JP2008147433A - Surface light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface light source which decreases a large Schottky barrier between an electrode and a semiconductor layer, and increases an implantation efficiency of electrons and holes into a light emitting layer. <P>SOLUTION: The surface light source comprises: a substrate; a planar back face electrode formed on the substrate; a planar transparent electrode formed opposing to the back face electrode; a planar light emitting layer of at least one layer formed to be put between the back face electrode and the transparent electrode; and at least one buffer layer formed to be put between the back face electrode or the transparent electrode and the light emitting layer. The formation of the buffer layer causes to set a level of the potential barrier between the electrode and the light emitting layer pinching the buffer layer smaller than a level of the Schottky barrier when the electrode comes into direct contact with the light emitting layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a planar light source using an electroluminescence (hereinafter abbreviated as EL) element.

  A conventional semiconductor light emitting device has a low voltage and a high luminance, but is a point light source and is difficult to use as a planar light source. Further, an expensive substrate is necessary for manufacturing the light emitting element, which is a cause of cost increase. Further, in the case of a thin film light emitting element, there is a problem that a Schottky barrier is generated at the joint surface between the light emitting layer and the electrode, thereby inhibiting carrier injection.

  FIG. 8 is a schematic configuration diagram showing a configuration of a conventional light emitting device 50. As the light-emitting layer 53, a light-emitting layer 53 having a two-layer structure of an n-type semiconductor layer 53a and a p-type semiconductor layer 53b is provided as a recombination-type light-emitting layer configuration. A transparent electrode 52 serving as an electron injection electrode and a back electrode 54 serving as a hole injection electrode are electrically connected via a DC power supply 55. When power is supplied from the DC power supply 55, a potential difference is generated between the transparent electrode 52 and the back electrode 54, and a voltage is applied to the light emitting layers 53a and 53b. Then, the light emitting layers 53 a and 53 b disposed between the transparent electrode 52 and the back electrode 54 emit light, and the light passes through the transparent electrode 52 and is extracted outside the light emitting element 50.

  Here, depending on the combination of the semiconductor and the electrode, a Schottky barrier is generated at the joint surface, and the efficiency of injection of electrons and holes into the light emitting layers 53a and 53b is lowered, which hinders efficiency. The problem of the Schottky barrier at the joint surface will be described with reference to the energy band diagrams of FIGS. 9A and 9B and FIGS. 10A and 10B.

  FIGS. 9A and 9B are energy band diagrams before and after contact when the n-type semiconductor layer 53a and the transparent electrode 52 are brought into contact with each other. Before contact, as shown in FIG. 9 (a), the Fermi level is different from the vacuum level. When the semiconductor and the electrode are brought into contact with each other, as shown in FIG. 9 (b). In addition, the band of the n-type semiconductor layer 53a is curved at the contact surface so that the respective Fermi levels coincide with each other, and a large Schottky barrier is generated between the n-type semiconductor layer 53a and the transparent electrode 52. For this reason, the injection efficiency of electrons from the transparent electrode 52 to the n-type semiconductor layer 53a is lowered. In addition, for example, a metal oxide such as ITO is used as the transparent electrode 52. Generally, since these work functions are relatively large as 4 to 5 eV, a large shot is formed between the n-type semiconductor layer 53a and the transparent electrode 52. A key barrier is created.

  FIGS. 10A and 10B are energy band diagrams before and after contact when the p-type semiconductor layer 53b and the back electrode 54 are brought into contact with each other. In the case of the p-type semiconductor layer 53b, as in the case of the n-type semiconductor layer 53a, when the semiconductor and the electrode are brought into contact with each other, the band of the p-type semiconductor layer 53b is curved at the contact surface so that the respective Fermi levels coincide with each other. Therefore, as shown in FIG. 10B, a large Schottky barrier is generated between the p-type semiconductor layer 53b and the back electrode 54, and the injection efficiency of holes from the back electrode 54 to the p-type semiconductor layer 53b is as follows. Lower.

In order to solve the above problems, the following methods are generally performed.
(1) A material having a high work function is used as the hole injection electrode. A material having a low work function is used as the electron injection electrode.
(2) A highly doped layer is formed at the interface between the electrode and the semiconductor. (For example, refer to Patent Document 1.)
(3) The Schottky barrier is made smaller than the alloying reaction between the electrode material and the semiconductor. (For example, refer nonpatent literature 1.)

JP 2005-294415 A J. et al. Crystal Growth 214/215, p1064 (2000)

  However, in the case of the method (1), for example, when a substance having a low work function is used as an electrode, a substance having a low work function generally has low stability in the air and cannot be practically used. In the case of the methods (2) and (3), it is highly likely that the processing conditions need to be reviewed every time the semiconductor material and composition as the light emitting layer changes.

  In order to solve these problems, the object of the present invention is to provide a planar light source that reduces the large Schottky barrier generated between the electrode and the semiconductor layer and increases the injection efficiency of electrons and holes into the light emitting layer. It is to be.

The above problems can be solved by the planar light source according to the present invention. That is, the planar light source according to the present invention includes a substrate,
A planar back electrode provided on the substrate;
A planar transparent electrode provided facing the back electrode;
At least one planar light emitting layer provided sandwiched between the back electrode and the transparent electrode;
Comprising at least one buffer layer provided between the back electrode or the transparent electrode and the light emitting layer,
By providing the buffer layer, the size of the potential barrier between the electrode and the light emitting layer sandwiching the buffer layer is such that the Schottky barrier in the case where the electrode and the light emitting layer are in direct contact with each other. It is characterized by being smaller than the size.

Further, a light source may be made to emit light by applying a DC voltage between the back electrode and the transparent electrode. In this case, either the back electrode or the transparent electrode functions as an electron injection electrode, and the other electrode functions as a hole injection electrode. Furthermore, the buffer layer includes
A first buffer layer provided between the first electrode which is an electron injection electrode and the light emitting layer;
You may provide the two buffer layers of the 2nd buffer layer provided between the 2nd electrode which is a positive hole injection electrode, and a light emitting layer. Alternatively, the buffer layer is
A first buffer layer provided between the first electrode which is an electron injection electrode and the light emitting layer;
You may provide at least one buffer layer among the 2nd electrode provided between the 2nd electrode which is a positive hole injection electrode, and a light emitting layer.

  Further, the first buffer layer may include a material having a work function of 3.5 eV or less.

  Furthermore, the second buffer layer may contain a substance having a work function of 5.0 eV or more.

  Further, the first buffer layer may contain an alkali metal oxide. Alternatively, the first buffer layer may be made of a substance having an electronegativity of 3 or more.

  Furthermore, the light emitting layer may be a two-layer light emitting layer in which an n-type semiconductor layer and a p-type semiconductor layer are stacked.

  Furthermore, the light emitting layer may be a three-layer light emitting layer including an n-type semiconductor layer, a p-type semiconductor layer, and an undoped semiconductor layer sandwiched therebetween.

  Further, a color conversion layer may be further provided in front of the back electrode and the transparent electrode and in front of the light emission extraction direction from the light emitting layer.

  According to the present invention, a thin planar light source that can be driven at a low voltage and has high luminance and high efficiency can be provided at low cost.

  Hereinafter, planar light sources according to embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.

(Embodiment 1)
<Schematic configuration of planar light source>
FIG. 1 is a schematic perspective view showing a schematic configuration of a planar light source 10 according to Embodiment 1 of the present invention. The planar light source 10 includes a substrate 1, a planar back electrode 4 provided on the substrate 1, a planar transparent electrode 2 provided to face the back electrode 4, and the back electrode 4 and transparent A planar light-emitting layer 3 provided between the electrode 2, a first buffer layer 6 provided between the transparent electrode 2 and the light-emitting layer 3, and a back electrode 4 and light emission And a second buffer layer 7 provided between the layer 3 and the second buffer layer 7.

  FIG. 2 is a schematic cross-sectional view showing a configuration of a cross section S viewed from the AA direction of FIG. FIG. 3 is a plan view of the planar light source 10. The planar light source 10 can be considered as one light emitting element (EL element) 10, which is formed on the substrate 1 on the back electrode 4, the second buffer layer 7, the light emitting layer 3, the first light emitting element 10. 1 buffer layer 6 and transparent electrode 2 are laminated in order. The transparent electrode 2 and the back electrode 4 are electrically connected via a DC power source 5. In this case, the transparent electrode 2 connected to the negative electrode side functions as an electron injection electrode (first electrode), and the back electrode 4 connected to the positive electrode side functions as a hole injection electrode (second electrode). To do. In the light-emitting element 10, the light-emitting layer 3 has a two-layer structure in which an n-type semiconductor layer 3a and a p-type semiconductor layer 3b are stacked, and an electron injection electrode is formed on the n-type semiconductor layer side with a hole. The injection electrode is installed on the p-type semiconductor layer side.

  The light emitting element 10 includes a first buffer layer 6 between the transparent electrode 2 that is an electron injection electrode (first electrode) and the n-type semiconductor layer 3a, and the p-type semiconductor layer 3b. A second buffer layer 7 is provided between the back electrode 4 which is a hole injection electrode (second electrode). Thus, by inserting the first and second buffer layers 6 and 7 between the semiconductor layers 3a and 3b constituting the light emitting layer 3 and the electrodes 2 and 4, respectively, the energy bands of FIGS. As shown in the figure, the height of the Schottky barrier between the transparent electrode 2 and the n-type semiconductor layer 3a and the Schottky barrier between the back electrode 4 and the p-type semiconductor layer 3b can be reduced. it can. Thereby, the injection efficiency of electrons and holes into the light emitting layer 3 can be increased. The effect of reducing the Schottky barrier at the joint surface by providing the first and second buffer layers 6 and 7 will be described later.

  Further, in the light emitting element 10, the transparent electrode 2 and the back electrode 4 are electrically connected via a DC power source 5. When power is supplied from the DC power supply 5, a potential difference is generated between the transparent electrode 2 and the back electrode 4, and a voltage is applied to the light emitting layer 3. Then, the light emitting layer 3 disposed between the transparent electrode 2 and the back electrode 4 emits light, and the light passes through the transparent electrode 2 and is extracted outside the light emitting element 10, that is, the planar light source 10.

  Furthermore, the light-emitting layer 3 may have a p-i-n type three-layer structure without being limited to the above-described configuration. The p-i-n type structure is a structure in which an intrinsic semiconductor layer is inserted between a p-type semiconductor and an n-type semiconductor. Furthermore, the light emitting layer 3 has a single layer structure, a plurality of pn junction films are provided, a plurality of pin type structures are stacked, and a thin dielectric layer is provided between the electrode and the light emitting layer for the purpose of current limitation. Provided in plural, driven by an alternating current power source, the back electrode is also made transparent, either the back electrode is a black electrode, and further includes a structure for sealing all or part of the planar light source 10, and in front of the light emission direction It is possible to make appropriate changes such as further including a structure for color-converting the color of light emitted from the light emitting layer 3. For example, a white planar light source can be formed by combining a blue light-emitting layer and a color conversion layer that converts blue into green and red.

  Hereinafter, each component of the planar light source 10 will be described in detail.

<Board>
The substrate 1 is made of a material that can support each layer formed thereon and has high electrical insulation. Moreover, when taking out light from the board | substrate 1 side, it is calculated | required that it is a material which has a light transmittance with respect to the wavelength of the light emitted from a light-emitting body. As such a material, for example, glass such as Corning 1737, quartz, ceramic, or the like can be used. It may be non-alkali glass or soda lime glass in which alumina or the like is coated on the glass surface as an ion barrier layer so that alkali ions contained in ordinary glass do not affect the light emitting element. Further, polyester, polyethylene terephthalate-based, a combination of polychlorotrifluoroethylene-based and nylon 6, a fluororesin-based material, a resin film such as polyethylene, polypropylene, polyimide, and polyamide can also be used. When using a resin film, it is preferable to use a material excellent in durability, flexibility, transparency, electrical insulation, and moisture resistance. In addition, description of the said material is an illustration, Comprising: The material of the board | substrate 1 is not specifically limited to these.

  Note that in the case of a structure in which light is not extracted from the substrate 1, the above-described light transmittance is unnecessary, and a material that does not have a light-transmitting property can also be used. Examples of these include metal substrates, ceramic substrates, silicon wafers, and the like having an insulating layer on the surface.

<Electrode>
As the electrodes, there are a transparent electrode 2 on the light extraction side and a back electrode 4 on the other side. Here, as shown in FIG. 2, the case where the back electrode 4 is provided on the substrate 1 will be described. However, the present invention is not limited to this. For example, the transparent electrode 2 is provided on the substrate 1 and light is emitted thereon. It is good also as a structure which laminates | stacks the layer 3 and the back electrode 4 in order. Alternatively, both the transparent electrode 2 and the back electrode 4 may be transparent electrodes.

  When a DC power source 5 is connected between the two electrodes and a DC voltage is applied between the two electrodes to emit light, one electrode connected to the negative electrode functions as an electron injection electrode and The other connected electrode functions as a hole injection electrode. In this case, whether the two electrodes function as an electron injection electrode or a hole injection electrode is determined by connection to a DC power source regardless of whether the electrode is the transparent electrode 2 or the back electrode 4. That is, whether it is the transparent electrode 2 or the back electrode 4 is determined by whether or not light is transmitted, and whether it functions as an electron injection electrode or a hole injection electrode is determined by connection with a DC power source. Is done. The electron injection electrode is disposed on the n-type semiconductor layer side, and the hole injection electrode is disposed on the p-type semiconductor layer side.

First, the transparent electrode 2 will be described. The material of the transparent electrode 2 may be any material as long as it has light transmittance so that light emitted in the light emitting layer 3 can be extracted to the outside, and preferably has high transmittance in the visible light region. Moreover, it is preferable that it is low resistance as an electrode, and also it is preferable that it is excellent in adhesiveness with the board | substrate 1 and the light emitting layer 3. FIG. As a material for the transparent electrode 2, a metal mainly composed of ITO (In ₂ O ₃ doped with SnO ₂ , also referred to as indium tin oxide), InZnO, ZnO, SnO ₂ or the like is mainly used. Examples include oxides, metal thin films such as Pt, Au, Pd, Ag, Ni, Cu, Al, Ru, Rh, and Ir, or conductive polymers such as polyaniline, polypyrrole, PEDOT / PSS, and polythiophene. It is not limited to these. These transparent electrodes 2 can be formed by a film forming method such as a sputtering method, an electron beam evaporation method, an ion plating method or the like for the purpose of improving the transparency or reducing the resistivity. Further, after film formation, surface treatment such as plasma treatment may be performed for the purpose of resistivity control. The film thickness of the transparent electrode 2 is determined from the required sheet resistance value and visible light transmittance.

The carrier concentration of the transparent electrode 2 is desirably in the range of 1E17 to 1E22 cm ⁻³ . In order to obtain performance as the transparent electrode 2, the volume resistivity of the transparent electrode 2 is preferably 1E-3 Ω · cm or less, and the transmittance is preferably 75% or more at a wavelength of 380 to 780 nm. The refractive index of the transparent electrode 2 is preferably 1.85 to 1.95. Furthermore, when the film thickness of the transparent electrode 2 is 30 nm or less, a dense and stable film can be realized.

The back electrode 4 may be any conductive material that is generally well known. Furthermore, it is preferable that the adhesiveness with the light emitting layer 3 is excellent. Suitable examples include, for example, metal oxides such as ITO, InZnO, ZnO, SnO ₂ , Pt, Au, Pd, Ag, Ni, Cu, Al, Ru, Rh, Ir, Cr, Mo, W, Ta, Metals such as Nb, laminated structures thereof, conductive polymers such as polyaniline, polypyrrole, PEDOT [poly (3,4-ethylenedioxythiophene)] / PSS (polystyrene sulfonic acid), or conductive carbon Can be used.

  For the purpose of area control as a backlight, the transparent electrode 2 and the back electrode 4 may be provided, for example, by dividing the transparent electrode 2 in the x direction and the back electrode 4 in the y direction so as to be orthogonal to each other. . Area control can further reduce power consumption.

<Light emitting layer>
Next, the light emitting layer 3 will be described. The light emitting layer 3 is a two-layer light emitting layer in which an n-type semiconductor layer 3a and a p-type semiconductor layer 3b are stacked.

The material of the n-type semiconductor layer 3a is an n-type semiconductor material in which majority carriers are electrons and exhibit n-type conduction. The material preferably has an optical band gap having a band gap size from the near ultraviolet region to the visible light region (1.7 eV to 3.6 eV), and further from the near ultraviolet region to the blue region (2.6 eV to 3. Those having 6 eV) are more preferred. Specifically, the above-described ZnS, Group 12-Group 16 compounds such as ZnSe, ZnTe, CdS, and CdSe, mixed crystals thereof (for example, ZnSSe), and Group 2 to Group 16 such as CaS and SrS. Intergroup compounds and mixed crystals thereof (for example, CaSSe), Group 13-15 compounds such as AlP, GaAs, GaN, GaP, and mixed crystals thereof (for example, InGaN), ZnMgS, CaSSe, CaSrS, etc. A mixed crystal of the above compound can be used. Furthermore, a chalcopyrite type compound such as CuAlS ₂ may be used. Furthermore, Cu, Ag, Au, Al, Ga, In, Mn, Cl, Br, I, Li, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb One or more kinds of atoms or ions selected from the group consisting of may be contained as an additive. The color of light emitted from the light emitting layer 3 is also determined by the type of these elements.

On the other hand, the material of the p-type semiconductor layer 3b is a p-type semiconductor material in which majority carriers are holes and exhibits p-type conduction. Examples of the p-type semiconductor material include compounds such as Cu ₂ S, ZnS, ZnSe, ZnSSe, ZnSeTe, and ZnTe. Among these p-type semiconductor materials, Cu ₂ S and the like inherently show p-type conduction, but other materials are added with one or more elements selected from nitrogen, Ag, Cu, and In as additives. Use. Further, chalcopyrite type compounds such as CuGaS ₂ and CuAlS ₂ exhibiting p-type conduction may be used. Furthermore, nitrides such as GaN and InGaN containing Zn or Mg as additives may be used.

<Buffer layer>
The first buffer layer 6 is provided between the first electrode 2 that is an electron injection electrode and the n-type semiconductor layer 3a. As the first buffer layer 6, it is preferable to select a material having a low work function, particularly a material having a work function of 3.5 eV or less, which forms an ohmic junction with the n-type semiconductor layer 3 a. In such a case, as shown in FIG. 4, the Schottky barrier between the first electrode (transparent electrode) 2 that is an electron injection electrode and the n-type semiconductor layer 3a becomes small, and Electron injection is performed efficiently. The composition of the first buffer layer 6 is preferably one or more of Al, Li, Al—Li, and the like.

  The second buffer layer 7 is provided between the second electrode 4 that is a hole injection electrode and the p-type semiconductor layer 3b. As the second buffer layer 7, it is preferable to select a material having a large work function that forms an ohmic junction with the p-type semiconductor layer 3 b, particularly a material having a work function of 5.0 eV or more. In such a case, as shown in FIG. 5, the Schottky barrier between the second electrode (back electrode) 4 that is a hole injection electrode and the p-type semiconductor layer 3b becomes small, and the second electrode 4 Hole injection is efficiently performed. The composition of the second buffer layer 7 is preferably made of one or more substances having a work function of 5 eV or more, such as Pt and Au.

<Manufacturing method>
Next, an example of a method for manufacturing the planar light source 100 according to Embodiment 1 in the case where ZnS is used as the light emitter material of each of the semiconductor layers 3a and 3b of the light emitting layer 3 will be described. A similar manufacturing method can also be used when using a light emitting layer made of the other materials described above.
(1) Prepare Corning 1737 as the substrate 1.
(2) A planar back electrode 4 is formed on the substrate 1. For example, Al is used and the film thickness is 200 nm.
(3) Pt is deposited on the back electrode 4 as a planar second buffer layer 7 by photolithography or the like. This thickness is 400 nm.
(4) Next, ZnS is deposited on the Pt layer 7 of the second buffer layer by a vapor deposition method. As conditions at this time, the substrate temperature is 600 ° C., and ZnS and Ag are deposited in a thickness of 1 μm in a gas containing NH ₃ , whereby a p-type ZnS layer can be formed as the p-type semiconductor layer 3b.
(5) ZnS and Ag were deposited on the p-type ZnS layer 3b by a vapor deposition method. As conditions at this time, the substrate temperature is set to 600 ° C., a thickness of 1 μm is deposited, and a planar n-type ZnS layer can be formed as the n-type semiconductor layer 3a.
(6) On the n-type ZnS layer 3a, Al is deposited as the first buffer layer 6 to a thickness of 200 nm by a sputtering method.
(7) Next, ITO is deposited as the transparent electrode 2 on the Al layer 6 of the first buffer layer by sputtering. The film thickness is 200 nm.
(8) Subsequently, a transparent insulator layer such as silicon nitride is formed as a protective layer (not shown) covering the whole.
Through the above steps, the planar light source 10 according to the first embodiment can be obtained.

In the planar light source 10 according to the first embodiment, the transparent electrode 2 and the back electrode 4 are connected to a DC power source 5 and a light emission is evaluated by applying a DC voltage therebetween. At 35V, the emission luminance was about 600 cd / m ² .
In the first embodiment, although both the first buffer layer 6 and the second buffer layer 7 are provided, either one may be used. Further, the method for forming each layer is not limited to the method described above.

<Effect>
The planar light source according to the present embodiment does not need to apply an AC high voltage unlike the conventional planar light source, and can obtain a necessary and sufficient light emission luminance with a DC low voltage.

(Embodiment 2)
<Schematic configuration of planar light source>
A planar light source according to the second embodiment will be described. The schematic structure of the planar light source is as shown in FIG. In the planar light source according to the second embodiment, compared to the planar light source according to the first embodiment, an alkali metal oxide such as CaO, BaO, or SrO is used as the first buffer layer 6a. And The present inventor has found that the alkali metal oxide has a characteristic of apparently lowering the work function of the metal that is the electron injection electrode, and the first electrode (transparent electrode) 2 that is the electron injection electrode and the light emitting layer. The first buffer layer 6a made of an alkali metal oxide is inserted between In this way, by using an alkali metal oxide as the first buffer layer 6a, the Schottky barrier between the first electrode 2 and the light emitting layer 3 can be reduced as shown in the energy band diagram of FIG. Can do. As a result, the efficiency of electron injection into the light emitting layer can be increased.

  FIG. 6 is an energy band diagram when an alkali metal oxide is used as the first buffer layer 6a. The cause of the apparent lowering of the work function of the metal by the alkali metal oxide is not yet clear, but the present inventor believes that this is because strong polarization occurs in the oxide. The work function of the transparent electrode 2 becomes apparently small, and the contact between the transparent electrode 2 and the n-type semiconductor layer 3a becomes ohmic. A first buffer layer 6a such as MgO exists between the transparent electrode 2 and the n-type semiconductor layer 3a. If the thickness of the first buffer layer 6a is sufficiently thin, electrons are tunneled. Due to the effect, it is possible to move from the transparent electrode 2 to the n-type semiconductor layer 3a.

<Manufacturing method>
Hereinafter, an example of the method for manufacturing the planar light source according to Embodiment 2 in the case where ZnS is used as the light emitter material of each of the semiconductor layers 3a and 3b of the light emitting layer 3 will be described. The same manufacturing method can be used for the light emitting layer made of the other materials described above.
(1) Prepare Corning 1737 as the substrate 1.
(2) A planar back electrode 4 is formed on the substrate 1. For example, Al is used and the film thickness is 200 nm.
(3) ZnS is deposited on the back electrode 4 in a planar shape by a gas phase growth method. As a condition at this time, a planar p-type ZnS layer is formed as the p-type semiconductor layer 3b by depositing ZnS and Ag in a thickness of 1 μm in a gas containing NH ₃ at a substrate temperature of 600 ° C. it can.
(4) Next, ZnS and Ag are deposited in a planar shape on the p-type ZnS layer 3b by the gas phase growth method. As conditions at this time, the substrate temperature is set to 600 ° C., a thickness of 1 μm is deposited, and an n-type ZnS layer can be formed as the n-type semiconductor layer 3a.
(5) Next, a CaO layer having a thickness of 2 nm is deposited as a first buffer layer on the n-type ZnS layer 3a by sputtering.
(6) Furthermore, ITO is deposited in a thickness of 200 nm as a transparent electrode 2 on the top of the CaO layer 6a of the first buffer layer by a sputtering method.
(7) Subsequently, a transparent insulating layer such as silicon nitride is formed as a protective layer (not shown) covering the whole.
Through the above steps, the planar light source according to the second embodiment can be obtained.

When the transparent electrode 2 and the back electrode 4 of the planar light source are connected to a DC power source 5 and a DC voltage is applied between them, the light emission is evaluated. As a result, light emission starts at an applied voltage of 15V and approximately 600 cd / m at 35V. An emission luminance of ² was shown.
In the second embodiment, the configuration includes only the first buffer layer 6a. However, the configuration may include not only the first buffer layer 6a but also the second buffer layer 7. Further, the method for forming each layer is not limited to the method described above.

(Embodiment 3)
<Schematic configuration of planar light source>
A planar light source according to Embodiment 3 will be described. The schematic structure of the planar light source is as shown in FIG. Compared with the planar light source according to the first embodiment, the planar light source according to the third embodiment is composed of a substance having a large electronegativity of about 3 or more, such as oxygen and fluorine, as the first buffer layer 6b. It is characterized by. The substance having an electronegativity of 3 or more forms an electric dipole at the interface between the n-type semiconductor layer 3a and the first buffer layer 6b. Due to the effect of this electric dipole, as shown in the energy band diagram of FIG. 7, the band on the transparent electrode 2 side is raised, and the height of the Schottky barrier with the n-type semiconductor layer 3a is reduced. The first buffer layer 6b does not need to be thick, and a thickness of one to several atomic layers is sufficient.

<Manufacturing method>
Hereinafter, an example of the method for manufacturing the planar light source according to Embodiment 3 in the case where ZnS is used as the light emitter material of each of the semiconductor layers 3a and 3b of the light emitting layer 3 will be described. The same manufacturing method can be used for the light emitting layer made of the other materials described above.
(1) Prepare Corning 1737 as the substrate 1.
(2) A planar back electrode 4 is formed on the substrate 1. For example, Al is used and the film thickness is 200 nm.
(3) ZnS is deposited on the back electrode 4 by a vapor deposition method. As conditions at this time, the substrate temperature is set to 600 ° C., and ZnS and Ag are deposited in a thickness of 1 μm in a gas containing NH ₃ to form a planar p-type ZnS layer as the p-type semiconductor layer 3b. Can be formed.
(4) Next, ZnS and Ag are deposited on the p-type ZnS layer 3b by a gas phase growth method. The conditions at this time are such that the substrate temperature is 600 ° C., a thickness of 1 μm is deposited, and a planar n-type ZnS layer can be formed as the n-type semiconductor layer 3a.
(5) Next, the sample is held in a high vacuum chamber, CH ₃ F gas is introduced, and then UV irradiation is performed so that the surface is covered with about one atomic layer of fluorine as the first buffer layer 6b. .
(6) ITO is deposited to a thickness of 200 nm as the transparent electrode 2 on the top of the fluorine serving as the first buffer layer 6b by sputtering.
(7) Subsequently, a transparent insulating layer such as silicon nitride is formed as a protective layer (not shown) covering the whole.
Through the above steps, the planar light source according to the third embodiment can be obtained.

When the transparent electrode 2 and the back electrode 4 of the planar light source according to the third embodiment are connected to a DC power source 5 and a DC voltage is applied between them, a light emission evaluation is performed, and light emission begins at an applied voltage of 15V. And an emission luminance of about 600 cd / m ² at 35V.
In the third embodiment, the configuration includes only the first buffer layer 6b. However, the configuration may include not only the first buffer layer 6b but also the second buffer layer 7. Further, the method for forming each layer is not limited to the method described above.

<Effect>
The planar light source according to the present embodiment can obtain necessary and sufficient light emission luminance at a low voltage by reducing the Schottky barrier between the light emitting layer and the electrode.

  The planar light source according to the present invention provides a thin planar light source capable of obtaining high brightness by low voltage driving at low cost. For example, it is useful as a backlight for a liquid crystal display.

It is a schematic perspective view which shows the structure of the planar light source which concerns on Embodiment 1 of this invention. It is a schematic sectional drawing which shows the structure of the cross section S perpendicular | vertical to the light emission surface seen from the AA direction of FIG. It is a top view of the planar light source of FIG. It is an energy band figure between the 1st electrode which is an electron injection electrode of FIG. 2, and an n-type semiconductor layer. It is an energy band figure between the 2nd electrode which is a hole injection electrode of FIG. 2, and a p-type semiconductor layer. FIG. 6 is an energy band diagram between a first electrode that is an electron injection electrode of a planar light source according to Embodiment 2 and an n-type semiconductor layer. FIG. 6 is an energy band diagram between a first electrode that is an electron injection electrode of a planar light source according to Embodiment 3 and an n-type semiconductor layer. It is a schematic block diagram of the light emitting element which concerns on the conventional embodiment. (A) is the energy band figure before making the 1st electrode and n-type semiconductor layer which are the electron injection electrodes of the conventional light emitting element contact, and (b) is the energy band figure after contact. (A) is an energy band figure before making the 2nd electrode which is a hole injection electrode of the conventional light emitting element, and a p-type semiconductor layer contact, (b) is an energy band figure after a contact. .

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Transparent electrode 3 Light emitting layer 3a n-type semiconductor layer 3b p-type semiconductor layer 4 Back electrode 5 DC power supply 6, 6a, 6b First buffer layer 7 Second buffer layer 10 Planar light source, light emitting element 50 Light emitting element 51 Substrate 52 Transparent electrode 53 Light-emitting layer 53a n-type semiconductor layer 53b p-type semiconductor layer 54 Back electrode 55 DC power supply

Claims (10)

A substrate,
A planar back electrode provided on the substrate;
A planar transparent electrode provided facing the back electrode;
At least one planar light emitting layer provided sandwiched between the back electrode and the transparent electrode;
Comprising at least one buffer layer provided between the back electrode or the transparent electrode and the light emitting layer,
By providing the buffer layer, the size of the potential barrier between the electrode and the light emitting layer sandwiching the buffer layer is such that the Schottky barrier in the case where the electrode and the light emitting layer are in direct contact with each other. A planar light source characterized by being smaller than the size. A DC voltage is applied between the back electrode and the transparent electrode to emit light, and either the back electrode or the transparent electrode functions as an electron injection electrode, and the other electrode is a positive electrode. Functions as a hole injection electrode,
The buffer layer is
A first buffer layer provided between the electron injection electrode and the light emitting layer;
The planar light source according to claim 1, further comprising two buffer layers, a second buffer layer provided between the hole injection electrode and the light emitting layer. A DC voltage is applied between the back electrode and the transparent electrode to emit light, and either the back electrode or the transparent electrode functions as an electron injection electrode, and the other electrode is a positive electrode. Functions as a hole injection electrode,
The buffer layer is
A first buffer layer provided between the electron injection electrode and the light emitting layer;
The planar light source according to claim 1, further comprising at least one buffer layer of a second buffer layer provided between the hole injection electrode and the light emitting layer.   The planar light source according to claim 2 or 3, wherein the first buffer layer contains a substance having a work function of 3.5 eV or less.   4. The planar light source according to claim 2, wherein the second buffer layer contains a substance having a work function of 5.0 eV or more.   4. The planar light source according to claim 2, wherein the first buffer layer contains an alkali metal oxide.   The planar light source according to claim 2 or 3, wherein the first buffer layer is made of a substance having an electronegativity of 3 or more.   The planar light source according to claim 1, wherein the light emitting layer is a two-layer light emitting layer in which an n-type semiconductor layer and a p-type semiconductor layer are stacked.   The light emitting layer is a three-layer light emitting layer including an n-type semiconductor layer, a p-type semiconductor layer, and an undoped semiconductor layer sandwiched between the n-type semiconductor layer and the p-type semiconductor layer. A planar light source according to claim 1.   The surface according to any one of claims 1 to 9, further comprising a color conversion layer facing the back electrode and the transparent electrode and in front of a direction in which light is extracted from the light emitting layer. Light source.

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