Display device

The invention provides a method and apparatus to obtain accurate gradation by obtaining an accurate ratio of light emitting parts in a display device which implements gradation by forming a plurality of TFTs and a plurality of OELDs in each pixel, directly connecting the TFTs and OELDs, and switching an on and off state of the TFTs, and controlling a light emitting area of the OELDs. A plurality of OELDs have the same shape, and gradation can be implemented by controlling the number of OELDs that emit light. A plurality of OELDs have a round shape. A plurality of OELDs are arranged at the same interval in a vertical and/or horizontal direction. According to this structure, because the light emitting areas of the plurality of OELDs become equal to each other, by controlling the number of OELDs, a ratio of the light emitting areas can be accurately obtained.

BACKGROUND OF THE INVENTION
 1. Field of Invention
 This invention relates to a display device, particularly a thin film
 transistor driven organic electro-luminescent display device (hereafter
 referred to as TFT-OELD) which is driven by a thin film transistor
 (hereafter referred to as TFT) and provided with an organic
 electro-luminescent element (hereafter referred to as OELD) of a high
 polymer system formed in a liquid phase process.
 2. Description of Related Art
 TFT-OELD is promising because it is a display device which realizes
 light-weightness, thinness, smallness, higher accuracy, wider view angle,
 lower electric consumption, and the like. FIG. 1 shows a conventional
 TFT-OELD. FIG. 2 shows a cross-sectional view of the conventional
 TFT-OELD. Here, there is only one pixel 11 depicted, but there are
 actually many pixels 11 in plural rows and lines. Here, OELD 18 is a high
 polymer system, formed by a liquid phase process, such as spin coating,
 blade coating, ink jet, or the like.
 In order to implement gradation, in the case of the structure shown in FIG.
 1, a gate voltage of a driving TFT 17 is made to change and conductance is
 changed, so electric current which flows in the OELD 18 needs to be
 controlled. However, according to this method, particularly in half tone,
 irregularity of transistor characteristics of the driving TFT 17 appears
 as brightness irregularity of the OELD 18, and there is a problem such
 that the screen becomes non-uniform.
 Therefore, as shown in FIG. 3, a method is considered which implements
 gradation by changing a light emitting area of the OELD 18 (Japanese
 Patent Application 9-233107). FIG. 4 shows a driving method of this
 method. A scanning electric potential 31 is applied to a scanning line 12,
 and a signal line 13 is formed of a signal line (lower bit) 131 and a
 signal line (upper bit) 132. A signal electric potential (lower bit) 321
 and signal electric potential (upper bit) 322 are respectively applied as
 a signal electric potential 32. A driving TFT 17 is formed of a driving
 TFT (lower bit) 171 and a driving TFT (upper bit) 172, and the OELD 18 is
 formed of an OELD (lower bit) 181 and an OELD (upper bit) 182. In this
 example, 2-bit 4 gradation is considered, so an area ratio between OELD
 (lower bit) 181 and OELD (upper bit) 182 is 1:2.
 In this method, the driving TFT 17 takes either a substantially completely
 on state or a substantially completely off state. In the on state, the
 resistance of the driving TFT 17 is small enough to be ignored, compared
 to the resistance of OELD 18, and the electric current amount which flows
 in the driving TFT 17 and OELD 18 is substantially determined by only the
 resistance of the OELD 17.
 Therefore, irregularity of transistor characteristics of the driving TFT 17
 does not appear as brightness irregularity of the OELD 18. Furthermore, in
 the off state, voltage applied to the OELD 18 becomes less than a
 threshold voltage, so the OELD 18 does not emit light at all, and,
 needless to say, irregularity of transistor characteristics of the driving
 TFT 17 does not appear as brightness irregularity of the OELD 18.
 FIG. 5 is a cross-sectional view of TFT-OELD which implements gradation
 display by changing a light emitting area of the OELD 18 shown in FIGS. 3
 and 4. FIG. 5(a) is a cross-sectional view of the OELD (lower bit) 181,
 and FIG. 5(b) is a cross-sectional view of the OELD (upper bit) 182. The
 ratio between the light emitting part 25 of the OELD (lower bit) 181 and
 the light emitting part 25 of the OELD (upper bit) 182 is preferably 1:2.
 A light emitting layer 22 is an OELD of a high polymer system and formed in
 a liquid phase process. A surface of a bank 24 is lyophobic and the light
 emitting layer 22 does not remain. Therefore, the area of the OELD 18 is
 determined by patterning. With respect to a side surface of the bank 24,
 the materials and processing determine whether the side surface of the
 bank 24 becomes lyophobic or lyophilic.
 FIG. 5 shows the case of a lyophilic side surface of the bank 24. As a
 phenomenon that is characteristic of a liquid phase process, the light
 emitting layer 22 has a cross-sectional shape which is pulled toward the
 side surface of the bank 24. In this case, electric current flows into a
 thinner part of the light emitting layer 22, and this part becomes a light
 emitting part 25. The cross-sectional shape of the light emitting layer 22
 described here is sensitive to liquid amount, liquid material, an initial
 position of the liquid, and a state, temperature, atmosphere, or the like
 of a substrate, and which are difficult to control. That is, it is
 difficult to obtain an absolute value of a desired light emitting area.
 Because of this, it is difficult to obtain an accurate ratio of 1:2,
 between the light emitting part 25 of the OELD (lower bit) 181 and the
 light emitting part 25 of the OELD (upper bit) 182, and ultimately, it is
 difficult to obtain accurate gradation.
 FIG. 6 is a cross-sectional view of OELD (lower bit) 181 (FIG. 6(a)) and a
 cross-sectional view of OELD (upper bit) 182 (FIG. 6(b)) in the same
 manner as in FIG. 5. In FIG. 6, the side surface of the bank 24 is
 lyophobic. As a phenomenon that is characteristic of a liquid phase
 process, the light emitting layer 22 has a cross-sectional shape which is
 distant from the side surface of the bank 24. In this case as well,
 electric current flows into the thinner part of the light emitting layer
 22, and this part becomes the light emitting part 25. In this case as
 well, in the same manner as in the case of FIG. 5, it is difficult to
 obtain an accurate ratio of 1:2 between the light emitting part 25 of the
 OELD (lower bit) 181 and the light emitting part 25 of the OELD (upper
 bit) 182, so it is difficult to obtain accurate gradation.
 SUMMARY OF THE INVENTION
 Therefore, one aspect of this invention is to obtain an accurate ratio of
 the light emitting parts, and accurate gradation. Therefore, the invention
 may provide a display device in which gradation is implemented by forming
 a plurality of TFTs and a plurality of OELDs in each pixel, directly
 connecting the TFTs and OELDs, switching an on and off state of the TFTs,
 and controlling an area of the OELDs, that emits light, wherein the
 plurality of OELDs have the same shape, and gradation is implemented by
 controlling the number of OELDs that are created to emit light.
 According to this structure, as a characteristic phenomenon of a liquid
 phase process, even if an OELD becomes a cross-sectional shape which is
 pulled in to a side surface of a bank or is distant from the side surface
 of the bank, the light emitting part of each OELD is the same area, and
 accurate gradation can be obtained. In this structure as well, it is
 difficult to obtain an absolute value of a desired light emitting area,
 but the light emitting area of a plurality of OELDs becomes equal, so the
 ratio of the light emitting areas can be accurate by controlling the
 number OELDs.
 The display device may also include a plurality of OELDs that have a round
 shape. According to this structure, the light emitting part of each OELD
 can reliably be the same area, and accurate gradation can be obtained. The
 reasons are as follows. When an OELD has a shape with a rectangular vertex
 (or vertices), there is a possibility that a phenomenon may occur, for
 example, that the vertex becomes pulled in or the vertex cannot be filled.
 This phenomenon prevents a user from obtaining accurate gradation for the
 same reason as in the problems of a cross-sectional shape as described
 above. This phenomenon is more sensitive to the liquid amount, liquid
 material, initial position of liquid, and the state, temperature, and
 atmosphere of a substrate, more so than the problems in a cross-sectional
 shape described above, and it is difficult to control this phenomenon
 between adjacent OELDs. By making the OELD round shaped, this phenomenon
 can be avoided.
 The display device may also include a plurality of OELDs are arranged at
 the same interval in a horizontal and/or vertical direction. According to
 this structure, the light emitting part of each OELD is made to be more
 reliably the same area, and accurate gradation can be obtained. The
 reasons are as follows. When OELD is formed by spin coating or blade
 coating, the light emitting layer which is coated over all the pixels, due
 to the lyophobicity of a surface of the bank, the light emitting layer
 naturally flows into a convex part of the bank. In the case of ink jet as
 well, this may sometimes happen. At this time, when a concave area
 surrounded by a bank convex part is large, the light emitting layer coated
 over this part flows into a convex bank portion, so the light emitting
 layer becomes thick. When the convex area surrounded by the bank concave
 part is small, the light emitting layer becomes thin. Ultimately,
 irregularity of film thickness of the light emitting layer is generated.
 This irregularity can be avoided by arranging a plurality of OELDs at the
 same interval in a horizontal or vertical direction.
 Additionally, according to this structure, when the OELDs are formed by an
 ink jet process, ink jetting can be performed at the same interval, so
 fabrication can be simplified.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The following explains preferred embodiments of this invention, based upon
 the drawings.
 FIG. 7 is a diagram showing a pixel of a TFT-OELD related to a first
 embodiment of this invention. Here, only one pixel 11 is depicted, but
 many pixels 11 actually exist in a plurality of rows and a plurality of
 lines.
 An OELD (lower bit) 181 is formed of OELD (lower bit.cndot.rectangular)
 1811, and an OELD (upper bit) 182 is formed of OELD (upper
 bit.cndot.first.cndot.rectangular) 18211 and an OELD (upper
 bit.cndot.second.cndot.rectangular) 18221. As indicated in claim 1, the
 OELD (lower bit.cndot.rectangular) 1811, the OELD (upper
 bit.cndot.first.cndot.rectangular) 18211, and the OELD (upper
 bit.cndot.second.cndot.rectangular) 18221 have the same shape, so the same
 light emitting area can be obtained, and accurate gradation can be
 obtained by changing the number of OELDs that are caused to emit light.
 FIG. 8 is a diagram showing a pixel of a TFT-OELD related to another
 embodiment of this invention. Here, only one pixel 11 is depicted, but
 many pixels 11 actually exist in a plurality of lines and a plurality of
 rows.
 An OELD (lower bit) 181 is formed of an OELD (lower bit.cndot.round shape)
 1812, and an OELD (upper bit) 182 is formed of OELD (upper
 bit.cndot.first.cndot.round shape) 18212 and OELD (upper
 bit.cndot.second.cndot.round shape) 18222. As indicated in claim 2, the
 OELD (lower bit.cndot.round shape) 1812, the OELD (upper
 bit.cndot.first.cndot.round shape) 18212, and the OELD (upper
 bit.cndot.second.cndot.round shape) 18222 have the same round shape, so
 the same light emitting area can be reliably obtained, and accurate
 gradation can be obtained.
 FIG. 9 is a diagram showing a pixel of a TFT-OELD related to another
 embodiment of this invention. Here, only one pixel 11 is depicted, but
 many pixels 11 actually exist in a plurality of lines and a plurality of
 rows.
 An OELD (lower bit) 181 is formed of an OELD (lower bit.cndot.round shape)
 1812, and an OELD (upper bit) 182 is formed of an OELD (upper
 bit.cndot.first.cndot.round shape) 18212 and an OELD (upper
 bit.cndot.second.cndot.round shape) 18222. As indicated in claim 3, the
 OELD (lower bit.cndot.round shape) 1812, the OELD (upper
 bit.cndot.first.cndot.round shape) 18212, and the OELD (upper
 bit.cndot.second.cndot.round shape) 18222 are arranged at the same
 interval in horizontal and vertical directions within the pixel 11, and
 also with respect to the adjacent pixel 11. Because of this, the light
 emitting part of each OELD can more reliably have the same area, and
 accurate gradation can be obtained.
 Furthermore, as an EL element formed in each pixel, in first embodiment
 (FIG. 7), a rectangular element is shown as an example, and in the second
 and third embodiments (FIGS. 8 and 9, respectively), a round-shaped
 element is shown, but this invention is not limited to these. Accurate
 gradation can also be obtained in a polygonal-or elliptic-shaped element.
 In particular, an elliptic element, as in the case of a round shape, does
 not have a vertex such as is present in the case of a rectangular shape,
 so there is no problem such that the vertex cannot be filled by the light
 emitting layer.
 As described above, according to this invention, by controlling the area of
 an electro-luminescent element that emits light, accurate gradation can be
 realized.