WO2006097566A1 - Methods and arrangements for acquiring and utilising enhanced electronic conduction in an organic thin film transistor - Google Patents

Methods and arrangements for acquiring and utilising enhanced electronic conduction in an organic thin film transistor Download PDF

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Publication number
WO2006097566A1
WO2006097566A1 PCT/FI2005/000159 FI2005000159W WO2006097566A1 WO 2006097566 A1 WO2006097566 A1 WO 2006097566A1 FI 2005000159 W FI2005000159 W FI 2005000159W WO 2006097566 A1 WO2006097566 A1 WO 2006097566A1
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thin film
film transistor
dielectric layer
transistor device
layer
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PCT/FI2005/000159
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French (fr)
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Tomas BÄCKLUND
Henrik Sandberg
Henrik Stubb
Ronald ÖSTERBACKA
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Avantone Oy
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Publication of WO2006097566A1 publication Critical patent/WO2006097566A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having a potential-jump barrier or a surface barrier
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/464—Lateral top-gate IGFETs comprising only a single gate
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having a potential-jump barrier or a surface barrier
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/466—Lateral bottom-gate IGFETs comprising only a single gate
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having a potential-jump barrier or a surface barrier
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/468—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics
    • H10K10/471—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics the gate dielectric comprising only organic materials
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K19/00—Integrated devices, or assemblies of multiple devices, comprising at least one organic element specially adapted for rectifying, amplifying, oscillating or switching, covered by group H10K10/00
    • H10K19/10—Integrated devices, or assemblies of multiple devices, comprising at least one organic element specially adapted for rectifying, amplifying, oscillating or switching, covered by group H10K10/00 comprising field-effect transistors

Definitions

  • the invention concerns generally the technology of using organic materials in manufacturing thin film transistors, and utilising thin film transistors so manufactured. Especially the invention takes advantage of certain properties of organic thin film transistors that are related to electronic conduction, and shows how appropriate selection of materials and manufacturing methods can lead to even completely new areas of application for organic thin film transistors.
  • Fig. 1 is a cross section through one basic OFET structure, known as the top-gate structure.
  • a substrate 101 constitutes a smooth, nonconductive support surface on which the other layers reside.
  • source and drain electrodes 102 and 103 are made of a highly conductive material, such as a thin metallic layer or a conductive polymer.
  • An active layer 104 connects the source and drain electrodes 102 and 103 together.
  • Some textbook sources also designate the active layer as the channel layer.
  • the active layer 104 is made of a semiconductive organic polymer; conjugated polymers are preferred.
  • an insulation layer 105 On top of the active layer 104 there is an insulation layer 105, the purpose of which is to act as an electric insulator. Consequently the insulation layer 105 has good electric insulation properties, and is made of polymer (e.g. polystyrene) or inorganic material (e.g. SiO 2 ). It is possible to build the insulation layer 105 from several component layers one upon the other, in order to acquire specific results such as surface modification, diffusion barrier or solvent compatibility.
  • a gate electrode layer 106 lies on top of the insulation layer 105 and is made of metal or a highly conductive polymer. The structure also comprises various interconnect lines and contact pads connected to electrodes, but these are not shown in fig. 1 in order to enhance graphical clarity.
  • the structure of fig. 1 functions as a field-effect transistor so that the electric potential of the gate 106 gives rise to an increased number of charge carriers within the active layer 104, which affects the possibility of an electric current to flow therethrough.
  • the OFET of fig. 1 functions as a switch, so that at one gate potential value an electric current may pass between the source and drain, while at another gate value electric current is kept from flowing.
  • One of the central factors that affect the operation of an OFET is the inherent mobility of charge carriers in the channel material. Typically high mobility is aimed at, because high mobility of charge carriers in the channel translates as sensitivity in terms of fast switching time and strong correlation between gate voltage and source-drain current.
  • a majority of the operational features of a transistor are related to the formation of a current channel through the active layer between the source and drain electrodes. Impurities or dopants in semiconductor materials have a strong influence on their properties, so especially the interface between the active layer and the gate insulator should be kept free from all contamination.
  • the publication US 2003/0059984 Al explains how only the source and drain electrodes can be produced onto the substrate in air, after which the substrate must be taken into an inert (nitrogen) atmosphere for the rest of the process. Said publication suggests that even the measurements made for characterising the behaviour of the OFETs should take place in an inert atmosphere.
  • An additional objective of the invention is to present practical applications of organic thin film transistors where laboratory conditions are not necessary.
  • a further objective of the invention is to enable cheap mass production of organic thin film transistor based circuits in accordance with the invention.
  • An thin film transistor device comprises:
  • the dielectric layer is made of a hygroscopic polymer and arranged to allow humidity in the form of a polar solvent absorbed in the dielectric layer to enhance the operation of the thin film transistor device through ionic effects.
  • an electronic measurement circuit which comprises:
  • a supply voltage circuit for providing a supply voltage to said thin film transistor device
  • a detector circuit coupled to said thin film transistor device, which detector circuit is arranged to detect a change in a current modulation capability of said thin film transistor device, which change results from changing environmental conditions.
  • the invention applies to a method for manufacturing a thin film transistor device.
  • the method comprises the steps of:
  • the step of forming a dielectric layer involves using a hygroscopic polymer as a material for the dielectric layer and
  • the method comprises allowing the dielectric layer to absorb humidity in the form of a polar solvent in order to enhance the operation of the thin film transistor device through ionic effects.
  • a hygroscopic polymer is one that absorbs moisture from its environment. According to a very limited interpretation, the terms “hygroscopicity”, “moisture” and “humidity” are only associated with absorbing water vapour. In the context of the present invention, however, also other solvents than water should be considered. Throughout this description and the associated claims, hygroscopicity is taken to mean the capability of absorbing solvent vapour in a way that an electrolyte is allowed to form in the hygroscopic material and/or at a surface thereof if ions are present. Similarly this document assumes the concepts moisture and humidity to refer to the presence of vaporous and/or liquid solvent, specific examples of which include but are not limited to water, ethanol and methanol. Later in this description we will also address the effect of the polarity of such a solvent.
  • the gate insulation layer of an organic thin film transistor should contain a hygroscopic polymer.
  • certain characteristics of OFETs were greatly enhanced when moisture from ambient air was allowed to get absorbed into the insulation layer.
  • the observed effects suggest that the presence of moisture within the insulation layer causes phenomena that in measurements appear as if they were related to exceptionally good charge carrier mobility within the channel, which in turn causes the device to exhibit large electric current at low gate voltage as well as excellent current modulation by the gate voltage and a stable source-drain current in the saturation regime under a range of environmental parameters.
  • the saturation current appears to be stable as long as the environmental parameters are maintained constant during the measurement. Some other stable current will be measured if e.g. the humidity level is changed to some other constant value. Similar well-defined behaviour is observed through a large range of humidity levels and other environmental parameters.
  • a manufacturing technique of OFETs was also found to increase the orientation of molecules in the channel material, which in general is an advantageous feature.
  • a top-gate structure was formed in which a generally round gate electrode is located on top of the insulation layer, which in turn covers the channel.
  • the round gate electrode was produced by depositing a drop of liquid polymer solution at the appropriate location and allowing the solvent to evaporate. During evaporation the gate electrode contracted, which caused a mechanical force that drew the insulation layer, which mechanical force was strong enough to even mechanically cause the molecules of the active layer to get somewhat organised.
  • Fig. 1 illustrates a conventional OFET structure
  • fig. 2 illustrates the principle of a layered structure according to the invention
  • fig. 3 illustrates certain method steps according to the invention
  • fig. 4 is a schematic representation of a birefringence pattern
  • fig. 5 is a schematic cross section of an OFET according to the invention
  • fig. 6 shows the OFET of fig. 5 from above
  • fig. 7 illustrates certain I- V curves of an OFET according to the invention
  • fig. 8 illustrates certain other I-V curves of an OFET according to the invention
  • fig. 9 illustrates switching speed in an OFET according to the invention
  • fig. 10 illustrates switching speed in a prior art OFET
  • fig. 11 illustrates a humidity detector according to an embodiment of the invention.
  • Fig. 2 illustrates a cross section of a sandwich structure of organic polymer layers according to an embodiment of the present invention.
  • an active layer 201 At the bottom there is an active layer 201, on top of which there are two other layers that in this description will be designated as the mediator layer 202 and the modifier layer 203.
  • the horizontal dimension of the active layer 201 and the mediator layer 202 are not important, but we assume that the modifier layer 203 has a limited dimension at least in the horizontal direction that coincides with the cross-sectional plane shown in fig. 2.
  • Typical ways of fulfilling such a criterion include but are not limited to making the modifier layer 203 appear as a round dot, in which case the dimension d shown in fig.
  • the diameter of the dot or making the modifier layer 203 appear as a line across the surface of the modifier layer, in which latter case the dimension d is the width of the line.
  • the modified part of the active layer can be separately designated as the modified active layer part 204. It is shown fig. 2 as a cross-hatched area.
  • the key features of the layers of fig. 2 are as follows: - the active layer 201 consists of a semiconductive organic polymer
  • the mediator layer 202 consists of an insulating, hygroscopic organic polymer
  • step 301 an active layer is formed.
  • step 301 involves spin coating a semiconductive organic polymer film onto a substrate from a solution having a concentration of 2-10 mg/ml at a speed of 1000-3000 rpm. The thickness of the resulting film is in the order of 10-100 nm.
  • Step 302 involves covering the active layer formed at step 301 with a hygroscopic insulator layer, typically by spin coating from a solution having a concentration of 50-150 mg/ml at a speed of 500- 3000 rpm.
  • the thickness of the resulting hygroscopic insulator layer is between a few hundred nanometers and a few micrometers, however typically less than 2 micrometers.
  • steps 301 and 302 could also be accomplished through solution casting or printing.
  • spin coating is used, other spin parameters can be used, depending e.g. on initial solution concentrations.
  • the parameter ranges described above should be construed as exemplary only, without limiting the scope of the present invention. It is within the capability of a person skilled in the art to experiment with other parameter ranges in order to find other ways of arriving at comparable operational characteristics of the resulting organic thin film semiconductor devices.
  • Step 303 involves forming a modifier layer on top of the hygroscopic insulator layer formed in step 302.
  • the modifier layer is formed by depositing a limited amount of a solution onto the surface of the hygroscopic insulator layer at substep 304, and allowing said limited amount of a solution to dry at substep 305.
  • Typical ways of depositing a limited amount of a solution include but are not limited to pipeting and ink jet printing.
  • the solution from which the modifier layer is formed may include surfactants that determine the way in which the solution behaves in the depositing process.
  • the drying of the modifier layer at substep 305 means that solvent evaporates and consequently the modifier layer shrinks.
  • the shrinking of the modifier layer 203 in the horizontal direction causes a horizontal mechanical force that draws the mediator layer 202 and therethrough also the surface of the active layer 201.
  • the drawing force acts in the radial directions of the round dot.
  • the drawing force coincides essentially with a direction perpendicular to the direction of the line.
  • the force that draws the surface of the active layer causes the molecules within the modified active layer part 204 to become somewhat oriented in the direction of the force.
  • the adhesion properties between the layers are important; the better is the adhesion between layers 201 and 202 on one hand and layers 202 and 203 on the other hand, the stronger is the effect of conveying some of the horizontally drawing force into the active layer 201.
  • Fig. 4 is a schematic line drawing representation of an image which was taken from a modified active layer surface with an optical microscope using crossed polarizers.
  • the imaged sample was prepared by first manufacturing a layered polymer structure in accordance with figs. 2 and 3, and later removing the modifier and mediator layers so that the surface of the active layer became visible.
  • the generally round pattern 401 coincides with the top surface of the modified active layer part 204; in other words it is that part of the active layer surface that was directly under the
  • the cross polarized image shows a birefringence pattern which indicates a modification of the molecular order of the active layer material.
  • a narrow band 402 of intense birefringence appears at the outer rim of the round pattern 401.
  • the birefringence pattern is similar to that of spherulites with a typical "Maltese cross" pattern. Spherulites grow from a center point outwards. In the process described above the starting point is a solid, homogeneous, amorphous or semicrystalline polymer film in which a rearrangement of molecules occurs.
  • the birefringence pattern 401 can only be due to ordered domains in the polymer material of the active layer.
  • Investigating the different parts of the imaged sample with atomic force microscopy confirms that outside the clearly visible outer birefringence ring, e.g. at part 411 seen in the partial enlargement, the topography of the active layer surface shows unordered, essentially spherical grains of unevenness.
  • the hygroscopicity and electric conduction/isolation properties are important. Basically it is possible to first use some other mediator and modifier materials to produce the ordering and deformation effects in the active layer, then remove these "preliminary" mediator and modifier layers by applying suitable etching or other removing process that does not damage the active layer, and only thereafter deposit the final gate insulator and gate electrode layers with the desired hygroscopicity and electric conduction and isolation properties.
  • Electrochemical processes and effects are basically known in the field of organic thin film semiconductor devices.
  • the subject has been treated for example in D Nilsson et al. "Bi-stabled and dynamic current modulation in electrochemical organic transistors", Advanced Materials 2002, 14 page 51, and Epstein, AJ. et al: "Electric-field induced ion-leveraged metal-insulator transition in conducting polymer-based field effect devices", Current Applied Physics Vol. 2, Issue 4, August 2002, pp. 339-343.
  • the mode of operation of an electrochemical FET described therein differs from that of a traditional FET.
  • an electrochemical transistor is of the so-called “always on” type, where a gate voltage is only used to lower or quench a source-drain current that otherwise would flow freely.
  • the active layer of a conventional electrochemical FET consists of a highly doped conductive polymer, in order to make the maximum attainable conductivity of the channel sufficiently high.
  • Other related structures are organic FET sensors where exposing the channel to the environment causes chemical oxidation or reduction of the semiconductive polymer.
  • the gate dielectric of such known devices has been a traditional insulator, and otherwise such organic FET sensors have behaved like conventional FETs.
  • Another prior art publication that considers electrochemical FETs is V. Rani and K. S.V. Santhanam: "Polycarbazole based electrochemical transistor", J. Solid state Electrochem., 2:99 (1998).
  • the mode of operation of a prior art electrochemical FET depends on the "natural" redox state of the polymer used.
  • the modulation of a current through the active layer by a voltage applied to the modifier layer can be greatly enhanced through electrochemical processes and other ionic effects.
  • electrochemical processes and other ionic effects enhance the modulation of a source-drain current by a gate voltage.
  • the relative order of importance of electrochemical processes and other ionic effects is not accurately known, but since both result from a certain selection of materials and method steps, they fall within the scope of the invention and are described in more detail below.
  • Electrochemical processes and ionic movement may occur in a polymer when there are salts or ions present, and the following two criteria are met:
  • electrochemical processes are fully reversible and an equilibrium state is reached after a time that depends on the electric field within the polymer as well as the concentrations of the different species.
  • An electrochemical process in an organic semiconductor can be generally described as
  • polymer is a conjugated polymer
  • C is a cation
  • A is an anion
  • e is an electron and the plus and minus superscripts designate electric charge.
  • Equation (1) means that a salt gets dissociated into an anion and a cation in the presence of moisture, the anion oxidizes the polymer chain, and a cation and an electron are left free.
  • the conjugated polymer is left in an oxidized state, which typically means that it becomes more conductive.
  • the cation is either trapped in the material or moves freely within the structure under the influence of the prevailing electric field.
  • RR-P3HT regioregular poly(3-hexylthiophene)
  • PEDOT poly(2,3-dihydrothieno-[3,4-b]-l,4-dioxm)
  • PEDOT is also known as poly(dihydrothienodioxine) or polyethylenedioxythiophene.
  • RR-P3HT should not be confused with regiorandom poly(3-hexylthiophene), for which the simplified notation PHT is typically used.
  • PEDOT poly(styrenesulfonate)
  • PSS poly(styrenesulfonate)
  • C is a cation, for example a metal ion.
  • C is a cation, for example a metal ion.
  • the conductivity of a semiconductive polymer like RR-PAT is significantly modified by an electrochemical process under the influence of electric fields, since inherently the material is in a (close to) neutral state. The equation for the process is
  • the active layer 201 is made of RR-P3HT
  • the modifier layer 203 is made of PEDOT:PSS and there is a negative bias voltage applied to the modifier layer 203, giving rise to an electric field.
  • the active layer 201 is in touch with other electrode(s). That is the case for example if the layered structure of fig. 2 is a part of an organic thin film transistor, where a part of the active layer 201 constitutes a channel between a source and a drain, and the modifier layer 203 is the gate electrode (compare to figure 5 which illustrates a FET device and where the electrical connections used to characterize the device are indicated).
  • the hygroscopicity of the mediator (or insulator) layer 202 means that an electrolyte is present, at least if the hygroscopic polymer has been allowed to absorb moisture.
  • PEDOT:PSS is hygroscopic, which means that an electrolyte is present also within the modifier layer 203. Salts and/or ions are present due to intentional doping. Unintentional doping is also likely to result from residues from polymer synthesis as well as contamination during processing.
  • Equation (2) The process described by equation (2) is taking place within the modifier layer 203. Electrons are introduced at the modifier layer 203 by the negative bias voltage. Simultaneously the process of equation (3) is taking place at or near the insulator/semiconductor interface.
  • a negative bias voltage means that electrons will be extracted by that or those electrode(s) that are in contact with the active layer 201.
  • An electrolyte in the hygroscopic insulator 202 will separate salts into negative and positive ions.
  • the negative ions (anions) will oxidize the PHT chains and leave the active layer more conductive.
  • the electrons that appear on the right side of equation (3) will be extracted from the system by the above-mentioned electrode(s) or trapped e.g. in interface states in the insulator.
  • the cations are transported into the insulator 202 and possibly all the way into the modifier layer 203, where they may take part in the simultaneous reaction according to equation (2).
  • the active layer is a p-type semiconducting material and there is a negative bias on the gate electrode (modifier 203) while the drain voltage is swept from zero to a negative value, pinch-off will occur at the drain electrode at a voltage determined by the gate voltage and a threshold voltage.
  • the current through the channel has a purely electronic nature and is therefore very fast. In other words it should not pose any temporal limitations to the applicability of such a device.
  • the ionic processes described above rely on the dissociation of salts and the movement of charged species of ions. This is a slow process and may potentially limit modulation frequency, i.e. the frequency at which certain changes can be observed in the device as a response to changed operating conditions.
  • solvents the molecules of which have inherent polarity cause significant enhancement of the ionic effects.
  • solvents include but are not limited to water, ethanol and methanol.
  • non- polar solvents such as anhydrous p-xylene do not seem to cause similar enhanced ionic effects.
  • FIGs. 5 and 6 illustrate a top-gate thin film transistor structure according to an embodiment of the present invention.
  • Fig. 5 is a cross section and fig. 6 is a top view.
  • the relative dimensions in the drawing are not realistic but were mainly chosen for reasons of graphical clarity.
  • the structure resembles that shown in fig. 1 : there is a non-conductive substrate 501, conductive source and drain electrodes 502 and 503 respectively on a surface of the substrate 501, an active layer 504 connecting the source and drain electrodes 502 and 503, a gate insulator 505 covering the active layer 504, and a gate electrode 506 on top of said gate insulator 505.
  • the material of the substrate 501 is glass or a passive, non-conductive polymer such as PET (polyethylene terephthalate) or PI (polyimide).
  • the source and drain electrodes 502 and 503 are made of thin metal films, for example gold, silver or aluminium, or of conductive polymers such as doped PANI (polyaniline). Generally any material forming an essentially ohmic contact with the semiconductor material can be used.
  • regiorandom PHT seems to not work can be the fact that its conductivity is very low, and consequently the currents measured in a regiorandom PHT based organic thin film transistor are about the order of the leakage currents (most probably due to ionic drift).
  • Regioregular poly(alkylthio- phene)s with longer side chains than RR-PHT show the same behaviour as RR-PHT but with less profound saturation and modulation, which is well in accord with the established knowledge that regioregular poly(alkylthiophene)s with longer side chains have a lower conductivity than RR-PHT.
  • the gate insulator 505 is made of a hygroscopic polymer, such as PVP (polyvinylphenol). Also other hygroscopic polymers can be used, as long as the dielectric constant of the material is sufficiently high: other candidate polymers include but are not limited to polyvinylalcohol.
  • the material of the gate electrode 506 it would basically be possible to make the gate electrode 506 of a metal like the source and drain electrodes.
  • a conductive polymer is the most appropriate, if the all-organic nature of the structure with all associated advantages in processing is to be maintained.
  • a primary candidate for such a conductive polymer is PEDOT:PSS.
  • An additional advantage that has been observed is that since PEDOT:PSS is hygroscopic, a gate electrode made thereof can act as an additional humidity reservoir that enhances the phenomena that are otherwise related to the hygroscopicity of the gate insulator 505.
  • Advantageous solvents for RR-P3HT include chloroform (i.e. trichloromethane or methyl trichloride) and xylene.
  • the solvent for the hygroscopic polymer of the gate insulator should not dissolve the active layer; good candidates for such solvents are alcohols such as 2-propanol, as well as ethylacetate and acetone.
  • An organic gate electrode may be advantageously formed from a dispersion in water.
  • a known commercially available PEDOT:PSS dispersion with 1.3 weight percentage comes under the trade name Baytron P, which is a registered trade mark of H.C.Starck GmbH.
  • the thickness of the active layer 504 is between 5 and 500 nanometers, preferably between 10 and 100 nanometers. According to the invention the conjugated polymer material of the active layer 504 does not need to be purified, which also limits the useful thickness of the active layer: with thicknesses larger than 100 nanometers performance will degrade due to bulk properties of the film.
  • the thickness of the hygroscopic gate insulator 505 is in the range of 300-2000 nanometers, and the thickness of the gate electrode 506 is in the range of 100-5000 nanometers.
  • the length L of the channel region between the source and drain electrodes 502 and 503 is in the order of some tens of micrometers, and the width W of the channel is typically in the order of millimetres.
  • L values in the range from 10 to 400 micrometers, and W values in the range from 1 to 100 millimeters.
  • the longer W values are most easily achieved by using a finger structure.
  • Said exemplary values should not be construed as limiting the applicability of the invention to L and W values also outside said ranges.
  • the structure may also contain other layers, especially on top of those layers illustrated here. If the purpose is to expose the thin film transistor device to environmental conditions and take advantage of the changes in operation of the device that result from changes in the environmental conditions, any outer protective layers or decorative layers should be permeable to humidity in the form of at least one polar solvent. If the structure is covered with a hermetically sealing protective layer, care must be taken to allow humidity in the form of a polar solvent to get absorbed into the structure before the hermetically sealing coating is applied.
  • Fig. 7 illustrates certain I-V curves of an actual test device having the structure of figs. 5 and 6, with a PET substrate, PANI source and drain electrodes, RR-P3HT active layer, PVP gate insulator and PEDOT:PSS gate electrode.
  • the channel length in said actual test device was 20 micrometers and the width W of the channel was 7.2 millimeters.
  • the horizontal axis of fig. 7 is the applied source-drain voltage, and the vertical axis is the measured drain current.
  • the curves of fig. 7 show that an organic thin film transistor according to an embodiment of the invention is capable of exhibiting strong current modulation as well as a relatively high drain current at drive (gate) voltages that are essentially lower than the typical values of 20—30 volts known from prior art devices.
  • the turn on voltage which reflects the gate voltage at which the current through the semiconductor layer starts to increase under influence of the gate voltage, is about 0.0-0.5 volts.
  • the so-called sub threshold swing is less than 1 volt per decade, and the ON/OFF ratio is from 100 to 1000.
  • Standard techniques described in e.g. S.M. Sze: "Physics of semiconductor devices", 2 nd ed., John Wiley & son, New York 1981, can be applied to estimate the charge carrier mobility, which appears to be in the order of one to a few hundred cm 2 /Vs.
  • the estimated charge carrier mobility values are remarkably higher than according to known prior art, up to the limit of being unphysically high.
  • Typical intrinsic mobility values for polymers lie in the range 10 5 — 10 "2 cm 2 /Vs depending on deposition techniques and material purity.
  • a fourth piece of evidence is an observed temperature dependency of the current modulation effect: low temperatures, which are known to hamper ion movement, can also be shown to decrease current modulation in an organic thin film transistor according to an embodiment of the invention.
  • low temperatures which are known to hamper ion movement, can also be shown to decrease current modulation in an organic thin film transistor according to an embodiment of the invention.
  • the curve 801 of fig. 8 is obtained.
  • the curve 801 exhibits largest current values at drain voltages between -0.5 and -1.0 volts.
  • Curves 802 and 803 represent subsequent measurements with gate voltages 0.0 V and -0.5 volts respectively.
  • the "bump" in curve 801 would indicate some initial rearranging of the ions inherently present in the device.
  • FIG. 9 illustrates measurement results from an arrangement where an OFET with a PVP gate electrode was subjected to a sequence of gate voltage values of 0.0 V, -1.0 V, 0.0 V, -1.0 V, 0.0 V, +0.5 V, 0.0 V, -1.0 V and 0.0 V so that the gate voltage remained constant for 60 seconds at each step of the sequence.
  • the source-drain voltage was 1.0 V.
  • the horizontal axis in fig. 9 is time in seconds, and the vertical axis is drain current in amperes.
  • fig. 10 illustrates measurement results from a similar arrangement with a non-hygroscopic polystyrene gate insulator. This time the source-drain voltage was 10.0 V and the sequence of gate voltage values was 0.0 V, -10.0 V, 0.0 V, -10.0 V, 0.0 V, +5.0 V, 0.0 V, -10.0 V and 0.0 V.
  • Curve 1001 is essentially rectangular, showing much faster settling of the drain current. However, one should note that even with ten times larger gate and drain voltage values, the drain currents measured in the latter case (with no hygroscopic gate insulator like in the present invention) remain one decade smaller than in the case of fig. 9.
  • Fig. 11 illustrates schematically a device in which an OFET unit 1101 is coupled to a bias circuit 1102, a supply voltage circuit 1103 and a detector circuit 1104.
  • the OFET unit 1101 comprises two serially coupled organic thin film transistors 1111 and 1112 according to the invention.
  • the source of the upper transistor 1111 is coupled to ground potential, and the drain of the lower transistor 1112 is coupled to a negative supply voltage.
  • the gate and source of the lower transistor 1112 are coupled together to the drain of the upper transistor 1111, from which there is also a connection to the detector circuit 1104.
  • the bias circuit 1102 is coupled to the gate of the upper transistor 1111.
  • the bias circuit 1102 is adapted to supply a slowly oscillating bias voltage to the gate of the upper transistor 1111.
  • the time constant for the slowly oscillating bias voltage is typically in the order of several seconds or tens of seconds, and it oscillates between voltage values the absolute values of which are relatively close to zero, like between 0.0 V and +0.5 V or 0.0 V and -0.5 V.
  • the detector circuit 1104 comprises a peak to peak detector that is adapted to measure the maximum observable amplitude of the oscillating voltage that appears at the point between the transistors 1111 and 1112.
  • the OFET unit 1101 If the OFET unit 1101 is in a very dry atmosphere, the current modulation capability of the transistors 1111 and 1112 is small and consequently also the amplitude of the output voltage measured in the detector circuit 1104 is small. On the other hand, if the OFET unit is subjected to humidity levels such as those observed in normal room atmosphere, the transistors 1111 and 1112 exhibit strong current modulation capability, resulting in relatively large oscillations in the output voltage. It is easy to build a digital look-up table arrangement within the detector circuit 1104 that maps each detected output voltage amplitude into a certain estimated humidity value.
  • the exemplary embodiments described above should not be construed so that they would be the only way of reducing the invention into practice.
  • the exemplary description of an OFET according to the invention only considered the top-gate structure, which is only one of the known basic structures of thin film transistors.
  • the ionic effect based enhancement of the operation of an organic thin film transistor device can be achieved also in other kinds of structures.
  • a transistor structure is known where the gate electrode is at the bottom, covered by the gate dielectric and the active layer.
  • the source and drain electrodes are at the top. If such a structure is formed so that the gate dielectric is made of a hygroscopic polymer and left exposed to ambient humidity, it can function essentially like the top-gate structured ones described above.
  • One way of ensuring that the gate dielectric is exposed is to make the active layer and the source and drain electrodes small enough, so that humidity may enter the gate dielectric from around the active layer and the source and drain electrodes.
  • Another possibility is to make the active layer and the source and drain electrodes look like a mesh, with holes therethrough to allow humidity to get in touch with the gate dielectric.
  • the top-gate structure can be modified from that illustrated in fig. 5 by placing the source and drain electrodes between the active layer and the gate insulator layer, instead of between the substrate and the active layer as in fig. 5.
  • Experiments with such a modified structure suggest that much less saturation will occur compared to the structure of fig. 5, which could be interpreted as less efficient pinch-off.
  • Dopant impurities can occur in the active layer due to many reasons. Firstly, an unpurified original solution can be used, with impurities inherently present. Secondly, it is possible to use a purified original solution with intentionally added dopant substance(s). Thirdly, the impurities may come from the adjacent insulator layer, which can similarly be made from an unpurified material or from a purified material with some intentionally added ionic substance(s).
  • the hygroscopic gate dielectric layer (and the potentially hygroscopic gate electrode) would be sealed from the environment under certain other layers, it is possible to achieve enhanced operation according to the invention by ensuring that enough humidity was absorbed in said hygroscopic layer(s) before the structure was completed. This is easy if the manufacturing process is carried out in room atmosphere, where humidity is naturally present, or under some at least slightly controlled atmosphere where humidity is intentionally provided.
  • the absorption of humidity into the hygroscopic layers can be enhanced during manufacture by deliberately exposing said layers to water or other solvent vapour before covering them with other layers.
  • the electrochemical and other ionic enhancement of the transistor operation is naturally independent of environmental conditions.

Abstract

A thin film transistor device comprises an organic semiconductor layer (504), a source electrode (502) and a drain electrode (503). The source and drain electrodes are coupled to each other through the organic semiconductor layer (504). Additionally there is a gate electrode (506) and a dielectric layer (505) between the organic semiconductor layer (504) and the gate electrode (506). The dielectric layer (505) is made of a hygroscopic polymer and arranged to allow humidity absorbed in the dielectric layer (505) to enhance the operation of the thin film transistor device through ionic effects.

Description

Methods and arrangements for acquiring and utilising enhanced electronic conduction in an organic thin film transistor
The invention concerns generally the technology of using organic materials in manufacturing thin film transistors, and utilising thin film transistors so manufactured. Especially the invention takes advantage of certain properties of organic thin film transistors that are related to electronic conduction, and shows how appropriate selection of materials and manufacturing methods can lead to even completely new areas of application for organic thin film transistors.
The basic technology of organic thin film field effect transistors, known as OFETs for short, is well known and established. Fig. 1 is a cross section through one basic OFET structure, known as the top-gate structure. A substrate 101 constitutes a smooth, nonconductive support surface on which the other layers reside. On top of the substrate 101 there are source and drain electrodes 102 and 103 respectively, which are made of a highly conductive material, such as a thin metallic layer or a conductive polymer. An active layer 104 connects the source and drain electrodes 102 and 103 together. Some textbook sources also designate the active layer as the channel layer. The active layer 104 is made of a semiconductive organic polymer; conjugated polymers are preferred.
On top of the active layer 104 there is an insulation layer 105, the purpose of which is to act as an electric insulator. Consequently the insulation layer 105 has good electric insulation properties, and is made of polymer (e.g. polystyrene) or inorganic material (e.g. SiO2). It is possible to build the insulation layer 105 from several component layers one upon the other, in order to acquire specific results such as surface modification, diffusion barrier or solvent compatibility. A gate electrode layer 106 lies on top of the insulation layer 105 and is made of metal or a highly conductive polymer. The structure also comprises various interconnect lines and contact pads connected to electrodes, but these are not shown in fig. 1 in order to enhance graphical clarity.
The structure of fig. 1 functions as a field-effect transistor so that the electric potential of the gate 106 gives rise to an increased number of charge carriers within the active layer 104, which affects the possibility of an electric current to flow therethrough. In its simplest form the OFET of fig. 1 functions as a switch, so that at one gate potential value an electric current may pass between the source and drain, while at another gate value electric current is kept from flowing. One of the central factors that affect the operation of an OFET is the inherent mobility of charge carriers in the channel material. Typically high mobility is aimed at, because high mobility of charge carriers in the channel translates as sensitivity in terms of fast switching time and strong correlation between gate voltage and source-drain current.
The state of the art in OFET technology is discussed for example in publications US 2003/0059984 Al, WO02/095805, US 6,380,558, US 6,506,438, US 6,429,450, WO99/10939 and US 6,344,660. Additionally certain methods for manufacturing semiconductive thin film structures are treated in EP-Al-O 701 290, US 6,150,692 and US 2002/0158574 Al . It is possible to combine organic and inorganic materials in manufacturing thin film transistors, but the use of inorganic substances and small organic molecules tends to require complicated vacuum deposition processing and expensive machinery, which means loosing many advantages associated with organic materials that come in solutions.
A majority of the operational features of a transistor are related to the formation of a current channel through the active layer between the source and drain electrodes. Impurities or dopants in semiconductor materials have a strong influence on their properties, so especially the interface between the active layer and the gate insulator should be kept free from all contamination. For example the publication US 2003/0059984 Al explains how only the source and drain electrodes can be produced onto the substrate in air, after which the substrate must be taken into an inert (nitrogen) atmosphere for the rest of the process. Said publication suggests that even the measurements made for characterising the behaviour of the OFETs should take place in an inert atmosphere.
Other disadvantageous features of known manufacturing processes include extensive cleaning and surface preparation steps, which are required especially if thermal SiO2 is used as the gate insulator. The selection of materials is far from trivial: for example the source and drain electrode materials must bond well to the underlying substrate on one hand and make good electrical contact with the conjugated polymer material of the active layer on the other hand. In order to allow the gate voltage to effectively modulate the channel current, the gate insulator material should have a high dielectric constant, and it should be possible to make the gate insulator as thin as possible. The conventional properties of organic insulators make it difficult to achieve these aims, which means that relatively large voltages are often needed to operate the device. High voltages in turn increase the risk of breakdown discharges and reduce the practical applicability of the circuits. According to an article of Bao, Z. et al: "Soluble and processable regioregular poly(3-hexylthiophene) for thin film field-effect transistor applications with high mobility", published in Applied Physics Letters, 69 (26) pp. 4108-4110 (1996), standard polymer transistors require a gate voltage of 20-30 volts or higher to achieve saturation of the drain current and typical transistor-like I-V curves (current vs. voltage curves) that show how gate voltage modulates the source-drain current.
It is an objective of the present invention to present methods and arrangements for preparing and utilising (especially all-organic) thin film transistors with low voltage requirements. An additional objective of the invention is to present practical applications of organic thin film transistors where laboratory conditions are not necessary. A further objective of the invention is to enable cheap mass production of organic thin film transistor based circuits in accordance with the invention.
The objectives of the invention are achieved by using a hygroscopic polymer in the gate insulation layer of an organic thin film transistor, and by allowing ambient humidity to affect the electronic current within the channel of the organic thin film transistor. Certain aspects of the invention are also achieved by allowing the contraction of an upper layer during manufacture to mechanically pull the molecules of the active layer into a certain direction, causing increased orientation within the active layer.
An thin film transistor device according to the invention comprises:
- an organic semiconductor layer,
- a source electrode and a drain electrode, which source and drain electrodes are coupled to each other through the organic semiconductor layer, - a gate electrode and
- a dielectric layer between the organic semiconductor layer and the gate electrode; it is characteristic to the device that the dielectric layer is made of a hygroscopic polymer and arranged to allow humidity in the form of a polar solvent absorbed in the dielectric layer to enhance the operation of the thin film transistor device through ionic effects.
The invention also applies to an electronic measurement circuit, which comprises:
- a thin film transistor device of the kind described above, which thin film transistor device is exposed to environmental conditions, - a biasing circuit for providing a bias voltage to a thin film transistor within said thin film transistor device,
- a supply voltage circuit for providing a supply voltage to said thin film transistor device, and - a detector circuit coupled to said thin film transistor device, which detector circuit is arranged to detect a change in a current modulation capability of said thin film transistor device, which change results from changing environmental conditions.
Additionally the invention applies to a method for manufacturing a thin film transistor device. The method comprises the steps of:
- forming an organic semiconductor layer to connect a pair of source and drain electrodes,
- forming a dielectric layer to cover a part of the organic semiconductor layer, and
- forming a gate electrode to cover a part of the dielectric layer and to coincide in location with a channel region between said source electrode and said drain electrode; and is characterised in that:
- the step of forming a dielectric layer involves using a hygroscopic polymer as a material for the dielectric layer and
- the method comprises allowing the dielectric layer to absorb humidity in the form of a polar solvent in order to enhance the operation of the thin film transistor device through ionic effects.
A hygroscopic polymer is one that absorbs moisture from its environment. According to a very limited interpretation, the terms "hygroscopicity", "moisture" and "humidity" are only associated with absorbing water vapour. In the context of the present invention, however, also other solvents than water should be considered. Throughout this description and the associated claims, hygroscopicity is taken to mean the capability of absorbing solvent vapour in a way that an electrolyte is allowed to form in the hygroscopic material and/or at a surface thereof if ions are present. Similarly this document assumes the concepts moisture and humidity to refer to the presence of vaporous and/or liquid solvent, specific examples of which include but are not limited to water, ethanol and methanol. Later in this description we will also address the effect of the polarity of such a solvent.
Conventionally the hygroscopicity of certain polymers has been regarded as a nuisance in thin film transistor technology: according to the established technology in this technical field the building materials of OFETs should be kept as pure as possible to achieve controlled and predictable functional characteristics. Manufacturing and measurements have taken place within an inert atmosphere, and/or OFET structures have been covered with impermeable coatings before exposing them to uncontrolled environmental conditions.
According to the invention the gate insulation layer of an organic thin film transistor should contain a hygroscopic polymer. In the research that led to the invention it was found that certain characteristics of OFETs were greatly enhanced when moisture from ambient air was allowed to get absorbed into the insulation layer. The observed effects suggest that the presence of moisture within the insulation layer causes phenomena that in measurements appear as if they were related to exceptionally good charge carrier mobility within the channel, which in turn causes the device to exhibit large electric current at low gate voltage as well as excellent current modulation by the gate voltage and a stable source-drain current in the saturation regime under a range of environmental parameters. In more exact terms, the saturation current appears to be stable as long as the environmental parameters are maintained constant during the measurement. Some other stable current will be measured if e.g. the humidity level is changed to some other constant value. Similar well-defined behaviour is observed through a large range of humidity levels and other environmental parameters.
A manufacturing technique of OFETs was also found to increase the orientation of molecules in the channel material, which in general is an advantageous feature. In said manufacturing technique a top-gate structure was formed in which a generally round gate electrode is located on top of the insulation layer, which in turn covers the channel. The round gate electrode was produced by depositing a drop of liquid polymer solution at the appropriate location and allowing the solvent to evaporate. During evaporation the gate electrode contracted, which caused a mechanical force that drew the insulation layer, which mechanical force was strong enough to even mechanically cause the molecules of the active layer to get somewhat organised.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Fig. 1 illustrates a conventional OFET structure, fig. 2 illustrates the principle of a layered structure according to the invention, fig. 3 illustrates certain method steps according to the invention, fig. 4 is a schematic representation of a birefringence pattern, fig. 5 is a schematic cross section of an OFET according to the invention, fig. 6 shows the OFET of fig. 5 from above, fig. 7 illustrates certain I- V curves of an OFET according to the invention, fig. 8 illustrates certain other I-V curves of an OFET according to the invention, fig. 9 illustrates switching speed in an OFET according to the invention, fig. 10 illustrates switching speed in a prior art OFET, and fig. 11 illustrates a humidity detector according to an embodiment of the invention.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
Fig. 2 illustrates a cross section of a sandwich structure of organic polymer layers according to an embodiment of the present invention. At the bottom there is an active layer 201, on top of which there are two other layers that in this description will be designated as the mediator layer 202 and the modifier layer 203. The horizontal dimension of the active layer 201 and the mediator layer 202 are not important, but we assume that the modifier layer 203 has a limited dimension at least in the horizontal direction that coincides with the cross-sectional plane shown in fig. 2. Typical ways of fulfilling such a criterion include but are not limited to making the modifier layer 203 appear as a round dot, in which case the dimension d shown in fig. 2 is the diameter of the dot, or making the modifier layer 203 appear as a line across the surface of the modifier layer, in which latter case the dimension d is the width of the line. Certain physical and chemical interactions between the layers, which interactions will be described later in detail, cause a certain part of the active layer 201 to be modified. The modified part of the active layer can be separately designated as the modified active layer part 204. It is shown fig. 2 as a cross-hatched area.
According to the present invention, the key features of the layers of fig. 2 are as follows: - the active layer 201 consists of a semiconductive organic polymer
- the mediator layer 202 consists of an insulating, hygroscopic organic polymer
- the properties of the modifier layer 203, and/or the process of forming the modifier layer 203 on top of a stack comprising the active layer 201 and the mediator layer 202, cause the modifications that give rise to the modified active layer part 204.
We will first describe certain physical modifications that are obtained in the modified active layer part 204 by following certain manufacturing steps. For the time being we neglect the fact that certain other structural elements may be present, and just consider the three layers of fig. 2. Let us assume that a manufacturing method of an organic thin film semiconductor device comprises the steps of fig. 3. At step 301, an active layer is formed. Typically step 301 involves spin coating a semiconductive organic polymer film onto a substrate from a solution having a concentration of 2-10 mg/ml at a speed of 1000-3000 rpm. The thickness of the resulting film is in the order of 10-100 nm. Step 302 involves covering the active layer formed at step 301 with a hygroscopic insulator layer, typically by spin coating from a solution having a concentration of 50-150 mg/ml at a speed of 500- 3000 rpm. The thickness of the resulting hygroscopic insulator layer is between a few hundred nanometers and a few micrometers, however typically less than 2 micrometers.
Instead of spin coating, steps 301 and 302 could also be accomplished through solution casting or printing. Also when spin coating is used, other spin parameters can be used, depending e.g. on initial solution concentrations. The parameter ranges described above should be construed as exemplary only, without limiting the scope of the present invention. It is within the capability of a person skilled in the art to experiment with other parameter ranges in order to find other ways of arriving at comparable operational characteristics of the resulting organic thin film semiconductor devices.
Step 303 involves forming a modifier layer on top of the hygroscopic insulator layer formed in step 302. The modifier layer is formed by depositing a limited amount of a solution onto the surface of the hygroscopic insulator layer at substep 304, and allowing said limited amount of a solution to dry at substep 305. Typical ways of depositing a limited amount of a solution include but are not limited to pipeting and ink jet printing. The solution from which the modifier layer is formed may include surfactants that determine the way in which the solution behaves in the depositing process. The drying of the modifier layer at substep 305 means that solvent evaporates and consequently the modifier layer shrinks. With a modifier layer in the form of a round dot, shrinking of the order of 4-12 per cent from the area of the dot has been observed. The thicker the layer of deposited solution, the stronger is the shrinking effect. Typical thicknesses of the complete, dried modifier layer are between 100 and 5000 nanometers. Related issues are discussed in detail in a publication L. F. Francis, A. V. McCormick, D. M. Vaessen, and J. A. Payne: "Measurement and Development of Stress in Polymer Coatings", Journal of Materials Science, 37 4717-4731 (2002).
According to the present invention, it is not necessary to perform steps 301 to 305 in any particularly controlled atmosphere. Ordinary room conditions are enough. Unlike many beliefs that were common earlier, impurities and moisture that are inevitably introduced into the polymer layers in a non-controlled atmosphere do not degrade the performance of the obtained organic thin film device; quite on the contrary they appear to have a remarkably positive effect on certain measurable characteristics of the device.
The shrinking of the modifier layer 203 in the horizontal direction causes a horizontal mechanical force that draws the mediator layer 202 and therethrough also the surface of the active layer 201. In the case of a modifier layer shaped like a round dot, the drawing force acts in the radial directions of the round dot. In the case of a modifier layer shaped like a line across the surface of the mediator layer, the drawing force coincides essentially with a direction perpendicular to the direction of the line. The force that draws the surface of the active layer causes the molecules within the modified active layer part 204 to become somewhat oriented in the direction of the force. The adhesion properties between the layers are important; the better is the adhesion between layers 201 and 202 on one hand and layers 202 and 203 on the other hand, the stronger is the effect of conveying some of the horizontally drawing force into the active layer 201.
Fig. 4 is a schematic line drawing representation of an image which was taken from a modified active layer surface with an optical microscope using crossed polarizers.
The imaged sample was prepared by first manufacturing a layered polymer structure in accordance with figs. 2 and 3, and later removing the modifier and mediator layers so that the surface of the active layer became visible. The generally round pattern 401 coincides with the top surface of the modified active layer part 204; in other words it is that part of the active layer surface that was directly under the
(round dot shaped) modifier layer 203 before removing the modifier and mediator layers. The cross polarized image shows a birefringence pattern which indicates a modification of the molecular order of the active layer material. A narrow band 402 of intense birefringence appears at the outer rim of the round pattern 401.
The birefringence pattern is similar to that of spherulites with a typical "Maltese cross" pattern. Spherulites grow from a center point outwards. In the process described above the starting point is a solid, homogeneous, amorphous or semicrystalline polymer film in which a rearrangement of molecules occurs. The birefringence pattern 401 can only be due to ordered domains in the polymer material of the active layer. Investigating the different parts of the imaged sample with atomic force microscopy confirms that outside the clearly visible outer birefringence ring, e.g. at part 411 seen in the partial enlargement, the topography of the active layer surface shows unordered, essentially spherical grains of unevenness. Inside the birefringence ring, e.g. at part 412 seen in the partial enlargement, similar grains exist but they appear to be stretched in the radial direction of the round birefringence pattern. At the intense birefringence ring, e.g. at part 413 of the partial enlargement, the topography of the surface shows very strong mechanical modification, which in atomic force microscopy images is seen as an arrangement of tightly packed lines in the radial direction.
As a conclusion from the cross polarized optical microscopy imaging and atomic force microscopic studies we may state that the shrinking effect of a round dot shaped modifier layer during manufacture causes both molecular (crystalline) ordering and mechanical topographic deformation within the active layer film. Ordering generally has an advantageous effect on charge carrier mobility within the active layer. An important thing to note is that if only the ordering and deformation aspects described above are aimed at, the requirements for using hygroscopic polymers with some specifically determined electric conduction and isolation properties can be lifted. Similar contraction-based ordering phenomena may be achieved by using also other kinds of materials in the "mediator" and "modifier" layers. However, in order to achieve the OFET structures and enhanced transistor operation aimed at in the present invention, also the hygroscopicity and electric conduction/isolation properties are important. Basically it is possible to first use some other mediator and modifier materials to produce the ordering and deformation effects in the active layer, then remove these "preliminary" mediator and modifier layers by applying suitable etching or other removing process that does not damage the active layer, and only thereafter deposit the final gate insulator and gate electrode layers with the desired hygroscopicity and electric conduction and isolation properties.
Next we will describe certain ionic effects that also appear in the layered structure of fig. 2 and that apparently have an even stronger influence on the performance of e.g. organic thin film transistors than the ordering and deformation phenomena described above, if said organic thin film transistors are made to include such a layered structure.
Electrochemical processes and effects are basically known in the field of organic thin film semiconductor devices. The subject has been treated for example in D Nilsson et al. "Bi-stabled and dynamic current modulation in electrochemical organic transistors", Advanced Materials 2002, 14 page 51, and Epstein, AJ. et al: "Electric-field induced ion-leveraged metal-insulator transition in conducting polymer-based field effect devices", Current Applied Physics Vol. 2, Issue 4, August 2002, pp. 339-343. The mode of operation of an electrochemical FET described therein differs from that of a traditional FET. According to said publications, an electrochemical transistor is of the so-called "always on" type, where a gate voltage is only used to lower or quench a source-drain current that otherwise would flow freely. The active layer of a conventional electrochemical FET consists of a highly doped conductive polymer, in order to make the maximum attainable conductivity of the channel sufficiently high. Other related structures are organic FET sensors where exposing the channel to the environment causes chemical oxidation or reduction of the semiconductive polymer. The gate dielectric of such known devices has been a traditional insulator, and otherwise such organic FET sensors have behaved like conventional FETs. Another prior art publication that considers electrochemical FETs is V. Rani and K. S.V. Santhanam: "Polycarbazole based electrochemical transistor", J. Solid state Electrochem., 2:99 (1998). The mode of operation of a prior art electrochemical FET depends on the "natural" redox state of the polymer used.
According to the present invention, the modulation of a current through the active layer by a voltage applied to the modifier layer can be greatly enhanced through electrochemical processes and other ionic effects. Translated into transistor terminology, this means that electrochemical processes and other ionic effects enhance the modulation of a source-drain current by a gate voltage. At the time of writing this description the relative order of importance of electrochemical processes and other ionic effects is not accurately known, but since both result from a certain selection of materials and method steps, they fall within the scope of the invention and are described in more detail below.
Electrochemical processes and ionic movement may occur in a polymer when there are salts or ions present, and the following two criteria are met:
- there is an electrolyte to accommodate mobile ions
- there is an electrical current supplying electric charges (electrons).
The electrochemical processes are fully reversible and an equilibrium state is reached after a time that depends on the electric field within the polymer as well as the concentrations of the different species. An electrochemical process in an organic semiconductor can be generally described as
polymer + C+A" <-> polymer"1" A" + C+ + e" (1)
where "polymer" is a conjugated polymer, C is a cation, A is an anion, e is an electron and the plus and minus superscripts designate electric charge. Equation (1) means that a salt gets dissociated into an anion and a cation in the presence of moisture, the anion oxidizes the polymer chain, and a cation and an electron are left free. The conjugated polymer is left in an oxidized state, which typically means that it becomes more conductive. The cation is either trapped in the material or moves freely within the structure under the influence of the prevailing electric field.
As specific examples we will consider electrochemical processes and other ionic effects in RR-P3HT (regioregular poly(3-hexylthiophene)), which is a polymeric semiconductor, and PEDOT (poly(2,3-dihydrothieno-[3,4-b]-l,4-dioxm)), which is a conductive polymer. Depending on the applied naming standard, PEDOT is also known as poly(dihydrothienodioxine) or polyethylenedioxythiophene. RR-P3HT should not be confused with regiorandom poly(3-hexylthiophene), for which the simplified notation PHT is typically used. On the other hand, from the viewpoint of the present invention all regioregular poly(alkyl-thiophene)s are believed to work similarly, so in order to achieve more generality, the term RR-PAT can be used, meaning regioregular poly(alkyl-thiophene). PEDOT is used in a highly oxidized state in a dispersion in water together with a negatively charged other polymer, typically PSS (poly(styrenesulfonate)). This makes it useful as an electric conductor regardless of small changes in the redox state of the material that may arise from electrochemical processes. The combination name PEDOT :PS S is frequently used. An equation for the electrochemical process is PEDOT+PSS" + C+ + e" <-> PEDOT + C+PSS" (2)
Where C is a cation, for example a metal ion. On the other hand, the conductivity of a semiconductive polymer like RR-PAT is significantly modified by an electrochemical process under the influence of electric fields, since inherently the material is in a (close to) neutral state. The equation for the process is
RR-PAT + C+A" <-> RR-PAT+A" + C+ + e" (3)
The electron appears in both equations (2) and (3), indicating that the processes must be driven by an electric current to occur.
Let us assume that in the layered structure of fig. 2 the active layer 201 is made of RR-P3HT, the modifier layer 203 is made of PEDOT:PSS and there is a negative bias voltage applied to the modifier layer 203, giving rise to an electric field. We may further assume that due to certain other structural factors that are present, the active layer 201 is in touch with other electrode(s). That is the case for example if the layered structure of fig. 2 is a part of an organic thin film transistor, where a part of the active layer 201 constitutes a channel between a source and a drain, and the modifier layer 203 is the gate electrode (compare to figure 5 which illustrates a FET device and where the electrical connections used to characterize the device are indicated). The hygroscopicity of the mediator (or insulator) layer 202 means that an electrolyte is present, at least if the hygroscopic polymer has been allowed to absorb moisture. Actually also PEDOT:PSS is hygroscopic, which means that an electrolyte is present also within the modifier layer 203. Salts and/or ions are present due to intentional doping. Unintentional doping is also likely to result from residues from polymer synthesis as well as contamination during processing.
Taken the assumptions above, both processes of equations (2) and (3) may be occurring simultaneously, but they are not necessarily dependent on each other. A process where the same cation would travel through the thickness of the mediator layer 202 and take part in both redox processes would potentially provide better long time stability than completely independent processes.
The process described by equation (2) is taking place within the modifier layer 203. Electrons are introduced at the modifier layer 203 by the negative bias voltage. Simultaneously the process of equation (3) is taking place at or near the insulator/semiconductor interface. A negative bias voltage means that electrons will be extracted by that or those electrode(s) that are in contact with the active layer 201. An electrolyte in the hygroscopic insulator 202 will separate salts into negative and positive ions. The negative ions (anions) will oxidize the PHT chains and leave the active layer more conductive. The electrons that appear on the right side of equation (3) will be extracted from the system by the above-mentioned electrode(s) or trapped e.g. in interface states in the insulator. The cations are transported into the insulator 202 and possibly all the way into the modifier layer 203, where they may take part in the simultaneous reaction according to equation (2).
The reactions described by equations (l)-(3) above only represent some of the possible explanations of why should an organic thin film transistor according to the invention exhibit such remarkably advantageous characteristics. This description should not be construed as trying to exclude other plausible explanations. Other ionic effects that are supposed to play a certain role include but are not limited to direct ionic movement (drift and diffusion) and doping of the semiconductor, as well as dissociation of a polar solvent (such as e.g. water, ethanol, methanol, isopropanol or a mixture thereof). The last-mentioned effect gives rise to charged species that can dope the semiconductor. In the case of water, the charged species are H+ and/or OH" ions. A process involving the dissociation of a polar solvent should be pH-sensitive.
If there are two electrodes making contact with the active layer 201 and constituting a source-drain pair (compare to fig. 5 below), and if a drain voltage is applied, mobile charges in the active layer 201 will also drift within the active layer between said two electrodes. Thus a channel is formed within the active layer 201. Sweeping the drain voltage from zero towards a non-zero value will eventually cause pinch- off at either the source or the drain contact, which should be visible in an ideally shaped I/V curve (i.e. show saturation of channel current at voltages determined by a turn on voltage). More specifically, if the active layer is a p-type semiconducting material and there is a negative bias on the gate electrode (modifier 203) while the drain voltage is swept from zero to a negative value, pinch-off will occur at the drain electrode at a voltage determined by the gate voltage and a threshold voltage. The current through the channel has a purely electronic nature and is therefore very fast. In other words it should not pose any temporal limitations to the applicability of such a device. However, the ionic processes described above rely on the dissociation of salts and the movement of charged species of ions. This is a slow process and may potentially limit modulation frequency, i.e. the frequency at which certain changes can be observed in the device as a response to changed operating conditions. In experiments with different solvents that might be absorbed into the hygroscopic layer(s) of the structure, it has been found that solvents the molecules of which have inherent polarity cause significant enhancement of the ionic effects. Such solvents include but are not limited to water, ethanol and methanol. On the contrary, non- polar solvents such as anhydrous p-xylene do not seem to cause similar enhanced ionic effects.
We will next take the relatively abstract level discussion presented above further and describe a specific example of an organic thin film transistor. Figs. 5 and 6 illustrate a top-gate thin film transistor structure according to an embodiment of the present invention. Fig. 5 is a cross section and fig. 6 is a top view. The relative dimensions in the drawing are not realistic but were mainly chosen for reasons of graphical clarity. In principle the structure resembles that shown in fig. 1 : there is a non-conductive substrate 501, conductive source and drain electrodes 502 and 503 respectively on a surface of the substrate 501, an active layer 504 connecting the source and drain electrodes 502 and 503, a gate insulator 505 covering the active layer 504, and a gate electrode 506 on top of said gate insulator 505. The material of the substrate 501 is glass or a passive, non-conductive polymer such as PET (polyethylene terephthalate) or PI (polyimide). The source and drain electrodes 502 and 503 are made of thin metal films, for example gold, silver or aluminium, or of conductive polymers such as doped PANI (polyaniline). Generally any material forming an essentially ohmic contact with the semiconductor material can be used.
The active layer 504 consists of a semiconductive conjugated polymer, such as RR- P3HT. More generally we may define that material of the active layer 504 is from a group of semiconductive polymers comprising at least poly(alkylthiophene)s and poly(fluorene-co-thiophene)s, without excluding also other semiconductive polymers. It is possible that a major requirement for the semiconductive polymer is a low ionisation potential, which enables easy oxidation (=doping) by charged ions. On the other hand it has been observed that regiorandom PHT does not give as advantageous results, even though the ionisation potential thereof is the same as for RR-PHT. One reason why regiorandom PHT seems to not work can be the fact that its conductivity is very low, and consequently the currents measured in a regiorandom PHT based organic thin film transistor are about the order of the leakage currents (most probably due to ionic drift). Regioregular poly(alkylthio- phene)s with longer side chains than RR-PHT show the same behaviour as RR-PHT but with less profound saturation and modulation, which is well in accord with the established knowledge that regioregular poly(alkylthiophene)s with longer side chains have a lower conductivity than RR-PHT.
As a difference to prior art organic thin film transistors the gate insulator 505 is made of a hygroscopic polymer, such as PVP (polyvinylphenol). Also other hygroscopic polymers can be used, as long as the dielectric constant of the material is sufficiently high: other candidate polymers include but are not limited to polyvinylalcohol. Regarding the material of the gate electrode 506, it would basically be possible to make the gate electrode 506 of a metal like the source and drain electrodes. However, as a material of the gate electrode 506 a conductive polymer is the most appropriate, if the all-organic nature of the structure with all associated advantages in processing is to be maintained. A primary candidate for such a conductive polymer is PEDOT:PSS. An additional advantage that has been observed is that since PEDOT:PSS is hygroscopic, a gate electrode made thereof can act as an additional humidity reservoir that enhances the phenomena that are otherwise related to the hygroscopicity of the gate insulator 505.
The hygroscopic properties of the gate insulator 505 (and possibly also of the gate electrode 506), the presence of salts and ions, the electrochemical processes and other ionic effects that begin when a gate voltage is applied and the horizontal forces that resulted from the shrinking of the gate electrode during solvent evaporation all cause a modified active layer part 507 to be formed at the interface between the active layer 504 and the gate insulator 505.
The selection of materials is not only a question of conductive properties, but also of solvent compatibility. Advantageous solvents for RR-P3HT include chloroform (i.e. trichloromethane or methyl trichloride) and xylene. The solvent for the hygroscopic polymer of the gate insulator should not dissolve the active layer; good candidates for such solvents are alcohols such as 2-propanol, as well as ethylacetate and acetone. An organic gate electrode may be advantageously formed from a dispersion in water. A known commercially available PEDOT:PSS dispersion with 1.3 weight percentage comes under the trade name Baytron P, which is a registered trade mark of H.C.Starck GmbH.
The physical dimensions of the structure deserve certain consideration. The thickness of the active layer 504 is between 5 and 500 nanometers, preferably between 10 and 100 nanometers. According to the invention the conjugated polymer material of the active layer 504 does not need to be purified, which also limits the useful thickness of the active layer: with thicknesses larger than 100 nanometers performance will degrade due to bulk properties of the film. The thickness of the hygroscopic gate insulator 505 is in the range of 300-2000 nanometers, and the thickness of the gate electrode 506 is in the range of 100-5000 nanometers. The length L of the channel region between the source and drain electrodes 502 and 503 is in the order of some tens of micrometers, and the width W of the channel is typically in the order of millimetres. Experiments have produced satisfactory results at least with L values in the range from 10 to 400 micrometers, and W values in the range from 1 to 100 millimeters. The longer W values are most easily achieved by using a finger structure. Said exemplary values should not be construed as limiting the applicability of the invention to L and W values also outside said ranges.
Although not illustrated in fig. 5, the structure may also contain other layers, especially on top of those layers illustrated here. If the purpose is to expose the thin film transistor device to environmental conditions and take advantage of the changes in operation of the device that result from changes in the environmental conditions, any outer protective layers or decorative layers should be permeable to humidity in the form of at least one polar solvent. If the structure is covered with a hermetically sealing protective layer, care must be taken to allow humidity in the form of a polar solvent to get absorbed into the structure before the hermetically sealing coating is applied.
Fig. 7 illustrates certain I-V curves of an actual test device having the structure of figs. 5 and 6, with a PET substrate, PANI source and drain electrodes, RR-P3HT active layer, PVP gate insulator and PEDOT:PSS gate electrode. The channel length in said actual test device was 20 micrometers and the width W of the channel was 7.2 millimeters. The horizontal axis of fig. 7 is the applied source-drain voltage, and the vertical axis is the measured drain current. The different curves represent different gate voltages Vg so that curve 701 represents Vg = -0.4 volts, curve 702 represents Vg = -0.2 volts, curve 703 represents Vg = -0.0 volts, curve 704 represents Vg = +0.2 volts and curve 705 represents Vg = +0.4 volts. The curves of fig. 7 show that an organic thin film transistor according to an embodiment of the invention is capable of exhibiting strong current modulation as well as a relatively high drain current at drive (gate) voltages that are essentially lower than the typical values of 20—30 volts known from prior art devices. The capability of operating with low voltages is very advantageous, because the production and use of low voltages is simple in practical applications, and since the risk for electric breakdown is reduced even if the dielectric characteristics of the appropriate insulating materials would deteriorate due to e.g. environmental conditions.
The turn on voltage, which reflects the gate voltage at which the current through the semiconductor layer starts to increase under influence of the gate voltage, is about 0.0-0.5 volts. The so-called sub threshold swing is less than 1 volt per decade, and the ON/OFF ratio is from 100 to 1000. Standard techniques described in e.g. S.M. Sze: "Physics of semiconductor devices", 2nd ed., John Wiley & son, New York 1981, can be applied to estimate the charge carrier mobility, which appears to be in the order of one to a few hundred cm2/Vs.
The estimated charge carrier mobility values are remarkably higher than according to known prior art, up to the limit of being unphysically high. Typical intrinsic mobility values for polymers lie in the range 10 5— 10"2 cm2/Vs depending on deposition techniques and material purity. An article CD. Sheraw et al: "Organic Thin-Film Transistor-Driven Polymer-Dispersed Liquid Crystal Displays on Flexible Polymeric Substrates," Applied Physics Letters, vol. 80, n. 6, February 2002 announces highest charge carrier mobilities for organic semiconductors to be in the order of 0.1-1 cmVVs for small molecules or oligomers. Another article Sirringhaus et al: "Two-dimensional charge transport in self-organized, high- mobility conjugated polymers", Nature 401, pp. 685-688 (1999) presents a best obtainable figure of 0.1 cm2/Vs for intrinsically conducting polymers, obtained with highly oriented poly(3-hexylthiophene). It is known that high mobility values can also be obtained by increasing the bulk (intrinsic) conductivity of a semiconductor material, but the high intrinsic conductivity typically results in poor current modulation as well as current saturation.
There are several indications about the special importance of the hygroscopicity of the gate insulator and even that of the gate electrode for the measurement results described above. Firstly, it has been observed that when an organic thin film transistor like that described above is taken from room atmosphere into either intentionally dried air or a controlled atmosphere of pure nitrogen, many of the advantageous effects disappear: no current modulation, no saturation of the I- V curves, no capability for operation with low drive voltages. Said advantageous effects reappear when the device is brought back into room atmosphere. Secondly, if a polymer gate electrode and the original gate insulator are etched away and replaced with a fresh gate insulator and a metallic gate electrode, the originally observed advantageously strong current modulation property is greatly reduced even if the ordering effects evidenced by the birefringence patterns are still present. A related observation is that if the gate electrode was ink-jet printed as a line and not pipeted as a round dot, little (or no) birefringence can be observed while the strong current modulation and low drive voltage effects still remain. Thirdly, the estimated charge carrier mobility values are clearly highest with structures where both the gate insulator and the gate electrode are hygroscopic. It should be noted, though, that the standard methods used to estimate charge carrier mobility values do not take into account any ionic or electrochemical effects; the estimated values above merely serve to qualitatively illustrate the superiority of the devices over prior art organic thin film transistors.
A fourth piece of evidence is an observed temperature dependency of the current modulation effect: low temperatures, which are known to hamper ion movement, can also be shown to decrease current modulation in an organic thin film transistor according to an embodiment of the invention. Fifthly, if an initial I-V measurement is made with a positive gate voltage of +0.5 volts, the curve 801 of fig. 8 is obtained. The curve 801 exhibits largest current values at drain voltages between -0.5 and -1.0 volts. Curves 802 and 803 represent subsequent measurements with gate voltages 0.0 V and -0.5 volts respectively. The "bump" in curve 801 would indicate some initial rearranging of the ions inherently present in the device.
Measuring simply the conductivity of the RR-P3HT layer before and after applying a PVP dielectric layer onto it gives results that are in accord with the other observations. The conductivity of the RR-P3HT increases directly when the PVP is applied, which indicates that dopant species are present in the PVP and they cause doping of the RR-P3HT. The conductivity can be decreased by vacuum annealing the sample but it will not reach the initial conductivity of a bare RR-P3HT film. From the values measured after applying the PVP layer the conductivity increases further when a PEDOT:PSS layer is applied on top of the PVP in order to produce a gate electrode. Without any voltage brought to the PEDOT:PSS layer an OFF state conductivity of approximately 10"4 S/cm is measured. With a gate voltage applied, the measured conductivity is of the order of 10"2 S/cm.
It has been observed that in humid conditions there is a leakage current through even a PVP film with a PEDOT:PSS layer on top of it. Such a leakage current will set an ultimate lower limit for a leakage current through an organic thin film transistor according to the invention. The conductivity of a PVP layer was measured in a plate capacitor arrangement with one gold plate and one PEDOT:PSS plate with a PVP dielectric therebetween. The measurement gives a conductivity of approximately 10"n-10"12 S/cm in room air and a negligibly small conductivity in a dry nitrogen atmosphere. These observations further support the explanations given above based on humidity-assisted ion mobility and electrochemical effects.
Another observed feature that is most likely associated with other than purely electronic phenomena is the modest frequency tolerance of an organic thin film transistor device according to an embodiment of the invention. Fig. 9 illustrates measurement results from an arrangement where an OFET with a PVP gate electrode was subjected to a sequence of gate voltage values of 0.0 V, -1.0 V, 0.0 V, -1.0 V, 0.0 V, +0.5 V, 0.0 V, -1.0 V and 0.0 V so that the gate voltage remained constant for 60 seconds at each step of the sequence. The source-drain voltage was 1.0 V. The horizontal axis in fig. 9 is time in seconds, and the vertical axis is drain current in amperes. It is easy to see from the rounded edges of curve 901 that after each change in gate voltage it took several tens of seconds before drain current stabilised. As a comparison, fig. 10 illustrates measurement results from a similar arrangement with a non-hygroscopic polystyrene gate insulator. This time the source-drain voltage was 10.0 V and the sequence of gate voltage values was 0.0 V, -10.0 V, 0.0 V, -10.0 V, 0.0 V, +5.0 V, 0.0 V, -10.0 V and 0.0 V. Curve 1001 is essentially rectangular, showing much faster settling of the drain current. However, one should note that even with ten times larger gate and drain voltage values, the drain currents measured in the latter case (with no hygroscopic gate insulator like in the present invention) remain one decade smaller than in the case of fig. 9.
The fact that current modulation in an organic thin film transistor according to the invention exhibits remarkable sensitivity to ambient humidity suggests that one possible practical application of the invention could be a humidity sensor. Fig. 11 illustrates schematically a device in which an OFET unit 1101 is coupled to a bias circuit 1102, a supply voltage circuit 1103 and a detector circuit 1104. The OFET unit 1101 comprises two serially coupled organic thin film transistors 1111 and 1112 according to the invention. The source of the upper transistor 1111 is coupled to ground potential, and the drain of the lower transistor 1112 is coupled to a negative supply voltage. The gate and source of the lower transistor 1112 are coupled together to the drain of the upper transistor 1111, from which there is also a connection to the detector circuit 1104. The bias circuit 1102 is coupled to the gate of the upper transistor 1111.
The bias circuit 1102 is adapted to supply a slowly oscillating bias voltage to the gate of the upper transistor 1111. Taken the presently known form and performance of the organic thin film transistors according to the invention, the time constant for the slowly oscillating bias voltage is typically in the order of several seconds or tens of seconds, and it oscillates between voltage values the absolute values of which are relatively close to zero, like between 0.0 V and +0.5 V or 0.0 V and -0.5 V. The detector circuit 1104 comprises a peak to peak detector that is adapted to measure the maximum observable amplitude of the oscillating voltage that appears at the point between the transistors 1111 and 1112. If the OFET unit 1101 is in a very dry atmosphere, the current modulation capability of the transistors 1111 and 1112 is small and consequently also the amplitude of the output voltage measured in the detector circuit 1104 is small. On the other hand, if the OFET unit is subjected to humidity levels such as those observed in normal room atmosphere, the transistors 1111 and 1112 exhibit strong current modulation capability, resulting in relatively large oscillations in the output voltage. It is easy to build a digital look-up table arrangement within the detector circuit 1104 that maps each detected output voltage amplitude into a certain estimated humidity value.
The exemplary embodiments described above should not be construed so that they would be the only way of reducing the invention into practice. For example, the exemplary description of an OFET according to the invention only considered the top-gate structure, which is only one of the known basic structures of thin film transistors. The ionic effect based enhancement of the operation of an organic thin film transistor device can be achieved also in other kinds of structures. For example a transistor structure is known where the gate electrode is at the bottom, covered by the gate dielectric and the active layer. The source and drain electrodes are at the top. If such a structure is formed so that the gate dielectric is made of a hygroscopic polymer and left exposed to ambient humidity, it can function essentially like the top-gate structured ones described above. One way of ensuring that the gate dielectric is exposed is to make the active layer and the source and drain electrodes small enough, so that humidity may enter the gate dielectric from around the active layer and the source and drain electrodes. Another possibility is to make the active layer and the source and drain electrodes look like a mesh, with holes therethrough to allow humidity to get in touch with the gate dielectric.
The top-gate structure can be modified from that illustrated in fig. 5 by placing the source and drain electrodes between the active layer and the gate insulator layer, instead of between the substrate and the active layer as in fig. 5. Experiments with such a modified structure suggest that much less saturation will occur compared to the structure of fig. 5, which could be interpreted as less efficient pinch-off.
Dopant impurities can occur in the active layer due to many reasons. Firstly, an unpurified original solution can be used, with impurities inherently present. Secondly, it is possible to use a purified original solution with intentionally added dopant substance(s). Thirdly, the impurities may come from the adjacent insulator layer, which can similarly be made from an unpurified material or from a purified material with some intentionally added ionic substance(s).
Even if the hygroscopic gate dielectric layer (and the potentially hygroscopic gate electrode) would be sealed from the environment under certain other layers, it is possible to achieve enhanced operation according to the invention by ensuring that enough humidity was absorbed in said hygroscopic layer(s) before the structure was completed. This is easy if the manufacturing process is carried out in room atmosphere, where humidity is naturally present, or under some at least slightly controlled atmosphere where humidity is intentionally provided. The absorption of humidity into the hygroscopic layers can be enhanced during manufacture by deliberately exposing said layers to water or other solvent vapour before covering them with other layers. In a structure where the hygroscopic layers are sealed from environmental effects, the electrochemical and other ionic enhancement of the transistor operation is naturally independent of environmental conditions.

Claims

Claims
1. A thin film transistor device, comprising:
- an organic semiconductor layer (201, 504),
- a source electrode (502) and a drain electrode (503), which source and drain electrodes are coupled to each other through the organic semiconductor layer (201,
504),
- a gate electrode (203, 506) and
- a dielectric layer (202, 505) between the organic semiconductor layer (201, 504) and the gate electrode (203, 506); characterised in that the dielectric layer (202, 505) is made of a hygroscopic polymer and arranged to allow humidity in the form of a polar solvent absorbed in the dielectric layer (202, 505) to enhance the operation of the thin film transistor device through ionic effects.
2. A thin film transistor device according to claim 1, characterised in that a part of the dielectric layer (202, 505) is directly exposed to ambient humidity.
3. A thin film transistor device according to any of claims 1 or 2, characterised in that the gate electrode (203, 506) is made of a hygroscopic polymer and exposed to ambient humidity.
4. A thin film transistor device according to any of claims 2 or 3, characterised in that it comprises:
- an electrically non-conductive substrate (501),
- electrically conductive patterns on a surface of said substrate (501), which electrically conductive patterns comprise the source (502) and drain (503) electrodes, - an organic semiconductor layer (201, 504) that covers a part of the source electrode (502), a part of the drain electrode (503) and a part of the surface of said substrate (501) between the source and drain electrodes,
- a gate dielectric layer (202, 505) made of a hygroscopic polymer, which gate dielectric layer covers said organic semiconductor layer (201, 504) at a location that coincides with a part of the source electrode (502), a part of the drain electrode (503) and a part of the surface of said substrate (501) between the source and drain electrodes, and
- a gate electrode layer (203, 506) that covers a part of said gate dielectric layer (202, 505) at a location that coincides with a part of the source electrode (502), a part of the drain electrode (503) and a part of the surface of said substrate (501) between the source and drain electrodes.
5. A thin film transistor device according to claim 4, characterised in that the thickness of said organic semiconductor layer (201, 504) is between 5 and 500 nanometers, the thickness of said gate dielectric layer (202, 505) is between 300 and 2000 nanometers, and the thickness of said gate electrode layer (203, 506) is between 100 and 5000 nanometers.
6. A thin film transistor device according to any of claims 4 or 5, characterised in that the material of said organic semiconductor layer (201, 504) is from the group poly(alkylthiophene)s and poly(fluorene-co-thiophene)s; the material of said gate dielectric layer (202, 505) is from the group polyvinylphenol and polyvinylalcohol; and the material of said gate electrode layer (203, 506) is a compound of polyethylenedioxythiophene and poly(styrenesulfonate).
7. A thin film transistor device according to claim 6, characterised in that the material of said organic semiconductor layer (201, 504) is regioregular poly(3- hexylthiophene) and the material of said gate dielectric layer (202, 505) is polyvinylphenol.
8. A thin film transistor device according to claim 6, characterised in that the material of at least one of said organic semiconductor layer (201, 504) and said dielectric layer (202, 505) is unpurified.
9. A thin film transistor device according to claim 6, characterised in that the material of at least one of said organic semiconductor layer (201, 504) and said dielectric layer (202, 505) is a purified polymer with an intentionally added ionic or dopant substance.
10. A thin film transistor device according to any of the preceding claims, characterised in that a surface of the organic semiconductor layer (201, 504), which surface is against the dielectric layer (202, 505), comprises a zone (204, 401, 507) of molecular ordering and mechanical topographic deformation at a location that coincides with the gate electrode (203, 506).
11. A thin film transistor device according to claim I5 characterised in that it comprises absorbed humidity in the dielectric layer (202, 505) and a sealing arrangement for sealing the dielectric layer (202, 505) from an environment of the thin film transistor device.
12. An electronic measurement circuit, characterised in that it comprises:
- a thin film transistor device (1101) according to claim 1, which thin film transistor device is exposed to environmental conditions,
- a biasing circuit (1102) for providing a bias voltage to a thin film transistor (1111) within said thin film transistor device (1101),
- a supply voltage circuit (1103) for providing a supply voltage to said thin film transistor device (1101), and
- a detector circuit (1104) coupled to said thin film transistor device (1101), which detector circuit (1104) is arranged to detect a change in a current modulation capability of said thin film transistor device (1101), which change results from changing environmental conditions.
13. An electronic measurement circuit according to claim 12, characterised in that:
- the thin film transistor device (1101) comprises a first organic thin film transistor (1111) and a second organic thin film transistor (1112), coupled in series so that a drain electrode of the first organic thin film transistor (1111) is coupled to a source electrode and a gate electrode of the second organic thin film transistor (1112)
- said biasing circuit (1102) is coupled to provide a bias voltage to a gate electrode of said first organic thin film transistor (1111), - said supply voltage circuit (1103) is coupled to provide a supply voltage over the series coupling of said first (1111) and second (1112) organic thin film transistors, and
- said detector circuit (1104) is coupled to detect changes in the potential of a point between said first (1111) and second (1112) organic thin film transistors in said series coupling.
14. An electronic measurement circuit according to any of claims 12 or 13, characterised in that said biasing circuit (1102) is adapted to feed an oscillating bias voltage to the thin film transistor device (1101), and said detector circuit (1104) is adapted to detect a change in a peak to peak voltage value at a measurement point within the thin film transistor device (1101).
15. A method for manufacturing a thin film transistor device, comprising the steps of:
- forming (301) an organic semiconductor layer to connect a pair of source and drain electrodes, - forming (302) a dielectric layer to cover a part of the organic semiconductor layer, and
- forming (303) a gate electrode to cover a part of the dielectric layer and to coincide in location with a channel region between said source electrode and said drain electrode; characterised in that:
- the step of forming (302) a dielectric layer involves using a hygroscopic polymer as a material for the dielectric layer and
- the method comprises allowing the dielectric layer to absorb humidity in the form of a polar solvent in order to enhance the operation of the thin film transistor device through ionic effects.
16. A method according to claim 15, characterised in that allowing the dielectric layer to absorb humidity involves leaving a part of the dielectric layer exposed to environment in a completed thin film transistor device.
17. A method according to any of claims 15 or 16, characterised in that the step of forming (303) a gate electrode comprises the substeps of:
- depositing (304) an amount of polymer solution onto a part of the dielectric layer and
- allowing (305) said amount of polymer solution to dry and shrink, thus causing a force in a planar direction of the dielectric layer in order to mechanically modify a surface of the organic semiconductor layer.
18. A method according to any of claims 15 to 17, characterised in that said steps (301, 302, 303, 304, 305) are performed in room atmosphere.
19. A method according to any of claims 15 to 18, characterised in that at least one of said steps (301, 302, 303, 304) involves applying a printing technique with a polymer solution in order to form a layer.
20. A method according to claim 15, characterised in that allowing the dielectric layer to absorb humidity involves exposing a part of the dielectric layer to humidity during the manufacturing of a thin film transistor device, and later sealing the dielectric layer from the environment.
21. A method according to claim 15, characterised in that at least one of the steps of forming (301) an organic semiconductor layer and forming (302) a dielectric layer involves using an unpurified polymer as a layer material.
22. A method according to claim 15, characterised in that at least one of the steps of forming (301) an organic semiconductor layer and forming (302) a dielectric layer involves using a purified polymer with intentionally added ionic or dopant substance as a layer material.
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