Organic Light Emitting Device and a Compound for Use Therein

Abstract

The present invention relates to a organic light emitting device comprising a cathode, an anode, a light emitting layer, at least one first electron transport layer and at least one second electron transport layer, wherein the light emitting layer, the first electron transport layer and the second electron transport layer are arranged between the cathode and the anode, wherein the first electron transport layer comprises a compound of formula (I) and the second electron transport layer comprises a compound of formula (II) as well as to a compound for use in an organic electronic device.

##STR00001##

Claims

1. Organic light emitting device comprising a cathode, an anode, a light emitting layer, at least one first electron transport layer and at least one second electron transport layer, wherein the light emitting layer, the first electron transport layer and the second electron transport layer are arranged between the cathode and the anode, wherein the first electron transport layer comprises a compound of formula (I)
L-M wherein in formula (I) L is selected from the C.sub.6 to C.sub.60 unsubstituted or substituted aryl, wherein the one or more substituents, if present, are independently selected from C.sub.2 to C.sub.24 alkenyl, C.sub.1 to C.sub.20 alkyl, C.sub.7 to C.sub.32 aryl-alkyl, C.sub.7 to C.sub.32 aryl-alkenyl; M is substituted pyrimidine or substituted triazine, wherein the one or more substituents, are independently selected from C.sub.6 to C.sub.24 aryl wherein the C.sub.6 to C.sub.24 aryl may be unsubstituted or substituted with C.sub.1 to C.sub.3 alkyl; L and M may be connected via a direct bond or via a spacer moiety selected from non-condensed C.sub.6 to C.sub.18 aryl which may be unsubstituted or substituted wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.12 aryl, C.sub.2 to C.sub.20 heteroaryl, and phosphine oxide group; and the compound of formula (I) has a molecular dipole moment of ≤5 Debye; wherein the second electron transport layer comprises a compound of formula (II) ##STR00043## wherein in formula (II) A.sup.1 is selected from unsubstituted or substituted C.sub.10 to C.sub.24 aryl comprising at least one aromatic ring system comprising at least two condensed 6-membered aromatic rings, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.24 aryl, C.sub.2 to C.sub.20 heteroaryl and these substituents may be further substituted with a phosphine oxide group; X.sup.1 is selected from unsubstituted or substituted C.sub.6 to C.sub.24 aryl, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.12 aryl, C.sub.2 to C.sub.10 heteroaryl and these substituents may be further substituted with a phosphine oxide group or CN; Y.sup.1 to Y.sup.3 are independently selected from N and CR.sup.2 provided that at least two of Y.sup.1 to Y.sup.3 are N, wherein the R.sup.2 is selected from H or C.sub.1 to C.sub.3 alkyl; Z is selected from unsubstituted or substituted C.sub.6 to C.sub.24 aryl or unsubstituted or substituted C.sub.8 to C.sub.24 heteroaryl, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.24 aryl, C.sub.1 to C.sub.20 alkyl, phosphine oxide group and CN; and R.sup.1 is selected from unsubstituted or substituted C.sub.6 to C.sub.24 aryl, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.24 aryl, C.sub.1 to C.sub.20 alkyl, phosphine oxide group and CN.

2. Organic light emitting device according to claim 1, wherein M is substituted triazine wherein the one or more substituents, are independently selected from C.sub.6 to C.sub.24 aryl which may be unsubstituted or substituted with C.sub.1 to C.sub.3 alkyl.

3. Organic light emitting device according to claim 1, wherein the spacer moiety is phenylene or biphenylene.

4. Organic light emitting device according to claim 1, wherein A.sup.1 is substituted anthracene, wherein the one or more substituents, are independently selected from C.sub.6 to C.sub.24 aryl and C.sub.2 to C.sub.20 heteroaryl and these substituents may be further substituted with a phosphine oxide group or CN.

5. Organic light emitting device according to claim 1, wherein X.sup.1 is phenylene.

6. Organic light emitting device according to claim 1, wherein ah Y.sup.1 to Y.sup.3 are N.

7. Organic light emitting device according to claim 1, wherein Z is selected from unsubstituted or substituted C.sub.6 to C.sub.24 aryl.

8. Organic light emitting device according to claim 1, wherein the at least one second electron transport layer comprises a metal, a metal salt or a metal complex.

9. Organic light emitting device according to claim 1, wherein the at least one second electron transport layer does not comprise a metal or a metal salt or a metal complex.

10. Compound having the general formula (III) ##STR00044## wherein A.sup.2 is selected from unsubstituted or substituted C.sub.10 to C.sub.24 aryl comprising at least two condensed 6-membered aromatic rings, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.24 aryl, C.sub.2 to C.sub.20 heteroaryl and these substituents may be substituted with a phosphine oxide group or CN; X.sup.2 is either a single bond or unsubstituted or substituted C.sub.6 to C.sub.24 aryl not comprising condensed 6-membered aromatic rings, wherein the one or more substituents, if present, are selected from C.sub.6 to C.sub.12 aryl, C.sub.2 to C.sub.10 heteroaryl and these substituents may be substituted with a phosphine oxide group or CN; Y.sup.4 to Y.sup.6 are independently selected from N and CR.sup.4 provided that at least two of R.sup.4 to R.sup.6 are N, wherein R.sup.4 is selected from H or C.sub.1 to C.sub.3 alkyl; G is selected from unsubstituted or substituted C.sub.8 to C.sub.24 heteroaryl comprising at least one 5-membered ring, wherein the one or more substituents, if present, are independently selected from C.sub.6 to C.sub.24 aryl, C.sub.1 to C.sub.20 alkyl, phosphine oxide group and CN and the heteroatom is selected from O, S, Se, N; and R.sup.3 is selected from unsubstituted or substituted C.sub.6 to C.sub.24 aryl, wherein the substituents are selected from C.sub.6 to C.sub.24 aryl, C.sub.1 to C.sub.20 alkyl, phosphine oxide group and CN.

11. Compound according to claim 10, wherein A.sup.2 is substituted anthracene wherein the one or more substituents are independently selected from C.sub.6 to C.sub.24 aryl and C.sub.2 to C.sub.20 heteroaryl and these substituents may be further substituted with a phosphine oxide group or CN.

12. Compound according to claim 10, wherein X.sup.2 is phenylene.

13. Compound according to claim 10, wherein all Y.sup.4 to Y.sup.6 are N.

14. Compound according to claim 10, wherein G is selected from the group consisting of dibenzofurane, benzofurane, dibenzothiophene and benzothiophene.

15. Compound according to claim 10, wherein R.sup.3 is phenyl.

16. The compound according to claim 10, wherein the heteroatom is selected from O, S, or Se.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0180] These and/or other aspects and advantages of the present invention will become apparent and more readily appreciated, from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, of which:

[0181] FIG. 1 is a schematic sectional view of a tandem OLED comprising a charge generation layer (CGL), according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0182] Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below, in order to explain the aspects of the present invention, by referring to the figures.

[0183] 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.

[0184] Referring to FIG. 1, the OLED 200 includes a substrate 110, an anode 120, a first hole injection layer (HIL) 130, a first hole transport layer (HTL) 140, a first electron blocking layer (EBL) 145, a first emission layer (EML) 150, a first electron transport layer or hole blocking layer (HBL) 155, a second electron transport layer 160, an n-type charge generation layer (n-type CGL) 185, a hole generating layer (p-type charge generation layer; p-type GCL) 135, a second hole transport layer (HTL) 141, a second electron blocking layer (EBL) 146, a second emission layer (EML) 151, a further first electron transport layer or hole blocking layer (HBL) 156, a further second electron transport layer 161, a second electron injection layer (EIL) 181 and a cathode 190.

[0185] While not shown in FIG. 1 a sealing layer may further be formed on the cathode electrodes 190, in order to seal the OLED 200. In addition, various other modifications may be applied thereto.

[0186] 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.

EXPERIMENTAL PART

[0187] The invention is furthermore illustrated by the following examples which are illustrative only and non-binding.

Example Structures

[0188]

TABLE-US-00004 Respective Compound Formula in terms Name Structure of the chains ET-1 [00021] Compound of formula (I) ET-2 [00022] Compound of formula (I) ET-15 [00023] Compound of formula (I) ET-3 [00024] Compound of formula (I) ET-11 [00025] Compound of formula (II) ET-12 [00026] Compound of formula (II) ET-13 [00027] Compound of formula (II) ET-14 [00028] Compound of formula (II) ET-16 [00029] Compound of formula (II) ET-4 [00030] Compound of formula (III) ET-5 [00031] Compound of formula (III) ET-6 [00032] Compound of formula (III) ET-7 [00033] Compound of formula (III) ET-8 [00034] Compound of formula (III) ET-9 [00035] Compound of formula (III) ET-10 [00036] Compound of formula (III)

Experimental Data

Melting Point

[0189] The melting point (mp) is determined as peak temperatures from the DSC curves of the above TGA-DSC measurement or from separate DSC measurements (Mettler Toledo DSC8226, heating of samples from room temperature to completeness of melting with heating rate 10 K/min under a stream of pure nitrogen. Sample amounts of 4 to 6 mg are placed in a 40 μL Mettler Toledo aluminum pan with lid, a <1 mm hole is pierced into the lid).

Glass Transition Temperature

[0190] The glass transition temperature (Tg) is measured under nitrogen and using a heating rate of 10 K per min in a Mettler Toledo DSC 822e differential scanning calorimeter as described in DIN EN ISO 11357, published in March 2010.

Rate Onset Temperature

[0191] The rate onset temperature (TRO) is determined by loading 100 mg compound into a VTE source. As WE source a point source for organic materials may be used as supplied by Kurt J. Lesker Company (www.lesker.com) or CreaPhys GmbH (http://www.creaphys.com). The VTE source is heated at a constant rate of 15 K/min at a pressure of less than 10-5 mbar and the temperature inside the source measured with a thermocouple. Evaporation of the compound is detected with a QCM detector which detects deposition of the compound on the quartz crystal of the detector. The deposition rate on the quartz crystal is measured in Angstrom per second. To determine the rate onset temperature, the deposition rate is plotted against the VTE source temperature. The rate onset is the temperature at which noticeable deposition on the QCM detector occurs. For accurate results, the VTE source is heated and cooled three time and only results from the second and third run are used to determine the rate onset temperature.

[0192] To achieve good control over the evaporation, rate of an organic compound, the rate onset temperature may be in the range of 200 to 255° C. If the rate onset temperature is below 200° C. the evaporation may be too rapid and therefore difficult to control. If the rate onset temperature is above 255° C. the evaporation rate may be too low which may result in low tact time and decomposition of the organic compound in VTE source may occur due to prolonged exposure to elevated temperatures.

[0193] The rate onset temperature is an indirect measure of the volatility of a compound. The higher the rate onset temperature the lower is the volatility of a compound.

Reduction Potential

[0194] The reduction potential is determined by cyclic voltammetry with potenioststic device Metrohm PGSTAT30 and software Metrohm Autolab GPES at room temperature. The redox potentials given at particular compounds were measured in an argon de-aerated, dry 0.1M THF 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 the scan rate 100 mV/s. 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 behaviour.

Dipole Moment

[0195] The dipole moment |{right arrow over (μ)}| of a molecule containing N atoms is given by:

[00001]$\stackrel{.fwdarw.}{\mu }=\underset{i}{\overset{N}{.Math.}}{q}_i\stackrel{.fwdarw.}{r_1}$$.Math.\stackrel{.fwdarw.}{\mu }.Math.=\sqrt{{\mu }_x^2+{\mu }_y^2+{\mu }_z^2}$

where q.sub.i and {right arrow over (r.sub.1)} are the partial charge and position of atom i in the molecule.

[0196] The dipole moment is determined by a semi-empirical molecular orbital method.

[0197] The geometries of the molecular structures are optimized using the hybrid functional B3LYP with the 6-31G* basis set in the gas phase as implemented in the program package TURBOMOLE 6.5 (TURBOMOLE GmbH, Litzenhardtstrasse 19, 76135 Karlsruhe, Germany). If more than one conformation is viable, the conformation with the lowest total energy is selected to determine the bond lengths of the molecules.

Calculated HOMO and LUMP

[0198] 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.

Synthesis Procedures

[0199] General synthetic procedure for the preparation the coupling between unsymmetrically substituted chloro-triazine (A) and boronic add (B):

Synthesis of 2-(dibenzo[b,d]furan-3-yl)-4-phenyl-6-(10-phenylanthracen-9-yl)-1,3,5-triazine (ET-4)

[0200] ##STR00037##

[0201] In a 500 mL 3-necked flask, were placed following compounds: 2-chloro-4-(dibenzo[b,d]furan-3-yl)-6-phenyl-1,3,5-triazine (A, log, 28 mmol, 1 eq.), phenylanthranylboronic acid (B, 9.2 g, 31 mmo, 1.1 eq.) and potassium carbonate (7.7 g, 55.9 mmol, 2 eq.) together with THF (224 mL) and dest. water (56 mL). The yellow suspension was degassed by bubbling N.sub.2 for 60 minutes. Then Pd(dppf)Cl.sub.2 (0.1 g, 0.14 mmol, 0.5% molar) was added under nitrogen counter flow. The reaction was then heated 19 h at 75° C. (bath temperature) under nitrogen. TLC control shows reaction completion. After cooling to rt and evaporation of the solvent, residue was extracted with 4 L DCM/Chloroform and the organic layer washed in portions with in total 7.5 L of water until pH neutral. After filtering over Florisil layer, the organic layer was evaporated and the residue treated by 200 mL hexane. Precipitate was filtered and dried under high vacuum two times at 120° C. (melting). Obtained 14.4 g (89%). MS [M+H]+ 576.

[0202] According to this procedure, the synthesis of following molecules was performed:

2-([1,1′-biphenyl]-3-yl)-4-phenyl-6-(3-(10-phenylanthracen-9-yl)phenyl)-1,3,5-triazine (ET-3)

[0203] ##STR00038##

[0204] A: 15 g 43.6 mmol (1 eq.) and B 17.9 g 47.99 mmol (1.1 eq.). Obtained 20.3 g (73%) MS [M+H]+ 638.

2-(dibenzo[b,d]furan-3-yl)-4-phenyl-6-(3-(10-phenylanthracen-9-yl)phenyl)-1,3,5-triazine (ET-6)

[0205] ##STR00039##

[0206] A: 7.91 g 22.1 mmol (1 eq.) and B: 10.6 g 23.23 mmol (1.05 eq.), Obtained 13.5 g (94%). [M+H]+ 652.

2-(dibenzo[b,d]furan-3-yl)-4-phenyl-6-(4-(10-phenylanthracen-9-yl)phenyl)-1,3,5-triazine

[0207] ##STR00040##

[0208] A: 8.14 g 22.7 mmol (1 eq.) and B: 8.94 g 23.9 mmol (1.05 eq.). Obtained 11.6 g (78%) MS [M+H]+ 652.

2-(benzo[b]thiophen-2-yl)-4-(4-chlorophenyl)-6-phenyl-1,3,5-triazine

[0209] ##STR00041##

[0210] A: 50 g 166 mmol (1 eq.) and B: 30.93 g 174 mmol (1.05 eq.). Obtained 9.3 g. Material was farther reacted as follows:

2-(benzo[b]thiophen-2-yl)-4-phenyl-6-(4-(10-phenylanthracen-9-yl)phenyl)-1,3,5-triazine

[0211] ##STR00042##

[0212] In a 1000 mL 2-necked flask equipped with, a mechanical stirrer, were placed following compounds: 2-(benzo[b]thiophen-2-yl)-4-(4-chlorophenyl)-6-phenyl-1,3,5-triazine (A, 9 g, 22.5 mmol, 1 eq.), 4,4,5,5-tetramethyl-2-(10-phenylanthracen-9-yl)-1,3,2-dioxaborolane (B, 10.3 g, 27 mmol, 1.2 eq.) and dissolved with the addition of 225 mL THF. In Parallel, an aqueous solution of potassium phosphate (9.6 g, 45 mmol, 2 eq.) in 22.5 mL distillated water was degassed by bubbling N.sub.2 for 30 minutes. The base solution was transferred into the THF solution and the mixture degassed again. Then as catalyst Pd-172 [1798781-99-3] (0.27 g, 0.45 mmol, 2% molar) was added under nitrogen counter flow. The orange solution was then heated 18 h at 55° C. (bath temperature) under nitrogen. TLC control (Eluent DCM/hexane 1:1) shows total consumption of B. After cooling to rt, the precipitate was isolated by filtration, washed with 20 mL THF and 20 mL water. Dissolution in 1 L of hot chloroform followed by hot filtration over Florisil pad, washed off with 3×100 mL of hot chloroform afforded a bright yellow solution that was reduced to around 200 mL and treated at room temperature with 200 mL of hexane. The precipitate was isolated by filtration. Re-crystallisation two times in 150 mL then 250 mL followed by drying under vacuum overnight afforded 9.4 g (67%) of product. ESI-MS [M+H]+ 618.

General Procedure for Fabrication of OLEDs

[0213] For the top emission OLEDs 1 to 3 a substrate with dimensions of 150 mm×150 mm×0.7 mm was ultrasonically cleaned with a 2% aquatic solution of Deconex FPD 211 for 7 minutes and then with pure water for 5 minutes, and dried for 15 minutes in a spin rinse dryer. Subsequently, Ag was deposited as anode at a pressure of 10-5 to 10-7 mbar.

[0214] Then, HT-1 and D-1 were vacuum co-deposited on the anode to form a HIL. Then, HT-1 was vacuum deposited on the HIL, to form an HTL. Then, HT-2 was vacuum deposited on the HTL to form an electron blocking layer (EBL).

[0215] Afterwards the emission layer was formed on the EBL by co-deposition of HOST-1 and EMITTER-1.

[0216] Then, the at least one first electron transport layer was vacuum deposited onto the emission layer to form the hole blocking layer (HBL). Then, the at least one second electron transport layer is formed on the hole blocking layer.

[0217] Then, the electron injection layer is formed on the electron, transporting layer by deposing Yb.

[0218] Ag is evaporated at a rate of 0.01 to 1 Å/s at 10-7 mbar to form a cathode.

[0219] A cap layer of HT-1 is formed on the cathode.

[0220] OLED 1 comprises the compound ET-1 (compound of formula (I)) in the at least one first electron transport layer and the compound ET-3 (compound of formula (II)) in the at least one second electron transport layer. The details of the layer stack in the devices are given below. A slash “/” separates individual layers. Layer thicknesses are given in squared brackets [ . . . ], mixing ratios in wt % given in round brackets ( . . . ):

Layer stack details of OLED 1; silver [100 nm]/HT-1:D-1 (92:8)[10 nm]/HT-1 [118 nm]/HT-2 [5 nm]/HOST-1:EMITTER-1 (97:3) [20 nm]/ET-1 [5 nm]/ET-3:LiQ (1:1) [31 nm]/Yb [2 nm]/silver:magnesium (9:1) [13 nm]/HT-1 [70 nm]

[0221] OLED 2 comprises the compound ET-2 (compound of formula (I)) in the at least one first electron transport layer and the compound ET-3 (compound of formula (II)) in the at least one second electron transport layer. The details of the layer stack in the devices are given below. A slash “/” separates individual layers. Layer thicknesses are given in squared brackets [ . . . ], mixing ratios in wt % given in round brackets ( . . . ):

Layer stack details of OLED 2; silver [100 nm]/HT-1:D-1 (92:8)[10 nm]/HT-1 [118 nm]/HT-2 [5 nm]/HOST-1:EMITTER-1 (97:3) [20 nm]/ET-2 [5 nm]/ET-3:LiQ (1:1) [31 nm]/Yb [2 nm]/silver:magnesium (9:1) [13 nm]/HT-1 [70 nm]

[0222] OLED 3 comprises the compound ET-1 (compound of formula (I)) in the at least one first electron transport layer and the compound ET-4 (compound of formula (III)) in the at least one second electron transport layer. The details of the layer stack in the devices are given below. A slash “/” separates individual layers. Layer thicknesses are given in squared brackets [ . . . ], mixing ratios in wt % given in round brackets ( . . . ):

Layer stack details of OLED 3: silver [100 nm]/HT-1:D-1 (92:8)[10 nm]/HT-1 [118 nm]/HT-2 [5 nm]/HOST-1:EMITTER-1 (97:3) [20 nm]/ET-1 [5 nm]/ET-4:LiQ (1:1) [31 nm]/Yb [2 nm]/silver:magnesium (9:1) [13 nm]/HT-1 [70 nm]

[0223] OLED 4 comprises the compound ET-1 (compound of formula (I)) in the at least one first electron transport layer and the compound ET-16 (compound of formula (II)) in the at least one second electron transport layer. The details of the layer stack in the devices are given below, A slash “/” separates individual layers. Layer thicknesses are given in squared brackets [ . . . ], mixing ratios in wt % given in round brackets ( . . . ):

Layer stack details of OLED 4: silver [100 nm]/HT-1:D-1 (92:8)[10 nm]/HT-1 [118 nm]/HT-2 [5 nm]/HOST-1:EMITTER-1 (97:3) [20 mm]/ET-1 [5 nm]/ET-3:ET-16 (7:3) [31 nm]/Yb [2 nm]/silver:magnesium (9:1) [13 nm]/HT-1 [70 nm]

Technical Effect of the Invention

[0224] The OLED devices according to the invention show high EQE efficiency at low voltage. Using the particularly preferred compounds of formula (III) for the at least one second electron transport layer further improves the EQE efficiency at a similar operating voltage.

TABLE-US-00005 List of compounds used IUPAC name Reference HT-1 N-([1,1′-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9- US2016322581 phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine [CAS 1242056-42-3] HT-2 N-(4-(dibenzo[b,d]furan-4-yl)phenyl)-N-(4-(9-phenyl- WO2015174640 9H-fluoren-9-yl)phenyl)-[1,1′-biphenyl]-4-amine [CAS 1824678-59-2] D-1 4,4′,4,″-((1E,1′E,1″E)-cyclopropane-1,2,3- US2008265216 triylidenetris(cyanomethanylylidene))tris(2,3,5,6- tetrafluorobenzonitrile) [CAS 1224447-88-4] HOST-1 H09 (Fluorescent-blue host material) Commercially available from Sun Fine Chemicals, Inc, S. Korea EMITTER-1 BD200 (Fluorescent-blue emitter material) Commercially available from Sun Fine Chemicals, Inc, S. Korea ET-1 2,4-diphenyl-6-(4′,5′,6′-triphenyl- WO2016171358 [1,1′:2′,1″:3″,1″′,3″′,1″″-quinquephenyl]-3″″- yl)-1,3,5-triazine [CAS 2032364-64-8] ET-2 2-(3′-(9,9-dimethyl-9H-fluoren-2-yl)-[1,1′- WO2016105141 biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine [CAS 1955543-57-3] ET-3 2-([1,1′-biphenyl]-3-yl)-4-phenyl-6-(3-(10- phenylanthracen-9-yl)phenyl)-1,3,5-triazine ET-4 2-(dibenzo[b,d]furan-3-yl)-4-phenyl-6-(10- phenylanthracen-9-yl)-1,3,5-triazine LiQ 8-Hydroxyquinolinolato-lithium WO2013079217 [CAS 850918-68-2]

TABLE-US-00006 TABLE 1 Properties of compounds ET-1 to ET-4, both used in the at least one second electron transport layer Dipole moment simulated by DFT (B3LYP_Gaussian/ mp Tg TRO 6-31G*, gas phase) [Debye] [oC] [oC] [oC] ET-1 0.30 — 141 267 ET-2 0.31 167 95 220 ET-3 0.37 286 127 243 ET-4 0.78 273 145 224 ET-5 0.55 333 165 282 ET-6 1.72 330 158 262 ET-8 0.94 321 159 265

TABLE-US-00007 TABLE 2 Performance of an organic electroluminescent device comprising the compounds of formula (I) in the at least one first electron transport layer and compounds of formula (II) and formula (III) in the at least one second electron transport layer. First Second OLED electron electron Operating device transport transport layer voltage at EQE at examples layer (wt ratio) 10 mA/cm.sup.2 (V) 10 mA/cm.sup.2 OLED-1 ET-1 ET-3:LiQ (1:1) 3.5 15.5 OLED-2 ET-2 ET-3:LiQ (1:1) 3.6 14.9 OLED-3 ET-1 ET-4:LiQ (1:1) 3.6 16.1 OLED-4 ET-1 ET-3:ET-16 (7:3) 3.5 15.4

[0225] The features disclosed in the foregoing description and in the dependent claims may, both separately and in any combination thereof, be material for realizing the aspects of the disclosure made in the independent claims, in diverse forms thereof.