US20070278478A1 - Ambipolar, Light-Emitting Field-Effect Transistors - Google Patents

Ambipolar, Light-Emitting Field-Effect Transistors Download PDF

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US20070278478A1
US20070278478A1 US10/586,244 US58624405A US2007278478A1 US 20070278478 A1 US20070278478 A1 US 20070278478A1 US 58624405 A US58624405 A US 58624405A US 2007278478 A1 US2007278478 A1 US 2007278478A1
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light
electron
emitting transistor
transistor according
injecting electrode
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Jana Zaumseil
Henning Sirringhaus
Lay-Lay Chua
Peter Ho
Richard Friend
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Cambridge Enterprise Ltd
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Definitions

  • the present invention concerns a new transistor capable of ambipolar conduction and light-emission from a well defined zone within the channel of the transistor and a method for making the same.
  • Field-effect transistors are three-terminal devices that comprise a source contact, a drain contact, and a gate contact.
  • a semiconductive layer (the “channel”) bridges the source and drain contacts, and is itself spaced from the gate contact by an insulating layer called the gate dielectric.
  • the semiconductive layer is fabricated from a semiconductive organic material.
  • the semiconductive organic layer is fabricated from a semiconductive polymer, typically a n-conjugated organic polymer. This layer may be deposited in the device by a precursor route or directly by solution processing.
  • a voltage is applied across the source contact and the drain contact. Further, in a field effect transistor, a voltage is applied to the gate contact. This voltage creates a field that alters the current-voltage characteristics of the semiconductive layer lying directly next to the gate dielectric by causing accumulation or depletion of charge carriers there. This in turn modulates the channel resistance and the rate at which charges pass from the source to the drain contact (that is, the source-drain current) for a given source-drain voltage.
  • organic field effect transistors can operate in two modes; either as an n-channel device (where the charges accumulated in the channel are electrons) or a p-channel device (where the charges accumulated in the channel are holes).
  • the insulator materials investigated range from silicon dioxide, poly(vinyl phenol) (PVP), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), poly(perfluoroethylene-co-butenyl vinyl ether), cyanopullulane, polyisobutylene, poly(4-methyl-1-pentene), and a copolymer of polypropylene:poly[propylene-co-(1-butene)].
  • n-channel organic FETs are limited to special classes of very-high electron-affinity (EA) semiconductors, such as those containing perylenetetracarboxylate diimide/dianhydride, naphthalenetetracarboxylate diimide/dianhydride or phthalocyanine units; or those with very small bandgaps (less than or equal to 1.6 eV), which by virtue of their small bandgaps do have very high electron-affinity.
  • EA electron-affinity
  • n-channel semiconductive materials Some specific examples of small molecule n-channel semiconductive materials that have been used are:
  • n-channel semiconductive materials examples include Katz et al, Nature 404 (2000) pg. 478-481. This document in fact mentions the possibility of an organic polyimide dielectric. However, no example of a suitable polyimide is given and no information is provided regarding how a suitable polyimide may be selected. Further, typically polyimides contain 1-5% residual —COOH groups.
  • Some examples of high electron-affinity oligomer n-channel semiconductive materials that have been used with an inorganic dielectric are:
  • Very high-electron affinity materials can have their own limitations. For example, they may be unintentionally doped by adventitious impurities, for example, H+, ammonium and metal ions, especially under bias to a permanently conducting state. Therefore it would be advantageous to be able to develop n-channel and ambipolar transistors from a general class of materials with more moderate bandgaps and electron-affinities.
  • ambipolar transistors that may be made according to this teaching are restricted.
  • small bandgap semiconductors with ⁇ - ⁇ * gap ⁇ 1.6 eV tend to be relatively less stable since they are vulnerable to unintentional doping, and facile photochemical reactions.
  • electron-conducting blends with C 60 networks are not only unstable, due to facile chemical reactions of the C 60 anions and rapid oxygen trapping but suffer also from general instability of the percolation paths due to the recrystallisation of the small molecule electron-acceptor and transporter.
  • LFETs light-emitting FETs
  • LFETs are of interest for applications, such as optical signal transduction and communication, and the incorporation of display functions into an integrated transistor circuit without requiring the realization of special light-emitting device structures, such as vertical light-emitting diodes, which might not be compatible with the process flow required for the transistor circuit, or might require additional process step.
  • LFETs Light-emitting transistors
  • the associated high electric field near the drain contact causes electrons to be injected from the drain terminal.
  • the light was emitted from a zone very close to the drain electrode due to the very low mobility of electrons in this device, and it was not possible to bias the device in such a way that the recombination zone could be moved into the channel of the device.
  • FIG. 3 a shows the schematic of an ambipolar LFET. If the device is biased such that the gate voltage is set at a value in between the source and drain voltage, a hole accumulation layer is induced near the source electrode and an electron accumulation is induced near the drain electrode.
  • the device operates by flow of holes from the source, and flow of electrons from the drain with recombination in a well-defined recombination zone ( 8 ) somewhere along the channel.
  • the position of the recombination zone depends on the relative electron and hole mobilities. If the electron and hole mobilities are well balanced the recombination zone can be moved to the middle of the channel, and in fact to any position from the source to the drain electrodes by changing the bias voltages applied to the device.
  • the N-channel transistor invention discloses that n-channel field-effect conduction can be stably supported at a dielectric/semiconductor interface only if the dielectric does not present too high a concentration of chemical moieties that can trap the negative charge-carriers induced in the semiconductor channel by the field effect.
  • the dielectric interface is the more important of the two, the dielectric bulk preferably also should be considered, since bulk trap states can still be populated albeit very slowly. The induced charge carriers travel along the interface and therefore are most severely affected by the traps they encounter at the interface. In contrast, the charge carriers must tunnel into the bulk to be trapped there. Nevertheless, the bulk trap states are capable of retaining charge for a long time, which is detrimental to transistor behavior.
  • the N-channel transistor invention discloses that by appropriate choice of the organic gate insulating material, it is indeed possible to obtain n-channel FETs from a much broader range of organic semiconductors than has been known hitherto. As such, it provides for the first time the opportunity for n-channel conduction using an organic semiconductor and an organic dielectric.
  • a broad range of poly(p-phenylenevinylene) and poly(fluorene) derivatives and copolymers have been tested successfully by the present inventors.
  • the key is that the organic gate dielectric layer must not contain trapping groups (that lie in energy near to or below the electron-transport level of the organic semiconductor) above a critical concentration.
  • the scope for obtaining n-channel organic FETs and ambipolar organic FETs is considerably expanded. It is no longer limited to the organic semiconductor being a very-high electron-affinity semiconductor.
  • the N-channel transistor invention has identified that the reason why n-channel FETs have been so elusive to date is that the gate dielectrics that have been tested to date (most notably, silicon oxide, poly(methyl methacrylate), poly(vinyl phenol) and poly(imide)) do not satisfy the specifications of the N-channel transistor invention.
  • the present invention discloses that the use of a trapping-free dielectric according to the N-channel transistor invention enables realization of an ambipolar LFET in which the recombination zone can be swept across the entire channel length of the transistor by variation of the bias voltages applied to the source, drain and gate electrodes of the transistor.
  • an ambipolar, light-emitting field-effect transistor comprising a trapping-free gate dielectric that emits light from a well-defined recombination zone within the channel of the transistor when operated in a biasing regime in which negative electrons are injected from one electrode into the organic semiconductive layer, and positive holes are injected from another electrode into the organic semiconductive layer.
  • an ambipolar, light-emitting field-effect transistor in which the light-emission zone can be moved to any position across the channel of the transistor by varying the voltages applied to the source, drain and/or gate electrode of the transistor.
  • a method for moving the light-emission zone in an ambipolar, light-emitting field-effect transistor to any position along the channel of the transistor.
  • a fourth aspect of the present invention the use of an ambipolar light-emitting transistor for light-emission from a transistor is provided.
  • a method for fabricating an ambipolar, light-emitting field-effect transistor is provided.
  • a trapping free-dielectric characterised in that the bulk concentration of trapping groups in the gate dielectric layer is less than 10 18 cm ⁇ 3 , where a trapping group is a group having (i) an electron affinity EA x greater than or equal to EA semicond and/or (ii) a reactive electron affinity EA rxn greater than or equal to (EA semicond. ⁇ 2 eV)
  • Reactive traps are those that undergo a subsequent chemical reaction so that the electron becomes trapped in a new (and deep) state that cannot ordinarily re-emit the electron. Hence, the trapping is not reversible.
  • Particularly ubiquitous examples of reactive traps are those with active (acidic) hydrogens, such as —COOH, and —CR 2 OH that can irreversibly trap electrons to expel hydrogen.
  • Non-reactive traps are those that can re-emit the trapped electrons. Reactive traps consume the induced carriers leading to static charging of the dielectric interface and gigantic shifts in threshold voltages, while non-reactive traps lead to loss of charge-carrier mobility. Both of these are detrimental to transistor devices. It is to be noted that the distinction between the two kinds of traps is not always clear-cut, since non-reactive traps in the presence of some impurities (such as H 2 O) can turn into reactive traps.
  • the present invention requires the total concentration of both reactive traps and non-reactive traps to be below a critical concentration. It will be appreciated that, according to the present invention, whether or not a group is a trapping group must be determined by reference to the EA semicond. of the organic semiconductive material which forms the semiconductive layer. Knowing the semiconductive material and thus EA semicond. , a person skilled in the art will be able to use the analysis and the definitions for EA x and EA rxn given below, to order all chemical groups present in an organic gate dielectric layer under consideration into two series with respect firstly to EA x (i.e. according to their non-reactive trapping properties) and secondly to EA rxn (i.e. according to their reactive trapping properties).
  • a person skilled in the art then can use the electron affinity of the semiconductor (EA semicond ) to provide the cut-off values to identify which groups are not trapping groups and so may be permitted to exist in the gate dielectric layer, and which other groups are trapping groups and so may not be present above the critical concentration in the gate dielectric layer.
  • EA semicond the electron affinity of the semiconductor
  • the critical concentration of reactive and non-reactive traps at the semiconductor/gate dielectric interface (C interf ) needs to be less than 10 12 cm ⁇ 2 , preferably less than 10 11 cm ⁇ 2 and more preferably less than 10 10 cm ⁇ 2 .
  • the bulk concentration of trapping groups in the dielectric layer must be less than 10 18 cm ⁇ 3 .
  • the bulk concentration preferably is at least 1-2 orders of magnitude lower. A preferred bulk concentration therefore is below 10 17 cm ⁇ 3 .
  • FTIR FTIR-sensitive infrared spectroscopy
  • suitable methods that can be used to measure the bulk concentration (c bulk ) are given below. These examples take OH groups as the trapping group of interest.
  • Electron affinity is the energy released when the material accepts electrons from vacuum. Electron affinity is not directly related to the polarity of a material nor is there any correlation between electron affinity and dielectric constant.
  • EA semicond. can be determined from cyclic voltammetry experiments for the organic semiconductor or from its measured ionization energy.
  • EA electron affinity
  • the bandgap ( ⁇ E) is measured using optical absorption for example.
  • the exciton binding energy (BE) is widely held to be 0.4 eV for a number of conjugated polymers.
  • the EA of the organic semiconductor can be measured in a more direct way by inverse photoemission, or by standard cyclic voltammetry as the midpoint potential (E o ′) of the coupled reduction and oxidation peaks or the onset reduction potential (E onset ), in a reduction scan.
  • EA is a relatively local property of the chemical groups or moieties present in organic materials. In contrast, this is not the case for inorganic materials. Therefore, for the purposes of the present invention, whether an organic gate dielectric layer is compatible with the n-channel activity of a particular semiconductor is determined in part by comparing the individual EA's of the constituent moieties of the dielectric (EA x and EA rxn ) against EA semicond . Because gas-phase EAs of a wide range of groups are available in the literature, this provides a useful a priori means to determine the EA x of a wide range of chemical groups present in candidate dielectrics for screening purposes.
  • the trapping is reversible and so the criteria for a group to be non-trapping is for the solid-state EA for the chemical group in the dielectric material (EA x ) to be less than EA semicond .
  • EA x values for common groups are shown below. In accordance with the present invention, these values are calculated from the gas-phase EA data.
  • the solid state EA x value is calculated from the gas phase EA by adding the solid-state polarization energy which is taken here to be 1.8 eV as given by M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers (Oxford University Press, 1999.
  • Solid-state Moiety X EA x (eV) a (a) Aliphatic carbonyl (—CO—, —COO—, —CONR—) 1.8-2.0 and CN (b) Aromatic carbonyl (—CO—, —COO—, —CONR—) 2.0-2.4 (c) Aromatic fluorocarbons 2.3 (d) Quinoxalines 2.5 (e) Aliphatic fluorocarbons 2.8-2.9 (f) Quinones 3.4-3.6 a Obtained by adding the polarization energy (taken to be 1.8 eV for the purposes of the present invention) to the gas-phase EA of appropriate model compounds.
  • Such traps undergo a coupled reaction that makes detrapping impossible.
  • An example of a coupled reaction is the expulsion of a hydrogen atom from moieties with active hydrogens. Once this hydrogen atom is lost (through some other radical reaction or recombination to give hydrogen gas), the electron charge is irreversibly trapped on the moiety until some charge neutralization event occurs. In any case, the initially trapped electron is not re-emitted, and the capacitive charge density of the gate dielectric becomes filled by such immobile charges.
  • the criteria for a group to be non-trapping requires a consideration of the reaction free energy.
  • the gas phase EA values for reactive traps are not available. Thus, for the purposes of the present invention, it is necessary only to consider the reactive electron affinity for such groups.
  • this moiety is present as an impurity at the sub % level in poly(methyl methacrylate); and at the few % level in polyimides due to incomplete conversion of the precursor material.
  • the reaction being considered is: Semicond ⁇ (s)+Diel-COOH(s) ⁇ Semicond(s)+Diel-COO ⁇ (s)+H(s) which represents trapping of the induced electron initially present in the semiconductor (semicond ⁇ ) onto —COO ⁇ in the dielectric (Diel-COO ⁇ ) with lost of an H atom.
  • Reaction (ii) involves the expulsion of a hydrogen atom.
  • the energetics of reaction (ii) is denoted as the negative of the reactive electron-affinity (EA rxn ).
  • the present invention therefore, defines an EA rxn for reactive traps, which can be estimated using a Born-Haber thermodynamic cycle for a corresponding small molecule model, an example of which is as follows: (ii)(a) Diel-COOH(s) ⁇ Diel-COOH(g) ⁇ G subl (ii)(b) Diel-COOH(g) ⁇ Diel-COO ⁇ (g) + H + (g) ⁇ G deprot (ii)(c) H + (g) + e ⁇ (g) ⁇ H (g) ⁇ G ion, H (ii)(d) Diel-COO ⁇ (g) ⁇ Diel-COO ⁇ (s) ⁇ G polar ⁇ G subl (ii)(e) H
  • Sublimation energies ⁇ G subl,H are sufficiently small (probably less than 0.1 eV) to be omitted.
  • ⁇ G deprot can be obtained from data tables. The primary factor governing whether a chemical group containing active hydrogen may act as a reactive trap is thus its deprotonation energy.
  • EA rxn values obtained in this way for a range of common moieties that may act as reactive traps are tabulated below.
  • the present inventors have determined that (e) and (f) generally are incompatible with n-channel FET conduction for a range of organic semiconductors (where EA semicond. ⁇ 2-2.5 eV). Therefore, the present inventors propose that in order for a moiety to be non-trapping, its EA rxn should be less than that of EA semicond by at least 2 eV, i.e. EA rxn ⁇ (EA semicond ⁇ 2 eV ).
  • the insulating material preferably does not include more than 0.1% by weight —OH groups and other hydrogen-bonding groups. A very small concentration of —OH groups and other hydrogen-bonding groups may be tolerated in the insulating material.
  • the insulating polymer contains less than 0.01% by weight —OH groups and other hydrogen-bonding groups, more preferably less than 0.001% by weight —OH groups and other hydrogen-bonding groups. Most preferably the insulating polymer is substantially free of —OH groups and other hydrogen-bonding groups
  • Organic dielectrics that contain these groups as defects, chain ends, stabilizers or contaminants need to be rigorously purified so that they are present below the critical concentration as discussed above.
  • the light-emitting transistor may have a top-gate or bottom-gate configuration.
  • the gate dielectric layer may be considered to comprise an organic gate insulating material together with any impurities.
  • the gate dielectric layer preferably does not contain any trapping groups. This ensures that the concentration of trapping groups is below the critical concentration.
  • the organic gate insulating material per se does not comprise a repeat unit comprising a trapping group. Most preferably, the organic gate insulating material per se does not contain any trapping groups.
  • EA semicond. will be greater than or equal to 2 eV, although the present invention is not so limited. Also typically, EA semicond. will be in the range of from 2 eV to 4 eV, more typically in the range of from 2 eV to 3 eV.
  • trapping groups in the insulating material may be a part of the insulating material per se (i.e. excluding impurities) or may be present as an impurity. Trapping groups also may be present as part of a residue unit that is present in the organic insulating material due to incomplete formation of the organic insulating material during preparation. It will be appreciated that whilst a trapping group may be tolerated as an end group, defect, stabilizer or impurity, it usually will not be tolerated as a repeating unit in the insulating material. This is because, when present as a repeating unit, it may bring the concentration of trapping groups to above the critical concentration. Trapping groups may or may not be tolerated in a residue unit depending on the concentration of residue units in the organic insulating material.
  • the insulating material does not contain a repeat unit or residue unit comprising a group having (i) an electron affinity EA x greater than or equal to 3 eV and/or (ii) a reactive electron affinity EA rxn greater than or equal to 0.5 eV.
  • the insulating material does not contain a repeat unit or residue unit comprising a quinone, aromatic —OH, aliphatic —COOH, aromatic —SH, or aromatic —COOH group.
  • the insulating material does not comprise a repeat unit or residue unit having an electron affinity EA x greater than or equal to 2.5 eV, more preferably greater than or equal to 2 eV.
  • the insulating material preferably does not contain a repeat unit and/or residue unit comprising an aliphatic fluorocarbon group. More preferably, the insulating material does not contain a repeat unit and/or residue unit comprising an aromatic carbonyl, quinoxaline or aromatic fluorocarbon group.
  • the insulating material per se contains one or more groups independently selected from alkene, alkylene, cycloalkene, cycloalkylene, siloxane, ether oxygen, alkyl, cycloalkyl, phenyl, and phenylene groups. These groups may be substituted or unsubstituted. These groups optionally may be part of a repeat unit of the insulating material.
  • the insulating material per se may contain one or more groups independently selected from aliphatic carbonyl, cyano, aliphatic —NHR, , aromatic —NHR, and. Again, these groups optionally may be part of a repeat unit of the insulating material.
  • the insulating material may be made from a precursor to the insulating material. Such a precursor may be converted to the final insulating material by appropriate reaction.
  • a precursor insulating material may contain crosslinkable groups and the crosslinked insulating material may be formed from the precursor by heating, for example. Desirable groups to be present in a precursor insulating material include alkene and styrene groups.
  • the gate dielectric layer preferably comprises an organic insulating polymer.
  • organic insulating polymers which can be used after appropriate purification, are given below:
  • repeat units in the above polymers may be substituted or unsubstituted provided that the final insulating polymers comply with the above design rules.
  • Substituents include functional substituents to enhance particular properties of the polymer such as solubility.
  • Crosslinked derivatives of the above polymers also are within the scope of the present invention.
  • the insulating material is not a poly(imide).
  • the insulating polymer comprises an Si(R) 2 —O—Si(R) 2 unit where each R independently comprises a hydrocarbon.
  • a preferred poly(siloxane) as mentioned above is an insulating polymer where the backbone of the polymer comprises a repeat unit comprising —Si(R) 2 —O—Si(R) 2 — where each R independently is methyl or substituted or unsubstituted phenyl.
  • a polymer has general formula: and may be substituted or unsubstituted.
  • This polymer may be made by crosslinking monomers having general formula:
  • the insulating material preferably comprises an insulating oligomer or insulating small molecule.
  • the interface desirably is chemically stable, molecularly abrupt and molecularly smooth.
  • dielectric layer preferably should show high dielectric breakdown strength, and very low electrical conductivity.
  • the gate dielectric polymer must be compatible with the overall designated processing scheme of organic (particularly polymer) FETs. For example, its formation must not destroy earlier formed layer integrities, while it itself has to survive subsequent solvent and thermal processing (if any).
  • the insulating material has low bulk electrical conductivity and high dielectric breakdown strength.
  • the insulating material has a glass transition temperature of greater than 120° C., most preferably greater than 150° C.
  • the bulk resistance of the insulating material preferably is greater than 10 14 Ohm cm, most preferably greater than 10 15 Ohm cm).
  • the insulating material desirably should be processable into a high-quality defect-free ultrathin film.
  • the dielectric breakdown strength advantageously may be greater than 1 MV/cm, preferably greater than 3 MV/cm.
  • the gate organic gate insulating material in one embodiment preferably is crosslinked.
  • the dielectric layer may consist of a single layer of a single insulating material or may comprise more than one layer of insulating materials or a blend of insulating materials.
  • Semiconductive materials that are usable in the present invention include small molecules, oligomers and polymers.
  • suitable semiconductive polymers are: poly(fluorene) homopolymers and copolymers, poly(p-phenylenevinylene) homopolymers and copolymers, poly(oxadiazole) homopolymers and copolymers, poly(quinoxaline) homopolymers and copolymers, and homopolymers and copolymers that include one or more groups selected from perylenetetracarboxylic diimide, naphthalenetetracarboxlic dianhydride, quinoline, benzimidiazole, oxadiazole, quinoxalines, pyridines, benzothiadiazole, acridine, phenazine, and tetraazaanthracene.
  • repeat units in the above polymers may be substituted or unsubstituted provided that the final semiconductive polymers comply with the above design rules.
  • Substituents include functional substituents to enhance particular properties of the polymer such as solubility.
  • the semiconductive polymer may be made from a precursor polymer. Such a precursor may be converted to the final semiconductive polymer by appropriate reaction.
  • a precursor semiconductive polymer may contain crosslinkable groups and the crosslinked semiconductive polymer may be formed from the precursor by heating, for example.
  • suitable semiconductive small molecules are: pentacene, perylenetetracarboxylic dianhydride and diimide, naphthalenetetracarboxlic dianhydride and diimide.
  • charge-carrier mobility is as high as possible.
  • typical values obtainable with the present invention are in the range 10 ⁇ 5 -10 ⁇ 1 cm 2 /Vs.
  • processing conditions must be selected so that trapping groups are not present in the dielectric layer above the critical concentration. This will involve the selection of appropriate processing conditions particularly when forming the dielectric layer, for example so that the final dielectric layer does not contain residual units that may act as traps above the critical concentration.
  • the dielectric layer and the/or the semiconductive layer preferably are formed by solution processing.
  • a solution containing reactant material for making the crosslinked insulating material may be deposited by solution processing.
  • the reactant material is then cured to make the crosslinked insulating material.
  • One common mechanism for curing is a condensation reaction which crosslinks the reactant material. This condensation reaction typically involves the loss of —OH leaving groups from the reactant material. However, where curing proceeds via a condensation reaction with the loss of —OH leaving groups, this typically will not remove all —OH leaving groups that were present in the reactant material. Thus, the final crosslinked insulating material will include residual —OH leaving groups. As described above this is disadvantageous and as such, the reactant material for making the crosslinked insulating material preferably does not include any —OH leaving groups.
  • the crosslinking group in the insulating material is derivable from a crosslinkable group in the reactant material that can be cured without the loss of a leaving group.
  • examples of such reactions include Diels-Alder reactions between a diene and a dienophile (as exemplified by the reaction between benzocyclobutene and alkene), and a hydrosilylation reaction between Si—H and alkene.
  • the source and drain electrodes need to be constructed such that efficient electron injection is possible from one of said source and drain contacts, and efficient hole injection is possible from the other of said source and drain electrodes.
  • ambipolar bottom-gate TFTs with BCB as gate dielectric, poly-3-hexylthiophene as active semiconducting layer, and top-contact calcium source-drain electrodes.
  • BCB gate dielectric
  • poly-3-hexylthiophene active semiconducting layer
  • top-contact calcium source-drain electrodes Depending on the sign of the gate voltage with respect to the source/drain voltage these devices exhibit ambipolar transport characteristics.
  • ambipolar transport has been observed in top-gate poly-dioctylfluorene-co-bithiophene (F8T2) transistors with gold source-drain contacts.
  • an LFET is constructed with a source electrode that comprises a different conducting material than the drain electrode.
  • the two metals have different workfunctions.
  • the lower workfunction metal is selected to enable efficient electron injection into the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor, while the higher workfunction metal is selected to enable efficient hole injection into the highest occupied molecular orbital (HOMO).
  • LUMO unoccupied molecular orbital
  • HOMO highest occupied molecular orbital
  • the work function difference between the two contacts is at least 0.5 eV. More preferably the difference in workfunction between the source and drain metals is higher than 1.0 eV, most preferably the workfunction difference is higher than 1.5 eV.
  • Suitable hole injecting contacts include gold (workfunction 5.0 eV) silver, platinum, palladium, indium tin oxide or conducting polymers such as PEDOT/PSS.
  • Suitable electron injecting contacts are low-workfunction metals such as calcium (workfunction 2.9 eV), magnesium, barium, copper or aluminum.
  • the source and drain contacts can be fabricated from the same bulk material, but prepared to exhibit different surface composition as a result of a surface treatment process or a sequence of surface treatment steps which has/have a different effect on the source and the drain electrodes. Modification of a metal electrode with self-assembled monolayers can change the workfunction of the electrode significantly.
  • source and drain contacts can be fabricated by different methods.
  • source and drain contacts are formed in the same plane of the substrate.
  • One method for forming source and drain contacts from different materials is to employ two-step shadow mask evaporation.
  • the first material such as a thin film of calcium
  • the second material such as a thin film of gold
  • the first opening might be kept open if the device structure is such that the second material which is evaporated on top of the first material in the area of the first opening does not impede the injection of the respective charges from the first material into the channel of the transistor.
  • the second material (Au) can then provide some encapsulation to the first material (Ca).
  • techniques such as two-step photolithography or two subsequent direct-write printing steps, such as, but not limited to, inkjet printing, screen printing, offset printing, laser forward transfer printing or gravure printing can be used.
  • first step the first material is deposited and patterned, and then in a second step the second material is deposited and patterned adjacent to the first one in accurate registration with the edge of the first material.
  • the channel length of the transistor is short, i.e. on the order of a few microns, or even of submicrometer dimension. In this way the transistor ON current (proportional to 1/L) is high, and the intensity of emitted light is high.
  • a process is being used which is self-aligned, i.e., the position of the egde of the second electrode with respect to that of the first electrode is defined not by a registration or an alignment step.
  • One method according to the present invention to define a self-aligned channel length between a low-workfunction electrode and a high-workfunction electrode in the same plane of the substrate is based on surface-energy assisted inkjet printing (PCT/GB00/04942).
  • the surface of the substrate is prepatterned into lyophobic and lyophilic surface regions.
  • the channel length is defined by a narrow lyophobic rib separated by two adjacent lyophilic regions on both sides.
  • a first ink comprising a high workfunction metallic material, such as a gold nanoparticle ink or a PEDOT/PSS conducting polymer ink is inkjet printed into the first lyophilic substrate region.
  • the spreading of the ink is confined by the lyophobic rib, such that the ink does not come in contact with the second lyophilic region. Then a second ink comprising a lower workfunction metal is inkjet printed into the second lyophilic region with the lyophobic rib again confining the spreading of the ink. In this way a self-aligned channel is formed with the channel length defined by the width of the lyophobic rib.
  • Liquid-based patterning techniques require metallic inks with a low workfunction exhibiting sufficient stability against oxidation during printing/processing.
  • a suitable low-workfunction ink might be a carbon nanotube-based or carbon black dispersion ink. It has been shown that alkali-intercalated carbon nanotube bundles have workfunctions less 4.5 eV suitable for electron injection into many organic semiconductors. Alternative choices include metal nanoparticles or electroless plating inks of low workfunction metals such as copper.
  • Another preferred method for defining the self-aligned channel of an LFET is based on the self-aligned printing technique disclosed in UK0130485.6.
  • the self-aligned printing technique is based on printing a first material onto the substrate, modifying the surface of the first material to expose a repulsive surface to the ink of the second material.
  • a self-aligned gap of sub-100 nm dimension between the two subsequently printed materials which can be of different bulk or surface composition, can be defined. Rectifying diodes based on the self-aligned printing technique have been described in PCT/GB2004/003879.
  • Another method for self-aligned definition of the channel according to the present invention is based on two step evaporation as discussed above, where the second material is evaporated as a collimated beam at an oblique angle, such that a channel is defined by a shadowing effect with a length defined by the angle at which the second material is impinging onto the substrate.
  • Yet another technique for definition of a submicrometer gap between two dissimilar materials is based on underetching of a first metal layer protected by a resist pattern, followed by evaporation of a second material and lift-off of the resist pattern.
  • a related technique for definition of a submicrometer channel length between to identical metal electrode is disclosed in Scheinert et al., Appl. Phys. Lett. 84, 4427 (2004). This technique can be applied analogously to the definition of a gap between two dissimilar materials.
  • the low-workfunction metal is processed in such a way that oxidation or other chemical degradation of the reactive low-workfunction surface required for electron injection is avoided. This might require that the processing is performed under inert gas atmosphere, or that the metal is subjected to a final surface treatment or etching step to remove any surface oxide which might have been performed during the processing, before it is brought in contact with the organic semiconductive layer. For techniques where the semiconductor is formed on top of the source-drain structure oxidation will be more easily avoided if the low-workfunction metal is deposited second.
  • active material preferably a material with a high photo/electroluminescence efficiency as well as relatively high field-effect mobilities for both electrons and holes is selected.
  • the material should also be chosen such that efficient electron and hole injection can be achieved with available metal contact materials that have adequate stability under the processing conditions employed.
  • the electron affinity of the organic semiconducting material is preferably higher than 2.5 eV, more preferably higher than 3 eV, and the ionization potential of the organic semiconductive material is preferably less than 5.8 eV, more preferably less than 5.5 eV.
  • suitable materials are light-emitting polymers, including but not limited to polyfluorene-based systems such as poly-dioctylfluorene (F8), F8T2, or polyphenylenevinylene (PPV) based polymers, such as poly(2-methoxy-5-(3,7-dimethyl)octoxy-p-phenylenevinylene) (“OC1C10-PPV”).
  • polyfluorene-based systems such as poly-dioctylfluorene (F8), F8T2, or polyphenylenevinylene (PPV) based polymers, such as poly(2-methoxy-5-(3,7-dimethyl)octoxy-p-phenylenevinylene) (“OC1C10-PPV”).
  • OC1C10-PPV poly(2-methoxy-5-(3,7-dimethyl)octoxy-p-phenylenevinylene)
  • O1C10-PPV poly(2-methoxy-5-(
  • active semiconducting systems include mixtures of several components, for example an organic semiconductive material as a matrix selected for its charge transporting properties with a light-emitting, highly fluorescent or phosphorescent guest chromophore (Baldo, et al., Nature 403, 750 (2000), such as a triplet-emitting dye, and or a blend of high-mobility electron and hole transporting organic semiconductive layers (Appl. Phys. Lett. 85, 1613 (2004)).
  • an organic semiconductive material as a matrix selected for its charge transporting properties with a light-emitting, highly fluorescent or phosphorescent guest chromophore (Baldo, et al., Nature 403, 750 (2000), such as a triplet-emitting dye, and or a blend of high-mobility electron and hole transporting organic semiconductive layers (Appl. Phys. Lett. 85, 1613 (2004)).
  • the bias voltage applied to the gate electrode needs to be set at a value in between the voltage applied to the source and drain electrodes.
  • the recombination zone from which the light is emitted can be made to be located anywhere along the channel of the FET by varying the source, drain and gate voltages accordingly. For example, for a fixed negative gate voltage by increasing the negative drain voltage light-emission from near the drain electrode is first observed when the drain voltage becomes more negative then the gate voltage. At this point the transistor enters into an ambipolar transport regime as evidenced by a sudden increase of the transistor current above the hole saturation current for the respective gate voltage (see FIG. 3 c ). As the drain voltage becomes more and more negative the well-defined recombination zone can be moved to any position along the channel until it reaches the source electrode.
  • the present invention allows realization of light-emitting transistors in which the light emission zone is located well away from the source and drain contacts.
  • the recombination zone can be made to be more than 1 micron, and even more than 5 microns away from both the source and drain electrodes, and can in fact be moved all the way from the source to the drain contact as the source-drain voltage is increased.
  • FIGS. 1 a and 1 b show the transfer and output characteristics respectively for a transistor using F8BT as the semiconductive layer in accordance with Example 1;
  • FIG. 1 c shows the transfer characteristics for a transistor using CN-PPV as the semiconductive layer in accordance with Example 1;
  • FIG. 1 d shows the transfer characteristics for a transistor using OC1C10-PPV as the semiconductive layer in accordance with Example 1;
  • FIG. 1 e shows the transfer characteristics for a transistor using PPV as the semiconductive layer in accordance with Example 1;
  • FIGS. 2 a and 2 b show the transfer and output characteristics respectively for a transistor using F8BT as the semiconductive layer in accordance with Example 2;
  • FIGS. 2 c and 2 d show the transfer and output characteristics respectively of a transistor using CN-PPV as the semiconductive layer in accordance with Example 2.
  • FIG. 3 a shows a schematic diagram of a general LFET (top) and of a specific embodiment (bottom).
  • FIG. 3 c shows the output characteristics of an ambipolar OC1C10-PPV transistor which is operated in the hole accumulation regime at low V d .
  • V d When V d is increased further electrons are injected and cause a steep increase in the output current as indicated by the arrow. We observe light emission from that point onwards.
  • FIG. 3 d shows an optical micrograph of a light-emitting transistor in operation.
  • the light emission is observed as a thin line parallel to the electrodes in the middle of the channel (indicated by the arrow).
  • the exact position can be controlled with the applied bias voltages.
  • a p-doped silicon substrate with 200-nm SiO 2 layer is overcoated with a 50-nm-thick BCB layer by spinning a 4.4 w/v % BCB (Cyclotene®, The Dow Chemical Company) solution in mesitylene, and then rapid-thermal-annealed on a hotplate set at 290° C. 15 s under nitrogen (pO 2 ⁇ 5 ppm).
  • the required organic semiconductor is then spin cast over the substrate as a thin film 50-80-nm thick from a 1.3-1.8 w/v % solution in the appropriate solvent.
  • FIGS. 1 ( a ) and ( b ) show the transfer and output characteristics respectively for F8BT.
  • the linear-regime mobility ( ⁇ FET,e ) is estimated to be 5 ⁇ 10 ⁇ 3 cm 2 /Vs.
  • n-channel FET mobility is very poor, with ⁇ FET,e typically less than 10 ⁇ 6 cm 2 /Vs in the poly(fluorene) derivatives.
  • ⁇ FET typically less than 10 ⁇ 6 cm 2 /Vs in the poly(fluorene) derivatives.
  • poly(vinyl phenol) is used as the gate dielectric, no n-channel FET behaviour can be obtained.
  • hexamethyldisilazane-treated SiO 2 is used as the gate dielectric, no n-channel FET behaviour can be obtained either. All of these observations are consistent with the design rules of the present invention.
  • a p-doped silicon substrate with 200-nm SiO 2 layer is overcoated with a 50-nm-thick BCB layer by spinning a 4.4 w/v % BCB (Cyclotene®, The Dow Chemical Company) solution in mesitylene, and then rapid-thermal-annealed on a hotplate set at 290° C. 15 s under nitrogen (pO 2 ⁇ 5 ppm).
  • the required organic semiconductor is then spin cast over the substrate as a thin film 50-80-nm thick from a 1.3-1.8 w/v % solution in the appropriate solvent.
  • FIGS. 2 ( a ) and ( b ) show the transfer and output characteristics respectively for F8BT.
  • the linear-regime mobility ( ⁇ FET,e ) is estimated to be 4 ⁇ 10 ⁇ 4 cm 2 /Vs. This is one order of magnitude lower than with. Ca electrodes.
  • the output characteristics also saturate too early. Both of these are indicative of high contact resistance with the aluminium electrodes. But nevertheless n-channel activity has still been obtained.
  • the glass substrate is coated with 50-80-nm thick poly(9,9-dioctylfluorene-alt-benzo-2-thia-1,3-diazole) from a 1.7 w/v % solution in mixed xylenes.
  • the gate dielectric layer is then deposited by spinning a 200-nm-thick BCB layer from a 12.7 w/v % BCB (extracted from Cyclotene®, The Dow Chemical Company) solution in decane at 30-40° C., and then rapid-thermal-annealed on a hotplate set at 290° C.
  • PEDT:PSSR surfactant-ion-exchanged poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) complex
  • the electron mobility is ca. 10 ⁇ 4 cm 2 /Vs, and is limited by injection. Therefore further improvements can be made by appropriately functionalising the gold electrode to improve its effective work-function, using dipolar self-assembled monolayers.
  • the same structure as described in example 1 was used with calcium as the drain electrode 14 and gold as the source electrode 13 ( FIG. 3 a (bottom)).
  • the two metals were evaporated subsequently through a shadow mask with a channel width of 3 mm and a channel length of 100 ⁇ m.
  • Calcium was evaporated first through a first window in the shadow mask defining the drain electrode, while a second window in the shadow mask defining the source electrode was blocked off.
  • Gold was then evaporated with both the first and second window opened up. During the second evaporation the calcium contact is capped with gold, which provides some encapsulation of the reactive calcium contact, while not affecting its electron injecting properties.
  • OC1C10-PPV As semiconducting material we used “OC1C10-PPV”) spun from an anhydrous mixed isomer xylene solution (5 w %). In contact with BCB OC1C10-PPV combines a high photoluminescence efficiency with relatively high, and well balanced field effect mobilities for electron and holes (hole mobility of 5 ⁇ 10 ⁇ 4 cm 2 /Vs, electron mobility of 2 ⁇ 10 ⁇ 3 cm 2 /Vs).
  • FIG. 3 c shows the output characteristics of an ambipolar OC1C10-PPV transistor which is operated in the hole accumulation regime at low V d .
  • V d When V d is increased further electrons are injected and cause a steep increase in the output current as indicated by the arrow. We observe light emission from that point onwards.
  • FIG. 3 d shows two optical micrographs of a light-emitting transistor in operation with different source-drain voltages, and constant gate voltage.
  • the light emission is observed as a thin line parallel to the electrodes in the middle of the channel (indicated by the arrow).
  • the exact position can be controlled with the applied bias voltages.
  • the difference in source-drain voltage between the two images is 2V, and with the more negative value the recombination zone moves closer towards the source electrode.

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