Light-emitting element and display device

Abstract

This light-emitting element (1) comprises, in the following order, a first transparent electrode (2), a light-emitting layer (3), a second transparent electrode (4), and a reflective layer (5) that reflects light which is emitted from the light-emitting layer (3) and transmitted through the second transparent electrode (4). The first transparent electrode (2) and the second transparent electrode (4) apply a voltage to the light-emitting layer (3), and the light-emitting layer (3) emits light in which the full width at half maximum of the emission spectrum is 50 nm or less.

Description

Light-emitting element and display device

[0001] The present disclosure relates to a light-emitting element and a display device.

[0002] A light-emitting element including a first transparent electrode, a light-emitting layer including an OLED (organic light-emitting diode, Organic Light Emitting Diode), a second transparent electrode, and a reflective layer that reflects light emitted from the light-emitting layer including the OLED and transmitted through the second transparent electrode is known (Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2011-60549

[0004] However, in a light-emitting element including a QLED (quantum dot light-emitting diode, Quantum dot Light Emitting Diode) element using a light-emitting layer including quantum dots with a peaky emission wavelength, since the full width at half maximum of the emission spectrum is much narrower than that of a light-emitting element including an OLED, the inventors have first found a problem that even if there is a slight deviation in the emission wavelength or a deviation in the microcavity length, the light extraction efficiency is greatly reduced.

[0005] A light-emitting element according to one aspect of the present disclosure includes a first transparent electrode, a light-emitting layer, a second transparent electrode, and a reflective layer that reflects light emitted from the light-emitting layer and transmitted through the second transparent electrode in this order, the first transparent electrode and the second transparent electrode apply a voltage to the light-emitting layer, and the light-emitting layer emits light having a full width at half maximum of the emission spectrum of 50 nm or less.

[0006] A display device according to one aspect of the present disclosure includes the light-emitting element according to one aspect of the present disclosure.

[0007] According to one aspect of the present disclosure, it is possible to provide a light-emitting element and a display device that can reduce the wavelength dependence of the light extraction efficiency.

[0008] This is a cross-sectional view of a light-emitting element according to Embodiment 1. This is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer provided in the light-emitting element. This is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer provided in a comparative example light-emitting element. This is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer provided in another comparative example light-emitting element. This is a cross-sectional view of a modified example of Embodiment 1. This is a cross-sectional view of a light-emitting element according to Embodiment 2. This is a cross-sectional view showing the reflective surface of the reflective layer provided in the light-emitting element. This is a cross-sectional view showing the light-emitting layer and reflective layer provided in the light-emitting element. This is a cross-sectional view of a display device according to Embodiment 3. This is a plan view of the display device.

[0009] (Embodiment 1) Figure 1 is a cross-sectional view of the light-emitting element 1 according to Embodiment 1.

[0010] The light-emitting element 1 comprises, in this order, a first transparent electrode 2, a light-emitting layer 3, a second transparent electrode 4, and a reflective layer 5 that reflects the light emitted from the light-emitting layer 3 and transmitted through the second transparent electrode 4. The first transparent electrode 2 and the second transparent electrode 4 apply a voltage to the light-emitting layer 3. The light-emitting layer 3 emits light with a full width at half maximum of 50 nm or less. This allows the light extraction efficiency to be maintained even if there is a slight shift in the emission wavelength or microcavity length in the light-emitting element 1, which includes a light-emitting layer 3 with a very narrow full width at half maximum of the emission spectrum. The light-emitting element 1 further comprises a transparent layer 6 located between the second transparent electrode 4 and the reflective layer 5. This allows the light emitted from the light-emitting layer 3 and transmitted through the second transparent electrode 4 to pass through the transparent layer 6 and be reflected by the reflective layer 5. As will be described later, the transparent layer 6 is a layer for adjusting the distance between the reflective surface 7 of the reflective layer 5 and the light-emitting layer 3. If a layer that obstructs light transmission exists between the reflective surface 7 of the reflective layer 5 and the light-emitting layer 3, the light extraction efficiency of the light-emitting element 1 decreases. Therefore, it is preferable that only one transparent layer 6 exists between the second transparent electrode 4 and the reflective layer 5. The light-emitting element 1 further comprises a first functional layer 9 located between the first transparent electrode 2 and the light-emitting layer 3, and a second functional layer 10 located between the light-emitting layer 3 and the second transparent electrode 4. The first functional layer 9 and the second functional layer 10 each include a carrier transport layer. For example, the first functional layer 9 includes an electron transport layer (ETL). The second functional layer 10 includes, for example, a hole injection layer (HIL) and a hole transport layer (HTL).

[0011] In light-emitting devices such as OLEDs, in the case of a top-emission structure, the light-emitting layer and various thin-film functional layers suitable for the light-emitting layer are laminated on a reflective electrode that is common to the reflective surface, and an electrode is formed on top of that. In contrast, the light-emitting element 1 according to this embodiment is formed by forming a transparent layer 6 on a reflective layer 5 (for example, Ag) that will be the reflective surface 7, and then laminating a second transparent electrode 4, a light-emitting layer 3, a first functional layer 9 suitable for the light-emitting layer 3, a second functional layer 10, and a first transparent electrode 2 on top of that.

[0012] The first transparent electrode 2 can be made of a conductive material suitable for light-emitting devices, and can be made of a thin metal film of a thickness sufficient to provide visible light transmission. For example, thin metal films of Ag, Ma, Al, etc., can be used. It is desirable that this thin metal film be made of a metal with a low work function. The second transparent electrode 4 can be made of a transparent conductive material such as ITO (indium tin oxide). The reflective layer 5 can be made of Ag, for example. The transparent layer 6 can be made of a resin or a conductive transparent material such as ITO.

[0013] The light-emitting layer 3 preferably contains quantum dots. The light-emitting layer 3 may be an OLED, not limited to a QLED (Quantum Dot Light Emitting Diode), as long as it emits light with a full width at half maximum of 50 nm or less in its emission spectrum.

[0014] The distance between the light-emitting layer 3 and the reflective layer 5 is preferably 1 μm or more. This ensures that the optical path length difference, which is the distance the light emitted from the light-emitting layer 3 travels back and forth between the light-emitting layer 3 and the reflective surface 7, is 10 times or more the wavelength of the light emitted from the light-emitting layer 3, thereby sufficiently reducing wavelength selectivity. The distance between the light-emitting layer 3 and the reflective layer 5 is preferably 2.5 times or more the peak wavelength of the light emitted from the light-emitting layer 3. This ensures that, when the wavelength of the light emitted from the light-emitting layer 3 is 400 nm, the distance between the light-emitting layer 3 and the reflective layer 5 is 1 μm or more. The optical path length difference, which is the distance the light emitted from the light-emitting layer 3 travels back and forth between the light-emitting layer 3 and the reflective surface 7, is twice the distance between the light-emitting layer 3 and the reflective surface 7. This optical path length difference is preferably 10 times or more the wavelength of the light emitted from the light-emitting layer 3. The optical path length difference is calculated as the distance traveled by the light emitted from the light-emitting layer 3 to the reflective surface 7 multiplied by the refractive index, and is twice the distance between the light-emitting layer 3 and the reflective surface 7. The transparent layer 6 has a high refractive index of about 2.

[0015] By increasing the optical path length difference to about 10 times or more the emission wavelength, wavelength selectivity can be sufficiently reduced even with a QLED with a full width at half maximum of about 50 nm. This reduces the change in light extraction efficiency caused by deviations such as a ±5 nm shift in the emission peak wavelength or a ±5 nm shift in the distance from the second transparent electrode 4 (lower electrode) to the light-emitting layer 3.

[0016] For example, if the refractive index of the transparent layer 6 is 1.5 and the wavelength of the light emitted from the light-emitting layer 3 is 600 nm, then the wavelength of that light within the transparent layer 6 will be 600 nm / 1.5 = 400 nm.

[0017] Figure 2 is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer 3 provided on the light-emitting element 1. Figure 3 is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer provided on a light-emitting element according to a comparative example. Figure 4 is a graph showing the relationship between the wavelength and intensity of light emitted from the light-emitting layer provided on another comparative example light-emitting element.

[0018] In self-luminescent devices, it is common practice to enhance the light extraction effect by utilizing the resonance phenomenon of light emission between the light-emitting layer and the reflective electrode within the self-luminescent device (microcavity effect).

[0019] Conventionally, in OLEDs, as shown in Figure 4, the full width at half maximum of the emission wavelength from the light-emitting layer was broad. Therefore, even if there were manufacturing variations in emission wavelength shift or microcavity length (distance between the light-emitting layer and the reflective electrode), it did not affect the light extraction efficiency, and it had the effect of improving color purity by making the emission wavelength steeper.

[0020] However, through diligent research, the inventors discovered for the first time that, as shown in Figure 3, in QLED elements utilizing quantum dots with peaky emission wavelengths, even slight deviations in emission wavelength or microcavity length can significantly reduce the light extraction efficiency. Therefore, this embodiment solves a novel problem: reducing the wavelength dependence of light extraction efficiency due to the element structure, thereby reducing the impact of variations on efficiency.

[0021] The aforementioned problem will be explained in detail below. When a light-emitting element is fabricated with a simple structure of sandwiching it between a reflective electrode and a transparent electrode, the distance between the light-emitting layer and the reflective electrode is short, resulting in a strong microcavity effect where the light is strengthened or weakened due to interference between the light emitted directly from the light-emitting layer towards the transparent electrode (direct light) and the light reflected by the reflective electrode (reflected light).

[0022] The microcavity effect is a type of interference that occurs when the difference in optical path length between direct light and reflected light is an integer multiple of the wavelength of light, resulting in the most constructive interference, and when it is an integer multiple plus 1 / 2, resulting in the most destructive interference.

[0023] The degree of interference due to the microcavity effect decreases as the difference in optical path length increases. Since the strength of optical interference depends on the wavelength of light, the degree to which interference enhances light will vary even when emitted from the same light source.

[0024] As shown in Figure 4, since the emission wavelength is broad in OLED elements, even if the film thickness is slightly off, the effect of the microcavity effect is limited to a slight shift in the wavelength peak of the synchrotron radiation, and conversely, it can be used to improve the color purity of the synchrotron radiation.

[0025] However, as shown in Figure 3, in QLED elements with peak emission wavelengths, even a slight deviation in film thickness can cause a significant change in the intensity of the synchrotron radiation due to the shift in wavelengths that reinforce each other through the microcavity effect.

[0026] Furthermore, since variations in quantum dot size directly lead to variations in emission wavelength, in areas where the microcavity effect is strong, it will affect the optimal distance between the light-emitting layer and the reflective electrode.

[0027] As described above, when a device combines a QLED element with the microcavity effect, the variation in light emission efficiency becomes large due to two manufacturing variations: (1) variations in emission wavelength due to variations in quantum dot fabrication, and (2) variations in the peak wavelength of the microcavity due to variations in film thickness when fabricated as a light-emitting element. To reduce the effects of variations in manufacturing, which is a phenomenon unique to QLED elements, it is desirable to reduce the influence of the microcavity effect.

[0028] Therefore, in this embodiment, as shown in Figure 1, the distance between the reflective electrode and the light-emitting layer 3 is widened to make the distance sufficiently longer than the wavelength of light emitted by the light-emitting layer 3. The reflective electrode is made of the reflective layer 5, and a transparent layer 6 is formed on this reflective layer 5 with a thickness of several times the wavelength or more, and a second transparent electrode 4 is formed on the transparent layer 6. This makes it possible to widen the distance between the reflective surface 7 of the reflective layer 5 and the light-emitting layer 3 while keeping the film thickness between the light-emitting layer 3 and the second transparent electrode 4 of the light-emitting element 1, which is a QLED element, thin.

[0029] One possible method to increase the optical path length is to widen the distance between the reflective electrode and the light-emitting layer of the QLED element and increase the film thickness. However, this method has significant disadvantages because (1) the transparency of the functional layer between the reflective electrode and the light-emitting layer of the QLED is not high, and (2) the resistance component increases as the film thickness of the functional layer increases, leading to a higher driving voltage for the light-emitting element. In contrast, this embodiment makes it possible to reduce the wavelength dependence of the light extraction efficiency while maintaining a low driving voltage for the light-emitting element.

[0030] The transparent layer 6 should preferably have a thickness of at least five times the wavelength of light, including the effect of the refractive index. For example, in a transparent layer 6 with a refractive index of 2, light with a wavelength of 500 nm becomes 500 nm / 2 = 250 nm. Therefore, for a material with a refractive index of 2, the thickness of the transparent layer 6 that is at least five times the wavelength of light for light with a wavelength of 500 nm is 250 nm × 5 = 1250 nm.

[0031] The components of the light-emitting element 1 according to this embodiment 1 are a reflective layer 5 (which may also serve as an electrode, wiring, etc.), a transparent layer 6, a second transparent electrode 4 (which may also serve as the transparent layer 6 by making it thicker), a light-emitting layer 3, a first functional layer 9, a second functional layer 10, and a first transparent electrode 2, which sandwich the light-emitting layer 3 and have the function of causing the light-emitting layer 3 to emit light. Figure 5 is a cross-sectional view of a light-emitting element 1A according to a modified example of embodiment 1. In the configuration shown in Figure 1, the layers are in the order of first transparent electrode 2 (metal such as Ag), first functional layer 9 (electron transport layer, etc.), light-emitting layer 3, second functional layer 10 (hole transport layer, etc.), second transparent electrode 4 (ITO, etc.), transparent layer 6, and reflective layer 5. However, as shown in Figure 5, the layers may be rearranged to the order of second transparent electrode 4, second functional layer 10, light-emitting layer 3, first functional layer 9, first transparent electrode 2, transparent layer 6, and reflective layer 5.

[0032] In top-emission light-emitting devices, the light emitted from the light-emitting layer is produced by the interference of light directly emitted to the outside (direct light) and light reflected by the reflective electrode (reflected light). When the phases of the direct light and reflected light are aligned, the intensity of the emitted light increases, and when the phases are reversed, the intensity of the emitted light decreases. The degree of phase alignment is determined by the emission wavelength and the difference in optical path lengths between the direct light and reflected light. Therefore, the parameter that has the effect on the intensity of the emitted light is the distance between the light-emitting layer and the reflective layer in terms of device structure.

[0033] The phase difference between the light emitted from the light-emitting layer and the light emitted from the reflective electrode is determined by the distance between the light-emitting layer and the reflective electrode. As shown in Figure 4, if the spectrum of the light emitted from the light-emitting layer is broad, the wavelength selectivity of the light resonance due to the microcavity structure can also have the effect of making the radiant light from the light-emitting device steeper.

[0034] However, as shown in Figure 3, if the spectrum of light emitted from the light-emitting layer is steep, there is a problem that if the resonant spectrum of light due to the microcavity structure and the spectrum of light emitted from the light-emitting layer are misaligned, light will not be emitted to the outside. Since the color purity of the light emitted from QLEDs is higher than that of OLEDs, the effect of improving color purity due to the microcavity effect is smaller for QLEDs than for OLEDs, and in practical terms, there is no problem even without the improvement in color purity due to the microcavity effect.

[0035] Therefore, as in this embodiment, by increasing the distance between the light-emitting layer 3 and the reflective surface 7, the wavelength selectivity due to the light-emitting device structure can be weakened.

[0036] (Embodiment 2) Figure 6 is a cross-sectional view of the light-emitting element 1B according to Embodiment 2. Figure 7 is a cross-sectional view showing the reflective surface 7B of the reflective layer 5B provided on the light-emitting element 1B. Figure 8 is a cross-sectional view showing the light-emitting layer 3 and the reflective layer 5B provided on the light-emitting element 1B. Components similar to those described above are denoted by the same reference numerals. Detailed descriptions of these components will not be repeated.

[0037] The light-emitting element 1B includes a reflective layer 5B that reflects light emitted from the light-emitting layer 3 and transmitted through the second transparent electrode 4. This reflective layer 5B has a reflective surface 7B with irregularities formed on it. This further reduces the cavity effect. Thus, irregularities are made on the reflective surface 7B compared to the structure of Embodiment 1. These irregularities do not have to be formed regularly, but can be formed randomly. The dimension H of the irregularities is preferably about half the wavelength of light passing through the transparent layer 6 along the direction perpendicular to the light-emitting layer 3. That is, it is desirable to make irregularities on the reflective surface 7B of the transparent layer 6 with a dimension of about half the wavelength of the light to be extracted from the light-emitting layer 3. This makes the distance between the light-emitting layer 3 and the reflective surface 7B uneven, and by increasing this distance, the already mitigated cavity effect can be further reduced.

[0038] Although the cavity effect is weaker because the distance between the light-emitting layer 3 and the reflective surface 7B is wider than in the comparative example, the cavity effect is not completely eliminated. The formation of irregularities on the reflective surface 7B causes the distance between the surface of the reflective surface 7B and the light-emitting layer 3 to become uneven, resulting in a random distribution of areas where direct light and reflected light reinforce and areas where they cancel each other out, thus averaging out the effect.

[0039] As a result, the distance over which light emitted from the light-emitting layer 3 is reflected by the reflective surface 7B and returns to the light-emitting layer 3 becomes non-uniform along the surface of the light-emitting layer 3. Because the distance to the reflective surface 7B and the dimensions of the irregularities on the reflective surface 7B are close to the wavelength of light, the light behaves quantum mechanically, unlike reflection considered in general optical engineering. Therefore, the direction of reflection at the reflective surface 7B is considered to be omnidirectional. In other words, a physical phenomenon similar to the phenomenon of light bending around a diffraction grating appears.

[0040] Since reflective surfaces can sometimes reinforce each other in the forward direction while weakening each other in the diagonal direction, and vice versa, adding irregularities to the reflective surface 7B also has the effect of introducing randomness.

[0041] The effect is enhanced when the height of the irregularities on the reflective surface 7B is 100 nm or more. For example, if the wavelength of red light is 600 nm and the refractive index of the transparent layer 6 is 1.5, the difference in optical path length between the concave and convex parts is {100 nm × 2} (distance difference) ÷ 1.5 (refractive index) = 300 nm.

[0042] This difference in optical path length corresponds to half a wavelength for 600 nm light, and the phase difference is only inverse phase. Therefore, since it includes all phase differences, it is considered sufficient in terms of randomness.

[0043] (Embodiment 3) Figure 9 is a cross-sectional view of the display device 11 according to Embodiment 3. Figure 10 is a plan view of the display device 11. Components similar to those described above are denoted by the same reference numerals. Detailed descriptions of these components will not be repeated.

[0044] The light-emitting element 1C comprises a first transparent electrode 2, a light-emitting layer 3, a second transparent electrode 4C, and a reflective layer 5C that reflects light emitted from the light-emitting layer 3 and transmitted through the second transparent electrode 4C.

[0045] The reflective layer 5C has an area wider than that of the light-emitting layer 3 in plan view. The second transparent electrode 4C has an extending portion 8 extending in a direction away from the light-emitting layer 3 in plan view. The reflective layer 5C and the extending portion 8 are connected at a connecting portion 13. Thereby, the reflective layer 5C and the second transparent electrode 4C can be electrically connected. The display device 11 includes a light-emitting element 1C and a pixel circuit 12 provided on the opposite side of the second transparent electrode 4C with respect to the reflective layer 5C in order to supply a voltage to the second transparent electrode 4C of the light-emitting element 1C. Thereby, power can be supplied from the pixel circuit 12 to the second transparent electrode 4C through the reflective layer 5C. When patterning the transparent layer 6 on the reflective layer 5C, the edge of the transparent layer 6 is made to have a gentle slope. When placing the second transparent electrode 4C thereon, the second transparent electrode 4C and the reflective layer 5C are electrically connected.

[0046] By doing so, even if the transparent layer 6 is a non-conductive material, the second transparent electrode 4C and the pixel circuit 12 of the circuit board can be electrically connected. Therefore, the circuit board used in a display of the type in which light-emitting elements are arranged can be used as it is.

[0047] Since the reflective layer 5 can be used as a wiring, even if the conductivity of the second transparent electrode 4C on the transparent layer 6 is low, the distance between the reflective layer 5C and the light-emitting layer 3 can be increased while compensating with the reflective layer 5C.

[0048] Considering only the optical effect, a method of making the transparent electrode thicker can be considered. However, since the transmittance decreases when the transparent electrode is thickened and the resistance of the transparent electrode itself may not be so high, this structure is considered to be more advantageous.

[0049] Further, when the reflection surface 7B of the second embodiment has irregularities, by forming the transparent layer 6 with resin, the irregularities of the reflection surface 7B can be flattened, so that the light-emitting device portion can be formed with a flat film.

[0050] This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0051] 1. Light-emitting element 2. First transparent electrode 3. Light-emitting layer 4. Second transparent electrode 5. Reflective layer 6. Transparent layer 7. Reflective surface 8. Extendable portion 11. Display device

Claims

1. A light-emitting element comprising, in this order, a first transparent electrode, a light-emitting layer, a second transparent electrode, and a reflective layer that reflects light emitted from the light-emitting layer and transmitted through the second transparent electrode, wherein the first transparent electrode and the second transparent electrode are connected when a voltage is applied to the light-emitting layer, and the light-emitting layer emits light with a full width at half maximum of 50 nm or less of the emission spectrum.

2. The light-emitting element according to claim 1, wherein the light-emitting layer includes quantum dots.

3. The light-emitting element according to claim 1 or 2, wherein the distance between the light-emitting layer and the reflective layer is 1 μm or more.

4. The light-emitting element according to claim 1, wherein the distance between the light-emitting layer and the reflective layer is 2.5 times or more the peak wavelength of the light emitted by the light-emitting layer.

5. The light-emitting element according to any one of claims 1 to 4, further comprising a transparent layer located between the second transparent electrode and the reflective layer.

6. The light-emitting element according to claim 5, wherein only one layer of the transparent layer exists between the second transparent electrode and the reflective layer.

7. The light-emitting element according to any one of claims 1 to 6, wherein the reflective layer has a reflective surface on which irregularities are formed.

8. The light-emitting element according to any one of claims 1 to 7, wherein the reflective layer has a larger area than the light-emitting layer in a plan view, the second transparent electrode has an extended portion that extends away from the light-emitting layer in a plan view, and the reflective layer and the extended portion are connected.

9. A display device comprising a light-emitting element according to any one of claims 1 to 8.

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