KR20080090349A - Organic light-emitting diode element and optical interconnection module - Google Patents

Abstract

An OLED(Organic Light-Emitting Diode) element and an optical interconnection module are provided to prevent deterioration due to oxygen or moisture by forming an electron transport layer with inorganic material, thereby extending a lifetime. An OLED element(1) is formed by sequentially stacking an anode(3), a hole transport layer(4), a light emitting layer(5), an electron transport layer(6), an electron injection layer(7), and a cathode(8) on a substrate(2). The electron transport layer is made of semiconductor material. The semiconductor material contains a periodic table Group II-VI compound. The periodic table Group II-VI compound is ZnS. A thickness of the electron transport layer made of ZnS is in a range of 10nm to 300nm. The semiconductor material contains a reductive dopant. The reductive dopant has a work function of less than 2.9eV.

Description

Organic light-emitting diode element and optical interconnection module

The present invention relates to a light emitting device using an organic light emitting diode (OLED), and more particularly to an organic light emitting diode device (hereinafter referred to as an OLED device) with improved response speed and an optical communication module using the same.

An OLED element has the basic structure by which the 1st electrode layer (anode), the organic layer, and the 2nd electrode layer (cathode) were laminated | stacked on the surface of transparent glass or a transparent resin substrate. OLED devices are characterized by high contrast ratios, wide viewing angles, and thinness, and are beginning to be applied in fields such as displays. However, in the display using the OLED element, since the light emitting portion is formed on the driving transistor circuit, there is a problem that the light emitted from the transistor portion is absorbed or scattered in the normal element structure, and the extraction efficiency to the outside is deteriorated. . In order to solve this problem, the structure called the top emission structure laminated | stacked in order of a cathode, an organic layer, and an anode on a glass substrate is also examined.

The first electrode layer (anode) is formed of a transparent conductive material represented by ITO (indium tin oxide) or IZO (indium zinc oxide). The organic layer is composed of a plurality of layers, such as a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer.

OLED devices have been researched and developed in many research institutes until now, and their light emission characteristics (light emission efficiency, maximum brightness, power consumption, etc.) have been dramatically improved. For example, a lot of research and development have been carried out such as phosphorescent material having a higher luminous efficiency than a conventional fluorescent material, a cathode material having a low work function, optimization of the carrier balance between electrons and holes in the light emitting layer. Moreover, as a manufacturing method which can realize cost reduction, the devacuation process using screen printing, gravure printing, the inkjet method, etc. as well as the conventional vacuum deposition is examined.

On the other hand, as a new application of this OLED element, the light source for optical wiring modules is expected. The optical wiring module has a structure in which a light emitting element or a light receiving element is mounted at both ends of an optical fiber or a polymer optical waveguide. The optical wiring module converts an electrical signal into an optical signal using the light emitting element, and passes the optical signal through the optical fiber or polymer optical waveguide. Send it to the light-receiving element. Finally, the light receiving element converts an optical signal into an electrical signal to communicate with each other.

As a prior art of the optical wiring module using an OLED element, Unexamined-Japanese-Patent No. 2003-149541 and Unexamined-Japanese-Patent No. 2003-14995 are mentioned, for example.

Using these prior art techniques, an OLED element can be used as a light emitting element for transmitting light to an optical fiber or a polymer optical waveguide. In addition, an OLED element can be formed directly on a substrate on which a polymer optical waveguide with poor heat resistance is formed using a method such as vapor deposition. Therefore, there is an advantage that the optical waveguide and the OLED element can be easily combined without requiring complicated optical axis adjustment or processing of the optical waveguide cross section. In addition, it is also possible to integrally form an optical waveguide or an OLED element and to monolithically integrate it, so that the mounting process of the optical wiring module can be greatly shortened and the cost can be realized.

As a prior art which aimed at improving the response speed of OLED element, Unexamined-Japanese-Patent No. 5-29080, Unexamined-Japanese-Patent No. 2003-243157, and Unexamined-Japanese-Patent No. 2002-313553 are mentioned, for example.

In the method disclosed in Japanese Patent Laid-Open No. 5-29080, the response speed of the OLED element can be improved by reducing the capacitance of the OLED element. In addition, Japanese Patent Laid-Open No. 2003-243157 discloses a response speed of 100 MHz by applying a voltage obtained by superposing a bias voltage and a pulse voltage. In Japanese Patent Laid-Open No. 2002-313553, a hole barrier layer or an electron injection layer is provided next to the light emitting layer to improve the response speed.

In addition, examples of the prior art for improving the luminous efficiency of OLED devices include Japanese Patent Laid-Open Nos. 11-354283 and 2000-164363. These prior arts disclose, for example, providing an inorganic compound layer at the interface between the organic layer and the cathode, or mixing an inorganic compound in the organic compound layer near the cathode. As this inorganic compound, it is preferable that it is at least 1 compound chosen from an alkali metal oxide, a rare earth metal halide, and an alkali metal complex. Moreover, as an form of an inorganic compound, it is preferable to form in layer shape or an island shape. In the case of the layered phase, an ultra-thin film made of electron-injectable alkaline earth metal oxide, alkali oxide or alkali fluoride and having a film thickness of about 0.4 to 10 nm is preferable. Examples of the alkaline earth metal oxides include BaO, SrO, CaO, Ba _x Sr _1-x O (0 <x <1), Ba _x Ca _{1- x} O (0 <x <1), and the like. Examples of the alkali oxides and alkali fluorides include LiF, Li ₂ O, NaF, and the like. As a method of forming an alkaline earth metal oxide, while depositing an alkaline earth metal by a resistive heating deposition method, oxygen is introduced into a vacuum chamber to make the vacuum degree between 10 ^-3 and 10 ^-4 Pa, and vaporized while reacting oxygen and the alkaline earth metal. The method of making it preferable is preferable, and the method of forming an alkaline-earth metal oxide by the electron beam vapor deposition method can be employ | adopted. As the formation method of the alkali oxide, the same method as the formation method of the alkaline earth metal oxide mentioned above can be used. As a formation method of an alkali fluoride, an electron beam vapor deposition method or a resistive heating vapor deposition method is mentioned.

Further, as described in Japanese Patent Application Laid-Open Nos. 2002-164178 and 2003-238534, the device characteristics can be improved by containing an inorganic compound in the electron injection layer. As such an inorganic compound, an insulator or a semiconductor is preferable. When the electron transport layer contains an insulator or a semiconductor, leakage of current can be effectively prevented and electron transport can be improved. As the insulator which can be contained in the electron transport layer, at least one metal compound selected from the group consisting of alkali metal chalcogenides, alkaline earth metal chalcogenides, alkali metal halides and alkaline earth metal halides is preferable. If an electron carrying layer contains these alkali metal chalcogenides etc., an electron transport property can be improved further. Preferred alkali metal chalcogenides include, for example, Li ₂ O, LiO, Na ₂ S, Na ₂ Se, and NaO. Preferred alkaline earth metal chalcogenides include, for example, CaO, BaO, SrO, BeO, BaS and CaSe. Moreover, as a preferable alkali metal halide, LiF, NaF, KF, LiCl, KCl, NaCl etc. are mentioned, for example. Further, as the preferable alkali earth metal halide, and examples thereof include a halide other than _{CaF 2, BaF 2, SrF 2} , MgF 2 and BeF _2, such as a fluoride or fluorides. In addition, as the semiconductor which can be contained in the electron transport layer, an oxide containing at least one of Ba, Ca, Sr, Yb, Al, Ga, In, Li, Na, Cd, Mg, Si, Ta, Sb and Zn, 1 type, or a combination of 2 or more types, such as nitride or oxynitride, is mentioned.

However, the above-described prior art has the following problems.

Regarding the improvement of the response speed of the OLED device, the response speed of the OLED device can be obtained by using the methods disclosed in Japanese Patent Application Laid-Open Nos. 5-29080, 2003-243157, and 2002-313553. Is improved, but it is a phenomenon that the response speed is still insufficient for practical use as an optical wiring module. The practical response speed is 100 MHz or more as the cutoff frequency, but the method described in the prior art can realize only a cutoff frequency of about 10 to 20 MHz.

As disclosed in JP-A-11-354283 and JP-A-2000-164363, by inserting an inorganic compound layer at the interface between the organic layer and the cathode, the response speed of the OLED element is slightly improved. However, since the organic compound is used for the electron transport layer, and the organic material has low electron mobility, the improvement of the response speed of the OLED element is limited.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an OLED device having improved response speed and an optical communication module using the same.

The present invention provides an OLED device comprising at least an anode, a light emitting layer, an electron transport layer, and a cathode, wherein a semiconductor material is used for the electron transport layer.

In the OLED device of the present invention, it is preferable that the semiconductor used in the electron transport layer is a II-VI compound semiconductor.

In the OLED device of the present invention, it is preferable that the group II-VI compound semiconductor is ZnS.

In the OLED device of the present invention, the film thickness of the electron transport layer made of ZnS is preferably in the range of 10 nm to 300 nm.

In the OLED device of the present invention, the semiconductor material used for the electron transport layer preferably contains a reducing dopant.

As the reducing dopant, one having a work function of 2.9 eV or less is preferable.

Further, the reducing dopant may be an alkali metal, an alkaline earth metal, a rare earth metal, an oxide of an alkali metal, an halide of an alkali metal, an oxide of an alkaline earth metal, a halide of an alkaline earth metal, an oxide of a rare earth element, or a halide of a rare earth metal. It is preferably one or two or more selected from the group consisting of.

Moreover, this invention provides the module for optical communication which uses the OLED element which concerns on this invention mentioned above as a light source for transmission.

The OLED device of the present invention can improve the response speed of the OLED device by using a light emitting layer material having a high electron mobility for the electron transport layer.

1 is a view showing an embodiment of an OLED device of the present invention. The OLED element 1 of the present embodiment is made of an inorganic material such as an anode 3 made of an ITO thin film, a hole transport layer 4, a light emitting layer 5, ZnS, or the like on a substrate 2 made of a transparent material such as glass. The electron transport layer 6, the electron injection layer 7, and the cathode 8 made of a metal thin film are laminated in this order. In the OLED element 1 of the present embodiment, an inorganic material, in particular, a semiconductor material such as ZnS is used for the electron transport layer 6 having an electron transport function.

In general, the OLED device emits light by electrons injected into the organic layer from the cathode by recombination with holes in the emission layer. That is, the response speed of the OLED element is determined by the time from the injection of electrons from the cathode to the recombination. This time is governed by the time that electrons move in the electron transport layer, which is obtained from the electron mobility and the film thickness of the material used for the electron transport layer. The film thickness of the electron transport layer is preferably a film thickness of several nm in order to optimize the balance between holes and electrons in the light emitting layer. Therefore, in order to improve the response speed of OLED element, it is necessary to improve the electron mobility of an electron carrying layer. Since inorganic materials have higher electron mobility than organic materials, OLED devices using inorganic materials for the electron transport layer have a higher response speed than organic ELs using organic materials for the electron transport layer.

In addition, an OLED element is generally produced using the vapor deposition method. It is not preferable to form a film by a manufacturing method other than the electron transport layer because it leads to an increase in cost. As a vapor deposition method, there exist methods, such as vapor deposition of resistance line heating, electron beam vapor deposition, etc., As an inorganic material which can be formed into a film by these vapor deposition methods, the II-VI group semiconductor material of a periodic table is mentioned. In particular, a compound of zinc (Zn) and sulfur (S) is preferable in view of film thickness reproducibility during deposition. In addition, by using an inorganic material for the electron transport layer, it is possible to prevent deterioration of the OLED device due to oxygen or moisture, which leads to a longer life of the OLED device.

(Example 1)

The effect of this invention is demonstrated based on a specific Example.

A glass substrate on which a transparent electrode made of ITO (Indium Tin 0xide) having a thickness of 32 mm x 25 mm x 1 mm was ultrasonically cleaned for 5 minutes in isopropyl alcohol, followed by UV ozone cleaning for 5 minutes. The glass substrate after washing | cleaning was fixed to the board | substrate holder, and the organic layer and the cathode were formed into a film using the vapor deposition machine of resistance line heating.

The organic layer and the cathode were composed of 4-4'-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (α-NPD) and a hole transport layer having a thickness of 50 nm, and 0.5 mass%. Made of tris (8-hydroxyquinoline) aluminum (Alq ₃ ) doped with 5, 6, 11, and 12-tetraphenyltetracene (rubrene), a light emitting layer having a thickness of 20 nm, and an electron having a thickness of 50 nm It is a cathode which consists of a transport layer, LiF, an electron injection layer of 0.4 nm in thickness, and Al, and is 150 nm in thickness.

The response speed of the fabricated OLED device was evaluated. In evaluating the response speed, the frequency (blocking frequency) at which the output light intensity decreased by half when a sinusoidal wave voltage having an amplitude of 5 V and a bias voltage of 5 V was applied was measured. The measurement result is shown in FIG.

As can be seen from Fig. 2, in the device of Example 1, the cutoff frequency was 20 MHz.

(Comparative Example 1)

An OLED device of Comparative Example 1 was produced in the same manner as in Example 1 except that the ZnS used as the electron transport layer was replaced with an Alq ₃ layer (film thickness of 50 nm), and the response speed of the device was evaluated. In this device, the cutoff frequency was 5 MHz when the amplitude of the sine wave voltage was 5V and the bias voltage was 5V.

Comparing the results of Example 1 and Comparative Example 1, it can be seen that in the OLED device of Example 1, the response speed is greatly improved by using ZnS in the electron transport layer.

2 is a graph showing the frequency dependence of the output light intensity of OLED devices produced in Example 1 and Comparative Example 1, respectively. As can be seen from FIG. 2, it was demonstrated that the OLED device of Example 1 can obtain a certain level of output light intensity even at a higher frequency than the device of Comparative Example 1.

(Examples 2-8, Comparative Examples 2-3)

In the device structure of Examples, the OLED devices of Examples 2 to 8 and Comparative Examples 2 to 3 in which the film thickness of ZnS was changed as shown in Table 1 were produced, and the respective cutoff frequencies were evaluated. The amplitude of the sine wave voltage at this time was 5V, and the bias voltage was 5V. Table 1 shows the measurement results together with the results of Example 1.

As shown in Table 1, emission was not observed in the OLED device (Comparative Example 3) in which the film thickness of ZnS exceeded 300 nm. This is considered to be due to the fact that ZnS has a large film thickness, so that electrons do not easily move from the cathode to the light emitting layer. In addition, in an OLED device (Comparative Example 2) having a film thickness of ZnS of 5 nm, the device was destroyed when voltage was applied, and light emission could not be observed. This is considered to be because the electron transport layer is too thin and the carrier (holes or electrons) is excessively flowed in a large amount and the device is destroyed.

On the other hand, in OLED devices (Examples 1 to 8) in which the film thickness of ZnS is in the range of 10 nm to 300 nm, light emission can be normally performed, and the cutoff frequency is improved compared with the device of Comparative Example 1, and the response by inserting ZnS Speed improvement was confirmed.

Moreover, by containing a reducing dopant in the electron transport layer, the response characteristics and the luminous efficiency can be further improved. In particular, by using a material having a work function of 2.9 eV or less, it is possible to realize an effect of improving response characteristics and luminous efficiency. Here, the reducing dopant is defined as a substance capable of reducing the electron transport material (electron transport compound). Therefore, various things can be used as long as it has a constant reducing property, for example, alkali metal, alkaline earth metal, rare earth metal, oxide of alkali metal, halide of alkali metal, oxide of alkaline earth metal, halide of alkaline earth metal At least one material selected from the group consisting of oxides of rare earth metals or halides of rare earth metals can be suitably used. Preferred reducing dopants include at least one alkali selected from the group consisting of Na (work function: 2.4 eV), K (work function: 2.3 eV), Rb (work function: 2.2 eV), Cs (work function: 2.0 eV). Metal, or at least one alkaline earth metal selected from the group consisting of Ca (work function: 2.9 eV), Sr (work function: 2.5 eV), and Ba (work function: 2.5 eV). By using a material having a work function of 2.9 eV or less for the dopant of the electron transport layer, the luminous efficiency and response speed are improved. Among these materials, the more preferred reducing dopant is at least one alkali metal selected from the group consisting of K, Rb and Cs, even more preferably Rb or Cs, most preferably Cs. In particular, these alkali metals have a high reducing ability and can improve the light emission luminance and increase the life of the OLED element by a relatively small amount of addition to the electron injection layer. In addition, as a reducing dopant having a work function of 2.9 eV or less, a combination of two or more of these alkali metals is also preferable, and particularly a combination containing Cs, for example, Cs and Na, Cs and K, Cs and Rb or Cs and Na and K It is preferable that it is a combination of. By including Cs in combination, the reduction ability can be efficiently exhibited, and the response characteristic and the luminous efficiency of the OLED element can be improved by doping the electron transport layer.

(Examples 9 and 10)

A glass substrate on which a transparent electrode made of ITO (Indium Tin 0xide) having a thickness of 32 mm x 25 mm x 1 mm was ultrasonically cleaned for 5 minutes in isopropyl alcohol, followed by UV ozone cleaning for 5 minutes. The glass substrate after washing | cleaning was fixed to the board | substrate holder, and the organic layer and the cathode were formed into a film using the vapor deposition machine of resistance line heating.

The organic layer and the cathode consist of α-NPD, a hole transport layer having a thickness of 50 nm, Alq ₃ doped with 0.5 mass% of rubrene, a ZnS doped with a light emitting layer having a thickness of 20 nm, and an alkali metal of 10 mass%. An electron transport layer consisting of 50 nm thick, an electron injection layer consisting of LiF, 0.4 nm thick, and a cathode 150 nm thick. Here, LiF (Example 9) and CsF (Example 10) were used as the alkali metal.

The response speed of the fabricated OLED device was evaluated. In evaluating the response speed, the frequency (blocking frequency) at which the output light intensity decreased by half when a sinusoidal wave voltage having an amplitude of 5 V and a bias voltage of 5 V was applied was measured. The cut-off frequency was 25 MHz in both OLED elements, and the response speed was improved by using ZnS doped with alkali metal in the electron transport layer.

(Examples 11-16)

In the OLED devices of Examples 9 and 10, except that alkali metals or alkaline earth metals doped in the electron transport layer were made to Na, K, Nb, Cs, Sr, and Ba, respectively, and produced as in Examples 9 and 10, It was set as the OLED element of Examples 11-16. In addition, the doping with Na was carried out in Example 11, the K doped in Example 12, the Nb doped in Example 13, the Cs doped in Example 14, the Sr doped in Example 15, and the Sr doped in Example 15 and Ba One is Example 16.

About the produced OLED element of Examples 11-16 and Example 1, response speed was evaluated by measuring a cutoff frequency like Example 1-10. The results are shown in Table 2.

Alkali metal or alkaline earth metal Cutoff Frequency (MHz) Example 1 - 20 Example 11 Na 24 Example 12 K 22 Example 13 Nb 25 Example 14 Cs 26 Example 15 Sr 24 Example 16 Ba 28

From Table 2, in the OLED device of Examples 11 to 16, the cutoff frequency was 22 MHz to 28 MHz, which was larger than the cutoff frequency of Example 1. Therefore, by using ZnS doped with Na, K, Nb, Cs, Sr, and Ba as the alkali metal or the alkaline earth metal, respectively, in the electron transport layer, an improvement in response speed could be realized.

(Example 17)

In the OLED devices of Examples 9 and 10, except that the alkali metal doped in the electron transport layer was set to Rb, it was produced in the same manner as in Examples 9 and 10 to obtain the OLED device of Example 17.

About the produced OLED element of Example 17, response speed was evaluated by measuring the cut-off frequency like Example 1-10. The results are shown in Table 3.

alkali Cutoff Frequency (MHz) Example 17 Rb 25

By using ZnS doped with Rb as the alkali metal in the electron transport layer, it was possible to realize an improvement in response speed.

As mentioned above, although preferred Example of this invention was described, this invention is not limited to these Examples. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit of the invention. The present invention is not limited by the above description, but only by the appended claims.

1 is a cross-sectional view showing an embodiment of the OLED device of the present invention.

2 is a graph showing frequency dependence of output light intensities of Example 1 and Comparative Example 1. FIG.

Claims (8)

In an organic light emitting diode device composed of at least an anode, a light emitting layer, an electron transport layer, a cathode, An organic light emitting diode device using a semiconductor material for the electron transport layer. The method of claim 1, An organic light emitting diode device in which the semiconductor used in the electron transport layer is a II-VI compound semiconductor. The method of claim 2, The organic light emitting diode device of which the Group II-VI compound semiconductor is ZnS. The method of claim 3, The organic light emitting diode device having a film thickness of the electron transport layer made of ZnS is in the range of 10nm to 300nm. The method according to any one of claims 1 to 4, An organic light emitting diode device in which a semiconductor material used in the electron transport layer contains a reducing dopant. The method of claim 5, The organic light emitting diode device of which the reducing dopant has a work function of 2.9 eV or less. The method of claim 6, The reducing dopant consists of an alkali metal, an alkaline earth metal, a rare earth metal, an oxide of an alkali metal, a halide of an alkali metal, an oxide of an alkaline earth metal, a halide of an alkaline earth metal, an oxide of a rare earth element, or a halide of a rare earth metal. 1 type or 2 or more types of organic light emitting diode element selected from the group. The optical communication module which uses the organic light emitting diode element in any one of Claims 1-7 as a light source for transmission.

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