Abstract:
According to one aspect of the present invention, it is possible to sufficiently perform the discharging of charge without lowering the light emitting efficiency of an organic EL device and hence, the device can exhibit the light emitting efficiency higher than a conventional organic EL device and, at the same time, can prevent the degradation of the device. As an organic EL device to which the present invention is applied, on a glass transparent substrate, a transparent electrode, a hole injection layer and a hole transport layer which function as a hole transport function layer, a light emitting layer, an electron transport function layer, and a metal electrode are formed sequentially, and a drive power sources are connected to the transparent electrode and the metal electrode. Further, from the drive power source, as an applying voltage, a voltage which is obtained by overlapping any one of a sine wave, a pulse wave, a triangle wave and a sawtooth wave having two cycles or more to a drive signal or a voltage which is obtained by overlapping a sine wave having two cycles or more to the drive signal is supplied.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The disclosure of Japanese Patent Application No. P2005-058442 filed on 2005/03/03 (yyyy/mm/dd) including the claims, the specification, the drawings and the abstract is incorporated herein by reference in its entirety.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of driving an organic EL device and a display device which can prolong a lifetime of luminance, can prevent the elevation of a drive voltage or the increase of a drive current at the time of driving of the organic EL device with a fixed current.  
         [0004]     2. Description of Related Art  
         [0005]     An organic EL device, in general, has the structure in which an organic thin film containing a light emitting layer is sandwiched by an anode and a cathode, and by applying a DC voltage to the organic EL film, holes are injected from the anode and electrons are injected from the cathode thus emitting light. When the balance of electrons and holes is collapsed due to the influence of the charge transfer and the energy barrier of the materials which constitute these layers, the charge stored stage continues. Due to a portion of the charge stored in the organic thin film, the organic material is degenerated or the structure of the organic layer is changed. Such degeneration of the organic material and change of the structure of the organic layer has been one of causes of the deterioration of the organic EL device.  
         [0006]     In Japanese Laid-open publication 2000-30862 (patent document 1), there exists a description that by driving a single-layered or stacked-layered organic EL device by applying a sinusoidal AC voltage between an anode and a cathode and by periodically changing the voltage applied to the device so as to periodically repeating an ON state (emission of light) and an OFF state (non-emission of light) of the device, the deterioration is recovered at the OFF state thus prolonging a driving lifetime.  
         [0007]     In Japanese Laid-open publication 2000-36383 (patent document 2), there exists a description that by driving a single-layered or stacked-layered organic EL device by applying a pulse voltage of frequency of 5 kHz or more between an anode and a cathode and by setting the frequency at the time of performing this pulse driving to 5 kHz or more, a recovery effect of the deterioration at the time of OFF state is increased and hence, the deterioration of the device is suppressed.  
       SUMMARY  
       [0008]     In the above-mentioned patent document 1, to obtain the same luminance also with respect to the DC voltage driving, it is necessary to increase the applied voltage thus lowering the efficiency of emission of light. Further, a large amount of current flows momentarily and a larger quantity of charge is stored and hence, a deterioration prevention effect is low.  
         [0009]     In the above-mentioned patent document 2, the deterioration prevention effect is increased by the number of times that the voltage of pulse wave is off. That is, the patent document 2 does not refer to the inverse bias voltage or the like when the light emitting signal is off and hence, there may be a case that the discharging of the stored charge is not sufficient.  
         [0010]     Accordingly, the present invention focuses on a method of applying an AC voltage as a means to apply the inverse bias to the organic EL device for discharging the stored charge.  
         [0011]     That is, as the main reason of the deterioration of the organic EL device, when the organic EL device is driven with the voltage in the forward direction, there exist drawbacks that a driving force is increased due to the storing of charge in a short period and the degradation and the deterioration of the organic material which forms the device are generated due to the stored charge in a long period thus lowering the luminance.  
         [0012]     Accordingly, as means to prevent such drawbacks, to discharge the stored charge, a positive and negative voltage in turn which is smaller than an absolute value of a light emitting start voltage (hereinafter referred to as “built-in-voltage”) of the organic EL device may be applied or overlapped to the drive signal during a period in which the drive signal is off. Here, the drive signal is a voltage which is applied between the anode and the cathode and drives the organic EL element.  
         [0013]     Further, with respect to the applied or overlapped voltage, after examining the voltage dependency of the organic EL device on the electrostatic capacitance, by applying the voltage (which is smaller than built-in-voltage) which generates a peak electrostatic capacitance, it is possible to perform the efficient discharging of the stored charge. Here, the applied or overlapped voltage is a voltage which is smaller than the built-in-voltage and hence, light is not emitted.  
         [0014]     By setting the frequency of the applied voltage smaller than the frequency which corresponds to a response speed of the device and by setting the frequency to a frequency which assumes two cycles or more during time in which the drive signal is off, it is possible to perform the efficient discharge of a carrier.  
         [0015]     These operations are performed irrespective of the structure and the material of the device.  
         [0016]     In this manner, according to the present invention, in addition to the drive signal, by applying the voltage which is smaller than the absolute value of the built-in-voltage between devices, by applying the positive and negative signal in turn which is equal to the absolute value of the voltage at which the device assumes the maximum electrostatic capacitance, by applying the frequency which is smaller than the frequency corresponding to the response speed of the device and takes two cycles or more during the time that the drive signal is off or by applying a voltage waveform which is obtained by a plurality of these application modes, it is possible to sufficiently discharge the charge without lowering the light emitting efficiency of the organic EL device thus providing the organic EL device which can prevent the deterioration thereof while exhibiting the high light emitting efficiency.  
         [0017]     That is, by applying the voltages of positive and negative signals when the emission of light (drive signal) of the organic EL device is off, the reverse potential is generated and hence, the stored charge can be discharged whereby the deterioration of the organic layer can be suppressed.  
         [0018]     That is, as a cause of the deterioration of the organic layer, in an experiment which injects only the electrons and the holes, when the balance between the electron and the hole in the organic layer collapses, the increase of the resistance value or the lowering of the luminance is observed. Accordingly, it is considered that the cause of the deterioration of the organic layer attributes to the presence of the extra charge (stored charge) in the organic layer.  
         [0019]     It is considered that the storage of charges is generated on an interface due to the energy barrier between the organic layers and hence, when the storage of the charge is generated on all respective interfaces, the storage quantity of the charge becomes largest.  
         [0020]     Further, the electrostatic capacitance of the device is inversely proportional to the film thickness and when the electrostatic capacitance of the device is measured while gradually increasing the voltage, the charge is gradually injected by getting over the energy barrier and hence, an effective film thickness is decreased whereby the electrostatic capacitance of the device is increased.  
         [0021]     Since the storage of the charge becomes largest at the voltage whose electrostatic capacitance is maximum, by applying the same voltage in the reverse direction, the stored charge can be discharged.  
         [0022]     Here, when the voltage having the electrostatic capacitance which exceeds the maximum value is applied, the storage of undesired charge and the undesired emission of light are generated, while when the voltage having the electrostatic capacitance which is below the maximum value is applied, the discharging of the stored charge becomes insufficient. Accordingly, due to the application of the voltage of the maximum electrostatic capacitance, it is possible to prevent the discharging of the stored charge.  
         [0023]     Further, since the stored charge is discharged during the time that the emission of light (drive signal) is off, it is necessary to turn the applied voltage into the inverse bias, and the larger the number of turning the applied voltage into the inverse bias, the charge can be discharged effectively and hence, it is desirable to adopt the frequency which sets the number of the inverse bias at least two cycles or more during the time that the emission of light (drive signal) is off.  
         [0024]     Here, when the cycle of the applied AC voltage is shorter (the frequency of the applied voltage is larger) than the response time of the organic EL device, the movement of the charge cannot follow the change of the applied voltage and hence, the discharging of the stored charge becomes insufficient. Accordingly, although the response time differs depending on the structure of the device, according to an experiment on the transitional responsiveness, the response time is approximately 10 −8  to 10 −7  seconds and hence, it is desirable to set the frequency of the applied AC voltage to 10 MHz or less.  
         [0025]     As described above, when the organic EL device is driven, it is possible to suppress the deterioration of the organic layer, the elevation of voltage (the lowering of movement of the charge, the lowering of the charge injection efficiency due to the degeneration of the respective organic layer and the electrode interface).  
         [0026]     In this manner, it is possible to suppress the deterioration of the organic layer attributed to the storage of the charge and hence, it is possible to suppress the lowering of the luminance of the organic EL device and the elevation of the drive voltage. Since the lowering of the luminance and the elevation of the voltage can be suppressed, it is possible to enhance the lifetime of the device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a schematic structural cross-sectional view of an organic EL device according to the present invention;  
         [0028]      FIG. 2  is a voltage-current characteristic diagram of the organic EL device;  
         [0029]      FIG. 3  is a waveform diagram of an applied voltage;  
         [0030]      FIG. 4  is a waveform diagram of an applied voltage;  
         [0031]      FIG. 5  is a view showing an experimental method of a charge balance;  
         [0032]      FIG. 6  is a PL intensity change diagram due to the injection of charge;  
         [0033]      FIG. 7  is a view showing the storage of the charge due to the voltage;  
         [0034]      FIG. 8  is a view showing the relationship between the voltage and the electrostatic capacitance;  
         [0035]      FIG. 9  is a view expressing a response speed of the organic EL device;  
         [0036]      FIG. 10  is a schematic view of the display device using the organic EL device according to the present invention; and  
         [0037]      FIG. 11  is a table showing driving conditions of respective experiments and respective measuring effects. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     Hereinafter, embodiments of the present invention are explained in conjunction with drawings.  
       Embodiment 1  
       [0039]     Hereinafter, an organic EL device according to the present invention can selectively use known materials and, at the same time, can properly adopt the known structure.  
         [0040]     Here, first of all, experimental examples on the storage of charge, the electrostatic capacitance and the response speed of the organic EL device are explained.  
         [0041]      FIG. 5  is a view showing an experimental method of charge balancing and shows the cross-sectional structure of the device in which an organic layer is sandwiched between two electrodes and a state in which a voltage is applied to the electrodes using a drive power source.  FIG. 6  shows a PL intensity change diagram due to the injection of electrons and holes, that is, the injection of the charge.  FIG. 7A  and  FIG. 7B  are views which schematically show the charge stored states for every magnitudes of applied voltage, wherein  FIG. 7A  schematically shows the charge stored state when a voltage which is sufficiently lower than a voltage with which the electrostatic capacitance becomes maximum is applied and  FIG. 7B  schematically shows the charge stored state when a voltage which is sufficiently lower than a voltage with which the electrostatic capacitance becomes maximum is applied.  
         [0042]     As shown in  FIG. 5 , a device having the structure in which a dielectric layer  52  is formed on one surface of an organic layer  51  and the dielectric layer  52  is sandwiched by electrodes  53 ,  54  is prepared and an experiment which applies a DC voltage from a drive power source  55  is performed. Here, by changing the polarity of the applied voltage, it is possible to inject only electrons or holes to the inside of the organic layer  51  from the organic layer  51  side with which the electrode  53  is directly brought into contact. The reason that only the electrons and the holes can be injected is attributed to the presence of the dielectric layer  52  on one surface of the organic layer  51 .  
         [0043]     By performing a comparison of a phosphor intensity (PL intensity) before and after the above-mentioned injection of the charge, it is possible to confirm whether the change of the phosphor intensity attributed to the storage of charge is generated or not. When this experiment is performed using the device having the structure in which known CuPc (copper phthalocyanine), α-NPD (α-naphthyl phenyl diamine) and Alq3 (tris(8-quinolinol) aluminum are sequentially stored as an organic layer, as shown in  FIG. 6 , although the phosphor intensity is not changed substantially due to the injection of electrons, while the phosphor intensity is largely lowered due to the injection of holes. From this result, it is understood that, in this device structure, the phosphor intensity is degraded in the excessive hole state.  
         [0044]     Next, as shown in  FIG. 7A  and  FIG. 7B , when the applied voltage  54  is gradually increased, the charged stored state is changed from a state shown in  FIG. 7A  to a state shown in  FIG. 7B  and the electrons are injected by getting over an energy barrier between the respective layers. Here, a portion of the charge is stored in an interface. Then, a maximum amount of charge is stored immediately before the starting of the emission of light. Since an effective film thickness of the electrostatic capacitance becomes minimum, the electrostatic capacitance becomes maximum.  
         [0045]     With respect to the device which is constituted by sequentially stacking CuPc, α-NPD and Alq3, a device  3  (OLED3) which is constituted of CuPc having a film thickness of 40 nm, α-NPD having a film thickness of 40 nm and Alq3 having a film thickness of 40 nm, a device  4  (OLED4) which is constituted of CuPc having a film thickness of 40 nm, α-NPD having a film thickness of 80 nm and Alq3 having a film thickness of 40 nm, and a device  5  (OLED5) which is constituted of CuPc having a film thickness of 40 nm, α-NPD having a film thickness of 80 nm and Alq3 having a film thickness of 80 nm are prepared, and the electrostatic capacitances of these devices are measured by changing the voltage.  FIG. 8  shows a result of the measurement. As shown in  FIG. 8 , the change of the electrostatic capacitance differs depending on the film thickness of the constituting layers. However, the energy barriers of the respective layers are equal and hence, the voltages which generate the maximum electrostatic capacitance are substantially equal.  
         [0046]     Next, although the response time differs due to the structure of the organic EL device, according to a result of an experiment on the transitional responsiveness, as shown in  FIG. 9 , the response time is approximately 10 −8  to 10 −7  seconds. Accordingly, it is preferable that the frequency of the applied voltage  54  at the AC current is 10 MHz or below.  
         [0047]     In view of the above-mentioned experimental result, this embodiment is explained hereinafter.  
         [0048]      FIG. 1  is a view showing the structure of an organic EL device, wherein an ITO film is formed on a glass transparent substrate  1  by sputtering and, thereafter, the patterning for forming lines and electrodes is performed so as to form a transparent electrode  2  which constitutes an anode.  
         [0049]     On this transparent electrode  2 , CuPc which constitutes a hole injection layer  31  and α-NPD which constitutes a hole transport layer  32  are formed as first and second hole transport function layers  3 .  
         [0050]     Next, Alq3 which constitutes a host material of a light emitting layer  4 , known TPB (tetra phenyl butadiene) which constitutes a dopant material of the light emitting layer  4 , Alq3 which constitutes an electron transport function layer  5 , and lithium fluoride or aluminum which constitutes a metal electrode  6  as a cathode are sequentially formed by a vapor deposition method in this order.  
         [0051]     To drive the organic EL device formed in this manner, a drive power source  7  is connected to the transparent electrode  2  and the metal electrode  6 , and a voltage supplied from the drive power source  7  is applied to the organic EL device.  
         [0052]     Here, an organic EL device which respectively sets a film thicknesses of the hole injection layer (CuPc), the hole transport layer (α-NPD) and the light emitting layer (Alq3+TPB), and the electronic transport function layer (Alq3) in the organic layer to 40 nm, 40 nm, 40 nm, 40 nm is used as an “organic EL device  1 ” or an “organic electroluminescent device  1  (OLED1), and an organic EL device which respectively sets a film thicknesses of the hole injection layer (CuPc), the hole transport layer (α-NPD) and the light emitting layer (Alq3+TPB), and the electronic transport function layer (Alq3) in the organic layer to 40 nm, 40 nm, 80 nm, 40 nm is used as an “organic EL device  2 ” or an “organic electroluminescent device  2  (OLEDD2).  
         [0053]      FIG. 2  is a voltage-current characteristic diagram of these organic EL devices. As shown in the drawing, with respect to both organic EL devices, an electric current does not flow in a range of the applied voltage from minus voltage to 4V and the electric current starts flowing at the voltage of 4V or more and the emission of light starts. That is, the built-in-voltage is 4V.  
         [0054]     Here, the voltage (Vmc, see  FIG. 3  and  FIG. 4 ) which generates the maximum electrostatic capacitance of the organic material is 3.8V. Accordingly, the applied positive and negative voltages are set to ±3.8V.  
         [0055]     Waveforms of the applied voltage are shown in  FIG. 3  and  FIG. 4 . Symbol  3 E in  FIG. 3  is a waveform referred to as a drive signal which is a voltage for controlling the turning ON and OFF of an organic EL element. In this embodiment, the waveform  3 E is a square waveform.  
         [0056]     Symbols  3 A to  3 D in  FIG. 3  and symbols  4 A to  4 C in  FIG. 4  are waveform charts in each of which a given waveform is overlapped to the drive signal only during an OFF state or the given waveform is overlapped to the drive signal in both of the ON period and the OFF period.  
         [0057]     Symbol  3 A in  FIG. 3  is the waveform chart in which a sine wave  1  is overlapped to the drive signal during the OFF period.  
         [0058]     Symbol  3 B in  FIG. 3  is the waveform chart in which a pulse wave is overlapped to the drive signal during the OFF period.  
         [0059]     Symbol  3 C in  FIG. 3  is the waveform chart in which a sine wave  2  which is a limited peak voltage is overlapped to the drive signal during the OFF period.  
         [0060]     Symbol  3 D in  FIG. 3  is the waveform chart in which a triangle wave is overlapped to the drive signal during the OFF period.  
         [0061]     Symbol  4 A in  FIG. 4  is the waveform chart in which a sawtooth wave  1  is overlapped to the drive signal during the OFF period.  
         [0062]     Symbol  4 B in  FIG. 4  is the waveform chart in which a sawtooth wave  2  which has a phase opposite to a phase of the sawtooth wave  1  is overlapped to the drive signal during the OFF period.  
         [0063]     Symbol  4 C in  FIG. 4  is the waveform chart in which a sine wave  3  is overlapped to the drive signal not only during the OFF period but also during the ON period.  
         [0064]     This embodiment can adopt any one of these waveforms.  
         [0065]     In these waveforms, when the drive signal (a) is off, any one of the periodical sine wave, pulse wave, triangle wave and sawtooth wave is applied during two cycles or more.  
         [0066]     In experimental examples described hereinafter, a DC voltage of the drive power source  7  is adjusted such that the luminance assumes 1000 cd/m 2  with respect to the above-mentioned organic EL device.  
       EXPERIMENTAL EXAMPLE 1  
       [0067]     In the above-mentioned organic EL device  1 , when the sine wave voltage is set to 3.8V and the frequency is set to 1000 HZ, the peak current is 17 mA/Cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3600 h.  
       EXPERIMENTAL EXAMPLE 2  
       [0068]     In the above-mentioned organic EL device  1 , when the triangle wave voltage is set to ±3.8V and the frequency is set to 1000 Hz, the peak current is 15 mA/cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3700 h.  
       EXPERIMENTAL EXAMPLE 3  
       [0069]     In the above-mentioned organic EL device  1 , when the pulse wave voltage is set to ±3.8V and the frequency is set to 1000 Hz, the peak current is 16 mA/cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3500 h.  
       EXPERIMENTAL EXAMPLE 4  
       [0070]     In the above-mentioned organic EL device  1 , when the sawtooth wave voltage is set to ±3.8V and the frequency is set to 1000 Hz, the peak current is 14 mA/cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3400 h.  
       EXPERIMENTAL EXAMPLE 5  
       [0071]     In the above-mentioned organic EL device  1 , when the organic EL device  1  is driven by overlapping the sine wave to the DC current corresponding to the light emitting signal, the sine wave voltage is set to ±3.8V and the frequency is set to 1000 Hz, the peak current is 24 mA/cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3300 h.  
       EXPERIMENTAL EXAMPLE 6  
       [0072]     In the above-mentioned organic EL device  2 , when the sine wave voltage is set to 3.8V and the frequency is set to 1000 Hz, the peak current is 21 mA/cm 2 . When the organic EL device  2  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 3100 h.  
       EXPERIMENTAL EXAMPLE 7  
       [0073]     In the above-mentioned organic EL device  1 , when only the DC voltage is applied to the organic EL device  1  and the DC voltage is adjusted to set the luminance to 1000 cd/m 2 , the peak current is 15 mA/cm 2 . When the organic EL device  1  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 2100 h.  
       EXPERIMENTAL EXAMPLE 8  
       [0074]     In the above-mentioned organic EL device  2 , when only the DC voltage is applied to the organic EL device  2  and the DC voltage is adjusted to set the luminance to 1000 cd/m 2 , the peak current is 22 mA/cm 2 . When the organic EL device  2  is driven by controlling the DC voltage such that the current value always assumes a fixed value, the luminance half-life time is 1700 h.  
         [0075]     To sum up these relationships, it is possible to obtain a result shown in a table in  FIG. 11 .  
         [0076]     In this manner, in the driving of the organic EL device, by applying the positive and negative signal in turn corresponding to the voltage which provides the maximum electrostatic capacitance value of the device in addition to the drive signal, the life time property of the organic EL device can be enhanced.  
       Embodiment 2  
       [0077]     In  FIG. 10A  and  FIG. 10B ,  FIG. 10A  is a schematic view of an active matrix display device which uses the organic EL device according to the present invention, and  FIG. 10B  is an enlarged view of a pixel portion  300  shown in  FIG. 10A .  
         [0078]     In  FIG. 10A , in response to a scanning line  101  which is selected by a scanning line driving circuit  100 , a data signal is supplied to the pixel portion  300  of a display panel  400  from the data line driving circuit  200  by way of a data line  201 . To the pixel portion  300 , an applying voltage which is formed by adding the sine wave, the pulse wave, the triangle wave or the sawtooth wave to the drive signal is supplied from a drive power source  500  by way of a driving line  501 . Here, a common electrode  502  of the drive power source  500  is connected to a common electrode of the display panel  400 .  
         [0079]     In  FIG. 10B , a first thin film transistor  10  is provided to an intersection of the scanning line  101  and the data line  201 , the scanning line  101  is connected to a gate electrode  11  of the first thin film transistor  10 , and the data line  201  is connected to a source electrode (or a drain electrode)  12  of the first thin film transistor  10 , and one electrode of a holding capacitance  20  which temporarily holds the data signal is connected to the drain electrode (or the source electrode)  13  of the first thin film transistor  10 . Further, the drain electrode  13  of the first thin film transistor  10  is connected to the gate electrode  31  of the second thin film transistor  30 .  
         [0080]     To a source electrode (or a drain electrode)  32  of the second thin film transistor  30 , a driving line  501  is connected, while to the drain electrode (or the source electrode)  33  of the second thin film transistor  30 , one electrode of an organic EL device  40  is connected. Another electrode of the organic EL device  40  is connected to a common electrode  502  together with another electrode of the holding capacitance  20 .  
         [0081]     In the display device having such a constitution, due to the scanning line driving circuit  100  and the data line driving circuit  200 , the data signal is temporarily held in the holding capacitance  20  in the selected pixel portion  300 , the applying voltage from the drive power source  500  is supplied to the organic EL device  40  in response to the data signal held in the holding capacitance  20 , and the organic EL device  40  emits light. Here, the organic EL device  40  in the non-selected pixel portion  300  emits light in response to the data signal held by the holding capacitance  20 .