IMPROVED LIGHT EMISSION IN OLEDS

Abstract

Improved light emission in OLEDs The invention relates to an organic light-emitting diode (OLED) system comprising a multi-layered structure having a semiconducting organic layer (12) sandwiched between first and second electrodes (3a, 3b); further comprising a barrier layer (6) interposed between the semiconducting organic layer and a polymer substrate (1) having formed an random nanopillar structure thereon having a pillar height dimension between 50 and 1000 nanometer and a pitch in a range of 50-1000 nanometer.

Claims

1. A method of manufacturing a barrier substrate for an organic light-emitting diode (OLED) system comprising: providing a transparent polymer substrate; forming a random nanopillar structure on the transparent polymer substrate wherein pillars of the nanopillar structure are formed using an ablation process, wherein the random nanopillar structure has: a pillar height dimension in a range of 50 to 1000 nanometer; and a pitch in a range of 50 to 1000 nanometer; providing a transparent coating having a thickness in a range of 100 nm to 30 microns and having a refractive index matching an inorganic barrier layer; and providing the inorganic barrier layer.

2. The method according to claim 1, further comprising providing a multi-layered structure having a semiconducting organic layer sandwiched between a first electron and a second electrode; wherein the inorganic barrier layer is interposed between the first electrode and the second electrode and the transparent polymer substrate on a second side of the inorganic barrier layer.

3. The method according to claim 2, wherein the inorganic barrier layer is provided in contact with the first electrode layer such the first electrode follows the topology imparted by the random nanopillar structure.

4. The method according to claim 1 wherein the ablation process is a Reactive Ion Etching (REI) process.

5. The method according to claim 4 wherein the RIE process is carried out by a plasma taken from the group consisting of: CHF3, Ar, O2; and wherein the plasma is delivered at a power setting of 50-500 W, and time duration range of 0.1-10 minutes.

6. A method of manufacturing a barrier substrate for an organic light-emitting diode (OLED) system, carried out in a roll to roll process, comprising: providing a transparent polymer substrate on a roll; unrolling the transparent polymer substrate; forming a random nanopillar structure on the transparent polymer substrate wherein pillars of the nanopillar structure are formed using an ablation process, wherein the random nanopillar structure has: a pillar height dimension in a range of 50 to 1000 nanometer; and a pitch in a range of 50 to 1000 nanometer; providing a transparent coating having a thickness in a range of 100 nm to 30 microns and having a refractive index matching an inorganic barrier layer; providing the inorganic barrier layer to render a finished barrier substrate on a roll.

7. The method according to claim 6 wherein the transparent polymer substrate comprises a dispersion of inorganic shielding particles.

8. The method according to claim 7, wherein the inorganic shielding particles shield the nano pillars from the ablation process and are substantially made of an oxide of at least one element selected from the group consisting of: Si, Al, Ti and Zr.

9. The method according to claim 8, wherein an average particle diameter of the shielding particles is in a range of 5-100 nm.

10. A method according to claim 6 wherein the ablation process is a laser process.

11. The method of claim 6 wherein the transparent polymer substrate is made from polyethylene terephthalate (PET).

12. The method of claim 6 wherein the transparent polymer substrate is made from polyethylene naphthalate (PEN).

13. The method according to claim 1 wherein the transparent polymer substrate comprises a dispersion of inorganic shielding particles.

14. The method according to claim 13, wherein the inorganic shielding particles shield the nano pillars from the ablation process and are substantially made of an oxide of at least one element selected from the group consisting of: Si, Al, Ti and Zr.

15. The method according to claim 14, wherein an average particle diameter of the shielding particles is in a range of 5-100 nm.

16. The method according to claim 1 wherein the ablation process is a laser process.

17. The method of claim 1 wherein the transparent polymer substrate is made from polyethylene terephthalate (PET).

18. The method of claim 1 wherein the transparent polymer substrate is made from polyethylene naphthalate (PEN).

17. The method of claim 5 wherein the plasma is delivered at a power setting of between 100 and 300 W.

18. The method of claim 5 wherein the plasma is delivered for a duration range of between 0.5 and 5 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1A presents in a schematic way an embodiment of a cross-section of an OLED according to the invention;

[0022] FIG. 1B presents an exemplary OLED stack that also is the reference stack;

[0023] FIG. 2 (A+B) shows two SEM images of a polymer substrate, treated with an ablation process according to an aspect of the invention.

[0024] FIG. 3 (A+B+C) shows exemplary k-space plots for periodic and random structures;

[0025] FIG. 4 shows a measured outcoupling for the exemplary embodiment, in comparison with an untreated comparative embodiment.

[0026] FIG. 5 shows an example, wherein nano pillar structures are not planarized by the refractive coating and barrier layer.

DETAILED DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 presents in a schematic way an embodiment of a cross-section of an OLED according to the invention. In the organic light-emitting diode (OLED) a multi-layered structure 10 is provided having a semiconducting organic layer 2 sandwiched between first and second electrodes 3a, 3b. FIG. 1A illustrates a bottom emission OLED device that emits through a transparent anode 3a that is created on a plastic substrate. Electrode 3b serves as the cathode is highly reflective. Alternatively, cathode 3b could be formed by a layer combination that generates a semi-transparent reflective layer, or even a fully transparent layer, e.g. that is formed by a transparent conductive layer, which may include layers formed from transparent conductive oxides, nanowires, nanoparticles, and other materials combinations that provide the same n a further aspect of the invention, a flexible substrate 1 is provided with a surface texture that forms an irregular random nanopillar structure obtainable by selective etching or thermal/radiative treatment of the organic substrate surface. In one example, the PET or PEN can be laminated onto a glass substrate with a glue by a sheet-to-sheet process. The PET or PEN is then put in a RIE chamber and etched. The structure may be post- processed to remove debris (e.g. remove with sacrificial sticky foil). A refractive coating 5 is then put onto the substrate 1. This coating 5 is preferably of similar or high refractive index, e.g. n>1.5 (n is refractive index), preferably even of n larger or equal to 1.8 (also 1.7 may be used, e.g. polyimide). Coating 5 preferably does not absorb the visible OLED radiation. Polyimide was proven to be sufficiently transparent (special type as was commercially obtained from Brewer Sci). The n=1.8 layer was also fully transparent. The refractive coating 5 was subsequently coated with an inorganic barrier layer 6 a with matching refractive index, e.g. PE-CVD SiN. SiN is preferably adapted to have a relatively low refractive index and near 0 extinction coefficient by incorporating hydrogen. Barrier coating 6 may be formed by a stack of inorganic/organic/inorganic barrier layers e.g. of the type as disclosed in EP2924757. Barrier coating 6 or coatings is then covered with an anode 3a, e.g. ITO, and the OLED. In the example as disclosed, the OLED is green light-emitting, but the random nature of the RIE texture makes it suitable for any wavelength in the visible spectrum or even beyond that. The organics of the OLED are covered with a cathode 3a, which may be highly reflective (e.g. Al, Ag; here used is Al) or (semi-) transparent (e.g. TCO, metal nano-particles, nano-wires, graphene, etc.) in order to create a transparent device. The device 10 is sealed, for instance with a (thin film) encapsulation stack 4. Optional is to add external light diffusing layers, but this will affect the appearance.

[0028] The invention is not specifically tied to a particular OLED structure, that can be top emissive or bottom emissive. By way of illustration, Figure lb gives an exemplary stack that also functions as a reference stack when the substrate is glass. The OLED stack used in the experiment emits green light from an Ir(ppy)3 emitter that is co-evaporated in TPBI and TCTA. The stack consists of HAT-CN as hole-injection material, NPB as hole transport layer, TCTA (5 nm pure, 5 nm co-evaporated with the dye) and TPBI (co-evaporated with the dye) as the emissive hosts (where the first transports the holes and the latter the electrons), BAlQ as hole and exciton blocking layer and AlQ3 as the electron transport layer. Aluminum was used as cathode, in combination with the electron injection material LiF. The stack was applied in OLEDs on standard glass for modelling purposes, but also to serve as a reference to the structured OLEDs. Such green devices are fabricated regularly and have an efficacy of ?45 cd/A at 1000 cd/m2 without further modifications.

[0029] Alternatively, embodiment cathode 3b may be formed by a metal, a combination of multiple metals, metal oxides, a metalorganic compound or even one or more organic layers, and may comprise an electron injection layer part formed by one or more optically reactive materials that facilitate charge injection. For instance, a 15 nm layer may be provided of a transparent layer sequence of Ba/Al/Ag, which can be capped with 20-30 nm of a high index organic, such as ZnS or ZnSe. Other suitable electron injection materials may include Ca, LiF, CsF, NaF, BaO, CaO, Li.sub.2O, CsCO.sub.3. Organic layers that facilitate electron injection may be based on variety of mechanisms, including, but not limited to, the formation of radicals when doped in an organic layer (N-DMBI) or the formation of a dipole layer that shifts the work function of the adjacent layer. The stack may be capped by a dense layer of SiN of about 100-200 nm, which provides a barrier to moisture and gasses. The top layers may be provided by an alternating stack of OCP (Organic Coating for Planarization) and SiN layers 6, ending in one or more layers to shield SiN layer 6 from outside influences such as scratches, for instance another OCP layer.

The stack 2 may be formed by a multi-layered structure comprising hole injection layers that may, by way of example, be formed by any of the following materials PEDOT:PSS; Polyaniline; m-MTDATA (4,4,4-Tris[(3-methylphenyl) phenylamino]triphenylamine); carbonitriles, such as HAT-CN, PPDN; phenazines (HATNA); quinoclimethanes, such as TCNQ and F4TCNQ; Phthaocyanine metalcomplexes (including Cu, Ti, Pt complexes); Aromatic amines including fluorene moieties, such as MeO-TPD, MeO-Spiro-TPD; benzidines (such as NTNPB, NPNPB). The OLED stack may furthermore comprise material layers known to the skilled person, e.g. hole transport layers; material layers for emissive phosphorescent dyes (e.g. Ir(III) emitters) and electron transport & hole blocking layers e.g. formed of Quinolinolato metal complexes, like Liq, BAlq; Benzimidazoles (such as TPBi, N-DMBi); Oxadiazoles (such as PBD, Bpy-OXD, BP-OXD-Bpy); Phenanthrolines (such as BCP, Bphen); Triazoles (such as TAZ, NTAZ); Pyridyl compounds (such as BP4mPy, TmPyPB, BP-OXD-Bpy); Pyridines (such as BmPyPhB, TpPyPB); Bathocuproines and Bathophenanthrolines, oxadiazoles, triazoles, quinoline aluminum salts.

[0030] In FIG. 2A it is disclosed how the aperiodic pillar structure looks like in a SEM image of a polymer that is treated with a RIE-process, in particular, the structure in region R. Conditions of RIE may be adjusted to create higher structurese.g. structures of several 100 nm highup to or even beyond 1 micrometer. FIG. 2B shows a SEM image at 1000? of a polymer that exposed to laser irradiation of a KrF-excimer laser (248 nm), just below an ablation threshold; examples of such irradiation are found in H. Pzokian et al, J. Michromech, 22 (2012)035001.

[0031] It can be seen that on the substrate is formed a random nanopillar structure having a pillar height dimension between 50 and 1000 nanometer and a pitch in a range of 50-1000 nanometer.

[0032] FIG. 5 shows an example, wherein these nano pillar structures are not planarized by the refractive coating and barrier layer 6, but will create a so-called corrugated OLED wherein the OLED including cathode 3a follows the topology imparted by the aperiodic, random nanopillar structure 8. The other layers in the OLED stack 2 are not shown for reasons of intelligibility. To this effect, the barrier layer is provided with at least 1 dyad or tryad of transparent inorganic and transparent organic layers with a total thickness of a few hundred nanometers to at most 20 microns, such that the nano-topology is not planarized and a non-planar interface remains with a height of the topology of at least 10% of the original height, more preferably 30%, even more preferably 50% of the nanopillar structure.

[0033] By such enhanced structures, the outcoupling efficiency of the device can be further enhanced because surface plasmons will be harvested (e.g. will counter-act cathode quenching). This effect may already be present for structures below 200 nm. The RIE may also (have to) be tuned to create less debris. Also, the RIE may be tuned to be a faster process. Also, the RIE may be tuned to have a higher or lower periodicity by tuning the density of particles.

[0034] Various ablation processes can be used to obtain similar results wherein the shielding particles shield the nanopillars from the ablation process, e.g. [0035] 3 min; 100 W; corresponding homogeneous etch rate HPR504 34 nm/min (100 sccm Ar, 15 sccm O2 and 5 sccm CHF3) [0036] 3 min; 300 W; corresponding homogeneous etch rate HPR504 69 nm/min (15 sccm O2 and 5 sccm CHF3) [0037] 9 min; 300 W; corresponding homogeneous etch rate HPR504 113 nm/min (15 sccm O2 and 5 sccm CHF3)

[0038] In another embodiment, the ablation process can be carried out by laser irradiation, to obtain a substrate having formed a random nanopillar structure thereon having a pillar height dimension between 50 and 1000 nanometer and a pitch in a range of 50-1000 nanometer.

[0039] In the range of 200-500 nm and above an increasing risk of shorts is present because the corrugation may be more difficult to conformably cover by the active layers of the OLED (all layers of the OLED, e.g. from bottom electrode to top electrode). Imperfect layer coverage may lead to irregular lateral electric field strengths, which may cause higher parasitic currents and eventually catastrophic shorts during device operation. On the other hand an irregular surface due to anti-reflective properties may also lead to enhanced outcoupling.

[0040] FIG. 4 shows a comparative example of increased electroluminescence of the OLED through use of the polymer substrate as provided. The example is provided by a RIE etching treatment of PEN for a period of about 3 minutes at 100 W, in an Oxygen plasma. Alternatively results may be obtained at 300 W*3min. The graph shows a measured brightness (cd/m2) as a function of external viewing angle (excluding cosine theta dependence). The electroluminescence response shows a clear signs of a redirection of emission into forward directed angles.

[0041] Data was obtained with a Display Metrology System (DMS, Autronic Melchers GmbH). The angle dependent luminance (in cd/m2) follows from the integration over the visible wavelength range of the overlap of the measured angle dependent spectral radiance S(?,?) (in W/sr m.sup.2 nm) with the photopic curve S.sub.y(?) that has a power efficiency of 683 lm/W through the definition of the Candela SI unit. The measurement occurred at a current density of 5 mA/cm2.

[0042] Without being bound to theory, it is surmised that the process of creating the nanopillar structure by the ablation process is enhanced by the polymer substrate having a dispersion of inorganic shielding particles, wherein a periodic nanopillar structure is obtainable by an ablation process of the polymer substrate, wherein the shielding particles shield the nanopillars from the ablation process. While the particles may be selected and tuned to obtain a specific dimensioning of the nanopillar structure, the inorganic shielding particles are substantially made of an oxide of at least one element selected from the group consisting of Si, Al, Ti and Zr similar to the materials described in EP 1724613 A1. A compound that is available in organic substrate materials that are commercially available, e.g. a Dupont Q65 PEN foil, or other suitable substrate, e.g. a normally has a sufficient catalyst particle substance to obtain a relevant effect.

[0043] Particles may consist of polycondensation catalyst particles comprised of metallic components selected from the group consisting of antimony, lithium, germanium, cobalt, titanium, selenium, tin, zinc, aluminum, lead, iron, manganese, magnesium and calcium; and, employed in an amount ranging from 0.005 to 1% by weight based on the weight of the naphthalenic reactant, e.g. in the form of metal acetates of the type disclosed in U.S. Pat. No. 5,294,695

[0044] To demonstrate the impact of nano-pillar structure on the output of an OLED, full wave calculations were performed. The results are shown in FIG. 3, showing a calculation scheme for periodic structures of increasing periodicity, in this case, 1000 nm, 2000 nm and random structures. The figures are based on the visualization of the k-space and the determination of the number of modes present inside the light cone which would incouple light from free space modes into the modes of the multilayered system.

[0045] A figure of merit is defined by counting the number of modes in k-space inside the light cone for light of 532 nm wavelength (emission wavelength). The comparison is done between equally weighted k-space figures with following results:

TABLE-US-00001 Periodicity (nm) Figure of Merit 1000 0.342 2000 0.498 Random 1.3453

[0046] It is noted that the numbers indicate a better outcoupling for structures with larger periods. Although a very large periodicity (p?>inf) could result in a similar k-space structure as for the random structure we need to take into account that light being emitted from the OLED has a certain coherence length on the order of 1 micron and therefore far spaced scatterers could present little to zero effect on the light emission.

[0047] It will be appreciated that while specific embodiments of the invention have been described above, that the invention may be practiced otherwise than as described. In addition, isolated features discussed with reference to different figures may be combined.