Formulations of luminescent compounds

Abstract

The present invention relates to a formulation comprising organic materials for the production of organic electronic devices having a low failure rate.

Claims

1. A formulation comprising at least one type of organic luminescent compound (TADF compound) and an organic solvent or a mixture of multiple organic solvents, wherein the TADF compound has a separation between the lowest triplet state T.sub.1 and the first excited singlet state Si of less than or equal to 0.15 eV; wherein the formulation comprises a first organic matrix material and a second organic matrix material, wherein the first organic matrix material is an electron-transporting material or a hole-transporting matrix material, and the second organic matrix material is a compound which has a band gap between HOMO and LUMO of greater than or equal to 3.5 eV.

2. The formulation of claim 1, wherein the separation between S.sub.1 and T.sub.1 of the TADF compound is less than or equal to 0.10 eV.

3. The formulation of claim 1, wherein the TADF compound is an aromatic compound which contains both donor and acceptor substituents.

4. The formulation of claim 1, wherein the surface tension of the solvent or solvents is at least 28 mN/m.

5. The formulation of claim 1, wherein the boiling or sublimation point of the solvent or solvents is less than 300 C.

6. The formulation of claim 1, wherein the viscosity of the solvent or of the individual solvents of a solvent mixture is greater than 3 mPa*s.

7. The formulation of claim 1, wherein the molecular weight of the solvent or of the solvents used in the solvent mixture is less than or equal to 1000 g/mol.

8. The formulation of claim 1, wherein the concentration of the TADF compound in the formulation, based on the entire formulation, is in the range from 1 to 20% by weight.

9. The formulation of claim 1, wherein the solvent is selected from the group consisting of toluene, anisole, o-, m- or p-xylene, methylbenzoate, mesitylene, tetralin, veratrol, THF, methyl-THF, THP, chlorobenzene, dioxane, phenoxytoluene, ()-fenchone, 1,2,3,5-tetramethylbenzene, 1,2,4,5-tetramethylbenzene, 1-methylnaphthalene, 2-methylbenzothiazole, 2-phenoxyethanol, 2-pyrrolidinone, 3-methylanisole, 4-methylanisole, 3,4-dimethylanisole, 3,5-dimethylanisole, acetophenone, -terpineol, benzothiazole, butyl benzoate, cumene, cyclohexanol, cyclohexanone, cyclohexylbenzene, decalin, dodecylbenzene, ethyl benzoate, indane, methyl benzoate, NMP, p-cymene, phenetole, 1,4-diisopropylbenzene, dibenzyl ether, diethylene glycol butyl methyl ether, triethylene glycol butyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, diethylene glycol monobutyl ether, tripropylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 2-isopropylnaphthalene, pentylbenzene, hexylbenzene, heptylbenzene, octylbenzene, 1,1-bis(3,4-dimethylphenyl)-ethane and mixtures thereof.

10. The formulation of claim 1, characterised in that the formulation comprises a matrix material, where the following applies for the lowest unoccupied molecular orbital (LUMO) of the TADF compound LUMO(TADF) and the highest occupied molecular orbital (HOMO) of the matrix HOMO(matrix):
[LUMO(TADF)HOMO(matrix)] is greater than or equal to [S.sub.1(TADF)0.4 eV], where S.sub.1(TADF) is the first excited singlet state S.sub.1 of the TADF compound.

11. A method for producing one or more layers of an organic electronic device from solution comprising utilizing the formulation of claim 1.

12. A process for the production of an organic electronic device, comprising producing at least one layer of the electronic device from solution with the aid of the formulation of claim 1.

13. The process of claim 12, characterised in that the electronic device is an organic electroluminescent device.

Description

EXAMPLES

Example 1

Determination Methods

(1) Determination of HOMO, LUMO, Singlet Level and Triplet Level

(2) The energy levels of the molecular orbitals and the energy of the lowest triplet state T.sub.1 or the lowest excited singlet state S.sub.1 of the materials are determined via quantum-chemical calculations. To this end, the Gaussian-09, Revision D.01 software package (Gaussian Inc.) is used in the present application. For the calculation of organic substances without metals (denoted by org. method), firstly a geometry optimisation is carried out using the semi-empirical method AM1 (Gaussian input line # AM1 opt) with charge 0 and multiplicity 1. This is followed by an energy calculation (single point) for the electronic ground state and triplet level on the basis of the optimised geometry. The TDDFT (time dependent density functional theory) method B3PW91 with the 6-31G(d) base set (Gaussian input line # B3PW91/6-31G(d) td=(50-50,nstates=4)) is used here (charge 0, multiplicity 1). For organometallic compounds (denoted by org.-m method), the geometry is optimised using the Hartree-Fock method and the LanL2 MB base set (Gaussian input line # HF/LanL2 MB opt) (charge 0, multiplicity 1). The energy calculation is carried out, as described above, analogously to that of the organic substances, with the difference that the LanL2DZ base set is used for the metal atom and the 6-31 G(d) base set is used for the ligands (Gaussian input line # B3PW91/gen pseudo=lanl2 td=(50-50,nstates=4)), The energy calculation gives, for example, the HOMO 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 stand for the HOMO energy in hartree units and the LUMO energy in hartree units respectively. The energies of the other energy levels, such as HOMO1, HOMO2, . . . LUMO+1, LUMO+2 etc., in hartree units are obtained analogously.

(3) For the purposes of this application, the values calibrated with reference to CV measurements ((HEh*27.212)0.9899)/1.1206) in eV are regarded as the energy levels of the occupied orbitals.

(4) For the purposes of this application, the values calibrated with reference to CV measurements ((LEh*27.212)2.0041)/1.385) in eV are regarded as the energy levels of the unoccupied orbitals.

(5) 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 arises from the quantum-chemical energy calculation.

(6) 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 arises from the quantum-chemical energy calculation.

(7) The method described herein is independent of the software package used and always gives the same results. Examples of frequently used programmes for this purpose are Gaussian09W (Gaussian Inc.) and Q-Chem 4.1 (Q-Chem, Inc.). In the present application, the Gaussian09W software package is used for the calculation of the energies.

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

(9) Determination of Molecular Orbital Overlaps

(10) The overlap of the molecular orbitals which are involved in certain electronic transitions (charge-transfer states) is described with the aid of the parameter . The meaning of the parameter is well known to the person skilled in the art. Determination of the parameter by means of methods which are described in the prior art presents the person skilled in the art with absolutely no difficulties. For the purposes of the present invention, the parameter is determined from the PBHT method in accordance with D. J. Tozer et al. (J. Chem. Phys. 128, 044118 (2008)), which is implemented, for example, in the Q-Chem 4.1 software package from Q-Chem, Inc. The molecular orbitals are calculated here by the method described above. The spatial overlaps for all possible pairs of occupied molecular orbitals, .sub.i, and unoccupied (virtual) molecular orbitals, .sub.a, are subsequently determined from the following equation:
O.sub.ia=|.sub.i|.sub.a|
where the absolute values of the orbitals are used for the calculation. The parameter then arises from the weighted sum over all pairs ia of occupied and unoccupied molecular orbitals in accordance with

(11) $=\frac{{.Math.}_{ia}{}_{\mathrm{ia}}^2{O}_{\mathrm{ia}}}{{.Math.}_{ia}{}_{\mathrm{ia}}^2}$
where the value of .sub.ia is determined by the method of Tozer et al. from the orbital coefficients in the excitation vectors of the resolved TD (time-dependent) eigenvalue equation and where 01.
Determination of the PL Quantum Efficiency (PLQE)

(12) A 50 nm thick film of the emission layers used is applied to a quartz substrate. This film comprises the same materials in the same concentrations as in the emission layer of the corresponding OLED, unless the emission layer comprises one or more further components (for example quantum dots, inorganic semiconductors or organic semiconductors). In this case, the film for measurement of the PLQE comprises all materials apart from the further components, and the mixing ratios of the materials present correspond to those in the emission layer of the OLED. The same production conditions as in the production of the emission layer for the OLEDs are used in the production of the films for measurement of the PLQE. An absorption spectrum of the film is measured in the wavelength range from 350-500 nm. To this end, the reflection spectrum R() and the transmission spectrum T() of the sample are determined at an angle of incidence of 6 (i.e. virtually perpendicular incidence). The absorption spectrum in the sense of this application is defined as A()=1R()T().

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

(14) The PLQE is determined using a Hamamatsu C9920-02 measurement system. The principle is based on excitation of the sample by light of defined wavelength and measurement of the absorbed and emitted radiation. The sample is located in an Ulbricht sphere (integrating sphere) during measurement. The spectrum of the excitation light is approximately Gaussian with a full width at half maximum of less than 10 nm and a peak wavelength .sub.exc as defined above.

(15) The PLQE is determined by the evaluation method which is usual for the said measurement system. It is vital to ensure that the sample does not come into contact with oxygen at any time, since the PLQE of materials having a small energetic separation between S.sub.1 and T.sub.1 is reduced very considerably by oxygen (H. Uoyama et al., Nature 2012, Vol. 492, 234). The measurement is carried out at room temperature.

(16) Determination of the Decay Time

(17) The decay time is determined using a sample 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 J, beam diameter 4 mm). The sample is located in a vacuum (less than 10.sup.5 mbar) here. After the excitation (defined as t=0), the change in the intensity of the emitted photoluminescence over time is measured. The photoluminescence exhibits a steep drop at the beginning, which is attributable to the prompt fluorescence of the TADF compound. As time continues, a slower drop is observed, the 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 t.sub.a in the sense of this application is the decay time of the delayed fluorescence and is determined as follows: a time t.sub.d is selected at which the prompt fluorescence has decayed significantly below the intensity of the delayed fluorescence, so that the following determination of the decay time is not influenced by the prompt fluorescence. This choice can be made by a person skilled in the art and belongs to his general expert knowledge. For the measurement data from time t.sub.d, the decay time t.sub.a=t.sub.et.sub.d is determined, t.sub.e here is the time after t=t.sub.d at which the intensity has for the first time dropped to 1/e of its value at t=t.sub.d.

Example 2

Device Examples

(18) The materials required for the following examples are shown in Table 1. The associated HOMO and LUMO energy levels as well as S.sub.1 and T.sub.1 are indicated in Table 2.

(19) Vacuum-Processed OLEDs

(20) Glass plates coated with structured ITO (indium tin oxide) in a thickness of 50 nm are wet-cleaned (laboratory dishwasher, Merck Extran detergent), subsequently dried by heating at 250 C. in a nitrogen atmosphere for 15 min and treated with an oxygen plasma for 130 s before the coating. These plasma-treated glass plates form the substrates to which the OLEDs are applied. The substrates remain in the vacuum before the coating. The coating begins at the latest 10 min after the plasma treatment.

(21) The materials are applied by thermal vapour deposition in a vacuum chamber. The emission layer here always consists of at least one matrix material (host material) and the emitting material. This is admixed with the matrix material or matrix materials in a certain proportion by volume by coevaporation. An expression such as IC1(60%):WB1(30%):D1(10%) here means that material IC1 is present in the layer in a proportion by volume of 60%, WB1 is present in the layer in a proportion of 30% and D1 is present in the layer in a proportion of 10%. Analogously, the electron-transport layer may also consist of a mixture of two materials.

(22) For characterisation of the OLEDs, current/voltage/luminous density characteristic lines are measured. The luminous density is determined using a calibrated photodiode. Furthermore, the electroluminescence spectrum is measured at a luminous density of 1000 cd/m.sup.2. The external quantum efficiency (EQE, measured in percent) is calculated therefrom assuming Lambert emission characteristics.

Example V1 in Accordance with the Prior Art

(23) The layer sequence 40 nm of BPA1, then 60 nm of IC1(60%):WB1(30%):D1(10%), then 10 nm of ST1, then 40 nm of ST1(50%):LiQ(50%) and finally 100 nm of aluminium as cathode is applied to the prepared substrates by thermal evaporation.

(24) The emission layer exhibits a PLQE of 84% (.sub.exc=350 nm) and a decay time of 4.9 s (t.sub.d=6 s).

(25) The OLEDs exhibit green emission, 15.1% EQE at 1000 cd/m.sup.2 and require a voltage of 7.2 V for this luminous density. 64 components are produced from this example. On operation at 20 mA/cm.sup.2 over a period of 200 h, seven of the components fail, i.e. they no longer exhibit any emission at all.

Example V2 in Accordance with the Prior Art

(26) The layer sequence 40 nm of BPA1, then 60 nm of IC1(30%):WB1(60%):D1(10%), then 10 nm of ST1, then 40 nm of ST1(50%):LiQ(50%) and finally 100 nm of aluminium as cathode is applied to the prepared substrates by thermal evaporation.

(27) The emission layer exhibits a PLQE of 79% (.sub.exc=350 nm) and a decay time of 5.1 s (t.sub.d=7 s).

(28) The OLEDs exhibit green emission, 13.9% EQE at 1000 cd/m.sup.2 and require a voltage of 6.8 V for this luminous density. 64 components are produced from this example. On operation at 20 mA/cm.sup.2 over a period of 200 h, five of the components fail, i.e. they no longer exhibit any emission at all.

(29) OLEDs Having a Solution-Processed Emission Layer

(30) Layers applied by a solution-based method and by a vacuum-based method are combined within an OLED in the example discussed below, so that the processing up to and including the emission layer is carried out from solution and in the subsequent layers by thermal vacuum evaporation.

(31) Cleaned glass plates (cleaning in Miele laboratory dishwasher, detergent Merck Extran) which have been coated with structured ITO (indium tin oxide) in a thickness of 50 nm are treated with a UV ozone plasma for 20 min and subsequently coated with 20 nm of PEDOT:PSS (poly(3,4-ethyl-enedioxythiophene) poly(styrenesulfonate), purchased as CLEVIOS P VP Al 4083 from Heraeus Precious Metals GmbH, Germany, applied by spin coating from aqueous solution). These substrates are subsequently dried by heating at 180 C. for 10 min.

(32) A hole-transport layer with a thickness of 20 nm is applied to these substrates. It consists of a polymer of the following structural formula:

(33) ##STR00028##
which has been synthesised in accordance with WO 2010/097155. The material is dissolved in toluene. The solids content of the solution is 5 g/l. A layer with a thickness of 20 nm is applied therefrom by spin coating in a nitrogen atmosphere. The sample is subsequently dried by heating at 180 C. in a nitrogen atmosphere for 60 minutes.

(34) The emission layer is subsequently applied. This is always composed of at least one matrix material (host material) and an emitting dopant (emitter). An expression such as IC2(60%):WB1(30%):D1(10%) means that material IC2 is present in the solution from which the emission layer is produced in a proportion by weight of 60%, WB1 is present in a proportion by weight of 30% and D1 is present in a proportion by weight of 10%. A corresponding solid mixture for the emission layer is dissolved in toluene. The solids content is 18 g/l. The emission layer is applied by spin coating in a nitrogen atmosphere and dried by heating at 180 C. in a nitrogen atmosphere for 10 minutes.

(35) The samples are subsequently introduced into a vacuum chamber without contact with air, and further layers are applied by thermal evaporation. If a layer of this type consists of a plurality of materials, the nomenclature described above for layers applied by thermal vapour deposition applies to the mixing ratios of the individual components.

(36) The OLEDs are characterised as described for the vacuum-processed OLEDs.

Example E1 According to the Invention

(37) For the emission layer, a solid mixture IC2(60%):WB1(30%):D1(10%) is used. An emission layer with a thickness of 60 nm is produced therefrom as described above. A layer of material ST1 with a thickness of 10 nm and then a layer of ST1(50%):LiQ(50%) with a thickness of 40 nm is subsequently applied by thermal vacuum evaporation. An aluminium layer with a thickness of 100 nm is subsequently applied as cathode by vacuum evaporation.

(38) The emission layer exhibits a PLQE of 82% (.sub.exc=350 nm) and a decay time of 4.6 s (t.sub.d=5 s).

(39) The OLEDs exhibit green emission, 14.2% EQE at 1000 cd/m.sup.2 and require a voltage of 7.4 V for this luminous density. 64 components are produced from this example. On operation at 20 mA/cm.sup.2 over a period of 200 h, two of the components fail, i.e. they no longer exhibit any emission at all.

Example E2 According to the Invention

(40) The OLED corresponds to Example E1, with the difference that the mixture IC2(60%):WB1(30%):D1(10%) is replaced by the mixture IC2(30%):WB1(60%):D1(10%).

(41) The emission layer exhibits a PLQE of 75% (.sub.exc=350 nm) and a decay time of 4.7 s (t.sub.d=5 s).

(42) The OLEDs exhibit green emission, 12.9% EQE at 1000 cd/m.sup.2 and require a voltage of 6.9 V for this luminous density. 64 components are produced from this example. On operation at 20 mA/cm.sup.2 over a period of 200 h, none of the components fails.

Example V3 (in Accordance with the Prior Art) Having a Vacuum-Processed Emission Layer

(43) The OLED corresponds to that of Example E1, with the difference that the emission layer is vacuum-processed, i.e. the emission layer IC2(60%):WB1(30%):D1(10%) with a thickness of 60 nm is produced by vacuum evaporation.

(44) The emission layer exhibits a PLQE of 86% (.sub.exc=350 nm) and a decay time of 4.7 s (t.sub.d=5 s).

(45) The OLEDs exhibit green emission, 13.5% EQE at 1000 cd/m.sup.2 and require a voltage of 7.5 V for this luminous density. 64 components are produced from this example. On operation at 20 mA/cm.sup.2 over a period of 200 h, four of the components fail, i.e. they no longer exhibit any emission at all.

Comparison of the Examples

(46) In Examples V1, V2 in accordance with the prior art and Examples E1, E2 according to the invention, very similar materials are employed which are adapted for the respective processing type. In particular, compound D1 is employed as TADF material in all examples.

(47) The comparison of Examples V1 and E1 or V2 and E2 shows that comparable efficiency and voltage can be achieved with solution-processed emission layers as with vacuum-processed emission layers. However, the failure rate of the vacuum-processed OLEDs in operation is significantly higher.

(48) In Examples V3 and E1, identical materials are used, with the difference that in E1 the emission layer is produced from solution and in V3 the emission layer is produced by vacuum evaporation. The performance data are comparable, but the OLEDs having a solution-processed emission layer exhibit significantly fewer failures.

(49) OLEDs Having Emission Layers Containing Quantum Rods

(50) The OLED corresponds to Example E1, with the difference that the mixture IC2(60%):WB1(30%):D1(10%) is replaced by the mixture IC2(45%):WB1(23%):D1(7%):QRod(25%). QRod here is a red-emitting quantum rod which contains a CdSe core having a diameter of 3.9 nm, the surrounding rod with a length of 35 nm consists of CdS. The capping agent used is octadecylphosphonic acid. The peak wavelength of QRod is 635 nm, the half-value width is 30 nm.

(51) The emission layer without QRod exhibits a PLQE of 81% (.sub.exc=350 nm) and a decay time of 4.8 s (t.sub.d=5 s).

(52) The OLED exhibits yellow emission, i.e. a mixture of the green emission of D1 and the red emission of QRod. The EQE is 8.4% at 1000 cd/m.sup.2, a voltage of 9.4 V is required for this luminous density.

(53) TABLE-US-00002 TABLE 1 Structural formulae of the materials for the OLEDs LiQ 0 ST1 IC1 BPA1 WB1 IC2 D1

(54) TABLE-US-00003 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) D1 org. 6.11 3.40 2.50 2.41 IC1 org. 5.80 2.83 3.12 2.70 IC2 org. 5.78 2.84 3.04 2.69 WB1 org. 6.16 2.24 3.38 2.95 BPA1 org. 5.14 2.27 3.14 2.52 ST1 org. 6.03 2.82 3.32 2.68 LiQ org.-m 5.17 2.39 2.85 2.13