JP2006049578A - Organic semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for realizing an n-type organic transistor having high mobility and an organic semiconductor device using electrons as carriers and having high performance. <P>SOLUTION: The problem is solved by the organic semiconductor device. The organic semiconductor device has at least an organic semiconductor layer 40. The organic semiconductor device further has at least electrodes 2 and 3 forming a conductive charge transfer complex, by combining an electron donative molecular material with an electron acceptable molecular material having an ionizing energy the same as or similar to a molecular material constituting an organic semiconductor in the electrodes or parts of the electrodes on the sides brought into contact with the organic semiconductor, for improving the injection efficiency of electrons to the organic semiconductor as the electrodes constituting electrical contacts with the organic semiconductor layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode in an organic semiconductor device, and more particularly, an electrode serving as an electrical contact for passing an electric current to an organic semiconductor constituting the organic semiconductor device is a constituent element of a charge-transfer complex organic material having high conductivity. And an electrode for an organic semiconductor device having excellent electron injection efficiency into the organic semiconductor.

Electronic devices made of organic semiconductors are attracting attention as inexpensive alternatives to silicon semiconductor devices. In particular, an organic semiconductor device can be manufactured at a lower cost than a silicon semiconductor device that requires a process that requires a significant manufacturing cost, and is useful when economy is a priority.
Other advantages of the organic semiconductor device include that it is easy to make a large-area electronic device, that a high-temperature process is not required in the manufacturing process, and that it can be formed on a plastic substrate. Since it has characteristics such as not deteriorating element characteristics against simple bending, it is possible to manufacture a mechanically flexible electronic device having a large area, which is impossible with a silicon semiconductor device.

Among organic semiconductor devices, organic semiconductor thin film transistors have been greatly researched and developed in recent years, and various low molecular weight organic materials having a molecular weight of 1000 or less have been proposed as organic semiconductor materials constituting the organic semiconductor devices.
In particular, an organic semiconductor thin film transistor using pentacene as an organic semiconductor has obtained a P-type field effect transistor exhibiting a high hole mobility of 1 cm ² / Vs or more.
However, there are few examples of N-type field effect transistors whose carriers are negatively charged electrons, and they are not stable in the air, and there are cases where none show high electron mobility of 1 cm ² / Vs or higher. Is not fully developed.

For the above reasons, it is difficult to construct a complementary logic circuit composed of both a P-type field effect transistor and an N-type field effect transistor using only an organic semiconductor. Since complementary logic circuits are energy efficient and advantageous for miniaturization, they are indispensable in current integrated circuits. So far, N-type field effect transistors made of amorphous silicon have been used as P-type organic semiconductor field-effect transistors. Ingenuity has been made such as building in combination.
However, the method of combining N-type field effect transistors using amorphous silicon almost loses the advantages of organic semiconductor devices such as low manufacturing costs and mechanical flexibility. Realization of an N-type field effect transistor has become a major issue.

A necessary condition for realizing an N-type field effect transistor using an organic semiconductor is to realize injection of electrons from an electrode constituting an electrical contact with the organic semiconductor into the organic semiconductor with high efficiency. For this purpose, the characteristics between the organic semiconductor material used as the N channel and the highly conductive material used as the source / drain electrodes of the field effect transistor are matched, that is, a conduction band in which electrons serving as carriers in the organic semiconductor are accommodated. The Fermi level of the electrode must be matched at the semiconductor-electrode interface. However, the conduction band of an organic semiconductor usually has a small bandwidth, and it is difficult to match the Fermi level of an electrode with the conduction band of an organic semiconductor by using ordinary metal materials of limited types.
US Pat. No. 5,347,144 US Pat. No. 5,625,199 US Pat. No. 6,528,816 Appl. Phys. Lett. 82, 4581 (2003) Title: "Fabrication and characterization of C60 thin-film transistors with high field-effect mobility" Kobayashi, T. Takenobu, and S. Mori, A. Fujiwara, Y. Iwasa

  In view of the above-described conventional problems, an object of the present invention is to provide an electrode for realizing an N-type organic transistor having high mobility and a high-performance organic semiconductor device using electrons as carriers. .

  In order to solve the above-described problems, the present invention is an electrode that constitutes an electrical contact with an organic semiconductor layer that can be applied to various organic semiconductor devices, and increases the efficiency of electron injection into the organic semiconductor layer. Therefore, the charge transfer complex can be obtained by combining an electron-donating molecular material with an electron-accepting molecular material having the same or similar ionization energy as that of the molecular material constituting the organic semiconductor layer. Provided is an organic semiconductor device having at least an electrode that is obtained by forming an organic material having a high conductivity.

  The present invention also provides an organic semiconductor device wherein the molecular weight of the electron-accepting molecular material and the electron-donating molecular material is 1000 or less.

  The present invention also provides an organic semiconductor device in which the electrode has a conductivity higher than 100 Ωcm.

  The present invention also provides an organic semiconductor device in which the conductive charge transfer complex constituting the electrode is formed by a vacuum deposition method, a cast method, or a stamp method.

  Furthermore, the present invention provides an N-type organic semiconductor layer and source and drain electrodes that constitute an electrical contact between the N-type organic semiconductor layer, and the source is provided in order to increase the efficiency of electron injection into the organic semiconductor layer. And an electron-accepting molecular material in which a part of the source and drain electrodes in contact with the N-type organic semiconductor and a part of the source and drain electrodes in contact with the N-type organic semiconductor have the same or similar ionization energy as the molecular material constituting the organic semiconductor layer Provided is a field effect organic semiconductor device having at least a source electrode and a drain electrode in which a conductive charge transfer complex is formed by combining molecular materials.

  The present invention also provides an organic semiconductor device in which the N-type organic semiconductor layer is an N-type organic semiconductor single crystal layer or an N-type organic semiconductor polycrystalline layer.

  The present invention also provides a field effect organic semiconductor device further comprising a P-type organic thin film field effect transistor, and comprising a complementary logic circuit together with the N type organic thin film field effect transistor comprising the N type organic semiconductor layer.

  Furthermore, the present invention provides an organic solar cell or an organic electroluminescence diode by realizing high electron injection efficiency into the organic semiconductor layer.

  The present invention provides a high-performance organic semiconductor device such as a high-performance N-type organic thin film field effect transistor using electrons as carriers in order to enable formation of an electrode having high electron injection efficiency into an organic semiconductor material. can do.

  In general, an electronic device made of an organic semiconductor has a structure as shown in FIGS. These organic semiconductor devices include a substrate 10 made of glass, silicon, plastic, or the like. When an inexpensive or flexible device is required, a plastic substrate such as polyethylene naphthalate (PEN) is usually used.

In the organic semiconductor thin film transistor shown in FIGS. 1 and 2, a dielectric layer 20 is formed on a substrate 10. As the dielectric material, well-known materials such as silicon dioxide, silicon nitride, polyimide, polyethylene, polyparaxylylene (parylene), aluminum oxide and the like are used.
In the organic semiconductor single crystal thin film transistor shown in FIG. 3, the dielectric layer 20 is formed on the organic semiconductor single crystal layer 30.

  The organic semiconductor thin film transistor and the organic semiconductor single crystal thin film transistor have three spatially separated contacts 1, 2, and 3. The contact 1 corresponding to the gate electrode is in contact with the dielectric layer 20 via the substrate, the conductor film formed on the substrate, or directly. In the thin film transistor of FIG. 1, two spatially spaced contacts (2, 3) corresponding to the source / drain electrodes are formed directly on the dielectric layer 20. In the organic semiconductor thin film transistor of FIG. 2, the contacts (2, 3) are formed on the organic semiconductor layer 40. The thickness of the organic semiconductor layer 40 in the organic semiconductor thin film transistor of FIGS. 1 and 2 is usually about 150 nm to about 5 nm. Further, in the structure of FIG. 3 showing the organic semiconductor single crystal thin film transistor, the contacts (2, 3) are formed on the organic semiconductor single crystal layer 30.

On the other hand, the organic electroluminescence diode as shown in FIG. 4 is different from the transparent electrode layer 50 for forming a contact made of indium-tin oxide (ITO) on the substrate 10 and the hole transport layers 60 and 60 made of an organic semiconductor. It has a multilayer structure of an electron transport layer 70 made of an organic semiconductor and an electrode layer 4 that forms a contact.
Examples of organic materials constituting the organic semiconductor single crystal layer 30, the organic semiconductor layer 40, and the electron transport layer 70 include single-component organic semiconductors such as pentacene, rubrene, tetracyanoquinodimethane (TCNQ), phthalocyanine, and Alq3, or Organic charge transfer complex organic semiconductors such as dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) and bisethylenedithiotetrathiafulvalene-difluorotetracyanoquinodimethane (BEDTTTF-F2TCNQ) are used.

  The contacts (2, 3) according to the present invention correspond to the organic semiconductor single crystal layer 30, or the organic semiconductor layer 40, or the electron transport layer 70, and are made of a highly conductive charge transfer complex system that matches the material. An organic material is selected. That is, as shown in FIG. 5, the conduction band in which electrons serving as carriers in an organic semiconductor are accommodated is composed of the lowest unoccupied molecular orbitals (LUMO) of organic molecules as constituent elements. In order to efficiently inject electrons from the electrode to the conduction band of the organic semiconductor at the organic semiconductor-electrode interface, the energy level of the Fermi surface of the material forming the electrode must match the energy level of the conduction band. There is. In order to satisfy this condition, a charge transfer complex is formed by combining an electron donating molecular material with a molecular material having the same or similar ionization energy as the molecular material constituting the organic semiconductor layer as an electrode or a part thereof. If a molecular material having the same or similar ionization energy as the molecular material composing the organic semiconductor layer is put into a partial charge transfer state, the Fermi energy level of the electrode matches the energy level of the conduction band of the organic semiconductor. In addition, since an organic material having high conductivity suitable for an electrode can be obtained, it is most suitable.

For example, when the organic semiconductor layer is composed of tetracyanoquinodimethane (TCNQ) or dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ), tetracyanoquino Charge transfer-type complex using tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), or similar substances, by combining dimethane (TCNQ) with electron-donating molecule tetrathiafulvalene (TTF) Due to partial charge transfer within the complex (TTF-TCNQ has a charge transfer amount of about 0.59, ie TTF ^+0.59 -TCNQ ^-0.59 ), the Fermi energy level of the electrode or part thereof is the lowest of TCNQ. Appropriate because it is in a conduction band consisting of unoccupied molecular orbitals (LUMO) and is a highly conductive metallic material that matches the energy level of the conduction band of the organic semiconductor layer. is there.
In the present invention, it is known that the electron-accepting molecular material and the electron-donating molecular material are advantageous for forming a thin film by vacuum deposition or the like, are easy to synthesize molecules, and can obtain high conductivity. It is more preferable for the electrode to be a molecular material. Also, the specific resistance of the electrode is more preferably higher in conductivity than 100 Ωcm in order to increase the response speed of the organic semiconductor device and reduce energy consumption.

Examples of the present invention will be described below.
Here, an organic semiconductor crystal thin film transistor that operates on exactly the same principle as an organic semiconductor thin film transistor, in particular, the channel portion is made of a single crystal, so that the reproducibility of the results is good and it is easy to determine the effect of the electrode. Examples relating to the organic semiconductor single crystal thin film transistor having the structure 3 will be described.

Dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) single crystal, which is an organic semiconductor single crystal used in this example, is composed of each raw material molecule, that is, dibenzotetrathiafulvalene (DBTTF) molecule and tetracyanoquinodimethane. (TCNQ) molecules were obtained by sublimation together in a glass tube filled with 20 mbar nitrogen gas.
The charge transfer complex with extremely high conductivity used as an electrode was prepared by dissolving tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) in an organic solvent such as acetonitrile, and then mixing the two solutions. A complex sample was obtained as a powder sample.

The organic semiconductor single crystal thin film transistor shown in FIG. 3 was manufactured using the organic semiconductor single crystal and the organometallic material prepared by the above process. In the manufacture of the single crystal thin film transistor of FIG. 3, the contact channels of the contacts 2 and 3 are defined by a shadow mask, and adjusted to a flat crystal growth surface of the single crystal, tetrathiafulvalene-tetracyanoquinodimethane (TTF). -TCNQ) A thin film was formed by vacuum deposition so as to have a thickness of about 100 nm. The thin film can also be formed by a casting method or a stamp method.
The single crystal thin film transistor had a channel width of about 250 μm and a channel length of 25 to 200 μm.

  Next, a parylene polymer insulating film having a thickness of 1 μm was deposited as a gate insulating layer using a known gas phase polymerization method. Furthermore, a gate electrode was formed by vapor-depositing silver having a thickness of about 30 nm on the insulating layer, and a field effect transistor was obtained by connecting a gold wire. The DC field effect characteristics were evaluated using a semiconductor evaluation analyzer E5270 from Agilent Technologies under normal temperature and normal pressure, or by enclosing a sample in a cryostat. The electrical contact between the obtained electrode-semiconductor is far greater than that of an organic semiconductor crystal thin film transistor using a gold thin film or a silver thin film as an electrode instead of a tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) thin film. It was small.

Dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) single crystal thin film transistor with three types of single crystal thin film transistors with (a) TTF-TCNQ, (b) silver, and (c) gold source / drain electrodes 6 shows the result of measuring the gate voltage dependence in the b-axis direction with the drain voltage fixed at 5V.
The mobility was evaluated in the linear region using the standard field-effect transistor equation: μ = (dI _D / dV _G ) [L / (WC _i V _D )]. Tetrathiafulvalene-tetracyanoquinodimethane The mobility of the one using (TTF-TCNQ) as the electrode material was estimated to be 1.1 cm ² / Vs. Where C _i is the insulating capacity of the gate insulating layer, L and W are the channel length and channel width, V _G is the gate voltage, V _D is the drain voltage, I _D is the source-drain current, and μ is the mobility.
From FIG. 6, it can be confirmed that when the gate voltage is swept under a constant drain voltage, the drain current suddenly rises from near zero with the application of the positive gate voltage.

  FIG. 7 shows the results of measuring the current-voltage characteristics at various gate voltages in the organic semiconductor single crystal thin film transistor. Given the extra bulk current in the current-voltage characteristics, the results for devices using TTF-TCNQ electrodes are much better than those of typical N-type silicon field effect transistors that exhibit current saturation in the high drain voltage region. You can see that they match. On the other hand, an element using a gold or silver electrode exhibits nonlinear current-voltage characteristics. It can be seen that the feature is related to the barrier potential for electron injection at the metal-semiconductor junction.

From the above results, it can be seen that the electrode using TTF-TCNQ is very well aligned with the organic semiconductor layer and exhibits high electron injection efficiency compared to other ordinary metal electrodes.
The conduction band of the organic semiconductor crystal DBTTF-TCNQ which is the active layer is mainly composed of the lowest unoccupied molecular orbital (LUMO) of TCNQ, and electrons are injected or thermally excited into the LUMO level of TCNQ. Similar to the band diagram schematically shown in FIG. 5, since the Fermi energy of TTF-TCNQ is at the LUMO level of TCNQ, the use of the TTF-TCNQ electrode makes it efficient for the conduction band of DBTTF-TCNQ. It is considered that electron injection has become possible. Therefore, charge transfer complex materials can adjust the Fermi level of a system consisting of the same molecule from semiconducting to metallic by changing the type of the partner molecule. Because it is possible, it is the most promising material for realizing efficient carrier injection.

From the measurement result of the gate voltage dependency shown in FIG. 6, it is possible to distinguish the bulk conductivity from the field effect conductivity.
FIG. 8 shows the temperature dependence of conductivity in an activated plot (logarithm of current vs. reciprocal of temperature). In the transistor using TTF-TCNQ as an electrode, it can be seen that the activation energy (0.157 eV) of the bulk conductivity is much larger than the mobility (0.065 eV) due to the field effect. This is because the bulk conductivity depends on the number of thermally excited carriers, while the field effect conductivity nominally depends only on the number of carriers controlled by the gate voltage. From this, it can be considered that the difference in activation energy is determined by the mechanism in which the number of carriers governing conductivity is different as described above.

On the other hand, in the transistor using gold or silver as an electrode, as can be seen in FIG. 8, the activation energy of the conductivity due to the electric field effect is almost the same as the activation energy of the bulk conductivity. ing. That is, the number of carriers due to the field effect is actually derived from being much smaller than nominal due to the low electron injection efficiency resulting from the barrier potential at the metal-semiconductor junction.
From the above, it can be seen that the contact potential for electron injection is markedly suppressed in the transistor using the TTF-TCNQ electrode. That is, it can be seen that the TTF-TCNQ electrode is useful for improving the performance of an organic semiconductor device including an N-type organic semiconductor thin film field effect transistor.
In addition, said Example is for making an understanding of this invention easy, and is not limited to this Example. That is, modifications and other aspects based on the technical idea of the present invention are naturally included in the present invention.

The organic semiconductor device according to the present invention can be manufactured by a simpler method than before and can be used as an inexpensive organic semiconductor thin film transistor. In particular, it is extremely useful in manufacturing a switching device for a small and large screen display (display) device in the electronics field, or an organic semiconductor thin film field effect transistor for a complementary logic operation circuit used in its driving circuit.
In addition, since high electron injection efficiency into the organic semiconductor layer is realized, it can be used for an organic solar cell or an organic electroluminescence diode.

It is a cross-sectional schematic diagram of an organic semiconductor thin film transistor. It is a cross-sectional schematic diagram of another organic-semiconductor thin-film transistor. It is a cross-sectional schematic diagram of an organic semiconductor single crystal thin film transistor. It is a cross-sectional schematic explanatory drawing of an organic electroluminescent diode. It is a band figure of the organic-semiconductor-electrode interface at the time of using the highly conductive organic material which concerns on this invention as an electrode. It is a graph which shows the change of the drain current with respect to the change of a gate voltage in the state which applied fixed drain voltage in three types of organic-semiconductor crystal thin-film transistors. It is a graph which shows the result of having measured the current-voltage characteristic in the state which applied the fixed gate voltage in three types of organic-semiconductor crystal thin-film transistors. It is a graph which shows a mode that the electric current in a gate-off state and the electric current in a gate-on state change with temperature in three types of organic-semiconductor crystal thin-film transistors.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrical contact used as a gate electrode 2, 3 Electrical contact used as a source and drain electrode 4 Electrical contact used as an electron injection electrode 10 Substrate 20 Dielectric layer 30 Organic semiconductor single crystal layer 40 Organic semiconductor layer 50 Transparent electrode layer 60 Hole transport layer 70 made of organic semiconductor Electron transport layer made of organic semiconductor

Claims (9)

An electrode that constitutes an electrical contact between the organic semiconductor layer and the organic semiconductor layer, and in order to increase the efficiency of electron injection into the organic semiconductor, the electrode or a part of the electrode on the side in contact with the organic semiconductor Has at least an electrode in which a conductive charge transfer complex is formed by combining an electron-accepting molecular material with an electron-accepting molecular material having the same or similar ionization energy as the molecular material constituting the organic semiconductor. Semiconductor device. 2. The organic semiconductor device according to claim 1, wherein the electron-accepting molecular material and the electron-donating molecular material have a molecular weight of 1000 or less. The organic semiconductor device according to claim 1, wherein the electrode has a conductivity higher than 100 Ωcm. The organic semiconductor device according to claim 1, 2 or 3, wherein the conductive charge transfer complex constituting the electrode is formed by a vacuum deposition method, a cast method or a stamp method. A source and drain electrode constituting an electrical contact between the N-type organic semiconductor layer and the N-type organic semiconductor layer, wherein the source and drain electrodes are provided to increase the efficiency of electron injection into the N-type organic semiconductor layer. Or an electron-accepting molecular material in which a part of the source and drain electrodes on the side in contact with the N-type organic semiconductor layer has the same or similar ionization energy as the molecular material constituting the N-type organic semiconductor A field effect organic semiconductor device having at least a source electrode and a drain electrode formed by combining molecular materials to form a conductive charge transfer complex. 6. The organic semiconductor device according to claim 5, wherein the N-type organic semiconductor layer is an N-type organic semiconductor single crystal layer or an N-type organic semiconductor polycrystalline layer. 6. The field effect organic semiconductor device according to claim 5, further comprising a P-type organic thin film field effect transistor, and comprising a complementary logic circuit together with the N type organic thin film field effect transistor comprising the N type organic semiconductor layer. The organic semiconductor device according to claim 1, wherein the organic semiconductor device is an organic solar cell. 2. The organic semiconductor device according to claim 1, wherein the organic semiconductor device is an organic electroluminescence diode.