JP2010049056A - Organic el display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic EL panel that can perform operation in a linear area of a drive TFT in a wide range. <P>SOLUTION: A channel area of a thin film transistor is formed to have fixed channel width both in a longitudinal direction of crystal grain shape that crystalizes the thin film transistor and a direction orthogonal thereto. The sum L (L=L<SB>h</SB>+L<SB>V</SB>) of channel length L<SB>h</SB>in the longitudinal direction of the crystal grain shape and channel length L<SB>V</SB>in the direction orthogonal thereto in sub pixels 13R, 13G and 13B having different emitted light color is equal, and L<SB>h</SB>/L<SB>V</SB>is different. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an organic EL display, and more particularly to an active matrix organic EL display including a self-light emitting element such as an organic electroluminescence element and a thin film TFT.

  There are two types of flat panel displays: a self-luminous type that emits light itself and a non-luminous type that uses external light or a backlight. Unlike the non-light-emitting liquid crystal display panel, the organic display panel is a self-luminous type, and thus is a flat panel display having features of high contrast and high viewing angle.

  This organic EL display is configured by arranging a large number of organic EL elements as pixels and arranging them in a matrix. As a driving method of the organic EL element, there are passive driving and active driving. Comparing the passive drive and the active drive, the active drive is preferable for realizing a high-definition and high-speed response panel. A TFT used for a flat panel display such as an organic EL element or a liquid crystal display element is composed of a switching TFT for controlling the operation of each pixel and a driving TFT for driving the pixel.

  In an organic EL display, generally, the light emission efficiency differs depending on subpixels having different emission colors such as R, G, and B. For this reason, when the same current is applied, a sub-pixel of a certain emission color has high luminance, and a sub-pixel of another emission color has low luminance. Therefore, it is difficult to obtain an appropriate white balance.

  In order to solve such a problem, a method of applying different power supply voltages for each sub-pixel has been disclosed (see Patent Document 1). However, if possible, a method in which the power supply voltage is not changed between the sub-pixels is preferable.

  As another method for adjusting the amount of current, it is conceivable to create mobility that has a proportional relationship with the amount of current of the driving TFT. As an easier process, a method of adjusting white balance by changing the relative angle between the channel direction of the driving TFT and the growth direction of the anisotropic crystal is disclosed (see Patent Document 2). This method takes into account the grain boundary direction of polycrystalline silicon.

  In the driving TFT described in Patent Document 2, the amount of current is controlled by making different mobility by changing the angle between the crystal anisotropy direction and the channel direction. Therefore, in the method described in Patent Document 2, in adjusting the electron mobility, the direction of the active layer of silicon may not be arranged in parallel with any of the signal line, the scanning line, and the voltage supply line. For this reason, compared with the organic electroluminescent display of this invention, a useless area | region arises in a layout. Thus, it is clear that the present invention is preferable as a high-definition organic EL display.

Japanese Patent Laid-Open No. 5-107561 JP 2004-272193 A

  In the state where the same driving voltage is applied to the organic EL element, the current can be adjusted for each of the R, G, and B subpixels regardless of the difference in efficiency of each of the R, G, and B subpixels. An object of the present invention is to provide an organic EL display capable of adjusting the brightness.

In order to achieve the above-described object, the organic EL display of the present invention has the following features. That is, in the organic EL display of the present invention, a plurality of pixels having an organic light emitting element and a driving circuit including a thin film transistor that defines a driving current of the organic light emitting element are arranged on the substrate, Each of the pixels is an organic EL display including at least two subpixels having different emission colors,
The channel region of the thin film transistor is formed of a semiconductor active layer having an anisotropic crystal grain shape, and the channel length direction is formed both in the longitudinal direction of the crystal grain shape and in a direction intersecting therewith,
The sum L (L = L _h + L _v ) of the channel length L _{h in} the longitudinal direction of the crystal grain shape and the channel length L _{V in} the direction intersecting with it in at least two sub-pixels having different emission colors is the ratio L _h. / L _V are different from each other.

Since the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{v in} the direction perpendicular thereto differs for each subpixel, the average mobility in the channel length direction differs for each subpixel, and different current amounts are obtained. Can do. For this reason, it is possible to adjust the white balance even when the luminous efficiencies of the emission colors of the R, G, and B sub-pixels are different.

  Hereinafter, embodiments of the organic EL display of the present invention will be described with reference to the drawings.

<Outline of organic EL display>
In the organic EL display of the present invention, a plurality of pixels having an organic light emitting element and a driving circuit including a thin film transistor that defines a driving current of the organic light emitting element are arranged on a substrate, and each pixel emits light. Sub-pixels having different colors are included. The active layer constituting the thin film transistor has an anisotropic crystal grain shape, and the channel region is formed in both the longitudinal direction of the crystal grain shape and the direction intersecting the crystal grain shape with a constant channel width. Further, the channel length L _{h in} the longitudinal direction of the crystal grain shape and the sum L (L = L _h + L _v ) of the intersecting channel length L _{V in} the at least two subpixels having different emission colors are equal, and the ratio L _h / L _V is different.

  In the thin film transistor, the magnitude of the drive current can be changed by changing L or W. However, if L or W of the thin film transistor is made different between the sub-pixels constituting one pixel, the area occupied by the thin film transistor is different, so that the circuit layout of each sub-pixel is also different. As a result, a characteristic change that is difficult to predict due to a difference in parasitic capacitance occurs, which is not preferable. The present invention adjusts the mobility by distributing the channel length in both the longitudinal direction and the lateral direction of the crystal grain shape while keeping the overall channel length L of the thin film transistor of each subpixel equal. As a result, the current driving capability of the thin film transistor can be made different between the sub-pixels without changing L and W.

When a pixel displays white, that is, when the sub-pixels is maximum luminance, the more subpixels larger current value supplied to the organic light emitting device, it is preferable to increase the ratio L _h / L _V. The semiconductor active layer constituting the channel region is preferably mainly composed of polycrystalline silicon, and this polycrystalline silicon is preferably formed by a crystallization method using a laser.

As described above, according to the organic EL display of the present invention, the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{v in} the direction perpendicular to the laser irradiation is different for each sub-pixel. the average mobility mu _x of can be varied in the range of _{μ v ≦ μ x ≦ μ h} ( see FIG. 5).

Further, since the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{v in} the direction perpendicular to the longitudinal direction of the crystal grain shape differs for each sub-pixel, an average mobility μ _x that differs for each sub-pixel is obtained. be able to. Therefore, even if the pixel has a different efficiency for each sub-pixel, the amount of current is proportional to the average mobility. Therefore, white balance can be adjusted while supplying the same power supply voltage for each pixel.

<Embodiment>
FIG. 1 is a schematic plan view showing the arrangement of TFTs in each color of the organic EL display of the present invention. In FIG. 1, 10 is a scanning line, 11 is a signal line, 12 is a voltage supply line, 13R is a red subpixel, 13G is a green subpixel, 13B is a blue subpixel, 14 is a source, 15 is a drain, 16 Denotes a gate, and 17 denotes an active layer.

  In FIG. 1, only the driving transistor that defines the current of the organic light emitting element is illustrated in each sub-pixel, but includes other transistors, circuit elements such as capacitors and wirings.

The driving transistors of the three subpixels have the same channel width W and channel length L. L is divided into L _h and L _v , and L _h is shortest in R and longest in B. As will be described later, since the mobility in the longitudinal direction of the crystal grain shape is larger than the mobility in the direction perpendicular thereto, the average mobility is the smallest in R and increases in the order of G and B.

  As shown in FIG. 1, the organic EL display according to the embodiment of the present invention includes a red subpixel 13R, a green subpixel 13G, and a blue subpixel 13B. Each of the sub-pixels 13R, 13G, and 13B is provided with a source 14, a drain 15, a gate 16, and an active layer 17, and a scanning line 10, a signal line 11, and a voltage supply line 12 are disposed.

  FIG. 2A is a schematic plan view of an arbitrary sub-pixel constituting the organic EL display of the present invention. A planar pattern of the SD wiring metal 57 for making contact with the active layer 53, the gate 55, and the source 58 and the drain 59 is shown. A portion where the active layer 53 and the source 55 overlap is a channel region. FIG. 2B is a cross-sectional view taken along the dotted line passing through the center of the channel in FIG. The channel length in this specification is defined as the length along the dotted line shown in FIG.

  As shown in FIG. 2B, the sub-pixel of the organic EL display according to the present embodiment has a base film 52 made of silicon nitride (SiN) or the like on an insulating substrate 51 made of quartz glass, non-alkali glass or the like. And an active layer 53 of poly-Si is formed thereon. Further, a gate insulating film 54 and a gate 55 made of a refractory metal are formed on the active layer 53.

Then, an interlayer insulating film 56 made of silicon oxide (SiO ₂ ) or the like is formed on the entire upper surfaces of the gate insulating film 54 and the gate 55. Further, a contact hole is opened at a place where the source 58 and the drain 59 are formed, and a metal 57 such as Al is filled in the opening to form the source 58 and the drain 59. 2A and 2B, reference numeral 57 denotes an SD wiring, and the gate, source, and drain wirings are omitted.

In the organic EL display, since the light emission efficiency of each of the R, G, and B subpixels is different, a difference occurs in the luminance of each light emitting layer. Therefore, in the conventional organic EL display, white balance cannot be adjusted when the same current value flows. For example, the luminous efficiencies of the R, G, and B subpixels are R: 10.3 (cd / A), G: 16.7 (cd / A), and B: 2.6 (cd / A). is there. When trying to obtain a luminance of 500 (cd / m ² , an aperture ratio of 50%), the current value for adjusting the white balance in consideration of the visibility is 8 (mA / cm ² ), which is the smallest for the R subpixel, The G subpixel is 11 (mA / cm ² ), and the B subpixel is the largest, 15 (mA / cm ² ).

Such a difference in current value, as shown in FIG. 1, the ratio of L _v and L _h of the driving TFT, and can be corrected by separately forming the subpixels of different colors.

When μ _h / μ _v = 2, R is set as L _v : L _h = 1: 0, G is set as L _v : L _h = 3: 2, and B is set as L _v : L _h = 1: 9. , White balance can be adjusted. In the following description, an embodiment in which the sub-pixel is composed of each color of R, G, and B will be described, but the present invention is not limited to a mode in which the sub-pixel is composed of each color of R, G, and B.

  FIG. 3 is a schematic plan view showing the anisotropic crystal structure of the polycrystalline silicon thin film forming the active layer of the TFT. This is a method in which a continuous wave (CW) laser is used to scan a laser beam applied to a silicon film formed on a substrate or a stage on which a substrate on which a silicon film is formed is set in one direction with respect to the substrate surface. As laser irradiation conditions, the area of the laser irradiation portion is 5 μm in the scanning direction and the beam width is about 50 μm, and the scanning speed is determined by the silicon film thickness, the laser output intensity, the base insulating film thickness, and the like. Scan in a direction perpendicular to the long axis of the line beam. In order to perform crystal growth in a direction along the laser scanning direction, long crystal grains are formed mainly in a direction parallel to the laser scanning direction. The size of the crystal grains is about 0.2 μm in width and about 3 μm in length.

  In FIG. 3, 20 indicates a crystal grain boundary. In this specification, the average direction of the crystal grain boundary 20 is referred to as the longitudinal direction of the anisotropic crystal grain shape.

When crystallization is performed by continuous wave (CW) laser irradiation, anisotropic crystal grains that are long in one direction are formed, and the mobility in the longitudinal direction of the crystal grain shape is large. That is, the relationship between the magnitudes of mobility is μ _h > μ _v . In Patent Document 2 described above, the ratio of μ _h to μ _v is about twice. The ratio between μ _h and μ _v can be changed depending on the conditions for laser crystallization, the conditions for performing solid phase crystallization (SPC) before laser crystallization, and the like. In the N-channel TFT, the mobility μh in the longitudinal direction of the crystal grain shape is about 400 (cm ² / V · s), and the mobility μv in the direction perpendicular to the longitudinal direction of the crystal grain shape is about 200 (cm ² / V · s). s) degree.

  FIG. 4 is a conceptual diagram illustrating a configuration example of a pixel circuit. In FIG. 4, reference numeral 41 denotes a capacitor, 42 denotes a driving TFT, 43 denotes an organic EL element, 44 denotes GND, 10 denotes a scanning line, 11 denotes a signal line, and 12 denotes a voltage supply line.

  As shown in FIG. 4, a scanning line 10 is arranged between pixels in the row direction, and a signal line 11 and a voltage supply line 12 are arranged between the pixels in the column direction. The scanning line 10 extending in the row direction is connected to the gate of a switching TFT 40 that is an n-channel TFT. A signal line is connected to the drain of the switching TFT 40 in the column direction. A capacitor 41 is connected to the source of the switching TFT 40 and the voltage supply line 12. The connection point between the source of the switching TFT 40 and the capacitor 41 is connected to the gate of the drive TFT 42 which is a p-channel TFT. The source is connected to the voltage supply line 12 and the drain is connected to the organic EL element 43. The voltage supply line 12 may be shared by adjacent subpixel columns by making the arrangement symmetrical for each column.

  As shown in FIG. 4, the sub-pixel of the organic EL display according to the embodiment of the present invention includes two TFTs, a switching TFT 40 for switching and a driving TFT 42 for driving, a capacitor 41, and one organic EL element 43. Become. However, the number of TFTs and capacitors is not limited to the number described above, and more TFTs and capacitors may be provided depending on the purpose.

FIG. 5 is a conceptual diagram for explaining the average mobility μ _x determined by the ratio of the channel lengths L _h and L _V.

As shown in FIG. 5, when the mobility in the longitudinal direction of the crystal grain shape is μ _h and the mobility in the direction perpendicular to the longitudinal direction of the crystal grain shape is μ _V , the mobility of the driving TFT is μ _x = (L _h μ _h + L _v μ _v ) / (L _h + L _v ). That is, the average mobility between different subpixels is determined by the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{V in} the direction perpendicular to the longitudinal direction of the crystal grain shape. Since L _h + L _v = L and the channel length L _V is constant between the sub-pixels, μ _x = (L _h μ _h + L _v μ _v ) / L. By designing the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{v in} the direction perpendicular to the longitudinal direction of the crystal grain shape for each subpixel, μ _v ≦ μ _x ≦ μ _H The mobility can be changed within the range.

FIG. 6 is a conceptual diagram for explaining that the drain current I _d changes when the electron mobility μ _x changes.

As shown in FIG. 6, by designing the drive TFT so that the ratio of the channel length L _{h in} the longitudinal direction of the crystal grain shape to the channel length L _{v in} the direction perpendicular to the longitudinal direction of the crystal grain shape is different for each subpixel. The amount of current I _d to the organic EL element can be controlled. In this embodiment, by reducing the mobility of the driving TFT from the mobility μ _{h in} the longitudinal direction of the crystal grain shape to the mobility μ _{v in the} direction perpendicular to the longitudinal direction of the crystal grain shape, I _dsat ≦ I ≦ I _The drain current I _d can be changed within the range of _d ′.

From μ _h to μ _v , the mobility can be created separately for each subpixel. Accordingly, since the drain current amount changes from I _d to I _d ′, the white balance can be adjusted in accordance with the efficiency of each color of the sub-pixel in the drain current range I _d ≦ I ≦ I _{d ′} .

  Patent Document 2 described above discloses a method of applying a different power supply voltage for each sub-pixel as a white balance adjustment method that causes a problem due to the difference in efficiency between the sub-pixels of each color. However, if the power supply voltage is changed for each sub-pixel, it becomes impossible to share the power-supply voltage line between adjacent sub-pixels, which hinders high definition of the pixel area.

The plane schematic diagram of the drive TFT in each sub pixel of the organic EL display of this invention. (A) is the plane schematic diagram of the arbitrary subpixels which comprise the organic EL display of this invention, (b) is a cross-sectional schematic diagram of the dotted-line part in (a). The plane schematic diagram which shows the anisotropic crystal structure of the polycrystalline-silicon thin film which makes the active layer of TFT. The conceptual diagram which shows the structural example of a pixel circuit. Conceptual diagram illustrating the average mobility mu _x determined by the ratio of the channel length L _H and L _V. The conceptual diagram for demonstrating the relationship between source-drain voltage and drain current when the mobility (mu) _{x of a} thin-film transistor changes.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Scan line 11 Signal line 12 Voltage supply line 13R Red subpixel 13G Green subpixel 13B Blue subpixel 14 Source 15 Drain 16 Gate 17 Active layer 20 Grain boundary 40 Switching TFT
41 Capacitance 42 Drive TFT
43 Organic EL device 44 GND
51 insulating substrate 52 underlying film 53 active layer 54 gate insulating film 55 gate 56 interlayer insulating film 57 S-D wiring 58 source 59 drain L _h grain form longitudinal channel length L _v grain shape elongated in the vertical direction channel Long

Claims (3)

A plurality of pixels having an organic light emitting element and a driving circuit including a thin film transistor that defines a driving current for the organic light emitting element are arranged in a plane on the substrate, and each of the pixels has at least different emission colors. An organic EL display including two subpixels,
The channel region of the thin film transistor is formed of a semiconductor active layer having an anisotropic crystal grain shape, the channel width W is constant, and the channel length direction is either the longitudinal direction of the crystal grain shape or a direction intersecting with the longitudinal direction. Is also formed,
The sum L (L = L _h + L _v ) of the channel length L _{h in} the longitudinal direction of the crystal grain shape and the channel length L _{V in} the direction intersecting with it in at least two sub-pixels having different emission colors is the ratio L _h. Organic EL display characterized by / L _V being different. 2. The organic EL display according to claim 1, wherein L _h / L _V is larger in a sub-pixel having a larger current value supplied to the organic light-emitting element when the sub-pixel displays maximum luminance.   3. The organic EL display according to claim 1, wherein the semiconductor active layer is mainly composed of polycrystalline silicon.

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