JP3839616B2 - Surface modification method, organic thin film manufacturing method, and element substrate - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to the technology of display devices, and more particularly to the technical field of organic EL elements used in display devices.
[0002]
[Prior art]
When organic compounds are compared with inorganic compounds, organic compounds have more diverse reaction systems and characteristics, and surface treatment can be performed with low energy, so functional organic thin films have recently attracted attention.
[0003]
There are various types that use functional organic thin films, such as organic EL elements, piezoelectric sensors, pyroelectric sensors, and electrical insulating films. Of these, organic EL elements can be used as display panels. Attention has been paid.
[0004]
9 is a schematic configuration diagram of an organic EL element. On a glass substrate 110, an ITO (indium tin oxide) film (anode electrode film) 111, a hole injection layer 112, a hole A transport layer 113, a light emitting layer 114, an electron transport layer 115, an electron injection layer 116, and a cathode electrode film 117 are formed in this order.
[0005]
The hole injection layer 112 to the electron injection layer 116 are composed of organic thin films. When the organic EL element 101 is formed, first, the ITO thin film 111 is formed on the glass substrate 110 by a sputtering method, and then by a vapor deposition method. When the hole injection layer 112 to the electron injection layer 116 and the ITO thin film 111 are formed in this order, and finally a protective film (not shown) is formed, the organic EL element 101 is obtained.
[0006]
When a positive voltage is applied to the ITO thin film 111 and a negative voltage is applied to the cathode electrode film 117, holes and electrons are injected into the hole injection layer 112 and the electron injection layer 116, respectively. These holes and electrons are transported in the hole transport layer 113 and the electron transport layer 115, and when they reach the light emitting layer 114, the organic compound constituting the light emitting layer 114 emits light.
[0007]
Reference numeral 118 denotes light emitted from the light emitting layer 114. Since the ITO thin film 111 is transparent, the light 118 passes through the ITO thin film 111 and the glass substrate 110 and is emitted to the outside.
[0008]
The organic EL element as described above is required to be multicolored, highly efficient, and have a long lifetime for practical use. Among them, in order to increase the efficiency, it is considered that an organic material constituting the light emitting layer should be selected with a high light emitting efficiency.
[0009]
However, the conventional organic EL device has a problem that even if the luminance is high immediately after fabrication, the light emission efficiency is lowered due to a change with time, and the luminance gradually decreases.
[0010]
[Problems to be solved by the invention]
The present invention was created in order to solve the disadvantages of the prior art described above, and an object of the present invention is to provide an organic EL element with high brightness and long life.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, an invention according to claim 1 is an element that generates an oxygen plasma in a plasma generating device, emits ions in the plasma into a vacuum atmosphere, and is placed in the vacuum atmosphere. A surface modification method for modifying the surface of an ITO thin film exposed on the element substrate and exposing the element substrate, wherein a positive voltage is intermittently applied to the element substrate.
[0012]
According to a second aspect of the present invention, in the surface modification method according to the first aspect, a negative voltage is intermittently applied to the element substrate.
[0013]
The invention according to claim 3 is the surface modification method according to claim 2, wherein the positive voltage and the negative voltage are alternately applied.
[0014]
The invention according to claim 4 is the surface modification method according to claim 3, wherein the plasma generating device has a coil, an AC voltage is applied to the coil, and the gas introduced into the plasma generating device is turned into plasma. And the surface modification method characterized by making the frequency of the alternating voltage comprised by the said positive voltage and the said negative voltage lower than the frequency of the alternating voltage applied to the said coil.
[0015]
The invention according to claim 5 is a surface modification method, wherein the ITO thin film surface exposed on the surface of the element substrate is irradiated with ultraviolet rays in a vacuum atmosphere, and then is not exposed to the atmosphere. The surface modification method according to any one of the above is performed.
[0016]
A sixth aspect of the present invention is the surface modification method according to any one of the first to fifth aspects, wherein a rare gas and an oxygen gas are introduced into the plasma generator, and the rare gas is introduced. And after the generation of the mixed gas plasma of oxygen gas and the oxygen gas, the introduction of the rare gas is stopped to form the oxygen gas plasma.
[0017]
The invention according to claim 7 is an organic thin film manufacturing method, wherein after the surface modification method according to any one of claims 1 to 6 is performed, the surface of the ITO thin film is organically exposed without being exposed to the atmosphere. A thin film is formed.
[0018]
Invention according to claim 8, a glass substrate, and ITO thin film formed on the glass substrate, an element substrate having an organic thin film formed on the ITO thin film surface, the ITO thin film, wherein The surface modification method according to any one of Items 1 to 6 is performed.
[0019]
The inventors of the present invention pay attention to the fact that the work function of the ITO thin film formed by the prior art is −4.8 eV while the work function of the hole injection layer is −5.0 eV, and the organic EL element It was speculated that the cause of the low luminous efficiency was that the work function of the hole injection layer and the work function of the ITO thin film were large.
[0020]
When the work function of the hole injection layer is changed, the work function of the upper hole transport layer must also be changed. Therefore, in order to increase the luminous efficiency, the work function of the ITO thin film may be changed to be larger than or at least approximated to the work function of the hole injection layer.
[0021]
The present invention was created based on the above prediction, and in order to increase the work function of an oxide thin film such as an ITO thin film in the negative direction, an element substrate having an oxide thin film on its surface is placed in a vacuum atmosphere. Then, oxygen gas or ozone gas is introduced into the plasma generating apparatus, and an alternating current voltage is applied to a coil provided in the plasma generating apparatus to generate oxygen plasma.
[0022]
Since ions are generated in oxygen plasma, the ions, neutral molecules, and neutral atoms are released into a vacuum atmosphere, and an alternating voltage is applied to the element substrate on which the oxide thin film is formed. The work function is changed by making ions incident on the surface of the oxide thin film by cleaning the surface of the oxide thin film, repairing oxygen defects, and modifying the surface of the oxide thin film.
[0023]
Ions were incident on the ITO thin film formed on the glass substrate under different conditions, and the work function was measured.
First, the AC voltage (substrate bias voltage) applied between the element substrate and the plasma generation apparatus was changed. FIG. 3 shows the relationship between the applied AC voltage and the work function of the ITO thin film.
[0024]
In this surface modification, argon gas and oxygen gas are introduced into the plasma generator, an AC voltage is applied to the coil in the plasma generator, discharge starts, and argon and oxygen gas plasma is generated. Immediately, the introduction of argon gas was stopped, a plasma containing only oxygen was formed, and the ITO thin film was irradiated. No other gas is introduced into the vacuum chamber. The flow rate of oxygen gas introduced into the plasma generator was controlled so that the pressure in the vacuum chamber was 5 × 10 ⁻³ Torr during the oxygen plasma formation.
[0025]
The processing time is about 20 seconds, and the power supplied to the element substrate is 100 W. The thickness of the ITO thin film used is 2000 mm.
[0026]
In this graph, it can be seen that applying an AC voltage (substrate bias voltage) of about 20 V to the tissue plate is most effective. It can also be seen that the AC voltage applied to the element substrate is effectively in the range of 35V to 65V.
[0027]
Next, the work function was measured by fixing the voltage applied to the element substrate to 50 VAC and changing the processing time. Other conditions were set to be the same as those in FIG. The measurement results are shown in the graph of FIG.
[0028]
From this graph, it can be seen that the processing time is most effective when the processing time is about 20 seconds, and it is effective when the processing time is 10 seconds or more and 30 seconds or less.
[0029]
The processing time was set to 20 seconds, the oxygen gas introduction amount was changed, and the work function was measured. Other conditions were set to be the same as those in FIG. The measurement results are shown in the graph of FIG. The amount of oxygen gas introduced was indicated by the pressure (total pressure) in the vacuum chamber during processing.
[0030]
From this graph, it can be seen that the oxygen gas introduction amount (oxygen gas pressure in the vacuum chamber: total pressure) is most effective when it is 0.005 Torr, and is effective when it is 0.0025 Torr or more and 0.01 Torr or less.
[0031]
As described above, according to the present invention, the oxygen defects of the ITO thin film are repaired, and the work function which was -4.8 eV in the conventional ITO thin film can be reduced to about -5.5 eV. As a result, the work function of the ITO thin film becomes larger than the work function (−5.0 eV) of the hole injection layer, and holes are easily injected into the hole injection layer.
[0032]
When an AC voltage is applied to the element substrate to attract ions in the oxygen plasma, the oxygen plasma has a high O ⁻ activity and a high cleaning effect, so that the ground potential (the potential of the vacuum chamber or plasma generator) is high. It is preferable to apply a positive voltage to the element substrate side, draw O 2 ^- ions, and perform surface cleaning simultaneously with repair of oxygen defects. When the positive voltage is applied intermittently, the element substrate is not charged up.
[0033]
On the other hand, oxygen plasma contains O ⁺ ions, and this O ⁺ also has a cleaning effect. When attracting O ⁺ , a negative voltage may be applied to the element substrate. After all, it is appropriate to apply an alternating voltage to which positive and negative voltages are alternately applied to the element substrate.
[0034]
Since the AC voltage is for moving oxygen ions released into the vacuum atmosphere, the frequency should be low. While the plasma generator has a high frequency of 13.56 MHz, an AC voltage applied to the element substrate is suitably a low frequency of about 5 Hz to several kH.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
Reference numeral 1 in FIG. 1 is an example of an organic EL element manufacturing apparatus that can implement the method of the present invention.
The organic EL element manufacturing apparatus 1 includes a sputtering apparatus 2, a surface modification apparatus 3, an organic vapor deposition apparatus 4, and a metal vapor deposition apparatus 5.
A vacuum pump (not shown) is connected to each of the devices 2 to 5 and is configured so that it can be evacuated individually.
[0036]
First, each of the devices 2 to 5 is evacuated, a glass substrate to be processed is mounted in a loading chamber (not shown), and loaded into the sputtering apparatus 2 while maintaining a vacuum atmosphere.
[0037]
The sputtering apparatus 2 has a substrate holder 21 and a cathode 22, and a target 23 made of ITO is disposed on the cathode 22.
[0038]
The loaded glass substrate is held by the substrate holder 21 with the film formation surface facing the target 23. The code | symbol 12 of FIG. 1 has shown the glass substrate of the state. Sputtering of the target 23 is performed to form an oxide thin film made of an ITO thin film on the surface of the glass substrate 12, and then carried into the surface modification device 3 disposed in the subsequent stage.
[0039]
As shown in FIG. 2, the surface modification apparatus 3 has a vacuum chamber 30, a substrate holder 31 is disposed on the ceiling side of the vacuum chamber 30, and a plasma generation device 80 is disposed on the bottom side. Has been.
[0040]
Reference numeral 9 denotes an element substrate having a glass substrate 12 and an oxide thin film 13 made of an ITO thin film formed on the glass substrate 12. The element substrate 9 is disposed with the oxide thin film 13 facing the plasma generation device 80 side.
[0041]
The plasma generation device 80 includes a container 81 and a coil 83 provided around the container 80. A gas pipe 86 is connected to the container 81, and at least two kinds of gases can be introduced into the container 80 by operating valves 87 and 88. Here, argon gas and oxygen gas can be introduced.
[0042]
The glass substrate 12 is disposed in close contact with the substrate holder 31. The heater 9 (not shown) provided in the substrate holder 31 is energized, the element substrate 9 is heated to 215 ° C., and the oxide thin film 13 is annealed.
[0043]
A high frequency power supply 84 is connected to the coil 83 provided in the container 81, and after annealing, the valves 87 and 88 are opened, and argon gas and oxygen gas are introduced into the container 80 while controlling the flow rate. Here, the ratio of argon gas to oxygen gas was set to 2: 8 in flow rate ratio.
[0044]
Next, when the high frequency power supply 84 is activated and one end of the coil 83 is grounded and a high frequency voltage of 13.56 MHz is applied to the other end, the argon gas present in the container 81 is ionized and discharged in the container 81.
Once the discharge occurs, the oxygen gas is also ionized, so that a mixed plasma of argon gas and oxygen gas is formed in the container 81.
[0045]
Next, when the argon gas side valve 87 is closed and the introduction of the argon gas is stopped, the argon gas plasma is extinguished from the plasma of the mixed gas, and a plasma composed of oxygen is formed.
[0046]
On the other hand, a low-frequency power source 34 is connected to the substrate holder 31, and the vacuum chamber 30 and the plasma generator 80 are connected to the ground potential. When the low frequency power supply 34 is activated and an AC voltage of 50 Hz (an alternating voltage positive and negative with respect to the ground potential) is applied to the element substrate 9 through the substrate holder 31, the low frequency power supply 34 is reduced between the element substrate 9 and the plasma generation device 80. A frequency alternating voltage is applied.
[0047]
In the oxygen plasma, anions (O ⁻ ), cations (O ⁺ ), ozone (O ₃ ) and ions of oxygen gas (O ₂ ) are generated, and these ions, neutral molecules, When the active atoms are released into the vacuum chamber 30 from the opening 82 of the container 81, the neutral molecules and atoms are irradiated to the oxide thin film 13 without being affected by the AC voltage applied to the element substrate 9. The anions are irradiated to the oxide thin film 13 when a positive voltage is applied to the element substrate 9, and the cations are irradiated when a negative voltage is applied.
[0048]
When an organic substance adheres to the oxide thin film 13, it causes a reduction in luminous efficiency of the formed organic EL element. However, when an anion or a cation released from the container 81 enters the oxide thin film 13, The surface is cleaned.
[0049]
Further, the oxide thin film 13 becomes deficient in oxygen during annealing or cleaning, but when oxygen, ozone ions, neutral molecules, or neutral atoms are incident, the oxygen defects in the oxide thin film 13 are repaired. Is done.
[0050]
As described above, as a result of the modification of the surface of the oxide thin film, the work function which was -4.6 eV in the conventional ITO thin film becomes about -5.55 eV, and the work function of the hole injection layer becomes -5. It becomes larger in the negative direction than 0 eV, the interface barrier is lowered, and the hole injection efficiency is increased.
[0051]
After the surface modification of the oxide thin film 13 as described above is performed for a predetermined time, the element substrate 9 is carried into the organic vapor deposition apparatus 4. Here, when the organic thin film is laminated by one organic vapor deposition apparatus 4, the organic vapor deposition apparatus 4 has a plurality of organic vapor deposition sources (in FIG. 1, two organic vapor deposition sources 42 ₁ and 42 ₂ are shown. And the oxide thin film 13 side of the loaded element substrate 9 is held by the substrate holder 41 toward the organic vapor deposition sources 42 ₁ and 42 ₂ .
[0052]
A hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are formed in this order on the surface of the oxide thin film 13 and then carried into the metal vapor deposition apparatus 5.
[0053]
A metal vapor deposition source 52 in which a metal vapor deposition material 53 is housed is disposed in the metal vapor deposition apparatus 5, the electron injection layer is directed toward the metal vapor deposition source 52, and is disposed on the substrate holder 51. Evaporate to form an anode electrode film.
[0054]
The element substrate 9 on which the anode electrode film is formed is taken out from the organic EL element manufacturing apparatus 1 and carried into the protective film forming apparatus, and an organic EL element is obtained by forming a protective film on the anode electrode.
[0055]
The symbol L _{1 in} the graph of FIG. 6 is a graph showing the relationship between the elapsed time and the luminance when a voltage is applied to the organic EL element to emit light. Symbol L ₂ is a graph showing the relationship between the elapsed time and the luminance of the conventional organic EL element. In the case of the prior art, the word whose luminance has decreased since the start of light emission has a constant value, but it can be seen that the organic EL element of the present invention can maintain high luminance without a decrease in luminance.
[0056]
The sign L ₃ in FIG. 7, an organic EL device of the present invention, is a graph showing the relationship between the elapsed time and the applied voltage in the case where light is emitted by the brightness constant. The symbol L _{4 in} the figure is a graph in the case of a conventional organic EL element. It can be seen that the applied voltage is low in the organic EL element of the present invention.
[0057]
In the above embodiment, the case where the protective film forming apparatus is provided separately from the organic EL element manufacturing apparatus 1 has been described. However, when the organic EL element manufacturing apparatus having the protective film forming apparatus is used, the anode electrode film is formed. The element substrate is unloaded from the metal deposition device, loaded into the protective film forming device provided at the later stage, and processed from the formation of the ITO thin film to the formation of the protective film in a vacuum atmosphere consistently without exposure to the atmosphere. May be.
[0058]
On the other hand, when purchasing an element substrate on which an ITO thin film is formed to produce an organic EL element, an ultraviolet irradiation device is provided in place of the sputtering device 2 to irradiate the ITO thin film with ultraviolet rays in a vacuum atmosphere. After removing the organic matter adsorbed on the thin film surface as CO ₂ gas, it is preferable to carry it into the surface modification device 3 without exposing it to the atmosphere and to modify the surface of the ITO thin film.
[0059]
As described above, according to the present invention, the oxygen defects of the ITO thin film are repaired and the surface is cleaned. In particular, when the oxide thin film is an ITO thin film, the work function is higher than the work function of the hole injection layer. Increases in the negative direction, improving the hole injection efficiency and increasing the brightness.
[0060]
Moreover, since the organic compound adsorbed on the surface of the ITO thin film is decomposed and removed, there is little luminance deterioration.
[0061]
In the above embodiment, oxygen gas is introduced into the plasma generation apparatus 80 to generate oxygen plasma. However, oxygen gas may be generated by introducing ozone gas. Alternatively, oxygen plasma may be generated without using a rare gas such as argon gas.
[0062]
【The invention's effect】
An organic EL element with high brightness and long life can be obtained.
[Brief description of the drawings]
FIG. 1 shows an example of an organic EL element manufacturing apparatus that can perform the surface modification method of the present invention. FIG. 2 shows an example of a surface modification apparatus provided in the organic EL element manufacturing apparatus. FIG. 4 is a graph showing the relationship between the processing time and the work function value. FIG. 5 is a graph showing the relationship between the pressure and the work function value. FIG. 6 is a graph showing the relationship between the application time and the luminance change. [Figure 7] Graph showing the relationship between applied time and applied voltage [Figure 8] Structure of organic EL element [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Organic EL element manufacturing apparatus 2 ... Surface modification apparatus 9 ... Element substrate 12 ... Glass substrate 13 ... Oxide thin film (ITO thin film) 80 ... Plasma generator 83 ... Coil

Claims (8)

Generate oxygen plasma in the plasma generator,
Discharging ions in the plasma into a vacuum atmosphere;
A surface modification method for modifying the surface of an ITO thin film exposed on the element substrate by reaching the surface of the element substrate placed in the vacuum atmosphere,
A surface modification method comprising intermittently applying a positive voltage to the element substrate. The surface modification method according to claim 1, wherein a negative voltage is intermittently applied to the element substrate. The surface modification method according to claim 2, wherein the positive voltage and the negative voltage are alternately applied. The surface generation method according to claim 3, wherein the plasma generation device has a coil, an alternating voltage is applied to the coil, and the gas introduced into the plasma generation device is turned into plasma.
The surface modification method characterized by making the frequency of the alternating voltage comprised by the said positive voltage and the said negative voltage lower than the frequency of the alternating voltage applied to the said coil. The surface modification method according to any one of claims 1 to 4, wherein the surface of the ITO thin film exposed on the surface of the element substrate is irradiated with ultraviolet rays in a vacuum atmosphere, and then exposed to the atmosphere without performing exposure to the atmosphere. A surface modification method. Introducing rare gas and oxygen gas into the plasma generator, generating a plasma of a mixed gas of the rare gas and oxygen gas, then stopping the introduction of the rare gas, and plasma of the oxygen gas The surface modification method according to claim 1, wherein the surface modification method is formed. An organic thin film manufacturing method comprising: forming an organic thin film on the surface of an ITO thin film without performing exposure to the air after performing the surface modification method according to any one of claims 1 to 6. A glass substrate;
An ITO thin film formed on the glass substrate;
An element substrate having an organic thin film formed on the surface of the ITO thin film ,
7. The element substrate according to claim 1, wherein the ITO thin film is subjected to the surface modification method according to claim 1.

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