Classifications

• • 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 potential barriers
  • H10K10/40—Organic transistors
  • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
  • H10K10/462—Insulated gate field-effect transistors [IGFETs]
  • H10K10/484—Insulated gate field-effect transistors [IGFETs] characterised by the channel regions
  • H10K10/486—Insulated gate field-effect transistors [IGFETs] characterised by the channel regions the channel region comprising two or more active layers, e.g. forming pn heterojunctions
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
  • H10K50/14—Carrier transporting layers
  • H10K50/15—Hole transporting layers
  • H10K50/155—Hole transporting layers comprising dopants
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
  • H10K50/14—Carrier transporting layers
  • H10K50/16—Electron transporting layers
  • H10K50/165—Electron transporting layers comprising dopants
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/30—Organic light-emitting transistors

Definitions

• the present invention relates to an organic light emitting field effect transistor (OLEFET) including a layered structure comprising two transport layers at least one of which is doped sandwiching an active layer.
• OLEFET organic light emitting field effect transistor
• the OLEFET of the invention exhibits light emission from the whole transistor channel area with high output current at low applied gate/drain voltage.
• Organic materials represent a new class of semiconductor materials for flexible, low- cost, light-weight electronic devices, such as organic thin-film transistors (OTFTs) and circuits, organic solar cells and organic light-emitting diodes (OLEDs).
• OFTs organic thin-film transistors
• OLEDs organic light-emitting diodes
• An organic field effect transistor comprises an organic semiconducting layer, in which charges move, by hopping through disordered localized states, between source and drain electrodes. Their movement is modified by the potential applied to a third electrode, or gate, separated from the active material by an insulating layer.
• Charge carriers can be accumulated or depleted within a conducting channel, few nanometer thick, located next to the dielectric layer. Only the first two molecular layers next to the dielectric interface contribute to the charge transport, as it has been demonstrated by F. Dinelli, et alii, Phys. Rev. Lett., 92, 116802 (2004).
• the gate dielectric could drastically affect the transistor behaviour: lower dielectric constant ensures higher carrier mobility, a dielectric material with high number of -OH groups could act as trap for electrons.
• different gate insulators are reported, in order to avoid charge trapping and Frohlich polaron quenching phenomena at the dielectric/organic semiconductor interface, as disclosed for example in I. N. Hulea, S. Fratini, H . Xie, C.L. Mulder, N. N. Iossad, G. Rastelli, S. Ciuchi, A.F. Morpurgo, Nat. Mat., 5, 982 (2006).
• OLEDs organic light-emitting field-effect transistors
• ambipolar transport properties i.e. both holes and electrons are injected and transported
• Ambipolar light emitting transistors with a lateral structure have been fabricated with blends and bilayers of evaporated p- and n-type small molecules, or employing ambipolar light emitting polymers, both in a bottom gate/top contacts and in a top gate/ bottom contact configuration.
• An overview of the state of the art in this field is given for example in F. Cicoira and C. Santato, "Organic Light Emitting Field effect transistors: Advances and Perspectives", Adv. Funct. Mater. 2007, 17, pages 3421-3434.
• both electrons and holes can flow through the transistor channel and, potentially, the exciton density profile and the recombination zone position could be accurately controlled by modulating the gate bias and the relative hole/electron injection.
• OLEFETs Due to the low field effect carrier mobility of organic materials, OLEFETs have been so far characterised by low current output even at high working voltages, thus by low brightness, power and luminous efficiency. The maximum external quantum efficiencies so far reported are less than 1%, both for polymeric and molecular devices (see for example J. Zaumseil et al., Adv. Mat. 2006, vol. 18, page 2708 and T. Oyamada et alii, JAP, vol . 98, page 74506 (2005)).
• the emissive area is strongly reduced, as described in M .Muccini, Nat. Mat. 2006, 5, 605, thus preventing the use of OLEFETs in most practical applications.
• An object of the present invention is an organic light emitting field effect transistor (OLEFET) as defined in Claim 1.
• OEFET organic light emitting field effect transistor
• the OLEFET of the present invention is capable of emitting light along the whole large area transistor channel at relative low applied gate/drain voltage.
• the present invention relates to an ambipolar organic light emitting field effect transistor (in short "OLEFET”) which comprises a layered stack including an organic active layer inserted between two organic transport layers with different charge transport characters, at least one of which is a doped layer.
• OEFET ambipolar organic light emitting field effect transistor
• a layer is defined as a region of a material having a given predominant charge transport character, i.e. a layer can be a predominantly hole or electron transport layer (generally referred as n-type or p-type layer), or none of the above. Therefore, an OLEFET including two different organic layers having different charge transport characters indicates that in the OLEFET (at least) two regions having different transport characters, regardless whether they are realized in the same material or not, are present.
• the fact that the OLEFET of the invention as said comprises two organic transport layers, at least one of which doped, and an organic active layer therebetween means that the OLEFET might either comprise a single material physical layer which might include three different zones stacked one on top of the other, each zone having a different charge transport character, or three different layers of three different materials realizing a heterostructure or any combination therebetween.
• the structure so formed is a so-called PIN heterostructure.
• the active layer is a layer having light emissive properties, for example in a case of an active layer including a guest/host system i.e. upon receiving exciton energy by Forster or Dexter energy transfer or more generally by forming an exciton either electrically or optically, undergoes radiative decay to produce light. In this layer therefore the light is emitted .
• This active layer can also have either a hole or an electron predominant transport character, and it can be doped or not.
• the active layer comprises a material which has a high photoluminescent efficiency so that most of the excitons (singlets or triplets) recombine determining fluorescence or phosphorescence, respectively.
• the active layer may also have fluorescent or phosphorescent properties.
• At least one of the two organic layers sandwiching the active layer is in the OLEFET of the invention doped : i.e. the predominantly hole transport layer is doped in such a way to enhance the transport of holes and/or in a similar way the predominantly electron transport layer is doped in such a way to enhance the transport of electrons. Therefore, the doped one of the two transport layers in the OLEFET of the invention is not simply intrinsically "p- type” or "n-type” (in the following an intrinsic p-type or n-type layer will be called in contrast "non-doped"). These two layers have the main goal of tuning the hole and electron current profiles and optimizing the electro- optical characteristics of the OLEFET structure.
• the active layer can be a single layer, it may include an active medium with bipolar or unipolar behaviour, i.e. it includes a guest/host system having a predominantly hole transporting character or a predominant electron transporting character, or it may comprise two organic layers, one with a predominant hole transporting character and the other one with a predominant electron transporting character in order to confine the emission zone in the middle of them, preventing exciton losses. Additionally, it might be a single photoluminescent material (i.e. no guest/host) with bipolar or unipolar behavior.
• the matrix of the guest/host system in which the active layer is realized might be the same matrix in which one of the two transport layers is realized, doped in a different way and with different dopants.
• the doping of the active layer has nothing to do with the doping of the p-type and n-type layers: the latter is a doping in order to enhance the charge transport behavior of the layer, while the doping of the active layer is a doping in order to enhance its photoluminescence efficiency by Forster or Dexter transfer process both in the case of fluorescence or phosphorescence radiative decay
• additional layer(s) interposed between the doped layer(s) and the active layer can also be present.
• the doping of at least one of the transport layers of the layered stacked structure is performed in the OLEFET of the invention in order to improve the charge injection from the electrodes due to tunneling effect and to enhance the conductivity of the transport layers of several order of magnitudes, as it will be better detailed below.
• both layers are doped .
• only the n-layer is doped while the p-layer is intrinsically of the p-type.
• the doped layers are preferably realized in the following material.
• Possible materials for the p-layer and/or the n-layer structure, which can be used in the OLEFET of the invention, can be those described in Wellmann, M . Hofmann, O. Zeika, A. Werner, J. Birnstock, R. Meerheim, G. He, K. Walzer, M . Pfeiffer and K. Leo, J. Soc. Inf. Disp.
• the OLEFET of the invention includes a gate, a drain and a source electrode. Drain and source are preferably substantially coplanar.
• the device geometry can be of any of the known types: a bottom gate with top contact configuration can be used, as well as a bottom contact and top gate configuration. Between the gate and one of the doped layers a dielectric (insulating) layer is also interposed.
• the layered stack structure "doped layer - active layer - doped layer" is disposed between the gate and the source-drain electrodes.
• the OLEFET device of the invention is realized on a substrate.
• the substrate may be any suitable substrate, preferably characterized by well defined surface properties, in particular with regards to its roughness.
• the maximum roughness is of about 10 nm. Even more preferably the roughness of the substrate is lower than 7 nm. If the supporting substrate material has a roughness of the order of or greater than the maximum roughness permitted it hinders the formation of a continuous organic layer on top, preventing a good current conduction.
• the substrate may be substantially smooth, transparent or opaque, flexible or rigid . Glass and plastic are preferred substrate, even if a silicon wafer can also be utilized if a bottom gate/top contact configuration is fabricated.
• the OLEFET structure includes, as bottom contacts, a source and a drain electrode spatially interdigitated on a glass substrate.
• the organic layered stack comprising the p-doped layer, the active medium and the n-doped layer, are defined by shadow mask.
• the p-layer is in contact with the substrate, however also the reversed embodiment is possible, in which the n-layer is in contact with the substrate.
• the doping of the same can be better controlled and quenching can be minimized.
• the insulating layer, that cover the full lateral structure, is deposited before the gate electrode (top contact).
• n-doped with the p-doped layer n-doped layer / active layer / p-doped layer
• gate and source-drain electrodes position in a bottom gate / top contact configuration
• the OLEFET of the present invention is an ambipolar device: both n and p types of charge carriers are transported across the transistor channels. Indeed more than one channel is formed in which the holes and electrons can be transported .
• the electrons form a channel close to the insulating layer and partly migrate within the active layer, while the holes move at the interface between the active layer and the p-layer.
• Hole and electron charge carriers are injected from the electrodes and the injection is improved by the doping of at least one of the p or/and n transport organic layers of the stack structure. Under a proper density of dopant states (DOS) value, the injected hole and electron charge carriers flow near and across the active layer, which is the basic condition for light emission .
• DOS dopant states
• the doping profiles in at least one of the layers sandwiching the active layer, modulating the DOS values are important for the capability of emitting light along the whole large area transistor channel at relative low applied gate/drain voltage, as well as to increase the conductivity of both transport layer of around four orders of magnitude with respect to the known OLEFET devices.
• the doping level of the p and/or n layer(s) moving the Fermi level closer to the transport level with respect to the undoped layers, increases the conductivity of the layers themselves.
• both doped layers have intrinsically high and isotropic conductivity, thus current is not vertically confined, and current flow can occur inside the layers without requiring a conductive channel to build up by population inversion, confined to the gate dielectric as in usual enhancement MOSFETs.
• current is mainly controlled by injection efficiency at the contacts, which depends on the drain and gate potential .
• the gate is also involved in controlling charge carrier balancing at the active layer interface, putting the ground for high external quantum efficiency devices.
• the doping concentration inside the p or n doped layers mod ulating the density of dopant states ( DOS), controls the charge carrier flow near and across the active layer, putting the ground for the light emission .
• DOS density of dopant states
• the function ing of the OLEFET device is the following : Appl icants have found that, tuning the density of states ( DOS), it is possible to move both the injected holes and electrons charges inside the active layers.
• DOS density of states
• both electrons and holes are respectively injected from the sou rce and d rain electrodes (in case of positive gate voltage and d rain voltage V g , Vd and g rounded source) by tunnel ling effect.
• V g and V d negative V g and V d , considering the holes injected from the source and the electrons from the d rain electrode .
• the l ig ht emission takes place in a comparatively broad area, i .e. the excitons spread within the active layer and consequently recombinations take place d ue to the hig h photoluminescent efficiency of the active layer itself.
• the charge recombination usually takes place in a rather small area near the drain electrode, due to different mobilities of the injected charges. Although this could be modulated by changing the source- drain charge in order to move the recombination zone within the channel, the area would remain rather small.
• the doping of the two layers sandwiching the active layers has preferably a maximum value.
• light emission occurs in case of "light” (the term “light” will be explained below) doping scenario, which means a light doping of at least one of the n-doped and p-doped layers i.e. for low values of DOS.
• the simulation has been performed using a commercial two/three dimensional semiconductor device simulator (Atlas version 2.10.4. R), provided by Silvaco International, assuming that the density of states linearly increases with the doping concentration and it approaches the density of free carriers that effectively participate to the transport processes.
• the presence of a maximum doping level is due to the fact that, due to the relative position of the Fermi levels, hole and electron accumulation layers are present on the two sides of the p-doped/n-doped interface. Consequently, charge transport mainly occurs close to the interface, making recombination possible.
• a depletion of the p and n doped layers around the interface occurs, due to the higher Fermi level in the n-doped layer with respect the p-doped one. This determines negligible current densities in the region close the interface, thus the suppression of charge recombination and light emission. DOS values can be determined experimentally by measuring Seebeck coefficient.
• DOS values can be found by means of simulations fixing HOMO and LUMO values, which are known as characteristic of the materials, mobility and changing conductivity, hence DOS, and calculating the maximum value.
• this simulation can be made on a p-doped /n-doped bilayer structure.
• the DOS in the p-layer or in the n-layer of the layered stack structure of the OLEFET of the invention is comprised between 1X10 16 cm “3 and 2X10 18 cm “3 .
• the doping of the p and/or n layer renders the choice of a suitable material for the realization of the layer itself less troublesome enhancing the transport character of the same.
• FIG. 1 is a schematic lateral view in section of a preferred embodiment of an organic light emitting field effect transistor according to the invention
• FIG. 2 is a graph showing the electrical output characteristics in the light emission Vg range, evidencing ambipolarity and charge carrier recombination. Inset: output characteristics for 9V ⁇ Vg ⁇ 16V;
• FIG. 3 is a graph showing the Recombination rate vs. DOS, as obtained by the simulations. Inset: relative Fermi level alignment in the case of "heavy” or "light” doping scenario;
• FIGS. 4(a)-(d) are current density maps for electrons inside the n- doped layer (a) and holes inside the p-doped layer (b) along the whole channel length, in the light-doping scenario, (c) Total current density in the heavy-doping scenario, evidencing a depletion zone along the p-doped, n-doped interface and preventing recombination , (d) Map of potential .
• FIG. 5 is a scheme of the interdigitated source-drain electrodes
• FIG 7 is a graph of the transfer characteristics of the device of fig . 1. Preferred embodiments of the invention
• 10 indicates an organic light emitting field effect transistor (OLEFET) according to the present invention.
• the OLEFET of the invention includes a source S and drain D electrodes as well as a gate electrode G.
• it includes a stack layered structure 1 comprising a n-doped layer 2, an active light emitting layer 3 and a p- doped layer 4.
• the whole structure is realized on a substrate GS.
• different configurations can be used and only one of the layers 2, 4 can be doped .
• the materials and realization process of the OLEFET 10 are as follows:
• an interdigitated source- drain electrode configuration has been photolithographically pre-patterned, by standard lift-off process.
• the source S and drain D electrodes are realized in gold .
• the source and drain electrodes are preferably formed by chrome/gold metallization, taking the form of rectangular plates interdigitated as proposed in the fig . 5.
• the transistor micrometric channel length so defined is equal to 112 microns, while the channel width has been respectively fixed to 10000 microns. According to different embodiment of the invention, it is possible to consider different electrode shapes or different length/width ratios.
• the gate electrode G is preferably made of gold and it has been deposited, utilizing a shadow mask, on top of the layered stack structure 1 made of organic material and an insulating layer 5 realized between the gate electrode G and the stack layered structure 1.
• the total organic layered stack is of about 100 nm and it comprises the p-doped layer 4 of 30 nm, directly deposited on top of the interdigitated source-drain electrodes, followed by the active layer 3 of 20 nm and the n-doped layer 2 of 30 nm.
• the insulating layer 5 is preferably realized in lithium fluoride (LiF), alternatively Si0 2 or the materials described in I. N. Hulea et al, Nature Materials, vol . 5, December 2006, page 982 can be used .
• the layer 5 is preferably 300 nm-thick, and it is realized on top of the layered stack 1 before the gate electrode G.
• the gate G electrode is preferably realized in semitransparent gold, deposited by thermal evaporation and it is 18 nm-thick.
• the full layered device structure has been fabricated by high vacuum thermal evaporation in a Kurt J. Lesker multiple chamber system with at a base pressure around 10 "8 mbar, without breaking the vacuum.
• the organic active layer 3 preferably comprises a guest/host system of 4,4'- bis[N-(l-naphthyl)-N-phenyl-amino] biphenyl (NPB) doped with a 2 wt% concentration of 5,6,11,12-tetraphenyl-naphthacene (rubrene), has been deposited between the p-doped 4 and n-doped layer 2. It has a predominant hole transporting character, having the host matrix and the guest dopant the same behaviour.
• NPB 4,4'- bis[N-(l-naphthyl)-N-phenyl-amino] biphenyl
• rubberrene 5,6,11,12-tetraphenyl-naphthacene
• guest/host systems both with predominant hole transporting characters and electron transporting character, like for example aluminum tris (8- hydroxyquinoline) (Alq 3 ) as host matrix and [2-methyl-6-[2,2,3,6,7- tetrahydro-lH,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2) as guest dopant with concentration of 5 wt%.
• DCM2 propane-dinitrile
• the above mentioned guest/host systems have Alq3 : DCM2 electron transporting character, while NPB: rubrene predominant hole transporting character.
• the p/n doped layers 4,2 can be respectively obtained by the chemical combination of an organic matrix and an electron attracting/donor dopant; the optimized doping level can be reached of by thermal co-evaporation of matrix and dopant. In fact, their evaporation rates can be controlled independently by measuring them with separate quartz thickness monitors. Modulating the doping ratio, it is possible to control the density of states (DOS) the conductivity of the organic layers and the charge carrier mobilities if a good film morphology has been obtained .
• DOS density of states
• organic molecule p-dopant 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ), is used, in the concentration 5,5 wt.% inside the ⁇ , ⁇ , ⁇ ', ⁇ '-tetrakis (4-methoxyphenyl)-benzidine (MeO-TPD) matrix.
• MeO-TPD ⁇ , ⁇ , ⁇ ', ⁇ '-tetrakis (4-methoxyphenyl)-benzidine
• alkali metals are commonly used, in particular caesium dispersed inside a wide energy gap organic matrix, like 4,7-diphenyl-l,10- phenanthroline (BPhen) in the ratio of 1 : 1.
• the doping concentration is determined in order to allow a density of dopant states (DOS) in the range Ixl0 16 -2xl0 18 (cm "3 ) and a conductivity in the range of 8xl0 "5 - 8xl0 "3 S/cm, that represent, in accordance with the simulation results discussed below, the optimum conditions to obtain efficient light emission. Therefore the doping concentration depends on the material(s) used for the p-layer and n-layer. In the simulation, the doping of the p layer is comprised between 2 and 8 wt%.
• the DOS in the p-layer or in the n-layer of the layered stack structure of the OLEFET of the invention is comprised between 1X10 16 cm “3 and 2X10 18 cm “3 .
• Fig.2 reports the drain current characteristics, as output characteristics, with positive drain voltage V d gate V g biases in the common source layout.
• V d gate V g positive drain voltage
• the OLEFET 10 of the invention emits on a large area.
• simulations considered a bilayer structure (i.e. the active layer is simply an interface) comprising a p-doped and a n-doped layer in contact therebetween.
• Figure 3 shows a plot of the simulated recombination rate, as function of the density of dopant states in a region 20 microns-wide central region of the transistor channel.
• the active layer can be thought as a lightly doped layer and assimilated as a part of a unique p-doped or n-doped layer with variable doping profile.
• the method used in the invention is the following .
• simulations are performed in order to determine the DOS range within which the recombination rate is maximized.
• a simplified p-doped/n-doped structure is considered (i.e. no active layer present).
• the value of the recombination rate are measured in the center of the channel (i.e. given a channel of length L, it is calculated at L/2 and more in particular in an interval of 20 nm around L/2.
• the DOS can be obtained using the Seebeck coefficient and/or conductivity given the mobility.
• a molecular dopant concentration is preferably comprised within 2 and 10 wt% and even more preferably between 4 and 8 wt%, or in case of alkaline metals, i.e Caesium, the ratio is preferably 1 : 1.
• a dopant concentration is preferably comprised within 2 and 10 wt% and even more preferably between 4 and 8 wt%.
• the working mechanism of the OLEFET 10 of the invention is considerably different with respect to the traditional field-effect devices, because it is not required to build up a conduction channel by population inversion below the insulating layer, as in usual enhancement MOSFETs. In this way, the dielectric - charge carrier interactions are reduced and in turn the Frohlich polaron quenching phenomena do not occur.
• the dielectric properties have not a fundamental role in this sense.
• Both doped layers 2 and 4, as shown in fig . 1 have intrinsically high and isotropic conductivity, the current is not vertically confined, and flow of charge carriers, in lateral or perpendicular direction with respect to the substrate plan, can be controlled by the doping concentration and by the drain - gate potential.
• the gate is also involved in controlling the charge balancing at the interface with the active layer, optimizing the charge recombination and potentially improving the external quantum efficiency of the OLEFET 10.

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