Charge carrier modulation for color and brightness coordination in organic light-emitting diodes

Abstract

The device for charge carrier modulation is a current-controlled component, which has semiconductor layers arranged on top of each other. The organic semiconductor layers arranged on top of each other are an electron transport layer, which is arranged between a first and a second hole transport layer, and/or a hole transport layer, which is arranged between a first and a second electron transport layer. The respective central layer is the modulation layer having a contact for a modulation voltage. By applying a modulation voltage, a modulation current flow is generated over the modulation layer. The modulation current flow influences the component current flow which flows from the first into the second hole or electron transport layer via the respective modulation layer.

Claims

1. A current-controlled component comprising: a light emission region including at least one emission layer configured to emit light of at least one color; and at least one of: a hole modulator having a multiple hole transport layer arrangement including a first hole transport layer, a second hole transport layer, an electron transport layer arranged between the first hole transport layer and the second hole transport layer and configured as a first modulation layer, the first modulation layer having a first modulation layer contact for a first modulation voltage, generated by a first modulation voltage source, by which a first modulation current across the first modulation layer is generated, and a first modulation current flow path, including the first modulation voltage source connected to the first modulation layer contact, through the first modulation layer and the first hole transport layer, wherein the light emission region is arranged outside the first modulation current flow path, and wherein the first modulation current controls a component current from the first hole transport layer to the second hole transport layer through the first modulation layer, or an electron modulator having a multiple electron transport layer arrangement including a first electron transport layer, a second electron transport layer, a hole transport layer arranged between the first electron transport layer and the second electron transport layer and configured as a second modulation layer, the second modulation layer having a second modulation layer contact for a second modulation voltage generated by a second modulation voltage source, by which a second modulation current across the second modulation layer is generated, a second modulation current flow path, including the second modulation voltage source connected to the second modulation layer contact, through the second modulation layer and the first electron transport layer, wherein the light emission region is arranged outside the second modulation current flow path, and wherein the second modulation current controls a second component current from the first electron transport layer to the second electron transport layer through the second modulation layer.

2. The current-controlled component as claimed in claim 1, wherein the organic semiconductor layers lie on one another in a vertical layer stack.

3. The current-controlled component as claimed in claim 1, further comprising one of: heterojunctions formed between the organic semiconductor layers lying on one another, or intrinsic interlayers arranged between the organic semiconductor layers.

4. The current-controlled component as claimed in claim 1, wherein the first modulation layer in the multiple hole transport layer arrangement has a thickness which is less than a diffusion length of minority charge carriers in a semiconductor material of the first modulation layer, or the second modulation layer in the multiple electron transport layer arrangement has a thickness which is less than a diffusion length of minority charge carriers in a semiconductor material of the second modulation layer.

5. The current-controlled component as claimed in claim 1, wherein in the multiple hole transport layer arrangement, the first modulation voltage is applied to the first modulation layer and to the first hole transport layer so that majority charge carriers in a semiconductor material of the first hole transport layer and the second hole transport layer and minority charge carriers in a semiconductor material of the first modulation layer are moved from the first hole transport layer into the first modulation layer, or in the multiple electron transport layer arrangement, the second modulation voltage is applied to the second modulation layer and to the first electron transport layer so that majority charge carriers in a semiconductor material of the first electron transport layer and the second electron transport layer and minority charge carriers in a semiconductor material of the second modulation layer are moved from the first electron transport layer into the second modulation layer.

6. The current-controlled component as claimed in 1, wherein: in the multiple hole transport layer arrangement, at least the first modulation layer and one of the first and second hole transport layers have a doping concentration of between 0.01 vol % and 10 vol %, or in the multiple electron transport layer arrangement, at least the second modulation layer and one of the first and second electron transport layers have a doping concentration of between 0.01 vol % and 10 vol %.

7. The current-controlled component as claimed in claim 1, wherein: in the multiple hole transport layer arrangement, the first modulation layer has a doping concentration of between 0.01 vol % and 10 vol %, or in the multiple electron transport layer arrangement, the second modulation layer has a doping concentration of between 0.01 vol % and 10 vol %.

8. An organic light-emitting diode comprising at least one current-controlled component as claimed in claim 1.

9. The organic light-emitting diode as claimed in claim 8, comprising an emission zone comprising at least two emission layers which emit light of different colors.

10. An organic light-emitting diode comprising at least one current-controlled component as claimed in claim 1, wherein in the multiple hole transport layer arrangement, a color rendering of the organic light-emitting diode is varied by application of the first modulation voltage, or in the multiple electron transport layer arrangement, a color rendering of the organic light-emitting diode is varied by application of the second modulation voltage.

11. An organic light-emitting diode comprising a current-controlled component as claimed in claim 1, wherein: in the multiple hole transport layer arrangement, efficiency is variable by application of the first modulation voltage, or in the multiple electron transport layer arrangement, efficiency is variable by application of the second modulation voltage.

12. An organic light-emitting diode comprising: a multiple hole transport layer arrangement including a first hole transport layer, a second hole transport layer, a first modulation layer arranged between the first hole transport layer and the second hole transport layer and being an electron transport layer, the first modulation layer having a first contact for a first modulation voltage, generated by a first modulation voltage source, by which a first modulation current across the first modulation layer is generated, and a first modulation current flow path, including the first modulation voltage source connected to the first modulation layer contact, through the first modulation layer and the first hole transport layer, and wherein the first modulation current controls a component current from the first hole transport layer to the second hole transport layer through the first modulation layer; a multiple electron transport layer arrangement including a first electron transport layer, a second electron transport layer, and a second modulation layer arranged between the first electron transport layer and the second electron transport layer and being a hole transport layer, the second modulation layer having a second contact for a second modulation voltage, generated by a second modulation voltage source, by which a second modulation current across the second modulation layer is generated, a second modulation current flow path, including the second modulation voltage source connected to the second modulation layer contact, through the second modulation layer and the first electron transport layer, and wherein the second modulation current controls a second component current from the first electron transport layer to the second electron transport layer through the second modulation layer; and an emission zone comprising at least one emission layer and interposed between the multiple hole transport layer and the multiple electron transport layer arrangement, the emission zone configured to receive a hole current from the multiple hole transport layer arrangement and an electron current from the multiple electron transport layer arrangement wherein the emission zone is arranged outside both the first modulation current flow path and the second modulation current flow path.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

(2) FIG. 1 shows a hole modulator,

(3) FIG. 2 shows an electron modulator,

(4) FIG. 3 shows a diagram with a plot of the layer thickness and diffusion length

(5) FIG. 4 shows an organic light-emitting diode having a hole modulator,

(6) FIG. 5 shows an organic light-emitting diode having an electron modulator, and

(7) FIG. 6 shows an organic light-emitting diode having an electron modulator and a hole modulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(8) Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

(9) The hole modulator 10 shown in FIG. 1 includes three organic semiconductor layers, which are arranged above one another. For the production of such a three-layer system, a first layer which is a hole transporter ht.sub.2, a second layer which is an electron transport layer et.sub.m, and a third layer which is a hole transport layer ht.sub.2, are deposited. There is thus an electron transport layer et.sub.m between two hole transport layers ht.sub.1 and ht.sub.2.

(10) This electron transport layer et.sub.m constitutes the modulation layer. The three organic semiconductor layers are respectively contactable. The first hole transport layer ht.sub.1 is connected via a voltage source U.sub.1 to the second hole transport layer ht.sub.2. By the voltage supply U.sub.1, a voltage U.sub.1 can be applied to the two outer hole transport layers ht.sub.1, ht.sub.2. The voltage U.sub.1 is applied in such a way that hole transport takes place from the first hole transport layer ht.sub.1 through the modulator layer et.sub.m into the second hole transport layer ht.sub.2. The first hole transport layer ht.sub.1 is connected via a further contact to a second voltage source U.sub.h and via the latter to the electron transport layer et.sub.m. A further voltage U.sub.h is thus applied to the electron transport layer, or modulation layer, et.sub.m and to the first hole transport layer ht.sub.1.

(11) The electron transport layer et.sub.m is a heavily doped n-type conducting layer. The hole current from the first hole transporter ht.sub.1 to the second hole transporter ht.sub.2 is influenced by this modulator layer et.sub.m. Owing to the excess of holes in the electron transport layer et.sub.m, where these are minority charge carriers, and the concomitant concentration gradient, a diffusion current flows so that the holes are transported to the second hole transporter ht.sub.2. The holes injected into the first hole transport layer ht.sub.1 are in this case accelerated in the strong electric field of the electron transporter et.sub.m. The first hole transporter ht.sub.1 is preferably heavily doped, so that there is a high conductivity. This improves the property of the hole modulator for modulation of the hole current. The number of free charge carriers in an undoped organic transport material is usually very low, and lies in the range of from 10.sup.5/cm.sup.3 to 10.sup.9/cm.sup.3. The doping is selected in such a way that the number of free charge carriers is increased by up to 10 orders of magnitude to a range of from 10.sup.15/cm.sup.3 to 10.sup.19/cm.sup.3. To this end, dopant concentrations in the range of from 0.01 vol % to 50 vol % are used.

(12) The thickness d.sub.e of the electron transport layer, or modulator layer, et.sub.m must be less than the diffusion length L.sub.diff of the minority charge carriers, which in this case are holes. The diffusion length L.sub.diff can be determined from the charge carrier mobility and the lifetime of the free charge carriers.

(13) For example, the electron transport material is Alq (tris(8-hydroxyquinolinato)aluminum). The field-dependent hole mobility .sub.h in Alq is

(14) ${}_h\left(\mathrm{Alq}\right)=1.2.Math.{10}^{-7}\frac{{\mathrm{cm}}^2}{V.Math.s}.Math.\exp \left(\sqrt{\frac{U_h/d_e}{1959\mathrm{kV}\text{/}\mathrm{cm}}}\right)$

(15) With the assumption of a free charge carrier lifetime of 50 ns, a maximum value of 10 nm is obtained for the layer thickness d.sub.e of an Alq modulator layer. This value is calculated from the hole mobility .sub.h in Alq, from the lifetime of the charge carriers and from the electric field, which is directly proportional to the applied modulator voltage U.sub.h.

(16) FIG. 2 shows an electron modulator. It is in turn constructed from three organic semiconductor layers. The organic semiconductor layers are arranged vertically above one another, so that the current flows vertically through the component. A hole transport layer ht.sub.m is arranged above a second electron transport layer et.sub.2. A first electron transport layer et.sub.1 is in turn arranged above. The three layers lie on one another in such a way that large-area contact of the electron transport layer with the hole transport layer is respectively established. The hole transport layer ht.sub.m has a substantially smaller layer thickness d.sub.u than the electron transport layers et.sub.1, et.sub.2. The hole transport layer ht.sub.m constitutes the modulation layer for the electron flow. A voltage U.sub.2 is applied to the two outer electron transport layers et.sub.1, et.sub.2 in such a way that an electron flow takes place from the first electron transporter et.sub.1 to the second electron transporter et.sub.2, i.e. through the hole transport layer ht.sub.m. The latter, however, initially constitutes a barrier. The electron transport layer et.sub.1 is provided with a further contact, so that a voltage U.sub.e can be applied between the first electron transport layer et.sub.1 and the hole transport modulator layer ht.sub.m. The voltage is applied in such a way that a flow of electrons takes place from the first electron transporter et.sub.1 across the hole transporter ht.sub.m. The barrier property of the modulator layer ht.sub.m decreases with an increase in the voltage U.sub.e. By the current from the first electron transporter et.sub.1 across the hole transporter ht.sub.m, the total current from the first electron transporter et.sub.1 to the second electron transporter et.sub.2 can therefore be controlled.

(17) An electron modulator is thus constructed in a similar way to the hole modulator construction in the vertical arrangement of a three-layer system that includes two electron transport layers et.sub.1, et.sub.2 and a hole transport layer ht.sub.m arranged between them. In order to ensure the best possible properties of the electron modulator layer ht.sub.m, high charge carrier densities must be respected in the electron transport layer. To this end, above all, the second electron transport layer et.sub.2 is heavily doped. In order to prevent recombination of charge carriers in the hole transport layer ht.sub.m, i.e. the modulator layer of the electron modulator, the layer thickness d.sub.h of the modulator layer must again be kept so small that it does not exceed the diffusion length L.sub.diff of the electrons in the material, i.e. of the minority charge carriers in the modulator layer. Often, in this case, high charge carrier mobilities are associated with long diffusion lengths L.sub.diff.

(18) One example of a hole conductor, which may be used as a modulator layer in the electron modulator, is -NPD (N,N-di(naphthalen-1-yl)-N,N-diphenylbenzidine). The field-dependent electron mobility .sub.e in -NPD is

(19) ${}_e\left(-\mathrm{NPD}\right)=6.1.Math.{10}^{-6}\frac{{\mathrm{cm}}^2}{V.Math.s}.Math.\exp \left(\sqrt{\frac{U_e/d_h}{1500\mathrm{kV}\text{/}\mathrm{cm}}}\right)$

(20) The maximum layer thickness for this hole transport layer is calculated from the electron mobility .sub.e and lifetime , as well as from the applied field, which is directly proportional to the modulation voltage U.sub.e. If a free charge carrier lifetime of 50 ns is in turn assumed, the maximum value for the layer thickness d.sub.h of the modulator layer is calculated as 100 nm.

(21) Examples of electron transport materials are: 2,2,2-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline 8-hydroxyquinolinolato-lithium 4-(naphthalen-1-yl)-3,5-diphenyl-4H-1,2,4-triazole 1,3-bis[2-(2,2-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene 4,7-diphenyl-1,10-phenanthroline 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum 6,6-bis[5-(biphenyl-4-yl)-1,3,4-oxadiazo-2-yl]-2,2-bipyridyl 2-phenyl-9,10-di(naphthalen-2-yl)-anthracene 2,7-bis[2-(2,2-bipyridin-6-yl)-1,3,4-oxadiazo-5-yl]-9,9-dimethylfluorene 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene 2-(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane 1-methyl-2-(4-(naphthalen-2-yl)phenyl)-1H-imidazo[4,5f][1,10]phenanthroline

(22) In order to increase the electron conductivity of these electron transport materials, they may be doped.

(23) Examples of dopants are: alkali metals, alkaline earth metals, lanthanides, such as Li, Na, K, Cs, Ca, Mg, Sr, Ba or Sm inorganic salts which increase the electron conductivity: Cs.sub.2CO.sub.3 metalorganic complexes with a strongly reducing effect, such as W.sub.2(TBD).sub.4, Mo.sub.2(TBD).sub.4, where TBD is the anion of 1,5,7-triazabicyclo[4.4.0]dec-5-ene, or metal(O) complexes such as Mo(CO).sub.6 or W(CO).sub.6.

(24) Examples of hole transport materials are: N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)-9,9-dimethylfluorene N,N-bis(3-methylphenyl)-N,N-bis(phenyl)-9,9-diphenylfluorene N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)-9,9-diphenylfluorene N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)-2,2-dimethylbenzidine N,N-bis(3-methylphenyl)-N,N-bis(phenyl)-9,9-spirobifluorene 2,2,7,7-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)-benzidine N,N-bis(naphthalen-2-yl)-N,N-bis(phenyl)-benzidine N,N-bis(3-methylphenyl)-N,N-bis(phenyl)-benzidine N,N-bis(3-methylphenyl)-N,N-bis(phenyl)-9,9-dimethylfluorene N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)-9,9-spirobifluorene di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane 2,2,7,7-tetra(N,N-di-tolyl)amino-spiro-bifluorene 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene 2,2,7,7-tetrakis[N-naphthalenyl(phenyl)-amino]-9,9-spirobifluorene 2,7-bis[N,N-bis(9,9-spiro-bifluorene-2-yl)-amino]-9,9-spirobifluorene 2,2-bis[N,N-bis(biphenyl-4-yl)amino]-9,9-spiro-bifluorene N,N-bis(phenanthren-9-yl)-N,N-bis(phenyl)-benzidine N,N,N,N-tetra-naphthalen-2-yl-benzidine 2,2-bis(N,N-di-phenyl-amino)-9,9-spirobifluorene 9,9-bis[4-(N,N-bis-naphthalen-2-yl-amino)phenyl]-9H-fluorene 9,9-bis[4-(N,N-bis-naphthalen-2-yl-N,N-bis-phenyl-amino)-phenyl]-9H-fluorene titanium oxide phthalocyanine copper phthalocyanine 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyano-quinodimethane 4,4,4-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine 4,4,4-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine 4,4,4-tris(N-(1-naphthyl)-N-phenyl-amino)triphenylamine 4,4,4-tris(N,N-diphenyl-amino)triphenylamine pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile N,N,N,N-tetrakis(4-methoxyphenyl)benzidine 2,7-bis[N,N-bis(4-methoxy-phenyl)amino]-9,9-spirobifluorene 2,2-bis[N,N-bis(4-methoxy-phenyl)amino]-9,9-spirobifluorene N,N-di(naphthalen-2-yl)-N,N-diphenylbenzene-1,4-diamine N,N-di-phenyl-N,N-di-[4-(N,N-di-tolyl-amino)phenyl]-benzidine N,N-di-phenyl-N,N-di-[4-(N,N-di-phenyl-amino)phenyl]-benzidine

(25) In addition, however, metalorganic complexes, such as tris(phenylpyridinato)iridium(III) or related compounds, are suitable as hole transport materials.

(26) In order to increase the hole conductivities of these hole transport materials, they may be doped.

(27) Examples of dopants are: inorganic salts or oxides which increase the hole conductivity: MoO.sub.3, WO.sub.3, Re.sub.2O.sub.7, FeCl.sub.3, metalorganic complexes with a strong Lewis acid effect, such as Rh.sub.2 (O.sub.2CCF.sub.3).sub.4, or Ru compounds, organic acceptor molecules such as F.sub.4-TCNQ.

(28) FIG. 3 shows a diagram which illustrates the calculation of the maximum layer thickness d.sub.max of the modulator layer et.sub.m,ht.sub.m. The layer thickness d.sub.e/h of the modulator layer et.sub.m,ht.sub.m is plotted on the abscissa. The modulator layer thicknesses d.sub.e/h are between 110.sup.10 m and 110.sup.6 m. The diffusion length L.sub.diff of the minority charge carriers in the modulator layer et.sub.m,ht.sub.m is plotted on the ordinate. The diffusion length L.sub.off of the minority charge carriers is between 110.sup.10 m and 110.sup.4 m. In the diagram, the modulator layer thickness values d.sub.e/h are first plotted. To this end, the diffusion lengths L.sub.diff of the minority charge carriers were plotted as a function of the layer thickness d.sub.e/h of the modulator layer et.sub.m/ht.sub.m. The dependency of the layer thickness d.sub.e/h is dictated by the field dependency of the diffusion length L.sub.diff, or the charge carrier mobility .

(29) The maximum layer thickness d.sub.max of a modulator layer et.sub.m,ht.sub.m is denoted by a dashed line and d.sub.max in the diagram. For layer thicknesses less than d.sub.max, a functional charge carrier modulator can be produced. For layer thicknesses d.sub.B which lie above d.sub.max, the electron or hole transport layer respectively between the hole or electron transport layers forms a barrier layer, which constitutes a charge barrier, and throttles or reduces the charge flow irrespective of an applied voltage.

(30) FIG. 4 shows a schematic structure of an organic light-emitting diode 30 having a hole modulator 10. From bottom to top, the layer stack first shows a substrate 31, which is for example a glass substrate. An anode 32 is applied on the glass substrate 31. The anode is preferably transparent, for example formed of indium tin oxide.

(31) The light generated in the organic light-emitting diode 30 can be output from the component through the transparent anode 32 on the glass substrate 31. The light output 50 is indicated by an arrow. A hole injection layer h.sub.i is applied on the anode 32. Thereon there is a hole modulator 10, i.e. a first hole transport layer ht.sub.1, an electron transport layer et.sub.m thereon and in turn a second hole transport layer ht.sub.2 thereon. The modulator voltage U.sub.h is applied to the electron transport layer et.sub.m and to the anode 32. The hole modulator 10 is followed by the emission region 40. The emission region 40 may include a plurality of emitter layers. For example, a red emission layer 43, a green emission layer 42, and a blue emission layer 41 thereon, follow on from the hole transporters ht.sub.2. Recombination 45 of the charge carriers preferably takes place in this emission region 40. The recombination zone 45 can be displaced by the hole and electron current. Thus, depending on the ratio of electrons to holes reaching the emission zone 40, more recombinations can take place in one of the emitter layers 41-43. For example, a hole blocker hb follows on above the emitter layers 41-43. An electron transport layer et.sub.m is deposited thereon. An electron injection layer e.sub.i is applied thereon. Above this, there is a cathode 33. A voltage U.sub.d, by which the component is operated, is applied to the cathode 33 and anode 32. The voltage U.sub.d is applied in such a way that electrons are injected into the component at the cathode 33 and holes are injected into the component at the anode 32. The electron current I.sub.e through the injection and electron transport layers e.sub.i, e.sub.t into the emitter layers 41-43 is denoted by an arrow from the cathode 33 in the direction of the recombination center 45. The hole current I.sub.h is similarly denoted by an arrow from the anode 32 through the hole modulator layers 10 into the recombination zone 45. The hole current I.sub.h which reaches the recombination zone 45 can be adjusted by the hole modulator 10.

(32) FIG. 5 in turn shows an organic light-emitting diode 30. It again comprises a layer stack of organic semiconductor layers, which lies between a cathode 33 and an anode 32 on a glass substrate 31. An arrow again indicates the light output 50 through the transparent anode 32 and the transparent substrate 31. The organic layers follow on from this substrate 31 and the anode 32: first a hole injection layer h.sub.i, and a hole transporter ht thereon. The latter is followed by the emission region 40 comprising, for example, three emitter layers 43, 42, 41. A hole blocker hb is, for example, arranged above the emitter layers 41-43. Above the hole blocker is the electron transport region. The latter is an electron modulator 20. That is to say, first a second electron transporter et.sub.2, a hole transporter ht.sub.m thereon for modulating the electrons, and a first electron transporter et.sub.1 thereon, are arranged on the hole blocker layer hb. Above the electron modulator 20, there is furthermore an electron injection layer ei below the cathode 33. The component voltage U.sub.d is in turn applied to the cathode 33 and anode 32. This voltage U.sub.d is applied in such a way that an electron flow I.sub.e takes place from the cathode 33 into the organic light-emitting diode, and hole transport I.sub.u takes place from the anode 32 into the organic light-emitting diode. The modulator voltage U.sub.e is applied to the cathode 33 and to the hole transport modulator layer ht.sub.m. It is applied directionally in such a way that an electron flow takes place from the cathode 33 through the electron injection layer e.sub.i and the first electron transport layer et.sub.1 into the hole transporter ht.sub.m. By application of the voltage U.sub.e, the barrier property of the hole transporter ht.sub.m between the electron transport layers et.sub.1/et.sub.2 is reduced and the electron current I.sub.e is thus increased. The electron injection current I.sub.e modulated in this way is indicated by an arrow from the cathode 33 in the direction of the emission layers 40. The hole transport I.sub.u takes place from the anode 32 in the direction of the emission layers 40, and is also indicated by an arrow. Where electrons and holes encounter one another, recombination 45 of the charge carriers can take place. This position can be influenced by the ratio of electrons to holes. By the electron modulation, the recombination zone 45 within the emission region 40 can be displaced into the different emission layers 41-43.

(33) Lastly, FIG. 6 again shows an organic light-emitting diode 30 having an electron modulator 20 and a hole modulator 10. That is to say, the layer stack of organic semiconductor layers comprises a hole modulator 10, an electron modulator 20 and an emission layer system 40. The hole modulator is in turn applied on a hole injection layer h.sub.i, which is itself applied on an anode 32 on a glass substrate 31. Above the hole modulator 10 there are the emission layers 40, a hole blocker hb thereon, and above this there is the electron modulator 20. The electron injection from the cathode 33 into the organic light-emitting diode takes place across an electron injection layer e.sub.i, which is arranged between the cathode 33 and the electron modulator 20. In order to modulate the charge carriers, the component voltage U.sub.d is applied to the cathode 33 and the anode 32, the hole modulation voltage U.sub.h is additionally applied to the electron transport layer et.sub.m of the hole modulator 10, and the electron modulation voltage U.sub.e is additionally applied to the hole transport layer ht.sub.m of the electron modulator 20. In the organic light-emitting diode 30, a hole blocker hb may be introduced on the cathode side and an electron blocker may be introduced on the anode side, in order to deliberately confine the individual charge carriers in the emitting layer 40.

(34) The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase at least one of A, B and C as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).