Organic electroluminescent device

Abstract

The present invention relates to organic electroluminescent devices, the emitting layer thereof containing a blend of a luminescent material having a narrow singlet-triplet gap and a fluorescent emission material having high steric shielding.

Claims

1. An organic electroluminescent device comprising cathode, anode and at least one emitting layer comprising a sterically shielded fluorescent compound, wherein the emitting layer comprises a luminescent organic compound having a gap between the lowest triplet state T.sub.1 and the first excited singlet state S.sub.1 of <0.30 eV (TADF compound), where the peak emission wavelength of the sterically shielded fluorescent compound is greater than or equal to the peak emission wavelength of the TADF compound; wherein the sterically shielded fluorescent compound comprises a fluorescent base skeleton comprising a rigid n system substituted by one or more sterically demanding substituents, where the sterically demanding substituents are one or more fused-on aliphatic groups of the formulae (Ring-1A) to (Ring-7A) ##STR00338## ##STR00339## ##STR00340## where the dotted bonds indicate the linkage of the two carbon atoms within the fluorescent base skeleton and in addition: G is an alkylene group which has 1, 2 or 3 carbon atoms and optionally substituted by one or more R.sup.b radicals or an ortho-bonded arylene group which has 5 to 14 aromatic ring atoms and optionally substituted by one or more R.sup.b radicals; R.sup.a is in each case independently selected from the group consisting of H, D, F, a straight-chain alkyl group having 1 to 40 carbon atoms and a branched or cyclic alkyl group having 3 to 40 carbon atoms, each of which is optionally substituted by one or more R.sup.b radicals, an aromatic ring system having 5 to 60 aromatic ring atoms, each of which is optionally substituted by one or more R.sup.b radicals, or an aralkyl group which has 5 to 60 aromatic ring atoms and is optionally substituted by one or more R.sup.oradicals, where it is optionally possible for two or more adjacent R.sup.a substituents to form a ring system which may be substituted by one or more R.sup.b radicals; R.sup.b is selected from the group consisting of H, D, F, an aliphatic hydrocarbyl radical having 1 to 20 carbon atoms, and an aromatic ring system having 5 to 30 aromatic ring atoms, where two or more adjacent R.sup.b substituents together may form a ring system; and wherein the emitting layer further comprises at least one matrix compound, which is one or more compounds selected from the group consisting of triarylamines, carbazoles, dibenzofurans, indolocarbazoles, indenocarbazoles, azacarbazoles, bipolar matrix materials, silanes, azaboroles, diazasiloles, diazaphopholes, triazines, pyrimidines, quinoxalines, and bridged carbazoles, with the proviso that the matrix is not a TADF compound; and wherein T.sub.1(matrix)−T.sub.1(TADF)≥0.1 eV, where T.sub.1(matrix) and T.sub.1(TADF) are respectively the lowest triplet energy of the matrix compound and the lowest triplet energy of the TADF compound.

2. The organic electroluminescent device of claim 1, wherein the peak emission wavelength of the sterically shielded fluorescent compound is at least 10 nm greater than the TADF compound.

3. The organic electroluminescent device as of claim 1, wherein the TADF compound has a luminescence quantum efficiency of at least 40%.

4. The organic electroluminescent device of claim 1, wherein the TADF compound has a decay time of ≤50 μs.

5. The organic electroluminescent device of claim 1, wherein the gap between S.sub.1 and T.sub.1 of the TADF compound is ≤0.25 eV.

6. The organic electroluminescent device of claim 1, wherein the sterically shielded fluorescent compound has a luminescence quantum efficiency of at least 60%.

7. The organic electroluminescent device of claim 1, wherein the fluorescent base skeleton is selected from the group consisting of formulae (1) to (70): ##STR00341## ##STR00342## ##STR00343## ##STR00344## ##STR00345## ##STR00346## ##STR00347## ##STR00348## ##STR00349## ##STR00350## ##STR00351## ##STR00352## ##STR00353## where optionally one, two, three or four carbon atoms in the fused aromatic base skeleton in the groups of formulae (1) to (50) are replaced by nitrogen.

8. The organic electroluminescent device of claim 1, wherein the (Ring-1A) too (Ring-7A) groups do not have any acidic benzylic protons.

9. A process for producing the organic electroluminescent device of claim 1, comprising applying at least one layer by a sublimation method and/or applying at least one layer by an organic vapor phase deposition method or with the aid of a carrier gas sublimation and/or applying at least one layer from solution, by spin-coating or by a printing method.

10. The organic electroluminescent device of claim 1, wherein the gap between S.sub.1 and T.sub.1 of the TADF compound is ≤0.15 eV.

11. The organic electroluminescent device of claim 1, wherein the gap between S.sub.1 and T.sub.1 of the TADF compound is ≤0.10 eV.

12. The organic electroluminescent device of claim 1, wherein the sterically shielded fluorescent compound has a luminescence quantum efficiency of at least 80%.

13. The organic electroluminescent device of claim 1, wherein the sterically shielded fluorescent compound has a luminescence quantum efficiency of at least 90%.

14. The organic electroluminescent device of claim 1, wherein R.sup.a in the benzylic positions is not H or D.

Description

EXAMPLES

Synthesis Examples

Synthesis of a Sterically Shielded Fluorescent Compound

(1) The syntheses which follow, unless stated otherwise, are conducted under a protective gas atmosphere in dried solvents. The metal complexes are additionally handled with exclusion of light or under yellow light. The solvents and reagents can be purchased, for example, from Sigma-ALDRICH or ABCR. The respective figures in square brackets or the numbers quoted for individual compounds relate to the CAS numbers of the compounds known from the literature.

Synthesis Example 1

a) Synthesis of Compound 1

(2) ##STR00325##

(3) To a well-stirred mixture of 15.8 g (100 mmol) of 2,6-diaminonaphthalene [2243-67-6], 47.6 g (250 mmol) of 2,6-bis(1-methylethyl)benzaldehyde [179554-06-4], 27.1 g (250 mol) of bicyclo[2.2.2]oct-2-ene [931-64-6] and 300 mL of dichloromethane are added dropwise 2.8 g (20 mmol) of boron trifluoride etherate [60-29-7], and then the mixture is heated under reflux for 80 h. After cooling, the reaction mixture is washed twice with 200 mL each time of water, the organic phase is dried over magnesium sulfate and then the dichloromethane is removed under reduced pressure. The residue is taken up in 300 mL of o-dichlorobenzene, 87 g (1 mol) of manganese dioxide are added and the mixture is heated under reflux on a water separator for 16 h. After cooling, 500 mL of ethyl acetate are added, the manganese dioxide is filtered off with suction through a Celite layer, the manganese dioxide is washed with 200 mL of ethyl acetate and the combined filtrates are freed of the solvents under reduced pressure. The residue is chromatographed on silica gel with n-heptane/ethyl acetate (2:1). Yield: 6.4 g (9 mmol) 9%; purity about 97% by .sup.1H NMR.

b) Synthesis of Compound 2

(4) ##STR00326##

(5) A mixture of 6.4 g (9 mmol) of compound 1, 4.3 g (22 mmol) of 2-chloro-1,3-bis(1-methylethyl)benzene [54845-36-2], 2.5 g (25 mmol) of sodium tert-butoxide, 44 mg (0.1 mmol) of dirhodium tetraacetate [85503-41-3], 52 mg (0.2 mmol) of 1,3-diphenyl-1H-imidazolium chloride [26956-10-5], 50 g of glass beads (diameter 3 mm) and 200 ml of o-xylene is heated under reflux for 48 h. After cooling, the salts are filtered off through Celite, the solvent is removed under reduced pressure and the residue is chromatographed on silica gel (n-heptane/ethyl acetate (9:1)) until a purity of >99.8% by HPLC has been attained. Yield: 1.1 g (1.1 mmol), 12%.

(6) General Description of the Determination of the Relevant Parameters

(7) 1) Determination of the Energy Levels of the Molecular Orbitals and Singlet and Triplet Levels

(8) The energy levels of the molecular orbitals and the energy of the lowest triplet state T.sub.1 and of the lowest excited singlet state S.sub.1 of the materials are determined via quantum-chemical calculations. For this purpose, in the present case, the “Gaussian09, Revision D.01” software package (Gaussian Inc.) is used. For calculation of organic substances without metals (referred to as the “org.” method), a geometry optimization is first conducted by the semi-empirical method AM1 (Gaussian input line “#AM1 opt”) with charge 0 and multiplicity 1. Subsequently, on the basis of the optimized geometry, a single-point energy calculation is effected for the electronic ground state and the triplet level. This is done using the TDDFT (time dependent density functional theory) method B3PW91 with the 6-31G(d) basis set (Gaussian input line “#B3PW91/6-31G(d) td=(50-50,nstates=4)”) (charge 0, multiplicity 1). For organometallic compounds (referred to as the “M-org.” method), the geometry is optimized by the Hartree-Fock method and the LanL2 MB basis set (Gaussian input line “#HF/LanL2 MB opt”) (charge 0, multiplicity 1). The energy calculation is effected, as described above, analogously to that for the organic substances, except that the “LanL2DZ” basis set is used for the metal atom and the “6-31G(d)” basis set for the ligands (Gaussian input line “#B3PW91/gen pseudo=lanl2 td=(50-50,nstates=4)”). From the energy calculation, the HOMO, for example, is obtained as the last orbital occupied by two electrons (alpha occ. eigenvalues) and LUMO as the first unoccupied orbital (alpha virt. eigenvalues) in Hartree units, where HEh and LEh represent the HOMO energy in Hartree units and the LUMO energy in Hartree units respectively. The energies in Hartree units are obtained in an analogous manner for the other energy levels such as HOMO-1, HOMO-2, . . . LUMO+1, LUMO+2 etc.

(9) In the context of this application, the ((HEh*27.212)−0.9899)/1.1206 values in eV, calibrated using CV measurements, are considered to be the energy levels of the populated orbitals.

(10) In the context of this application, the ((LEh*27.212)−2.0041)/1.385 values in eV, calibrated using CV measurements, are considered to be the energy levels of the unpopulated orbitals.

(11) The lowest triplet state T.sub.1 of a material is defined as the relative excitation energy (in eV) of the triplet state having the lowest energy which is found by the quantum-chemical energy calculation.

(12) The lowest excited singlet state S.sub.1 is defined as the relative excitation energy (in eV) of the singlet state having the second-lowest energy which is found by the quantum-chemical energy calculation.

(13) The method described herein is independent of the software package used and always gives the same results. Examples of frequently utilized programs for this purpose are “Gaussian09” (Gaussian Inc.) and Q-Chem 4.1 (Q-Chem, Inc.). In the present case, the energies are calculated using the software package “Gaussian09, Revision D.01”.

(14) Table 2 states the HOMO and LUMO energy levels and S.sub.1 and T.sub.1 of the various materials.

(15) 2) Determination of the Photoluminescence Quantum Efficiency (PLQE) Of the TADF Compound

(16) A 50 nm-thick film of the layer comprising a TADF material is applied to a quartz substrate. This film comprises the same materials in the same concentration ratios as the corresponding layer in the OLED except for the fluorescent compound. If the layer in the OLED contains, for example, the material IC1 to an extent of 85%, the material D1 to an extent of 10% and the material SE1 (fluorescent compound) to an extent of 5%, the film for the determination of the PLQE contains the materials IC1 and D1 in a volume ratio of 85:10. To produce the film, the same production conditions are used as for production of the emission layer for the OLEDs.

(17) An absorption spectrum of this film is measured in the wavelength range of 350-500 nm. For this purpose, the reflection spectrum R(λ) and the transmission spectrum TOO of the sample are determined at an angle of incidence of 6° (i.e. incidence virtually at right angles). The absorption spectrum in the context of this application is defined as A(λ)=1−R(λ)−T(λ).

(18) If A(λ)≤0.3 in the range of 350-500 nm, the wavelength corresponding to the maximum of the absorption spectrum in the range of 350-500 nm is defined as λ.sub.exc. If, for any wavelength, A(λ)>0.3, λ.sub.exc is defined as being the greatest wavelength at which A(λ) changes from a value of less than 0.3 to a value of greater than 0.3 or from a value of greater than 0.3 to a value of less than 0.3.

(19) The PLQE is determined using a Hamamatsu C9920-02 measurement system. The principle is based on the excitation of the sample with light of a defined wavelength and the measurement of the radiation absorbed and emitted. During the measurement, the sample is within an Ulbricht sphere (“integrating sphere”). The spectrum of the excitation light is approximately Gaussian with a half-height width of <10 nm and a peak wavelength λ.sub.exc as defined above.

(20) The PLQE is determined by the evaluation method customary for said measurement system. The measurement is effected at room temperature. It should be strictly ensured that the sample does not come into contact with oxygen at any time, since the PLQE of materials having a small energy gap between S.sub.1 and T.sub.1 is very greatly reduced by oxygen (H. Uoyama et al., Nature 2012, Vol. 492, 234).

(21) The PLQE is reported in the respective examples together with the excitation wavelength used.

(22) 3) Determination of Peak Emission Wavelength λ.sub.max

(23) To determine the peak emission wavelength of the TADF compounds and the fluorescent compound, the particular material is dissolved in toluene. In this case, a concentration of 0.5 mg/100 mL is used. The solution is excited with a wavelength of 350 nm in a Hitachi F-4500 fluorescence spectrometer. The measurement is effected at room temperature. The peak emission wavelength λ.sub.max is the wavelength at which the emission spectrum obtained reaches its maximum value.

(24) 4) Determination of the PLQE of the Fluorescent Compound

(25) To determine the PLQE of the fluorescent compound, the material is dissolved in toluene. In this case, a concentration of 1 mg/100 mL is used. To determine the PLQE, the solution is analyzed in a Hamamatsu 09920-02 measurement system (for description see above). The measurement is effected at room temperature. The excitation wavelength used is the wavelength 0.27*λ.sub.max+300 nm, where λ.sub.max represents the peak emission wavelength of the fluorescent compound as defined above.

(26) 5) Determination of Decay Time

(27) The decay time is determined using a sample which is produced as described above under “Determination of the PL quantum efficiency (PLQE)”. The sample is excited at room temperature by a laser pulse (wavelength 266 nm, pulse duration 1.5 ns, pulse energy 200 μJ, beam diameter 4 mm). At this time, the sample is under reduced pressure (<10.sup.−5 mbar). After excitation (defined as t=0), the profile of the intensity of the photoluminescence emitted against time is measured. The photoluminescence exhibits a steep drop at the start, which is attributable to the prompt fluorescence of the TADF compound. Later on, a slower drop is observed, delayed fluorescence (see, for example, H. Uoyama et al., Nature, vol. 492, no. 7428, 234-238, 2012 and K. Masui et al., Organic Electronics, vol. 14, no. 11, pp. 2721-2726, 2013). The decay time to in the context of this application is the decay time of the delayed fluorescence and is determined as follows: A time t.sub.d at which the prompt fluorescence has abated to well below the intensity of the delayed fluorescence is chosen, such that the determination of the decay time that follows is not affected by the prompt fluorescence. This choice can be executed by a person skilled in the art and forms part of his common art knowledge. For the measurement data from the time t.sub.d, the decay time t.sub.a=t.sub.e−t.sub.d is determined. In this formula, t.sub.e is that time after t=t.sub.d sat which the intensity has for the first time dropped to 1/e of its value at t=t.sub.d.

(28) 6) Determination of the Shielding Parameter S of the Fluorescent Compound

(29) a) Determination of the active atoms of a molecular orbital First of all, for each individual molecular orbital, a determination of which atoms are active is conducted. In other words, a generally different set of active atoms is found for each molecular orbital. There follows an illustrative description of how the active atoms of the HOMO molecular orbital are determined. For all other molecular orbitals (e.g. HOMO-1, LUMO, LUMO+1, etc.), the active atoms are determined analogously.

(30) For the fluorescent compounds, a quantum-chemical calculation as described above is conducted. This calculation gives the different molecular orbitals from which, with the aid of SCPA Population Analysis, the participation of all atoms in the molecule in the different molecular orbitals is determined. SCPA Population Analysis, also called C-squared Population Analysis, is described in Ros, P.; Schuit, G. C. A. Theoret. Chim. Acta (Berl.) 1966, 4, 1-12 and is implemented, for example, in the AOMix software package.

(31) Hereinafter, N denotes the number of atoms in the molecule, and P′.sub.HOMO(a) the participation of the atom numbered a (a=1 . . . N) in the HOMO molecular orbital (analogously for all other molecular orbitals). The participation levels are defined such that

(32) $\underset{a=1}{\overset{N}{.Math.}}{P^{\prime }}_{\mathrm{HOMO}}\left(a\right)=1$

(33) i.e. the sum total of all participation levels is one. The atomic participation levels normalized to one are designated P.sub.HOMO(a), i.e.
P.sub.HOMO(a)=P′.sub.HOMO(a)/MAX(P′.sub.HOMO(a), a=1 . . . N)
If
P.sub.HOMO(a)≥0.2,

(34) the atom a is considered to be active with respect to the HOMO molecular orbital. An analogous definition applies to all other molecular orbitals.

(35) b) Charge-Exchanging Molecular Orbitals of the Fluorescent Compound

(36) Charge-exchanging molecular orbitals are considered to be the HOMO and LUMO, and all molecular orbitals that are separated in energy by 75 meV or less from the HOMO or LUMO.

(37) In addition, active populated molecular orbitals are considered to be all of those whose energy E satisfies the following condition: E(LUMO)−E≤gap(TADF). In addition, active unpopulated molecular orbitals are considered to be all of those whose energy E satisfies the following condition: E−E(HOMO)≤gap(TADF). E(LUMO) here is the LUMO energy level and E(HOMO) the HOMO energy level of the fluorescent compound. In addition, gap(TADF)=E(LUMO,TADF)−E(HOMO,TADF). E(LUMO,TADF) here is the LUMO energy level of the TADF compound and E(HOMO,TADF) the HOMO energy level of the TADF compound. If the OLED contains two or more TADF compounds, gap(TADF) is the greatest of the values for the TADF compounds present.

(38) c) Determination of the Active Atoms in the Fluorescent Compound

(39) If one atom in at least one charge-exchanging molecular orbital is active, it is considered to be active in respect of the fluorescent compound. Only atoms that are inactive (non-active) in all charge-exchanging molecular orbitals are inactive in respect of the fluorescent compound.

Example

Active and Inactive Atoms of Rubrene

(40) The material rubrene is shown below with the numbers a of the carbon atoms present. The numbers of the hydrogen atoms are not stated, since these are inactive without exception.

(41) ##STR00327##

(42) The table which follows shows the participations P and the normalized participations P of the carbon atoms in the HOMO and LUMO molecular orbitals, calculated by the method described above. The participations of the hydrogen atoms are not shown. In addition, the last column shows whether the atoms are active (“A”) or inactive (“I”), assuming that only HOMO and LUMO are charge-exchanging.

(43) TABLE-US-00010 a P′.sub.HOMO (a) P′.sub.LUMO (a) P.sub.HOMO (a) P.sub.LUMO (a) Active/inactive 1 0.035 0.034 0.31 0.29 A 2 0.042 0.040 0.38 0.33 A 3 0.020 0.025 0.18 0.20 A 4 0.020 0.025 0.18 0.20 A 5 0.042 0.040 0.38 0.33 A 6 0.035 0.034 0.31 0.29 A 7 0.112 0.120 1.00 1.00 A 8 0.112 0.120 1.00 1.00 A 9 0.004 0.002 0.04 0.01 I 10 0.004 0.002 0.04 0.01 I 11 0.112 0.120 1.00 1.00 A 12 0.020 0.025 0.18 0.20 A 13 0.020 0.025 0.18 0.20 A 14 0.112 0.120 1.00 1.00 A 19 0.042 0.040 0.38 0.33 A 21 0.042 0.040 0.38 0.33 A 23 0.035 0.034 0.31 0.29 A 25 0.035 0.034 0.31 0.29 A 27 0.003 0.003 0.03 0.02 I 28 0.012 0.007 0.10 0.06 I 29 0.012 0.009 0.10 0.07 I 30 0.004 0.002 0.03 0.01 I 32 0.002 0.004 0.02 0.03 I 34 0.004 0.006 0.04 0.05 I 38 0.003 0.003 0.03 0.02 I 39 0.012 0.007 0.10 0.06 I 40 0.012 0.009 0.10 0.07 I 41 0.004 0.002 0.03 0.01 I 43 0.002 0.004 0.02 0.03 I 45 0.004 0.006 0.04 0.05 I 49 0.003 0.003 0.03 0.02 I 50 0.012 0.007 0.10 0.06 I 51 0.012 0.009 0.10 0.07 I 52 0.004 0.002 0.03 0.01 I 54 0.002 0.004 0.02 0.03 I 56 0.004 0.006 0.04 0.05 I 60 0.003 0.003 0.03 0.02 I 61 0.012 0.007 0.10 0.06 I 62 0.012 0.009 0.10 0.07 I 63 0.004 0.002 0.03 0.01 I 65 0.002 0.004 0.02 0.03 I 67 0.004 0.006 0.04 0.05 I

(44) d) Determination of V(D.sub.cut)

(45) The active surface refers to the van der Waals surface of the active atoms of the fluorescent compound. This is the surface of the van der Waals volume of the active atoms. If a sphere having the van der Waals radius corresponding to the particular type of atom is placed around each active atom, with the atom forming the center of the sphere, the union of all these spheres is the van der Waals volume of the active atoms. The van der Waals radii Now of the different elements are reported in angstroms in the following table:

(46) TABLE-US-00011 Element H He Li Be B C N O F Ne Na Mg Al r.sub.VDW (Å) 1.2 1.22 1.52 1.7 2.08 1.85 1.54 1.4 1.35 1.6 2.31 1.73 2.05 Element Si P S Cl Ar K Ca Sc Ti C Cr Mn Fe r.sub.VDW (Å) 2 1.9 1.85 1.81 1.91 2.31 1.97 1.7 1.7 1.7 1.7 1.7 1.7 Element Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y r.sub.VDW (Å) 1.7 1.7 1.7 1.7 1.7 1.7 2 2 2.1 2.1 1.7 1.7 1.7 Element Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te r.sub.VDW (Å) 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 2.2 2.2 Element I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb r.sub.VDW (Å) 2.15 2.16 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 Element Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt r.sub.VDW (Å) 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.72 Element Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa r.sub.VDW (Å) 1.66 1.55 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 Element U Np Pu Am Cm Bk Cf r.sub.VDW (Å) 1.7 1.7 1.7 1.7 1.7 1.7 1.7

(47) The Connolly surface (also referred to as solvent-excluded surface) of the fluorescent compound as a whole refers to the surface of the solvent-excluded volume (Michael L. Connolly, “Computation of Molecular Volume”, J. Am. Chem. Soc., 1985, Vol. 107, p. 1118-1124). If the van der Waals volume of the fluorescent compound as a whole (defined analogously to the above-described van der Waals volume of the active atoms) is considered to be hard, i.e. to be a volume which cannot be penetrated, the solvent-excluded volume in the context of the present invention is the portion of the space that cannot be occupied by a hard sphere with radius 0.4 nm. The area of the Connolly surface of the fluorescent compound as a whole is referred to as A.sub.con.

(48) The area of that proportion of the Connolly surface having a distance of not more than d.sub.cut from the active surface is referred to as A.sub.cut. For a given value of d.sub.cut, the parameter V(d.sub.cut) is defined as A.sub.cut/A.sub.con.

(49) e) Determination of the Shielding Parameter S for the Fluorescent Compound

(50) The parameter S which describes the steric shielding of the fluorescent compounds of this invention is defined as
S=(V(0.2 nm)/2+V(0.3 nm)/3+V(0.4 nm)/4+V(0.5 nm)/5+V(0.6 nm)/6).Math.(20/29)

(51) The parameter S depends on the structure of the fluorescent compound and of the TADF compound used (see “Charge-exchanging molecular orbitals of the fluorescent compound” further up).

(52) A fluorescent compound is considered to be sterically shielded in the context of the present application when the above-defined shielding parameter S≤0.6.

(53) Examples of Organic Electroluminescent Devices

(54) Production of the OLEDs

(55) Glass plaques coated with structured ITO (indium tin oxide) of thickness 50 nm are subjected to wet cleaning (dishwasher, Merck Extran detergent), then baked in a nitrogen atmosphere at 250° C. for 15 min and, prior to coating, treated with an oxygen plasma for 130 s. These plasma-treated glass plaques form the substrates to which the OLEDs are applied. The substrates remain under reduced pressure prior to coating. The coating begins no later than 10 min after the plasma treatment. After the production, the OLEDs are encapsulated for protection against oxygen and water vapor. The exact layer structure of the OLEDs can be found in the examples. The materials required for production of the OLEDs are shown in table 1.

(56) All materials are applied by thermal vapor deposition in a vacuum chamber. The emission layer(s) always consist(s) of at least one matrix material (host material) to the emitting material. The latter is added to the matrix material(s) in a particular proportion by volume by coevaporation. Details given in such a form as IC1:D1:SE1 (92%:5%:3%) mean here that the material IC1 is present in the layer in a proportion by volume of 92%, D1 in a proportion of 5% and SE1 in a proportion of 3%. Analogously, the electron transport layer may also consist of a mixture of two materials.

(57) The OLEDs are characterized in a standard manner. For this purpose, the electroluminescence spectra at 1000 cd/m.sup.2 and current-voltage-luminance (UIL) characteristics are measured, from which it is possible to determine the external quantum efficiency (EQE, measured in %) assuming Lambertian emission characteristics. The parameter U1000 refers to the voltage which is required for a luminance of 1000 cd/m.sup.2. EQE1000 refers to the external quantum efficiency at an operating luminance of 1000 cd/m.sup.2.

(58) The lifetime LT is defined as the time after which the luminance drops from the starting luminance to a certain proportion L1 in the course of operation with constant current. Figures of j0=10 mA/cm.sup.2, L1=80% mean that the luminance in the course of operation at 10 mA/cm.sup.2 falls to 80% of its starting value after the time LD.

(59) The TADF material used is the compound D1. This has an energy gap between S.sub.1 and T.sub.1 of 0.09 eV.

(60) For the two materials rubrene and SE1, in conjunction with material D1, the HOMO and LUMO molecular orbitals are charge-exchanging. The shielding parameter is S=0.755 for rubrene and 0.518 for SE1.

(61) For the emission maximum of compound D1, λ.sub.max=511 nm, for rubrene λ.sub.max=555 nm, and for SE1 Δ.sub.max=572 nm.

Example 1

Fluorescent Compounds Having Different Shielding Parameters S in the Emission Layer Comprising a TADF Compound

(62) The OLEDs consist of the following layer sequence which is applied to the substrate after the plasma treatment: 85 nm SpMA1, 15 nm IC1:D1:emitter (92%:5%:3%), 10 nm IC1, 45 nm ST2:LiQ (50%:50%), aluminum (100 nm).

(63) The PLQE of the layer containing the TADF compound is 94% (λ.sub.exc=350 nm), the decay time 5.4 μs (t.sub.d=7 μs).

(64) If rubrene is used as emitter (S=0.755), this gives EQE1000=4.6%, U1000=4.6 V and LD=220 h for j0=10 mA/cm.sup.2, L1=85%. With the emitter SE1 (S=0.518), much better values of EQE1000=8.2%, U1000=4.4 V and LD=440 h are obtained for j0=10 mA/cm.sup.2, L1=85%. In both cases, orange emission is obtained.

(65) For an OLED of otherwise identical construction which does not contain any fluorescent compound in the emission layer, i.e. IC1 and D1 in a volume ratio of 92:5 only, a lifetime LD=37 h for j0=10 mA/cm.sup.2, L1=85%, is obtained.

(66) The PLQE of rubrene is 94%, excitation wavelength 450 nm. The PLQE of SE1 is 97%, excitation wavelength 454 nm.

Example 1a

Higher Concentration of the Fluorescent Compound

(67) The OLEDs are produced as in example 1, except using an IC1:D1:SE1 (89%:5%:6%) layer for the 15 nm-thick IC1:D1:emitter (92%:5%:3%) layer. There is a distinct increase in the lifetime compared to example 1; LD 1120 h is obtained for j0=10 mA/cm.sup.2, L1=85%. In addition, there is an improvement in color purity. While the residual emission of the TADF compound at 500 nm is 9% of the peak emission (at 567 nm) in example 1, this is reduced to only 3% of the peak emission (at 571 nm) for 6% SE1.

Example 1b

Higher Concentration of the TADF Compound

(68) The OLEDs are produced as in example 1, except using an IC1:D1:SE1 (87%:10%:3%) layer for the 15 nm-thick IC1:D1:emitter (92%:5%:3%) layer. There is a distinct increase in the lifetime compared to example 1; LD=655 h is obtained for j0=10 mA/cm.sup.2, L1=85%.

(69) The PLQE of the layer containing the TADF compound is 87% (λ.sub.exc=350 nm) for this example, the decay time 4.9 μs (t.sub.d=7 μs).

Example 1c

Higher Concentration of the Fluorescent and TADF Compounds

(70) The OLEDs are produced as in example 1, except using an IC1:D1:SE1 (84%:10%:6%) layer for the 15 nm-thick IC1:D1 (92%:5%:3%) layer. There is a distinct increase in the lifetime compared to example 1; LD=1605 h is obtained for j0=10 mA/cm.sup.2, L1=85%. There is an improvement in the color purity over example 1 to the same degree as described in example 1a.

Example 1d

Use of a Different Electron Transport Layer

(71) The OLEDs are produced as in example 1, with the difference that the 45 nm-thick ST2:LiQ (50%:50%) layer is replaced by the sequence of 45 nm ST2, 3 nm LiQ.

(72) If rubrene is used as emitter (S=0.755), this gives EQE1000=4.8%, U1000=4.6 V and LD=115 h for j0=10 mA/cm.sup.2, L1=85%. With the emitter SE1 (S=0.518), much better values of EQE1000=8.4%, U1000=3.6 V and LD=150 h are obtained for j0=10 mA/cm.sup.2, L1=85%. In both cases, orange emission is obtained.

Example 2

Comparison with an OLED Comprising an Anthracene Matrix

(73) The OLEDs are produced with the fluorescent compound SE1 as in example 1, with the difference that the 15 nm-thick IC1:D1:SE1 (92%:5%:3%) layer is replaced by a 30 nm-thick AM1:SE1 (97%:3%) layer. The layer thickness of the emission layer is optimized for the AM1 matrix material used and is better than in the case of use of a 15 nm-thick emission layer. Anthracene-containing materials such as AM1 are matrix materials that are very frequently used in the prior art for fluorescent compounds, for example the material SE1. Nevertheless, with U1000=4.9 V, EQE1000=4.8%, much poorer values are obtained than in example 1.

Example 3

Fluorescent Compounds Having Different Shielding Parameters S in a Layer Adjoining the Layer Containing the TADF Material

(74) The OLEDs are produced as in example 1, with the difference that the 15 nm-thick IC1:D1:emitter (92%:5%:3%) layer is replaced by the sequence of 7.5 nm IC1:D1 (95%:5%), 7.5 nm IC1:emitter (97%:3%), i.e. two adjoining layers.

(75) The PLQE of the layer containing the TADF compound is 92% (λ.sub.exc=350 nm), the decay time 5.4 μs (t.sub.d=7 μs).

(76) If rubrene is used as emitter (S=0.755), this gives EQE1000=8.8%, U1000=3.8 V and LD=130 h for j0=10 mA/cm.sup.2, L1=80%. With the emitter SE1 (S=0.518), a much better efficiency is found; EQE1000=14.2%, U1000=3.7 V and LD=135 h are obtained for j0=10 mA/cm.sup.2, L1=80%.

Example 4

(77) The OLEDs are produced as in example 3, with the difference that the 85 nm SpMA1 is replaced by 75 nm SpMA1 and 10 nm SpMA2.

(78) If rubrene is used as emitter (S=0155), this gives EQE1000® 8.0%, U1000=3.8 V and LD=150 h for j0=10 mA/cm.sup.2, L1=80%. With the emitter SE1 (S=0.518), a much better efficiency is found; EQE1000=15.1%, U1000=3.8 V and LD=150 h are obtained for j0=10 mA/cm.sup.2, L1=80%.

(79) TABLE-US-00012 TABLE 1 Structural formulae of the materials for the OLEDs 0

(80) TABLE-US-00013 TABLE 2 HOMO, LUMO, T.sub.1, S.sub.1 of the relevant materials HOMO LUMO S.sub.1 T.sub.1 Material Method (eV) (eV) (eV) (eV) C-D1 org. −6.11 −3.40 2.50 2.41 IC1 org. −5.79 −2.83 3.09 2.69 SpMA1 org. −5.25 −2.18 3.34 2.58 SpMA2 org. −5.35 −2.34 3.14 2.62 ST2 org. −6.03 −2.82 3.32 2.68 LiQ M-org. −5.17 −2.39 2.85 2.13

Comparative Example According to the Prior Art

(81) US 2012/248968 describes OLEDs comprising a TADF compound and a fluorescent emitter material. The compound GH-4 used has an energy gap of 0.04 eV between S.sub.1 and T.sub.1. For the fluorescent emitter GD-1 with GH-4 as TADF compound, S=0.834. The external quantum efficiency shown for this combination is EQE=5.04% for 1 mA/cm.sup.2 (corresponding to 174 cd/m.sup.2) and EQE=4.59% for 10 mA/cm.sup.2 (1585 cd/m.sup.2). Since efficiency generally decreases toward higher luminances, the EQEs exhibited in the present invention for 1000 cd/m.sup.2 are thus distinctly higher.

(82) The compounds GH-4 and GD-1 used in the prior art are shown below:

(83) ##STR00337##