Patent Application
20090001327
The present invention relates to a doped organic semiconductor material and to an organic light-emitting diode (OLED) comprising this semiconductor material.
Since the demonstration of organic light-emitting diodes and solar cells in 1989 [C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)], devices which are composed of organic thin layers have formed the subject of intensive research. Such layers have advantageous properties for the aforementioned applications, such as, for example, efficient electroluminescence for organic light-emitting diodes, high absorption coefficients in the range of visible light for organic solar cells, cost-effective preparation of the materials and manufacture of the devices for extremely simple electronic circuits, among others. The use of organic light-emitting diodes for display purposes is already commercially important.
The performance features of (opto)electronic multilayer components are determined inter alia by the ability of the layers to transport the charge carriers. In the case of light-emitting diodes, the ohmic losses in the charge transport layers during operation are closely linked to the conductivity, which on the one hand has a direct influence on the required operating voltage but on the other hand also determines the thermal strain or stress of the component. Moreover, depending on the charge carrier concentration of the organic layers, band bending occurs in the vicinity of a metal contact, which facilitates the injection of charge carriers and can therefore reduce the contact resistance. Similar considerations in respect of organic solar cells also lead to the conclusion that their efficiency is also determined by the transport properties for charge carriers.
By doping hole transport layers with a suitable acceptor material (p-doping) and/or by doping electron transport layers with a donor material (n-doping), the charge carrier density in organic solids (and therefore the conductivity) can be considerably increased. Moreover, by analogy with experience with inorganic semiconductors, applications are to be expected which are based on the use of p- and n-doped layers in a component and which would otherwise not be conceivable. U.S. Pat. No. 5,093,698 describes the use of doped charge carrier transport layers (p-doping of the hole transport layer by adding acceptor-type molecules, n-doping of the electron transport layer by adding donor-type molecules) in organic light-emitting diodes.
To date, the following approaches are known for improving the conductivity of organic layers applied by vapour deposition:
1. Increasing the charge carrier mobility by
While there are already sufficiently strong organic dopants for p-doping (F4TCNQ), only inorganic materials such as caesium for example are available for n-doping. By using these, it has already been possible to achieve an improvement in the performance parameters of OLEDs. For example, by doping the hole transport layer with the acceptor material F4TCNQ, a drastic reduction in the operating voltage of the light-emitting diode is achieved (X. Zhou et al., Appl. Phys. Lett., 78 (4), 410 (2001)). A similar success can be achieved by doping the electron transport layer with Cs or Li (J. Kido et al., Appl. Phys. Lett., 73 (20), 2866 (1998); J.-S. Huang et al., Appl. Phys. Lett., 80, 139 (2002)).
For a long time, one significant problem with n-doping lay in the fact that only inorganic materials were available for this. However, the use of inorganic materials has the disadvantage that, due to their small size, the atoms or molecules that are used can easily diffuse into the component and therefore make it more difficult to produce in a defined manner e.g. sharp transitions from p-doped to n-doped areas.
By contrast, when organic molecules which take up a lot of space are used as dopants, diffusion should play a subordinate role since place-changing processes are possible only by overcoming relatively high energy barriers.
Particularly in the case of organic polymeric semiconductor materials, it has been known for many years that effective electron transfer from a dopant (for example sodium) to the organic matrix (for example polyacetylene) is possible only if the difference between the HOMO energy level (=ionization potential) of the dopant and the LUMO energy level (=electron affinity) of the matrix is as small as possible.
For determining the ionization potential, ultraviolet photoelectron spectroscopy (UPS) is the preferred method (e.g. R. Schlaf et al., J. Phys. Chem. B 103, 2984 (1999)). A related method, inverse photoelectron spectroscopy (IPES), is used to determine electron affinities (e.g. W. Gao et al., Appl. Phys. Lett. 82, 4815 (2003)), but this is a less established method. Alternatively, the solid potentials can be estimated by means of electrochemical measurements of oxidation potentials Eox and/or reduction potentials Ered in the solution, e.g. by means of cyclic voltammetry (CV) (e.g. J. D. Anderson, J. Amer. Chem. Soc. 120, 9646 (1998)). A number of works provide empirical formulae for converting the electrochemical voltage scale (oxidation potentials) into the physical (absolute) energy scale (ionization potentials), e.g. B. W. Andrade et al., Org. Electron. 6, 11 (2005); T. B. Tang, J. Appl. Phys. 59, 5 (1986); V. D. Parker, J. Amer. Chem. Soc. 96, 5656 (1974); L. L. Miller, J. Org. Chem. 37, 916 (1972); Y. Fu et al., J. Amer. Chem. Soc. 127, 7227 (2005). No correlation between reduction potential and electron affinity is known, since electron affinities can be measured only with difficulty. For the sake of simplicity, therefore, the electrochemical and physical energy scale are converted into one another using IP=4.8eV+e*Eox (vs. ferrocene/ferrocenium) or EA=4.8 eV+e*Ered (vs. ferrocene/ferrocenium), as described in B. W. Andrade, Org. Electron. 6, 11 (2005) (see also Refs. 25-28 therein). The conversion of various standard potentials and redox pairs is described for example in A. J. Bard, L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2nd edition 2000.
It can thus be seen from what has been mentioned above that it is at present impossible to precisely determine all the energy values, and the given values can be considered only as indications.
In the case of n-doping, the dopant acts as electron donor and transfers electrons to a matrix which is characterized by a sufficiently high electron affinity. The matrix is therefore reduced. Due to the transfer of electrons from the n-dopant to the matrix, the charge carrier density of the layer is increased. The extent to which an n-dopant is able to donate electrons to a suitable matrix with electron affinity and thereby increase the charge carrier density and thus also the electrical conductivity depends in turn on the relative situation between the HOMO of the n-dopant and the LUMO of the matrix. If the HOMO of the n-dopant is above the LUMO of the matrix with electron affinity, electron transfer can take place. If the HOMO of the n-dopant is below the LUMO of the matrix with electron affinity, electron transfer can also take place, provided that the energy difference between the two orbitals is small enough to allow a certain thermal population of the higher energy orbital. The smaller this energy difference, the higher the conductivity of the resulting layer should be. However, the highest conductivity is to be expected in the case where the HOMO level of the n-dopant is above the LUMO level of the matrix with electron affinity. The conductivity can in practice be measured and is an indication as to how well the electron transfer from the donor to the acceptor operates, provided that the charge carrier mobilities of different matrices are comparable.
The conductivity of a thin layer sample is measured using the 2-point method. In this method, contacts made of a conductive material, e.g. gold or indium tin oxide, are applied to a substrate. The thin layer to be tested is then applied over the surface of the substrate, so that the contacts are covered by the thin layer. After applying a voltage to the contacts, the current then flowing is measured. Based on the geometry of the contacts and the layer thickness of the sample, the conductivity of the thin layer material is obtained from the resistance thus determined. The 2-point method can be used when the resistance of the thin layer is much greater than the resistance of the supply lines or the contact resistance. In experiments, this is ensured by a sufficiently large spacing between the contacts and can be checked by the linearity of the current/voltage characteristic.
Studies by the inventors have shown that metal complex dopants of the structure IV
can advantageously be used as dopants for an organic matrix material, since such a dopant solves the above-described diffusion problem. For this reason, a dopant having the structure IVa
was tested as dopant for conventional electron transport materials, such as Alq3 (tris(8-hydroxyquinoline) aluminium) or BPhen (4,7-diphenyl-1,10-phenanthroline).
The gas phase ionization potential of the dopant having the structure IVa is 3.6 eV. The corresponding ionization potential of the solid can be estimated according to Y. Fu et al. (J. Am. Chem. Soc. 2005, 127, 7227-7234) and is approximately 2.5 eV.
The results are shown in Table 1 below.
As can be seen from Table 1, the conductivities achieved with the known matrix materials are still insufficient and very low.
The object of the present invention is to provide a doped organic semiconductor material, which overcomes the disadvantages of the prior art. In particular, a semiconductor material is to be provided which has an increased charge carrier density and effective charge carrier mobility and also an improved conductivity. The semiconductor material is also intended to exhibit a high level of thermal stability, which results for example from higher glass transition points, higher sublimation temperatures and higher decomposition temperatures.
An organic light-emitting diode, for which the semiconductor material according to the invention can be used, is also to be provided.
The object of the invention is achieved by a doped organic semiconductor material comprising at least one organic matrix material, which is doped with at least one dopant, the matrix material being selected from the group consisting of phenanthroline derivatives of the following structures I, II and III:
wherein:
R3 and R6 are independently selected from the group: H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl,
aryl, heteroaryl, oligoaryl, oligoheteroaryl and oligoarylheteroaryl, wherein all the sp2-hybridized carbon atoms that are not used for ring bonding may be substituted independently of one another with H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl; or combinations thereof;
R1, R2, R4, R5, R7 and R8 are not simultaneously hydrogen, but otherwise are independently selected from the group: H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20
alkyl, C1-C20 alkenyl, C1-C20 alkynyl, aryl, heteroaryl, oligoaryl, oligoheteroaryl and oligoarylheteroaryl, wherein all the sp2-hybridized carbon atoms that are not used for ring bonding may be substituted independently of one another with H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl; or combinations thereof;
wherein structure II is a dimer in which two phenanthroline units are bonded to one another via a single bond, and on each phenanthroline unit independently this bond can be located at the 2, 3, 4, 5, 6, 7, 8 or 9 position of the respective phenanthroline ring,
wherein
the substituents R2, R3, R4, R5, R6, R7 and R8 are not simultaneously hydrogen, but otherwise are independently selected from the group: H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl,
aryl, heteroaryl, oligoaryl, oligoheteroaryl and oligoarylheteroaryl, wherein all the sp2-hybridized carbon atoms which are not used for ring bonding may be substituted independently of one another with H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl; or combinations thereof;
wherein X is bonded to each phenanthroline unit via a single bond, and on each phenanthroline unit independently this bond can be located at the 2, 3, 4, 5, 6, 7, 8 or 9 position of the respective phenanthroline ring,
wherein X is a central element which is selected from the group B, C, Si, Ge, Sn, N, O, S, P, wherein n is an integer between 1 and the maximum valency of the selected central element, or wherein X is a central structural unit, which may be a simple or bridged cycloalkane, an aromatic, a heteroaromatic, or a spiro-bonded polycyclic system, and n is an integer between 1 and the maximum number of phenanthroline units,
wherein
the substituents R2, R3, R4, R5, R6, R7 and R8 are not simultaneously hydrogen, but otherwise are independently selected from the group: H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl,
aryl, heteroaryl, oligoaryl, oligoheteroaryl and oligoarylheteroaryl, wherein all the sp2-hybridized carbon atoms which are not used for ring bonding may be substituted independently of one another with H, C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl, alkyloxy —ORx, dialkylamino —NRxRy, alkylthio —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, —SO2Rx, wherein Rx and Ry=C1-C20 alkyl, C1-C20 alkenyl, C1-C20 alkynyl; or combinations thereof.
In relation to structure II illustrated above, it must be noted that the bonding shown therein is given solely by way of example. The bonding of the two phenanthroline units may take place independently of one another via any position of the respective phenanthroline ring, i.e. the 2, 3, 4, 5, 6, 7, 8 or 9 position. Structure II as illustrated above shows by way of example a bonding of two phenanthroline units in which a bond is produced in each case via an external ring element, and in each case the radical R1 (1st structure I) has been removed for the bond. The same also applies in respect of structure III shown above, in which a bond between X and the phenanthroline unit is possible via any suitable position, i.e. the 2, 3, 4, 5, 6, 7, 8 or 9 position, of the phenanthroline unit.
If n in structure III does not correspond to the maximum valency, one or more radicals R0 for X can be selected from R1-R8, as defined above.
As the central structural unit X, mention may be made for example of adamantane or spirobifluorene.
It is preferred that the matrix material can be reversibly reduced.
Alternatively, it is proposed that, during a reduction, the matrix material breaks down into stable, redox-inactive constituents.
The dopant may be a metal complex.
Preferably, the metal complex has a structure IV:
wherein M is a transition metal, preferably Mo or W; and wherein
It is particularly preferred if the dopant has the following structure IVa:
As an alternative, it is also preferred if the dopant is an alkali metal and/or alkaline earth metal, preferably caesium.
It is also proposed that the matrix material has an energy level for the lowest unoccupied molecular orbital (LUMO), which differs by 0-0.5 V from the ionization potential (HOMO) of the dopant, preferably by 0-0.3 V, particularly preferably by 0-0.15V.
One embodiment is characterized in that the matrix material has an electron affinity (LUMO energy level), which is lower than the ionization potential (HOMO) of the dopant. Here, “lower” means that the LUMO energy level has a greater numerical value than the HOMO energy level. Since the two parameters are given negatively from the vacuum level, this means that the absolute HOMO energy value is greater than the absolute LUMO energy value.
It is also preferred if the concentration of the dopant is 0.5 to 25 percent by mass, preferably 1 to 10 percent by mass, particularly preferably 2.5 to 5 percent by mass.
It may furthermore be provided that the glass transition temperature Tg of the matrix material is higher than that of 4,7-diphenyl-1,10-phenanthroline.
Preferably, the matrix material has an evaporation temperature of at least 200° C.
The invention also provides an organic light-emitting diode, which comprises a semiconductor material according to the invention.
It has surprisingly been found that certain phenanthroline derivatives can be used as matrix materials which can if necessary be doped with metal complex dopants. By incorporating suitable electron-attracting substituents at certain positions in the phenanthroline skeleton, electronic properties of the semiconductor material can be set in a targeted manner, so that the LUMO of the phenanthroline derivate falls within a range which means that it can be doped with metal complex dopants, in particular having the structure IV.
Up to date, phenanthroline derivatives have been used for example as undoped electron conductors (EP 0 564 224 A2) or hole blockers (EP 1 097 981, EP 1 097 980). The phenanthroline derivative BPhen has also already been used as a caesium-doped layer in organic light-emitting diodes (U.S. Pat. No. 6,013,384), but with the aforementioned disadvantage of diffusion of the metal when a voltage is applied, which leads to premature failure of the light-emitting diode.
The matrix materials used according to the invention for the semiconductor material moreover have an improved level of thermal stability compared to the prior art, which can be attributed in particular to increased sublimation and decomposition temperatures. The efficiency of an OLED according to the invention is also increased.
Further features and advantages of the invention will become apparent from the following detailed description of preferred examples of embodiments, with reference to the appended drawings, in which
Selected phenanthroline derivatives, which are shown in Table 2 below, were used as matrix material for doping with W2(hpp)4 (structure IVa), and the conductivity was measured. The results are summarized in Table 2.
Comparison with the conductivity values for the matrix materials Alq3 and BPhen known from the prior art (Table 1) shows considerably improved conductivity values for the phenanthroline derivatives used according to the invention.
A glass substrate is provided with contacts made of indium tin oxide. A layer of Ie (Table 2) doped with a dopant of structure IVa is then produced on the substrate. The doping concentration of the dopant IVa is 4 mol %. The conductivity of the layer is 2*10−6 S/cm at room temperature. The sample is then subjected to a temperature cycle. The temperature cycle consists of individual steps. In each step, the sample is heated to the target temperature, held at the target temperature for 20 min and then cooled to room temperature. Successive steps differ in that the target temperature is increased by 10° C. in each case. During the temperature cycle, the conductivity of the layer is measured. The results are shown in
Comparative Example 2 was carried out in a manner analogous to the procedure according to Example 2, but BPhen was used instead of the matrix material Ie. The resulting conductivity of the layer is 4*10−7 S/cm. As can be seen from
By comparing
A glass substrate is provided with contacts made of indium tin oxide. The layers spiro-TTB doped with p-dopant 2-(6-dicyanomethylene-1,3,4,5,7,8-hexafluoro-6H-naphthalen-2-ylidene)malononitrile (50 nm, 4 mol %), 2,2′,7,7′-tetrakis(N,N′-diphenylamino)-9,9′-spirobifluorene, N,N′-di(naphthalen-2-yl)-N,N′-diphenylbenzidine doped with emitter dopant iridium(III)bis(2-methyldibenzo[f,h]quinoxaline)-(acetylacetonate) (20 nm, 10% by weight), Id (10 nm), Id doped with IVa (60 nm, 4 mol %)/Al (100 nm) are then deposited thereon one after the other. The organic light-emitting diode thus produced emits orange-red light.
The organic light-emitting diode (OLED) is subjected to a stability test. For this test, the OLED is encapsulated and operated at 80° C. in an oven. The decrease in brightness over time at a constant current density is measured.
After 10 hours, the OLED exhibits a decrease in luminance from for example 5000 cd/m2 to 3300 cd/m2, as can be seen from the filled-in symbols shown in
An OLED was produced as in Example 3, but BPhen was used instead of Id. The decrease in luminance for this OLED during continuous operation is also shown in
The features disclosed in the description, the claims and the appended drawings may be essential both individually and in any combination with one another for implementing the invention in its various embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05024515.8 | Nov 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP06/10816 | 11/10/2006 | WO | 00 | 7/21/2008 |
