This invention relates to optoelectronic devices, namely, to optoelectronic devices based on organic light- emitting diodes, which include the active electroluminescent layer containing quantum dots with the modified surface.

Modern organic light-emitting diodes (OLED) , as a rule, are multilayer electroluminescent devices that include at least the following elements: glass or polymer transparent substrate coated with a transparent anode layer (as a rule, indium tin oxide (ITO)), polymer or low molecular organic transport layer with a hole conductivity, active luminescent layer based on organic compounds, organic transport layer with electron conductivity and metal cathode.

Also, organic light-emitting diodes with a more complex configuration of elements are known, where between the anode and transport layer with hole conductivity a layer of holes injection is located, and between transport layer with hole conductivity and active luminescent layer an electron blocking layer or a hole blocking layer is located, at that between transport layer with electron conductivity and active electroluminescent layer the electron blocking layer or hole blocking layer is located.

Presently, the main disadvantage of all organic light-emitting diodes, and OLED technology in general, is short life of used light-emitting materials based on organic luminescent materials because of their rapid degradation .

One of the ways to solve this problem, according to authors, is the creation of optoelectronic device, in which as the active luminescent layer the colloidal quantum dots are used that have a number of significant advantages comparing to conventional organic low molecular and polymer luminescent materials. Besides, the use of quantum dots can substantially increase the service life of devices made on their basis.

Today, the colloidal quantum dots are obtained through high temperature synthesis in the inert atmosphere in a non-coordinating high-boiling organic solvent, using a different sets of surfactants, which provide the controlled growth of nanocrystals . As a result of such synthesis the semiconductor nanocrystals are obtained, which consist of semiconductor nano-size core coated with one or more semiconductor shells, and an outer organic layer of adsorbed surface-active ligands oriented with their polar ends to the surface of quantum dot and with hydrophobic part to the nonpolar solvent side, in which they dispersed (Fig. 1) .

From the prior art the light-emitting devices are known (US 2009/0039764 Al, H01J 1/63, 12.02.2009; US 2010/0109521 Al, H01J 1/62, 06.05.2010), in which instead of organic phosphors the quantum dots are used as a light-emitting layer. To use the quantum dots in such organic light-emitting diode systems it is necessary to apply the dispersion of quantum dots in the solvent to organic conductive layers with subsequent formation of active electroluminescent layer. However, during such applying and formation of active electroluminescent layer a problem occurs concerned with the breach of integrity of previous layers, while applying the layer of quantum dots and subsequent layers in situations, where during applying the layer of quantum dots or subsequent layers the solvent is used capable to dissolve substances of previous layer. Authors of US 7132787 B2, H01J 1/72, 07.11.2006 patent faced the same problem at creating their invention a multilayer polymer light-emitting diode based on quantum dots. To solve it, it was suggested to use solvents of different polarity during the applying of adjacent layers in order to avoid breaching the integrity of the previous layer. However, the proposed solution may require dispersion of quantum dots into a polar solvent. In this case, presently it is possible to obtain the quantum dots dispersing into polar solvents by means of three basic ways:

synthesize quantum dots directly in the polar phase; replace hydrophobic ligands present on the surface of quantum dots with hydrophilic ligands;

- additionally modify the surface of quantum dots by amphiphilic compound not replacing any of ligands that are already on the surface of quantum dots.

It is known from the scientific literature (http://dx.doi.org/10.1039/B606572B; http://dx.doi.org/ 10.1002/smll.200700654) that the first two methods have a number of disadvantages, which result in occurrence of defects on the surface as well as inside of the crystal structure of the quantum dots. These defects, in their turn, are the reason for lowered quantum yield of fluorescence and photostability of the nanocrystals . Presently, only the quantum dots produced in nonpolar solvents have a fluorescence quantum yield of more than 90% and the service life measured by decades.

Therefore, the use of water soluble quantum dots produced by the first two methods, in organic light- emitting diode systems results in low luminous efficiency of these systems as well as their lowered service life.

Third method for modification of surface of quantum dots using amphiphilic ligands is devoid of disadvantages described above, but the application of quantum dots obtained by this method in multilayer electroluminescent devices also shows some flaws. Thus, conventional hydrophobic ligands used by the synthesis of quantum dots, and, respectively, located on their surface, represent the classes of compounds as follows: fatty carboxylic acids, fatty amines, fatty phosphines, fatty phosphine oxides, etc. These compounds do not possess conductivity and in electroluminescent systems serve as energy barrier for injection of electrons and holes. But if even for trioctylphosphine oxide and trioctylphosphine (TOPO and TOP) this barrier is not very large (due to the relatively short length of these molecules) then formation of one more insulating organic layer over them (while modifying the surface of quantum dots by amphiphilic ligand) results in a substantial increase of energy barrier for the penetration of charge carriers into the semiconductor structure, as well as reducing the efficiency of resonant energy transfer to the quantum dots. As a result, the efficiency of such electroluminescent systems remains quite low, and control voltages required for "turning-on" a device are too high, which ultimately leads to rapid degradation of organic layers and outage of the device as a whole.

The purpose of this invention is creation of active electroluminescent layer of optoelectronic device that includes the layer of quantum dots, deposited from the polar solvent, in which they can be dispersed through modifying their surface by amphiphilic conductive polymer, at that the layer of quantum dots should be positioned between the layers of organic polymers possessing high solubility in nonpolar and low solubility in polar solvents. The technical result is the elimination of integrity breach of previous layers, while applying the layer of quantum dots and subsequent layers in situations, where during applying the layer of quantum dots or subsequent layers a solvent capable of dissolving the substances of previous layer is used. At that the most efficient and photostable nanocrystals with minimal number of defects in their crystal structure are used, and the problem of formation of thick non-conductive layer on its surface during modification by amphiphilic conductive polymer is eliminated. From the practical point of view, this allows to obtain devices with higher light intensity and low operating voltages.

The problem is solved, and the technical result is achieved due to the fact that the active electroluminescent layer of optoelectronic device includes the first organic hole transport layer soluble in the nonpolar solvent, second organic electron transport layer soluble in a nonpolar solvent, between which the layer of semiconductor quantum dots is located; those dots are applied from solution in the polar solvent, at that the surface of quantum dots was preliminary modified by amphiphilic conductive polymer and prior to modification the surface of semiconductor quantum dots was hydrophobic and contained the layer of surfactant over its entire surface, which thickness was from 0.7 nm to 3 nm.

In order to obtain quantum dots with high fluorescence quantum yield and higher photostability, the semiconductor nanocrystals structure of "core/first semiconductor shell" or "core/first semiconductor shell/second semiconductor shell" type are usually used. Further in the text the following symbol for semiconductor nanocrystals is used: core/first semiconductor shell or core/first semiconductor shell/second semiconductor shell. For example, InP/ZnS or CdSe/CdS/ZnS.

Semiconductor materials for cores and shells as well as their thicknesses are selected so that the formed nanocrystals possess the minimum amount of defects, and are not exposed to aggressive environmental effect.

Semiconductor core is a semiconductor compound selected from the group: CdSe, ZnSe, CdZnSe, CdS, CdZnS, CdSSe, CdZnSSe, InP, GaP, InGaP, InZnP, InAs, InAsP, CdTe, ZnTe, CdZnTe, PbS, PbSe, CuInS ₂ , CuInSe ₂ , AgInS ₂ , but not limited to the listed compounds.

First semiconductor shell is a semiconductor compound selected from the group: CdSe, ZnSe, CdZnSe, CdS, CdZnS, CdSSe, CdZnSSe, InP, GaP, InGaP, InZnP, InAs, InAsP, CdTe, ZnTe, CdZnTe, PbS, PbSe, CuInS ₂ , AgInS ₂ , but not limited to the listed compounds.

Second semiconductor shell is a semiconductor compound selected from the group: CdS, CdZnS, ZnS, but not limited to the listed compounds.

It is necessary to understand that both the general description above and the following detailed description are approximate and explanatory descriptions only and are intended to provide additional explanation of the claimed invention.

As the first organic conductive layer the polymer compounds with p-type conductivity and soluble in nonpolar solvent or weak polarity solvent can be used. Such compounds, for example, may include poly-p- phenylenevinylene (MEH-PPV) , poly-3-hexylthiophene

(P3HT), Ν,Ν'-bis ( 3-methylphenyl ) -N, N ' -bis (phenyl) benzidine (TPD) or poly ( 3, -ethylenedioxythiophene ) - tetramethacrylate (PEDOT-TMA) , whose structural formula is shown below.

As second organic conductive layer the polymer compounds with n-type conductivity and soluble in nonpolar solvents and insoluble in polar ones are used. Such compounds, for example, may include tris(8- hydroxyquinolinato) aluminium (Alq3), 2 , 9-dimethyl-4 , 7- diphenyl-1, 10-phenanthroline (BCP) , 3- (biphenyl-4-yl) -5- (4-tretbutylphenyl)-4-phenyl-4H-[l,2,4]triazole (TAZ) .

For example, toluene, chloroform, n-alkanes, such as, hexane or octane, or cycloalkanes, such as, cyclohexane are used as nonpolar solvents. It is also possible to use compounds with intermediate polarity, for example, propylene carbonate.

Water, alcohols, for example, isopropyl, or ethers, for example, diethyl are used as polar solvents. It should be noted that it is undesirable to use water and water-containing solvents with compounds sensitive to moisture in both organic layers.

As amphiphilic conductive polymer the polyfluorene derivatives, including block copolymers of polyfluorene with hydrophilic polymers, for example, polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, etc. as well as statistical copolymers with fluorene derivatives, with hydrophilic substituents at position 9 were selected.

The fatty phosphines, fatty phosphine oxides, fatty amines or fatty carboxylic acids are used as surfactant. For example, it is possible to use hexadecylamine or oleic acid.

The claimed invention is explained by drawings with the figures as follows:

Fig.l - Schematic image of quantum dot before modification of its surface.

Fig. 2 - Schematic image of quantum dot after modification of its surface in polar solvent.

Fig. 3 - Scheme of device with a layer of modified quantum dots.

Fig.4 - The current-voltage characteristics and current efficiency of the device based on modified quantum dots.

Fig. 1 schematically shows a quantum dot, which is a semiconductor nanocrystal consisting of a semiconductor core (pos. 1), coated with the first semiconductor shell (pos. 2) and the second semiconductor shell (pos. 3) as well as an outer organic layer of chemisorbed surface- active ligands giving layer thickness of 0.7-3 nm (pos. 4 ) oriented with polar area to the quantum dot surface, and with hydrophobic area to the side of nonpolar solvent, in which this quantum dot is dispersed.

In order to obtain quantum dots with modified surface (Fig. 2) the authors of this invention had synthesized at the first stage the quantum dots of different configuration; the methodology of that procedure is given below.

Example 1. Obtaining of quantum dots with CdSe/CdS/ZnS structure.

It is necessary to load stirring 3 g of cadmium oleate in octadecene into a flask under argon. Obtained mixture should be heated to 240 °C and 2 ml of a 1M solution of trioctylphosphineselenide in trioctylphosphine should be injected. After 3-5 minutes the mixture should be cooled, and cores CdSe (d = 3-4 nm) should be isolated through depositing the mixture by isopropanol and methyl alcohol as well as re-dissolving in toluene.

To build first semiconductor layer CdS and second semiconductor layer ZnS the standard SILAR procedure is used. The known amount of semiconductor cores CdSe, defined based on exciton absorption is loaded into the flask, octadecene and hexadecylamine are added in a ratio of 1: 4 by weight, then the obtained mixture is heated, while being stirred under argon, and solutions of cadmium oleate and sulfur are sequentially injected, afterwards solutions of zinc oleate and sulfur are injected in volumes required for layered growth of first shell CdS and second shell ZnS.

The obtained quantum dots are isolated by the method described above; an aliquot of solution in toluene is evaporated to determine the mass concentration. Photoluminescence quantum yield equals to 90%. Example 2. Obtaining the quantum dots with CuInS ₂ /ZnS structure .

Copper iodide (I) and indium acetate (III) are mixed in the flask in a ratio of 1 to 5 by weight with addition of dodecanethiol, then the mixture is vacuumized and the temperature gradually increased to 240 °C. After one hour heating, the zinc myristate suspension in octadecene is added by portions in total weight twice more than used indium acetate. The obtained quantum dots CuInS ₂ / nS are isolated with butyl alcohol through re-dissolving the deposits of quantum dots in toluene. An aliquot of solution is evaporated to determine the mass concentration. Photoluminescence quantum yield equals to 80%.

Example 3 .· Obtaining of quantum dots with

InP/ZnSe/ZnS structure.

A solution of indium acetate and myristic acid in octadecene is heated to 200 °C in inert atmosphere, and then is degassed for 1 hour at a temperature of 120 °C. The solution is then reheated and precursor of phosphorous - tris (trimethylsilyl ) phosphine - in mixture with octadecene and hexadecylamine is injected. For the growth of ZnSe shell the solution is heated to 240 °C, the solution of zinc oleate in octadecene is entered dropwise into it, and then the solution of selenium in trioctylphosphine is also added into it dropwise. For the growth of ZnS shell the similar procedure is conducted, but as a precursor of sulfur the solution of sulfur in octadecene is used. After one hour the mixture is cooled and the quantum dots are isolated with butyl alcohol, re- dissolving the deposit of quantum dots in toluene. An aliquot of solution is evaporated to determine the mass concentration .

Photoluminescence quantum yield equals to 80%. In order to obtain quantum dots with other semiconductor, one of the methods described above is used, which is selected depending on specified objectives .

In order to obtain semiconductor nanocrystals with the structure - semiconductor core/first semiconductor shell it is not necessary to build the second semiconductor shell, and the desired semiconductor compound is immediately isolated.

Thus, it is possible to obtain semiconductor quantum dots with the following structures: CdSe/CdS, CdSe/ZnS, ZnSe/ZnS, ZnSe/CdS, ZnSe/CdSe, CdSe/ZnSe, CdS/ZnS, ZnSe/CdSe/CdS, CdSe/CdS/ZnS , CdSe/ZnSe/ZnS, InP/ZnS, InP/ZnSe, InP/CdS/ZnS, CdTe/CdS, CdTe/CdSe/CdS/ZnS , CuInS ₂ / nS, CuInSe ₂ /ZnS, etc., as well as solid solutions with interpenetration of shells, in other words, any combinations of semiconductor materials described above, through using the corresponding precursors during the synthesis and change of synthesis temperature depending on reactivity of the specific chemical compound.

Further, to prepare quantum dots with modified surface it is necessary to preliminarily synthesize an amphiphilic conductive polymer based on one of known techniques, e.g., based on the technique described below; afterwards it is necessary to prepare a solution with known concentration of quantum dots obtained at the first stage, - 1-10· 10 ¹⁶ pes. per ml (measured by optical means according to absorption per 350 nm) and used amphiphilic conductive polymer dissolved, for example, in toluene, at that tenfold excess of polymer relatively to the weight of quantum dots should be taken. The solution should be evaporated on a rotary evaporator at 50 °C and re- dissolved in a volume of isopropyl alcohol or any other polar solvent listed above sufficient to obtain a concentration of quantum dots of about 1-10·10 ¹⁶ .

Fig. 2 schematically shows a quantum dot obtained after modifying its surface in the polar solvent. The quantum dot is a semiconductor core 1 covered with first semiconductor shell 2 and second semiconductor shell 3, whose surface is coated with an outer organic layer from adsorbed surface-active ligands 4, oriented with polar area to the surface of quantum dot, and with hydrophobic area to the side of amphiphilic conductive polymer. The hydrophilic part of amphiphilic conductive polymer chain is designated by position 5, and the hydrophobic part of amphiphilic polymer chain is designated by position 6 in Figure 2.

Amphiphilic conductive polymer suggested in this invention is a block copolymer of 9,9-dialkyl substituted polyfluorene with polyethylene glycol (PEG) . This block copolymer is obtained by covalent binding of PEG hydrophilic block with hydrophobic block of polyfluorene through amide formation. Binding of carboxyl group and amino group (preliminarily entered as a substituent to the end of chains of polyfluorene and PEG, respectively) was carried out by means of activation with carbonyldiimidazole . The molecular weight of polyfluorene and PEG blocks was about 10 ³ g/mol each.

Fig. 3 schematically shows the construction of optoelectronic device, which is a multilayer structure, whose distinguishing feature is the use of quantum dots as an active electroluminescent layer, whose surface was previously modified by amphiphilic conductive polymer.

The device includes a transparent substrate (pos. 7) coated with a layer of transparent anode (pos. 8), to which the first polymer layer is applied (pos. 9), and which is, in fact, the transport layer with hole conductivity. Preliminarily modified samples of quantum dots (pos. 10) were applied onto the first polymer layer 9 using centrifugation technology. Then the second polymer layer was applied (pos. 11), which is, in fact, transport layer with electron conductivity. The device also includes a cathode (pos. 12).

The device operates as follows. When voltage is applied between electrodes, the movement and recombination of charge carriers occurs, which leads to the formation of singlet and triplet excitons in the area of +/- 20 nm between first organic conductive layer 9 and second organic conductive layer 11. With high probability the excitons formed in the layer of organic semiconductor through the process of resonance energy transfer give it to acceptors - quantum dots. Also in this area the direct injection of electrons and holes into quantum dots is taking place. Relaxation of energy obtained in that way, leads to the appearance of exciton in quantum dots, and on the levels of quantum dots lS _e and lS _h - electron and hole, respectively. In case of presence of photoexcitation ion-acceptors in the structure of quantum dot, the energy is transferred to their levels. During the time period from 100 ps to 10 μΞ the recombination of charge carriers in quantum dots occurs, with emission of a light quantum that has energy close to the difference between energies of levels lS _e and lS _h .

The specific procedure of making an optoelectronic device with the structure ( ITO/PEDOT : A/TPD: KT (15 nm)/Alq ₃ (30 nm)/Al) with a layer of quantum dots is given below.

Example 4. Onto the glass with a layer of ITO in a dry box a drop of solution of PEDOT : TMA in propylene carbonate is poured and the centrifuge is switched on. The number of revolutions is regulated depending on the required thickness of layer. Then, during 20 minutes the film is getting dried at 110 °C and cooled to the indoor temperature. Then a solution of modified quantum dots, for example, CdSe/CdS/ZnS is dropped onto this film, centrifuged, and a layer with quantum dots of 15 nm thickness is obtained. The concentration of the quantum dots was selected so that per the unit of area of the layer with first organic conductive material the number of quantum dots was sufficient to form a monolayer. After one hour incubation in a dry box at 40°C, a substrate with layers is transferred to the sputtering chamber Alq3, and a thermal spraying is carried out at pressure of 10 ^~5 mbar. After sputtering of 30 nm Alq3 layer through magnetron sputtering, the aluminum layer is applied - the portion with area of 0.5 cm ² .

Afterwards, in dry box an electric current was supplied to the manufactured device. By means of a precision voltmeter and ammeter as well as calibrated spectrometer, whose optical input was placed directly over the light-emitting platform of device, the electroluminescence spectra, the current-voltage characteristics, and device efficiency indicators were obtained. The absorption spectra were measured by means of the spectrophotometer Perkin Elmer 45 in the spectral range of 300-800 nm, the spectra of photo-and electroluminescence - by means of the spectrometer Maya Pro 2000, Ocean Optics.

Organic conductive compounds can be applied from the gas phase through sputtering as well as from solutions.

Obtained in that way devices based on quantum dots demonstrated a bright glow that reached 600 cd/m ² (Fig. 4). Turn-on voltage - 2.5 V, the external quantum efficiency - 1%. The device made as described in Example 4 , where instead of quantum dots with surface modified by means of amphiphilic conductive polymer the water soluble quantum dots of CdSe / CdS / ZnS type were taken, obtained through replacing the surface fatty amines with the mercaptopropionic acid, has demonstrated the emission brightness less than 50 cd/m ² at turn-on voltage about 4V .

It is proposed to use the quantum dots synthesized in nonpolar solvent medium and possessing a luminescence quantum yield of more than 90%, with photostability over 5 years, dispersed in polar solvent through modification of their surface by amphiphilic polymer.