DE102011117357A1 - Preparing solution of n-doped organic low molecular semiconductor comprises dissolving organic semiconductor in aprotic solvent, adding alkali metal, and optionally separating/purifying semiconductor solution under inert gas atmosphere - Google Patents

Abstract

Preparing a solution of n-doped organic low molecular semiconductor with alkali metals, where the ionization potential of the alkali metal is lower than electron affinity of the semiconductor, comprises: (a) dissolving an organic semiconductor in an aprotic solvent; (b) adding the alkali metal or a solution of the alkali metal in a Solvent to the semiconductor solution; and (c) optionally separating or purifying the n-doped semiconductor solution under inert gas atmosphere. Independent claims are included for: (1) producing thin layers of the low molecular n-doped organic semiconductor material comprising preparing the solution of n-doped organic low molecular semiconductor as above per se, and applying the solution on a substrate; and (2) thin layer of the n-doped organic semiconductor material produced by the method.

Description

The present invention relates to a method for n-doping low molecular weight organic semiconductors with the aid of alkali metals in the liquid phase. Furthermore, the invention relates to a method for the production of n-doped semiconductor layers from the liquid phase, as well as their use in semiconductor devices, such. As organic solar cells, light emitting diodes, lasers, photodetectors, thermoelectric generators or field effect transistors.

The electrical doping is used in semiconductor technology to increase the free carrier density and thus to increase the conductivity of semiconductors. In the case of organic semiconductors, the doping is carried out by oxidation reactions (p-doping) or reduction reactions (n-doping) of the semiconductor molecule, whereby the molecule emits or takes up electrons.

In the n-doping of organic semiconductor molecules, an electron transfer into the LUMO (lowest unoccupied molecular orbital) of this molecule takes place. Accordingly, reducing agents are used for n-doping. Alkali metals are suitable as reducing agents for the n-doping of organic semiconductors, since they can easily release valence electrons into the LUMO of the organic materials due to the low ionization potential.

The doping of organic semiconductors with alkali metals is known in the literature. However, the doping is carried out in vacuo by coevaporation or vacuum sublimation of the alkali metals. The vacuum technology is expensive and expensive. More advantageous would be a method for doping in the liquid phase. This would allow easier application of a liquid-phase semiconductor layer to a substrate.

In US 4,204,216 describes a method for n-doping of polyacetylene, in which a polyacetylene film is brought into contact with a doping solution. The doping solution comprises an organometallic compound having an organic radical anion and a metal having an electronegativity which may not be greater than 1.6, dissolved in an inert organic solvent. As an alternative dopant, an alkali metal dissolved in ammonia is mentioned, whereby the doping process has to be carried out at very low temperatures of -78.degree. The central disadvantage of this method is the fact that the substance to be doped, ie the polyacetylene, on the finished component is contacted with the dopant and must diffuse into the substance. This process of diffusing is tedious and takes several hours, with a cleaning step followed by another. Furthermore, the diffusion of the dopant is not homogeneous, so that the polymer layer is not uniform in its morphology and semiconductor property over the entire layer thickness. Rather, a doping gradient is to be expected.

Hua et al. ( MY Hua, GW Hwang, YH Chuang, SA Chen, 2000, Macromolecules 33, 6235-38 ) describe the synthesis of a soluble n-doped polyaniline. For this purpose, polyaniline is dissolved in DMSO, filtered off and brought into contact with NaH or KH, which is also dissolved in DMSO. The result is a yellow polymer, which is applied as a layer on a substrate is electrically conductive. The use of the n-doped polyaniline in semiconductor devices is not mentioned. The doping of low molecular weight compounds is not mentioned.

It would be desirable if the in-situ preparation of an n-doped low molecular weight organic semiconductor in solution would be possible. This would make a process for the production of n-doped semiconductor layers accessible, without having to use a complex vacuum technique. Such coating processes are simple and quick to perform, which makes the production processes of organic semiconductor devices much less expensive.

To overcome these disadvantages of the prior art, it is an object of the invention to propose a method for the preparation of a solution of n-doped organic low molecular weight semiconductor materials, which are n-doped in-situ and can be used directly or after purification. Furthermore, a coating method is proposed with which an organic semiconductor layer can be easily applied to a semiconductor device. It is likewise an object of the invention to provide a semiconductor layer which, according to the o. Procedure was made. Ultimately, it is an object of the invention to show uses of these semiconductor layers.

The present invention achieves the object by the methods claimed in claims 1 and 5, the semiconductor layer claimed in claim 7 and the uses claimed in claim 12. Preferred embodiments of the method and the layers are described in the dependent claims.

The invention relates to a method for producing a solution of n-doped organic low molecular weight semiconductors with alkali metals. For the doping to proceed efficiently, the ionization potential of the alkali metal should be less than the electron affinity.

In the context of the present invention, a small molecule (small molecule) is a non-polymeric compound whose molecular weight does not exceed 1000 g / mol.

Low molecular weight semiconductor compounds have the advantage over polymers of being more thermally stable than polymers. They are easier to process and purify, especially in the liquid process according to the invention. Low molecular weight semiconductor compounds are purer than layers of polymers that differ in both their chain length and degree of polymerization as well as their degree of doping.

The ionization energy is defined as the energy needed to ionize an atom, i. H. to separate an electron from the atom. In the present case of ionization of alkali metals, the ionization energy refers to the valence electron.

Electron affinity (EA) means the energy difference between the electronic ground state of a neutral molecule and the electronic ground state of the associated negatively charged ion. The ground state is the name for the lowest-energy state of the molecules. The electron affinity is a measure of how strongly a neutral molecule can bind an additional electron.

The doping process according to the invention takes place in solution. The low molecular weight organic semiconductor molecule is initially charged in an anhydrous aprotic solvent.

Agrotic solvent is the name given to nonaqueous solvents that do not contain an ionizable proton in the molecule. This includes in addition to aliphatic and aromatic hydrocarbons and polar solvents such. As ketones, ethers, tertiary amines, pyridines, furans, nitroalkanes, nitriles or sulfoxides.

Subsequently, a suitable alkali metal is added to this semiconductor solution. For a suitable alkali metal, the ionization potential should be less than the electron affinity of the semiconductor molecule.

In Table 1, the principal dopability by alkali metals is shown by way of example on some organic molecules.

The greater the energy gain of the alkali metal valence electrons in the transition to the organic semiconductor, the better the doping works. BPhen TPBi TAZ BuPDB EA = -3.5 eV EA = -2.8 eV EA = -2.6 eV EA = -2.6 eV Li Φ _a = 2.9 eV + - - - N / A Φ _a = 2.75 eV + + - - K Φ _a = 2.3 eV + + + + Cs Φ _a = 2.14 eV + + + + Table 1: Potability of BPhen (4,7-diphenyl-1,10-phenanthroline), TPBi (2,2 ', 2''- (1,3,5-benzenetriyl) tris-1-phenyl-1H-benzimidazole) , TAZ (3- (4-biphenyl) -4-phenyl-5-tert-butylphenyl-1,2,4-triazole) or BuPBD (2- (biphenyl-4-yl) -5-phenyl-1,3, 4-oxadiazole) with alkali metals, comparing the ionization potential (Φ _a ) and the electron affinity (EA).

Alternatively, the alkali metal in a suitable solvent, such as. As ammonia, pyridine or aliphatic amines are dissolved before it is added to the semiconductor solution.

The dissolution of the metal allows a better concentration control of the reactants and a targeted stoichiometric adjustment of the doping reaction.

The doping reaction takes place spontaneously in the resulting mixture. Preferably, the mixture is incubated at a temperature of 10 ° C to 80 ° C, more preferably at room temperature of 15 ° C to 30 ° C, for 5 min to 120 min. Subsequently, the n-doped semiconductor solution is separated under an inert gas atmosphere. Optionally, a separation or purification step (filtration, chromatography, etc.) can take place. Since the doping reaction is usually reversible in the air, it is advantageous if the semiconductor solution is stored in oxygen-free and anhydrous environment and further processed.

Preferably, in this process, as the organic semiconductor, BPhen (4,7-diphenyl-1,10-phenanthroline), TPBi (2,2 ', 2 "- (1,3,5-benzenetriyl) tris-1-phenyl-1H benzimidazole), TAZ (3- (4-biphenyl) -4-phenyl-5-tert-butylphenyl-1,2,4-triazole) or BuPBD (2- (biphenyl-4-yl) -5-phenyl-1 , 3,4-oxadiazole).

In a preferred embodiment of the method, the molar ratio of semiconductor molecule to alkali metal is set from 100: 1 to 1:10, more preferably from 10: 1 to 1: 3.

The preparation obtained, ie the solution of an n-doped low molecular weight organic semiconductor, can subsequently be applied directly to a substrate in order to produce amorphous and / or polycrystalline and / or monocrystalline layers of low molecular weight n-doped organic semiconductor material. The morphology of the layer obtained depends primarily on the semiconductor molecule used.

Amorphous here means a test material that does not form ordered structures, i. H. the n-type semiconductor molecules do not form a crystal structure. Polycrystalline layers mean an irregular accumulation of crystallites that grow to contact and prevent each other from forming planar interfaces. It can also be a mixed layer of amorphous and partially crystalline morphology, which sets a solid with different near and distant order.

The substrate in the context of the present invention means a material having a substantially planar or curved surface which serves as a support for the semiconductor material. In this context, the material may be, for example, glass, a foil or a wafer.

Preferably, the substrate is a semiconductor device. This semiconductor component is particularly preferably an organic solar cell, an organic photodetector, an organic light-emitting diode, an organic thermoelectric generator, an organic laser or an organic field-effect transistor.

The term application describes in this context the application of the solution to the substrate and the subsequent evaporation of the solvent, so that a homogeneous layer of the doped semiconductor is formed on the substrate.

Preferred application methods are printing (inkjet, gravure printing, flexographic printing, screen printing, etc.) or coating (doctoring, spin-coating, slot casting, etc.). The process parameters depend on the particular method used and can be adjusted accordingly by the person skilled in the art. Furthermore, process additives can be used to increase the layer homogeneity.

It should be emphasized at this point that semiconductor layers can be produced in a simple manner with these application methods. In contrast to the vacuum deposition processes, this application method can also be integrated in so-called roll-to-roll processes.

The thus prepared amorphous, poly or monocrystalline or mixed ordered layer of n-doped organic low molecular weight semiconductors is also part of the present invention. It is homogeneous in its structure and in its electrical properties. A more homogeneous doping is easier to produce by the method according to the invention compared to diffusion processes.

Preferably, this amorphous, poly or monocrystalline or mixed ordered thin layer has a layer thickness of 10 nm to 1000 nm.

The present invention further relates to the use of a layer of low molecular weight n-doped organic semiconductor material, which was prepared by the above method as a charge carrier transport or matching layer in organic solar cells, as an electron injection layer in organic Light emitting diodes, in organic photodetectors in organic thermoelectric generators, in organic lasers or as semiconductor material in organic field effect transistors (OFET).

The invention will be explained below with exemplary embodiments and the following figures.

1 Absorption spectra of Na-doped and undated TPBi.

2 Architecture or layer sequence of an organic solar cell

3 Current density-voltage characteristic of an organic solar cell on the cathode side without n-type matching layer (without), with undoped matching layer (0 vol%) and with n-doped matching layer (5 vol%).

4 Schematic structure of a tandem solar cell.

5 Layer sequence of an organic light-emitting diode.

6 Characteristics and efficiencies of the OLEDs mentioned in Example 3, a) current density-voltage characteristic, b) luminous efficacy over luminance, c) luminance over the current density, d) current efficiency over the luminance.

Example 1: Preparation of an n-doped low molecular weight compound

• a) im ungelösten Zustand: Eintauchen des Alkalimetalls in die organische Halbleiter (OHL) Lösung. Auch ein Übergießen des Alkalimetallstückes mit der Halbleiter-Lösung führt zu einer verwertbaren Dotierung der Lösung. Dabei wird die Alkalimetalloberfläche oxidiert und die organischen Halbleiter Moleküle reduziert. Die Dotierkonzentration des organischen Halbleiters wird über die Größe und die Kontaktzeit des Alkalimetallstücks mit der organischen Halbleiter-Lösung eingestellt. Durch Rühren der Lösung kann der Prozess beschleunigt werden.
• b) im gelösten Zustand: Ein Alkalimetallstück wird in einem Lösungsmittel gelöst, das Alkalimetalle lösen kann (z. B. Pyridin, Ammoniak). Ggf. ungelöste Alkalimetallrückstände können anschließend z. B. mit einer Zentrifuge oder einem PTFE-Filter herausgefiltert werden. Die Alkalimetall-Lösung wird dann einer OHL Lösung hinzu gegeben. Auf diese Weise lässt sich die Dotierkonzentration einfacher einstellen als im Fall (a). Die im 2. Schritt erhaltene n-dotierte OHL Lösung lässt sich nun z. B. durch geeignete Applikationsverfahren wie Drucken (Tintenstrahl, Tiefdruck, Flexodruck, Siebdruck, etc.) oder Beschichten (Aufschleudern, Rakeln, Schlitzgießen, etc.) zu einer Schicht verarbeiten und somit in ein organisches Halbleiterbauelement integrieren. Die Prozessparameter hängen dabei von dem jeweils verwendeten Verfahren ab.

An organic semiconductor is dissolved in a suitable anhydrous solvent capable of dissolving the organic semiconductor (OHL) at the desired concentration. Subsequently, an alkali metal is introduced into this OHL solution, whereby two alternative procedures are possible.

• a) in the undissolved state: immersion of the alkali metal in the organic semiconductor (OHL) solution. A pouring over of the alkali metal piece with the semiconductor solution also leads to a usable doping of the solution. The alkali metal surface is oxidized and the organic semiconductor molecules are reduced. The doping concentration of the organic semiconductor is adjusted by the size and contact time of the alkali metal piece with the organic semiconductor solution. By stirring the solution, the process can be accelerated.
• b) in the dissolved state: An alkali metal piece is dissolved in a solvent capable of dissolving alkali metals (eg, pyridine, ammonia). Possibly. undissolved alkali metal residues can then z. B. be filtered out with a centrifuge or a PTFE filter. The alkali metal solution is then added to an OHL solution. In this way, the doping concentration can be set more easily than in the case (a). The obtained in the 2nd step n-doped OHL solution can now be z. B. by suitable application methods such as printing (ink jet, gravure, flexographic printing, screen printing, etc.) or coating (spin-coating, knife coating, slot casting, etc.) to process a layer and thus integrate into an organic semiconductor device. The process parameters depend on the particular method used.

The doping of the OHL in solution or after the deposition in the solid can be recognized, for example, in the absorption spectrum. 1 shows by way of example the normalized absorption spectra of the OHLs 2,2 ', 2''- (1,3,5-benzene triyl) tris (1-phenyl-1-H-benzimidazoles) (TPBi) in various solvents. The doping with the alkali metal sodium (Na) according to both methods described above produces an n-doping with the formation of typical polaron bands whose energy position varies as a result of the interaction with the solvents. The exact location depends on the interaction of the OHL and the alkali metal with the solvent. The doping of the OHL with solid (s) or dissolved in pyridine (f) alkali metal (here sodium) leads in both cases to form the same absorption bands.

In further experiments z. Example, the formation of polaron bands in the doping with potassium or in the case of 4,7-diphenyl-1,10-phenanthroline (BPhen) additionally be shown with lithium by the method described above. A similar change in absorption is also observed in the deposited layers (solid). This is a proof that the doping is retained even after the deposition. Exposing the solutions and layers to air (oxygen) causes a dedoping of the OHLs. This manifests itself in a decolorization of the solutions and layers or the recovery of the absorption spectrum to the absorption spectrum of the intrinsic OHL.

Example 2: Organic solar cells

In 2 the schematic structure of an organic solar cell is shown. Light enters through a semitransparent electrode and generates strongly bound, charge-neutral electron-hole pairs (excitons) that can not dissociate into free charge carriers by thermal excitation alone. Therefore, one resorts to the absorber layer of organic solar cells (OSC) mostly on combinations (mixtures and multilayers) of z. B. conjugated polymers such as poly (3-hexylthiophene-2,5-diyl) (P3HT) as a donor and z. B. [6,6] -phenyl-C61-butyl acid methyl ester (PCBM) as acceptor back. With sufficient energy, the incident photon excites an electron from the HOMO into the LUMO of the donor. At the donor / acceptor interfaces, exciton dissociation may occur if the LUMO energy level of the donor is at least the binding energy of the exciton above the LUMO of the acceptor. The electrons are then passed over the acceptor material to one electrode ("cathode") while the holes in the donor travel to the other electrode ("anode"). In order to improve the electrical contact to the electrode and thus better dissipate the charge carriers from the active layer, electrically (n- and p-) doped semiconductor matching layers are used.

a) Preparation of a cell with n-doped matching layer:

A glass substrate coated with indium tin oxide (ITO) as a semitransparent cathode was patterned by etching and then cleaned. The n-doped matching layer used was sodium doped 2,2 ', 2 "- (1,3,5-benzene triyl) tris (1-phenyl-1-H-benzimidazole) (TPBi). TPBi was dissolved in pyridine at room temperature at a concentration of 5 mg / ml. The alkali metal sodium was dissolved in pyridine in an amount sufficient for doping at room temperature during one day under a nitrogen atmosphere. The resulting sodium solution was freed from any undissolved residues with a filter prior to mixing with the TPBi solution. Subsequently, the mixture of the two solutions in the ratio of the desired doping concentration, in this case by adding 5 vol% sodium solution in the TPBi solution.

The TPBi, absorber and p-doped layers were applied in this example directly from the solution by spin coating. The layer thickness of the doped TPBi layer was adjusted to 20 nm by the suitable choice of the spin-on parameters. The absorber layer consisting of a mixture of P3HT: PCBM at 40 mg / ml was dissolved in 1,2-dichlorobenzene. Poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) was used as the p-doped matching layer.

In 3 the current density-voltage characteristics of the processed solar cells are shown. Without an adaptation layer on the cathode side (without), a contact barrier is formed, which causes a reduction of the open circuit voltage and a deformation of the diode behavior of the solar cell due to the additional contact resistance. This effect can not be eliminated with an undated TPBi layer (0% by volume). Only by n-doping (5 vol%), the open-circuit voltage can be increased and the deformation of the diode characteristic can be eliminated. The doping was carried out by adding sodium dissolved in pyridine to the TPBi solution.

From the example described above, the insert, the layers produced by the described doping method, is derived as n-doped matching layers. By using alkali metals with a low ionization potential, organic semiconductors with a relatively low electron affinity can be doped from the liquid phase, which are necessary for an energetic adaptation of the LUMO energy levels of common acceptor materials to the ionization potential of stable metal electrodes. The use of reactive electrode materials for electron extraction such. B. Calcium can thus be bypassed.

b) Tandem solar cell:

4 shows the schematic structure of an organic tandem solar cell, a monolithic interconnection of two organic solar cells. For an interconnection of the tandem solar cell, a so-called recombination zone of a p- and an n-doped semiconductor is required, in which electrons and holes from the two solar cells can recombine (lossless). In the course of process development for a completely liquid processable (eg printable) tandem solar cell, an n-doped buffer layer can be applied to a red-absorbing OHL layer by the method presented here, for B. according to the process described in Example 2a). This is followed by a p-doped buffer layer of, for example, PEDOT: PSS and then the second solar cell.

Example 3: Organic Light Emitting Diodes

Holes are injected at the anode side of an organic light-emitting diode (OLED) and electrons are injected at the cathode side into the emitter layer. For this purpose, the charge carriers must thermally overcome an energy barrier ΔΦ or tunnel through quantum mechanically. The charge carriers move via a hopping transport through the emitter material and form excitons, which can then radiantly recombine. However, such an OLED is inefficient because of the energy barrier to be overcome in the injection. Efficient OLEDs therefore do not only consist of these three layers (anode, emitter, cathode), but are optimized in terms of the charge carrier transport in their structure as in 5 shown. The sequence of layers is reversible. Under the anode, an HIL (hole injection layer) and applied to the cathode an EIL (English: Electron Injection Layer). These injection layers serve to adapt the ionization potential of the corresponding electrode to the HOMO or LUMO level of the emitter, and thus to reduce the injection barrier of the charge carriers. The injection layer is followed by a charge transport layer HTL (narrow hole transport layer) or ETL (narrowed Electron Transport Layer). In order to prevent the charge carriers from passing through the emitter layer without forming excitons, an electron block layer EBL (hole blocking layer) is inserted between the emitter layer and HTL / ETL. In practice, the number of layers can often be reduced because one layer performs several tasks.

manufacturing

The electron injection layer was realized by the n-doped OHL layers according to the invention.

A glass substrate with sputtered and patterned ITO as the cathode is cleaned and deposited with a sodium-doped TPBi layer of solution. The doping was carried out beforehand by adding 15% by volume of a pyridine solution saturated with sodium and filtered into a 5 mg / ml solution of TPBi dissolved in pyridine. The following layers can either be deposited by suitable solution-based processes or evaporated in vacuo. In this example, the following layers are vacuum applied in the following order: The emitter consists of 4,4'-bis (N-carbazolyl) -1,1'-biphenyl (CPB): Tris [2-phenylpyridinato-C2, N ) iridium (III) (Ir (ppy) 3). As an electron block layer, N, N'-bis (naphthalen-1-yl) -N, N'-bis (phenyl) -2,2'-dimethylbenzidine (NPD) and as a hole injection layer 4,4 ', 4' Tris (N-3-methylphenyl-N-phenylamino) triphenylamine (MTDATA): 7,7,8,8-tetracyano-2,3,5,6-tetrauoroquinodimethane (F4TCNQ) sublimated.

6 shows the optoelectronic characteristics of the OLED (TPBi: sodium) thus prepared in comparison to a reference with a liquid-processed, undated TPBi layer (TPBi). The brightness and efficiency of the OLED is here in the case of the sodium-doped compared to the undated injection layer by an order of magnitude higher. The threshold voltage is lowered. Without an injection layer between the ITO cathode and the emitter, the OLED does not light up and has an ohmic resistance behavior.

Example 4: Thermoelectric Generators

Thermoelectric generators (TEGs) are used to convert thermal energy into electrical energy. They use alternating p- and n-doped semiconductor structures that are exposed to a temperature difference. The majority charge carriers of the doped semiconductors diffuse to the colder side of the device and recombine there. In this way, a voltage is built up, which can be tapped from the outside. The n-doped material according to the invention can also be used in thermoelectric generators.

With the use of the n-doping of organic semiconductors according to the invention with metals from the liquid phase, it is possible to adjust the doping level of the required n-doped semiconductors and to provide a method for large-area liquid-phase deposition.

Example 5: Humidity sensor

All investigated alkali metals doped organic semiconductors (BPhen, TPBi) show due to the forming polaron bands in the band gap of the semiconductor discoloration of previously clear semiconductor solutions. This discoloration is reversible upon contact with water. The discoloration takes place in air due to the humidity within a few seconds and can thus be used as a moisture sensor use. The applied layer shows a discoloration without contact with water, which regresses in air. Thus, the material according to the invention comes as a disposable sensor for moisture determination in question. Technically, this sensor can be realized by introducing small doped amounts of solution that lose their previously characteristic color after water or oxygen contamination in the gas-tight sealed volume to be monitored. It is also conceivable to administer the n-doped OHL in solid form (as an applied layer or in a silica gel soaked in it).

In the case of a solid, the change in doping and thus the contamination by oxygen or water can also be detected by measuring the electrical resistance of the layer, since a change in the doping also brings about a change in the conductivity of the OHLs. In this way, the sensors can be integrated directly into electrical circuits.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4204216 [0005]

Cited non-patent literature

• MY Hua, GW Hwang, YH Chuang, SA Chen, 2000, Macromolecules 33, 6235-38 [0006]

Claims (11)

A method for producing a solution of n-doped organic low molecular weight semiconductors with alkali metals, wherein the ionization potential of the alkali metal is smaller than electron affinity of the semiconductor, comprising the following steps: a) dissolving an organic semiconductor in an aprotic solvent, b) adding an alkali metal or a solution of an alkali metal in a suitable solvent to this semiconductor solution, c) Optional separation or purification of this n-doped semiconductor solution under an inert gas atmosphere. The process of claim 1, wherein the semiconductor is BPhen (4,7-diphenyl-1,10-phenanthroline), TPBi (2,2 ', 2 "- (1,3,5-benzenetriyl) tris-1-phenyl-1H benzimidazole), TAZ (3- (4-biphenyl) -4-phenyl-5-tert-butylphenyl-1,2,4-triazole) or BuPBD (2- (biphenyl-4-yl) -5-phenyl-1 , 3,4-oxadiazole). The method of claim 1 or 2, wherein the alkali metal is added in step b) dissolved in pyridine or ammonia to the semiconductor solution. A method according to any one of claims 1 to 3, wherein the molar ratio of semiconductor to alkali metal is from 100: 1 to 1:10. Method for producing thin layers of low molecular weight n-doped organic semiconductor material comprising the steps: a) Preparation of a solution according to the method steps according to one of claims 1 to 4. b) applying this solution to a substrate. The method of claim 5, wherein the applying in step b) is a printing or coating process. The method of claim 5 or 6, wherein the substrate is a semiconductor device. The method of claim 7, wherein the semiconductor device is an organic solar cell, an organic light emitting diode, an organic thermoelectric generator, an organic laser, an organic photodetector or an organic field effect transistor. Thin layer of low molecular weight n-doped organic semiconductor material prepared by a process according to claim 5 to 8. A thin layer of low molecular weight n-doped organic semiconductor material according to claim 9, wherein the material has a layer thickness of 1 nm to 1000 nm. Use of a layer of low molecular weight n-doped organic semiconductor material according to claim 9 or 10 as charge carrier transport or adaptation layer in organic solar cells, as electron injection layer in organic light emitting diodes, in organic photodetectors in organic thermoelectric generators, in organic lasers or as semiconductor material in organic field effect transistors.

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