JP2006303456A - Light emitting device and laser equipment using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting device that produces an inverted distribution state by subjecting an organic compound to current excitation at low current density and a current excitation laser oscillator using an organic compound as a laser medium. <P>SOLUTION: Molecules existing in a normal state are not excited directly by external energy but configured by current excitation where current flow on a first layer and a second layer, in a way that icon storage takes place in an area where a first layer adjoins a second layer. Namely, the first and second layers are provided, an energy barrier is provided between the lowest unoccupied molecule (LUMO) orbit level of the first layer and the LUMO level of the second layer and between the highest occupied molecule orbit level of the first later and the HOMO level of the second layer, and the energy barrier between the HOMO levels is larger than that between the LUMO levels. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light emitting element that performs stimulated emission by current excitation, and a laser apparatus using the light emitting element.

  Laser light is light having monochromaticity, high coherence, and strong directivity, and is indispensable in fields such as optical communication technology, optical recording technology, and optical information processing technology. Devices that generate laser light, that is, laser devices, are mainly classified into solid lasers, dye lasers, and gas lasers depending on the type of laser medium. Among these, a semiconductor laser, which is one of solid lasers, is a laser that has rapidly developed in recent years. The feature of the semiconductor laser is that the apparatus can be miniaturized first. For this reason, it is easy to combine with other components such as an optical module for connecting to an optical fiber. Second, laser light can be obtained by current excitation. Therefore, it is possible to oscillate instantaneously by passing a current, the output can be adjusted according to the amount of current, and a stable output can be obtained. Thirdly, since it can be produced by diverting a semiconductor manufacturing process that has already been established technically, mass production is possible. Furthermore, it is a great feature that the output wavelength can be converted by changing the semiconductor medium.

  A semiconductor laser composed of an inorganic compound semiconductor has a P-type semiconductor layer 1003, an N-type semiconductor layer 1004, and an active layer 1005 that emits light between electrodes 1001 and 1002, as shown in FIG. is there. As the active layer 1005, a compound semiconductor such as InGaAsP, GaAs, or InGaN is often used. A semiconductor laser is manufactured by sandwiching the active layer 1005 between a P-type semiconductor layer 1003 called a cladding layer and an N-type semiconductor layer 1004. Examples of the compound semiconductor used for the cladding layer include InP, AlGaAs, ZnSSe, and GaN. Holes are injected from the P-type semiconductor layer 1003 and electrons are injected from the N-type semiconductor layer 1004, and these carriers reach the active layer 1005. In the active layer 1005, holes and electrons recombine, and light corresponding to the energy difference between the valence band and the conduction band is generated. Most of the generated light is confined in the active layer 1005 and output from the cleavage plane. The reflectance of the cleavage plane is determined by the refractive index of the active layer 1005 and air. For example, when GaAs is used for the active layer, light is reflected by about 30% and returns to the active layer 1005. The reflected light is amplified while being reflected by the two cleavage planes of the active layer 1005, and only light having a wavelength determined by the length of the active layer is amplified. Here, when the current value is increased, an inversion distribution is formed at a certain current density. The current density at this time is called a threshold, and light whose amplitude is amplified by stimulated emission after this threshold is oscillated from the cleavage plane as laser light.

  As described above, the semiconductor lasers developed so far are composed of inorganic semiconductors and are called inorganic semiconductor lasers. On the other hand, development of a laser using an organic compound as a laser medium (referred to as an organic laser) has been extremely difficult and has not yet been put into practical use. However, if an organic laser is put into practical use, characteristics that cannot be obtained with an inorganic semiconductor laser can be imparted. For example, a flexible laser can be produced based on the flexibility of the material, the manufacturing process can be simplified and the cost can be reduced, and the manufacturing process can be varied (e.g., vapor deposition, spin coating, printing, dip coating, etc. Etc.).

  The main factor that has hindered the development of organic lasers so far is that it was difficult to form an inversion distribution state indispensable for laser oscillation when organic compounds were used.

As shown in Non-Patent Document 1, the light energy density required to oscillate a laser beam using an organic compound (that is, to form an inverted distribution state) is 5 μJ / cm ² . In order to obtain energy corresponding to this by current injection, it is necessary to flow a current having a current density of several thousand A / cm ² . However, when a current having a current density of several thousand A / cm ² is passed through the organic compound, the device is destroyed. Therefore, in order to realize current-excitation laser oscillation using an organic compound as a laser medium, it has been necessary to develop a technique that can form an inversion distribution state at a low current density.
Kozlov, V.M. G. Et al., Applied Physics Letters, 1998, 72, 144-146.

  This invention is made | formed in view of the said problem, and makes it a subject to provide the light emitting element which can form an inversion distribution state by carrying out current excitation of the organic compound with a low current density. It is another object of the present invention to provide a current excitation type laser oscillator (that is, a laser device) using an organic compound as a laser medium.

  In a conventional laser element, an inversion distribution is obtained by exciting a ground state molecule to an excited state by external energy and increasing the number of molecules in the ground state than the number of molecules in the excited state. On the other hand, in the present invention, the ground state molecule is made to have an electrochemical carrier state, and the number of ground state molecules is relatively reduced, whereby the number of excited state molecules is less than the number of ground state molecules. By increasing the number, an inversion distribution is formed. In order to form such an inversion distribution, a plurality of layers are provided between a pair of electrodes.

  The structure of the light emitting element of the present invention will be described below.

  The light-emitting element of the present invention includes at least a first layer and a second layer between a pair of electrodes, and the first layer and the second layer are connected to the first layer and the second layer on one side. When holes are injected and electrons are injected into the other, it is provided adjacent to the first layer so that ion species are accumulated in a region in contact with the second layer.

  Specifically, the light-emitting element of the present invention has a first layer and a second layer adjacent to each other, and the lowest unoccupied molecular orbital (LUMO) level and the second layer of the first layer. Energy barriers between the LUMO level of the first layer and between the highest occupied molecular orbital (HOMO) level of the first layer and the HOMO level of the second layer. It is characterized by that. The energy barrier between HOMO levels is larger than the energy barrier between LUMO levels.

  In the light-emitting element of the present invention having another form, the proportion of molecules in the ground state is electrochemically reduced in the first layer by reducing the proportion of molecules present as a ground state in a region in contact with the second layer. An inversion distribution state is generated by increasing the proportion of molecules in an excited state.

  The light-emitting element of the present invention having another form includes a first energy barrier for blocking the movement of holes from the first layer to the second layer, and electrons from the second layer to the first layer. It has the 2nd energy barrier for stopping movement, It is characterized by the above-mentioned. The second energy barrier is secondly larger than the energy barrier.

  In the light-emitting element of the present invention having another form, the first layer and the second layer have different polarities of carriers transported preferentially, and the HOMO level of the first layer is the HOMO level of the second layer. The LUMO level of the first layer is higher than the level of the LUMO of the second layer. In addition, the first layer has a higher hole-transport property than electrons, and the second layer has a higher electron-transport property than holes.

  Note that the absolute value of the difference between the HOMO levels of the first layer and the second layer is preferably 0.5 eV or more, more preferably 0.5 eV or more and less than 3.0 eV. Further, the absolute value of the difference between the LUMO levels of the first layer and the second layer is 0.1 eV or more and is larger than the absolute value of the difference between the HOMO levels of the first layer and the second layer. It is preferably small, more preferably 0.1 or more and less than 3.0 eV.

  With the above configuration, when holes are injected into one of the first layer and the second layer and electrons are injected into the other, the number of molecules in the ground state in the light emitting region is greater than the number of molecules in the excited state. Less. That is, an inversion distribution state is likely to occur. An element that emits light by injecting holes in one side and injecting electrons in the other side is called a current-excitation light-emitting element.

  Further, in order to amplify the light emission in the region where the inversion distribution state is formed, the light-emitting element of the present invention has a structure for resonating the light emission.

  Specifically, the light-emitting element of the present invention has the two layers described above between a pair of reflectors, and the distance between the reflectors is 2 of the emission wavelength in the region where the inversion distribution state is formed. It is characterized by being an integral multiple of a fraction.

  Note that the reflector can function as an electrode of the light-emitting element.

Examples of the substance constituting the first layer of the light-emitting element include 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (hereinafter referred to as α-NPD), 4 , 4 ′, 4 ″ -tris (N, N-diphenyl-amino) -triphenylamine (hereinafter referred to as TDATA), 4,4′-bis [N- (3-methylphenyl) -N-phenylamino ] -Biphenyl (hereinafter referred to as TPD), 4,4 ′, 4 ″ -tris (N-carbazolyl) triphenylamine (hereinafter referred to as TCTA) and the like are preferable. However, the substances other than those listed here may have a hole mobility of 1 × 10 ⁻⁶ cm ² / V · sec or more. The mobility is a value measured at room temperature.

As a substance constituting the second layer of the light-emitting element, for example, a metal complex having a quinoline skeleton or a benzoquinoline skeleton represented by a tris (8-quinolinolato) aluminum complex (hereinafter referred to as Alq ₃ ), or a metal complex thereof Mixed ligand complexes are preferred. In addition to metal complexes, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (hereinafter referred to as PBD), 1,3-bis [ Oxadiazole derivatives such as 5- (p-tert-butylphenyl) -1,3,4-oxadiazol-2-yl] benzene (hereinafter referred to as OXD-7), 3- (4-tert-butyl Phenyl) -4-phenyl-5- (4-biphenylyl) -1,2,4-triazole (hereinafter referred to as TAZ), 3- (4-tert-butylphenyl) -4- (4-ethylphenyl)- Triazole derivatives such as 5- (4-biphenylyl) -1,2,4-triazole (hereinafter referred to as p-EtTAZ), bathophenanthroline (hereinafter referred to as BPhen), bathocuproin (hereinafter referred to as “p-EtTAZ”) Phenanthroline derivatives such as indicated as BCP), 4, 4 '- (N-carbazolyl) biphenyl (hereinafter referred to as CBP) can be used. However, any substance other than those listed here may have an electron mobility of 10 ⁻⁸ cm ² / V · sec or more. The mobility is a value measured at room temperature.

  Note that one or both of the first layer and the second layer may be a layer containing an inorganic compound.

In such a light-emitting element, the change in emission spectrum intensity with respect to the density of the current flowing to the light-emitting element can be divided into two linear regions having different inclinations. On the high current density side. The threshold values of the two linear regions having different inclinations are ² mA / cm ² or more and 50 mA / cm ² or less.

  According to the present invention, a light-emitting element capable of forming an inversion distribution state by exciting an organic compound with a low current density can be obtained. Further, it is possible to obtain a light-emitting element that can resonate and amplify the stimulated emission in the region where the inversion distribution state is formed. In addition, a current excitation type laser apparatus using an organic compound as a laser medium can be obtained.

  Embodiments of the light-emitting element of the present invention will be described below.

  In a conventional laser element, an inversion distribution is obtained by exciting a ground state molecule to an excited state by external energy and increasing the number of molecules in the ground state than the number of molecules in the excited state. On the other hand, in the present invention, the ground state molecule is made to have an electrochemical carrier state, and the number of ground state molecules is relatively reduced, whereby the number of excited state molecules is less than the number of ground state molecules. By increasing the number, an inversion distribution is formed.

  Specifically, as shown in FIG. 3, the light emitting element has at least two layers 311 and 312 provided adjacent to each other in which the polarities of carriers transported preferentially are different. Includes molecules 301a and 302a, respectively. By injecting holes or electrons into the two layers 311, 312 respectively, almost all of the molecules 301a, 302a in the ground state are brought into a state having carriers. The state having carriers is a state in which ionic species are formed. The ionic species includes a cation radical 301b or an anion radical 302b. Other ionic species include dications and dianions. The formation of ionic species reduces the concentration of electrochemically ground state molecules in the film. Then, for example, electrons are injected into a region where almost all of the molecules 301a in the ground state become cation radicals 301b. At this time, excited state molecules 301c are formed by recombination, but the density of the ground state molecules 301a is very low, and therefore the number of excited state molecules 301c is relatively larger than the number of ground state molecules 301a. As a result, an inversion distribution state is formed. In FIG. 3, 302 c represents a molecule that has emitted light and has reached the ground state.

  An energy structure of a light-emitting element capable of forming such an electrochemical inversion distribution state will be described with reference to FIG.

  FIG. 1 is an energy band diagram of the light emitting device of the present invention. In FIG. 1, the first layer 101 is a layer having a higher hole-transport property than electrons, and the second layer 102 is a layer having a higher electron-transport property than holes. Holes are injected into the first layer 101 from the anode 103 side and transported to the cathode 104 side.

Here, the HOMO level (HOMO level) 107 of the first layer 101 is located above the HOMO level 108 of the second layer 102, and the HOMO levels of the first layer 101 and the second layer 102. The absolute value of the level difference (ΔE ₁ ) 106 is configured to be as large as possible. Thus, the second layer 102 functions as a layer for blocking holes, and as many holes as possible can be accumulated in the first layer 101. The absolute value of the HOMO level difference 106 is preferably 0.5 eV or more, more preferably 0.5 eV or more and less than 3.0 eV.

The LUMO level (LUMO level) 109 of the first layer 101 is located above the LUMO level 110 of the second layer 102, and the LUMO levels of the first layer 101 and the second layer 102 are also set. The absolute value of the level difference (ΔE ₂ ) 105 is configured to be as large as possible. Thus, a large amount of electrons injected from the cathode 104 side can be prevented from being injected into the first layer 101. Accordingly, it is possible to suppress the accumulation of holes due to recombination of electrons and holes.

The difference between HOMO levels or LUMO levels is called an energy barrier.

In order to emit light, the energy barrier for electron transport is smaller than the energy barrier for hole transport. That is, the absolute value of the LUMO level difference 105 (ΔE ₂ ) is configured to be smaller than the absolute value of the HOMO level difference 106 (ΔE ₁ ). This is because the carrier injection speed largely depends on the size of the energy barrier. Note that the magnitude of the LUMO level difference 105 is preferably 0.1 or more and less than 3.0 eV. By setting it to such a value, it is possible to control so that slight electron injection into the first layer 101 is performed.

  In the light-emitting element of the present invention having the above structure, the number of molecules in the ground state in the light-emitting region can be reduced as much as possible to increase the density of molecules having holes (cation radical molecules). That is, almost no ground state molecules exist on the side of the first layer 101 that is in contact with the second layer 102, and the cation radical that is an ionic species occupies. Note that a slight amount of electrons are injected into the first layer 101 in which holes are accumulated. When electrons are injected, they recombine with holes and excitons are generated in the light emitting region. That is, an excited state is formed. At this time, since there are almost no ground state molecules in the light emitting region, the number of molecules in the ground state becomes smaller than the number of molecules in the excited state, and an inversion distribution state is formed.

Note that although a light-emitting element having a configuration in which an inversion distribution state is formed in the first layer 101 has been described in this embodiment mode, light emission having a configuration in which an inversion distribution state is formed in the second layer 102 can be performed. It may be an element. In this case, a light-emitting element having a configuration opposite to that of the light-emitting element described above may be used. Specifically, the absolute value of ΔE ₂ may be configured to be larger than the absolute value of ΔE ₁ . In the light-emitting element having such a structure, electrons are accumulated in the second layer 102.

  Note that many organic compounds have better hole transportability than electrons, and the mobility of holes in the first layer 101 is higher than the mobility of electrons in the second layer 102. A simple light-emitting element is easier to manufacture. Therefore, the light emitting element preferably has a structure in which an inversion distribution state is formed in the first layer 101 as described above.

  The structure of the light-emitting element having the energy band structure as described above will be described below with reference to FIG.

  Note that in this embodiment, a light-emitting element having a structure in which light emission can be extracted from an end surface (edge portion) of the light-emitting element as indicated by an arrow will be described. Needless to say, the element structure of the present invention can be applied even to a light-emitting element having a structure in which light is extracted from the upper surface of the light-emitting element.

  In FIG. 2, reference numeral 11 denotes a substrate for supporting the element. There is no particular limitation on the material of the substrate 11. Not only glass, quartz and plastic but also flexible substrates such as paper and cloth can be used.

  A first electrode 12 is formed on the substrate 11. The first electrode 12 may function as an anode and have a function as a reflector for reflecting light emission. In the light-emitting element described in this embodiment, the first electrode 12 can be formed of two layers (12a and 12b). For example, the first electrode 12a may be formed with a high conductivity. The first electrode 12 b is made of a material that is in contact with the first layer 13 and has a function of injecting holes into the first layer 13. For example, a metal oxide having a high work function such as indium tin oxide (ITO) or ZnO can be used. Further, the first electrode 12b may function as a reflector. Therefore, it is preferable to use a metal or an alloy such as Al, Ag, Pt, or Au having a high work function and reflectivity. In consideration of the fact that the first electrode 12b also functions as a reflector, it is preferable to form the first electrode 12b using a material having a low visible light absorptivity such as Ag and a high reflectance. The film thickness is controlled so that the first electrode 12b can function as a reflector. This is because if it is very thin, it cannot function as a reflector. Note that since the first electrode 12b is formed of a material having a high work function, the work function of the first electrode 12a is not particularly limited. The first electrode 12a may be made of a reflective material (Al, Ag, etc.), and may be a dielectric multilayer film. Thus, by making an electrode into a laminated structure, the selection range of an electrode material can be expanded and each function of the laminated | stacked electrode can be improved. In addition, the first electrode 12 is not necessarily formed of two layers, and may have a single layer structure or a stacked structure of three or more layers.

  The first layer 13 formed on the first electrode 12b is a layer for transporting holes and a layer that emits light. Note that in the light-emitting element of this embodiment, the first layer 13 is preferably formed with a material having a higher hole transportability than electrons, an excellent hole injection property, and a large energy band gap. In addition, since it is also a light emitting layer, a material having a large quantum yield of light emission is preferable. For example, an aromatic amine is preferable. Specifically, 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (hereinafter referred to as α-NPD), 4,4 ′, 4 ″ -tris ( N, N-diphenyl-amino) -triphenylamine (hereinafter referred to as TDATA), 4,4′-bis [N- (3-methylphenyl) -N-phenylamino] -biphenyl (hereinafter referred to as TPD) 4,4 ′, 4 ″ -tris (N-carbazolyl) triphenylamine (hereinafter referred to as TCTA) and the like can be used. On the other hand, as the polymer material, poly (vinyl carbazole) or the like showing good hole transportability may be used. Triphenylamine derivatives are particularly preferable because they have a large energy band gap and a high HOMO level, that is, a low ionization potential. Note that the first layer 13 is not limited to a single layer, and may be a layer having a laminated structure of two or more layers made of the materials described above.

A second layer 14 is formed on the first layer 13. The second layer is a layer for transporting electrons. The second layer 14 is preferably formed using a material having higher electron transportability than holes, excellent electron injection properties, and a large ionization potential. For example, it is preferable to use a metal complex having a quinoline skeleton or a benzoquinoline skeleton represented by a tris (8-quinolinolato) aluminum complex (hereinafter referred to as Alq ₃ ), a mixed ligand complex thereof, or the like. In addition to metal complexes, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (hereinafter referred to as PBD), 1,3-bis [ Oxadiazole derivatives such as 5- (p-tert-butylphenyl) -1,3,4-oxadiazol-2-yl] benzene (hereinafter referred to as OXD-7), 3- (4-tert-butyl Phenyl) -4-phenyl-5- (4-biphenylyl) -1,2,4-triazole (hereinafter referred to as TAZ), 3- (4-tert-butylphenyl) -4- (4-ethylphenyl)- Triazole derivatives such as 5- (4-biphenylyl) -1,2,4-triazole (hereinafter referred to as p-EtTAZ), bathophenanthroline (hereinafter referred to as BPhen), bathocuproin (hereinafter referred to as “p-EtTAZ”) Phenanthroline derivatives such as indicated as BCP), 4, 4 '- (N-carbazolyl) biphenyl (hereinafter referred to as CBP) can be used. Note that the material constituting the second layer 14 is preferably a material having a larger band gap and a larger ionization potential than the material constituting the first layer 13. Specifically, phenanthroline derivatives such as CBP and BCP, carbazole derivatives, and the like. Note that the second layer 14 is not limited to a single layer, and may be a layer having a laminated structure of two or more layers made of the materials described above.

  The HOMO level of the first layer 13 is located above the HOMO level of the second layer 14, and the LUMO level of the first layer 13 is higher than the LUMO level of the second layer 14. It is preferable that it is located above. When the first layer 13 has a stacked structure of two or more layers, the HOMO level and the LUMO level of the layer in contact with the second layer 14 in the first layer 13 are the HOMO levels of the second layer 14 respectively. And preferably above the LUMO level. When the second layer 14 has a stacked structure of two or more layers, the HOMO level and the LUMO level of the layer in contact with the first layer 13 in the second layer 14 are the HOMO of the first layer 13 respectively. It is preferably located below the level and the LUMO level. Further, when the first layer 13 and the second layer 14 have a stacked structure of two or more layers, the HOMO level and the LUMO level of the layer in contact with the second layer 14 in the first layer 13 are The second layer 14 is preferably located above the HOMO level and the LUMO level of the layer in contact with the first layer 13.

  A second electrode 15 is formed on the second layer 14. The second electrode 15 may function as a cathode and have a function as a reflector for reflecting light emission. In the light-emitting element described in this embodiment mode, the second electrode 15 can be formed of two layers (15a and 15b). For example, the second electrode 15 a is in contact with the second layer 14 and injects electrons into the second layer 14. Therefore, the second electrode 15a is a group 1 or group 2 typical element, that is, an alkali metal such as Li or Cs, an alkaline earth metal such as Mg, Ca, or Sr, and an alloy containing these (Mg / Ag, In addition to (Al / Li), it is preferable to form with a low work function such as transition metals including rare earth metals. The second electrode 15a functions as a reflector. Therefore, it is preferable to use a metal having low visible light absorption and high reflectance such as Al, Ag, Mg, or an alloy thereof. The film thickness is controlled so as to function as a reflector. This is because if it is very thin, it cannot function as a reflector. Since the second electrode 15a is formed of a material having a low work function, the work function of the second electrode 15b is not particularly limited. The second electrode 15b may be made of a reflective material (Al, Ag, etc.), and may be a dielectric multilayer film. Thus, by making an electrode into a laminated structure, the selection range of an electrode material can be expanded and each function of the laminated | stacked electrode can be improved. In addition, the second electrode 15 is not necessarily formed of two layers, and may have a single layer structure or a stacked structure of three or more layers.

  Note that the first layer 13 and the second layer 14 may be formed by applying either a wet method or a dry method. In the case of a polymer material, a spin coating method, an ink jet method, a dip coating method, a printing method, or the like is suitable. On the other hand, a low molecular material can be formed not only by dip coating and spin coating, but also by vacuum deposition. The formation method of the first electrode 12 and the second electrode 15 is not particularly limited, and can be formed by an evaporation method, a sputtering method, or the like.

  The optical distance between the first electrode 12 and the second electrode 15 functioning as a reflector (hereinafter simply referred to as the distance) is used to resonate and amplify the light emission in the region where the inversion distribution state is formed. It is good that it is an integral multiple of half the wavelength. This is because it is possible to resonate and amplify light by forming a standing wave. Note that the distance can be controlled by changing the integrated film thickness of the first layer 13 and the second layer 14.

  In this embodiment, the layer provided between the first electrode 12 and the second electrode 15 is composed of two layers of the first layer 13 and the second layer 14. Not limited to this, it may have a configuration of three or more layers provided with other functional layers. For example, functions such as an electron injection layer, a hole injection layer, and a hole blocking layer may be provided.

In the above light-emitting element, as shown in Example 1 below, the threshold of stimulated emission that can be seen from the change in the emission intensity with respect to the current density can be set to a current density of 50 mA / cm ² or less, preferably 15 mA / cm ² or less. That is, an inversion distribution state is formed when a current is supplied so as to have a current density equal to or higher than this threshold value. The threshold has preferably Considering durability is 2 mA / cm ² or more 50 mA / cm ² or less of the light-emitting element, the threshold value 2 mA / cm ² or more 50 mA / cm ² or less in the light emitting device of the present invention be able to. In the region where the state is formed, electrons and holes injected from each electrode are recombined, and part of the emitted light is resonated between the reflectors (first and second electrodes in this embodiment). Amplified. Note that the emission spectrum of the emitted light has a light emission wavelength that can resonate within the light emitting element and has a relatively sharp peak. The light emitting element can be used as, for example, a laser oscillator, that is, a laser device.

  The light-emitting element of the present invention and various characteristics of the light-emitting element will be described.

  As shown in FIG. 4, an electrode 71 is formed by depositing ITO on a glass substrate 70. After forming α-NPD as a first layer 72 on the electrode 71 by vacuum deposition, CBP and BCP are sequentially formed on the first layer 72 as second layers 73 (73a, 73b). Filmed. After forming calcium fluoride as a third layer 74 on the second layer 73, aluminum was further formed as an electrode 75 to obtain a light emitting element. The film thicknesses of the first layer 72 and the second layers 73a and 73b are 100, 30, and 130 nm, respectively, and the total film thickness thereof is 260 nm. In this light emitting element, the electrodes 71 and 75 function as reflectors, respectively. As described above, the light emitting element in this embodiment is configured to resonate the emitted light.

  Note that α-NPD is a substance excellent in hole transportability, and has a LUMO level of −2.4 eV and a HOMO level of −5.3 eV. CBP is a substance having a high electron transporting property, and the LUMO level is −2.5 eV and the HOMO level is −5.9 eV. BCP is a substance having an excellent electron transporting property, and has a LUMO level of −1.7 eV and a HOMO level of −6.7 eV. Note that FIG. 5 illustrates an energy band structure of the light-emitting element. As can be seen from FIG. 5, the LUMO level of the first layer 72 is higher than the LUMO level of the second layer 73a. The HOMO level of the first layer 72 is higher than the HOMO level of the second layer 73a. The LUMO level of the second layer 73b is higher than the LUMO level of the second layer 73a, and the HOMO level of the second layer 73b is lower than the HOMO level of the second layer 73a. Yes.

FIG. 6 shows an emission spectrum of the light-emitting element. From FIG. 6, it can be seen that light emission having a narrow peak at 465 nm is obtained. Further, FIG. 7 shows the change in the emission intensity with respect to the current density. Note that the plot of the example was divided into two regions having different slopes, and the approximate straight line for each region was shown by a solid line. From FIG. 7, it can be seen that the emission intensity increases linearly as the current density increases, but the slope increases with the current density being 12 mA / cm ² as the inflection point (that is, the threshold value). This indicates that spontaneous radiation is dominant in the region of current density smaller than 12 mA / cm ² , whereas stimulated radiation occurs in the region of current density larger than 12 mA / cm ^2. Conceivable.

  This is considered to be due to the following mechanism. The holes injected into the first layer are blocked from being injected into the second layer 73a by the energy barrier (difference in the HOMO level) between the first layer 72 and the second layer 73a. Accumulated in one layer 72. The electrons easily injected from the second layer 73b to the second layer 73a are caused by the energy barrier (difference in LUMO level) between the first layer 72 and the second layer 73a. Is blocked from injection. However, electrons are injected into the first layer with a small probability. Accordingly, in the region of the first layer 72 on the second layer 73a side, recombination of holes and electrons occurs and an excited state is formed. However, since the concentration of molecules having holes in the region is high, the number of molecules in the excited state is larger than the number of molecules in the ground state, and as a result, an inversion distribution state is formed.

(Comparative example)

  A comparative example for the light-emitting element will be described.

  As shown in FIG. 8, an electrode 81 is formed by depositing ITO on a glass substrate 80. An α-NPD film was formed as a first layer 82 on the electrode 81 by vacuum deposition, and then a BCP film was formed as a second layer 83 on the first layer 82. After forming calcium fluoride as a third layer 84 on the second layer 83, aluminum was further formed as an electrode 85 to obtain a light emitting element. The film thicknesses of the first layer 82 and the second layer 83 are 100 and 160 nm, respectively, and the total film thickness thereof is 260 nm. Note that the electrodes 81 and 84 each function as a reflector in the light-emitting element. Such a light emitting element in this comparative example is configured to resonate emitted light.

  FIG. 10 shows an energy band structure of the light-emitting element of the comparative example. As can be seen from FIG. 10, the LUMO level of the first layer 82 is lower than the LUMO level of the second layer 83. The HOMO level of the first layer 82 is higher than the HOMO level of the second layer 83.

  An emission spectrum of the light-emitting element is shown in FIG. It can be seen from FIG. 11 that light emission having a narrow peak at 460 nm is obtained. However, in the dependence of the light emission intensity on the current density (FIG. 7), the light emission intensity increases linearly as the current density increases, but there is an inflection point as seen in the light emitting device of the present invention. You can see that they are not.

  This is considered to be due to the following mechanism. The holes injected into the first layer are blocked from being injected into the second layer 83 by the energy barrier (difference in HOMO level) between the first layer 82 and the second layer 83, Accumulated in one layer 82. However, since the LUMO level of the second layer 83 is higher than that of the first layer 82, electrons are rapidly injected from the second layer 83 into the first layer 82, and recombination of electrons and holes is frequent. It happens. As a result, the number of molecules having carriers is small, and a state in which the number of molecules that have reached the ground state is larger than the number of molecules in the excited state is formed, and an inversion distribution state cannot be formed.

3A and 3B each illustrate an energy band structure of a light-emitting element of the present invention. 4A and 4B illustrate a stacked structure of a light-emitting element of the present invention. 8A and 8B illustrate a process for forming an inversion distribution state in a light-emitting element of the present invention. 4A and 4B illustrate a stacked structure of a light-emitting element of the present invention. 3A and 3B each illustrate an energy band structure of a light-emitting element of the present invention. The emission spectrum of the light emitting element of this invention. FIG. 5 shows current density dependence of emission spectrum intensity in the light-emitting element of the present invention and the light-emitting element of the comparative example. 8A and 8B illustrate a stacked structure of a light-emitting element of a comparative example. The figure explaining the semiconductor laser of a prior art. 6A and 6B illustrate an energy band structure of a light-emitting element of a comparative example. The emission spectrum of the light emitting element of a comparative example.

Explanation of symbols

11 substrate 12 first electrode 12a first electrode 12b first electrode 13 first layer 14 second layer 15 second electrode 15a second electrode 15b second electrode 70 glass substrate 71 electrode 72 first Layer 73 second layer 73a second layer 73b second layer 74 third layer 75 electrode 80 glass substrate 81 electrode 82 first layer 83 second layer 84 third layer 85 electrode 101 first Layer 102 second layer 103 anode 104 cathode 105 LUMO level difference 106 HOMO level difference 107 first layer 101 HOMO level 108 second layer 102 HOMO level 109 first layer 101 LUMO level 110 Second layer 102 LUMO level 301a molecule 301b cation radical 301c molecule 302a molecule 302b anion radical 302c molecule 311 layer 312 layer 1001 1002 electrodes 1003 P-type semiconductor layer 1004 N-type semiconductor layer 1005 active layer

Claims (22)

Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer A light-emitting element, which is provided adjacent to each other so that ion species are accumulated in a region in contact with the layer. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
And energy barriers between the LUMO level of the first layer and the LUMO level of the second layer, and between the HOMO level of the first layer and the HOMO level of the second layer. A light-emitting element including: Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
And energy barriers between the LUMO level of the first layer and the LUMO level of the second layer, and between the HOMO level of the first layer and the HOMO level of the second layer. Have
The energy barrier between the HOMO level of the first layer and the HOMO level of the second layer is an energy barrier between the LUMO level of the first layer and the LUMO level of the second layer. A light emitting element characterized by being larger. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
The proportion of molecules in the excited state is made larger than the proportion of molecules in the ground state by electrochemically reducing the proportion of molecules present as a ground state in a region of the first layer in contact with the second layer. Thus, an inversion distribution state is generated. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
A first energy barrier for preventing movement of holes from the first layer to the second layer;
And a second energy barrier for preventing movement of electrons from the second layer to the first layer. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
A first energy barrier for preventing movement of holes from the first layer to the second layer;
A second energy barrier for preventing movement of electrons from the second layer to the first layer;
The light emitting device, wherein the first energy barrier is larger than the second energy barrier. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
The first layer and the second layer have different polarities of carriers transported preferentially,
The HOMO level of the first layer is higher than the HOMO level of the second layer,
The light emitting element, wherein the LUMO level of the first layer is higher than the LUMO level of the second layer. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
The first layer has a higher hole mobility than electrons;
The second layer has higher electron mobility than holes;
The HOMO level of the first layer is higher than the HOMO level of the second layer,
The light emitting element, wherein the LUMO level of the first layer is higher than the LUMO level of the second layer. 9. The light-emitting element according to claim 7, wherein an absolute value of a difference between HOMO levels of the first layer and the second layer is 0.5 eV or more. The absolute value of the difference between the HOMO levels of the first layer and the second layer is the difference between the LUMO levels of the first layer and the second layer. A light emitting element characterized by being larger than the absolute value of. 11. The light-emitting element according to claim 10, wherein an absolute value of a difference in LUMO level between the first layer and the second layer is 0.1 eV or more and less than 3.0 eV. In any one of Claims 1 thru | or 11, It has the 1st electrode and 2nd electrode which reflect the light light-emitted from the said light emitting element,
The light-emitting element, wherein the first layer and the second layer have such a thickness that the light resonates in the first electrode and the second electrode. 13. The first layer according to claim 5, wherein the first layer is formed using 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl. A light emitting element. 14. The light-emitting element according to claim 5, wherein the second layer is formed using 4,4 ′-(N-carbazolyl) biphenyl. 14. The light-emitting element according to claim 5, wherein the second layer is formed using 4,4 ′-(N-carbazolyl) biphenyl and bathocuproine. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
A change in emission spectrum intensity with respect to current density is divided into two linear regions having different slopes, and a region having a large slope is on a higher current density side than a region having a small slope. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
And between the LUMO level of the first layer and the LUMO level of the second layer, and between the HOMO level of the first layer and the HOMO level of the second layer, respectively. Have an energy barrier,
The change in emission spectrum intensity with respect to the current density can be divided into two linear regions with different slopes, and the region with a large slope is on the high current density side with respect to the region with a small slope. element. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
And between the LUMO level of the first layer and the LUMO level of the second layer, and between the HOMO level of the first layer and the HOMO level of the second layer, respectively. Has an energy barrier,
The energy barrier between the HOMO level of the first layer and the HOMO level of the second layer is between the LUMO level of the first layer and the LUMO level of the second layer. Bigger than the energy barrier,
The change in emission spectrum intensity with respect to the current density can be divided into two linear regions with different slopes, and the region with a large slope is on the high current density side with respect to the region with a small slope. element. Having a first layer and a second layer between a pair of electrodes;
When the first layer and the second layer inject holes into one of the first layer and the second layer and inject electrons into the other, the second layer in the first layer So as to accumulate ionic species in the region in contact with the layer,
In the first layer, the proportion of molecules in the excited state is made larger than the proportion of molecules in the ground state by electrochemically reducing the proportion of molecules in the region in contact with the second layer as a ground state. To produce an inversion distribution state,
The change in emission spectrum intensity with respect to the current density can be divided into two linear regions with different slopes, and the region with a large slope is on the high current density side with respect to the region with a small slope. element. 20. The light-emitting element according to claim 16, wherein a threshold value of the two linear regions having different inclinations is ² mA / cm ² or more and less than 50 mA / cm ² . A laser device using the light emitting element according to claim 1. 21. A laser device using the light emitting element according to claim 1 as a laser oscillator.

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