electrodes 12, 14 is deposited by such a method as printing or evaporation. Using resistive electrodes in the switching element considerably reduces the current flowing through a potential though-hole in the intrinsically pressure- sensitive organic layer 16 with respect to the short circuit current, so that the switching element 10 keeps his functional ability.

[0042] Fig. 1 shows a most simple switching element 10a with two resistive electrodes 12.1 , 14.1 sandwiching the intrinsically pressure-sensitive organic layer 16.

[0043] Fig. 2 shows a pressure-sensitive switching element 10b, wherein the electrode 14 comprises an additional conductive layer 14.2 applied on the back surface of the resistive layer 14.1. The conductive layer 14.2 ascertains that the back surface of the resistive layer 14.1 is essentially at a same electric potential. As a consequence, the overall resistance of the pressure-sensitive switching element 10b is independent of the point of application of a force with respect to the position of a contact lead.

[0044] In Fig. 3, both electrodes 12, 14 comprise an additional conductive layer 12.2, 14.2 applied on the respective back surface of the resistive layers 12.1 and 14.1 , so that the intrinsically pressure-sensitive organic layer 16 is sandwiched between the resistive layers 12.1 , 14.1 , in turn sandwiched between the conductive layers 12.2, 14.2.

[0045] In Figs. 4 and 5, the electrode 12 is a conductive electrode, which comprises a conductive layer 12.2 in contact with the intrinsically pressure- sensitive organic layer 16. In case of a through-hole in the intrinsically pressure- sensitive organic layer 16, a short circuit is avoided because of the resistive layer 14.1 of the second electrode 14. In Fig. 5, the electrode 14 is provided with an additional conductive layer 14.2 applied on the back surface of the resistive layer 14.1.

[0046] With respect to Figs. 1-5, it should be noted that the conductive layers 12.2, 14.2 could be solid metal layers (e.g. metal plates). Such solid layers may play the role of substrates carrying the switching element. The intrinsically pressure-sensitive organic layer 16 may be deposited on top of one of the plates by a spin coating method. Alternatively, the intrinsically pressure- sensitive organic layer 16 can be deposited by printing, for example, by screen printing, ink-jet printing, off-set printing, gravure printing or flexography printing methods as well as by any other deposition method.

[0047] The conductive layers 12.2, 14.2 can also be deposited on the resistive layers 12.1 , 14.1 , the intrinsically pressure-sensitive organic layer 16 or on a substrate (such as e.g. in Figs. 6-10) by metal evaporation, chemical precipitation, any of the above printing methods or any other method of deposition.

[0048] Figs. 6-10 show pressure-sensitive switching elements arranged on a substrate 20. In Fig. 6, a switching element is analogous to that shown in Fig. 1 is arranged on the substrate 20. Fig. 7 shows a switching element as in Fig. 2 on the substrate 20. Fig. 8 represents a switching element as in Fig. 5 on the substrate 20, Fig. 9 a switching element as in Fig. 3 on the substrate 20. Application of a pressure P is again shown by arrows 18. The substrate 20 may be rigid as well as flexible. The construction of such a sensor may be achieved by first providing the substrate 20 and applying the different layers 12.2, 12.1 , 16, 14.1 , 14.2 one on top of the other on the substrate 20. Each layer may be applied by one of the above-mentioned techniques.

[0049] Fig. 10 shows a perspective view of a pressure-sensitive switching element 10 on a substrate 20. The first electrode is a comb-shaped electrode 12 applied on the substrate. The intrinsically pressure-sensitive organic layer 16 is applied on the first electrode 12, and the second electrode 14 is applied on top of the intrinsically pressure-sensitive organic layer 16. The second electrode 14 is a comb-shaped electrode, which is applied non-collinearly and preferably, as shown in Fig. 10, rotated by 90 degrees with respect to the first electrode 12. At least one of the said comb-shaped electrodes 12, 14 is a resistive electrode.

[0050] Assuming that p is the probability of formation of a through-hole anywhere in the intrinsically pressure-sensitive organic layer 16, the probability of formation of a through-hole between the "teeth" (or "fingers") 22, 24 of the first and the second comb-shaped electrodes 12, 14 (which would also be the probability of occurrence of a short circuit between the electrodes 12, 14 if they were both conductive), is equal to p d²/(D+d)², where d is the width of a tooth while D is the distance between two neighboring teeth. By choosing d/D«1 one can thus significantly reduce the probability of formation of a short circuit. Since at least one of the electrodes 12, 14 is a resistive electrode, the risk of a short circuit is further reduced.

[0051] Figs. 11-19 show several pressure-sensitive switching elements 30a-30d of construction substantially different from that of the previously discussed ones. The switching elements of Figs. 11 to 19 comprise, arranged on a substrate 20, which may be flexible or rigid, a first electrode 32 and a second electrode 34 having no direct electric contact with one another e.g. by being spaced from one another. At least one of the electrodes 32, 34 is resistive (i.e. comprises or consists of a resistive layer). An intrinsically pressure-sensitive organic layer 16 covers the first and the second electrodes 32, 34. Below a certain pressure threshold value, the intrinsically pressure-sensitive organic layer 16 behaves like an insulator so that the electrodes 32, 34 are essentially insulated from one another.

[0052] In the switching elements represented in Figs. 11 , 13 and 14, the material of the intrinsically pressure-sensitive organic layer 16 is chosen among those that turn electrically conductive in the directions parallel to the surface of the intrinsically pressure-sensitive organic layer as soon as the applied pressure exceeds the pressure threshold. For instance, the conductivity could vary isotropically with the applied pressure; in this case the intrinsically pressure- sensitive organic layer comprises or consists of a so-called isotropic intrinsically pressure-sensitive organic material. Alternatively, one could also choose an "anisotropic" material whose conductivity increases perpendicular to the direction of the compression. In both cases, as long as the applied pressure is below a certain pressure threshold, the intrinsically pressure-sensitive organic layer behaves like an electrical insulator. When pressure exceeding a certain pressure threshold is applied on the switching element, an electrical current may flow between the two electrodes through the shunting intrinsically pressure-sensitive organic layer. Such switching elements are therefore known as "shunt-mode" switching elements. [0053] There are intrinsically pressure-sensitive organic materials, whose conductivity remains low in the directions parallel to the pressure-sensitive layer; as mentioned hereinbefore, these are referred to as anisotropic intrinsically pressure-sensitive organic materials. As illustrated in Figs. 12, 15 and 16, intrinsically pressure-sensitive organic layers 16 comprising or consisting of such an anisotropic pressure sensing intrinsically pressure- sensitive organic can also be used in a shunt-mode switching element. In this case, an additional shunt electrode 36 is applied on top of the intrinsically pressure-sensitive organic layer 16, which at least partially overlaps with both the first and the second electrodes 32, 34. If pressure exceeding a certain pressure threshold is applied perpendicular to the intrinsically pressure- sensitive organic layer 16, the latter becomes conductive in this direction. Hence an electric current may flow from the first electrode 32, through the intrinsically pressure-sensitive organic layer 16, to the shunt electrode 36 and from the shunt electrode 36, through the intrinsically pressure-sensitive organic layer 16, to the second electrode 34. The shunt electrode 36 may be a resistive electrode or a conductive electrode as defined earlier.

[0054] Regarding application of the first and second electrodes 32, 34 on the substrate, respectively of the shunt electrode 36 on the intrinsically pressure- sensitive organic layer 16, any suitable method can be chosen. The electrodes 32, 34, 36 can e.g. be deposited by metal evaporation, chemical precipitation, any of the above-mentioned printing methods or any other suitable method of deposition. The intrinsically pressure-sensitive organic layer 16 may be applied on top of the said first and second electrodes 32, 34 e.g. by a spin coating method, by any of the said printing methods or by any other suitable deposition method.

[0055] Different shapes for the first and second electrodes 32, 34 are possible. They may, for instance be of simple rectangular shape, as shown in Figs. 11 and 12; preferably, however, they are comb-shaped with their respective teeth interdigitating as illustrated in Figs. 13-19. Fig. 14 represents a cross section through the switching element of Fig. 13 perpendicular to the teeth of the electrodes 32, 34. Fig. 16 represents a cross section through the switching element of Fig. 15 perpendicular to the teeth of the electrodes 32, 34.

[0056] Those skilled will appreciate that in contrast to switching elements having their first and second electrodes in facing relationship (through-mode switching elements), a single through-hole in the intrinsically pressure-sensitive organic layer 16 does not lead to an electrical shorting of the first and the second electrodes 32, 34. Shunt mode switching elements 30a-30d are thus inherently more robust against this failure than through-mode switching elements 10a-10e. In shunt-mode switching elements, a short circuit could only arise if a through- hole in the intrinsically pressure-sensitive organic layer were located above the first electrode and another through-hole were simultaneously located above the second electrode, if the first and the second electrodes came simultaneously into contact with the shunt electrode and if all of the electrodes were conductive.

[0057] For the switching element of Fig. 12, the probability of a short circuit between the first and second electrodes 32, 34 may be calculated as follows (for the hypothetical case that all electrodes are conductive). Denote the probability of formation of a small hole anywhere in the intrinsically pressure- sensitive organic layer 16 by p and the area of the switching element 30b by LD', where L is the length and D' is the width of the switching element. Denote further the width of each first and second electrode as d'. Note that d'<D'. One finds that the probability of formation of a single hole over one of the electrodes is p-d'/D' and the probability of simultaneous formation of a hole over each one of both electrodes 32, 34 is (p-d'/D')², which can be decreased by reducing the ratio d'/D'. The probability (p-d'/D')² is also the probability for a short circuit between the first and second electrodes 32, 34, provided that the shunt electrode 36 is conductive and covers substantially the area LD' of the switching element 30b. Nevertheless, since at least one of the electrodes is resistive (i.e. comprises or consists of a resistive layer) a short-circuit can almost certainly be excluded.

[0058] In case of the interdigitating electrodes of Figs. 15 and 16, the probability of a short circuit can also be calculated. The parameters of the comb-shaped electrodes are defined as in Fig. 7: d is the width of a tooth, D the distance between two neighboring teeth. The probability of formation of a through-hole anywhere in the intrinsically pressure-sensitive organic layer 16 is denoted by p. Since the probability of a small through-hole above any of the first and the second electrodes 32, 34 is equal to p d/(D+d) one finds the probability of formation of the short circuit (in the hypothetical case of all electrodes being conductive) to be p²d²/(D+d)². This probability is small if d/D«1.

[0059] A further reduction of the probability of formation of a short circuit (in the hypothetical case of all electrodes being conductive) can be achieved by fractionalizing of the shunting electrode 36. As illustrated in Fig. 17, the shunting electrode 36 may be arranged on the intrinsically pressure-sensitive organic layer 16 in the form of stripes with width b and oriented perpendicular to the teeth of the comb-shaped electrodes 32, 34 in such a way that each stripe overlaps all the teeth. In this case, the probability of formation of a small through-hole in the intrinsically pressure-sensitive organic layer 16 between the first electrode 32 and a particular stripe of the shunt electrode 36 is equal to p N b d/[N (D+d) L], where N>1 is the number of teeth, L is the length of each tooth, D and d are defined above. A short circuit arises as soon as at least two holes coexist in the same stripe. In order to shortcut the electrodes 32, 34 one of the through-holes must pass through the intrinsically pressure-sensitive organic layer 16 from the first electrode 32 to the stripe (the probability is p b d/[L (D+d)]) and the other one must pass through the intrinsically pressure- sensitive organic layer 16 from the stripe to the second electrode 34 (the probability is p b d/[L (D+d)]. Finally, the probability P' of such a configuration with two holes in a particular stripe is:

5' pbd

L{D + d)

[0060] The probability can be decreased by increasing the number of stripes M (M»1, i.e. choosing a design with many stripes), and making blL « \ (i.e. by choosing thin stripes) and dlD « 1 (i.e. by choosing thin teeth). [0061] Another embodiment of a pressure-sensitive switching element with reduced probability of a short circuit is shown in Fig. 18. In this embodiment, the stripes forming the shunt electrode 36 are oriented along the interdigitating teeth of the first and second comb-shaped electrodes 32, 34 in such a way that each stripe is situated above a part of a tooth belonging to the first electrode 32 and simultaneously above a part of the tooth belonging to the second electrode 34.

[0062] Yet another embodiment of a pressure-sensitive switching element with reduced probability of a short circuit is shown in Fig. 19, wherein the shunt electrode 36 is formed of patches that simultaneously overlap with the first and the second electrodes 32, 34.

[0063] Fig. 20 shows a pressure-sensitive switching element 30 with an additional functionalizing or protective layer 38. It should be noted that although the switching element 30 is represented as a shunt-mode switching element as in Figs. 15 and 16, the functionalizing or protective layer could also be applied on top of the other embodiments of shunt-mode switching elements as in Figs. 11-14, in Figs. 18 and 19, on top of a through-mode switching element as in Figs. 6-10 or around a through-mode switching element as in Figs. 1-5. The functionalizing or protective layer 38 can e.g. be applied by a spin coating method, by the printing methods mentioned above or by any other deposition method.

[0064] Figs. 21 and 22 show pressure sensors 40a, 40b that combine a pressure-sensitive switching element 10 as described above with an additional activation membrane 42. The pressure sensors 40a, 40b comprise a substrate and a flexible activation membrane, which are spaced from each other by a spacer 44. The spacer 44 has an opening or openings 46 arranged therein, which defines active zones of the sensors 40a, 40b. In each active zone, the sensors 40a, 40b comprise a pressure-sensitive switching element 10. When pressure is applied to the pressure sensor 40a, 40b, the activation membrane 42 leaves its neutral position 48 and bends towards the substrate 20 in the active zones. When the applied pressure becomes high enough, the activation membrane 42 is brought into contact with the pressure-sensitive switching element 10, which in turn is subjected to part of the applied pressure. Consequently, the external pressure P counterbalanced by the reaction of the activation membrane P_a and the reaction of the intrinsically pressure-sensitive organic material P_psp, so that P=P_a ⁺Ppsp- As soon as the pressure applied to the intrinsically pressure-sensitive organic layer 16 reaches a certain threshold P_cr the intrinsically pressure-sensitive organic layer goes into its conductive state. The overall turn-on point of the sensor Ptum-on, i-β. the external pressure at which the sensor 40a or 40b goes into the conductive state therefore takes the form Pturπ-oπ=Pa⁺ Per, which is higher than that of the intrinsically pressure- sensitive organic material as such. The value Ptum-on can be adjusted in a controlled manner by variation of the reaction of the activation membrane 42, which can be achieved by varying the size and/or the geometry of the active zone, the thickness and/or the constitution of the activation membrane 42 as well as the height of the spacer 44.

[0065] Fig. 22 illustrates an embodiment of a pressure sensor 40b with two active zones of different diameter. Each active zone therefore has its individual turn-on point, even though the intrinsically pressure-sensitive organic layers 16 used may be the same for the pressure-sensitive switching elements 10 arranged in the two active zones. This can be seen in Fig. 22: for the same applied pressure P, the activation membrane 42 is already in contact with the pressure-sensitive switching element 10 in right-hand active zone while it is not yet in contact with the pressure-sensitive switching element 10 in the left-hand active zone. The turn-on point of the right-hand active zone is thus lower than the turn-on point of the left-hand active zone.

It should be noted that although the pressure-sensitive switching element in Figs. 21 and 22 is represented as a specific through-mode pressure sensor 10, any one of the pressure-sensitive switching elements described above with respect to Figs. 1 -5, 6-10, 11 -19 or 20 may be placed in the active zones.

[0066] Figs. 23-25 are block schematic diagrams illustrating how a pressure- sensitive switching element 50 (which could also be any of the pressure- sensitive devices disclosed herein) can be combined with a light emitting diode 52 to form a pressure-sensitive light emitting device 54. With respect to Fig. 23, the light-emitting diode 52 is arranged in series with the pressure-sensitive switching element 50 and thus emits light if the pressure applied to the switching element 50 exceeds a certain pressure threshold.

[0067] A circuit as shown in Fig. 23 can be achieved, for instance, by arranging a intrinsically pressure-sensitive organic layer between two electrodes, at least one of which comprises a semiconductor material such as those used for the production of light-emitting diodes. As soon as the electrodes are connected to an electrical power supply and the so-formed circuit is closed, a current passes through the device when a pressure exceeding a certain threshold is applied to the device. In this embodiment, light is emitted due to recombination of the electron-hole pairs at the interface of the semiconductor layer and the layer adjacent to it, e.g. the intrinsically pressure-sensitive organic layer. The device may additionally be covered with a protective transparent, preferably elastic layer.

[0068] The circuit as shown in Fig. 23 can alternatively be achieved by arranging at least two mutually spaced electrodes, at least one of which comprises a semiconductor material such as those used for the production of light-emitting diodes, on an insulating substrate. A intrinsically pressure- sensitive organic layer at least partially overlaps with both of the electrodes. In case the intrinsically pressure-sensitive organic layer is an anisotropic intrinsically pressure-sensitive organic layer, a conductive layer is arranged on the upper surface of the intrinsically pressure-sensitive organic layer. In case the intrinsically pressure-sensitive organic layer is an isotropic intrinsically pressure-sensitive organic layer, such a conductive layer may be present but can also be omitted. An additional protective layer may cover the device.

[0069] With respect to the electric circuit shown in Fig. 24, a light emitting diode 52 is arranged in parallel to the pressure-sensitive switching element 50 and thus emits light when low pressures, at which the intrinsically pressure-sensitive organic layer is an insulator, act on the switching element 50. Light emission is reduced or stopped when the pressure applied to the sensor overcomes the pressure threshold at which the intrinsically pressure-sensitive organic layer becomes a conductor.

[0070] With respect to the electric circuit represented in Fig. 25, a first light emitting diode 52.1 is arranged in series with the pressure-sensitive switching element in a first branch of the electric circuit while a second light emitting diode 52.2 is arranged in a second branch of the electric circuit, in parallel to the first branch. The second light emitting diode 52.2 is on if the pressure applied to the switching element 50 is low, i.e. if the applied pressure is below the pressure threshold below which the intrinsically pressure-sensitive organic material is an insulator, while the first light emitting diode 50.1 is off. If the pressure applied to the switching element 50 is high, i.e. above the pressure threshold, above which the intrinsically pressure-sensitive organic material is a conductor, the second light emitting diode 52.2 emits only little light or is off while the first light emitting diode is on.

[0071] Pressure-sensitive light emitting devices 54 as described above may be used in any appliance that has components or parts subjected to pressure, e.g. in a keyboard, a keypad or any electronic appliance to indicate which button has been pressed or in a touch screen to indicate those locations, where a pressure is applied to the screen surface. These pressure-sensitive light emitting devices 54 might also be used in footwear, other pieces of wear or walking sticks to indicate the position of a pedestrian in the dark as well as in wheel-chairs or bicycles.

[0072] While the present patent application as filed in principle concerns the invention as defined in the claims attached hereto, the person skilled in the art will readily understand that the present patent application contains support for the definition of other inventions, which could e.g. be claimed as subject matter of amended claims in the present application or as subject matter of claims in divisional and/or continuation applications. Such subject matter could be defined by any feature or combination of features disclosed herein. References

1. S. Ruben, U.S. patent No. 2,375,178

2. R. S. Costanzo, U.S. patent No. 3,386,067

3. R. J. Mitchell, US patent No. 3,806,471

4. F. N. Eventoff, US patents No. 4,315,238 and 4,314,227

5. G. B. Anderson, U.S. Pat. No. 4,347,505.

6. Miyadera Yasuo, Hatakeyama Yuki, Morishita Yoshii, Japanese Patent 2002-241591 A2

7. lsao Tsukogoshi, Yasishi Gotou, Masami Yusa and Miyadera Yasuo, US patent application 2003/0178138 A1

8. Tomono et al. JP 2004-013451 A2.

9. http://www.nanosonic.com/MetalRubber.asp

10. J. H. LaIIi, A. Hill, S. Subrahmanayan, B. Davis, J. Mecham, R.M. Goff, and R.O. Claus, Metal Rubber TM sensors. Proc. of SPIE, 5758, 333-337 (2005)

11. A.N. Lachinov and S.N. Salazkin, RU 2 256 967 C1

12. Skaldin, O. A., Zherebov, A. Y., Lachinov, A. N., Chuvyrov, A. N. & Delev, V. A. Charge instability in thin films of organic superconductors. Pis'ma v Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki 51 , 141-144 (1990).

13. Lachinov, A. N., Zherebov, A. Y. & Kornilov, V. M. Anomalous electron instability of polymers under uniaxial pressure. Pis'ma v Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki 52, 742-5 (1990).

14. Lachinov, A. N., Zherebov, A. Y. & Kornilov, V. M. Electron instability in thin polymer films. Highly conducting state and its properties. Synthetic Metals 57, 5046-51 (1993).

15. Lachinov, A. N., Zherebov, A. Y. & Kornilov, V. M. High-conductivity state of thin polymer films: effect of an electric field and of a uniaxial pressure. Sov. Phys. JETP 75, 99-102 (1992). 16. Lachinov, A. N., Zherebov, A. Y. & Scaldin, O. A. Electronic instabilities in polyphthalidilidenarylene thin films. Possible applications. Synthetic Metals 41-43, 805-809 (1991 ).

17. Lachinov, A. N. Polymer films as a material for sensors. Sensors and Actuators A 39, 1-6 (1993).

18. Lyubovskii, R. B., Laukhina, E. E., Makova, M. K. & Yagubskii, E. B., Anomalous conductivity dependence of beta-(PT)₂l3 and beta-(PT)₂IBr₂ (PT = bis(propylenedithio)-tetrathiafulvalene ) organic conductors under high pressure, Synth. Met. 40, 155-160 (1991 )

Claims

1. A switching element responsive to pressure, comprising at least two electrodes, said electrodes being separate from one another; an intrinsically pressure-sensitive organic layer arranged between said electrodes and in contact with said electrodes in such a way that, in response to a pressure being applied to said switching element, the intrinsic electrical conductivity of said pressure-sensitive organic layer increases between at least one pair of said electrodes and the electrical resistance between said pair of electrodes decreases; characterized in that at least one of said electrodes comprises a resistive layer where said at least one of said electrodes faces another one of said electrodes with respect to said intrinsically pressure-sensitive organic layer.

2. The switching element as claimed in claim 1 , wherein said intrinsically pressure-sensitive organic layer comprises an intrinsically pressure- sensitive polymer.

3. The switching element as claimed in claim 1 , wherein said intrinsically pressure-sensitive organic layer comprises an intrinsically pressure- sensitive organic salt or an intrinsically pressure-sensitive organic crystal.

4. The switching element as claimed in any one of claims 1 to 3, wherein said at least two electrodes include a first electrode and a second electrode, wherein said intrinsically pressure-sensitive organic layer is sandwiched between said first and second electrodes and wherein said first and second electrodes are arranged in facing relationship with one another on opposite sides of said intrinsically pressure-sensitive organic layer.

5. The switching element as claimed in claim 4, wherein each one of said first and second electrodes comprises a resistive layer where it faces the other one of said first and second electrodes with respect to said intrinsically pressure-sensitive organic layer.

6. The switching element as claimed in claim 4 or 5, comprising a first substrate carrying said first electrode and/or a second substrate carrying said second electrode.

7. The switching element as claimed in any one of claims 1 to 3, wherein said at least two electrodes include a first electrode and a second electrode and wherein said first and second electrodes are arranged adjacent to one another on a first side of said intrinsically pressure-sensitive organic layer.

8. The switching element as claimed in claim 7, wherein said at least two electrodes include at least one third electrode arranged in facing relationship with said first and second electrodes on a second side of said intrinsically pressure-sensitive organic layer, said second side being opposite said first side, wherein, in response to a pressure being applied to said switching element, the intrinsic electrical conductivity of said pressure- sensitive organic layer increases between said first and said third electrodes as well as between said second and said third electrodes.

9. The switching element as claimed in claims 7 or 8, wherein said first and second electrodes interdigitate.

10. The switching element as claimed in claim 9, wherein the at least one of said electrodes that comprises a resistive layer comprises said at least one third electrode.

11. The switching element as claimed in any one of claims 7 to 10, comprising a substrate that carries said first and second electrodes.

12. The switching element as claimed in any one of claims 1 to 11 , wherein said intrinsically pressure-sensitive organic layer is an isotropic intrinsically pressure-sensitive organic layer.

13. The switching element as claimed in any one of claims 1 to 11 , wherein said intrinsically pressure-sensitive organic layer is an anisotropic intrinsically pressure-sensitive organic layer.

14. The switching element as claimed in any one of claims 1 to 13, comprising a protective layer.

15. The switching element as claimed in any one of claims 1 to 14, comprising a membrane arranged substantially parallel to said intrinsically pressure- sensitive organic layer, said membrane being arranged and configured so as to deflect towards said intrinsically pressure-sensitive organic layer when pressure is applied to said switching element, thereby reducing the pressure acting on said intrinsically pressure-sensitive organic layer.

16. The switching element as claimed in any one of claims 1 to 15, comprising a luminescent layer emitting light in response to a current flowing through said luminescent layer.

17. The switching element as claimed in any one of claims 1 to 16, connected in an electric circuit comprising a light emitting diode in such a way as to influence the emission of light by the light-emitting diode.

18. An electric appliance comprising a switching element as claimed in any one of claims 1 to 17.