Transparent, Thermally Stable Light-Emitting Component Having Organic Layers

The presently described subject matter relates to transparent and thermally stable light-emitting components having organic layers, and in particular to a transparent organic light-emitting diode having a charge carrier transport layer which is electrically doped with an organic dopant.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 10/496,414, filed Sep. 19, 2005, which is a national phase of and claims priority to International Application PCT/DE03/01021, filed Mar. 27, 2003, which claims priority to German Patent Application DE 102 15 210.1, filed Mar. 28, 2002, all of which are incorporated by reference in their entireties.

FIELD OF THE SUBJECT MATTER

The presently described subject matter relates to the organic semiconductor technology concerning transparent organic light-emitting diodes with doped charge carrier transport layers.

BACKGROUND

Ever since the demonstration, by Tang et al., 1987 [C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)], of low operating voltages, organic light-emitting diodes (OLED) have been promising candidates for the realization of large-area displays. They include a sequence of thin (typically 1 nm to 1 mu m) layers of organic materials, which can be vacuum-deposited or deposited from the solution, e.g., by a spin-on operation. For this reason, these layers are often more than 80% transparent in the visible spectral region. Otherwise, the OLED would have a low external light efficiency due to reabsorption. Contacting of the organic layers with an anode and a cathode is typically effected by means of at least one transparent electrode having, in many cases, a transparent oxide (e.g., indium tin oxide) and a metallic contact. This transparent contact (e.g., the ITO) can be located directly on the substrate. In the case of at least one metallic contact, the OLED as a whole is not transparent, but reflective or scattering (due to appropriate modifying layers, which do not belong to the actual OLED structure). In case of the typical structure with the transparent electrode on the substrate, the OLED emits through the substrate situated on its lower side.

In the case of organic light-emitting diodes, light is produced and emitted by the light-emitting diode by the injection of charge carriers (electrons from one side, holes from the other side) from the contacts into the organic layers situated there-between, as a result of an externally applied voltage, the subsequent formation of excitons (electron-hole pairs) in an active zone, and the radiant recombination of these excitons.

One feature of such organic components as compared with conventional inorganic components (semiconductors such as silicon, gallium arsenide) is that it is possible to produce very large-area display elements (visual displays, screens). Compared with inorganic materials, organic starting materials are relatively inexpensive (e.g., less expenditure of material and energy). Furthermore, these materials, because of their low processing temperature as compared with inorganic materials, can be deposited on flexible substrates, which opens up a wide variety of novel uses in display and illuminating technology.

The usual arrangement of such components having at least one non-transparent electrode includes a sequence of one or more of the following layers:

    • 1. Carrier, substrate;
    • 2. Base electrode, hole-injecting (positive pole), typically transparent;
    • 3. Hole-injecting layer;
    • 4. Hole-transporting layer (HTL);
    • 5. Light-emitting layer (EL);
    • 6. Electron-transporting layer (ETL);
    • 7. Electron-injecting layer;
    • 8. Cover electrode, in most cases a metal having a low work function, electron-injecting (negative pole);
    • 9. Encapsulation, to shut out environmental influences.

The above structure represents one general case; in some cases some layers are omitted (except 2, 5 and 8), or else one layer combines several properties.

In the case of the above-described layer sequence, the light emission takes place through the transparent base electrode and the substrate, whereas the cover electrode includes non-transparent metal layers. Some materials for the transparent base electrode include indium tin oxide (e.g., ITO) and related oxide semiconductors as injection contacts for holes (e.g., a transparent degenerate semiconductor). Used for electron injection are base metals such as aluminum (Al), magnesium (Mg), calcium (Ca) or a mixed layer of Mg and silver (Ag), or such metals in combination with a thin layer of a salt such as lithium fluoride (LiF).

These OLEDs are usually non-transparent. However, there are applications for which the transparency is of decisive importance. Thus, a display element may be produced which in the switched-off state appears transparent, i.e., the surroundings behind it can be perceived, but will, in the turned-on condition, provide the viewer with information. In this connection, one could think of car windshields or displays for persons who must not be limited in their freedom of movement by the display (e.g., head-on displays for surveillance personnel). Such transparent OLEDs, which represent the basis for transparent displays, are known, e.g., from

  • 1. G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest, Appl. Phys. Lett. 68, 2606 (1996);
  • 2. G. Gu, V. Khalfin, S. R. Forrest, Appl. Phys. Lett. 73, 2399 (1998);
  • 3. G. Parthasarathy et al., Appl. Phys. Lett. 72, 2138 (1997);
  • 4. G. Parthasarathy et al., Adv. Mater. 11, 907 (1997);
  • 5. G. Gu, G. Parthasarathy, S. R. Forrest, Appl. Phys. Lett. 74, 305 (1999).

In reference (1) above, the transparency is achieved by using the traditional transparent ITO anode as a base electrode (that is, directly on the substrate). Here, it should be mentioned that it is favorable for the operating voltage of the OLED if the ITO anode is pretreated in a special way (e.g., ozone sputter, plasma incineration) in order to increase the work function of the anode (e.g., C. C. Wu et al., Appl. Phys. Lett. 70, 1348 (1997); G. Gu et al., Appl. Phys. Lett. 73, 2399 (1998)). The work function of ITO can be varied, e.g., by ozonization, ozone or oxygen plasma treatment, and/or oxygen-plasma incineration from about 4.2 eV to about 4.9 eV. In that case, it is possible to inject holes from the ITO anode into the hole transport layer in a more efficient manner. However, this pretreatment of the ITO anode is mostly possible if the anode is situated directly on the substrate. This structure of the OLED is denoted as non-inverted, and the structure of the OLED with the cathode on the substrate as inverted. In (1), a combination of a thin, semitransparent layer, a base metal (magnesium, stabilized through the admixture of silver) and a conductive transparent layer of the known ITO is used as a cover electrode. The reason why this combination is necessary is that the work function of the ITO is too high for electrons to be efficiently injected directly into the electron transport layer and thereby make it possible to produce OLEDs having low operating voltages. This is avoided by means of the very thin magnesium intermediate layer. Because of the thin metallic intermediate layer, the resulting component is semitransparent (transparency of the cover electrode is about 50-80%), whereas the transparency of the ITO anode considered as fully transparent is over 90%. In reference (1), an additional ITO contact is deposited on the metallic intermediate layer by the sputter process, in order to ensure the lateral conductivity to the connection contacts of the OLED surroundings. The consequence of the ITO sputter process is that the metallic intermediate layer, in some embodiments, may not be thinner than 7.5 nm (1), as otherwise the sputter damage to the subjacent organic layers can be unacceptable. Structures of this type are also described in the following patents: U.S. Pat. No. 5,703,436 (S. R. Forrest et al.), applied for on Mar. 6, 1996; U.S. Pat. No. 5,757,026 (S. R. Forrest et al.), applied for on Apr. 15, 1996; U.S. Pat. No. 5,969,474 (M. Arai), applied for on Oct. 24, 1997. Two OLEDs, one on top of the other, with the cathodes described in reference (1), are described in reference (2). Here, a green and a red OLED arranged one upon the other (“stacked OLED”) are prepared. Since both OLEDs are semitransparent, it is possible, through suitable voltages at the now 3 electrodes, to choose the emission color in a targeted manner.

It is also known that an organic intermediate layer can be used to improve the electron injection (references 3-5). In this case, an organic intermediate layer is arranged between the light-emitting layer (e.g., aluminum tris-quinolate, Alq3) and the transparent electrode (e.g., ITO) used as a cathode. In some cases, this intermediate layer is copper phthalocyanine (CuPc). This material is a hole-transport material (higher hole mobility than electron mobility). It exhibits high thermal stability. Thus, the sputtered-on cover electrode cannot do as much damage to the subjacent organic layers. An additional feature of this CuPc intermediate layer is the small band gap (distance between HOMO—highest occupied molecular orbital—and LUMO—lowest unoccupied molecular orbital). Because of the low LUMO position, electrons can be injected from ITO relatively easily. However, because of the small band gap, the absorption in the visible region is high. For this reason, the thickness of the CuPc layer is limited to below 10 nm. Moreover, the injection of electrons from CuPc into Alq3 or another emission material is difficult, since their LUMOs lie generally higher. A further realization of the transparent cathode at the top of the OLED was proposed by Pioneer [U.S. Pat. No. 5,457,565 (T. Namiki), applied for on Nov. 18, 1993]. In this case, a thin layer of an alkaline earth metal oxide (e.g., LiO2) is used instead of the CuPc layer. This improves the otherwise poor electron injection from the transparent cathode into the light-emitting layer.

A further realization of the transparent OLED (G. Parthasarathy et al., Appl. Phys. Lett. 76, 2128 (2000), WO Patent 01/67825 A1 (G. Parthasarathy), applied for on Mar. 7, 2001, provides for an additional electron transport layer (e.g., BCP=bathocuproine having a high electron mobility) in contact with the transparent cathode (e.g., ITO). There is an approximately 1 nm thick pure layer of the alkali metal lithium (Li) either between the light-emitting layer and the thin (e.g., 10 nm) electron transport layer or between the electron transport layer and the ITO cathode. This Li intermediate layer drastically increases the electron injection from the transparent electrode. This effect is explained by a diffusion of the Li atoms into the organic layer and subsequent “doping,” with the formation of a highly conductive intermediate layer (e.g., degenerate semiconductor). Then, a transparent contact layer (e.g., mostly ITO) is placed on the latter.

The above studies make the following points clear:

    • 1. The choice of transparent electrodes includes ITO and similar degenerate inorganic semiconductors.
    • 2. The work functions of the transparent electrodes mainly favor hole injection, but for this, too, a special treatment of the anode is required, in order to further reduce its work function.
    • 3. Previous worked was aimed at finding a suitable intermediate layer which improved the injection of electrons into the organic layers.

SUMMARY

The presently described subject matter relates to transparent and thermally stable light-emitting components having organic layers, and in particular to a transparent organic light-emitting diode having a charge carrier transport layer which is electrically doped with an organic dopant.

It was determined that employing dopants which can dope electron transport materials (ETM) with a LUMO (of the ETM) less negative than Alq3 are useful for a low voltage, high efficient and long lifetime transparent OLED. Materials with a LUMO less negative than ETM-11 can be useful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-b show energy diagrams of the transparent OLED in one example embodiment.

FIG. 2a shows OLED structures according to some embodiments of the described subject matter.

FIG. 2b is an energy diagram of a transparent OLED according to another example embodiment.

FIG. 3 shows a luminance vs. voltage curve of Example 1.

FIG. 4 shows the normalized luminance over time in an accelerated aging test of one device of the presently described subject matter. The figure compares the bottom emitting device with the transparent device. The measured points almost completely overlap each other.

FIG. 5 shows the comparison of the optical transmittance of one device of the presently described subject matter (102) compared with other devices (103). The transmittance of the glass substrate with the ITO layer is also shown for comparison purposes (101).

FIG. 6 shows the luminance vs. voltage curve of a transparent OLED according to an embodiment with a non-inverted structure.

 DETAILED DESCRIPTION

It is known, that for light-emitting diodes from inorganic semiconductors, it is possible, through highly doped peripheral layers, to obtain thin space charge zones which, even in the presence of energy barriers, lead to efficient injection of charge carriers by tunneling. Here, the term “doping” includes the targeted influencing of the conductivity of the semiconductor layer through admixture of foreign atoms/molecules (as is possible for inorganic semiconductors). For organic semiconductors, the term “doping” includes the admixture, to the organic layer, of specific emitter molecules; here, a distinction should be made. The doping of organic materials was described in U.S. Pat. No. 5,093,698, applied for on Feb. 12, 1991. However, in the case of practical applications of the described doping, this leads to problems with the energy adaptation of the different layers and to reduction of the efficiency of the LEDs having doped layers.

In addition, electrical doping includes the phenomenon where a charge transfer occurs from the HOMO (LUMO) of the n-dopant (p-dopant) to the LUMO (HOMO) of the n-type (p-type) semiconductor which transports the charge carriers (also called matrix material). The charge density in equilibrium and the Fermi Level can be thus modified.

One object of the presently described subject matter is to provide a fully transparent (e.g., 70% transmission) organic light-emitting diode that can be operated at a low operating voltage, the organic light-emitting diode having a high light-emission efficiency. At the same time, the described subject matter includes the protection of organic layers, in particular of the light-emitting layers, against damage during preparation of the transparent cover contact. The described subject matter includes stable components (e.g., operating temperature range up to 80 degrees C., long-term stability).

According to the presently described subject matter, some objects are achieved in combination with the following features: a transparent, thermally stable light-emitting component having the following sequence of organic layers: a transparent substrate; a transparent anode; a hole transport layer adjacent to the anode; at least one light-emitting layer; a charge-carrier transport layer for electrons; and a transparent cathode; in such a way that the hole transport layer is p-doped with an acceptor-type organic material and the electron transport layer is n-doped with a donor-type organic material, and the molecular masses of the dopants are greater than 200 g/mole.

The presently described subject matter further includes a transparent, thermally stable light-emitting component, having the following organic layers: a transparent substrate; a transparent cathode; a charge transport layer for electrons adjacent to the cathode; at least one light-emitting layer; a charge-carrier transport layer for holes; and a transparent anode; in such a way that the electron transport layer is n-doped with a donor-type organic material and the hole transport layer is p-doped with an acceptor-type organic material, and the molecular masses of the dopants are greater than 200 g/mole.

As described in Patent Application DE 101 35 513.0 (Leo et al., submitted on Jul. 20, 2001), the layer sequence of the OLED can be reversed, thus the hole-injecting (transparent) contact (anode) can be a cover electrode. As a result, in the case of inverted organic light-emitting diodes the operating voltages can be considerably higher than with comparable non-inverted structures. One reason for this phenomenon is that the injection from the contacts into the organic layers is less efficient, because optimization of the work function of the contacts in a targeted manner can be more difficult.

In the solution according to the described subject matter, the injection of charge carriers from the electrodes into the organic layers (whether hole- or electron-transporting layers) does not depend so strongly on the work function of the electrodes itself. As a result it is also possible to use, on both sides of the OLED component, the same electrode type, thus, e.g., two equal transparent electrodes, e.g., ITO.

The term side includes extending along a plane parallel to the substrate. The term bottom includes a position of a layer that is closer to the substrate than another layer. The bottom electrode includes an electrode located somewhere between the substrate and at least one organic light-emitting layer. The term top includes a position of a layer that is further from the substrate than another layer. The top electrode includes an electrode located somewhere not between the substrate and at least one organic light-emitting layer.

Some embodiments include a transparent, thermally stable light-emitting component having organic layers, including a transparent substrate, a transparent anode, a hole transport layer adjacent to the anode, at least one light-emitting layer, a charge-carrier transport layer for electrons, and a transparent cathode, wherein the transparency in the visible spectral region is at least 75%, wherein the hole transport layer is p-doped with an acceptor organic material and the electron transport layer is n-doped with a donor organic material, and the molecular masses of the dopants are each greater than 200 g/mole, and wherein the transparent, thermally stable light-emitting component having organic layers is an organic light-emitting diode.

Some embodiments further include at least one of a hole-side blocking layer located between the doped hole transport layer and the light-emitting layer or an electron-side blocking layer located between the doped electron transport layer and the light-emitting layer. Some embodiments further include a electrode layer located between the anode and the hole transport layer and a electrode layer located between the charge-carrier transport layer and the cathode.

In some embodiments, the doping concentration of the organic dopants is such that an ohmic injection takes place from the anode into the charge-carrier transport layer or from the cathode into the hole transport layer. In some embodiments, the electrode layers comprise indium tin oxide (ITO) or a degenerate oxide other than ITO. In some embodiments, the cathode includes a metallic intermediate layer adjacent to the subjacent doped, charge-carrier transport layer when the cathode is located on top or the anode includes a metallic intermediate layer adjacent to the subjacent doped, hole transport layer when the anode is located on top and wherein the metallic layer has a nominal thickness between 0.1 nm and 3 nm.

In some embodiments, no metal layer is located between the doped hole transport layer and the anode when the anode is on top or between the doped electron transport layer and the cathode when the cathode is on top. The anode and cathode can be located between the substrate and encapsulation cover and the transparency can be at least 70% for each wavelength between at least 400 nm and 800 nm. The molar concentration of admixture in the hole transport layer or in the electron transport layer or in both the hole transport layer and the electron transport layer can be in the range of 1:100,000 to 1:10, calculated on the ratio of doping molecules to main-substance molecules. The molar concentration of admixture in the hole transport layer or in the electron transport layer, or in both the hole transport layer and the electron transport layer, can be at least 1 wt %, calculated on the ratio of doping molecules to main-substance molecules.

In some embodiments, the thickness of each of the hole transport layer or the electron transport layer, of the light-emitting layer and of the at least one of a hole-side blocking layer or an electron-side blocking layer lies in the range of 0.1 nm to 50 μm. In some embodiments, the cathode is in direct contact with a doped transport layer and is facing away from the substrate when the cathode is on top or the anode is in direct contact with a doped transport layer and is facing away from the substrate when the anode is on top and wherein the doped transport layer is a hole transport layer or an electron transport layer. In some embodiments, the organic n-dopant material is selected from the group consisting of heterocyclic radicals, diradicals, dimers, an oligomer, a polymer, a dispiro compound, and a polycycle thereof, having the structure according to one of the following formulae:

wherein structures 3 and 4 have one or more cyclic linkages A and/or A1 and/or A2,

wherein A, A1 and A2 are selected from the group consisting of carbocyclic, heterocyclic, polycyclic ring systems, and any combination thereof, which may be substituted or unsubstituted,

wherein A1 and A2 are present individually or together and A1 and A2 are selected as in structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S, and

wherein structure 7 has one or more bridge bonds Z and Z1, Z or Z1, Z1 and Z2, or Z1 or Z2, and Z, Z1 and Z2 are independently selected from the group consisting of alkyl, alkenyl, alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo, disulphide, heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl ether, polyether, primary alkylamine, arylamine, polyamine, aryl, and heteroaryl.

The organic acceptor organic material can be a quiniode derivative or a triylidene derivative, with a reduction potential in the range of 0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc+. In some embodiments, the n-doped, donor organic material is an asymmetrically substituted phenanthroline with the following structure

wherein:
R1 and R2 are selected from the group consisting of substituted or unsubstituted Aryl, Heteroaryl, and Alkyl; and
R3 is selected from the group consisting of H, CN, substituted or unsubstituted Aryl, Heteroaryl, and Alkyl;
R4 is selected from the group consisting of H, CN, COOR with R=Alkyl, Heteroalkyl, Aryl or Heteroaryl, substituted or unsubstituted Aryl, Heteroaryl, Alkyl mit C1-C20, and Cycloalkyl mit C3-C20.

In some embodiments, the n-doped, donor organic material has the structure:

wherein M is selected from the group consisting of Ti, Zr, Hf, Nb, Re, Sn and Ge,
each R is independently selected from the group consisting of hydrogen, C1-C20-Alkyl, C1-C20-Alkenyl, C1-C20-Alkinyl, Aryl, Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl, —ORx,
—NRxRy, —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, SO2Rx, and where Rx and Ry are selected from the group consisting of C1-C20-Alkyl, C1-C20-Alkenyl, and C1-C20-Alkinyl.

The n-doped, donor organic material can have the structure:

wherein R1, R2, R3, and R4 are independently selected from the group consisting of H, halogen, CN, substituted or unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy, and aryloxy.

In some embodiments, the anode is between the substrate and the at least one light-emitting layer. In some embodiments, the cathode is between the substrate and the at least one light-emitting layer. The electrode layers can include different transparent contact materials.

Some embodiments further include a contact-improving layer located between the electron transport layer and cathode and a contact-improving layer located between the anode and the hole transport layer, wherein the contact-improving layers are configured not to prevent charge from passing through. Some embodiments further include a contact-improving layer located between the electron transport layer and cathode or a contact-improving layer located between the anode and the hole transport layer, wherein the contact-improving layers are configured not to prevent charge from passing through. The light-emitting layer can include a mixed layer of several materials. The p-doped hole transport layer can include a mixture of an organic main substance and an acceptor doping substance and an acceptor doping substance and the molecular mass of the dopants can be greater than 200 g/mole. The electron transport layer can include a mixture of an organic main substance and a donor doping substance and an acceptor doping substance and the molecular mass of the dopants can be greater than 200 g/mole. In some embodiments, when the transparent cathode is on top, the transparent cathode includes a transparent protective layer or when the transparent anode is on top, the transparent anode includes a transparent protective layer. In some embodiments, when the transparent cathode is on top, the transparent cathode includes a metallic intermediate layer adjacent to the subjacent doped charge-carrier transport layer or when the transparent anode is on top, the transparent anode includes a metallic intermediate layer adjacent to the subjacent doped hole transport layer,

    • wherein the transparency of the metal intermediate layer in the visible spectral region is at least 75% and the thickness of the metal intermediate layer is between 0.3 nm and 3 nm. The sequence of p-doped hole transport layer and transparent anode can be repeated. The sequence of n-doped electron transport layer and transparent cathode can be repeated. Some embodiments further include a metallic electron-injection-promoting layer located between the doped electron transport layer and either the electron-side blocking layer or the light-emitting layer, wherein the transparency of the metallic electron-injection-promoting layer in the visible spectral region is at least 75%.

In some embodiments, the top transparent contact layer (which is facing away from the substrate) is in direct contact with the doped transport layer, which doped transport layer is a hole transport layer or an electron transport layer.

In some embodiments, the transparent organic light-emitting diode includes a thin (e.g., 1 to 10 nm thick) doped charge transport layer at the interface with the top electrode (this layer being localized between the light-emitting region and the electrode); the dopant concentration being greater than 40 wt %, in some embodiments greater or at least 50 wt %. In some embodiments, the transparent organic light-emitting diode includes a thin (e.g., 0.5 nm to 3 nm) pure dopant layer as a buffer layer at the interface with the top electrode (between the charge transport layer and the top electrode).

In some embodiments, ohmic injection occurs when the dependence of the current with the applied voltage is linear (e.g., can be measured in single carrier type devices (e.g., hole only devices)). In some embodiments, if a line fit (I=F(V)) to the I-V curve fits to at least 95% in a range of at least 1 V (layer thickness of at least 50 nm) then the injection is ohmic. For a layer thinner than 10 nm, a dopant concentration greater than or equal to 5%, perhaps greater than or equal to 10% may be required. For layers thicker than 10 nm, the concentration may be higher than 0.2%, perhaps higher than 1%, and if the layer is under the top electrode, then the doping concentration may be higher than 5%.

The cause of the increase of conductivity can be an increased density of equilibrium charge carriers in a layer. Here, the transport layer can have higher layer thicknesses than is possible with undoped layers (e.g., 20-40 nm), without drastically increasing the operating voltage. Similarly, the electron-injecting layer adjacent to the cathode can be n-doped with a donor-type molecule (e.g., an organic molecule or fragments thereof, see Patent Application DE 102 07 859.9). This n-doping leads to an increase in the electron conductivity due to higher intrinsic charge-carrier density. The transport layer can also be made thicker in the component than would be possible with undoped layers, since that would lead to an increase in the operating voltage. Thus, both layers are thick enough to protect the subjacent layers against damage during the production process (e.g., sputter process) of the transparent electrode (e.g., formed from ITO).

In the doped charge-carrier transport layers (holes or electrons) on the electrodes (anode or cathode), a thin space charge zone may be created through which the charge carriers can be injected in an efficient manner. Because of the tunnel injection, the injection is not hindered by the very thin space charge zone, even in case of an energetically high barrier. The charge-carrier transport layer can be doped by an admixture of an organic or inorganic substance (e.g., dopant). These large molecules are incorporated in a stable manner into the matrix molecule skeleton of the charge-carrier transport layers. As a result, a high degree of stability is obtained during operation of the OLED (e.g., no diffusion) as well as under thermal load.

In Patent Application DE 100 58 578.7, filed on Nov. 25, 2000 (see also X. Zhou et al., Appl. Phys. Lett. 78, 410 (2001)), it is described that organic light-emitting diodes having doped transport layers show an efficient light emission when the doped transport layers are combined with blocking layers in an appropriate manner. Hence, in an embodiment, the transparent light-emitting diodes are also provided with blocking layers. The blocking layer can be located between the charge-carrier transport layer and a light-emitting layer of the component, in which the conversion of the electric energy into light takes place. The electric energy of the charge carriers can be injected by current flow through the component. According to the described subject matter, the substances of the blocking layers can be selected so that when voltage is applied in the direction of the operating voltage, because of their energy levels, the majority charge carriers (HTL side: holes, ETL side: electrons) are not too strongly hindered at the doped charge-carrier transport layer/blocking layer interface (e.g., low barrier), but the minority charge carriers are efficiently arrested at the light-emitting layer/blocking layer interface (e.g., high barrier). Moreover, the barrier height for the injection of charge carriers from the blocking layer into the emitting layer can be small enough that the conversion of a charge-carrier pair at the interface into an exciton in the emitting layer is energetically advantageous. This prevents exciplex formation at the interfaces of the light-emitting layer, which reduces the efficiency of the light emission. Since the charge-carrier transport layers can have a high band gap, the blocking layers can be chosen to be very thin. In spite of this, no tunneling of charge carriers from the light-emitting layer in energy conditions of the charge-carrier transport layers is possible. This permits obtaining a low operating voltage despite blocking layers.

One embodiment of a transparent OLED according to the described subject matter includes the following layers (non-inverted structure) (FIG. 2a):

    • 1 Carrier, substrate;
    • 2 Transparent electrode, e.g., ITO, hole-injecting (anode=positive pole);
    • 3 p-Doped, hole-injecting and transporting layer;
    • 4 Thin hole-side blocking layer made of a material whose band positions match the band positions of the layers enclosing it;
    • 5 Light-emitting layer (possibly doped with emitter dye);
    • 6 Thin electron-side blocking layer of a material whose band positions match the band positions of the layers enclosing it;
    • 7 n-Doped electron-injecting and transporting layer;
    • 8 Transparent electrode, electron-injecting (cathode=negative pole);
    • 9 Encapsulation, to shut out environmental influences.

Another embodiment of a transparent OLED according to the described subject matter includes the following layers (inverted structure) (FIG. 2a):

    • 1 Carrier, substrate;
    • 2a Transparent electrode, e.g., ITO, electron-injecting (cathode=negative pole);
    • 3 n-Doped, electron-injecting and transporting layer;
    • 4a Thin electron-side blocking layer of a material whose band positions match the band positions of the layers surrounding it;
    • 5a Light-emitting layer (possibly doped with emitter dye);
    • 6a Thin hole-side blocking layer of a material whose band positions match the band positions of the layers surrounding it;
    • 7a p-Doped hole-injecting and transporting layer;
    • 8a Transparent electrode, hole-injecting (anode=positive pole), e.g., ITO;
    • 9 Encapsulation, to keep out environmental influences.

The described subject matter includes structures with one blocking layer, because the band positions of the injecting and transporting layer and of the light-emitting layer can match one another on one side. Furthermore, the functions of charge-carrier injection and of charge-carrier transport into layers 3 and 7 may be divided among several layers, of which at least one (namely that adjacent to the electrodes) is doped. When the doped layer is not directly located on the respective electrode, then layers between the doped layer and the respective electrode may be thin enough that they can efficiently be tunneled through by charge carriers (e.g., 10 nm). These layers can be thicker when they have a higher conductivity (the bulk resistance of these layers may be smaller than that of the neighboring doped layer). The intermediate layers can then be considered to be a part of the electrode. The molar doping concentrations can lie in the range of 1:10 to 1:10000. The dopants can include organic molecules having molecular masses above 200 g/mole.

The n-dopant, or dopant donor, can include a molecule or a neutral radical or combination thereof with a HOMO energy level (e.g., ionization potential in solid state) more positive than −3.3 eV, or more positive than −2.8 eV, or more positive than −2.6 eV and its respective gas phase ionization potential is more positive than −4.3 eV, or more positive than −3.8 eV, or more positive than −3.6 eV. The HOMO of the donor can be estimated by cyclo-voltammetric measurements. An alternative way to measure the reduction potential is to measure the cation of the donor salt. The donor can exhibit an oxidation potential that is smaller than or equal to −1.5 V vs Fc/Fc+ (Ferrum/Ferrocenium redox-pair), or smaller than −1.5 V, or smaller than or equal to approximately −2.0 V, or smaller than or equal to −2.2 V. The molar mass of the donor can be in a range between 200 and 2000 g/mole, or in a range from 300 and 1000 g/mole. The molar doping concentration is in the range of 1:10000 (dopant molecule:matrix molecule) and 1:2, or between 1:100 and 1:5, or between 1:100 and 1:10. Sometimes doping concentrations larger than 1:2 can be applied, e.g., if large conductivities are required. The donor can be created by a precursor during the layer forming (e.g., deposition) process or during a subsequent process of layer formation. The above given value of the HOMO level of the donor refers to the resulting molecule or molecule radical.

A p-dopant, or dopant acceptor, can include a molecule or a neutral radical or combination thereof with a LUMO level more negative than −4.5 eV, or more negative than −4.8 eV, or more negative than −5.04 eV. The LUMO of the acceptor can be estimated by cyclo-voltammetric measurements. The acceptor can exhibit a reduction potential that is larger than or equal to approximately −0.3 V vs Fc/Fc+ (Ferrum/Ferrocenium redox-pair), or larger than or equal to 0.0 V, or larger than or equal to 0.24 V. The molar mass of the acceptor can be in the range of 200 to 2000 g/mole, or between 250 and 1000 g/mole, or between 300 g/mole and 1000 g/mole. The molar doping concentration can be in the range of 1:10000 (dopant molecule:matrix molecule) and 1:2, or between 1:100 and 1:5, or between 1:100 and 1:10. Sometimes, doping concentrations larger than 1:2 can be applied, e.g., if large conductivities are required. The acceptor can be created by a precursor during the layer forming (e.g., deposition) process or during a subsequent process of layer formation. The above given value of the LUMO level of the acceptor refers to the resulting molecule or molecule radical.

An n-dopant of the following structure can be employed in the transparent p-i-n OLED:

where M is a transition metal, e.g., Mo or W; and where

    • the structural elements a-f can include: a=CR9R10, b=CR11R12, c=CR13R14, d=R15R16, e=CR17R18 and f=CR19R20, where R9R20 independently of one another are hydrogen, C1C20 alkyl, C1C20 cycloalkyl, C1C20 alkenyl, C1C20 alkynyl, aryl, heteroaryl, NRR or OR, where R=C1C20 alkyl, C1C20 cycloalkyl, C1C20 alkenyl, C1C20 alkynyl, aryl or heteroaryl, where R9, R11, R13, R15, R17, R19=H and R10, R12, R14, R16, R18, R20=C1C20 alkyl, C1C20 cycloalkyl, C1C20 alkenyl, C1C20 alkynyl, aryl, heteroaryl, NRR or OR, or
    • in the case of structural elements c and/or d, C can be replaced by Si, or
    • optionally a or b or e or f is NR, with R=C1C20 alkyl, C1C20 cycloalkyl, C1C20 alkenyl, C1C20 alkynyl, aryl, heteroaryl, or
    • optionally a and f or b and e are NR, with R=C1C20 alkyl, C1C20 cycloalkyl, C1C20 alkenyl, C1C20 alkynyl, aryl, heteroaryl,
    • where the bonds a c, b d, c e and d f, but not simultaneously a-c and c-e and not simultaneously b-d and d-f, may be unsaturated,
    • where the bonds a-c, b-d, c-e and d-f may be part of a saturated or unsaturated ring system, which may also contain the heteroelements O, S, Se, N, P, Se, Ge, Sn, or
    • the bonds a-c, b-d, c-e and d-f are part of an aromatic or condensed aromatic ring system, which may also contain the heteroelements O, S, Si, N,
    • where the atom E is a main group element, selected from the group C, N, P, As, Sb,
    • where the structural element a E-b is optionally part of a saturated or unsaturated ring system, which may also contain the heteroelements O, S, Se, N, P, Si, Ge, Sn, or
    • the structural element a E-b is optionally part of an aromatic ring system, which may also contain the heteroelements O, S, Se, N.

The dopant can have the following structure II:

Suitable n-dopant precursors include the heterocyclic radicals, diradical, a dimers, an oligomer, a polymer, a dispiro compound or a polycycle thereof, having the structure according to the following formulae:

where structures 3 and 4 have one or more cyclic linkages A and/or A1 and/or A2, where A, A1 and A2 may be carbocyclic, heterocyclic and/or polycyclic ring systems, which may be substituted or unsubstituted;

where A1 and A2 may be present individually or together and A1 and A2 are as defined for structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S;

where structure 7 has one or more bridge bonds Z and/or Z1 and/or Z2, and Z, Z1 and Z2 may independently be selected from alkyl, alkenyl, alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo, disulphide, heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl ether, polyether, primary alkylamine, arylamine and polyamine, aryl and heteroaryl;
Organic n-dopant compounds include the heterocyclic radicals or diradicals, the dimers, oligomers, polymers, dispiro compounds and polycycles of:

where the bridges Z, Z1 and Z2 can be independently selected from alkyl, alkenyl, alkinyl, cycloalkyl, silyl; alkylsilyl, diazo, disulfide, heterocycloalkyl, heterocyclyl, piperazinyl, dialkylether, polyether, alkylamine, arylamine, polyamine, Aryl and heteroaryl; X and Y can be O, S, N, NR21, P, or PR21; R0-19, R21, R22 and R23 are independently chosen from substituted or unsubstituted: aryl, heteroaryl, heterocyclyl, diarylamine, diheteroarylamine, dialkylamine, heteroarylalkylamine, arylalkylamine, H, F, cycloalkyl, halocycloalkyl, heterocycloalkyl, alkyl, alkenyl, alkinyl, trialkylsilyl, triarylsilyl, halogen, styryl, alkoxy, aryloxy, thioalkyl, thioaryl, silyl and trialkylsilylalkanyl, or R0-19, R21, R22 and R23, are part of a (hetero)aliphatic or (hetero)aromatic ring system alone or in combination.
Preferred n-dopants are those with the structure:

where R1 is methyl or isopropyl and R2 is phenyl or cyclohexyl.

Illustrative examples of suitable organic n-dopants include the following dimer structures, their diradical state and their monomer:

Other examples include (ED-9) 2,2′-diisopropyl-4,5-bis(2-methoxyphenyl)-4′,5′-bis(3-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole; (ED-10) 2,2′-Diisopropyl-4,5-bis(2-methoxyphenyl)-4′,5′-bis(4-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole; (ED-11) 2,2′-Diisopropyl-1,1′,3,3′-tetramethyl-2,2′,3,3′,4,4′,5,5′,6,6′,7,7′-dodecahydro-2,2′-bibenzo[d]imidazole; (ED-8) 2,2′-Diisopropyl-4,4′,5,5′-tetrakis(4-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-2,2′-biimidazole; (ED-12) 2-Isopropyl-1,3-dimethyl-2,3,6,7-tetrahydro-5,8-dioxa-1,3-diaza-cyclopenta[b]naphthene; (ED-13) Bis-[1,3-dimethyl-2-isopropyl-1,2-dihydro-benzimidazolyl-(2)]; (ED-14) 1,1′,2,2′,3,3′-hexamethyl-4,4′,5,5′-tetraphenyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole;

Electron transport materials (ETM) which can be used as host for the n-dopants include phenanthrolines, metal quinolinates, metal quinoxalinates, diazapyrenes and others.

Asymmetrically substituted phenanthrolines are described in the European patent application EP07400033.2. Asymmetrically substituted phenanthrolines which can be used as ETM can have the following structure

where:
R1 and R2 are chosen from substituted or unsubstituted Aryl, Heteroaryl, Alkyl;
R3 is chosen from H, CN, substituted or unsubstituted Aryl, Heteroaryl or Alkyl;
R4 is chosen from H, CN, COOR with R=Alkyl, Heteroalkyl, Aryl or Heteroaryl; substituted or unsubstituted Aryl, Heteroaryl, Alkyl mit C1-C20, Cycloalkyl mit C3-C20.

Examples of phenanthrolines to be used as n-doped ETM include:

Other ETM include metal complexes, such as metal chelates. A form of the metal chelates are metal quinolates and quinoxalines. Some materials are those with the structure:

where M is chosen from Ti, Zr, Hf, Nb, Re, Sn and Ge,
each R is independently chosen from hydrogen, C1-C20-Alkyl, C1-C20-Alkenyl, C1-C20-Alkinyl, Aryl, Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl, —ORx, —NRxRy, —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, SO2Rx, where Rx, and Ry are chosen from C1-C20-Alkyl, C1-C20-Alkenyl and C1-C20-Alkinyl.

Examples of quinoxalines include:

Other ETM include compounds according to the following formulae:

where R1, R2, R3, and R4 are in each occurrence independently selected from H, halogen, CN, substituted or unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy and aryloxy.

Examples of such ETM are:

Hole transport materials (HTM) that are used as host for the p-dopants include phenylamines, triphenyl-amines, fluorenes, benzidines.

Examples of such HTM include: 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA), 4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2-TNATA), MeO-TPD (N,N,N′,N′-tetrakis(4-methoxy-phenyl)benzidine), (2,2′,7,7′-tetrakis-(N,N-diphenylamino)-9,9′-spirobifluoren (spiro-TTB), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine, N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-9,9-spiro-bifluorene, 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorine, N,N′-bis(phenanthren-9-yl)-N,N′-bis(phenyl)-benzidine, 2,2′-bis[N,N-bis(biphenyl-4-yl)amino]9,9-spiro-bifluorene, 1,3,5-tris{4-[bis(9,9-dimethyl-fluorene-2-yl)amino]phenyl}benzene, and tri(terphenyl-4-yl)amine; N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPD).

The p-dopant can have a reduction potential in the range of 0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc+. Fc/Fc+, as usual the Ferrocene/Ferrocenium redox couple. Reduction potentials can be considered as measures for the LUMO of a molecule.

Examples of p-dopants include:

Name Chemical name MW OA-1 2,2′-(perfluorocyclohexa-2,5-diene-1,4- 276 (F4TCNQ) diylidene)dimalononitrile OA-2 (perfluoronaphthalene-2,6-diylidene)dicyanamide 314 OA-3 N,N′-bicyano-2,5-dichloro-1,4-chinodiimine(2,5-dichloro- 261 3,6-difluorocyclohexa-2,5-diene-1,4-diylidene)dicyanamide OA-4 N,N′-bicyano-2,5-dichloro-3,6-difluoro-1,4- 225 chinodiimine(2,5-dichlorocyclohexa-2,5-diene-1,4- diylidene)dicyanamide OA-5 N-(2,3,5,6-tetrafluoro-4-iminocyclohexa-2,5- 203 dienylidene)cyanamide OA-6 1,4,5,8-Tetrahydro-1,4,5,8-tetrathia-2,3,6,7- 384 tetracyanoanthrachinone OA-7 1,3,4,5,7,8-Hexafluoronaphtho-2,6- 362 chinontetracyanomethane OA-8 3,6-bis(cyano(4-cyano-2,3,5,6- 586 tetrafluorophenyl)methylene)-2,5-difluorocyclohexa-1,4- diene-1,4-dicarbonitrile OA-9 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2- 651 (perfluorophenyl)acetonitrile) OA-10 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2- 1095 (perfluorobiphenyl-4-yl)acetonitrile); OA-11 2,2′,2″-(Cyclopropane-1,2,3-triylidene) tris (2-(2,6- 672 dichloro-3,5-difluoro-4-(trifluoromethyl) phenyl)acetonitrile); OA-12 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2-(2,6- 902 dichloro-3,5-difluoro-4-(trifluoromethyl)phenyl)-acetonitrile) Some dopants have MW > 300. Some compounds have a MW > than 500.

Asymmetric phenanthrolines can be used as an electron transport layer in the devices of the described subject matter. Asymmetric phenanthrolines can also be used when they are n-doped with dopants that are, or that form, neutral radicals (or, e.g., diradicales, their dimers, oligomers).

The dopants that are, or that form, neutral radicals (or, e.g., diradicales, their dimers, oligomers) can form stable layers when used as dopants in a matrix having asymmetric phenanthrolines.

Metal quinoxalines can be used as electron transport materials doped with dopants that are, or that form, neutral radicals (or, e.g., diradicales, their dimers, oligomers). Precursor dopants can form stable layers when used as dopants in a matrix having metal quinoxalines.

Diazapyrenes can be used as electron transport materials doped with dopants that are, or that form, neutral radicals (or, e.g., diradicales, their dimers, oligomers). Precursor dopants can form stable layers when used as dopants in a matrix having metal quinoxalines.

Stability, low voltage, and high efficiency can be achieved in devices where organic mesomeric compounds are used as organic p-doping agents for the doping of an organic semiconductive hole transport matrix material. The organic mesomeric compound can be a radialene compound with the following formula:

in which each X is

where each R1 is independently selected from aryl and heteroaryl and aryl and heteroaryl are at least partially or completely substituted with electron acceptor groups.

Examples of emitter materials include Fluorescent emitters such as 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB); CBP, antracene, Metal chelates such as 3 quinoline Aluminum (Alq3); Phosphorescent emitters such as Ir-chelates; Ir(ppy)3 Fir-pic.

Emitter materials can be mixed with an emitter host. The host can also contribute to the emission. Examples of emitter hosts include: 3,9-di(naphthalen-2-yl)perylene+3,10-di(naphthalen-yl)perylene mixture (DNP); and NPD.

FIGS. 1a and 1b are energy diagrams of a transparent OLED in one embodiment of the described subject matter without doping. The position of the energy levels are shown in the upper part (HOMO and LUMO) without external voltage and in the lower part with applied external voltage. In this embodiment, both electrodes have the same work function. Here, for the sake of simplicity, the blocking layers 4 and 6 are also shown.

FIG. 2b is an energy diagram of a transparent OLED with doped charge-carrier transport layers and matching blocking layers according to an embodiment of the described subject matter. Note the band bending adjacent to the contact layers, here of ITO in both cases.

FIG. 3 shows the luminance vs. voltage curve of the embodiment presented in example 1; the monitor luminance of 100 cd/m 2 is attained already at 4 V. The efficiency is 2 cd/A. However, here, no transparent contact (e.g., ITO) is used as anode material. The transparent contact is simulated by a semitransparent (e.g., 50%) gold contact. Thus, this is a semitransparent OLED.

FIG. 4 shows the normalized luminance over time in an accelerated aging test (current density=60 mA/cm2). The figure compares the bottom emitting device with the transparent device. It can be seen that the devices have the same behavior; the measured points almost completely overlap each other. The extrapolated lifetime is in excess of 10,000 h.

FIG. 5 shows the comparison of the optical transmittance of an exemplary device of the described subject matter (102) compared with an existing device (103). The device 102 exhibits superior transmittance in comparison with the device 103. Note that the transmittance was measured through the glass substrate and through the encapsulation substrate and is greater than 70% in the visible range and greater than 75% between 460 nm and 800 nm. In contrast, the device 103 exhibits a transmittance less than 62% and largely less than 50% of a larger range of the visible spectrum. The transmission spectra of the device 103 is also more wavelength dependent (i.e., the spectra is less flat and has stronger color). The transmittance of the glass substrate with the ITO (101) is also shown for comparison purposes.

In the embodiment shown in FIGS. 1a-b, no space charge zone occurs at the contacts. This embodiment has a high energy barrier for the charge-carrier injection. This high energy barrier, under certain circumstances, cannot be overcome or overcome with difficulty when using available materials. Hence, the injection of charge carriers from the contacts is less effective. The OLED shows an increased operating voltage.

According to the described subject matter, increased performance is achieved, in some embodiments, by transparent OLEDs with doped injection and transport layers, optionally in combination with blocking layers. FIG. 2a shows one exemplary arrangement. In this embodiment, the charge-carrier-injecting and conducting layers 3 and 7 are doped, so that space charge zones are formed at the interfaces to contacts 2 and 8. The doping is sufficient to allow for the space charge zones to be easily tunneled through. Such doping has been shown to be possible for the p-doping of the hole transport layer for non-transparent light-emitting diodes (e.g., X. Q. Zhou et al., Appl. Phys. Lett. 78, 410 (2001); J. Blochwitz et al., Organic Electronics 2, 97 (2001)).

The foregoing arrangements exhibit various characteristics: (1) increased injection of charge carriers from the electrodes into the doped charge-carrier transport layers; (2) independence from the detailed preparation of the charge-carrier-injecting materials 2 and 8 (e.g., (I) injection layers may not be required if doping is used; (II) the layers which contact the electrodes may not need “special” treatment to improve injection (such as annealing, surface modification of ITO, etc); (III) arrangements such as inverted structures with the ETL on the bottom electrode, i.e., cathode on the substrate or non-inverted structures can be created without great constraints); (3) gives the option of choosing, for the electrodes 2 and 8, materials having comparatively high barriers for the charge-carrier injection (e.g., the same material in both cases such as ITO).

EXAMPLES Example 1

In the following example, the electron transport layer is not yet n-doped with stable large organic dopants. An embodiment with the nonstable n-doping of a known electron transport material (Bphen=bathophenanthroline) with Li demonstrates the effectiveness of the transparent OLED with doped organic transport layers (U.S. Pat. No. 6,013,384 (J. Kido et al.), applied for on Jan. 22, 1998; J. Kido et al., Appl. Phys. Lett. 73, 2866 (1998)). This approximately 1:1 mixture of Li and Bphen demonstrates the effectiveness of the doping. This layer is not stable thermally and operationally. It is assumed that the mechanism of doping is different because of the high doping concentration. On doping with organic molecules and doping ratios of between 1:10 and 1:10000, it can be assumed that the dopant does not significantly affect the structure of the charge-carrier transport layer. Where the concentration is 1:1 of doping metals, e.g., Li, the same cannot be assumed.

The OLED in Example 1 has the following layer structure (inverted structure):

    • 1 a Substrate, e.g., glass;
    • 2 a Cathode: ITO as purchased, untreated;
    • 3 a n-Doped electron-transporting layer: 20 nm Bphen:Li, 1:1 molecular mixing ratio;
    • 4 a Electron-side blocking layer: 10 nm Bphen;
    • 5 a Electroluminescent layer: 20 nm Alq 3, may be mixed with emitter dopants in order to in-create the internal quantum yield of the light production;
    • 6 a Hole-side blocking layer: 5 nm triphenyldiamine (TPD);
    • 7 a p-Doped hole-transporting layer: 100 nm Starburst m-MTDATA 50:1 doped with F 4-TCNQ dopant (thermally stable to about 80 degrees C.);
    • 8 a Transparent electrode (anode) indium tin oxide (ITO).

The mixed layers 3 and 7 are prepared by a vapor deposition process in vacuo by mixed evaporation. In principle, such layers can also be prepared by other processes as well, such as, e.g., vapor deposition of the substances one upon the other, followed by a possibly temperature-controlled diffusion of the substances into one another; or by another type of deposition (e.g., spin-on deposition) of the already mixed substances in or outside of vacuum. The blocking layers 3 and 6 are likewise vapor-deposited in vacuo, but can also be prepared by another process, e.g., by spin-on deposition in or outside of vacuum.

FIG. 3 shows the luminance vs. voltage curve of a semitransparent OLED. For test purposes, a semitransparent gold contact (e.g., 50% transmission) was used. For a luminance of 100 cd/M 2 an operating voltage of 4 V is used. This value represents a low operating voltage for transparent OLEDs, especially those with an inverted layer structure. This OLED demonstrates the feasibility of the described subject matter. Because of the semitransparent cover electrode, the external current efficiency is limited to a value of about 2 cd/A, short of 5 cd/A as expected for OLEDs with pure Alq3 as the emitter layer.

Devices of the described subject matter demonstrate increased efficiency, lifetime, and transparency and decreased voltage.

The described devices can be fabricated more easily and reliably than existing OLEDS. The use of doping layers allows for directly depositing transparent conductive oxides over a charge carrier transport layer without the necessity of buffer light absorbing buffer layers such as CuPc or metal layers. Also, a multi-step deposition procedure for the ITO is not necessary.

The following examples demonstrate the features of the OLEDs of the described subject matter. For reference, the OLED performance is compared with bottom emitting OLEDs that are made in the same batch. The bottom emitting OLEDs are produced in parallel with the transparent OLEDs. The difference is that, on the bottom emitting devices, Al is deposited as a cathode instead of ITO.

Using a non-optimized structure, a variation of the ETM doped with ED-8 demonstrates some favorable ETMs. The structure was made on Glass/ITO substrate with the following layer sequence: 50 nm of NPD p-doped with OA-11; 10 nm of NPD as EBL; emitter host doped with 0.5 wt % of rubrene; 10 nm of ETM-6 as HBL; the ETM in the following table doped with ED-8 followed by 100 nm of ITO.

Voltage increase at 10 mA/cm{circumflex over ( )}2, Doping compared to the reference (e.g., ETM concentration the reference cathode is Ag) Alq3 10    2 V ETM-4 8 1.8 V ETM-6 8 0.8 V ETM-11 8 0.8 V ETM-6 10% + 1 nm 0.4 V pure dopant ETM-6 10% + 1 nm 0.4 V pure dopant

The following layer sequence is a non-optimized OLED structure which was used for the experiments (the thickness is given in parenthesis):

ITO (90 nm)

NPD (50 nm) doped with 3 wt % of OA-11

NPD (10 nm)

DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

ETM-6 (10 nm)

ETL=n-doped ETM (70 nm)

top ITO (100 nm)

The comparative bottom emitting devices have a 100 nm Aluminum layer in place of the top ITO layer.

A variation of the n-doping concentration of the ETL demonstrates a favorable doping concentration. The structure was made on Glass/ITO substrate with the following layer sequence: 50 nm of NPD p-doped with OA-11; 10 nm of NPD as EBL; emitter host doped with 0.5 wt % of rubrene; 10 nm of ETM-6 as HBL; the ETM: dopant system in the following table; followed by 100 nm of ITO.

Voltage increase at 10 mA/cm{circumflex over ( )}2, ETM: dopant Doping compared to the reference (e.g., system concentration the reference cathode is Ag) ETM-4: ED-8 8 1.53 V  ETM-6: ED-8 2 2.7 V ETM-6: ED-8 4 1.5 V ETM-6: ED-8 8 0.9 V ETM-11: ED-14 8 0.8 V ETM-11: ED-8 8 1.1 V ETM-11: ED-14 10 0.76 V 

It can be seen in the table above that the optimum doping concentration to achieve a low voltage, with a comparative voltage increase of less than 1 V, compared to the bottom emitting device, is higher than or equal to 8%. A dopant concentration greater than 25% is less desirable in the ETL. However, a highly doped buffer can be used in addition to the doped ETL.

Another embodiment includes a thin (e.g., 1 to 15 nm thick) highly doped charge transport layer at the interface of the top electrode (this layer being localized between the light-emitting region and the electrode). Another embodiment includes a thin (e.g., 0.5 nm to 3 nm) pure dopant layer as a buffer layer at the interface of the top electrode (between the charge transport layer and the top electrode).

ED-8:doped ETM with 1 nm ED-8 interlayer

Voltage increase at Voltage increase at 10 mA/cm2, compared to the 100 cd/m2, compared to the reference (e.g., the reference reference (e.g., the reference ETM cathode is Al) cathode is Al) ETM-4 1 0.75 ETM-6 0.43 0.30 ETM-9 0.39 0.31 ETM-11 0.19 0.26 ETM-4 0.84 0.59 ETM-6 0.47 0.27 ETM-9 0.67 0.44 Voltage at 10 Voltage at 10 Voltage at 10 mA/cm2 for ED-8 mA/cm2 for ED-14 mA/cm2 for ED-8 ETM (100 cd/m2) (100 cd/m2) (100 cd/m2) ETM-4 (2.72) ETM-6 2.77 (2.4) 2.58 (2.27) ETM-9  3.09 (2.63) 2.76 (2.42) 2.6 (2.28) ETM-11 3.9 (3.5) 2.55 (2.79)  ETM-40 2.9 (2.69)

Similar results as those with ED-14 were obtained with ED-3 and ED-4.

The devices of the described subject matter exhibit a good life-time behavior. The time before the device exhibits half of the initial brightness can be more than 10,000 h, under accelerated aging (See FIG. 4).

Comparative Examples

Comparative devices were constructed according to known techniques, without using doped layers. The anode (e.g, ITO) was treated with oxygen plasma before the deposition of the organic layers, to enhance the hole injection. A thin layer of Mg:Ag with an atomic ratio of 40:1 was deposited, as part of the cathode (e.g., electron injection layer), on top of the organic layers. A sputtered ITO layer followed the thin metal layer.

The performance of the comparative device is poor, even if the same organic stack is used. The comparative devices exhibit a voltage (at a current density of 10 mA/cm2) more than 1 V higher. The (cd/A) efficiency is reduced due to the additional absorption of the thin metal layer. The overall power efficiency is further reduced because of the additional effects of the absorption of the metal layer and the increased operating voltage.

The samples with doped layers exhibit a higher yield, especially the samples using the described diazapyrenes, asymmetrical phenanthrolines, and metal quinoxalines as doped ETM.

The comparative devices exhibited a low yield. Many included short circuits immediately after being produced. The cause is believed to be due to metal diffusion and sputter damage during metal and ITO deposition. the doped layers can improve the robustness of the device, not only against the sputtering process. By using doped layers (with organic doping), the yield and device efficiency can be higher, e.g., because these layers offer protection against sputtering. The doping effect can be stable and strong such that even after sputtering, the device performs well.

Example Embodiment

An example OLED has the following layer structure (non-inverted structure):

Substrate, glass

Anode, ITO (90 nm)

doped hole transport layer, NPD (50 nm) doped with 3 wt % of OA-11

non doped interlayer NPD (10 nm) (optionally an electron blocking layer)

Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

non doped interlayer E™-6 (10 nm) (optionally a hole blocking layer)

electron transport layer, ETM-6 n-doped with 10 wt % ED-14 (70 nm)

Cathode, ITO (100 nm)

Another example OLED with an inverted structure has:

Substrate, glass

Cathode, ITO (e.g., 100 nm)

electron transport layer, ETM-6 n-doped with 4 wt % ED-14 (40 nm)

non doped interlayer E™-6 (10 nm) (optionally a hole blocking layer)

Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

non doped interlayer NPD (10 nm) (optionally an electron blocking layer)

doped hole transport layer, NPD (80 nm) doped with 8 wt % of OA-11

Anode, ITO (90 nm)

FIG. 6 shows the luminance vs. voltage curve of a transparent OLED according to an embodiment with a non-inverted structure. For a luminance of 100 cd/m2 an operating voltage of 2.14 V is used. This operating voltage is one of the lowest voltages for transparent OLEDs.

The high transparency and the flatness of the optical transmittance of the inventive OLEDs are especially useful for white OLEDs. White OLEDs were constructed by different methods, such as mixing multiple emitters in one light-emitting region, or stacking OLEDs through so-called connecting units.

The use of doped layers according to the described subject matter makes it possible to attain nearly the same low operating voltages and high efficiencies in a transparent structure as occur in a traditional structure with one-sided emission through the substrate. This is due, as described, to the efficient charge-carrier injection, which, thanks to the doping, is relatively independent of the exact work function of the transparent contact materials. In this way the same electrode materials (or, e.g., transparent electrode materials of only slightly different work functions) can be used as electron-injecting contacts and hole-injecting contacts.

From the examples and the knowledge of one ordinarily skilled in the art, it is obvious to a person skilled in the art that many modifications and variations of the described subject matter are possible which fall within the scope of the described subject matter. For example, transparent contacts other than ITO can be used as anode materials (e.g., as in H. Kim et al., Appl. Phys. Lett. 76, 259 (2000); H. Kim et al., Appl. Phys. Lett. 78, 1050 (2001)). Furthermore, some embodiments include transparent electrodes made by combining a sufficiently thin intermediate layer of a nontransparent metal (e.g., silver or gold) and a thick layer of the transparent conductive material. In that case, the thickness of the intermediate layer is thin enough so that the device is still transparent (e.g., 75% transparent in the entire visible spectral region). Because of the thick doped charge-carrier transport layers, no damage to the light-emitting layers is to be expected during sputter. A further embodiment uses, for the doped electron transport layer, a material whose LUMO level is too deep (in the sense of FIGS. 1a-b and 2a-b layers 7 or 3a) to be able to efficiently inject electrons into the blocking layer and light-emitting layer (6 or 4a, and 5 or 5a, respectively) (thus, greater barriers than those shown in FIG. 2a). In that case, it is possible to use between the n-doped electron transport layer (7 or 3a) and blocking layer (6 or 4a) or the light-emitting layer (5 or 5a) a thin (2.5 nm) layer of a metal having a lower work function than the LUMO level of the doped transport layer. The metal layer is thin enough so that the overall transparency of the component is mostly maintained (see L. S. Hung, M. G. Mason, Appl. Phys. Lett. 78, 3732 (2001)).

Claims

1. A transparent, thermally stable light-emitting component having organic layers, comprising:

a transparent substrate;
a transparent anode;
a hole transport layer adjacent to the anode;
at least one light-emitting layer;
a charge-carrier transport layer for electrons; and
a transparent cathode,
wherein the transparency in the visible spectral region is at least 75%,
wherein the hole transport layer is p-doped with an acceptor organic material and the electron transport layer is n-doped with a donor organic material, and the molecular masses of the dopants are each greater than 200 g/mole, and
wherein the transparent, thermally stable light-emitting component having organic layers is an organic light-emitting diode.

2. A light-emitting component according to claim 1, further comprising:

at least one of a hole-side blocking layer located between the doped hole transport layer and the light-emitting layer or an electron-side blocking layer located between the doped electron transport layer and the light-emitting layer.

3. A light-emitting component according to claim 1, further comprising:

a electrode layer located between the anode and the hole transport layer and a electrode layer located between the charge-carrier transport layer and the cathode.

4. A light-emitting component according to claim 1, wherein the doping concentration of the organic dopants is such that an ohmic injection takes place from the anode into the charge-carrier transport layer or from the cathode into the hole transport layer.

5. A light-emitting component according to claim 3, wherein the electrode layers comprise indium tin oxide (ITO) or a degenerate oxide other than ITO.

6. A light-emitting component according to claim 1, wherein the cathode includes a metallic intermediate layer adjacent to the subjacent doped, charge-carrier transport layer when the cathode is located on top or the anode includes a metallic intermediate layer adjacent to the subjacent doped, hole transport layer when the anode is located on top and wherein the metallic layer has a nominal thickness between 0.1 nm and 3 nm.

7. A light-emitting component according to claim 1, wherein no metal layer is located between the doped hole transport layer and the anode when the anode is on top or between the doped electron transport layer and the cathode when the cathode is on top.

8. A light-emitting component according to claim 1, where the anode and cathode are located between the substrate and encapsulation cover and the transparency is at least 70% for each wavelength between at least 400 nm and 800 nm.

9. A light-emitting component according to claim 1, wherein the molar concentration of admixture in the hole transport layer or in the electron transport layer or in both the hole transport layer and the electron transport layer is in the range of 1:100,000 to 1:10, calculated on the ratio of doping molecules to main-substance molecules.

10. A light-emitting component according to claim 1, wherein the molar concentration of admixture in the hole transport layer or in the electron transport layer, or in both the hole transport layer and the electron transport layer, is at least 1 wt %, calculated on the ratio of doping molecules to main-substance molecules.

11. A light-emitting component according to claim 2, wherein the thickness of each of the hole transport layer or the electron transport layer, of the light-emitting layer and of the at least one of a hole-side blocking layer or an electron-side blocking layer lies in the range of 0.1 nm to 50 μm.

12. A light-emitting component according to claim 1, wherein the cathode is in direct contact with a doped transport layer and is facing away from the substrate when the cathode is on top or the anode is in direct contact with a doped transport layer and is facing away from the substrate when the anode is on top and wherein the doped transport layer is a hole transport layer or an electron transport layer.

13. A light-emitting component according to claim 1, wherein the organic n-dopant material is selected from the group consisting of heterocyclic radicals, diradicals, dimers, an oligomer, a polymer, a dispiro compound, and a polycycle thereof, having the structure according to one of the following formulae: wherein structures 3 and 4 have one or more cyclic linkages A and/or A1 and/or A2, wherein A1 and A2 are present individually or together and A1 and A2 are selected as in structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S, and wherein structure 7 has one or more bridge bonds Z and Z1, Z or Z1, Z1 and Z2, or Z1 or Z2, and Z, Z1 and Z2 are independently selected from the group consisting of alkyl, alkenyl, alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo, disulphide, heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl ether, polyether, primary alkylamine, arylamine, polyamine, aryl, and heteroaryl.

wherein A, A1 and A2 are selected from the group consisting of carbocyclic, heterocyclic, polycyclic ring systems, and any combination thereof, which may be substituted or unsubstituted,

14. A light-emitting component according to claim 1, wherein the organic acceptor organic material is a quiniode derivative or a triylidene derivative, with a reduction potential in the range of 0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc+.

15. A light-emitting component according to claim 12 or 13, wherein the n-doped, donor organic material is an asymmetrically substituted phenanthroline with the following structure wherein:

R1 and R2 are selected from the group consisting of substituted or unsubstituted Aryl, Heteroaryl, and Alkyl; and
R3 is selected from the group consisting of H, CN, substituted or unsubstituted Aryl, Heteroaryl, and Alkyl;
R4 is selected from the group consisting of H, CN, COOR with R=Alkyl, Heteroalkyl, Aryl or Heteroaryl, substituted or unsubstituted Aryl, Heteroaryl, Alkyl mit C1-C20, and Cycloalkyl mit C3-C20.

16. A light-emitting component according to claim 12 or 13, wherein the n-doped, donor organic material has the structure: wherein M is selected from the group consisting of Ti, Zr, Hf. Nb, Re, Sn and Ge, each R is independently selected from the group consisting of hydrogen, C1-C20-Alkyl, C1-C20-Alkenyl, C1-C20-Alkinyl, Aryl, Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl, —ORx, —NRxRy, —SRx, —NO2, —CHO, —COORx, —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SORx, SO2Rx, and where Rx and Ry are selected from the group consisting of C1-C20-Alkyl, C1-C20-Alkenyl, and C1-C20-Alkinyl.

17. A light-emitting component according to claim 12 or 13, wherein the n-doped, donor organic material has the structure: wherein R1, R2, R3, and R4 are independently selected from the group consisting of H, halogen, CN, substituted or unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy, and aryloxy.

18. A light-emitting component according to claim 1, wherein the anode is between the substrate and the at least one light-emitting layer.

19. A light-emitting component according to claim 1, wherein the cathode is between the substrate and the at least one light-emitting layer.

20. A light-emitting component according to claim 3, wherein the electrode layers include different transparent contact materials.

21. A light-emitting component according to claim 1, further comprising a contact-improving layer located between the electron transport layer and cathode and a contact-improving layer located between the anode and the hole transport layer, wherein the contact-improving layers are configured not to prevent charge from passing through.

22. A light-emitting component according to claim 1, further comprising a contact-improving layer located between the electron transport layer and cathode or a contact-improving layer located between the anode and the hole transport layer, wherein the contact-improving layers are configured not to prevent charge from passing through.

23. A light-emitting component according to claim 1, wherein the light-emitting layer includes a mixed layer of several materials.

24. A light-emitting component according to claim 1, wherein the p-doped hole transport layer includes a mixture of an organic main substance and an acceptor doping substance and an acceptor doping substance and the molecular mass of the dopants is greater than 200 g/mole.

25. A light-emitting component according to claim 1, wherein the electron transport layer includes a mixture of an organic main substance and a donor doping substance and an acceptor doping substance and the molecular mass of the dopants is greater than 200 g/mole.

26. A light-emitting component according to claim 1, wherein when the transparent cathode is on top, the transparent cathode includes a transparent protective layer or when the transparent anode is on top, the transparent anode includes a transparent protective layer.

27. A light-emitting component according to claim 1, wherein when the transparent cathode is on top, the transparent cathode includes a metallic intermediate layer adjacent to the subjacent doped charge-carrier transport layer or when the transparent anode is on top, the transparent anode includes a metallic intermediate layer adjacent to the subjacent doped hole transport layer,

wherein the transparency of the metal intermediate layer in the visible spectral region is at least 75% and the thickness of the metal intermediate layer is between 0.3 nm and 3 nm.

28. A light-emitting component according to claim 1, wherein the sequence of p-doped hole transport layer and transparent anode is repeated.

29. A light-emitting component according to claim 1, wherein the sequence of n-doped electron transport layer and transparent cathode is repeated.

30. A light-emitting component according to claim 2, further comprising a metallic electron-injection-promoting layer located between the doped electron transport layer and either the electron-side blocking layer or the light-emitting layer, wherein the transparency of the metallic electron-injection-promoting layer in the visible spectral region is at least 75%.

Patent History
Publication number: 20100026176
Type: Application
Filed: Jul 31, 2009
Publication Date: Feb 4, 2010
Inventors: Jan Blochwitz-Nomith (Dresden), Karl Leo (Dresden), Martin Pfeiffer (Dresden), Xiang Zhou (Guangzhou)
Application Number: 12/533,891
Classifications
Current U.S. Class: Organic Phosphor (313/504)
International Classification: H01J 1/62 (20060101);