DISPLAY DEVICE COMPRISING A COMMON CHARGE GENERATION LAYER AND METHOD FOR MAKING THE SAME

Abstract

The present invention relates to a display device comprising a common charge generation layer and method for making the same.

Claims

1.-16. (canceled)

17. A display device, comprising a plurality of pixels, wherein each pixel of the plurality of pixels comprises at least two vertically stacked electroluminescent units and at least one charge generation layer arranged between the at least two vertically stacked electroluminescent units, wherein each electroluminescent unit comprises at least one light-emitting layer; wherein at least two pixels of the plurality of pixels share as the charge generation layer at least one common charge generation layer, wherein the at least one common charge generation layer comprises a common n-type charge generation layer comprising an organic electron transport compound and a metal dopant; and a common p-type charge generation layer comprising an organic hole transport compound and an organic p-dopant, wherein the organic hole transport compound is present in the p-type charge generation layer in an amount of ?60 wt %, based on the total weight of the p-type charge generation layer; wherein, the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?1.2 V to ?20 V, and the organic electron transport compound has an electron mobility, determined by measuring the voltage drop V.sub.EOD, in the range of ?0.1 V to ?20 V.

18. Display device according to claim 17, wherein the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?1.5 V to ?20 V.

19. Display device according to claim 17, wherein the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?1.7 V to ?20 V.

20. Display device according to claim 17, wherein the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?1.9 V to ?20 V.

21. Display device according to claim 17, wherein the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?2.1 V to ?20 V.

22. Display device according to claim 17, wherein the organic hole transport compound has a hole mobility, determined by measuring the voltage drop V.sub.HOD, in the range of ?2.3 V to ?20 V.

23. Display device according to claim 17, wherein the organic hole transport compound has an energy level of the HOMO, expressed in the absolute scale referring to vacuum energy level being zero, measured by cyclic voltammetry, in the range of ??6 eV to ??5.10 eV.

24. Display device according to claim 17, wherein the organic hole transport compound is present in the p-type charge generation layer in an amount of ?65 wt %, based on the total weight of the p-type charge generation layer.

25. Display device according to claim 17, wherein the organic p-dopant has an energy level of the LUMO, expressed in the absolute scale referring to vacuum energy level being zero, calculated using the hybrid functional B3LYP with a Gaussian 6-31G* basis set as implemented in the program package TURBOMOLE V6.5, in the range from ??5.7 eV to ??4.7 eV.

26. Display device according to claim 17, wherein the organic p-dopant is selected from a compound according to formula (I) ##STR00576## wherein in formula (I) A.sup.1 is independently selected from a group according to formula (II) ##STR00577## wherein Ar.sup.1 is independently selected from substituted or unsubstituted C.sub.6 to C.sub.36 aryl and substituted or unsubstituted C.sub.2 to C.sub.36 heteroaryl; wherein for the case that Ar.sup.1 is substituted, one or more of the substituents are independently selected from the group consisting of an electron-withdrawing group, F, CN, partially fluorinated or perfluorinated alkyl, and NO.sub.2; A.sup.2 and A.sup.3 are independently selected from a group of formula (III) ##STR00578## wherein Ar.sup.2 is independently selected from substituted or unsubstituted C.sub.6 to C.sub.36 aryl and substituted or unsubstituted C.sub.2 to C.sub.36 heteroaryl; wherein for the case that Ar.sup.2 is substituted, one or more of the substituents are independently selected from an electron-withdrawing group, F, CN, partially fluorinated or perfluorinated alkyl, and NO.sub.2; and wherein each R is independently selected from substituted or unsubstituted C.sub.6 to C.sub.18 aryl, C.sub.3 to C.sub.18 heteroaryl, electron-withdrawing group, partially fluorinated or perfluorinated C.sub.1 to C.sub.8 alkyl, halogen, F or CN.

27. Display device according to claim 17, wherein the common p-type charge generation layer has a thickness of ?30 nm.

28. Display device according to claim 17, wherein the organic electron transport compound has an energy level of the LUMO, expressed in the absolute scale referring to vacuum energy level being zero, calculated using the hybrid functional B3LYP with a Gaussian 6-31G* basis set as implemented in the program package TURBOMOLE V6.5, in the range from ??3.5 eV to ??1 eV.

29. Display device according to claim 17, wherein the organic electron transport compound comprises at least 15 covalently bound atoms.

30. Display device according to claim 17, wherein the organic electron transport compound is present in the common n-type charge generation layer in an amount of ?0.1 wt %, based on the total weight of the n-type charge generation layer.

31. Display device according to claim 17, wherein the metal dopant is a metal selected from the group consisting of Yb, or a metal alloys comprising a metal selected from the group consisting of Li or Yb.

32. Display device according to claim 17, wherein the common n-type charge generation layer has a thickness of ?30 nm.

33. Display device according to claim 17, wherein the common charge generation layer has a sheet resistance Rs in the range from ?0.2 giga ohms per square to ?10.sup.6 giga ohms per square, when measured alone.

34. Display device according to claim 17, wherein the display device comprises a driving circuit configured to separately driving the pixels of the plurality of pixels.

35. Display device according to claim 17, wherein the hole mobility is measured by measuring the voltage drop V.sub.HOD at a fixed current density of 10 mA/cm.sup.2 at 20? C. to 25? C. for a hole only device, and the electron mobility is measured by measuring the voltage drop V.sub.EOD at a fixed current density of 15 mA/cm.sup.2 at 20? C. to 25? C. for an electron only device; wherein the hole only device has a structure in the following order a glass substrate, 90 nm anode layer consisting of ITO with a sheet resistance of 15 ohm/square, 10 nm layer consisting of the organic hole transport compound and 10 wt % 4,4,4-((1E,1E,1E)-cyclopropane-1,2,3-triylidenetris(cyanomethanylylidene))tris(2,3,5,6-tetrafluorobenzonitrile), 400 nm layer consisting of the organic hole transport compound 10 nm layer consisting of CNHAT (Dipyrazino[2,3-f:2,3-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile), 100 nm cathode layer consisting of silver; and wherein the electron only device has a structure in the following order glass substrate 100 nm anode layer consisting of silver, 30 nm anode layer consisting of Mg:Ag (90:10 vol %), 1 mm layer consisting of 8-Hydroxyquinolinolato-lithium, 36 nm layer consisting of the organic electron transport compound, 1 nm layer consisting of 8-Hydroxyquinolinolato-lithium, 30 nm cathode layer consisting of Mg:Ag (90:10 vol %); wherein the layer of the structure for measuring the voltage drop are deposited by evaporation.

36. Method for the manufacture of a display device according to claim 17, the method comprising the step of manufacturing the at least one common charge generation layer, wherein each common layer is deposited onto the complete display area through one large mask opening in one processing step.

Description

DESCRIPTION OF THE DRAWINGS

[0355] The aforementioned components, as well as the claimed components and the components to be used in accordance with the invention in the described embodiments, are not subject to any special exceptions with respect to their size, shape, material selection and technical concept such that the selection criteria known in the pertinent field can be applied without limitations.

[0356] Additional details, characteristics and advantages of the object of the invention are disclosed in the dependent claims and the following description of the respective figures which in an exemplary fashion show preferred embodiment according to the invention. Any embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation as claimed.

FIGS. 1 to 8

[0357] FIG. 1 is a schematic sectional view of a test elements for measuring the sheet resistance;

[0358] FIG. 2 is a schematic sectional view of deposited layer stack.

[0359] FIG. 3 is a schematic sectional view of a test element;

[0360] FIG. 4 is a schematic sectional view of a test element;

[0361] FIG. 5 is a schematic sectional view of a test element for determining the voltage drop in an hole only device;

[0362] FIG. 6 is a schematic sectional view of a test element for determining the voltage drop in an electron only device;

[0363] FIG. 7 is a schematic sectional view of a display device according to one exemplary embodiment of the present invention; and

[0364] FIG. 8 is a schematic sectional view of a pixel in a display device according to one exemplary embodiment of the present invention.

[0365] Hereinafter, the FIGS. 1 to 8 are illustrated in more detail with reference to examples. However, the present disclosure is not limited to the following figures.

[0366] Herein, when a first element is referred to as being formed or disposed on or onto a second element, the first element can be disposed directly on the second element, or one or more other elements may be disposed there between. When a first element is referred to as being formed or disposed directly on or directly onto a second element, no other elements are disposed there between.

[0367] FIG. 1 shows a schematic representation of test elements or devices for measuring the sheet resistance of the charge generation layer. Particularly, the test devices comprise on a substrate charge generation bilayer consisting of a p-type charge generation layer and an n-type charge generation layer. Devices with 1, 2 or 3 bilayers are exemplarily shown.

[0368] FIG. 2 shows schematic representation of a deposited layer stack on a test element group for determining the sheet resistance. The test element consists of a channel (test element). The deposition layer (300) lies on top of the electrode.

[0369] FIG. 3 shows the test element comprising 2 highly conductive electrodes with a gap I between the 2 highly conductive electrodes and a width w for the respective highly conductive electrodes.

[0370] FIG. 4 shows the test element comprising 2 highly conductive electrodes with interdigitated finger pattern with a gap 1 between electrode fingers and total width w as product of interdigitation width we and number of interdigitation areas.

[0371] FIG. 5 shows a schematic sectional view of a test element for determining the voltage drop in an hole only device, wherein the device comprises a substrate 100. On the substrate 100 there is an anode layer 110 followed by a hole injection layer 120 consisting of the hole transport compound and 4,4,4-((1E,1E,1E)-cyclopropane-1,2,3-triylidenetris(cyanomethanylylidene))tris(2,3,5,6-tetrafluorobenzonitrile) and a hole transport layer 130 deposited on the hole injection layer 120. The hole transport layer 130 is followed by a layer comprising 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HAT-CN) to prevent electron injection from cathode 150.

[0372] FIG. 6 shows a schematic sectional view of a test element for determining the voltage drop in an electron only device, wherein the device comprises a substrate, a first anode layer 210 followed by a second anode layer 220. On the second anode layer 220, there is an electron injection layer 230 followed by a layer 240 consisting of electron transport compound. Said layer is contacted by an electron injection layer 250 and the cathode 260.

[0373] FIG. 7 shows a schematic sectional view of a display device 400 according to one exemplary embodiment of the present invention. In particular the display device 400 includes a substrate 410, the anode layers 420 and 420, the first vertically stacked electroluminescent units 510 and 510, a common n-type charge generation layer 485c, a common p-type charge generation layer 435c, the second vertically stacked electroluminescent units 520 and 520, and a cathode layer 490.

[0374] FIG. 8 shows a schematic sectional view of a pixel 401 in a display device 400 according to one exemplary embodiment of the present invention. In particular, the pixel 401 includes a substrate 410, an anode layer 420, a hole injection layer (HIL) 430, a first vertically stacked electroluminescent unit 510 including a first hole transport layer (HTL) 440, a first electron blocking layer (EBL) 445, a first emission layer (EML) 450, a first hole blocking layer (HBL) 455, and a first electron transport layer (ETL) 460; an n-type charge generation layer (n-CGL) 485, a p-type charge generation layer (p-GCL) 435 which may comprise compound of formula (I), a second vertically stacked electroluminescent unit 520 including a second hole transport layer (HTL) 441, a second electron blocking layer (EBL) 446, a second emission layer (EML) 451, a second hole blocking layer (EBL) 456, and a second electron transport layer (ETL) 461; an electron injection layer (EIL) 480 and a cathode layer 490. The HIL may comprise a compound of Formula (I). The p-type charge generation layer 435 and the n-type charge generation layer 485 are a common p-type charge generation layer 435c and a common n-type charge generation layer 485c according to the invention.

[0375] Hereinafter, one or more exemplary embodiments of the present invention will be described in detail with, reference to the following examples. However, these examples are not intended to limit the purpose and scope of the one or more exemplary embodiments of the present invention.

DETAILED DESCRIPTION

[0376] The invention is furthermore illustrated by the following examples which are illustrative only and non-binding. The compound may be prepared as described in the literature or alternative compounds may be prepared following similar compounds as described in the literature.

Calculated HOMO and LUMO

[0377] The HOMO and LUMO are calculated with the program package TURBOMOLE V6.5 (TURBOMOLE GmbH, Litzenhardtstrasse 19, 76135 Karlsruhe, Germany). The optimized geometries and the HOMO and LUMO energy levels of the molecular structures are determined by applying the hybrid functional B3LYP with a 6-31G* basis set in the gas phase. If more than one conformation is viable, the conformation with the lowest total energy is selected.

Measured HOMO

[0378] The energy level of the highest occupied molecular orbital is derived from cyclic voltammetry of molecules in solution and expressed in the physical absolute scale against vacuum taken as zero energy level. The given HOMO levels were calculated from redox potential V, (measured by cyclic voltammetry (CV) as specified below and expressed in the scale taking the potential of standard redox pair ferricenium/ferrocene (Fc.sup.+/Fc) equal zero) according to equation E.sub.HOMO=?q*V.sub.cv?4.8 eV, wherein q* stands for the charge of an electron (1e).

[0379] The redox potential can be determined by cyclic voltammetry, e.g. with a potentiostatic device Metrohm PGSTAT30 and software Metrohm Autolab GPES at room temperature. The redox potentials given at particular compounds was measured in an argon de-aerated, dry 0.1M THF (Tetrahydrofuran) solution of the tested substance, under argon atmosphere, with 0.1M tetrabutylammonium hexafluorophosphate supporting electrolyte, between platinum working electrodes and with an Ag/AgCl pseudo-standard electrode (Metrohm Silver rod electrode), consisting of a silver wire covered by silver chloride and immersed directly in the measured solution, with a scan rate of 100 mV/s. In the measurement, the first run was done in the broadest range of the potential set on the working electrodes, and the range was then adjusted within subsequent runs appropriately. The final three runs were done with the addition of ferrocene (in 0.1M concentration) as the standard. The average of potentials corresponding to cathodic and anodic peak of the studied compound, after subtraction of the average of cathodic and anodic potentials observed for the standard Fc+/Fc redox couple, afforded finally the values reported above. All studied compounds as well as the reported comparative compounds showed well-defined reversible electrochemical behavior. Alternatively, dichloromethane can be used as solvent.

[0380] A simple rule is very often used for the conversion of redox potentials into electron affinities (EA) and ionization potential (IP): IP (in eV)=4.80 eV+e*Eox (wherein Eox is given in Volt vs. ferrocene/ferrocenium (Fc/Fc+) and EA (in eV)=4.80 eV+e*Ered (Ered is given in Volt vs. Fc/Fc+), respectively (see B. W. D'Andrade, Org. Electron. 6, 11-20 (2005)), e* is the elemental charge. It is common practice, even if not exactly correct, to use the terms energy of the HOMO E(HOMO) and energy of the LUMO E(LUMO), respectively, as synonyms for the ionization energy and electron affinity (Koopmans Theorem).

Sheet Resistance Measurement

[0381] Determination of sheet resistance of a single CGL requires the measurement of the sheet resistance using the transmission line method on CGL test devices with a varied number of CGLs (FIG. 1). These CGL test devices are manufactured by deposition of a number of bilayers consisting of first a p-CGL followed by an n-CGL. By variation of number of bilayers compensation of interface effects can be done. Usually devices with 1, 2 and 3 bilayers are used. This gives 1, 3 and 5 p-n-interfaces. One single CGL is represented by half of the layer thickness of p- and n-CGL thickness used for bilayers. Single CGL sheet resistance is calculated by plotting reciprocal value of sheet resistance of CGL test devices versus number of p-n-interfaces and doing a linear fit. Sheet resistance is equal to reciprocal value of the slope. This calculation compensates substrate influence and interface to air on top of layer stack.

Transmission Line Method

[0382] Sheet resistance is measured for a given layer stack on a test element group (see FIG. 2) consisting of test elements called channel providing two highly conductive electrodes with gap l between each other and a width w of each (see FIG. 3). To improve noise level large w may be used and electrodes formed in interdigitated finger structure for reduced area requirement (see FIG. 4). At least three of such channels with different channel length l are needed for reliable sheet resistance calculation.

[0383] The measurement is done in following way:

[0384] First voltage is swept between ?5V to +5V with step size smaller than 1V and current is measured. Using a linear fit with current values on x-axis and voltage values on y-axis slope of the fit gives channel resistance R. This procedure is repeated for each channel n, resulting in an Rn for channel n with gap ln and width w. For Rs calculation width w must be constant over all channels. Fit quality index R.sup.2 must be larger than 0.9 for a reliable resistance value.

[0385] In the second step resistance Rn is plotted over channel with gap ln and linear fitted. Slope of this fit multiplied with channel width w gives sheet resistance of measured stack. This calculation removes influence of contact- and series resistance from measurement setup. Electrode gap should be close to target application gap, for example 20 ?m, 40 ?m, 60 ?m. Channel width for low conductive layers might be 100000 ?m.

[0386] The measurement is done on glass substrates (Corning Eagle XG) with ITO electrodes. After wet cleaning with water based tenside solution and drying by blowing with nitrogen followed by 200? C. 120 minutes baking in nitrogen atmosphere substrates are conditioned with nitrogen plasma. After organic layer deposition before electrical measurement samples are encapsulated using encapsulation glass lids including a desiccant.

Measurement of Hole Mobility

[0387] Hole mobility parameter for a given hole transport compound (HTM) is determined by measuring the voltage drop at fixed current density of 10 mA/cm.sup.2 (HOD voltage V.sub.HOD) at 22? C. for hole only device HOD with highly doped hole injection layer HIL and a thick hole transport layer HTL. Highly doped hole injection layer reduces voltage drop from anode to hole transport layer under test and thick hole transport layer ensures device voltage drop is dominated by hole transport layer. Hole only device stack made according to FIG. 5. On substrate 100 there is an ITO anode layer 110 with thickness of 90 nm and 15 ohm/sq sheet resistance. This is followed by a 10 nm hole injection layer 120 consisting of hole transport compound doped with 10 wt % of a compound with the following formula

##STR00510##

[0388] Adjacent 400 nm of the hole transport compound is deposited to form hole transport layer 130. This layer is followed by a 10 nm 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HAT-CN) layer 140, what prevents electron injection from cathode 150 consisting of a 100 nm silver layer.

[0389] HOD voltage V.sub.HOD is a measure for hole mobility allowing for comparison of different hole transport materials. Higher values mean lower mobility.

Measurement of Electron Mobility

[0390] Electron mobility parameter for a given electron transport compound (ETM) is given by measurement of voltage drop at fixed current density of 15 mA/cm.sup.2 at 22? C. for electron only device EOD according to stack shown in FIG. 6. On glass substrate 200 a first anode layer 210 with thickness 100 nm consisting of Silver is deposited. This layer is followed by second anode layer 220 consisting of Magnesium co-evaporated with 40 wt. % Silver. Next electron injection layer 230 consisting of 1 nm LiQ follows. Adjacent layer 240 consisting of 36 nm electron transport compound is deposited. This is followed by 1 nm electron injection layer (EIL) 250 consisting of LiQ for LiQ host or PO-containing LiQ-free ETMs. For non-PO LiQ-free ETMs 3-Phenyl-3H-benzo[b]dinaphtho[2,1-d:1,2-f]phosphepine-3-oxide:Yb(1 vol %) is used in this EIL. The EOD stack is finished by cathode 260 consisting of 30 nm Magnesium co-evaporated with 40 wt. % Silver.

[0391] EOD voltage V.sub.EOD is a measure for electron mobility allowing for comparison of different electron transport materials. Higher values mean lower mobility.

General Procedure for Fabrication of OLEDs

[0392] For the examples according to the invention and comparative examples in Table 2, a glass substrate with an anode layer comprising a first anode sub-layer of 10 nm ITO, a second anode sub-layer of 120 nm Ag and a third anode sub-layer of 8 nm ITO was cut to a size of 100 mm?100 mm?0.7 mm, ultrasonically washed with water for 60 minutes and then with isopropanol for 20 minutes. The liquid film was removed in a nitrogen stream, followed by plasma treatment, see Table 2, to prepare the anode layer. The plasma treatment was performed in an atmosphere comprising 97.6 vol.-% nitrogen and 2.4 vol.-% oxygen.

[0393] Then N-([1,1-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine was vacuum deposited with 8 wt.-% 2,2,2-(cyclopropane-1,2,3-triylidene)tris(2-(p-cyanotetrafluorophenyl)acetonitrile) to form a hole injection layer having a thickness 10 nm.

[0394] Then N-([1,1-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine was vacuum deposited, to form a first hole transport layer having a thickness of 29 nm

[0395] Then N,N-di([1,1-biphenyl]-4-yl)-3-(9H-carbazol-9-yl)-[1,1-biphenyl]-4-amine was vacuum deposited on the HTL, to form an electron blocking layer (EBL) having a thickness of 5 nm.

[0396] Then 97 wt.-% H09 (Sun Fine Chemicals, Korea) as EML host and 3 wt.-% BD200 (Sun Fine Chemicals, Korea) as fluorescent blue dopant were deposited on the EBL, to form a first blue-emitting EML with a thickness of 19 nm.

[0397] Then 2-(3-(9,9-dimethyl-9H-fluoren-2-yl)-[1,1-biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine was vacuum deposited to form a first hole blocking layer having a thickness of 5 nm.

[0398] Then, the electron transporting layer (ETL) having a thickness of 10 nm is formed on the hole blocking layer by depositing 2,2-(1,3-Phenylene)bis[9-phenyl-1,10-phenanthroline; Then a n-type CGL having a thickness of 7.5 nm is formed on the ETL1 by co-depositing 95 wt.-% of an electron transport compound (ETM of n-CGL) according to Table 2 and 5 wt.-% Yb.

[0399] Then a p-type CGL having a thickness of 10 nm is formed on the first n-type CGL by co-depositing a hole transport compound (HTM of p-CGL) according to Table 2 with 10 wt % 2,2,2-(cyclopropane-1,2,3-triylidene)tris(2-(p-cyanotetrafluorophenyl)acetonitrile) (compound B80) as organic p-dopant.

[0400] Then a second hole transport layer having a thickness of 44 nm is formed on the first p-type CGL by depositing a hole transport compound (HTM of second HTL) according to Table 2.

[0401] Then a second electron blocking layer having a thickness of 5 nm is formed on the second hole transport layer by depositing N,N-di([1,1-biphenyl]-4-yl)-3-(9H-carbazol-9-yl)-[1,1-biphenyl]-4-amine,

[0402] Then 97 wt.-% H09 (Sun Fine Chemicals, Korea) as EML host and 3 wt.-% BD200 (Sun Fine Chemicals, Korea) as fluorescent blue dopant were deposited on the second EBL, to form a second blue-emitting EML with a thickness of 19 nm.

[0403] Then 2-(3-(9,9-dimethyl-9H-fluoren-2-yl)-[1,1-biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine was vacuum deposited to form a second hole blocking layer having a thickness of 5 nm is formed on the second blue-emitting EML.

[0404] Then, 50 wt.-% 2-(2,6-diphenyl-[1,1:4,1-terphenyl]-4-yl)-4-phenyl-6-(3-(pyridin-4-yl)phenyl)-1,3,5-triazine and 50 wt.-% LiQ were vacuum deposited on the second hole blocking layer to form a second electron transport layer having a thickness of 31 nm.

[0405] Then Yb was evaporated at a rate of 0.01 to 1 ?/s at 10.sup.?7 mbar to form an electron injection layer with a thickness of 2 nm on the electron transporting layer.

[0406] Ag/Mg (1.8 wt %) is evaporated at a rate of 0.01 to 1 ?/s at 10.sup.?7 mbar to form a cathode with a thickness of 13 nm.

[0407] Then, N-({[1,1-biphenyl]-4-yl)-9,9,dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine} was vacuum deposited on the cathode layer to form a capping layer with a thickness of 75 nm.

[0408] To assess the performance of the inventive examples compared to the prior art, the current efficiency is measured at 20? C. The current-voltage characteristic is determined using a Keithley 2635 source measure unit, by sourcing a voltage in V and measuring the current in mA flowing through the device under test. The voltage applied to the device is varied in steps of 0.1V in the range between 0V and 10V. Likewise, the luminance-voltage characteristics and CIE coordinates are determined by measuring the luminance in cd/m.sup.2 using an Instrument Systems CAS-140CT array spectrometer (calibrated by Deutsche Akkreditierungsstelle (DAkkS)) for each of the voltage values. The cd/A efficiency at 10 mA/cm2 is determined by interpolating the luminance-voltage and current-voltage characteristics, respectively.

[0409] In bottom emission devices, the emission is predominately Lambertian and quantified in percent external quantum efficiency (EQE). To determine the efficiency EQE in % the light output of the device is measured using a calibrated photodiode at 10 mA/cm2.

[0410] In top emission devices, the emission is forward directed, non-Lambertian and also highly dependent on the mirco-cavity. Therefore, the efficiency EQE will be higher compared to bottom emission devices. To determine the efficiency EQE in % the light output of the device is measured using a calibrated photodiode at 10 mA/cm.sup.2.

[0411] Lifetime LT of the device is measured at ambient conditions (20? C.) and 30 mA/cm.sup.2, using a Keithley 2400 sourcemeter, and recorded in hours.

[0412] The brightness of the device is measured using a calibrated photo diode. The lifetime LT is defined as the time till the brightness of the device is reduced to 97% of its initial value.

[0413] The increase in operating voltage ?U is used as a measure of the operational voltage stability of the device. This increase is determined during the LT measurement and by subtracting the operating voltage after 1 hour after the start of operation of the device from the operating voltage after 100 hours.

?U=[U100 h)?U(1 h)] [0414] or the operating voltage after 1 hour after the start of operation of the device from the operating voltage after 400 hours.

?U=[U400 h)?U(1 h)]

[0415] The smaller the value of AU the better is the operating voltage stability.

Technical Effect of the Invention

[0416]

TABLE-US-00004 TABLE 1 Hole mobility and HOMO level (CV = Cyclic voltammetry) Structure of hole HOMO CV HOMO transport compound (ref ?4.80 B3LYP/ (HTM) V.sub.HOD eV) 6-31G Compar- ative [00511] 1,038 ?5,13 ?4.69 F2 [00512] 1.56 ?5,25 ?4.81 F4 [00513] 2.336 ?5,10 ?4.69 F1 [00514] 1.29 ?5.14 ?4.75 F12 [00515] 6.15 ?5.13 ?4.71 F11 [00516] 4.54 ?5.25 ?4.93 F3 [00517] 1.86 ?5.15 ?4.73 F14 [00518] 8.67 ?5.25 ?4.96 F5 [00519] 3.45 ?5.09 ?4.70 F19 [00520] 2.81 ?5.02 ?4.64 F20 [00521] 2.50 ?5.15 ?4.75 F8 [00522] 3.79 ?5.18 ?4.83 F13 [00523] 7.88 ?5.26 ?4.94 F9 [00524] 3.90 ?5.23 ?4.84 F7 [00525] 3.47 ?5.26 ?4.91 F6 [00526] 3.12 ?5.24 ?4.87 F10 [00527] 3.92 ?5.26 ?4.96

TABLE-US-00005 TABLE 2 Performance tests LUMO of LT97 ETM of Voltage [h] Life- n-CGL Rs at 15 Voltage 30 time V.sub.HOD ETM of n-CGL B3LYP/ V.sub.EOD [G?/ mA/ rel. mA/ Rel. HTM of second HTL HTM of p-CGL (hole transport compound) [V] (electron transport compound) 6-31G* [V] square] cm.sup.2 [%] cm.sup.2 [%] Comp. Ex. 1 [00528] [00529] 1.04 [00530] ?1.74 0.16 5 7.15 100 117 100 Comp. Ex. 2 [00531] [00532] >>20V [00533] ?1.74 0.16 >5000 Does not result in a working OLED, i.e. it does exhibit luminescence. Comp. Ex. 3 [00534] [00535] >>20V [00536] ?1.74 0.16 31 Does not result in a working OLED, i.e. it does exhibit luminescence. Comp. Ex. 4 [00537] [00538] 2.336 [00539] ?1.74 0.16 20 9.07 127 71 61 Ex. 1 [00540] [00541] 1.56 [00542] ?1.75 0.41 124 7.42 100 118 100 Ex. 2 [00543] [00544] 1.56 [00545] ?1.75 0.41 124 7.41 100 113 96 Ex. 3 [00546] [00547] 1.56 [00548] ?1.74 0.16 111 7.09 96 168 143 Ex. 4 [00549] [00550] 6.15 [00551] ?1.74 0.16 154 7.27 102 121 104 Ex. 5 [00552] [00553] 6.15 [00554] ?1.75 0.41 153 7.55 106 117 100 Ex. 6 [00555] [00556] 8.67 [00557] ?1.74 0.16 5280 7.37 99 161 137 Ex. 7 [00558] [00559] 8.67 [00560] ?1.75 0.41 19278 7.63 103 112 95 Ex. 8 [00561] [00562] 2.50 [00563] ?1.75 0.41 43 7.44 104 78 67 Ex. 9 [00564] [00565] 2.50 [00566] ?1.68 0.62 44 7.71 108 72 62 Ex. 10 [00567] [00568] 3.79 [00569] ?1.75 0.41 4454 7.75 1.08 81 69 Ex. 11 [00570] [00571] 4.54 [00572] ?1.75 0.41 3282 7.61 1.06 82 70 Ex. 12 [00573] [00574] 4.54 [00575] ?1.68 0.62 3534 7.87 110 75 64

[0417] When using a hole transport material with a low hole mobility in CGL (e.g. Ex. 7) in comparison to a hole transport material with a high hole mobility in the p-CGL (comparative example), the sheet resistance can be significantly increased, while a good OLED performance such as low operating voltage and a high device lifetime can be maintained.

[0418] This may be beneficial for reducing the cross-talk of pixels in display, while the low operating voltage is beneficial for the battery life or a low energy consumption, and a long lifetime may be beneficial for long-time stability of a display.

[0419] The particular combinations of elements and features in the above detailed embodiments are exemplary only; the interchanging and substitution of these teachings with other teachings in this and the patents/applications incorporated by reference are also expressly contemplated. As those skilled in the art will recognize, variations, modifications, and other implementations of what is described herein can occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the foregoing description is by way of example only and is not intended as limiting. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The invention's scope is defined in the following claims and the equivalents thereto. Furthermore, reference signs used in the description and claims do not limit the scope of the invention as claimed.