EP2545599A1

EP2545599A1 - Organic light emitting field effect transistor

Info

Publication number: EP2545599A1
Application number: EP11707864A
Authority: EP
Inventors: Vincenzo Maiorano; Giuseppe Gigli
Original assignee: Consiglio Nazionale delle Richerche CNR
Current assignee: Consiglio Nazionale delle Richerche CNR
Priority date: 2010-03-12
Filing date: 2011-03-11
Publication date: 2013-01-16
Also published as: WO2011110664A1; ITRM20100107A1

Abstract

The present invention relates to an organic light emitting field effect transistor, OLEFET, (10) comprising a gate (G), a source (S) and a drain (D) electrode; a layered organic stack structure (1) including an organic layer (4) with predominant hole transporting character, an organic layer (2) with predominant electron transporting character and an organic active layer (3) having light emitting properties,the organic active layer (3) being interposed between the predominant hole and electron doped transport layers (4,2). The organic layer (4) with predominant hole transporting character and/or the organic layer (2) with predominant electron transporting character is doped in order to enhance the transport of holes/electrons. In addition, the layered organic stack structure (1) is positioned between said gate (G) and said source-drain electrodes.

Description

Organic light emitting field effect transistor

Technical field

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. The OLEFET of the invention exhibits light emission from the whole transistor channel area with high output current at low applied gate/drain voltage. Technological background

Opto-electronic devices based on organic semiconductors have gained considerable industrial and academic interest in the last decades. 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).

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 fact that the channel is formed close to the dielectric interface creates a plurality of problems, such as Frohlich polaron quenching. In particular for the standard OLEFETs, 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. Usually, 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).

Recently, organic light-emitting field-effect transistors (OLEFETs) with ambipolar transport properties, i.e. both holes and electrons are injected and transported, have been demonstrated. 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.

J. Zaumseil et alii, Adv. Mat. 2006, vol . 18, page 2708 show that in ambipolar OLEFET devices charge recombination near the drain electrode occurs, due to the different motilities of the injected charges. This could be modulated by source-drain voltage in order to move the recombination zone within the channel in which the charges move (J. Zaumseil, R. H. Friend, H. Sirringhaus, Nat. Mat., vol, 5, page 69 (2006)). In addition, an OLEFET having a molecular layered structure has also been presented. This two-component layered structure has balanced ambipolar transport and mobility values as large as 3xl0^"2 cmVs^'1. These devices are realized by sequentially depositing p-type and n-type films, as described in "High Mobility Ambipolar Transport in organic Light-Emitting Transistors", written by F. Dinelli et al, Adv. Mater. 2006, vol. 18, pages 1416-1420.

In ambipolar charge conduction regime, 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. However, 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)). Moreover, due to the limited recombination zone within the channel length, 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.

Summary of the invention

An object of the present invention is an organic light emitting field effect transistor (OLEFET) as defined in Claim 1.

Advantageously, 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.

In more detail, 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.

The term "layer" has to be understood as follows: 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.

Given the above, 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. Preferably, 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. It has to be understood that 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

According to an additional embodiment of the invention, 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.

According to a preferred embodiment of the invention, both layers are doped . According to a different embodiment, 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. In order to obtain a p-doped or a n-doped organic transport layer used in the OLED of the invention, the material disclosed in K. Walzer, B. Maennig, M .Pfeiffer, K. Leo, Chem. Rev., 107, 1233 (2007). 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. 2005 13/5, page 393; or in B.W. D'Andrade, Stephen R. Forrest, Anna B. Chwang, Appl. Phys. Lett. 2003, vol . 83, page 3858 and G.He, O. Schneider, D. Qin, X. Zhou, M. Pfeiffer, and K. Leo, J. Appl . Phys. 2004, vol . 95, page 5773. All the materials present in these references can be used, i.e. host material and type of dopant, however the doping concentration can be varied according to the teaching of this invention.

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.

According to a preferred embodiment of the invention, 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. Preferably 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.

According to another preferred embodiment, the OLEFET structure includes, as bottom contacts, a source and a drain electrode spatially interdigitated on a glass substrate. As an example, on top of the intergiditated electrodes the organic layered stack, comprising the p-doped layer, the active medium and the n-doped layer, are defined by shadow mask. In this embodiment, 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. Depending on the layer which is on the top or on the bottom of the layered structure, 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). Other configurations can be used, reverting the n-doped with the p-doped layer (n-doped layer / active layer / p-doped layer) or the 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 . In particular, as it will be better detailed below, 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 . Applicants have found that 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. In more detail, 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.

As it will be better detailed below, it should be pointed out that, in spite of analogies with ambipolar field-effect transistors, the working mechanism of the OLEFET of the present invention is considerably different. Unlike in traditional field-effect devices, 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. Thus 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 .

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. Depending on the gate and d rain voltages, in particu lar if a positive drain-gate voltage is applied , 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. Analogously, the same happens for negative V_g and V_d, considering the holes injected from the source and the electrons from the d rain electrode .

As seen in the literature, a conducting layer of electrons is built up close to the dielectric layer interface however depend ing on the DOS in the p and n doped layers, the injected charges move lateral ly and vertically across the high conductive p and n doped layers and tend to accumulate near the active layer interface, guided by the electrostatic forces, following the gate - d rain potential configuration . The fact that the two charges are in proximity of course favours charge recombination that, in an active layer, in turn results in l ig ht emission .

Due to the fact that substantially the whole interface with the active layer is involved, 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. In standard OLEFET 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.

Applicants have shown that the doping of the two layers sandwiching the active layers has preferably a maximum value. According to performed simulation Applicants have found that 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. In heavily-doped devices, 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. Alternatively, 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. For example, this simulation can be made on a p-doped /n-doped bilayer structure.

Indeed changing the doping concentration (i.e. increasing the amount of dopant) in the n-layer and/or in the p-layer the Fermi level is shifted towards the LUMO and towards the HOMO respectively. It has been assumed that the DOS increase linearly with the doping and it is substantially equivalent to the density of charges. Possible methods to calculate the DOS of the p-layer and/or n-layer of the OLEFET of the present invention are described in O. Tal et al, Phys. Rev. Lett., Volume 95, page 256405 (2005), B. Maenning et al . Phys. Rev. B, volume 64, page 195208 (2001) and M. Pfeiffer et al., Applied Physics letters, Volume 73, Number 22, page 3202 (1998).

Preferably, 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 1X10¹⁶ cm^"3 and 2X10¹⁸ cm^"3. Preferably, this implies a molecular dopant concentration in the p and/or in the n doped layer comprised between 2 wt% and 10 wt%, more preferably between 4 and 8 wt%, or in the ratio of 1 : 1 for alkali metals, i.e Caesium. 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.

Brief description of the drawings

Further features and advantages of an organic light emitting field effect transistor according to the invention will become more clearly apparent from the following detailed description thereof, given with reference to the accompanying drawings, where:

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 . The derived qualitative sketch of the paths followed by charges shows that recombination is possible, provided that no depletion zone is present ("light" doping scenario). All plots are obtained imposing Vg = 15V and Vd = 25V;

FIG. 5 is a scheme of the interdigitated source-drain electrodes;

FIG. 6 is a picture of the device of FIG. 1 operating in the light- emitting voltage range (V_g = 15 V, V_d = 25 V);

FIG 7 is a graph of the transfer characteristics of the device of fig . 1. Preferred embodiments of the invention

With reference to Fig. 1, 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. In addition, 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. However 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:

Onto the substrate GS, preferably made of glass, an interdigitated source- drain electrode configuration has been photolithographically pre-patterned, by standard lift-off process. Preferably, 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 Si0₂ or the materials described in I. N. Hulea et al, Nature Materials, vol . 5, December 2006, page 982 can be used . In the present case, due to the fact that the recombination of excitons takes place relatively far from the insulating layer, quenching phenomena are minimized and the choice of a suitable insulating layer is not relevant. 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. It can be replaced by different guest/host systems, both with predominant hole transporting characters and electron transporting character, like for example aluminum tris (8- hydroxyquinoline) (Alq₃) 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%. 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 . Preferably, as 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 Ν,Ν,Ν',Ν'-tetrakis (4-methoxyphenyl)-benzidine (MeO-TPD) matrix. As n-dopant 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. It may be preferable, in order to improve the device stability and to better control the doping concentration, to utilize an organic molecular dopant for this purpose. In both cases, the doping concentration is determined in order to allow a density of dopant states (DOS) in the range Ixl0¹⁶-2xl0¹⁸ (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%.

In general, regardless of the material in which the p/n doped layer is realized, 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 1X10¹⁶ cm^"3 and 2X10¹⁸ cm^"3.

The working mechanism of the ambipoiar OLEFET of the invention is the following. 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. In a standard unipolar device this is a typical configuration in which the OLEFET should have a n-channel operation, with linear and saturation regions. However, in the OLEFET of the invention, an ambipoiar behaviour, characterised by a relative low driving voltage and high output current, is observed for V_g ranging from 9V to 16V, consisting in a superlinear current increase (hole current) superimposed on a significant saturation current (electron current). The current is visualized in the insert of fig. 2. This phenomenon is more evident for V_g = 11V: the hole current enhancement has a maximum around V_d = 25V, after that it quickly decreases due to presumable recombination with the electrons at the interface. It operates at low drain/gate voltages with high output currents overcoming the common problem of traditional organic lateral structure devices, characterised by relative high driving voltages and low output current.

For lower or higher V_g values, the hole or electron transport regime takes place separately and there is no further possibility of charge recombination. An unloaded voltage gain A_v = 1.25 was also estimated, according to the usual definition A_v = g_m/g_out, where g_m and g_out are the transconductance (dld/dVg) and the output conductance (dId/dV_d). These values have been calculated from the transfer characteristics which are plotted in fig . 7.

Fig.6 shows a picture of the OLEFET 10 at V_g = 15V and V_d = 25V, taken with a digital camera : it emits orange-red light, typically of rubrene emission, covering the whole channel length/width (112/10000 microns) area with good uniformity. Unlike standard lateral structures, where the recombination zone is spatially confined in a region of the channel of 100- 300 nm and moves by varying the gate bias, the OLEFET 10 of the invention emits on a large area.

The Applicants have performed several simulation to better understand the working mechanism of the OLEFET 10, using a simplified bilayer p-n doped structure. It has been assumed that the DOS linearly increases with the doping concentration and approaches the density of free carriers effectively involved in the transport. It also has been taken into account the fact that hole-electron mobilities are not significantly dependent on the doping, fixing their value in the order of magnitude of 10^"2 cm² V^'V¹. In additions 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.

Applicants have found that, for DOS values larger than 2xl0¹⁸ cm^"3 (which will be called "heavy" doping scenario) the recombination rate falls down, decreasing the probability of exciton formation. In this case light emission is quenched because the p and n layers are depleted at the interfaces with the active layer. The space charge zone, which builds up, prevents current flow and, then, charge recombination; consequently, light emission is generally inhibited with "heavy" doping; practical negative voltage values applied at the gate cannot revert this condition .

On the other hand, in the "light" doping scenario, corresponding to lower values of DOS, recombination takes place effectively. This happens because, due to the relative position of the Fermi levels (Fig .2, inset), 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 (Fig . 4 (a) and (b)), making recombination possible. The latter depends on V_d and V_g, whose effect can be opportunely visualized in terms of the potential map (Fig . 4d). For Vg = 15V and Vd = 25V, electrons tend to be accumulated below the gate dielectric and to the n/doped layer interface (Fig.4a) where also holes accumulate (Fig .4b). This proximity favors charge carrier recombination, and allows light emission if a proper active layer is inserted between the doped transport layers, as in the experimental case. Moreover, the simulations show that at sufficiently high gate voltage, the electron current density is maximized only below the gate dielectric, far away the n/p-doped layer interface where recombination processes can occur. This agrees with the experimental observation that no light emission takes place at gate biases above 20V. Analogous remarks can be done for negative Vg and Vd, considering the holes injected from the source and the electrons from the drain electrode.

In heavily-doped devices, 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 (Fig.2, inset). This determines negligible current densities in the region close the interface (Fig. 4c), thus the suppression of charge recombination and light emission.

The requirement of light doping for efficient light emission adds validity to our computational model in which the device structure is described as a p-n doped bilayer.

Hence, 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.

In order to obtain the doping concentration which is required for each material and OLEFET, the method used in the invention is the following . Using a suitable software, 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. These values are obtained fixing HOMO and LUMO of the matrix and of the dopant for each layer, the mobility and varying conductivity and DOS of both layers.

In a general way, DOS and conductivity are linked by the following formula : conductivity = mobility*n*q or conductivity = mobility*p*q for electrons and holes, respectively, where n and p are the number of electrons or holes; in case of low doping levels, the equation is conductivity = mobility*DOS*q. Obtained via simulation the curve recombination rate vs. DOS, the doping has been chosen so as to obtain the conditions of the simulation, i.e. the DOS for which the recombination rate is maximized . The DOS can be obtained using the Seebeck coefficient and/or conductivity given the mobility.

According to simulations, it has been found that 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.

According to simulations, it has been found that 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.

Claims

1) An organic light emitting field effect transistor, OLEFET, (10) comprising :

a) a gate (G), a source (S) and a drain (D) electrode;

b) a layered organic stack structure (1) including an organic layer (4) with predominant hole transporting character, an organic layer (2) with predominant electron transporting character and an organic active layer (3) having light emitting properties, said organic active layer (3) being interposed between the predominant hole and electron transport layers (4,2), wherein said organic layer (4) with predominant hole transporting character and/or said organic layer (2) with predominant electron transporting character is doped in order to enhance the transport of holes/electrons, and wherein the organic molecule dopant concentration in said organic layer (4) with predominant hole transporting character and/or in said organic layer (2) with predominant electron transporting character is comprised between 2 wt% and 10 wt% or in the ratio of 1 : 1 for alkali metals;

c) said layered organic stack structure (1) being positioned between said gate (G) and said source-drain electrodes.

2) The OLEFET (10) according to claim 1, further including an insulating layer (5) between said organic doped layer (4) with predominant hole transporting character or said organic doped layer (2) with predominant electron transporting character.

3) The OLEFET (10) according to any one of the preceding claims, in which both said organic layer (4) with predominant hole transporting character and said organic layer (2) with predominant electron transporting character are doped with dopant concentration comprised between 2 wt% and 10 wt%.

4) The OLEFET (10) according to any one of the preceding claims, in which the source and/or drain electrode includes a metal or a conductive oxide.

5) The OLEFET (10) according to claim 4, in which the gate-source-drain electrodes are made by the same metal or conductive oxide.

6) The OLEFET (10) according to any one of the preceding claims in which the active layer (3) includes an active layer with high photoluminescence efficiency.

7) The OLEFET (10) according to any one of the preceding claims in which the conductivity is comprised between 8xl0^"5 - 8xl0^"3 S/cm.

8) The OLEFET (10) according to any of the preceding claims in which the density of doped states, DOS, is comprised between 1X10¹⁶ cm^"3 and 2X10¹⁸ cm^"3.

9) The OLEFET (10) according to any of the preceding claims in which the organic molecule dopant concentration in said organic layer (4) with predominant hole transporting character and/or in said organic layer (2) with predominant electron transporting character is comprised between 4 wt% and 8 wt%.

10) The OLEFET according to any of the preceding claims in which the doping concentration of said organic doped layer (4) with predominant hole transporting character and/or organic doped layer (2) with predominant electron transporting character is smaller than the doping for which the Fermi level of the n-doping layer is substantially equal to the Fermi level of the p-doping layer.