LIGHT-EMITTING DIODE, METHOD FOR FABRICATING THE SAME, DISPLAY DEVICE

Abstract

A light-emitting diode, a method for fabricating the same, and a display device are disclosed. The light-emitting diode includes a first and second electrode; a first carrier transporting layer, a light emitting layer, and a second carrier transporting layer which are arranged between the first and second electrode in this order The light-emitting diode further includes a second carrier transporting layer which is arranged between the light emitting layer and the second electrode. The second carrier blocking layer blocks a portion of the second carrier from being transported to the light emitting layer. This decreases the injecting efficiency of the second carrier, improves an injecting balance between the second carrier and the first carrier with a low injecting efficiency, avoids energy consumption in the form of heat, and increases the light output efficiency and lifetime of the light-emitting diode.

Claims

1. A light-emitting diode comprising: a first electrode and a second electrode; a light emitting layer arranged between the first electrode and the second electrode; a first carrier transporting layer arranged between the first electrode and the light emitting layer, and is configured to transport a first carrier injected from the first electrode to the light emitting layer; a second carrier transporting layer arranged between the light emitting layer and the second electrode, wherein the second carrier transporting layer is configured to transport a second carrier injected from the second electrode to the light emitting layer; and a second carrier blocking layer arranged between the light emitting layer and the second electrode, wherein the second carrier blocking layer is configured to block a portion of the second carrier from being transported to the light emitting layer.

2. The light-emitting diode of claim 1, wherein the second carrier blocking layer is arranged between the light emitting layer and the second carrier transporting layer.

3. The light-emitting diode of claim 1, wherein the second carrier blocking layer is arranged between the second carrier transporting layer and the second electrode.

4. The light-emitting diode of claim 2, wherein the second carrier blocking layer has a refractive index smaller than that of the second carrier transporting layer.

5. (canceled)

6. The light-emitting diode of claim 1, wherein the first carrier is hole, and the second carrier is electron.

7. The light-emitting diode of claim 6, wherein the second carrier blocking layer comprises one or more of MgF.sub.2, AlF.sub.3, and SiO.sub.2.

8. The light-emitting diode of claim 6, wherein the first electrode is a transparent electrode.

9. The light-emitting diode of claim 6, wherein the second electrode is a reflective electrode.

10. The light-emitting diode of claim 6, wherein the second carrier blocking layer has a thickness of 0.5 nm-50 nm.

11. The light-emitting diode of claim 6, wherein the second carrier blocking layer has a thickness of 1%-20% of a thickness of the second carrier transporting layer.

12. The light-emitting diode of claim 1, wherein the light emitting layer comprises quantum dots.

13. A display device comprising the light-emitting diode of claim 1.

14. A method for fabricating a light-emitting diode, comprising: forming a first electrode; forming a first carrier transporting layer on the first electrode; forming a light emitting layer on the first carrier transporting layer; forming a second carrier transporting layer and a second carrier blocking layer on the light emitting layer; and forming a second electrode on the second carrier transporting layer or the second carrier blocking layer.

15. The method of claim 14, wherein the step of forming the second carrier transporting layer and the second carrier blocking layer on the light emitting layer comprises at least one of: forming the second carrier transporting layer on the light emitting layer, and forming the second carrier blocking layer on the second carrier transporting layer; and forming the second carrier blocking layer on the light emitting layer, and forming the second carrier transporting layer on the second carrier blocking layer.

16. The method of claim 14, wherein the first carrier is hole, and the second carrier is electron.

17. The method of claim 16, wherein forming the first carrier transporting layer on the first electrode comprises: forming a first sub-layer of the first carrier transporting layer by spin coating on the first electrode; and forming a second sub-layer of the first carrier transporting layer on the first sub-layer by spin coating.

18. The method of claim 17, wherein forming the light emitting layer on the first carrier transporting layer comprises: forming the light emitting layer comprising a quantum dot film on the second sub-layer of the first carrier transporting layer by spin coating.

19. The method of claim 18, wherein forming the second carrier transporting layer comprises: forming the second carrier transporting layer comprising a film comprising inorganic nano-particles by spin coating.

20. The method of claim 19, wherein the step of forming the second carrier blocking layer comprises at least one of: forming a film of AlF.sub.3 by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s; forming a film of MgF.sub.2 by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s; and forming a film of SiO.sub.2 by electron beam evaporation, under a vacuum degree <10.sup.−5 torr and an evaporation rate of 1-10 Å/s.

21. The method of claim 20, wherein forming the second electrode comprises: forming a metallic second electrode by vacuum evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s, and annealing the second electrode in a glovebox at 140-150° C. in an atmosphere of nitrogen for 30 min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 is a structural diagram for illustrating a light-emitting diode in an embodiment of the present disclosure;

[0040] FIG. 2 is a schematic view for illustrating the structure and the refractive indexes for functional layers of a light-emitting diode in an embodiment of the present disclosure;

[0041] FIG. 3 is a diagram for illustrating energy levels of a light-emitting diode in an embodiment of the present disclosure;

[0042] FIG. 4 is a flow chart for illustrating a method for fabricating a light-emitting diode in an embodiment of the present disclosure; and

[0043] FIG. 5 is a flow chart for illustrating a method for fabricating a QLED in an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0044] Embodiments of the present disclosure provide a light-emitting diode, a method for fabricating the same, and a display device. Electron injecting in the light-emitting diode is blocked, so as to improve the carrier injecting balance, the light output efficiency, the brightness, as well as the operation efficiency and lifetime of the light-emitting diode.

[0045] As shown in FIG. 1, in an embodiment of the present disclosure, a light-emitting diode comprises a first electrode 1 and a second electrode 6, a hole transporting layer 2 which is arranged between the first electrode and the second electrode, a quantum dot light emitting layer 3 which is arranged between the hole transporting layer and the second electrode, an electron transporting layer 4 which is arranged between the quantum dot light emitting layer and the second electrode, and an electron blocking layer 5 which is arranged between the electron transporting layer and the second electrode.

[0046] For example, the first electrode 1 is an anode, and the second electrode 6 is a cathode. Of course, it is also possible that the first electrode 1 is a cathode and the second electrode 6 is an anode.

[0047] For example, the anode is a transparent electrode, and the cathode is a metal electrode. Therefore, an embodiment of the present disclosure provides a QLED with a device configuration shown in FIG. 1. As shown, the QLED comprises for example a transparent anode, a hole transporting layer, a quantum dot light emitting layer, an electron transporting layer, an electron blocking layer, and a metal cathode which are arranged from top to bottom in this order. Since the electron blocking layer is arranged between the electron transporting layer and the metal cathode for blocking the electron from injecting, the balance of carrier injecting is improved, the light output efficiency of the device is improved, the brightness, efficiency and lifetime of the device is increased. Meanwhile, the device's resistance to water and oxygen erosion is improved.

[0048] For example, the electron blocking layer 5 is made from MgF.sub.2, AlF.sub.3, or SiO.sub.2, and has a refractive index smaller than that of the electron transporting layer 4 (which is made from e.g., ZnO), as shown in FIG. 2. For example, the transparent anode 1 is made from ITO or IZO, and has a refractive index n.sub.1=2.0. The hole transporting layer 2 has a dual-layer structure, each layer is made from polyvinylcarbazole (PVK) and WO.sub.3, and the dual-layer structure has a refractive index n.sub.2=1.7-2.0. The quantum dot light emitting layer 3 is made from QDs, and has a refractive index n.sub.3=2.5-3.5. The electron transporting layer 4 is made from ZnO, and has a refractive index n.sub.4=2.1. The electron blocking layer 5 is made from AlF.sub.3, and has a refractive index n.sub.5=1.38. The metal cathode 6 is made from Al, and has a refractive index n.sub.6=1.44. With such a configuration of refractive indexes, characteristics of the interface between the metal electrode and the electron transporting layer in the QLED are improved, and the light output efficiency of the device is improved.

[0049] In addition, the transparent anode is for example made from IZO or the like. The hole transporting layer is for example made from a dual-layer in which both layers are made from organic materials. Examples of such a dual-layer structure comprise PEDOT:PSS and TFB dual-layer, PEDOT:PSS and poly-TPD (Poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzi) dual-layer, or the like. The quantum dot light emitting layer is made from red (R), green (G), blue (B) QD materials. The electron transporting layer is for example made from TiO.sub.2, or TiO.sub.2 and ZnO nano-particles. The metal cathode is made from Li and Al.

[0050] Examples of the thicknesses for each layer follow. For example, the transparent anode has a thickness of 70 nm-200 nm. The hole transporting layer has a thickness of 50 nm-100 nm. The quantum dot light emitting layer has a thickness of 10 nm-60 nm. The electron transporting layer has a thickness of 40 nm-150 nm. The electron blocking layer has a thickness of 0.5 nm-50 nm. The metal cathode has a thickness of 80 nm-150 nm.

[0051] For example, the electron blocking layer has a thickness which is 1%-20% of the thickness of the electron transporting layer. In this manner, the electron blocking layer provides a better effect for electron blocking.

[0052] Furthermore, the electron blocking layer is formed by vacuum evaporation or electron beam evaporation. The thus formed electron blocking layer is denser than the electron transporting layer comprising nano-particles, has an improved stability, improves the device's resistance to water and oxygen erosion, and facilitate a device with a longer lifetime.

[0053] In the embodiment shown in FIG. 1, the electron blocking layer 5 is arranged between the electron transporting layer 4 and the second electrode 6. However, in other embodiments, the electron blocking layer 5 is arranged between the quantum dot light emitting layer 3 and the electron transporting layer 4. Theoretically, it is possible for the electron blocking layer 5 to arrange at any position between light emitting layer 3 and the second electrode 6.

[0054] In the QLED according to embodiments of the present disclosure, the semiconductor material in each layer has the following energy levels. According to the energy level theory, electrons can only move in specific and discrete orbitals. The electrons in each orbital have discrete energies, and the values of these energies are energy levels. As shown FIG. 3, for example, the ITO anode has a work function (the minimum energy which is required for an electron to move from the inside of a solid just to the surface) about −4.7 eV, the first hole transporting layer WO.sub.3 has a work function about −5.2 eV, and the second hole transporting layer PVK has a HOMO (highest occupied molecular orbital) energy level about −5.8 eV. As for QDs which emit in the visible region of 380 nm-720 nm, red QDs generally have a valence band energy level about −6.0 eV and a conduction band energy level about −4.0 eV; green QDs generally have a valence band energy level about −6.3 eV and a conduction band energy level about −3.9 eV; and blue QDs generally have a valence band energy level about −6.5 eV and a conduction band energy level about −3.6 eV. ZnO has a conduction band energy level about −4.0 eV and a valence band energy level about −7.5 eV. An Al electrode has a work function about −4.1 eV. The hole injecting barrier between WO.sub.3 and PVK is about 0.6 eV, and the hole injecting barrier between PVK and QDs is about 0.2-0.7 eV. Namely, holes in the device have to overcome a relatively large barrier of 0.8-1.3 eV to inject. The electron injecting barrier between ZnO and QDs is about 0-0.4 eV, so that ZnO not only efficiently injects electrons but also effectively blocks holes. Therefore, by adding an electron blocking layer EBL between ZnO and the Al electrode in the QLED device, electron injecting is blocked efficiently, and the injecting balance between electrons and holes in the device is improved. For example, the electron blocking layer EBL comprises one of MgF.sub.2, AlF.sub.3, SiO.sub.2 (all of these materials are insulating), or any combination thereof.

[0055] A method for fabricating a QLED in an embodiment of the present disclosure will be described hereinafter in detail.

[0056] As shown in FIG. 4, in an embodiment of the present disclosure, a method for fabricating a light-emitting diode comprises:

[0057] S401, forming a first electrode;

[0058] S402, forming a first carrier transporting layer on the first electrode;

[0059] S403, forming a light emitting layer on the first carrier transporting layer;

[0060] S404, forming a second carrier transporting layer and a second carrier blocking layer on the light emitting layer; and

[0061] S405, forming a second electrode on the second carrier transporting layer or the second carrier blocking layer.

[0062] The method will be described with reference to FIG. 5, by taking a QLED in which the electron has an injecting efficiency significantly higher than the hole as an example. For example, the method comprises:

[0063] S501, forming a first electrode;

[0064] S502, forming a hole transporting layer on the first electrode;

[0065] S503, forming a quantum dot light emitting layer on the hole transporting layer;

[0066] S504, forming an electron transporting layer on the quantum dot light emitting layer;

[0067] S505, forming an electron blocking layer on the electron transporting layer; and

[0068] S506, forming a second electrode on the electron blocking layer.

[0069] For example, the step S501 of forming the first electrode comprises: cleaning a patterned transparent conductive substrate by means of acetone, ethyl alcohol, de-ionized water, and isopropyl alcohol in this order; and irradiating the substrate with plasma or UV light to form the transparent first electrode.

[0070] For example, the step S502 of forming the hole transporting layer on the first electrode comprises: forming the first hole transporting layer on the transparent first electrode by spin coating; and forming the second hole transporting layer on the first hole transporting layer by spin coating.

[0071] For example, the step S503 of forming the quantum dot light emitting layer on the hole transporting layer comprises: forming a quantum dot film on the second hole transporting layer by spin coating, wherein the quantum dot film is a quantum dot light emitting layer.

[0072] For example, the step S504 of forming the electron transporting layer on the quantum dot light emitting layer comprises: forming a film comprising inorganic nano-particles on the quantum dot light emitting layer by spin coating, wherein the film comprising inorganic nano-particles is an electron transporting layer.

[0073] For example, in an embodiment, the step S505 of forming the electron blocking layer on the electron transporting layer comprises: forming a film of AlF.sub.3 on the electron transporting layer by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s (angstrom/sec). In another embodiment, step S505 comprises: forming a film of MgF.sub.2 on the electron transporting layer by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s. In another embodiment, step S505 comprises: forming a film of SiO.sub.2 on the electron transporting layer by electron beam evaporation, under a vacuum degree <10.sup.−5 torr and a evaporation rate of 1-10 Å/s.

[0074] For example, the step S506 of forming the second electrode on the electron blocking layer comprises: form a metallic second electrode on the electron blocking layer by vacuum evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s, and annealing the second electrode in a glovebox at 140-150° C. in an atmosphere of nitrogen for 30 min (minutes).

[0075] An example of the method for fabricating a QLED will be described hereinafter, in which the first electrode is a transparent anode and the second electrode is a metal cathode.

[0076] In a first step, a patterned transparent conductive substrate is cleaned by acetone, ethyl alcohol, de-ionized water, and isopropyl alcohol in this order, and is irradiated by plasma or UV to increase the work function or surface hydrophilicity of the conductive substrate. As a result, a transparent anode is obtained.

[0077] In a second step, a first hole transporting layer is formed on the above conductive substrate (i.e., transparent anode) by spin coating. For example, the first hole transporting layer is made from a material selected from the group consisting of WO.sub.3, NiO, MoO.sub.3, P—ZnO nano-particles, PEDOT:PSS. The spin coating is performed at a rotation rate of 5000 r.p.m. The first hole transporting layer is annealed in a glovebox at 80-150° C. in an atmosphere of nitrogen for 15 min. In case inorganic nano-particles are used, its concentration in ethyl alcohol is 10-50 mg/mL. In case PEDOT:PSS is used, its concentration in water is 6 wt. %. An organic molecule film is formed on the above film comprising inorganic nano-particles by spin coating, to form the second hole transporting layer. The material for the second hole transporting layer can be selected from PVK, poly-TPD, TFB. The spin coating is performed at a rotation rate of 3000 r.p.m., and the second hole transporting layer is annealed in a glovebox at 150° C. in an atmosphere of nitrogen for 30 min. The organic molecule used for forming the second hole transporting layer has a concentration in a chlorobenzene solution of 0.5-5 wt. %.

[0078] Namely, in an embodiment of the present disclosure, the hole transporting layer comprises a first hole transporting layer and a second hole transporting layer. During forming the hole transporting layer, the first hole transporting layer is formed, and then the second hole transporting layer is formed.

[0079] In a third step, a quantum dot film is formed on the above organic molecule film (i.e., the second hole transporting layer) by spin coating, to form a quantum dot light emitting layer. The quantum dot light emitting layer can be made from CdSe/CdS/ZnS, CdSe/ZnSe/ZnS, ZnCdS/ZnS, ZnSe/ZnS, InP/ZnSe/ZnS, or the like. The spin coating is performed at a rotation rate of 3000 r.p.m. The quantum dot light emitting layer is annealed in a glovebox at 80-110° C. in an atmosphere of nitrogen for 15 min. The quantum dots for forming the quantum dot light emitting layer has a concentration in n-hexane or methylbenzene solution of 10-30 mg/mL.

[0080] In a fourth step, a film comprising inorganic nano-particles is formed on the above sol-gel inorganic metal oxide film (i.e., the quantum dot light emitting layer) by spin coating, to form the electron transporting layer. The electron transporting layer has a surface roughness RMS <1 nm. The spin coating is performed at a rotation rate of 2000 r.p.m. The electron transporting layer is annealed in a glovebox at 80-110° C. in an atmosphere of nitrogen for 15 min. The inorganic nano-particles for forming the electron transporting layer have a concentration in ethyl alcohol of 10-50 mg/mL. It is annealed in an atmosphere of nitrogen for 30 min.

[0081] In a fifth step, an electron blocking layer is deposited on the electron transporting layer comprising inorganic metal oxide nano-particles by vacuum thermal evaporation or electron beam evaporation. This is realized by one of the following modes.

[0082] In a first mode, a film of AlF.sub.3 is formed by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and a evaporation rate of 1-5 Å/s.

[0083] In a second mode, a film of MgF.sub.2 is formed by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s.

[0084] In a third mode, a film of SiO.sub.2 is formed by electron beam evaporation, under a vacuum degree <10.sup.−5 torr and an evaporation rate of 1-10 Å/s.

[0085] In the sixth step, a metal film is formed as a cathode by vacuum evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s. It is annealed in a glovebox at 140-150° C. in an atmosphere of nitrogen 30 min.

[0086] Alternatively, in an embodiment of the present disclosure, a method for fabricating a QLED comprises: forming a first electrode; forming an electron blocking layer on the first electrode; forming an electron transporting layer on the electron blocking layer; forming a quantum dot light emitting layer on the electron transporting layer; forming a hole transporting layer on the quantum dot light emitting layer; and forming a second electrode on the hole transporting layer.

[0087] For example, forming the first electrode comprises: cleaning a patterned transparent conductive substrate by means of acetone, ethyl alcohol, de-ionized water, and isopropyl alcohol in this order; and irradiating the substrate with plasma or UV light to form the transparent first electrode.

[0088] For example, forming the hole transporting layer on the quantum dot light emitting layer comprises: forming a first hole transporting layer on the quantum dot light emitting layer by spin coating; and forming a second hole transporting layer on the first hole transporting layer by spin coating.

[0089] For example, forming the quantum dot light emitting layer on the electron transporting layer comprises: forming a quantum dot film on the electron transporting layer by spin coating, wherein the quantum dot film is a quantum dot light emitting layer.

[0090] For example, forming the electron transporting layer on the electron blocking layer comprises: forming a film comprising inorganic nano-particles on the electron blocking layer by spin coating, wherein the film comprising inorganic nano-particles is an electron transporting layer.

[0091] For example, forming the electron blocking layer on the first electrode comprises: forming a film of AlF.sub.3 on the first electrode by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s; or, forming a film of MgF.sub.2 on the first electrode by vacuum thermal evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s; or, forming a film of SiO.sub.2 on the first electrode by electron beam evaporation, under a vacuum degree <10.sup.−5 torr and a evaporation rate of 1-10 Å/s.

[0092] For example, forming the second electrode on the hole transporting layer comprises: forming a metallic second electrode on the hole transporting layer by vacuum evaporation, under a vacuum degree <10.sup.−6 torr and an evaporation rate of 1-5 Å/s, and annealing the second electrode in a glovebox at 140-150° C. in an atmosphere of nitrogen for 30 min.

[0093] A display device in an embodiment of the present disclosure comprises the QLED as described in the above embodiments.

[0094] For example, in each of the above embodiments, the first electrode and the second electrode have a layer form.

[0095] To sum up, embodiments of the present disclosure provide a light-emitting diode, a method for fabricating the same, and a display device. The second carrier blocking layer is arranged between the light emitting layer and the second electrode, and blocks a portion of the second carrier from being transported to the light emitting layer. By decreasing the injecting efficiency of the second carrier, an injecting balance between the second carrier and the first carrier which has a relatively low injecting efficiency is improved. This avoids energy consumption in the form of heat, and increases the light output efficiency and lifetime of the light-emitting diode.

[0096] Apparently, the person with ordinary skill in the art can make various modifications and variations to the present disclosure without departing from the spirit and the scope of the present disclosure. In this way, provided that these modifications and variations of the present disclosure belong to the scopes of the claims of the present disclosure and the equivalent technologies thereof, the present disclosure also intends to encompass these modifications and variations.