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  • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
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Definitions

• This invention relates to an organic light emitting field effect transistor with distributed light emission.
• the term distributed light emission refers to a light emission by a length greater than or equal to 20 ⁇ .
• OLET Organic electroluminescent field effect transistors
• OLED Organic Light Emitting Diodes
• ambipolar OLET have enhanced efficiency and luminosity, and also the possibility of using low cost production processes, once they have been optimized.
• This spatially confined emission does not cause problems in a whole class of organic- electronic devices, and it may be an advantage, for example in the field of sensing devices.
• This use is described for example in the international patent application WO2010049871. Nevertheless, this limits the possible application fields when a large or distributed source of light is needed, such as for example in the field of ambient lighting, of electroluminescent display technology, of the so-called Point of Care biomedical applications, of light sources with high brightness integrated on photonic chips.
• An objective of the present invention is thus to overcome the problems of prior art, making reference to the limited extension of the illuminated region of a field effect transistor, and according to a first aspect it consists in an organic ambipolar light emitting field effect transistor having an architecture with layers stacked one over the other, comprising:
• an ambipolar channel superposed to said dielectric layer comprising a P-type semiconductor layer whose energy band is determined by its highest occupied molecular orbital HOMO-SCp and lowest unoccupied molecular orbital LUMO-SCp, a N-type semiconductor layer whose energy band is determined by its highest occupied molecular orbital HOMO-SCn and lowest unoccupied molecular orbital LUMO-SCn and a light emitting layer adapted to allow recombination of charge carriers of opposite sign, interposed between said P-type semiconductor layer and said N-type semiconductor layer, whose energy band is determined by its highest occupied molecular orbital HOMO-R and lowest unoccupied molecular orbital LUMO-R, respectively;
• a source electrode adapted to inject charges of a first type and a drain electrode adapted to inject charges of a second type, said source electrode and drain electrode being in contact with a same layer of said P-type or N-type semiconductor layers, the other of said semiconductor layers being in contact with the dielectric layer.
• the novel field effect transistor is capable of generating a distributed light emission because the layers of the ambipolar channel are realized such that the ratio between the value of the effective field-effect mobility (m2) at the interface between one of said semiconductor layers (SCn; SCp) and said light emitting layer (R) and the value of the effective field-effect mobility (ml) at the interface between the other of said semiconductor layers (SCp; SCn) and said dielectric layer (Die) is in the range from 0.05 to 20;
• the difference between the levels HOMO-R and HOMO-SCn is comprised between 0.2 eV and 1 eV
• the difference between the levels LUMO-R and LUMO-SCn is comprised between 0.2 eV and 0.8 eV
• the difference between the levels HOMO-R and HOMO-SCp is comprised between 0 eV and 0.5 eV
• the difference between the levels LUMO-R and LUMO-SCp is comprised between -1 eV and 0 eV;
• the difference between the levels HOMO-R and HOMO-SCn is comprised between 0 eV and 1 eV
• the difference between the levels LUMO-R and LUMO-SCn is comprised between -0.5 eV and 0 eV
• the difference between the levels HOMO-R and HOMO-SCp is comprised between -0.2 eV and -0.8 eV
• the difference between the levels LUMO-R and LUMO-SCp is comprised between -0.2 eV and -1 eV.
• the light emitting layer is a Host-Guest (H-G) system, composed of a Host material and one or more Guest materials, each characterized by an energy band determined by its highest occupied molecular orbital HOMO-H (HOMO-G) and lowest unoccupied molecular orbital LUMO-H (LUMO-G).
• H-G Host-Guest
• HOMO-G highest occupied molecular orbital HOMO-H
• LUMO-G lowest unoccupied molecular orbital LUMO-H
• the novel field effect transistor in which the light emitting layer is a Host-Guest system is capable of generating a distributed light emission because the layers of the ambipolar channel are realized such that:
• the ratio between the value of the effective field-effect mobility (m2) at the interface between one of said semiconductor layers (SCn; SCp) and said light emitting layer (R) and the value of the effective field-effect mobility (ml) at the interface between the other of said semiconductor layers (SCp; SCn) and said dielectric layer (Die) is in the range from 0.05 to 20, and
• the difference between the levels HOMO-H and HOMO-SCn is comprised between 0.2 eV and 1 eV
• the difference between the levels LUMO-H and LUMO-SCn is comprised between 0.2 eV and 3 eV
• the difference between the levels HOMO-H and HOMO-SCp is comprised between 0 eV and 0.5 eV
• the difference between the levels LUMO-H and LUMO-SCp is comprised between -1 eV and 3 eV, and
• the difference between the levels LUMO-G and LUMO-SCn is comprised between 0.3 eV and -1 eV
• the difference between the levels HOMO-G and HOMO-H is comprised between 0 eV and 1 eV;
• the difference between the levels HOMO-H and HOMO-SCn is comprised between -3 eV and 1 eV
• the difference between the levels LUMO-H and LUMO-SCn is comprised between -0.5 eV and 0 eV
• the difference between the levels HOMO-H and HOMO-SCp is comprised between -0.2 eV and -3 eV
• the difference between the levels LUMO-H and LUMO-SCp is comprised between -0.2 eV and -1 eV, and
• the difference between the levels HOMO-G and HOMO-SCp is comprised between -0.3 eV and 1 eV
• the difference between the levels LUMO-G and LUMO-H is comprised between 0 eV and -1 eV.
• said ratio between the values of the effective field-effect mobilities ml and m2, defined hereinabove, is in the range from 0.7 to 1.3.
• the minimum value of said effective field-effect mobilities ml and m2 is 10 "3 cm 2 /Vs.
• figures 2A and 2B show a 3D digital elaboration of the illuminated channel of 2 different OLET devices operated in ambipolar conditions
• figure 3 shows the chemical structure of the molecular system SLV-131 used for realizing the layer of the N-type semiconductor material used in the comparative example shown in figure 7,
• FIGS 4A and 4B show other embodiments of an innovative OLET device in which the type of conductivity P and N, respectively, of the SCP and SCN semiconductor layer in contact with the dielectric layer is highlighted,
• figure 5 depicts an architecture of OLET device according to the invention with a P- type semiconductor layer in contact with the dielectric layer, the light emitting layer composed of a Host-Guest system, and the respective graphs of the optoelectronic characteristics,
• figure 6 depicts an architecture of OLET device according to the invention with a P- type semiconductor layer in contact with the dielectric layer, the light emitting layer composed of a Host-Guest system having the same Host material and a Guest material different from the material shown in figure 5, and the respective graphs of the optoelectronic characteristics,
• figure 7 shows an architecture of an OLET device not according to this invention with a P-type semiconductor layer (DH4T) in contact with the dielectric layer, a light emitting layer made of tris 8-hydroxyquinolato aluminum (Alq3), an N-type semiconductor layer (SLV131) located above the emitting layer, the energy bands of the used materials and the respective graphs of the optoelectronic characteristics,
• figures 8A-8D show the LUMO and HOMO levels of possible materials used in the layers of innovative structures of OLET devices with distributed illumination.
• the ambipolar channel of the transistors according to the invention is composed of a three-layer architecture, wherein the outermost layers SCI, SC2 are made of semiconductor materials, in the light emitting layer R charge carriers of opposite sign recombine with the consequent light emission enabled by the emission properties of the material or materials that constitute this layer.
• the inventors have found certain characteristics of an OLET architecture for obtaining, in a reproducible manner, a distributed light emission, that is, as previously defined, a broad (in geometrical sense) light emission spread over at least 20 ⁇ .
• the advantages tied to the present invention are even more evident in the embodiment of ambipolar OLET devices in which the channel is illuminated along hundreds of microns.
• the ambipolar channels must be sufficiently wide, that is the ambipolar channels have amplitude wider than or equal to the illuminated area.
• the amplitude of an ambipolar channel is determined by the distance between the source and drain electrodes of the transistor.
• An appropriate distance is also determined by the need of avoiding fabricating devices of size greater than what is needed, also taking into account the fact that, in the preferred embodiments of this invention, it is possible to illuminate a significant fraction of the ambipolar channel, typically not smaller than 33% and in certain cases, especially in the case of ambipolar channels smaller than 150 ⁇ , this fraction may also be close to 100% of its amplitude. Moreover, the fact that in wide channels dispersion phenomena and the effect of the difference between the effective mobilities become more relevant, pushes to prefer ambipolar OLET with light distributed emission having a distance between the source and drain electrodes comprised between 20 and 300 ⁇ .
• the field effect mobility indicates the property of the material of transporting charge carriers during functioning conditions in a field effect device; this value is not tied to the value of the bulk mobility, indeed in this case the mobility is essentially dominated by surface phenomena instead of bulk phenomena and superficial interaction.
• P3HT poly(3-hexylthiophene)
• MIS metal-insulator-semiconductor
• V D S VGS, with V D S that ranges from 0 to -100V
• OFET organic field effect transistor
• device substrate made of glass
• gate electrode made of a 150 nm layer of ITO
• dielectric layer made of a 450 nm layer of PMMA
• semiconductor layer made of a 10 nm layer of DH4T
• source and drain electrodes made of a 50 nm layer of gold.
• the field effect mobility is of about 0.1 cm 2 /Vs with a threshold voltage of about -25 V; on DH4T polycrystalline dust, mobility values of 0.015 cm 2 /Vs are obtained.
• the effective field effect mobility is given by the effective field effect mobility, this mobility being the one that is effectively obtained in the considered device under analysis, and may be obtained by its current- potential characteristic.
• it is different from the field effect mobility measured in the simplified reference structure, i.e. a field effect transistor (OFET) with a single semiconductor layer.
• OFET field effect transistor
• the value of field effect mobility may be compared with the effective field effect mobility in the considered device obtained by biasing in unipolar transport conditions a three-layer architecture in which, for example, the same layer of material acts as semiconductor SC2 positioned between the gold electrodes and the recombination layer R made of Alq3.
• the effective field effect mobility of DH4T is about 0.002 cm 2 /Vs with a threshold voltage of about - 60V.
• the differences between the LUMO and HOMO values of the materials of the layers of the ambipolar channel should satisfy the following constraints:
• the difference between the levels HOMO-R and HOMO-SCn is comprised between 0.2 eV and 1 eV
• the difference between the levels LUMO-R and LUMO-SCn is comprised between 0.2 eV and 0.8 eV
• the difference between the levels HOMO-R and HOMO-SCp is comprised between 0 eV and 0.5 eV
• the difference between the levels LUMO-R and LUMO-SCp is comprised between -1 eV and 0 eV;
• the difference between the levels HOMO-R and HOMO-SCn is comprised between 0 eV and 1 eV
• the difference between the levels LUMO-R and LUMO-SCn is comprised between -0.5 eV and 0 eV
• the difference between the levels HOMO-R and HOMO-SCp is comprised between -0.2 eV and -0.8 eV
• the difference between the levels LUMO-R and LUMO-SCp is comprised between -0.2 eV and -1 eV;
• the differences between the LUMO and HOMO values of the materials of the layers of the ambipolar channel must satisfy the following constrains:
• the difference between the levels HOMO-H and HOMO-SCn is comprised between 0.2 eV and 1 eV
• the difference between the levels LUMO-H and LUMO-SCn is comprised between 0.2 eV and 3 eV
• the difference between the levels HOMO-H and HOMO-SCp is comprised between 0 eV and 0.5 eV
• the difference between the levels LUMO-H and LUMO-SCp is comprised between -1 eV and 3 eV, and
• the difference between the levels LUMO-G and LUMO-SCn is comprised between 0.3 eV and -1 eV
• the difference between the levels HOMO-G and HOMO-H is comprised between 0 eV and 1 eV;
• the difference between the levels HOMO-H and HOMO-SCn is comprised between -3 eV and 1 eV
• the difference between the levels LUMO-H and LUMO-SCn is comprised between -0.5 eV and 0 eV
• the difference between the levels HOMO-H and HOMO-SCp is comprised between -0.2 eV and -3 eV
• - the difference between the levels LUMO-H and LUMO-SCp is comprised between -0.2 eV and -1 eV
• the difference between the levels HOMO-G and HOMO-SCp is comprised between -0.3 eV and 1 eV
• the difference between the levels LUMO-G and LUMO-H is comprised between 0 eV and -1 eV.
• the value of effective field effect mobility at the interfaces between the layers of the ambipolar channel is influenced by different factors that allow also to control it.
• the mobility of carriers may be increased/reduced by improving/worsening the purity of used materials and by increasing/reducing the size of crystal grains.
• Condition 2 may be verified by properly choosing the materials with which the ambipolar channel is realized.
• a skilled person will be capable of choosing, among the materials adapted to the fabrication of an OLET device, a P-type semiconducting material for the layer SCp, an N-type semiconducting material for the layer SCn, and one or more materials for the recombination layer R, that satisfy the condition 2).
• the layer SCp may be made of DH-4T
• the layer SCn may be made of DHF-4T
• the recombination layer R may be a Host-Guest system made of Alq3-PtOEP or of TCTA-PtOEP or yet of TCTA-DCM2 or yet made only of Alq3, or only of the complex l,3-di(2-pyridyl)-5-Methyl-benzene platinum(II) chloride.
• the roughness at the interfaces between the semiconducting layers SCI and SC2 and the light emitting layer R is comprised between 0.2 nm and 10 nm.
• the measure of the roughness expressed in nanometers indicates the arithmetic mean value of the deviation of the real profile of the surface from the mean line.
• said roughness at the interfaces is comprised between 0.2 nm and 5 nm.
• the purity of materials is controlled within certain levels.
• the purity interval useful for realizing a light emitting transistor with distributed emission should be determined, i.e. to fix the delicate compromise between the industrial feasibility (that may be jeopardized by the need of using materials with a very high purity level) and the use of materials that, not being sufficiently pure, hinder the attainment of the technical effect of the distributed illumination of the channel.
• this delicate balancing is satisfied when the purity of materials, that compose the layers of the ambipolar channel of the transistor according to the present invention, ranges between 99.8 and 99.999% and is preferably comprised between 99.99 and 99.999%.
• the ratio between the value of the field effect mobility ml at the interface between said first semiconductor layer SCI and said dielectric layer Die and the value of the effective field effect mobility m2 at the interface between said second semiconductor layer SC2 and said light emitting layer R ranges between 0.05 and 20. In other words, the ratio between the greatest and the smallest of the values of ml and m2 is comprised between 20 and 1.
• the light emitting transistor 10 comprises a substrate 11 that acts as a supporting layer, over which there is an electrode 12 that acts as control or gate electrode, and the layer of dielectric material Die, over which there is the first one of the two semiconductor layers SCI of the ambipolar channel of the transistor.
• the semiconductor layer SC 1 there is the light emitting layer R, over which on its turn the second semiconductor layer SC2 is deposited.
• FIG. IB A second alternative to which the teaching of the present invention applies, is shown in figure IB, in this case the OLET structure 20 contemplates the deposition of the source and drain electrodes 13 and 13', that are deposited directly on the substrate 11. These electrodes are in contact with the semiconductor layer SC2, that is surmounted by the light emitting layer R, over which on its turn is deposited the semiconductor layer SCI, surmounted by the dielectric layer Die on which the gate electrode 12 is deposited.
• the size and dimensional ratios of the various elements are out of scale because they have been altered to facilitate the comprehension of the figure.
• the depicted architectures are merely an example of the type of device that may be realized, because what is relevant for the present invention is the presence of the ambipolar channel comprising the semiconductor layers SCI and SC2 and the light emitting layer R with the previously described characteristics referring to the effective mobility as its characterizing element.
• a definitely equivalent variant of the structure shown in figure 1 A does not use an element 11 as supporting substrate, instead its function is performed by the gate electrode itself, that in this case has an amplitude greater than that depicted in figure 1, or the indicated substrate as single element 11 is in reality composed by a plurality of elements.
• the materials that compose the electrodes 13 and 13' may be optimized in function of the type of charge (electrons, holes) that should be injected.
• the type of charge electrospray, holes
• oligoacenes oligothiophenes and oligofluorenes
• pyrimidine derivatives of oligothiophenes tetrathiophenes substituted in positions a and ⁇ with alkylic chains, the latter being preferred because of the small surface roughness and the great field effect mobility.
• diimide derivatives of perylenes and oligothiophenes there are diimide derivatives of perylenes and oligothiophenes, pyrimidine derivatives of oligothiophenes, oligothiophenes with thiazole core, coronene derivatives and derivatives of tetrathiophene substituted in positions a and ⁇ with perfluorated chains, the latter being preferred because of the great field effect mobility.
• suitable materials adapted to the realization of the invention are Alq3, cyclometalated platinum complexes of the type 1 , 3 -di(2-pyridyl)-5 -Methyl-benzene platinum(II) chloride, or Host-Guest systems with quinoline Aluminum lattice doped in various manners with, for example, 4-(dicyanomethylen)-2-methyl-6-(p- dimethylaminostyryl)-4H-pyrane, Platinum octaethyl-porphyryn, phenylisoquinoline of Iridium acetylacetonate, or Host-Guest systems with Aluminum quinoline lattice doped with a combination of these molecules.
• Example 1 The invention is better described hereinafter through a non limiting example of an embodiment (Example 1). Counter-examples are also provided in which the light emission is spatially confined (Example 2, Example 3 and Example 4) because both the above conditions 1 and 2 are not verified at the same time.
• An organic ambipolar light emitting field effect transistor with distributed light emission is realized from a substrate that comprises a first layer of glass on which there is a layer of ITO (Indium Tin Oxide), on which there is a layer of polymethylmethacrylate (PMMA) as dielectric Die.
• the first layer of semiconductor material SCI is deposited over the layer of PMMA, in this case a layer of 15 nm made of dihexyl-tetrathiophene (DH4T) as layer for carrying holes.
• a 40 nm layer of light emitting material composed of a Host-Guest mixture of tris 8-hydroxyquinolato aluminum (Alq 3 ) containing 8% by weight of platinum octaethyl-porphyrin (PtOEP), and finally the last element of the three-layer stack, the semiconductor SC2 for carrying electrons, made of di-perflouro-hexyl-tetrathiophene (DHF4T), having a thickness of 20 nm.
• DHF4T di-perflouro-hexyl-tetrathiophene
• the stack of the three layers, DH4T, Alq3 + PtOEP and DHF4T constitutes the ambipolar channel of the transistor.
• the process for growing the structure has been optimized using techniques that are widespread in the field and well known to the skilled person, for not affecting the field effect mobility of the charge carriers, in particular the growing speed of the two layers of DH4T and DFH4T was controlled and set to 0.15 Angstrom/s, the growing speed of the layer of Alq 3 was been set to 5 Angstrom s. Moreover, the insertion of PtOEP in Alq3 took place through controlled co-deposition of this last element at the desired percentage (8%).
• DH4T was subjected to two re- crystallization processes, and the DHF4T was subjected to two sublimation processes at temperature with controlled gradient.
• the implemented processes have reduced impurities in the interval 0.02-0.001% for both materials; Alq3 is pure at 99.99%.
• the degree of purity of materials ensures that the effective field effect mobilities of both carries are sufficiently close to each other for having a wide illumination of the channel, using materials and related purification processes that allow the realization of devices on an industrial scale.
• the transistor in order to measure the effective field effect mobility, the transistor is operated in unipolar mode.
• the transistor realized as in the example 1 is operated in an ambipolar functioning condition, by setting a potential difference between gate and source Vgs of -60V, and a potential difference between drain and source Vds of -100V and its light emission is collected through a Nikon Eclipse 2000-E microscope with 60x magnification and 0.7 numerical aperture.
• An organic ambipolar light emitting field effect transistor is realized exactly with the same materials, the same structure and the same thicknesses used in the Example 1. Nevertheless, in this example, the growth rate in the two layers of DH4T and DFH4T was controlled and set to 0.1 Angstrom/s, and the growth rate for the Alq3 layer was set to 2 Angstrom/s. All the other characteristics of the device, including the insertion of PtOEP in Alq3 performed through controlled co-deposition in the percentage of 8%, remain unchanged.
• the overall structure of the device is thus ITO/PMMA/DH4T (15 nm, 0.1 A/s)/Alq 3 :PtOEP 8% (40 nm, 2 A/s)/DHF4T (20 nm, 0.1 A/s)/Au (50 nm).
• condition 1) the ratio between the values of the effective mobilities is not complied with, the emission of the ambipolar light emitting transistor is spatially confined.
• condition on the effective mobility is a cardinal element in order to achieve a distributed light emission as comparative example 2 clearly shows.
• Such condition was not disclosed or described in the already referenced 2010 Nature Materials paper "Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes", that shows OLET with narrow and well defined light spatial emission without no teaching or suggestion on how to alter or achieve a distributed light emission.
• an OLET architecture the scheme of which is depicted in figure 7, in which a layer of semiconductor material DH4T (30 nm thickness) with P-type mobility is deposited on the PMMA dielectric. Over the semiconductor SCI is deposited a layer of Alq 3 having a thickness of 30 nm, that acts as light emitting layer.
• the layer SC2 with N-type mobility in contact with the gold electrodes is composed of a layer having a thickness of 30 nm of an arylcarbonylic derivative of tetrathiophene (SLV-131), having the empirical formula C 30 H 14 O 2 S 4 F 4 and having the molecular structure depicted in Fig. 3.
• the architecture of figure 7 shows that the condition 2) about energy levels is complied with, though the structure of this example is characterized by a ratio of two orders of magnitude between the effective field effect mobility values at the respective interfaces of SCI and of SC2.
• the fact of not having satisfied even only the condition 1) caused a reduction of the amplitude of the light emitting area to about ⁇ , as demonstrated in Fig. 2B in which a 3D digital elaboration of the illuminated channel, in an ambipolar functioning condition, is depicted.
• An organic ambipolar light emitting field effect transistor is realized starting from a substrate that comprises a first layer of glass over which there is a layer of ITO (Indium Tin Oxide), over which there is a layer of polymethylmethacrylate (PMMA) that acts as dielectric Die.
• the first layer of semiconductor material SCI is deposited on the layer of PMMA, in this case a 10 nm layer made of di-hexyl-tetrathiophene (DH4T) as layer for carrying holes.
• TCTA Tris(4-carbazoyl-9-ylphenyl)amine
• DHF4T di- perfluoro-hexyl-tetrathiophene
• the stack of the three layers DH4T, TCTA and DHF4T constitutes the ambipolar channel of the transistor.
• the process for growing the structure has been optimized by using techniques that are widespread in the field and well known to the skilled persons for not affecting the field effect mobility of the charge carriers, in particular the growing rate of the two layers of DH4T and DFH4T was controlled and set to 0.1 Angstrom/s, the growing rate of the layer of TCTA was set to 5 Angstrom/s.
• the condition 1) about the ratio between the values of the effective mobilities was satisfied.
• the energy levels of the levels HOMO and LUMO of the used materials are not complying with condition 2) and the emission of the light emitting ambipolar transistor is spatially confined. Indeed, as it may be inferred from figures 8C and 8D, the conditions about the energy differences between LUMO-R and LUMO- SCn, that is 0.9 eV, thus out of the requested range [from 0.2 eV to 0.8 eV], is not satisfied.
• FIGS 4A and 4B depict possible architectures of an OLET device in which the layer in contact with the dielectric is respectively of P-type (SCP) or of N-type (SCN).
• Figure 5 shows an example of architecture of an OLET device with distributed illumination, experimentally tested, and the related graphs of the voltage-current and voltage-emission characteristics obtained with the described architectural features.
• the recombination layer in this case is composed of TCTA:DCM2. It is to be noticed that the effective mobilities differ between them by less than 20 times, i.e. the ratio of the maximum value, 0.28 cm 2 /Vs for the negative carriers, with the minimum value of 0.1 1 cm 2 /Vs for the positive carriers, is 2.54 cm 2 /Vs, well within the limit value of 20 as imposed by condition 1), . Also condition 2) is complied as shown in Fig. 8D.
• Figure 6 is similar to figure 5 and shows another exemplary architecture of an OLET device with distributed illumination and the related graphs of the voltage-current and voltage-emission characteristics.
• the recombination layer is composed of TCTA-PtOEP.
• the effective mobile of the negative carriers is the lowest one, 0.051 cm 2 /Vs, while the effective mobility of the positive carriers is 0.7 cm 2 /Vs, resulting in a ratio of 13.7.
• condition 2) is complied as shown in Fig. 8C.
• Figure 7 shows an architecture of an OLET device not according to this invention with a P-type semiconductor layer (DH4T) in contact with the dielectric, a light emitting layer made of tris 8-hydroxyquinolato aluminum (Alq3), a N-type semiconductor layer (SLV131) placed on the light emitting layer.
• a P-type semiconductor layer Diffraction-based light emitting layer
• Alq3 tris 8-hydroxyquinolato aluminum
• SLV131 N-type semiconductor layer
• the OLET device of figure 7 has therefore a spatially confined illumination.
• Figures from 8 A to 8D illustrate the LUMO and HOMO levels of materials usable for realizing the novel device, together with the Fermi level of the source and drain contacts.
• the condition 2) is satisfied thus they are suitable for realizing the novel OLET device with distributed illumination, provided that they are deposited in a way to make the effective field effect mobilities at the interfaces not differing from each other by more than 20 times.

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• 2012
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• 2013
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