Patent Grant
7460116
1. Field of the Invention
The present invention relates to display elements in which light emission is controlled by the amount of current flowing through an electro-optical element, such as an organic EL element or an inorganic EL element, as well as to display devices including such display elements.
2. Description of the Related Art
To date, there are display devices whose light emission sources are electro-optical elements whose optical characteristics are changed by applying electricity, such as organic EL (electroluminescent) elements, inorganic EL elements or light emitting diodes. In such display devices, a plurality of display elements each including an electro-optical element are arranged in a matrix, and the current necessary for lighting the electro-optical elements is supplied to the electro-optical elements from a power source supplying a predetermined current. These electro-optical elements are controlled to a predetermined luminance by applying a predetermined data signal voltage (or current).
Ordinarily, a display element includes a plurality of TFT (thin film transistor) elements, an electro-optical element, an auxiliary capacitance, a data signal line for applying data signals, a scanning signal line for applying scanning signals, and a power source line for supplying current from the power source. Here, the size (occupied surface area) of the electro-optical element in the display element is determined by the size (occupied surface area) of conductors and the elements other than the electro-optical element. For example, in order to make the numerical aperture of the pixels as large as possible, the size of the elements other than the electro-optical element should be made small and also the size of the conductors, in other words the width of the electrodes, should be small. If that is the case, then the numerical aperture can be made large, and the light emission efficiency of the electro-optical element increases, so that it is possible to drive with a comparatively low voltage. As a result, also the power consumption becomes relatively low.
However, the larger one tries to make the numerical aperture, the thinner the electrode width of the power source line needs to be made, so that the resistance of the power source line electrode increases. Consequently, when current is supplied from the power source line electrode to the display elements, the voltage drop at the display elements further away from the current supplying power source increases the greater the numerical aperture is made. This means that there are variations in the drop of the voltage applied by the current supplying power source to each of the display elements. When the voltage applied to the electro-optical element becomes small due to this voltage drop, then also the emitted luminance of the electro-optical element decreases. Therefore, the larger one tries to make the numerical aperture, the bigger are the variations that occur in the luminance of the pixels of the display device. Ordinarily, electro-optical elements such as organic EL elements have diode characteristics, so that the current flowing through the electro-optical element changes exponentially with changes in voltage. Thus, since the luminance of the electro-optical element is lowered exponentially due to the above-described voltage drop, the display device suffers from conspicuous variations or irregularities in luminance, lowering the display quality.
Conventionally, display devices address this problem by suppressing luminance variations on the display screen with the following configurations. For example, there are display devices that are provided with a memory for storing data expressing the image to be displayed and a memory for storing current correction data, which has been set in advance, for correcting the luminance variations, and image data that is corrected in accordance with the current correction data is applied to the display elements (see for example JP H11-344949A). With this configuration, luminance variations can be corrected. Moreover, there are display devices in which the conductors leading from a predetermined driving circuit to the display elements are provided with a predetermined resistance distribution (see for example JP 2001-83934A). This resistance distribution is set such that voltage drops due to the resistance of the conductors are suppressed. Thus, it is possible to reduce luminance variations due to voltage drops.
However, in the configuration disclosed in JP H11-344949A, a new correction circuit including a memory for storing the current correction data becomes necessary. When this correction circuit is provided in a driver portion incorporated in the display panel, the scale of the circuitry increases, which is undesirable. When the scale of the circuitry increases, there are disadvantages with regard to the manufacturing yield, for example. Moreover, with the configuration disclosed in JP 2001-83934A, the resistance distribution of the conductors must be set such that voltage drops due to the resistance of the conductors are suppressed, so that complex design and layout of the conductors becomes necessary.
To do so, the electrode width of the power source line should be increased in order to suppress luminance variations, but in this case, the numerical aperture is decreased, as mentioned above, worsening the light emission efficiency of the electro-optical elements. This causes the problems that the element lifetime is shortened, and the power consumption is increased.
Moreover, in conventional display devices, one pixel is configured by a plurality of sub-pixels, such as three sub-pixels for displaying the three colors for RGB display. In this case, the current for generating the necessary luminance depends on the (light emitting material of the) display elements forming the sub-pixels, so that when the luminance of the various colors deviates from the desired values when displaying white light for example, then a colored hue may be perceived on the display screen.
It is thus an object of the present invention to provide a display element and a display device, with which variations in the luminance are suppressed and colored hues are suppressed while ensuring a large numerical aperture, without providing special correction circuits or conductors having a complex resistance distribution
In order to attain this and other objects of the present invention, a display element in accordance with an aspect of the present invention comprises:
an electro-optical element that is driven by a current;
a control element for controlling the current flowing through the electro-optical element;
a selection element for applying to the control element a data signal for controlling the control element;
a scanning signal line electrode for applying to the selection element a scanning signal for controlling the selection element;
a data signal line electrode for applying to the selection element the data signal; and
a power source line electrode for applying to the control element the current for driving the electro-optical element;
wherein a resistance ratio Rx/Re between a resistance Re of the power source line electrode per display element and a resistance Rx of a path of the current flowing through the control element and the electro-optical element, starting at the power source line electrode, is at least 105.
In a display device comprising a plurality of these display elements, the balance of the current flowing from the power source line electrode through the electro-optical element can be adjusted, and the luminance variations can be kept within a predetermined tolerance range without increasing the scale of the circuitry. Therefore, a display element can be provided that has little luminance variations, a high numerical aperture, an excellent light emission efficiency, and a long lifetime.
It should be noted that the pixels of this display device may be constituted by a plurality of display elements. For example, one pixel may be constituted by three sub-pixels displaying the three colors R, G and B, or by four sub-pixels displaying R, G, B and W. In this case, the current for generating the necessary luminance is different for each of the display elements forming the sub-pixels. If the resistance ratios Rx/Re of the display elements forming the sub-pixels of different colors are set to be substantially the same, then the luminance variation among the display elements forming the various sub-pixels can be made small, and it is possible to achieve a uniform display without color hues when displaying white light, for example.
Moreover, it is preferable that the power source line electrode of the display element has a plurality of different electrode widths. With such a configuration, when trying to realize the above-noted resistance ratio in order to design the pixels such that they have a high numerical aperture, the above-noted resistance ratio condition can be met and the display elements can be designed to have the highest numerical aperture by suitably adjusting the electrode width of the power source line electrode.
Furthermore, it is preferable that the power source line electrode's electrode width at a portion adjacent to a region where the electro-optical element is disposed is larger than the power source line electrode's electrode width at other portions. With such a configuration, when trying to realize the above-noted resistance ratio in order to design the pixels such that they have a high numerical aperture, it is possible to set the electrode width to an extent that does not compromise the circuit configuration in regions with a relatively low degree of freedom, such as the TFT circuit portions, and to suitably adjust the electrode width in regions where the electro-optical elements are disposed, so that it becomes easy to design the pixels such that the above-noted resistance ratio condition is met.
Furthermore, when the tolerance value of the current variation is made comparatively large under the comparatively loose condition of making the resistance ratio Rx/Re at least 105, the current variation of display devices having a comparatively low pixel resolution and a comparatively small column pixel number can be kept within the range of tolerance values, but when the comparatively strict condition of making the resistance ratio Rx/Re at least 106 is set, then the current variations of almost all display devices can be kept within the range of tolerance values, even if the tolerance values for current variation are comparatively small.
In accordance with another aspect of the present invention, a display element comprises:
an electro-optical element that is driven by a current;
a control element for controlling the current flowing through the electro-optical element;
a selection element for applying to the control element a data signal for controlling the control element;
a scanning signal line electrode for applying to the selection element a scanning signal for controlling the selection element;
a data signal line electrode for applying to the selection element the data signal; and
a power source line electrode for applying to the control element the current for driving the electro-optical element;
wherein the display element forms one of a plurality of types of sub-pixels constituting one pixel; and
wherein a resistance ratio Rx/Re between a resistance Re of the power source line electrode per display element and a resistance Rx of a path of the current flowing through the control element and the electro-optical element, starting at the power source line electrode, is substantially the same as the resistance ratio Rx/Re of the other display elements forming the other types of sub-pixels in that one pixel.
In a display device comprising a plurality of these display elements, the resistance ratio Rx/Re of the display elements forming sub-pixels of different colors is set to substantially the same value, so that it is possible to attain a uniform display without color hues when displaying, for example, white.
Moreover, as in the above-described display element, it is preferable that the power source line electrode of this display element has a plurality of different electrode widths. Furthermore, it is preferable that the power source line electrode's electrode width at a portion adjacent to a region where the electro-optical element is disposed is larger than the power source line electrode's electrode width at other portions.
In accordance with yet another aspect of the present invention, a display device comprises an image display portion comprising an arrangement of a plurality of the above-described display elements, and a current supply portion for applying a predetermined current to the power source line electrodes comprised by the display elements.
In a display device including such display elements, a low luminance variation, a high numerical aperture, a superior light emission efficiency, and a long lifetime are achieved by adjusting a balance of the currents flowing from the power source line electrode through the electro-optical elements, without providing a separate circuit for correcting luminance variations or controlling the pixel luminance by adding a correction of the data signal. Furthermore, a uniform display without color hues can be performed.
Furthermore, it is preferable that the current supply portion of this display device applies a current from a plurality of current supply points to power supply line electrodes linked to a plurality of display elements.
With this configuration, the display element column can be expressed as a ladder circuit of multiple stages, and the further the display elements are from the current supply points, the lower is the applied voltage due to the electrode resistance. Thus, a large difference in luminance occurs when comparing display elements far from the current supply points and display elements near the current supply points. Therefore, by providing the power source line electrode with a plurality of current supply points, the positions of the display elements become closer to the current supply points and it is possible to further reduce the luminance variations. Furthermore, a uniform display without color hues can be performed.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the drawings.
The following is a description of preferred embodiments of the present invention, with reference to the accompanying drawings.
As shown in
In this display element, during the period in which the selection TFT 6′ is selected by the scanning signal on the scanning signal line 11, a data signal voltage DATA is applied to the data signal line 12, so that a voltage corresponding to this data signal voltage is held by the auxiliary capacitance 7′, and the organic EL control TFT 8′ is controlled in accordance with the voltage held by the auxiliary capacitance 7′ also in the subsequent period when the selection TFT 6′ is not selected. Thus, the light emission amount is controlled by letting a predetermined current Ioled flow through the organic EL element 9′ connected in series to the organic EL control TFT 8′.
Here, if the organic EL element 9′ is lit up, the ON resistance of the control TFT 8′ and the organic EL element 9′, which are connected in series, becomes the load for the power source line 13. For example, if (the organic EL elements 9′ of) all display elements arranged in one column of the display screen are lit up, then the current flowing through the power source line 13 is distributed among all display elements. In this case, with a circuit configuration using a p-channel TFT for the control TFT, the potential serving as a reference with which ON/OFF control is performed by the control TFT 8′ in each of the display elements (referred to as “reference potential” below) becomes the reference potential V1, . . . , Vn, in order, starting with the one closest to the current supply portion 2 (power source voltage VDD), as shown in
If the values of all the reference potentials V1 to Vn would coincide, then there would be no variations of the luminance of the lit pixels arranged in the column direction. However in practice, the current Ioled flowing through each of the organic EL elements 9′ from the current source 13 is subject to a distribution, so that the reference potentials V1 to Vn applied to the display elements differ due to the voltage drop caused by the resistance, and a variation occurs in the reference potentials V1 to Vn. Consequently, even when the gate potential Vg in the display elements is the same, the voltage applied to (i.e. between gate and source of) the control TFT 8′ varies, that is the held voltage varies, so that also the current that flows varies. As a result, the current Ioled flowing through the organic EL element 9′ varies with a predetermined distribution.
Here, the lower the resistance of the power source line 13 is, the smaller is the voltage drop between display elements, so that if the resistance of the power source line 13 is made smaller than shown in
As explained above, it is preferable that the resistance of the power source line 13 is reduced in order to reduce the variation of the current Ioled, but since in practice it is impossible to make the resistance of the power source line 13 substantially zero, it is necessary to provide a limit (referred to as “tolerance value” below) that can be realized in practice and with which the variation can be made as small as possible. Regarding this tolerance value, it is possible to refer to the tolerance value for luminance change in display screens of liquid crystal displays (referred to as “LCDs” below). Ordinarily, the human eye cannot perceive any differences when the luminance changes in the display screen of completely lit LCDs do not exceed a level of 2 %/cm, and this value is taken as the target for the tolerance value. In LCDs, the luminance distribution of the backlight serving as the light emitting portion directly affects the display luminance, so that measures are taken to ensure that the luminance of the backlight is at levels within this tolerance value. The following is an explanation of a display device (and display element) in which, while referring to this tolerance value, the tolerance value for the variation of the current Ioled (referred to as “current variation tolerance value” or simply “tolerance value” in the following) is set to X % and the variation of the current Ioled is kept within the range of this tolerance value.
It should be noted that since the organic EL element emits light at a luminance corresponding to the current Ioled, the distribution of the current Ioled, which depends on the distance from the current supply point as shown in
To keep the calculations simple, it is further assumed that the voltage VCOM of the common electrode is VCOM=0. Furthermore, V0 denotes the voltage applied to the display element that is furthest away from the current supply point of the current supply portion 2, that is, the voltage at the point where that display element is connected to the power source line 13. And i0 denotes the current flowing from that connection point to the organic EL element 9′. The pixel formation portion corresponding to this connection point is also referred to as “node 0”. Taking this node 0 as the starting point for the following calculation, when the current in and the voltage Vn at the node n, which corresponds to the n-th display element, are calculated in order toward the current supply point, then the current and the voltage at each node can be expressed by the following recursive equations:
i0=V0/Rx
I0=i0
V1=I0·(Re+Rx)
i1=V1/Rx
I1=i1+I0
V2=I1·Re+V1
in−1=Vn−1/Rx
In−1=in−1+In−2
Vn=In−1·Re+Vn−1
in=Vn/Rx
In=in+In−1
VDD=In·Re+Vn
Here, the maximum fluctuation rate Δi of the currents io to in in accordance with the current distribution of the currents io to in across all nodes can be expressed by the following Expression (1), taking the average value (intermediate value) of the maximum current and the minimum current in the current distribution as a reference:
Δi=2(in−i0)/(in+i0)×100% (1)
For example, when the maximum fluctuation rate Δi as given by Expression (2) is 200% (that is, when there is a fluctuation of ±100% around the average value), then this means that the current in at the node n is twice the average value, and that the current In at the node 0 is 0. However, in this case, a portion of the display elements is not properly lit up, so that the display device can be said to be defective.
Referring again to
Δin=2(in−in−1cm)/(in+in+1cm)×100%≦X % (2)
Here, the subscript “n−1 cm” of i in Expression (2) signifies the node number of the pixel that is closest to the position 1 cm removed from the node n.
For example, when the display screen of the display device has a pixel resolution of 100 PPI (pixels per inch) and has 176×RGB×220 pixels as used for example in mobile phones, then two pixels that are separated by 1 cm on the display screen are spaced apart by 39 pixels.
Referring to
When the equation for the voltage Vk in the above-mentioned recursive equations is calculated for k's in the range of 2≦k≦n, then Vk can be expressed as:
Vk=((Re/Rx)+2)·Vk−1−Vk−2
Moreover, for k=1, V1 can be expressed as:
V1=((Re/Rx)+1)·V0
It should be noted that ik=Vk/Rx.
From the above, if the resistance ratio of Rx/Re is given, the maximum fluctuation rate Δin defined by Expression (2) can be determined, so that the range of resistance ratios Rx/Re within the predetermined tolerance range of X % can be determined through a predetermined numerical calculation. Thus, when the display device uses the above-described display screen, namely a display screen whose pixel resolution is 100 PPI and whose number of pixels is 176×RGB×220, then the condition for the resistances Re and Rx at which the condition of Expression (2), which is based on the afore-mentioned recursive equations, is true, can be determined by a numerical calculation to be Rx/Re≧3.77×105.
Moreover, in this display screen, only one current supply point is provided for the power source line leading to a group of display elements of one column, but variations in the current Ioled can be suppressed by providing current supply points at both ends (upper end and lower end) of the power source line linked to the display element group of one column (referred to as “display element column” in the following). Therefore, it is possible to relax the condition for the resistance ratio Rx/Re, that is, to make the lower limit thereof smaller. The above-mentioned variation in the case that current is supplied from both ends of the display element column can be regarded as the variation when current is supplied only from one side but the number of nodes is cut in half. Consequently, if the above-described display screen is used and current is supplied from both ends of the display element columns, then the number of nodes for the calculation can be taken to be 110, which is half of the actual number of nodes 220. That is to say, the calculation can be made for the case that in the recursive equations and in Expression (2), n is 109. In this case, the condition for the resistances Re and Rx at which the condition of Expression (2), which is based on the afore-mentioned recursive equations, is true can be calculated by the same numerical calculation to be Rx/Re≧1.73×105.
It should be noted that since this way of calculating the resistance (resistance ratio) can be easily applied in accordance with the number and location of the current supply points, it is possible to apply it to display devices having any number of display pixels and any pixel resolution.
The following is a description of a configuration for disposing the electrode of the power source line having the above-noted resistance ratio in an appropriate manner within the display elements.
Here, the widths Wg, Ws and We of the scanning signal line electrode 11, the data signal line electrode 12, and the power source line electrode 13 can be set as appropriate and are ordinarily designed to values satisfying the operation characteristics of the overall display element. For example, the scanning signal line electrode 11 and the data signal line electrode 12 are set to such electrode widths that the waveform is not noticeably contorted at any position in the display screen and a sufficient pulse can be attained during the selection period. In contrast to these electrodes, the temporal variation ratio of the voltage applied to the power source line electrode 13 is small, because basically a direct current is applied to the power source line electrode 13. However, this power source line electrode 13 applies the current to the organic EL element included in the pixel aperture portion 15 via the TFT circuit portion 14, so that there is a voltage distribution that is subject to the voltage drop at each of the display elements. For example, when the width We of the power source line electrode 13 is comparatively small, then the voltage drop between the display elements becomes large, because the electrode resistance per display element becomes relatively large. Therefore, a comparatively large difference in the supply currents occurs between neighboring display elements in the direction in which the power source line electrode 13 is laid out (that is, the column direction). Conversely, when the width We of the power source line electrode 13 is comparatively large, then the difference in the supply currents between neighboring display elements becomes small, because the resistance of the power source line electrode 13 becomes relatively small. Thus, it can be seen that the electrode resistance Re of the power source line 13 is determined depending on the width of each of the electrodes in the display element shown in
The following is a description of the pixel aperture portion 15. As mentioned before, making the electrode resistance Re of the power source line 13 larger means making the electrode width We smaller. This means that the region to be occupied by the TFT circuit portion 14 and the pixel aperture portion 15 covers a relatively wide surface area. Conversely, to make the electrode resistance Re small, it becomes necessary to make the electrode width We large, so that the region to be occupied by the TFT circuit portion 14 and the pixel aperture portion 15 becomes relatively small. Ordinarily, it is not necessarily easy to change the surface area covered by the TFT circuit portion 14 freely in accordance with the size of the electrode width We. For this reason, if this surface area is regarded as a fixed surface area, then making the electrode resistance Re small means ultimately reducing the surface area of the pixel aperture portion 15.
It should be noted that in order to keep the influence of the electrode deterioration due to electromigration and the like to a minimum, it is preferable that ideally, the electrode width We of the power source line electrode 13 is uniform along the direction in which the electrode is laid out, but making the electrode width non-uniform does not cause any major problems. Moreover, the power source line electrode 13 may also have more than two electrode widths or the electrode width may be changed continuously, and there is no particular limitation to the shape of the power source line electrode 13.
Here, in order to set the luminance of the display screen to a predetermined value, an emitted luminance in accordance with the surface area of the pixel aperture portion 15 is necessary, so that a relatively large emitted luminance is required as the numerical aperture becomes smaller. For example, in electro-optical elements such as organic EL elements or LEDs, also the current flowing through the electro-optical element increases as the emitted luminance becomes large.
Thus, the size (width) of the power source line electrode 13 that is minimally necessary in order to suppress variations of the luminance within the display screen can be decided by determining the ratio between the electrode resistance Re of the power source line 13 and the combined resistance Rx of the control TFT 8′ and the organic EL element 9′ when the organic EL element 9′ is lit, such that the condition given by Expression (2) is satisfied. That is to say, if the electrode surface area of the power source line electrode 13 is set such that the numerical aperture becomes largest in accordance with the value of the thus obtained electrode resistance Re, then the luminance variation within the display screen can be suppressed and a display device and display element allowing display with a favorable light emission efficiency can be realized.
It should be noted that the tolerance value X of the luminance variation within the display screen of this display device was set to X=2% in view of the luminance variation tolerance per unit length that is ordinarily used for LCDs, but there is no limitation to this value, and it is also possible to change this value to a value (such as 5%, 8% or 10%) that is appropriate with regard to the display characteristics of the display element.
Moreover, if, in addition to this approach, the conventional approach of equalizing luminance variations by correcting the voltage (or current) of the data signal is used, then it is possible to enhance the above-described advantageous effect. However, this requires special circuitry for correcting the data signals, so that it is not preferable with regard to the increase of the scale of the overall circuitry.
Based on the foregoing, the following are commented examples of calculations of the minimally required resistance ratio Rx/Re and the resistance Re when setting the tolerance value X for current variation per unit length to 2%, 5% 8% and 10%. Numerical values are given for various pixel resolutions of the display screen of the display apparatus according to this embodiment, numbers of pixels in column direction (referred to as “column pixel number” in the following), numbers of current supply points, and combined resistances Rx.
As a first calculation example, the resistance ratio Rx/Re is calculated from the above-noted recursive equations as well as Expression (2) for the case that the pixel resolution and the pixel number in column direction of the display screen of the display device are set to various values. The results are listed in Table 1 below. It should be noted that the current flowing through the current line 13 is supplied from one point on one side (the upper side) for each display element column.
It can be seen from Table 1 that the higher the pixel resolution for the same column pixel number is, the larger is the minimally necessary resistance ratio Rx/Re for the same current variation tolerance value. For example, if the display screen of the display device is a VGA screen (of 640×RGB×480 pixels) with a pixel resolution of 100 PPI, then the column pixel number is 480. It can be seen that in this case, Rx/Re should be set to Rx/Re≧8.28×105 in order to keep the current variation (in other words the luminance variation) in the column direction within 2%. Furthermore, if a display device in which there are no large problems regarding variations of the display luminance is provided with a large current variation tolerance value, then the resistance ratio Rx/Re becomes smaller, so that the resistance ratio Rx/Re, that is, the electrode resistance Re of the power source line 13 can be determined with less strict conditions. For example, referring to Table 1, in a display device having a display screen with the same pixel resolution and column pixel number as above, when the current variation tolerance value is set to 10%, then it is sufficient to set Rx/Re to Rx/Re≧1.16×105. It can be further seen from Table 1 that when the current variation tolerance is set to 8% or to 10%, then the current variation can be kept within the tolerance range for display devices having a comparatively low pixel resolution and a comparatively small column pixel number, even when Rx/Re is set to Rx/Re≧1×105. Furthermore, it can be seen that when Rx/Re is set to Rx/Re≧1×106, and the current variation tolerance value is set to 5% to 10%, then the current variation is kept within the tolerance range for almost all display devices, and also when the current variation tolerance is set to 2%, the current variation can be kept within the tolerance range for display devices with a display screen having a comparatively small column pixel number.
As a second calculation example, the necessary resistance ratio Rx/Re is calculated from the above-noted recursive equations as well as Expression (2) for the case that the pixel resolution and the column pixel number of the display screen of the display device are set to various values. The obtained results are listed in Table 2. It should be noted that different from the first calculation example, the current flowing through the current line 13 is supplied from two points on both sides (upper and lower side) of each display element column, as shown in
It can be seen from Table 2 that, as in the first calculation example, the higher the pixel resolution for the same column pixel number is, the larger is the minimally necessary resistance ratio Rx/Re for the same current variation tolerance value. It also can be seen that the resistance ratios Rx/Re are substantially half those of the first calculation example. For example, if the display screen of the display device is a VGA screen with a pixel resolution of 100 PPI, then the resistance ratio Rx/Re that is minimally necessary in order to keep the current variation per unit length in the column direction within 2% is 4.13×105, which is about half of the value of the first calculation example. Thus, the electrode resistance Re of the power source line 13 may be about twice higher than in the first calculation example. Thus, it can be seen that the upper limit of the resistance that is minimally necessary in order to satisfy the current variation tolerance can be increased by providing more current supply points (voltage application points) from the current supply portion 2.
Here, referring to Table 2, it can be seen that when the current variation tolerance is set to 5% or to 10%, then the current variation can be kept within the tolerance range for display devices having a display screen with a comparatively low pixel resolution and a comparatively small pixel number, as well as for display devices having a display screen with a comparatively high pixel resolution and a comparatively large pixel number, even when Rx/Re is set to Rx/Re≧1×105. Furthermore, it can be seen that when the current variation tolerance is set to 5% or to 10% and Rx/Re is set to Rx/Re≧1×106, then the current variation is kept within the tolerance range even for display devices having a display screen resolution (1920×1080) corresponding to HDTV (high definition television). And when the current variation tolerance is set to 2%, then the current variation can be kept within the tolerance range for almost all display devices, except for display devices having a display screen with a high pixel resolution corresponding to HDTV.
As a third calculation example, the resistance Re when the display device used for the first calculation example is actually configured based on the equivalent circuit shown in
It should be noted that the combined resistance Rx [Ω] is determined from the above-listed numerical values by the following Expression (3):
Rx=10×103/((25.4)2/3×numerical aperture/100)+50×103 (3)
Referring to Table 3, it can be seen that when the number of column pixels is around 220, and the electrode resistance Re of the power source line 13 per display element is about 1.4 to 3Ω, then the current variation tolerance per unit length can be set to 2% or less. Here, for display elements with a numerical aperture of about 50% used for a display screen with, for example, a pixel resolution of 100 PPI and a column pixel number of 220, the length Lp of the power source line electrode 13 is about 20 times larger than the electrode width We of the power source line electrode 13, as shown in
Moreover, if the column pixel number is comparatively large, for example in the case of an SVGA screen (with 800×RGB×600 pixels and a column pixel number of 600), then, according to Table 3, the resistance Re should be about 0.5 to 1Ω, in order to keep the current variation (and thus the luminance variation) in column direction within 2%. In this case, it is necessary to use an electrode material having a sheet resistance that is even lower than in the above case.
Here, a lowering of the sheet resistance can be achieved by using an electrode material with a low sheet resistance, but it can also be achieved by increasing the electrode thickness. Moreover, to avoid the influence of level differences due to increasing the thickness of the electrode, it is also possible to decrease the current variation without increasing the thickness of the electrode by increasing the number of current supply points (voltage application points) in the display element column to two or more, as in the configuration of the second calculation example.
Considerations Regarding Sub-Pixels
Referring to
Here, due to differences in their light-emitting material, the display elements forming the sub-pixels of the colors R, G and B have different voltage/current conditions for displaying a predetermined luminance (here, a white luminance of 200 cd/m2). An example of these voltage/current conditions, the ON resistance of the organic EL elements included in the display elements forming the sub-pixels, the emitted luminance of the pixel, and the resistance ratio Rx/Re is listed in Table 4 below. It should be noted that here, for all display elements forming the sub-pixels of the three colors R, G and B, the sheet resistance of the power source line electrode is 0.1Ω/□, the power source line electrode width is 5 μm, the numerical aperture of the pixels is 50%, the pixel size of each of the RGB pixels is 84 μm×252 μm, and column pixel number of the display device including these display elements is 720. Moreover, the ON resistance of the current control TFT 8′ when the electro-optical element is lit is about 50 kΩ. Furthermore, as in the second calculation example, the current flowing through the power source line 13 is supplied from two points on both sides (upper and lower side) of each display element column, as shown in
Performing the same calculations as for the foregoing calculation examples, the maximum fluctuation ratio of the current per 1 cm differs among the sub-pixels for displaying the colors R, G and B.
As shown in
Here, in order to equalize the maximum current fluctuation rate in the display elements forming the sub-pixels of the various colors, the resistance ratio Rx/Re may be changed as described above. To change the resistance ratio Rx/Re, it is possible to adjust the resistance of the power source electrodes in the display elements forming the various sub-pixels. Here, the surface area of the pixel aperture portion is not changed and the average width of the power source line electrode is set as shown in Table 5 below. It should be noted that the resistance ratio Rx/Re in this case is set to at least 105, as noted above, but if it is not particularly necessary to consider the luminance variation within the display screen, then it is also possible to prevent the colored hue when the resistance ratio Rx/Re is lower than 105.
In this case, the maximum fluctuation rate of the current is substantially the same for the display elements forming the sub-pixels of each of the colors.
Rx(R)/Re(R)≈Rx(G)/Re(G)≈Rx(B)/Re(B) (4)
Here, it is preferable that the resistance ratios Rx/Re of the display elements forming the sub-pixels of the respective colors ideally are identical, but if the proportion of the variation with respect to the average resistance ratio Rx/Re shown in
In addition to this, the maximum fluctuation ratio of the current is consistently lower than 2%/cm (X=2%), which is the tolerance value in LCDs as noted above, so that the luminance variation in the display screen becomes small. Thus, it can be said that the display quality of this display device is sufficiently high.
It should be noted that the present invention is not limited to the display devices and display elements of the foregoing embodiments, and that the present invention can be applied to a variety of configurations with which the advantageous effect of the present invention can be achieved.
With the present invention as described above, the resistance ratio Rx/Re between the resistance Re of the power source line electrode 13 and the combined resistance Rx of a current path from the power source line electrode 13 through the organic EL element 9′ and the current control TFT 8′ with which each display elements is provided to the common electrode VCOM is set to at least a predetermined value, more specifically to at least 1×105. Thus, the balance of the current in the display elements can be adjusted, and it becomes possible to keep the luminance variation within a predetermined tolerance range without changing the scale of the circuitry. Therefore, it becomes possible to provide a display element and a display device having little luminance variation within the display screen, a high numerical aperture, an excellent light emission efficiency, and a long lifetime.
While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.
This application claims priority upon Japanese Patent Application 2003-173992 titled “DISPLAY ELEMENT AND DISPLAY DEVICE,” filed on Jun. 18, 2003, and Japanese Patent Application 2004-76284 titled “DISPLAY ELEMENT AND DISPLAY DEVICE,” filed on Mar. 17, 2004, which are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-173992 | Jun 2003 | JP | national |
| 2004-076284 | Mar 2004 | JP | national |
This application is a continuation of application Ser. No. 10/869,982 filed on Jun. 16, 2004, which claims priority to Japanese application no. 2003-173992 filed on Jun. 18, 2003 and Japanese application no. 2004-76284 filed on Mar. 17, 2004, the disclosures of which are expressly incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5077553 | Buzak | Dec 1991 | A |
| 5272472 | Buzak | Dec 1993 | A |
| 5627457 | Ishiyama et al. | May 1997 | A |
| 6897838 | Okamoto | May 2005 | B2 |
| 20040041761 | Sugita et al. | Mar 2004 | A1 |
| 20040076018 | Okamoto | Apr 2004 | A1 |
| 20050057580 | Yamano et al. | Mar 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 11-344949 | Dec 1999 | JP |
| 2001-083934 | Mar 2001 | JP |
| 2003-216108 | Jul 2003 | JP |
| 2003-303687 | Oct 2003 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20080150424 A1 | Jun 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10869982 | Jun 2004 | US |
| Child | 12033963 | US |
