Organic Electroluminescence Display Device

Abstract

An organic electroluminescence (EL) display device wherein pixel defects are reduced with a minimum increase in the number of thin film transistors used is to be provided. The organic EL display device has: a plurality of power supply lines each for supplying a current to one or another of pixel circuits disposed in an area surrounded by image signal lines and scanning lines; a plurality of split organic EL elements connected in parallel, each connected to one or another of the pixel circuits; a first thin film transistor of which the gate electrode is connected to the signal lines, the source electrode is connected in parallel to the anodes of the plurality of split organic EL elements, and the drain is connected to the power supply lines, and which controls the total amperage to be supplied during the light emitting period to the plurality of split organic EL elements with signals captured from the signal lines; and a plurality of second thin film transistors each disposed between the first thin film transistor and one or another of the split organic EL elements to control the current supplied to each of the split organic EL elements from the first thin film transistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows a one-pixel driving circuit in an organic EL display device, which is an embodiment of the present invention;

FIGS. 2A to 2C illustrate a pixel defect occurring when any leak path has been formed in the organic EL display device shown in FIG. 1;

FIGS. 3A and 3B illustrate examples of configuration of a bottom emission type organic EL display device;

FIGS. 4A and 4B illustrate examples of configuration of a top emission type organic EL display device;

FIGS. 5A and 5B illustrate a one-pixel driving circuit of the organic EL display device according to the earlier cited related art; and

FIGS. 6A to 6C illustrate a pixel defect that will arise when a leak occurs in a pixel in the organic EL display device of the configuration shown in FIGS. 5A and 5B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the invention will be described in detail below with reference to the accompanying drawings thereof.

Embodiment

FIG. 1 shows a one-pixel driving circuit in an organic EL display device, which is an embodiment of the invention. In FIG. 1, DTL denotes a signal line; RSL, a reset line (scanning line); PWL, a power supply line; and SWL, a light on/off switching signal line. The gate electrode of a first thin film transistor TFT1 is connected to the signal line DTL via a pixel capacitance CAP. A first electrode (lower electrode, which is an anode) of one pixel is split into four, and the resultant split organic EL elements OLED1, OLED2, OLED3 and OLED4 are connected in parallel to a first thin film transistor TFT1. Neither the organic EL light emitting layer nor a second electrode (upper electrode, which is a cathode) is not split.

The first thin film transistor TFT1 is a drive transistor, of which the drain electrode is connected to the power supply line PWL and the source electrode, to the split organic EL elements OLED1, OLED2, OLED3 and OLED4 of the first electrode via the drain-source of a second thin film transistor TFT2.

A third thin film transistor TFT3, connected between the connection point between a pixel capacitance CAP and the first thin film transistor TFT1 and the drain electrode of the first thin film transistor TFT1, discharges the accumulated electric charge of the pixel capacitance CAP at the end of one frame period to prepare for the next signal.

In this embodiment, second thin film transistors TFT21, TFT22, TFT23 and TFT24 are arranged intervening between the first thin film transistor TFT1 and the split organic EL elements OLED1, OLED2, OLED3 and OLED4, respectively. The gate electrodes of the second thin film transistors TFT21, TFT22, TFT23 and TFT24 are connected in common to a light on/off switching signal line SWL. The number of the increased transistors is the same as the number of anode splits.

FIGS. 2A to 2C illustrate a pixel defect occurring when any leak path is formed in the organic EL display device shown in FIG. 1. FIG. 2A shows the one-pixel driving circuit shown in FIG. 1, and FIG. 2B is an enlarged view of a pixel part PXC surrounded by broken lines in FIG. 2A. The quadrisected lower electrodes BEL1, BEL2, BEL3 and BEL4 in the pixel are simultaneously driven by the first thin film transistor TFT1. FIG. 2C shows the display state of the pixel in which a leak path is formed between the lower electrodes BEL1, BEL2, BEL3 and BEL4 and the upper electrode.

It is supposed here a case in which a leak path is formed in the area of BEL3 out of the lower electrodes BEL1, BEL2, BEL3 and BEL4 of the four split organic EL elements constituting one pixel. In this case, the split organic EL element configured in the area of the lower electrode 3 does not emit light. However, since the areas of the other split organic EL elements normally emit light, this pixel secures 75% brightness (luminance). Incidentally, since the on-resistances of the second thin film transistors TFT 21, 22, 23 and 24 are sufficiently higher than the resistances of the organic EL light emitting layer, there is no current concentration on the split organic EL element in which the leak path is formed, but the current is distributed to the remaining normal split organic EL elements.

The embodiment can provide an organic EL display device whose pixel defects are reduced with a minimum increase in the number of thin film transistors used. The number of pixel splits is not limited to the four in the above-described embodiment, but two or more splits can remedy almost any pixel defect (black point defect).

FIGS. 3A and 3B illustrate an example of configuration of a bottom emission type organic EL display device. FIG. 3A is a sectional view schematically illustrating the overall configuration, and FIG. 3B is a sectional view illustrating an exemplary structure of the unit pixel. The bottom emission type organic EL display device has a thin film transistor TFT over an insulating substrate SUB, for which a glass substrate is suitable, a first electrode or one electrode (hereinafter referred to as the lower electrode or a transparent electrode (ITO or the like) as the pixel electrode) BEL is formed through a contact hole bored in an insulating film INS. The lower electrode BEL is split into unit pixels, each constituting an independent split organic EL element.

A bank BNK formed of an insulator is disposed over the formation area of the thin film transistor TFT, and constitutes an accommodating part for an organic EL light emitting layer ILL, which emits light when an electric field is applied to it, by serving as partitioning between adjacent unit pixels. A reflective metal electrode as a second electrode (common electrode) or the other electrode, namely the upper electrode, is formed covering the organic EL light emitting layer ILL. The insulating substrate SUB having on its main face the organic EL element configured in this way is isolated from the external atmosphere by a sealing can CAV, and sealed with a sealing material, such as an adhesive. Incidentally, within the interior sealed by the sealing can CAV, a drying agent or a hygroscopic agent DSC is held.

Then, carriers (electrons and holes) are implanted into the organic EL element, configured of an organic multilayered film, to cause the organic multilayered film to emit light by applying an electric between the lower electrode BEL and the upper electrode UEL, which respectively may be the anode and the cathode, for instance. The luminescence (L) from the organic EL element is emitted through the insulating substrate SUB. The unit pixels of this organic EL element are color pixels of red (R), green (G) and blue (B), and full color image displaying is achieved by arranging these color pixels.

FIGS. 4A and 4B illustrate examples of configuration of a top emission type organic EL display device. FIG. 4A is a sectional view schematically illustrating the overall configuration, and FIG. 4B is a sectional view illustrating an exemplary structure of the unit pixel. In the top emission type organic EL display device, a reflective metal electrode is used as the lower electrode BEL which corresponds to one electrode in the bottom emission type described above, and a transparent electrode such as ITO is used as the upper electrode UEL, the counterpart of the other electrode. By applying an electric field between the two electrodes, an organic multilayered film is caused to emit light, and this luminescence L is emitted from the upper electrode UEL side. The lower electrode BEL in each unit pixel is split to constitute an independent split organic EL element. In the top emission type, a transparent plate, which suitably is a glass plate, is used as the counterpart of the sealing can in the bottom emission type, and a transparent material is used as the drying agent or the hygroscopic agent DSC, which, if not transparent, is arranged where it would not intercept display light. Other aspects of the configuration are substantially the same as their counterparts in FIG. 3.

Claims

1. An organic electroluminescence display device including a plurality of organic electroluminescence elements, wherein: the organic electroluminescence elements are arranged in an area surrounded by image signal lines and scanning lines, have a first thin film transistor, a light emitting unit split into a plurality of parts and a plurality of second thin film transistors;the plurality of light emitting subunits resulting from the splitting are driven by the first thin film transistor; andthe plurality of light emitting subunits resulting from the splitting are connected to the plurality of second thin film transistors.

2. The organic electroluminescence display device according to claim 1, wherein the gates of the plurality of second thin film transistors are connected to a common signal line.

3. The organic electroluminescence display device according to claim 1, Wherein: a third thin film transistor is formed between the source or drain electrode of the first thin film transistor and the gate of the first thin film transistor; andthe gate of the third thin film transistor is connected to the scanning lines.

4. The organic electroluminescence display device according to claim 1, wherein a capacitance is connected to the gate of the first thin film transistor and the other end of the capacitance is connected to the image signal lines.

5. The organic electroluminescence display device according to claim 1, wherein the light emitting unit is split into four subunits.

6. An organic electroluminescence display device including a plurality of organic electroluminescence elements, wherein: the organic electroluminescence elements are arranged in an area surrounded by image signal lines and scanning lines, have a first thin film transistor, a light emitting unit split into a plurality of parts and a plurality of second thin film transistors;the light emitting unit split into a plurality of parts includes an upper electrode, a lower electrode and a plurality of organic electroluminescence films formed between the upper electrode and the lower electrode; andeach of the plurality of subunits resulting from the splitting matches one of the second thin film transistors.

7. The organic electroluminescence display device according to claim 6, wherein a power supply line which supplies a current to the light emitting unit is connected to the source electrode of the first thin film transistor;the source electrode of each of the plurality of second thin film transistors is connected to the drain electrode of the first thin film transistor; andthe drain electrode of each of the plurality of second thin film transistors is connected to the split light emitting subunits.

8. The organic electroluminescence display device according to claim 6, wherein the gates of the plurality of second thin film transistors are connected to a common signal line.

9. The organic electroluminescence display device according to claim 6, Wherein: a third thin film transistor is formed between the drain electrode of the first thin film transistor and the gate electrode of the first thin film transistor; andthe gate electrode of the third thin film transistor is connected to the scanning lines.

10. The organic electroluminescence display device according to claim 6, wherein a capacitance is connected to the gate electrode of the first thin film transistor and the other end of the capacitance is connected to the image signal lines.

11. The organic electroluminescence display device according to claim 6, wherein the light emitting unit is split into four subunits.