JP5245220B2 - Display device - Google Patents

Description

  The present invention relates to a display device, and more particularly to a display device in which pixels including electro-optic elements are arranged in a matrix (matrix).

  In recent years, in the field of display devices that perform image display, a pixel circuit including a so-called current-driven electro-optical element, for example, an organic EL (electro luminescence) element, whose light emission luminance changes according to a flowing current value as a light-emitting element of a pixel. Organic EL display devices having a large number of arranged in a matrix have been developed and commercialized. Since the organic EL display device is a self-luminous device, the organic EL display device has higher image visibility than a liquid crystal display device that controls light intensity from a light source (backlight) using pixels including liquid crystal cells. Features such as no need for a backlight and fast response speed of the device.

  In the organic EL display device, as in the liquid crystal display device, a simple (passive) matrix method and an active matrix method can be adopted as the driving method. However, although a simple matrix display device has a simple structure, there is a problem that it is difficult to realize a large and high-definition display device. Therefore, in recent years, the current flowing in the electro-optical element is controlled by an active element provided in the same pixel circuit as the electro-optical element, for example, an insulated gate field effect transistor (generally, a thin film transistor (TFT)). Active matrix display devices have been actively developed.

  In an active matrix organic EL display device, a pixel (pixel circuit) includes, in addition to an organic EL element, a drive transistor that drives the organic EL element, a write transistor that samples an input signal voltage and writes the signal into the pixel, And a storage capacitor connected to the gate of the driving transistor and holding the input signal voltage written by the writing transistor (see, for example, Patent Document 1).

JP 2005-345722 A

In the organic EL display device having the above configuration, the drive transistor operates as a constant current source because it is designed to operate in a saturation region. As a result, a constant drain-source current Ids given by the following equation (1) is supplied from the drive transistor to the organic EL element having the anode electrode connected to the source of the drive transistor.
Ids = (1/2) · μ (W / L) Cox (Vgs−Vth) ² (1)
Here, Vth is the threshold voltage of the driving TFT 202, μ is the carrier mobility, W is the channel width, L is the channel length, Cox is the gate capacitance per unit area, and Vgs is the gate-source voltage.

In a color display device in which pixels are arranged in units of a plurality of light emission colors, for example, R (red), G (green), and B (blue), materials for organic EL elements that emit light of each color, The capacitance Coled of the organic EL element that emits each color varies depending on the film thickness. Here, if the capacitance value of the storage capacitor is Ccs and the increase amount of the gate potential of the drive transistor is ΔVg, the increase amount ΔVs of the source potential of the drive transistor is
ΔVs = ΔVg × {Ccs / (Coled + Ccs)} (2)
It becomes.

  As apparent from the above equation (2), when the capacitance Coled of the organic EL element is different between R, G, and B, the source of the driving transistor is the same even if the increase ΔVg of the gate potential of the driving transistor is the same between R, G, and B The potential increase ΔVs is different between R, G, and B. The source potential of the driving transistor is also the anode potential of the organic EL element. Therefore, when the capacitance Coled of the organic EL element is different between R, G, and B, a difference occurs in the driving voltage of the organic EL element between R, G, and B.

  Thus, even if the signal voltage Vsig of the same level (voltage value) is input to each of the R, G, and B pixels, the drive voltage of each of the R, G, and B organic EL elements corresponds to the signal voltage Vsig. The white balance is lost because it does not become. Here, “white balance is lost” means that even if the signal voltage Vsig for white display is input to each of the R, G, and B pixels, the display color of each of the R, G, and B pixels is complete. It is to say that it will not turn white. If the white balance is lost, it will not be possible to display a natural color image.

  SUMMARY An advantage of some aspects of the invention is that it provides a display device capable of realizing image display with white balance without being affected by the capacitance of electro-optic elements that differ for each emission color.

  In order to achieve the above object, the present invention provides an electro-optic element, a drive transistor that drives the electro-optic element, a write transistor that samples an input signal voltage and writes it into a pixel, and a gate and a source of the drive transistor And a pixel having a storage capacitor for holding the input signal voltage written by the writing transistor is arranged in units of a plurality of colors having different emission colors of the electro-optic element. In this configuration, the capacitance value of the storage capacitor is made different between pixels having different emission colors.

  In the display device having the above-described configuration, the electro-optical element has a capacitance, and the capacitance value differs for each emission color. When an input signal voltage is applied to the gate of the driving transistor, the source potential of the driving transistor is determined by the capacitance ratio between the storage capacitor and the capacitance of the electro-optic element. At this time, the capacitance value of the storage capacitor is set to be different between the pixels having different emission colors. Specifically, the capacitance value of the storage capacitor of each pixel is set corresponding to the difference in the capacitance value of the electro-optic element for each emission color. As a result, the driving voltage of the electro-optic element of each color becomes a voltage value corresponding to the input signal voltage with respect to the input of the signal voltage of the same level to each pixel having a different emission color. White balance can be maintained without being affected by the capacity of the optical element.

  According to the present invention, the white balance can be maintained without being affected by the capacitance of the electro-optic element that differs for each luminescent color, so that a more natural color image display can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a circuit diagram illustrating a configuration of an active matrix display device according to an embodiment of the present invention and a pixel circuit used in the display device.

(Pixel array part)
As shown in FIG. 1, the active matrix display device according to the present embodiment is a current-driven electro-optical element whose light emission luminance changes according to a current value flowing through the device, for example, an organic EL element 31 as a light emitting element of a pixel. , And a pixel circuit (pixel) 11 including the organic EL element 31 is two-dimensionally arranged in a matrix (matrix). Here, for simplification of the drawing, a specific circuit configuration of one pixel circuit 11 is shown.

  In the pixel array unit 12, a scanning line 13, a driving line 14, and first and second correction scanning lines 15 and 16 are wired for each pixel row for each pixel circuit 11, and for each pixel column. Data lines (signal lines) 17 are wired. Around the pixel array section 12, a writing scanning circuit 18 that scans and drives the scanning lines 13, a driving scanning circuit 19 that scans and drives the driving lines 14, and first and second correction scanning lines 15 and 16 are scanned. First and second correction scanning circuits 20 and 21 to be driven, and a data line driving circuit 22 for supplying a data signal (video signal) corresponding to luminance information to the data line 17 are arranged.

  In this example, the writing scanning circuit 18 and the driving scanning circuit 19 are arranged on one side (for example, the right side of the figure) with the pixel array unit 12 in between, and the first and second correction scanning circuits 20 and 21 are on the opposite side. Is arranged. However, these arrangement relationships are merely examples, and the present invention is not limited to these. The writing scanning circuit 18, the driving scanning circuit 19, and the first and second correction scanning circuits 20 and 21 scan the scanning line 13, the driving line 14, and the first and second correction scanning lines 15 and 16. The writing signal WS, the driving signal DS, and the first and second correction scanning signals AZ1 and AZ2 are output as appropriate.

  The pixel array section 12 is usually formed on a transparent insulating substrate such as a glass substrate, and has a flat (flat) panel structure. Each pixel circuit 11 of the pixel array unit 12 can be formed using an amorphous silicon TFT (thin film transistor) or a low-temperature polysilicon TFT. In the present embodiment, the case where the pixel circuit 11 is formed of a low-temperature polysilicon TFT will be described as an example. When the low-temperature polysilicon TFT is used, the writing scanning circuit 18, the driving scanning circuit 19, the first and second correction scanning circuits 20 and 21, and the data line driving circuit 22 are also on the panel on which the pixel array unit 11 is formed. Can be formed integrally.

(Pixel circuit)
The pixel circuit 11 has a circuit configuration including a drive transistor 32, a write transistor 33, switching transistors 34 to 36, and a storage capacitor 37 in addition to the organic EL element 31.

  In the pixel circuit 11, an N-channel TFT is used as the driving transistor 32, the writing transistor 33, and the switching transistors 35 and 36, and a P-channel TFT is used as the switching transistor 34. However, the combination of the conductivity types of the drive transistor 32, the write transistor 33, and the switching transistors 34 to 36 here is only an example, and is not limited to these combinations.

  The organic EL element 31 has a cathode electrode connected to the first power supply potential VSS (here, the ground potential GND). The drive transistor 32 is for driving the organic EL element 31 with current, and a source is connected to an anode electrode of the organic EL element 31 to form a source follower circuit. That is, as shown in FIG. 2, the source potential (source voltage) of the drive transistor 32 is determined by the operating point between the drive transistor 32 and the organic EL element 31, and has a voltage value that varies depending on the gate potential.

  The write transistor 33 has a source connected to the data line 17, a drain connected to the gate of the drive transistor 32, and a gate connected to the scanning line 13. The switching transistor 34 has a source connected to the second power supply potential VDD (in this case, a positive power supply potential), a drain connected to the drain of the drive transistor 32, and a gate connected to the drive line 14. The switching transistor 35 has a drain connected to the third power supply potential Vini1, a source connected to the drain of the write transistor 33 (gate of the drive transistor 32), and a gate connected to the first correction scanning line 15.

  The switching transistor 36 has a drain connected to a connection node N11 between the source of the drive transistor 32 and the anode electrode of the organic EL element 31, and a source connected to the fourth power supply potential Vini2 (here, a negative power supply potential). The gate is connected to the second correction scanning line 16. The storage capacitor 37 has one end connected to a connection node N12 between the gate of the drive transistor 32 and the drain of the write transistor 33, and the other end connected to a connection node N11 between the source of the drive transistor 32 and the anode electrode of the organic EL element 31. Has been.

  In the pixel circuit 11 in which the constituent elements are connected according to the connection relationship described above, the constituent elements have the following effects. That is, when the writing transistor 33 becomes conductive, the input signal voltage Vsig supplied through the data line 17 is sampled and written into the pixel. The written signal voltage Vsig is held in the holding capacitor 37. The switching transistor 34 supplies a current from the power supply potential VDD to the driving transistor 32 by becoming conductive.

  The drive transistor 32 drives the organic EL element 31 to emit light by supplying a current value corresponding to the signal voltage Vsig held in the holding capacitor 37 to the organic EL element 31 when the switching transistor 34 is in a conductive state. (Current drive). That is, duty driving for controlling light emission / non-light emission of the organic EL element 31 is performed by conduction / non-conduction of the switching transistor 34. The switching transistors 35 and 36 are appropriately turned on to detect the threshold voltage Vth of the drive transistor 32 prior to current driving of the organic EL element 31, and the detected threshold voltage Vth in order to cancel the influence in advance. Is held in the holding capacitor 37.

  In the pixel circuit 11, as a condition for guaranteeing normal operation, the fourth power supply potential Vini2 is set to be lower than the potential obtained by subtracting the threshold voltage Vth of the drive transistor 32 from the third power supply potential Vini1. Has been. That is, the level relationship is Vini2 <Vini1-Vth. The level obtained by adding the threshold voltage Vthel of the organic EL element 31 to the cathode potential Vcat (here, the ground potential GND) of the organic EL element 31 is obtained by subtracting the threshold voltage Vth of the drive transistor 32 from the third power supply potential Vini1. It is set to be higher than the level. That is, the level relationship is Vcat + Vthel> Vini1-Vth (> Vini2).

[Description of circuit operation]
Next, the circuit operation of the active matrix organic EL display device in which the pixel circuits 11 having the above configuration are two-dimensionally arranged in a matrix will be described with reference to the timing waveform diagram of FIG.

  In FIG. 3, when driving the pixel circuit 11 in an i-th row, the write signal WS supplied from the write scanning circuit 18 to the pixel circuit 11 via the scanning line 13, and from the driving scanning circuit 19 via the driving line 14. Drive signal DS supplied to the pixel circuit 11 and the first and second correction scanning circuits 20 and 21 supplied to the pixel circuit 11 via the first and second correction scanning lines 15 and 16. The timing relationship between the correction scanning signals AZ1 and AZ2 and the changes in the gate potential Vg and the source potential Vs of the driving transistor 32 are shown.

  Here, since the write transistor 33 and the switching transistors 35 and 36 are N-channel type, the write signal WS and the first and second correction scanning signals AZ1 and AZ2 are at a high level (in this example, the power supply potential VDD). Hereinafter referred to as “H” level) as an active state, and low level (in this example, power supply potential VSS (GND); hereinafter referred to as “L” level ”) as non-active. Set to active state. Further, since the switching transistor 34 is a P-channel type, with respect to the drive signal DS, an “L” level state is an active state, and an “H” level state is an inactive state.

  At time t1, the drive signal DS changes from “L” level to “H” level, and the switching transistor 34 becomes non-conductive, and at time t2, the second correction scanning signal AZ2 changes from “L” level to “H” level. The power supply potential Vini2 is applied to the source of the driving transistor 32 through the switching transistor 36 by transitioning to the “level” and the switching transistor 36 being in a conductive state.

  At this time, as described above, since the level relationship is Vini2 <Vcat + Vthel, the organic EL element 31 is in a reverse bias state. Therefore, no current flows through the organic EL element 31 and it is in a non-light emitting state.

  Next, at time t3, the first correction scanning signal AZ1 transitions from the “L” level to the “H” level, and the switching transistor 35 becomes conductive, so that the gate of the driving transistor 32 is connected to the gate through the switching transistor 35. The power supply potential Vini1 is applied. At this time, the gate-source voltage Vgs of the drive transistor 32 takes a value of Vini1-Vini2. Here, as described above, the level relationship of Vofs−Vini1> Vth is satisfied.

(Vth correction period)
Next, at time t4, the second correction scanning signal AZ2 changes from “H” level to “L” level, the switching transistor 36 becomes non-conductive, and then at time t5, the drive signal DS changes from “H” level. By transitioning to the “L” level and the switching transistor 34 becoming conductive, a current corresponding to the potential difference Vgs between the gate and the source flows through the driving transistor 32.

  At this time, the cathode potential Vcat (power supply potential VSS) of the organic EL element 31 is higher than the source potential Vs of the drive transistor 32, the organic EL element 31 is in a reverse bias state, and the current flowing from the drive transistor 32 is maintained at the node N11 → Since the current flows through the path of the capacitor 37 → the node N12 → the switching transistor 35 → the power supply potential Vini1, the charge corresponding to the current is charged in the holding capacitor 37, and the source potential Vs of the drive transistor 32 is supplied to the power supply potential along with this charging. Ascending gradually from Vini1 over time.

  Then, when a certain time has elapsed and the potential difference Vgs between the gate and the source (between N11 and N12) of the drive transistor 32 becomes equal to the threshold voltage Vth of the drive transistor 32, the drive transistor 32 is cut off, and the drive transistor Since the current does not flow to 32, the potential difference Vgs between the gate and source (between N11 and N12) of the driving transistor 32, that is, the threshold voltage Vth is held in the holding capacitor 37 as a potential for threshold correction.

  Thereafter, at time t6, the drive signal DS changes from the “L” level to the “H” level, and the switching transistor 34 becomes non-conductive. The period from time t5 to time t6 is a period during which the threshold voltage Vth of the driving transistor 32 is detected and held in the holding capacitor 38. Here, this fixed period t5-t6 is called a Vth correction period. Next, at time t7, the first correction scanning signal AZ1 changes from the “H” level to the “L” level, and the switching transistor 35 is turned off.

(Writing period)
After that, at time t8, the write signal WS transits from the “L” level to the “H” level, so that the input signal voltage Vsig is sampled by the write transistor 33 and written into the pixel. Vg becomes the input signal voltage Vsig. This input signal voltage Vsig is held in the holding capacitor 37.

  At this time, the source potential Vs of the drive transistor 32 rises due to the capacitive coupling of the storage capacitor 38 and the organic EL element 31 with respect to the amplitude of the gate potential Vg when the write transistor 33 is conductive. Here, if the capacitance value of the storage capacitor 38 is Ccs, the capacitance value of the organic EL element 31 is Coled, and the increase amount of the gate potential Vg of the drive transistor 32 is ΔVg, the increase amount ΔVs of the source potential Vs of the drive transistor 32 is It is expressed by the relational expression of the expression (2) described above.

  Further, the input signal voltage Vsig written by the write transistor 33 is held in the holding capacitor 37 in a form that is added to the threshold voltage Vth held in the holding capacitor 37. At this time, the holding voltage of the holding capacitor 37 is Vsig−Vini1 + Vth. Here, in order to facilitate understanding, when Vini1 = 0V, the gate-source voltage Vgs is Vsig + Vth.

  As described above, by holding the threshold voltage Vth in the storage capacitor 38 in advance, it is possible to correct variations and temporal changes in the threshold voltage Vth of the drive transistor 32 for each pixel. That is, when the driving transistor 32 is driven by the signal voltage Vsig, the threshold voltage Vth of the driving transistor 32 is canceled with the threshold voltage Vth held in the holding capacitor 38, in other words, the threshold voltage Vth is corrected. Therefore, even if the threshold voltage Vth varies from pixel to pixel and changes with time, the influence of the threshold voltage Vth on the driving of the organic EL element 31 by the drive transistor 32 can be canceled, and thus the threshold voltage Vth varies. In addition, the light emission luminance of the organic EL element 31 can be kept constant without being affected by changes over time.

(Mobility correction period)
After that, the drive signal DS changes from the “H” level to the “L” level at time t9 while the write transistor 33 remains conductive, and the switching transistor 34 becomes conductive, so that the power supply potential VDD changes to the drive transistor 32. Current supply is started. Note that the period from time t8 to time t9 is one horizontal period (1H). Here, by setting Vini1-Vth <Vthel, the organic EL element 31 is placed in a reverse bias state.

  When the organic EL element 31 is in the reverse bias state, the organic EL element 31 exhibits simple capacitance characteristics instead of diode characteristics. Therefore, the drain-source current Ids flowing through the drive transistor 32 is written into the capacitance C (= Cs + Coled) obtained by synthesizing the capacitance value Cs of the storage capacitor 37 and the capacitance value Coled of the capacitance component of the organic EL element 31. By this writing, the source potential Vs of the drive transistor 32 rises.

  The increase ΔVs of the source potential Vs is subtracted from the potential difference Vgs between the gate and the source of the driving transistor 32 held in the holding capacitor 38, in other words, acts to discharge the charge of the holding capacitor 38. Therefore, negative feedback is applied. That is, the increase ΔVs of the source potential Vs becomes a feedback amount of negative feedback. At this time, the potential difference Vgs between the gate and the source is Vsig−ΔVs + Vth.

  In this manner, the current flowing through the drive transistor 32 (drain-source current Ids) is negatively fed back to the gate input of the drive transistor 32 (potential difference between the gate and the source), so that the drain / source of the drive transistor 32 of each pixel is drained. It becomes possible to cancel the dependence of the inter-source current Ids on the mobility μ, that is, to correct the variation in the mobility μ of the drive transistor 32.

  A period T (t9-t10) in which the active period (“H” level period) of the write signal WS and the active period (“L” level period) of the drive signal DS overlap, that is, both the write transistor 33 and the switching transistor 34 The overlap period in which the conductive state is established is defined as a mobility correction period.

  Here, when considering a drive transistor having a high mobility μ and a drive transistor having a low mobility μ, a drive transistor having a high mobility μ in the mobility correction period T is compared to a drive transistor having a low mobility μ. The source potential Vs rises greatly. Further, as the source potential Vs rises significantly, the potential difference between the gate and the source of the driving transistor 32 becomes smaller, and the current hardly flows.

  That is, by adjusting the mobility correction period T, the same drain-source current Ids can be caused to flow in the drive transistors 32 having different mobility μ. The gate-source potential difference Vgs of the drive transistor 32 determined in the mobility correction period T is maintained by the storage capacitor 38, and a current (drain-source current Ids) corresponding to the gate-source potential difference Vgs is driven by the drive transistor. The organic EL element 31 emits light when 32 flows through the organic EL element 31.

(Light emission period)
At time t10, the write signal WS becomes “L” level and the write transistor 33 is turned off, so that the mobility correction period T ends and the light emission period starts. During this light emission period, the source potential Vs of the drive transistor 32 rises to the drive voltage of the organic EL element 31. As the source potential Vs rises, the gate of the drive transistor 32 is disconnected from the data line 17 and is in a floating state, so that the gate potential Vg also rises via the storage capacitor 37.

At this time, if the parasitic capacitance of the gate of the driving transistor 32 is Cg, the increase ΔVg of the gate potential Vg is expressed by the following equation (3).
ΔVg = ΔVs × {Ccs / (Ccs + Cg)} (3)
Meanwhile, the gate-source potential difference Vgs held in the holding capacitor 37 maintains the value of Vsig−ΔVs + Vth.

  Then, as the source potential Vs of the drive transistor 32 increases, the reverse bias state of the organic EL element 31 is canceled, and the constant drain / voltage given by the above-described formula (1) from the drive transistor 32 to the organic EL element 31 is reduced. Since the inter-source current Ids is supplied, the organic EL element 31 actually starts to emit light.

The relationship between the drain-source current Ids and the gate-source potential difference Vgs at this time is given by the following equation (4) by substituting Vsig−ΔVs + Vth into Vgs of the above equation (1).
Ids = kμ (Vgs−Vth) ²
= Kμ (Vsig−ΔV) ² (4)
In the above equation (4), k = (1/2) (W / L) Cox.

  As is clear from this equation (4), the term of the threshold voltage Vth of the drive transistor 32 is canceled, and the drain-source current Ids supplied from the drive transistor 32 to the organic EL element 31 is It can be seen that it does not depend on the threshold voltage Vth. Basically, the drain-source current Ids is determined by the input signal voltage Vsig. In other words, the organic EL element 31 emits light with a luminance corresponding to the input signal voltage Vsig without being affected by variations in the threshold voltage Vth of the drive transistor 32 and changes with time.

  Further, as apparent from the above equation (4), the input signal voltage Vsig is corrected by the feedback amount ΔVs by negative feedback of the drain-source current Ids to the gate input of the drive transistor 32. This feedback amount ΔVs acts so as to cancel the effect of the mobility μ located in the coefficient part of the equation (4). Therefore, the drain-source current Ids substantially depends only on the input signal voltage Vsig. That is, the organic EL element 31 emits light with a luminance corresponding to the input signal voltage Vsig without being affected by not only the threshold voltage Vth of the drive transistor 32 but also the variation in mobility μ of the drive transistor 32 and a change with time. As a result, uniform image quality without streaks or uneven brightness can be obtained.

  Here, in the active matrix display device in which the pixel circuits 11 including the organic EL elements 31 that are current-driven electro-optical elements are arranged in a matrix, when the light emission time of the organic EL elements 31 increases, The IV characteristic of the EL element 31 changes. For this reason, the potential of the connection node N11 between the anode electrode of the organic EL element 31 and the source of the drive transistor 32 also changes.

  In contrast, in the active matrix organic EL display device according to the present embodiment, since the gate-source potential difference Vgs of the drive transistor 32 is maintained at a constant value, the current flowing through the organic EL element 31 does not change. . Therefore, even if the IV characteristic of the organic EL element 31 deteriorates, the constant drain-source current Ids continues to flow through the organic EL element 31, so that the light emission luminance of the organic EL element 31 does not change ( Compensation function for characteristic variation of organic EL element 31).

  Further, by holding the threshold voltage Vth of the drive transistor 32 in the storage capacitor 37 in advance before the input signal voltage Vsig is written, the threshold voltage Vth of the drive transistor 32 is canceled (corrected), and the threshold voltage Vth of the threshold voltage Vth is set. Since a constant drain-source current Ids that is not affected by variations and changes over time can be passed through the organic EL element 31, a high-quality display image can be obtained (compensation function for Vth variation of the drive transistor 32). ).

  Further, during the mobility correction period t9-t10, the drain-source current Ids is negatively fed back to the gate input of the drive transistor 32, and the input signal voltage Vsig is corrected by the feedback amount ΔVs, thereby allowing the drain / source current Ids to be corrected. Since the dependency of the source-to-source current Ids on the mobility μ is canceled and the drain-source current Ids that depends only on the input signal voltage Vsig can be made to flow in the organic EL element 31, It is possible to obtain a display image with uniform image quality without streaks or luminance unevenness due to changes over time (compensation function for the mobility μ of the drive transistor 32).

  Up to this point, a color display device in which pixels are arranged in units of a plurality of emission colors, for example, R, G, and B, has been described as a technique common to pixels of each emission color. The capacitance Coled of the organic EL element that emits each color varies depending on the material and film thickness of the organic EL element that emits each color.

  FIG. 4 is a cross-sectional view showing a basic configuration example of the organic EL element. As shown in FIG. 4, in the organic EL element, a first electrode (for example, an anode) 42 made of a transparent conductive film is formed on a substrate 41 made of transparent glass or the like, and a hole transport layer 43 is further formed thereon. A light emitting layer 44, an electron transport layer 45, and an electron injection layer 46 are sequentially deposited to form an organic layer 47, and then a second electrode (for example, a cathode) 48 made of metal is formed on the organic layer 47. It has a configuration. Then, by applying a DC voltage E between the first electrode 42 and the second electrode 48, light is emitted when electrons and holes are recombined in the light emitting layer 44.

  In the organic EL element having such a configuration, the emission color is determined by the material of the light emitting layer 44, the film thickness of the organic EL element 47, and the like. That is, the material and film thickness t of the organic EL element differ for each organic EL element that emits each color of R, G, and B. When the material and film thickness t of the organic EL element are different, the capacitance Coled of the organic EL element is changed. That is, the capacitance Coled of the organic EL element that emits each color varies depending on the material and the film thickness t of the organic EL element that emits each color.

  When the capacitance Coled of the organic EL element 31 differs depending on the color of light emission, as is apparent from the above-described equation (2), the drive transistor 32 has the same amount of increase ΔVg in the gate potential of the drive transistor 32 in R, G, and B. The source voltage increase ΔVs is different for R, G, and B. As a result, even if the signal voltage Vsig of the same level (voltage value) is input to each of the R, G, and B pixels, the drive voltage of each of the R, G, and B organic EL elements corresponds to the signal voltage Vsig. The white balance is lost because it does not become.

  Therefore, in the present embodiment, in addition to the organic EL element 31, a driving transistor 32 that drives the organic EL element 31, a writing transistor 33 that samples the input signal voltage Vsig and writes it in the pixel, and a gate of the driving transistor 32 The pixel circuit 11 having at least a storage capacitor 37 that is connected between the source and the source and holds the input signal voltage Vsig written by the writing transistor 33 has a plurality of colors (in this example, different colors of light emitted from the organic EL element 31). , R, G, and B), the capacitance value Ccs of the storage capacitor 37 is made different between pixels having different emission colors.

  In the three colors of R, G, and B, the film thickness of the organic EL element is in a relationship of R> G> B due to the relationship between the wavelength length (R> G> B). As for the magnitude relationship, B is the largest, contrary to the film thickness, and B> G> R. As an example, the capacity ratio has a relationship of R: G: B = 1: 1.2: 1.5.

  Therefore, the capacitance value Ccs of the storage capacitor 37 of each of the R, G, and B pixels is determined according to the capacitance ratio of the capacitance Coled of each of the R, G, and B organic EL elements. Preferably, when the capacitance ratio of the capacitance Coled of the organic EL element is the above relationship (R: G: B = 1: 1.2: 1.5), the capacitance of the storage capacitor 37 is obtained from the above-described equation (2). The ratio is also determined to be R: G: B = 1: 1.2: 1.5.

  As described above, the capacitance ratio of the storage capacitor 37 is determined to be the same as the capacitance ratio of the capacitance Coled of the organic EL element, whereas the capacitance value Ccs for each of R, G, and B of the storage capacitor 37 is determined. Is preferably determined as follows.

  As shown in FIG. 5, between the capacitance value Ccs of the storage capacitor 37 and the gate-source potential difference Vgs of the drive transistor 32, the gate-source potential difference Vgs has a peak value when the capacitance value Ccs is a certain capacitance value Ccs0. The gate-source voltage Vgs is exponentially decreased as the capacitance value Ccs decreases from the capacitance value Ccs0 or increases. Therefore, the capacitance value Ccs for each of R, G, and B of the storage capacitor 37 is maintained according to the characteristics (drive voltage and capacitance Coled) of the organic EL element 31 while maintaining the above-described relationship of the capacitance ratio. The capacitance value Cgs0 (including the vicinity thereof) is determined so that the inter-voltage Vgs is the largest.

  Here, the effect by the capacitance value Ccs of the storage capacitor 37 being different between pixels having different emission colors will be described.

  First, when the write transistor 33 samples the input signal voltage Vsig and writes it in the pixel, as described above, the storage capacitor 37 when the organic EL element 31 emits light with respect to the amplitude of the gate potential Vg of the drive transistor 32. Due to the capacitive coupling of the organic EL element 31, the source potential Vs of the drive transistor 32 differs between the R, G, and B pixels. At this time, as is clear from the above-described equation (2), the smaller the capacitance value Ccs of the storage capacitor 37 is, the smaller the increase ΔVs of the source potential Vs of the drive transistor 32 is, so the gate-source potential difference Vgs of the drive transistor 32. Becomes larger.

  Further, when the organic EL element 31 emits light, the source potential Vs of the drive transistor 32 rises to the drive voltage of the organic EL element 31, and the amount of increase ΔVg of the gate potential Vg of the drive transistor 32 at that time is expressed by the above-described formula ( As apparent from 3), the larger the capacitance value Ccs of the storage capacitor 37, the greater the amount of increase ΔVs of the source potential Vs. At this time, the gate-source potential difference Vgs of the drive transistor 32 increases as the capacitance value Ccs of the storage capacitor 37 decreases during a signal write operation, and increases as the capacitance value Ccs of the storage capacitor 37 increases during light emission of the organic EL element 31. Become.

  Using this operating principle, the capacitance value Ccs of the storage capacitor 37 is made different for each pixel having a different emission color, and the gate-source potential difference Vgs at the time of signal writing and the gate-source potential difference Vgs at the time of light emission of the organic EL element 31. Is adjusted between the R, G, and B pixels so that the driving voltage of the organic EL element 31 of each color corresponds to the input signal voltage Vsig with respect to the input of the signal voltage Vsig of the same level. Therefore, the white balance can be maintained without being affected by the capacitance Coled of the organic EL element 31 that is different for each emission color. As a result, a more natural color image display can be realized.

  Further, when determining the capacitance value Ccs of the storage capacitor 37, as described with reference to FIG. 5, the gate-source potential difference Vgs of the drive transistor 32 is determined to be a capacitance value that can be most secured (including the vicinity thereof). Is preferred. By making the gate-source potential difference Vgs of the driving transistor 32 most secure, the amplitude of the input signal voltage Vsig can be reduced by that much, so that the data line driving circuit 22 that supplies the signal voltage Vsig (see FIG. 1). ) Can be reduced, and thus the overall power consumption of the display device can be reduced.

  In the above embodiment, the plurality of colors as the unit of image display are three colors of red, green, and blue. However, the present invention is not limited to the combination of these three colors, and for example, white is added to the combination of the three colors. It is also possible to make a combination of the added four colors or a combination of other colors.

In addition, the pixel circuit (pixel) of the organic EL display device to which the present invention is applied is not limited to the circuit example of the pixel circuit 11 shown in FIG. a drive transistor 32 for driving the EL element 31, a write transistor 33 for writing samples the input signal voltage Vsig, is connected between the gate and source of the driving transistor 32, the input signal voltage Vsig written by the write transistor 33 Any pixel circuit having a circuit configuration including a storage capacitor 37 that holds

  In the above embodiment, the case where the present invention is applied to an organic EL display device using the organic EL element 31 as the electro-optical element of the pixel circuit 11 has been described as an example. However, the present invention is limited to this application example. Instead, the present invention can be applied to all display devices using current-driven electro-optic elements (light-emitting elements) whose light emission luminance changes according to the current value flowing through the device.

1 is a circuit diagram illustrating a configuration of an active matrix display device according to an embodiment of the present invention and a pixel circuit used in the display device. FIG. 5 is a characteristic diagram of a drain-source voltage Vds−drain-source current Ids of a driving transistor. It is a timing waveform diagram for explaining circuit operation of an active matrix type organic EL display device. It is sectional drawing which shows the basic structural example of an organic EL element. It is a figure which shows the relationship between the capacitance value Ccs of a storage capacity | capacitance, and the gate-source voltage Vgs of a drive transistor.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Pixel circuit (pixel), 12 ... Pixel array part, 13 ... Scanning line, 14 ... Drive line, 15 ... First correction scanning line, 16 ... Second correction scanning line, 17 ... Data line (signal line) , 18 ... writing scanning circuit, 19 ... drive scanning circuit, 20 ... first correction scanning circuit, 21 ... second correction scanning circuit, 22 ... data line driving circuit, 31 ... organic EL element, 32 ... driving transistor, 33 ... Writing transistor, 34, 35, 36 ... Switching transistor, 37 ... Retention capacity

Claims (3)

An electro-optical element having a different film thickness for each emission color; a drive transistor connected in series to the electro-optical element to drive the electro-optical element; and a writing transistor that samples the input signal voltage and writes the signal into the pixel A pixel connected between a gate and a source of the driving transistor and having a storage capacitor for holding the input signal voltage written by the writing transistor, wherein a plurality of colors having different emission colors of the electro-optic element Arranged as
The storage capacitors have different capacitance values between pixels with different emission colors, and the capacitance ratio of the storage capacitors between pixels with different emission colors is smaller for pixels corresponding to emission colors with a larger film thickness ratio of the electro-optic element. Display device.   The capacitance value of the storage capacitor is determined so that the capacitance ratio between pixels having different emission colors is the same as the capacitance ratio of the electro-optic element corresponding to each emission color that emits the plurality of colors. The display device described in 1. The storage capacitor has one end connected to the gate of the drive transistor and the other end connected to the anode electrode of the electro-optic element.
3. The display device according to claim 2, wherein the capacitance value of the storage capacitor is determined to be a value that can most secure the gate-source potential difference in the relationship between the capacitance value and the gate-source potential difference of the driving transistor.

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