Active OLED Display, Method of Operating an Active OLED Display and Compound

Abstract

An active OLED display, comprising a plurality of OLED pixels, each of the OLED pixels comprising an anode, a cathode, and a stack of organic layers, wherein the stack of organic layers is provided between and in contact with the cathode and the anode and comprises an electron transport layer, a hole transport layer, and a light emitting layer provided between the hole transport layer and the electron transport layer, and a driving circuit configured to separately driving the pixels of the plurality of OLED pixels, wherein, for the plurality of OLED pixels, a common hole transport layer is formed by the hole transport layers provided in the stack of organic layers of the plurality of OLED pixels, the common hole transport layer comprising a hole transport matrix material and at least one electrical p-dopant, and the electrical conductivity of the hole transport material being lower than 1×10.sup.−3 S.Math.m.sup.−1 and higher than 1×10.sup.−8 S.Math.m.sup.−1.

Claims

1. An active OLED display, comprising a plurality of OLED pixels, each of the OLED pixels comprising an anode, a cathode, and a stack of organic layers, wherein the stack of organic layers is provided between and in contact with the cathode and the anode, and comprises an electron transport layer, a hole transport layer, and a light emitting layer provided between the hole transport layer and the electron transport layer, and a driving circuit configured to separately driving the pixels of the plurality of OLED pixels, wherein, for the plurality of OLED pixels, a common hole transport layer is formed by the hole transport layers provided in the stack of organic layers of the plurality of OLED pixels, the common hole transport layer comprising a hole transport matrix material and an electrical p-dopant, and wherein the electrical conductivity of the common hole transport layer is lower than 1×10.sup.−3 S.Math.m.sup.−1 and higher than 1×10.sup.−8 S.Math.m.sup.−1.

2. The active OLED display according to claim 1, wherein the LUMO energy level of the electrical p-dopant, expressed in the absolute scale referring to vacuum energy level being zero, is at least 150 meV higher than the highest HOMO energy level of the compounds forming the hole transport matrix material.

3. The active OLED display according to claim 1, wherein the LUMO energy level of the electrical p-dopant, expressed in the absolute scale referring to vacuum energy level being zero, is less than 600 meV above the highest HOMO energy level of the compounds forming the hole transport matrix material.

4. The active OLED display according to claim 1, wherein the hole transport matrix material consists of compounds having energies of their highest occupied molecular orbitals, expressed in the absolute scale referring to vacuum energy level being zero, in the range from −4.8 eV to −5.5 eV.

5. The active OLED display according to claim 1 wherein the common hole transport layer has a thickness of less than 50 nm.

6. The active OLED display according to claim 1, wherein the common hole transport layer has a thickness of more than 3 nm.

7. The active OLED display according to claim 1, wherein the work function of the anode, expressed in the absolute scale referring to vacuum energy level being zero, is less than 500 meV above the highest LUMO energy level of the compounds forming the p-dopant.

8. The active OLED display according to claim 1, wherein the stack of organic layers further comprises an electron blocking layer provided between the hole transport layer and the light emitting layer.

9. The active OLED display according to claim 8, wherein the electron blocking layer has a thickness of more than 30 nm.

10. The active OLED display according to claim 8, wherein the electron blocking layer has a thickness of less than 200 nm.

11. The active OLED display according to claim 8, wherein each compound forming the electron blocking layer has a HOMO level, expressed in the absolute scale referring to vacuum energy level being zero, higher than the HOMO level of any compound forming the hole transport matrix material of the common hole transport layer.

12. The active OLED display according to claim 8, wherein the hole transport matrix material of the common hole transport layer is provided with a hole mobility which is lower than a hole mobility of a matrix material of the electron blocking layer.

13. The active OLED display according to claim 1, wherein the hole transport matrix material of the common hole transport layer is selected from compounds comprising a conjugated system of delocalized electrons, the conjugated system comprising lone electron pairs of at least two tertiary amine nitrogen atoms.

14. The active OLED display according to claim 1, wherein the light emitting layer comprises a plurality of separated sub-regions, each of the sub-regions being assigned to one of the pixels from the plurality of OLED pixels.

15. The active OLED display according to claim 1, wherein, for the plurality of OLED pixels, a common electron transport layer is formed by the electron transport layers provided in the stack of organic layers of the plurality of OLED pixels.

16. The active OLED display according to claim 15, wherein the common electron transport layer comprises an electron transport matrix material and an electrical n-dopant.

17. A method of operating an active OLED display having a plurality of OLED pixels according claim 1, wherein a driving circuit applies a driving current to each pixel of the plurality of OLED pixels, the driving current being different for neighbor OLED pixels at an operation time.

18. Compound having formula ##STR00010##

Description

DESCRIPTION OF EMBODIMENTS

[0040] In the following, further embodiments will be described in further detail, by way of example, with reference to figures. In the figures show:

[0041] FIG. 1 a schematic representation of an active OLED display, the display having a plurality of OLED pixels,

[0042] FIG. 2 a graphical representation of the current density vs. voltage for the reference devices of the Comparative example 1 according line 1 of the Table 2 (solid squares) and for the devices according to line 2 of the Table 2 (open squares), respectively;

[0043] FIG. 3 a graphical representation of the quantum efficiency vs. current density for the reference devices of the Comparative example 1 according line 1 of the Table 2 (solid squares) and for the devices according to line 2 of the Table 2 (open squares), respectively;

[0044] FIG. 4 a graphical representation of the luminance vs. time for the reference devices of the Comparative example 1 according line 1 of the Table 2 (solid squares) and for the devices according to line 2 of the Table 2 (open squares), respectively; and

[0045] FIG. 5 a graphical representation of the forward voltage vs. time for the reference devices of the Comparative example 1 according line 1 of the Table 2 (solid squares) and for the devices according to line 2 of the Table 2 (open squares), respectively.

[0046] FIG. 1 shows a schematic representation of an active OLED display 1 having a plurality of OLED pixels 2, 3, 4 provided in an OLED display 1. In the OLED display 1, each pixel 2, 3, 4 is provided with an anode 2a, 3a, 4a being connected to a driving circuit (not shown). Various equipment able to serve as a driving circuit for an active matrix display is known in the art. In one embodiment, the anodes 2a, 3a, 4a are made of a TCO, for example of ITO.

[0047] A cathode 6 is provided on top of an organic stack comprising an electrically doped hole transport layer (HTL) 7, an electron blocking layer (EBL) 5, a light emitting layer (EML) having sub-regions 2b, 3b, 4b assigned to the pixels 2, 3, 4 and being provided separately in an electron transport layer (ETL) 9. For example, the sub-regions 2b, 3b, 4b can provide an RGB combination for a color display (R—red, G—green, B—blue). By applying individual drive currents to the pixels 2, 3, 4 via the anodes 2a, 3a, 4a and the cathode 6, the display pixels 2, 3, 4 are operated independently.

SYNTHESIS EXAMPLES

[0048] Synthesis of HT3

##STR00002##

STEP 1: Synthesis of N-(3-fluoro-4-methylphenyl)[1,1′-biphenyl]-4-amine

[0049] 4-Bromobiphenyl (20.0 g, 85.8 mmol), 3-fluoro-4-methylaniline (11.3 g, 90.1 mmol), Pd(OAc).sub.2 (578 mg, 2.57 mmol, 3 mol. %), 2,2′-bis(diphenylphosphino)-1,1′-binaphtalene ((BINAP) 2.40 g, 3.86 mmol, 4.5 mol. %), and Cs.sub.2CO.sub.3 (39.13 g, 0.12 mol, 1.4 eq.), were charged in a flask under nitrogen atmosphere. The solids were suspended in anhydrous 1,4-dioxane, and the suspension was refluxed for 22 h at 125° C. After cooling to room temperature, it was filtered over silica and the pad was rinsed with dichloromethane. The filtrate was evaporated to dryness and purified by chromatography (silica, elution with hexane/dichloromethane 2:1, R.sub.f in the corresponding TLC system 0.35). The product was isolated in two main fractions: (−1) 7.55 g (32% yield) with 99.73% purity according to HPLC; (−2) 3.75 g (16% yield) with 99.33% purity according to HPLC. Both fractions were mixed together for the next step.

[0050] .sup.1H NMR (CD.sub.2Cl.sub.2, 400 MHz): 7.58 (2H, dd, J=8.24 and 1.10 Hz), 7.54 (2H; m-AB; J=8.57 Hz), 7.43 (2H, t, J=7.75 Hz), 7.31 (2H, t, J=7.38 Hz), 7.14 (2H; m-AB; J=8.57 Hz), 7.09 (1H, t, J=8.47 Hz), 6.81 (2H, m), 5.86 (1H, bs), 2.22 (3H, s) ppm.

[0051] .sup.13C NMR (CD.sub.2Cl.sub.2, 100 MHz): 164.18, 163.29, 161.36, 143.03, 142.95, 142.86, 141.21, 134.34, 132.43, 132.37, 129.31, 128.44, 127.22, 126.95, 118.42, 117.52, 117.38, 114.02 (d, J=2.93 Hz), 105.10, 104.89, 14.12 (d, J=3.24 Hz) ppm.

STEP 2: Synthesis of N4,N4″-di([1,1′-biphenyl]-4-yl)-N4,N4″-bis(3-fluoro-4-methylphenyl)-[1,1′:4′,1″-terphenyl]-4,4″-diamine

[0052] 4,4″-Dibromo-1,1′:4′,1″-terphenyl (7.33 g, 18.9 mmol), N-(3-Fluoro-4-methylphenyl)-[1,1′-biphenyl]-4-amine (11.0 g, 39.7 mmol, 2.16 eq.), Pd(dba).sub.2 (217 mg, 0.57 mmol, 2.0 mol %), PtBu.sub.3 (115 mg, 0.57 mmol, 2.0 mol %), and KOtBu (6.36 g, 56.7 mmol, 3.0 eq.), were charged in a flask under nitrogen atmosphere. The solids were suspended in anhydrous toluene, and the suspension was refluxed for 22 h at 80° C. After cooling to room temperature, it was filtered over silica, the pad was abundantly rinsed with tetrahydrofuran, and the filtrate was evaporated to dryness. The resulting solid was triturated in refluxing methanol (150 mL) for 20 min, and the suspension filtered hot, yielding after drying 14.9 g of the title compound (98.9% yield) with 98.92% purity according to HPLC. The product was then sublimed to yield yellow amorphous solid with 99.51% purity according to HPLC.

[0053] Elemental analysis: C 85.88% (86.13% theor.), H 5.60% (5.42% theor.) N 3.61% (3.59% theor.)

[0054] Glass Transition Onset: Tg=114° C. (from DSC 10 K/min), no melting peak observed.

DEVICE EXAMPLES

Comparative Example 1

[0055] The active OLED display according to previous art was prepared on a glass substrate provided with transparent ITO anode (thickness 90 nm), by subsequent vacuum deposition of following layers: p-doped HTL (10 nm, HT1 doped with 8 wt. % PD2); EBL (HT1, 120 nm); fluorescent EML (ABH113:NUBD370 from SFC Co. Ltd., Korea, 20 nm, 97:3 wt. %,); ETL (ET1:LiQ, 36 nm, 50:50 wt. %); and cathode (aluminium, 100 nm). The obtained results are given in Table 2, line 1.

Working Example 1

[0056] The comparative example 1 was reproduced with p-doped HTL made of HT2 doped with 3 wt. % PD2. The obtained results are given in Table 2, line 2.

Working Example 2

[0057] Working example 1 was reproduced using HT3 instead of HT2. The obtained results are given in Table 2, line 3.

Working Example 3

[0058] Comparative example 1 was reproduced with p-doped HTL made of HT4 doped with 7 wt. % PD2. The obtained results are given in Table 2, line 4.

Comparative Example 2

[0059] Working examples 1 and 2 were reproduced using HT1 instead of HT2 or HT3. The obtained results are given in Table 2, line 5.

TABLE-US-00001 TABLE 1 Compound Structure HT1 N,N′-Di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″- terphenyl]-4,4″-diamine (CAS 139255-16-6) [00003] HT2 N3,N5-di([1,1′-biphenyl]-4-yl)-N3,N5-bis(4-(tert- butyl)phenyl)-3′-(trifluoromethyl)-[1,1′-biphenyl]-3,5- diamine (CAS 1602531-85-0) [00004] HT3 N4,N4″-di([1,1′-biphenyl]-4-yl)-N4,N4″-bis(3-fluoro-4- methylphenyl)-[1,1′:4′,1″-terphenyl]-4,4″-diamine [00005] HT4 N3,N3′-bis([1,1′-biphenyl]-4-yl)-N3,N3′-diphenyl-[1,1′- biphenyl]-3,3′-diamine (CAS 1242056-42-3) [00006] PD2 2,2′,2″-(cyclopropane-1,2,3-triylidene)-tris[2-(4- cyanoperfluorophenyl)-acetonitrile] (CAS 1224447-88-4) [00007] ET1 2-(4-(9,10-Di(naphthalen-2-yl)anthracen-2-yl)phenyl)-1- phenyl-1H-benzo[d]imidazole (CAS 561064-11-7) [00008] LiQ 8-Hydroxyquinolato lithium (CAS 850918-68-2) [00009]

TABLE-US-00002 TABLE 2 HTL Voltage HTL p-dopant HOMO at Voltage HOMO μ0 conductivity concentration EBL 15 mA/cm.sup.2 QE LT97 rise HTM1 (HTL) (eV) [10.sup.−7 cm.sup.2/Vs] [10.sup.−6 S/m] (wt. %) (eV) [V] at 15 mA/cm.sup.2 % at 15 mA/cm.sup.2 h at 15 mA/cm.sup.2 HT1 −5.25 7180 7630 8 −5.25 4.230 6.29 46 No HT2 −5.20 2.32 4.71 3 −5.25 4.290 6.49 40 No HT3 −5.28 2520 165 3 −5.25 4.139 6.36 55 No HT4 −5.33 2.32 60 7 −5.25 4.30 6.40 40 No HT1 −5.25 7180 177 3 −5.25 4.493 6.50 — yes

[0060] Following, with regard to terms used in Table 2, further explanation is provided.

[0061] The term “HOMO” refers to the Highest Occupied Molecular Orbital energy level 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.sub.cv (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).

[0062] 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.

[0063] A simple rule is very often used for the conversion of redox potentials into electron affinities (EA) and ionization potential (IP): IP (in eV)=4.84 eV+e*Eox (wherein Eox is given in Volt vs. ferrocene/ferrocenium (Fc/Fc+) and EA (in eV)=4.84 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).

[0064] The term “μ0” refers to zero field mobility. Mobility is determined in admittance spectroscopy from capacitance vs. frequency tracks and is described in detail in reference: Nguyen et al., Determination of charge-carrier transport in organic devices by admittance spectroscopy: Application to hole mobility in α-NPD.” Physical Review B 75.7 (2007): 075307.

[0065] The devices used for the hole mobility measurement had a layer structure of ITO (100 nm)/HT1:PD2 (10 nm)/assessed HTM (700 nm)/ HT1:PD2 (10 nm)/Au (10 nm)/Al 100 nm). The 10 nm hole injecting layers of HT1:PD2 (weight ratio 90:10) were provided to ensure ohmic contacts to the ITO anode and Au/Al cathode. Measurement of geometric capacitance was done by using a sample as given above without HILs. The following conditions and parameters applied: room temperature, amplitude: 20 mV, frequency: 110 Hz to 2 MHz. The voltage range was chosen appropriately, to allow the mobility estimation at relevant current densities in the range from 10 to 50 mA/cm.sup.2.

[0066] The column “conductivity” refers to electrical conductivity measured by standard four point method described e.g. in WO 2013/135237 A1, on a thin film of the chosen matrix comprising the PD2 dopant in concentration given in the next column of the Table 2. The films prepared for conductivity measurements were vacuum deposited onto glass substrate covered with ITO contacts; the conductivities were estimated at room temperature.

[0067] QE stands for quantum efficiency; LT97 stands for the timespan within the luminance of the device operated at given current density had not changed more than 3% of its initial value. “Voltage rise” is another important operational characteristic of OLEDs. In stable devices operated at constant current, the voltage remains constant. Should the voltage in a testing device raise more than 5% of its initial value during the desired lifetime, it is a sign that the tested material makes the device instable.

[0068] FIGS. 2 to 5 show that OLEDs proposed here allow for suppressed crosstalk in displays, have the same performance as previous art OLED of Comparative example 1 that comprises the redox-doped HTL with significantly higher conductivity than HTLs of Working examples 1 to 3. It is supposed that displays of the present disclosure show suppressed crosstalk due to redox-doped electrically doped hole transport layer having sufficient dopant concentration, still allowing good stability of the device and good charge injection from the anode and/or into the adjacent organic layer, but having significantly lower conductivity due to low charge carrier mobility and/or low actual charge carrier concentration.

[0069] An attempt to decrease the conductivity in a state-of-the-art OLED comprising the HT1 matrix with hole mobility above 5.10.sup.−4 cm.sup.2/Vs, as done in the Comparative example 2, resulted in a device lacking the necessary operational stability. These results surprisingly showed that sufficient concentration of a redox p-dopant is important not only for retaining good voltage, but also for the device stability. It was furthermore demonstrated that despite low conductivity of HTLs comprising matrices having low hole mobilities (below 5.10.sup.−4 cm.sup.2/Vs), in combination with a redox p-dopant, these matrices surprisingly allow construction of OLEDs having equal or better voltages and other performance parameters as the state-of-the-art devices comprising high-conductivity HTL, with the substantial advantage that the inventive OLEDs suppress the pixel crosstalk significantly, thanks to their low conductivity HTL, if used as pixels in state-of-the-art displays comprising a common HTL shared by plurality of pixels.

[0070] The features disclosed in the foregoing description and in the claims may, both separately and in any combination thereof, be material for realizing the aspects of the disclosure in diverse forms thereof.

[0071] Key symbols and abbreviations used throughout the application:

[0072] CV cyclic voltammetry

[0073] DSC differential scanning calorimentry

[0074] EBL electron blocking layer

[0075] EIL electron injecting layer

[0076] EML emitting layer

[0077] eq. equivalent

[0078] ETL electron transport layer

[0079] ETM electron transport matrix

[0080] Fc ferrocene

[0081] Fc.sup.− ferricenium

[0082] HBL hole blocking layer

[0083] HIL hole injecting layer

[0084] HOMO highest occupied molecular orbital

[0085] HPLC high performance liquid chromatography

[0086] HTL hole transport layer

[0087] p-HTL p-doped hole transport layer

[0088] HTM hole transport matrix

[0089] ITO indium tin oxide

[0090] LUMO lowest unoccupied molecular orbital

[0091] mol. % molar percent

[0092] NMR nuclear magnetic resonance

[0093] OLED organic light emitting diode

[0094] OPV organic photovoltaics

[0095] QE quantum efficiency

[0096] R.sub.f retardation factor in TLC

[0097] RGB red-green-blue

[0098] TCO transparent conductive oxide

[0099] TFT thin film transistor

[0100] T.sub.g glass transition temperature

[0101] TLC thin layer chromatography

[0102] wt. % weight percent