Patent Application
20120299482
This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2011-119459, filed May 27, 2011, the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a light emitting device, a display device, and a method of driving an organic electroluminescent device.
Recently, an organic electroluminescent device (hereinafter, also referred to as an organic EL device or an OLED) attracts attention for use in light emitting devices including illumination devices or flat surface light sources, and display devices. The organic electroluminescent device is configured by interposing a light emitting layer formed of organic material between a pair of electrodes including a cathode and an anode. When a voltage is applied to the device, electrons are injected from the cathode to the light emitting layer and holes are injected from the anode to the light emitting layer. In the light emitting layer, the holes and the electrons are recombined to generate excitons. When the excitons are radiatively deactivated, light is emitted.
In the manufacturing process and the like of the organic electroluminescent device, a foreign substance such as dust may be attached to a device panel. Foreign substances attached to the device panel may cause a leakage current, which may lead to defects and further to the device breakage. Therefore, preventing defects caused by the leakage current and preventing the lifetime of the device from being shortened by such defects are necessary.
According to an embodiment, there is provided a light emitting device including: an organic electroluminescent device that includes a first electrode, a light emitting layer arranged above the first electrode, and a second electrode arranged above the light emitting layer; a drive circuit that supplies a drive current between the first electrode and the second electrode to drive the organic electroluminescent device; and a drive stop circuit that stops driving of the organic electroluminescent device when a value of the drive current falls below a predetermined value.
Hereinafter embodiments will be described with reference to the drawings.
In this embodiment, the drive circuit for driving the OLED 10 monitors a current (drive current) for driving the OLED 10 and forcibly stops the driving when a value of the drive current falls below a predetermined value. Specifically, an emitter terminal of a transistor Q1 is connected to a positive terminal of the circuit power supply (Vcc). An anode of the OLED 10 is connected to a collector terminal of the transistor Q1. A cathode of the OLED 10 is connected to a collector terminal of a transistor Q2. An emitter terminal of the transistor Q2 is connected to a resistor R1. The other terminal of the resistor R1 is connected to a negative terminal of the circuit power supply (Vcc). An output terminal of an operational amplifier (differential amplifier) AMP 1 is connected to a base terminal of the transistor Q2. A non-inverting input terminal of the operational amplifier AMP1 is connected to resistors R2 and R3. An inverting input terminal of the operational amplifier AMP1 is connected to a non-inverting input terminal of an operational amplifier AMP2. An inverting input terminal of the operational amplifier AMP2 is connected to resistors R4 and R5. An output terminal of the operational amplifier AMP2 is connected to a base terminal of a transistor Q3. An emitter terminal of the transistor Q3 is connected to a base terminal of the transistor Q1 through a resistor. In the drive circuit illustrated in
Normally, when light is turned on, the operational amplifier AMP1 operates such that a reference voltage Vref2 input to the non-inverting input terminal of the operational amplifier AMP2 is the same as a voltage of the resistor R1 which detects a current flowing through the OLED 10. As a result, an output current of the transistor Q2 is controlled to be constant. The above-described operation is a normal operation as a constant current circuit, and the drive current is supplied between the terminals of the OLED 10.
As the circuit power supply, a battery such as a dry cell is assumed to be used, and a power supply voltage Vcc is gradually dropped by the discharge of the battery. Along with the drop of the power supply voltage Vcc, a collector-emitter voltage VCE of the transistor Q2 is also dropped gradually. Finally, the transistor Q2 enters the saturation region and is completely turned on. From this time, the drive circuit loses a current control function for voltage fluctuation and the like.
When the power supply voltage Vcc is further dropped, the current flowing through the OELD 10 also starts dropping and the current of the resistor R1 for detecting the output current of the OLED 10 is also dropped. The operational amplifier AMP2 monitors the current by monitoring the current of the resistor R1. That is, the voltage of the non-inverting input terminal of the operational amplifier AMP2 is equal to that of the resistor R1, and the voltage of the inverting input terminal of the operational amplifier AMP2 is equal to a setting reference voltage Vref1. In addition, the setting reference voltage Vref1 is set in advance according to a predetermined lower limit current. This lower limit current corresponds to a lower limit value of the current which is allowed to flow through the OLED 10, that is, a lower limit value of the drive current. The setting of the lower limit value of the drive current will be described below in detail.
The operational amplifier AMP2 outputs a High or Low (ON or OFF) signal according to the difference in potential between the non-inverting input terminal and the inverting input terminal. From the time when the voltage of the resistor R1 falls below the setting reference voltage Vref1, the operational amplifier AMP2 outputs the Low signal. As a result, the transistor Q3 is turned off, and the transistor Q1 is turned off. Therefore, the entire drive circuit is turned off and the driving of the OLED 10 is stopped.
The configuration illustrated in
Examples of the OLED 10 will be described. In Example 1-1 below, the OLED 10 is configured as an OLED panel using a single organic electroluminescent device. In Example 1-2, the OLED 10 is configured as an OLED panel in which plural organic electroluminescent devices are connected to each other in series.
In this example, alkali-free glass was used for the device substrate 20. Specific material examples of the device substrate 20 include transparent or semi-transparent fused quartz, transparent glass such as alkali or alkali-free glass, polyethylene terephthalate, polycarbonate, polymethyl methacrylate, polypropylene, polyethylene, polymer film formed of transparent resin such as amorphous polyolefin or fluororesin, and transparent ceramics. A glass substrate, a plastic substrate or the like with a high refractive index of about 1.6 to 1.9 may be used.
A 110 nm-thick indium tin oxide (ITO) layer (first electrode) 21 was formed above the device substrate 20. Furthermore, a 50 nm-thick molybdenum (Mo) layer, a 600 nm-thick aluminum (Al) layer, and a 50 nm-thick Mo layer were laminated to form a film (Mo—Al—Mo layer 28). Here, a thickness of the ITO layer 21 is determined by the design of a resistance value, a flatness, and a light extraction method, and varies depending on device design. In addition, the thicknesses of the respective layers of the Mo—Al—Mo layer 28 vary depending on device design. Here, Mo, Al, and Mo are used for the layer 28, but alloys of Mo and Nb, Al and Nd, and Mo and Nb may be used. That is, an alloy of molybdenum and niobium may be used instead of molybdenum and an alloy of aluminum and neodymium may be used instead of aluminum. Alternatively, instead of the layer 28 of molybdenum, aluminum, and molybdenum, a layer of titanium, aluminum, and titanium (Ti—Al—Ti layer) may be used. The layer 28 may not be formed by laminating layers.
Between the device substrate 20 and the'ITO layer 21, a barrier layer such as a silicon oxynitride (SiN) layer or a layer improving light extraction efficiency by scattering, refraction, diffraction and the like may be formed. Furthermore, the ITO layer 21 and the Mo—Al—Mo layer 28 may be laminated reversely.
The Mo—Al—Mo layer 28 and the ITO layer 21 were patterned by photolithography. Next, an insulating layer 22 is formed with a novolac-based positive resist. Materials of the insulating layer used herein preferably has a low-moisture adsorption and a strong tolerance to a pretreatment, such as oxygen plasma treatment, of organic EL deposition, and may be other resist, polyimide, acryl, or the like.
After UV-ozone cleaning, an organic layer 23 was formed and then a 150 nm-thick Al cathode 24 (second electrode) was formed by vapor deposition. Here, before the deposition, the insulating layer 22 may be annealed in vacuum or in a glove box under nitrogen atmosphere to evaporate moisture adsorbed to the insulating layer 22. Furthermore, before the deposition, oxygen plasma treatment or argon plasma treatment may be performed. When oxygen plasma treatment or argon plasma treatment is performed, UV-ozone cleaning treatment may be omitted.
After the deposition of the Al cathode 24, the layer was transported to a glove box under nitrogen atmosphere so as not to be exposed to the air and was sealed by bonding the layer with a countersunk glass 26. As a seal material 27, a UV curable resin was used. The sealing method may be any other method, for example, sealing which uses a SiN layer or the like, or Frit sealing.
In
Although not illustrated in the drawing, in order to increase light extraction efficiency, a microlens sheet (a φ30 μm-thick microlens array manufactured by Optmate Corporation) was bonded to a surface, which is opposite to the surface provided with the organic layer 23, of the device substrate 20. The light extraction sheet may use scattering or the like. In addition, a glass substrate may be processed directly instead of bonding the light extraction sheet.
The pad portions 28 and 29 of the first and second electrodes illustrated in
The present inventors found that leakage occurs when the OLED is driven at a low current. Hereinafter, a problem of leakage caused when the OLED 10 is driven at a low current will be described.
In this embodiment, the following is assumed for the deterioration of the OLED caused by low-current driving. That is, in a low-current (low-voltage) area, when there is a path of the same order of leakage current as that of a current required when light is emitted, a voltage is not applied to the organic electroluminescent device and only the leakage current flows. On the other hand, when the OLED is driven by a current (voltage) equal to or higher than a predetermined value, a leakage current path is burned out or a voltage is applied to the entire device for some reasons. As a result, the leakage disappears and light starts emitting.
When only a leakage current flows during low-current driving, a portion where there is a foreign substance causing the leakage current generates heat. As a result, there is a possibility that the device may be broken.
In this embodiment, a model in which an OLED deteriorates during the above-described low-current driving is assumed. Specific test results (which will be described below) performed by the present inventors found a phenomenon that a leakage easily occurs when an organic electroluminescent device is driven at a low current equal to or lower than 30 mA. When a foreign substance attached to an organic layer causes a leakage as described above, such a phenomenon may be avoidable if the foreign substance can be completely removed in the process of manufacturing the OLED panel. However, when a large-size OLED panel, for example, is manufactured in low-cost facility such as a clean room with a relatively low air cleanliness, removing the foreign substance or the like completely is difficult.
As a countermeasure for preventing such a device breakage, the OLED is driven by a current (voltage) equal to or higher than a predetermined value. In the configuration illustrated in
The definition of driving the OLED by a voltage (current) equal to or higher than a predetermined value will be described with reference to results of measuring device characteristics.
On the other hand, when the voltage is swept from +10 V to −5 V as illustrated in
When the driving is continued at a low voltage in which light is not emitted or low-luminance light is emitted, the current continuously flows through the leakage portion. By doing so, the temperature of the leakage portion rises due to Joule heat, which changes characteristics of the periphery organic layer and the like. As a result, the leakage portion is changed to be irreversible. The organic EL device has a large area, and in particular, has a large size of several square centimeters to several ten square centimeters for illumination. If there is even a single leakage portion in such a large-size device as described above, the device becomes a defective product and the yield ratio is significantly deteriorated. In addition, when countermeasures, for example, increasing the air cleanliness of a clean room, are taken in order to reduce a foreign substance or dust causing the defect, the manufacture cost is increased.
On the other hand, in this embodiment, the driving of the device is avoided at a low voltage in which light is not emitted or low-luminance light is emitted as much as possible, and can be performed in a normal operation range. Therefore, even a device having some foreign substances or dust can be used as a good product. Accordingly, the leakage is reduced while the manufacture cost is suppressed and thus the yield ratio may be greatly improved.
This embodiment focuses attention on the rate of the drive current rise (and (or) drive current fall) as well as on determining the low limit of the drive current of the OLED to forcibly stop the driving. That is, a time required for the drive current, which drives the OLED through the drive circuit, to rise to a predetermined current value (in this embodiment, 30 mA or more) is preferably at least within 2 milliseconds. Likewise, when light is turned off, a time required for the same drive current to fall from the predetermined current value (in this embodiment, 30 mA or more) to zero is preferably within at least 2 milliseconds. In such a configuration, the leakage which is caused by driving the OLED at a low current is suppressed, thereby preventing the device breakage. The rise and fall time is not necessarily 2 milliseconds exactly and may have some variations. Although also depending on costs of selected parts, a configuration in which the drive circuit operates within a high speed of about 2 milliseconds is preferable. In the configuration of this embodiment illustrated in
Generally, in illumination devices of the related art, the rise and fall time is relatively long. That is, it is preferable that the illumination is gradually turned on or off after a given time. In addition, while considering the manufacture cost, low-cost parts can be selected for the drive circuit and the like of the illumination device as much as possible. In this case, the rise time (fall time) of a constant current circuit is generally long.
Hereinafter, using a semi-transparent panel with a diameter of 150 mm which is configured by the serially connected devices as described in Example 1-2, results of a test of turning light on and off will be shown. The test time was 10 minutes, and the test of ON (1 second)-OFF (1 second) was performed. The luminance was measured by BM-7 with a view angle of 2.0. The luminance [cd/m2] when the drive current [mA] is changed, the actual luminance [cd/m], and the pass-fail are shown in the following tables. Here, the rise time of the drive current is 2 milliseconds. When the drive current is a low current of 5.8 mA and 22.6 mA, the leakage current occurs (a pass is indicated by a symbol O and a fail is indicated by a symbol X).
As an application example of the organic electroluminescent device, display devices are used in addition to the above-described light emitting device.
A display device 80 illustrated in
A drive current for driving the organic electroluminescent device 85 is monitored by the signal line drive circuit 82, the control line drive circuit 83, and (or) the controller 84. When the drive current falls below a given current (for example, 30 mA as in the first embodiment), the driving of the organic electroluminescent device 85 is stopped.
A leakage caused when the display device 80 is driven at a low current along with the voltage drop of a battery for driving the display device, can be prevented. Similarly to the case of the first embodiment, in this embodiment, the rise time and fall time of the drive current is preferably within 2 milliseconds.
According to the above-described embodiments and examples, defects caused by a leakage current can be suppressed. As a result, a light emitting device, a display device, and a method of driving an organic electroluminescent device which prevents the lifetime of the device from being shortened can be provided.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-119459 | May 2011 | JP | national |
