WO2020035481A1

WO2020035481A1 - Forming thin, crystalline layers from organic molecules having planar molecular structure in lying orientation

Info

Publication number: WO2020035481A1
Application number: PCT/EP2019/071682
Authority: WO
Inventors: Giuliano DUVA; Alexander HINDERHOFER; Frank Schreiber
Original assignee: Eberhard Karls Universität Tübingen
Priority date: 2018-08-15
Filing date: 2019-08-13
Publication date: 2020-02-20

Abstract

The aim of the invention is to form a crystalline layer from organic molecules on a surface of a substrate. This aim is achieved, according to the invention, in that the substrate having the surface to be coated is arranged in a vacuum chamber and is cooled to a deposition temperature in the range of -50°C to -273 °C. A substance composed of organic molecules, which, in a planar region, have a planar molecular structure and which can be arranged on the surface in a lying crystalline arrangement which is stable at room temperature and a standing crystalline arrangement which is stable at room temperature, is at least partially evaporated or sublimed, the molecules thus being converted to the gas phase, followed by deposition of the molecules on the cooled surface. After the deposition, the substrate is heated, starting from the deposition temperature, at a heating rate of at most 10°C/min to a maximum temperature greater than 0°C and less than the melting or sublimation temperature of the substance. The substrate thus coated is characterized in that the molecules are present on the substrate in the stable lying crystalline arrangement.

Description

Formation of thin, crystalline layers from organic molecules with a planar molecular structure in a lying orientation

The invention described below relates to a method for forming a crystalline layer of organic molecules on a surface of a substrate and the coated substrate obtained according to the method.

Understanding growth phenomena is of great importance for many technical applications, including different single-crystal systems and thin layers. The growth of molecular systems is of particular interest. In contrast to atomic systems, not only adsorption states but also molecular orientations have to be taken into account. If molecules such as diindenoperylene (DIP) are deposited on surfaces, they can assume a lying and a standing orientation on the surfaces, as described, for example, by Ko- warik et al. in Phys. Rev. Lett. 96, 125504 (2006) and in Appl. Phys. A, 95, 2009, 233-239. DIP has a planar, elongated aromatic molecular structure derived from perylene in a planar region. Through the plane of this planar area, a C2 axis can be placed lengthways and crossways. The longer of the two axes, the longitudinal axis of the molecule, forms a larger angle with the surface in the standing orientation than in the lying orientation. Kowarik et al. observed that DIP molecules can undergo a structural transition during the growth of a DIP layer, which may be associated with a change in this angle. In this regard, a dependency on the respective substrate as well as on the temperature and the growth rate was described. In many cases, the formation of mixed forms was observed, in which DIP occurred in a lying and in a standing orientation.

The formation of horizontal and vertical orientations can be of practical relevance, in particular in the formation of thin layers of organic semiconductor molecules, for example in the construction of organic solar cells or organic field effect transistors. As a rule, the optical properties of the layers largely depend on the orientation of the molecules. Conversely, this means that the optical properties of a layer made of organic semiconductors should be controllable through a targeted adjustment of the orientations. There is accordingly a need for a procedure which enables the targeted production of thin layers of organic semiconductor molecules in one of the orientations mentioned, that is to say lying or standing.

To achieve this object, the invention proposes the method with the features mentioned in claim 1. The substrate with the features of claim 10 is also encompassed by the invention. Developments of the invention are the subject of dependent claims.

The method according to the invention surprisingly represents a generic approach to the formation of a crystalline layer of organic molecules in a lying orientation on a surface of a substrate. It always comprises the immediately following six steps a. to f .: a. Providing the substrate with the surface to be coated in a vacuum chamber, b. Providing a substance from organic molecules that

have a planar molecular structure in a planar region

and can be arranged on the surface in a crystalline arrangement which is stable at room temperature and in a standing crystalline arrangement which is stable at room temperature,

in the vacuum chamber or a secondary chamber coupled to it, c. Cooling the substrate to a deposition temperature in the range of -50 ° C to -273 ° C, d. at least partial sublimation or evaporation of the substance with conversion of the organic molecules into the gas phase, e. Depositing the organic molecules from the gas phase on the cooled surface of the substrate, f. Heating the substrate with the coated surface, starting from the deposition temperature, with a heating rate of a maximum of 10 ° C / min to a maximum temperature> 0 ° C and <the melting or sublimation temperature of the substance.

The vacuum chamber is generally coupled to a high vacuum source, with the aid of which a vacuum of at least 10 ⁷ bar can be set in the vacuum chamber. A corresponding structure, in particular also an example of a suitable cooling system, was developed, for example, by Ritley et al. in Rev. Instr. 72 (2001), 1453. For the purpose of sublimation or evaporation, the substance can in principle be heated by means of any heating sources. Electrical heating is particularly preferably used, for example a heating wire installed in the secondary chamber. If the substance is provided in the secondary chamber, it may be preferred to provide a valve or a lock between the vacuum chamber and the secondary chamber in order to be able to better control the concentration of sublimed or vaporized organic molecules in the vacuum chamber. As soon as the sublimed or evaporated molecules can come into contact with the cooled substrate in the vacuum chamber, the deposition takes place all by itself, they are “captured” by its cooled surface.

The generic applicability mentioned means that the process result is fundamentally independent of the material properties of the substrate and the substance used. Surprisingly, the crystalline, lying orientation is always obtained after the heating process. This was by no means to be expected a priori. In general, it is true that the lying orientation at room temperature is not thermodynamically preferred. However, it cannot be inferred from this that the molecules used always align molecules in the lying arrangement at low temperatures.

Depending on the substance used, the deposited molecules usually have a so-called herringbone packing (face-to-edge configuration) in the crystalline layer after the heating step or they are stored in layers arranged parallel to one another (face-to-face Configuration). This can be controlled by an appropriate selection of the substance. Different crystalline configurations (polymorphs) are usually observed for the same molecule, which is why the crystal configuration can possibly be controlled by the treatment parameters, usually the maximum heating temperature.

In the simplest case, the planar region mentioned is a benzyl radical, more preferably a condensed aromatic ring system such as, for example, a naphthyl or anthracyl radical, or a larger condensed ring system as in the case of the DIP mentioned at the outset. Heteroaromatics with furan, thiophene, pyridine or pyrrole groups are also suitable. In particular preferred substances will be discussed below.

A distinction between the lying and the standing arrangement is preferably made by means of X-ray structure analysis. As mentioned at the beginning, in the case of DIP the longitudinal axis of the molecule closes when it is stationary Orientation a larger angle with the surface than with lying orientation. However, the respective angle can vary, especially depending on the substance used. The general rule is that straight lines can be laid in the plane of each planar area that form an angle with the surface. In the case of symmetrical molecules, these straight lines can be axes of rotation. There is a straight line for both the lying and the standing orientation, in which the respective angle that the straight line forms with the surface is maximum. In the case of DIP, this is the longitudinal axis of the molecule. In general, the maximum angle that a straight line forms with the surface in the case of a standing orientation is greater than in the case of a lying orientation.

The range given above for the deposition temperature is very broad. As a rule, cooling the substrate down to areas of absolute zero is neither necessary to achieve the process result nor is it expedient for cost reasons. Ideally, the deposition temperature should be chosen so that it can still be achieved with the help of liquid nitrogen. It is therefore preferably in the range from -50 ° C to -196 ° C. Within this range, ranges from -80 ° C to -196 ° C, in particular from -80 ° C to -160 ° C or from -80 ° C to -120 ° C, are further preferred.

Slow heating in step f. serves in particular to suppress undesirably strong diffusion processes. A too high heating rate above the 10 ° C / min is not conducive to the uniformity of the arrangement formed. In most cases, the heating rate is chosen even lower. It is preferably a maximum of 5 ° C / min, particularly preferably a maximum of 4 ° C / min, in particular a maximum of 2 ° C / min.

For obvious reasons, the maximum temperature must not exceed the sublimation or melting temperature of the substance used. It is preferably at least 10 ° C. below the sublimation or melting temperature of the substance used in each case, particularly preferably at least 25 ° C., in particular at least 50 ° C. Maximum temperatures in the range from 40 ° C. to 180 ° C. are preferred. Within this range, maximum temperatures in the range from 60 ° C. to 180 ° C. or in the range from 100 ° C. to 180 ° C., in particular in the range from 120 ° C. to 160 ° C., are further preferred.

If the parameters mentioned are observed, the coated substrate is characterized by a lying crystalline arrangement of the deposited molecules, which is stable even at room temperature and ambient air. In preferred developments, the method is characterized by at least one of the five immediately following additional features a. to e. from: a. The surface of the substrate on which the crystalline layer of organic molecules is formed is planar; the surface is particularly preferably a planar surface of a wafer, a sensor or an electrode. b. The surface of the substrate on which the crystalline layer of organic molecules is formed has a maximum roughness depth Rzlmax of 5 nm, more preferably 4 nm. c. The surface of the substrate consists of a metal, in particular from the group with Au (gold), Ag (silver), Cu (copper) and Si (silicon) or a metal oxide, in particular from the group with SiO _x with 1> x <2 , Indium tin oxide (ITO) and aluminum oxide, in particular corundum, or a carbon modification such as graphite. d. The surface of the substrate is already precoated with organic semiconductor molecules. e. The surface of the substrate is pretreated, in particular oxidized, hydrophilized or hydrophobized, for example by means of tetratetracontane.

Features a are particularly preferred. to c. or a. to d. realized in combination with each other.

The specified values for the maximum roughness depth refer preferably to values determined in accordance with DIN EN ISO 4287: 2010-0.

The precoating with organic semiconductor molecules can be preferred in particular in the production of organic electronics or photovoltaic components. Structures are then created with two or more layers of organic semiconductors.

In a particularly preferred embodiment, the surface to be coated is a silicon surface artificially oxidized in an oxygen atmosphere.

In further preferred developments, the method is characterized by at least one of the five immediately following additional features a. to e. out: a. The substrate is fixed in the vacuum chamber on a holder which is coupled to a coolant source. b. Before the molecules are deposited, a vacuum of <10 ⁷ mbar, preferably of <5 ^* 10 ⁸ mbar, is set in the vacuum chamber. c. The suppression remains constant or becomes lower during the deposition process. d. During the deposition process, a deposition rate of not less than 0.02 nm / min and / or a deposition rate of at most 2 nm / min, in particular a deposition rate in the range from 0.02 nm / min to 2 nm / min, is set. e. The maximum temperature is maintained over a period of up to 72 h, preferably up to 12 h, particularly preferably up to 1 h, in particular up to 10 min.

At least the features a., B., D. and e., in particular all features a. to e., realized in combination with each other.

Preferred procedures for cooling the substrate are described in the publication by Ritley et al. described.

It may be preferred that the negative pressure set in the vacuum chamber is also maintained during the heating step described above. But this is not absolutely necessary. Preferably, the ingress of oxygen into the vacuum chamber should be prevented during the heating step. This can also be achieved, for example, by flooding the vacuum chamber with an inert protective gas.

A drop in the growth rate below the lower limit of 0.02 nm / min should be avoided in many cases. Too slow growth can promote the formation of standing crystalline arrays.

In further preferred developments, the method is characterized by at least one of the five immediately following additional features a. to g. from: a. The organic molecules are molecules with semiconductor properties. b. The organic molecules are aniosotropic molecules. c. The molecular structure in the subarea is an aromatic structure. d. The organic molecules are molecules with a polycyclic ring structure, in particular a polycyclic aromatic ring structure, in particular a condensed polycyclic aromatic ring structure. e. The organic molecules are molecules with a linearly condensed ring system, in particular an oligoacene from the group with anthracene, anthracene derivatives, tetracene, tetracene derivatives, pentacene, pentacene derivatives (for example quinacridone), carbazole and carbazole derivatives , f. The organic molecules are molecules with a two-dimensional condensed ring system, in particular a molecule from the group with perylene or a perylene derivative (for example DIP or 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA)), triphe- nylene (9,10-benzophenanthrene) or a triphenylene derivative, coronene or a coronene derivative (eg hexabenzocoronene). G. The organic molecules are molecules with a molecular weight of up to 750 g / mol.

At least the features a., B., C. Are particularly preferred. and e. or a., b., c. and f. realized in combination with each other.

In some particularly preferred embodiments, the substance may also be rubrene or a rubrene derivative, sexithiophene or a sexithiophene derivative or a polythiophene a polythiophene derivative, the latter in particular with a chain length of up to 8 molecules, in particular with one Chain length from 5 to 8 thiophene molecules.

The molecular weight of the organic molecules is fundamentally not critical in the context of the present invention. However, the molecules should be sublimable or evaporable. This is usually the case with the preferred upper limit of 750 g / mol.

According to the invention, particular preference is given to molecules with a linearly condensed ring system in accordance with feature e above. and a molar mass in the range from 200 g / mol to 750 g / mol, more preferably in the range from 220 g / mol to 600 g / mol. In further particularly preferred embodiments, molecules with a two-dimensional condensed ring system according to feature f. and a molar mass in the range from 220 g / mol to 750 g / mol, more preferably in the range from 250 g / mol to 600 g / mol.

As stated above, the configuration in which the deposited molecules are present in the crystalline layer can be controlled by an appropriate selection of the substance used in the process according to the invention. A face-to-edge configuration can be obtained with DIP, for example, and a face-to-face configuration with PTCDA.

The substrate obtainable according to the described method is distinguished by a surface on which a crystalline layer composed of the described organic molecules is deposited, the molecules having the lying orientation likewise described.

The crystallinity of the layer can usually be proven by X-ray diffraction tests. As a result of the crystallinity, the Bragg peaks obtained, for example, by means of surface-sensitive X-ray diffraction, are broadened and are connected to one another by means of crystal truncation rods. The layer has anisotropic optical properties. Accordingly, it is particularly suitable for various optoelectronic applications.

In preferred developments, the substrate is characterized by at least one of the two immediately following additional features a. and b. from: a. The layer has a layer thickness in the range from 1 nm to 200 nm, particularly preferably in the range from 1 nm to 100 nm. b. The substrate is part of an optoelectronic device.

Both features a are particularly preferred. and b. realized in combination with each other.

Further features of the invention and advantages resulting from the invention result from the following exemplary embodiment, on the basis of which the invention is explained. The embodiment described below is used only for explanation and for a better understanding of the invention and is in no way to be understood as limiting. In a vacuum chamber which can be cooled with liquid nitrogen (analogous to the structure described by Ritley et al. in Rev. Sei. Instr. 72 (2001), 1453), a silicon substrate which has a naturally oxidized surface is bonded with a sufficient amount of adhesive Quantity of conductive silver varnish fixed on a sample holder. The chamber is then evacuated using a high vacuum pump. A suppression of 1 -2 ^* 10 ⁸ mbar is set. The sample holder and thus also the substrate can be cooled with the liquid nitrogen via pipe connections. Cooling is initiated and continued until the temperature of the substrate has stabilized at -150 ° C.

In a further step, "Diindenoperylene" (DIP) is deposited by means of "Organic Molecular Beam Deposition" (OMBD) on the oxidized surface of the cooled substrate. For this purpose, an auxiliary heater is used to sublimate DIP in a secondary chamber coupled to the vacuum chamber. When in contact with the substrate, DIP molecules in the gas phase suddenly lose their kinetic energy due to the low substrate temperature, which means that any diffusion processes on the substrate are minimized. The deposition process continues until a layer of DIP molecules of approximately 15 nm has deposited on the surface.

The crystalline arrangement of molecules captured in this way on the substrate surface is initially very small, and the deposited layer may even be completely amorphous at this point in time. There now follows a heating step in which the coated substrate is electrically heated at a heating rate of 2-3 ° C / minute. The heating is carried out until a temperature of 140 ° C is reached. The layer is gradually being provided with more and more thermal energy. Diffusion processes are then used in a controlled manner and the molecules can orient themselves in the desired crystalline arrangement, e.g. can be demonstrated with the help of X-ray examinations.

It is not only surprising that the DIP molecules always orient themselves spontaneously in the horizontal orientation. Rather, it is also surprising that the molecules maintain the described orientation in spite of the heating step. It is believed that the conversion to a standing arrangement is kinetically hindered. With the slow warm-up process, the molecules are not provided with enough thermal energy for this.

In summary, it can be stated that according to the method described, the formation of layers of organic molecules can be induced on substrate surfaces at very low temperatures, the molecules always aligning themselves in a lying orientation and ideally the layers are initially largely amorphous, possibly even completely amorphous, immediately after the deposition. The subsequent heating creates a gradual transition to a completely crystalline structure, while the lying orientation remains. Both the crystal structure and the horizontal orientation remain unchanged at room temperature and when in contact with room air.

A vivid illustration of the possible orientation control and induced crystalline arrangement according to the described invention can be found in FIG. 1.

The upper half of FIG. 1 shows a scattering pattern (left, la, measured) of a crystalline layer of DIP molecules in a standing arrangement and a comparison of optical properties (right, lb, from the publication by U. Heinemeyer et al. Phys. Rev. B 78 (2008), 085210) crystalline layers with DIP molecules in a standing orientation. The scatter pattern can be simulated with the "standing-oriented" DIP unit cell: the three-digit numbers represent the Miller indices for the designation of the different crystalline levels. The "out-of-plane" (ie perpendicular to the substrate) component of the imaginary part of the dielectric function is much stronger than the "in-plane" (ie parallel to the substrate) component: this is due to the orientation of the transition dipole moment ("mu") of the single molecule.

The lower half of FIG. 1 shows a scatter pattern (left, l.c, measured) of a crystalline layer of DIP molecules in a lying arrangement and a comparison of optical properties (right, l.d) of crystalline layers with DIP molecules in a lying orientation. The scatter pattern differs significantly from that shown in the upper half of FIG. 1. This can be explained by the horizontal orientation of the DIP molecules. The optical properties are reciprocal to those of crystals made from standing DIP molecules. The "in-plane" (i.e. parallel to the substrate) component of the imaginary part of the dielectric function is much stronger here than the "out-of-plane" (i.e. perpendicular to the substrate) component.

Claims

claims

1. A method for forming a crystalline layer of organic molecules on a surface of a substrate with the steps a. Provision of the substrate with the surface to be coated in a vacuum chamber, b. Providing a substance from organic molecules that

• have a planar molecular structure in a planar region

in the vacuum chamber or a secondary chamber coupled to it, c. Cooling the substrate to a deposition temperature in the range from -50 ° C to -273

° C, d. at least partial sublimation or evaporation of the substance with conversion of the molecules into the gas phase, e. Depositing the organic molecules from the gas phase on the cooled surface of the substrate, f. Heating the substrate with the coated surface, starting from the deposition temperature, with a heating rate of a maximum of 10 ° C / min to a maximum temperature> 0 ° C and <the melting or sublimation temperature of the substance.

2. The method of claim 1 with the following additional feature: a. The substrate with the coated surface is heated to a maximum temperature in the range from 40 ° C to 180 ° C.

3. The method of claim 1 with the following additional feature: a. The substrate with the coated surface is heated to a maximum temperature in the range from 100 ° C to 180 ° C.

4. The method of claim 1 with the following additional features: a. The surface of the substrate on which the crystalline layer of organic molecules is formed is planar. b. The surface of the substrate on which the crystalline layer of organic molecules is formed has a maximum roughness depth Rzlmax of 5 nm.

5. The method of claim 1 with the following additional feature: a. The surface of the substrate consists of a metal, in particular from the group with Au (gold), Ag (silver), Cu (copper) and Si (silicon) or a metal oxide, in particular from the group with SiOx with 1> x <2, Indium tin oxide (ITO) and aluminum oxide, in particular Ko-rund, or a carbon modification such as Graphite.

6. The method of claim 1 with the following additional feature: a. The surface of the substrate is already precoated with organic semiconductor molecules. b. The surface of the substrate is pretreated, in particular oxidized, hydrophilized or hydrophobized.

7. The method according to any one of the preceding claims with at least one of the following additional features: a. The substrate is fixed in the vacuum chamber on a holder which is coupled to a coolant source. b. Before the molecules are deposited, a vacuum of at least 10 ⁷ mbar, preferably of at least 5 ^* 10 ⁸ mbar, is set in the vacuum chamber. c. The suppression remains constant or becomes lower during the deposition process. d. The maximum temperature is maintained over a period of up to 72 h, preferably up to 12 h, particularly preferably up to 1 h, in particular up to 10 min.

8. The method according to any one of the preceding claims with at least one of the following additional features: a. A deposition rate in the range of 0.02 nm / min to 2 nm / min is set during the deposition process.

9. The method according to any one of the preceding claims with at least one of the following additional features: a. The organic molecules are molecules with semiconductor properties. b. The organic molecules are aniosotropic molecules. c. The molecular structure in the subarea is an aromatic structure. d. The organic molecules are molecules with a polycyclic ring structure, in particular a polycyclic aromatic ring structure, in particular a condensed polycyclic aromatic ring structure. e. The organic molecules are molecules with a linearly condensed ring system, in particular an oligoacene from the group with anthracene, anthropene derivatives, tetracene, tetracene derivatives, pentacene, pentacene derivatives, carbazole and carbazole derivatives. f. The organic molecules are molecules with a two-dimensional condensed ring system, in particular a molecule from the group with perylene or a perylene derivative, triphenylene or a triphenylene derivative, corones or a coronene derivative. G. The organic molecules are molecules with a molecular weight of up to 750 g / mol.

10. A substrate with a surface on which a crystalline layer of organic molecules is deposited, the molecules having a lying orientation and the layer being produced according to a method according to one of the preceding claims.

11. The substrate of claim 10 with at least one of the following additional features: a. The layer has a layer thickness in the range from 1 nm to 200 nm. b. The substrate is part of an optoelectronic device.