JP2012242675A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that deterioration of an organic EL element is accelerated due to a large flow of through current when an anode and a cathode of the element are short-circuited in an organic EL display device of voltage driving.SOLUTION: A display device has a plurality of pixel circuits, which are arranged thereon, each include a light emitting element having a pair of electrodes and a luminous layer sandwiched between the pair of electrodes and a switch connecting one of the electrodes of the light emitting element to a voltage source. The switch is set to on or off in response to a video signal given to the pixel circuit, and when the switch is turned on, voltage of the voltage source is applied between the pair of electrodes, and thereby light emitting element emits light. The display device comprises: a memory for storing a position of the pixel circuit including the light emitting element on which a short-circuit between the pair of electrodes occurs, among the arranged plurality of pixel circuits; and a signal processing section for reading output of the memory and applying a signal to turn off the switch regardless of the video signal to the pixel circuit including the light emitting element on which the short-circuit has occurred.

Description

  The present invention relates to a display device, and more particularly to a display device using a light emitting element such as an organic EL element.

  As a next-generation display device, an organic EL display device using an organic electroluminescence element (hereinafter referred to as an organic EL element) is known. An organic EL element is formed of a light emitting layer containing an anode, a cathode, and an organic compound sandwiched therebetween. When a voltage is applied between the anode and the cathode, electrons are injected from the cathode and holes from the anode are injected into the light emitting layer, and the energy generated by recombination of electrons and holes in the light emitting layer. Emits light.

  As a driving method of the organic EL display device, there are a current driving method and a voltage driving method. The current driving method is a method in which light is emitted by controlling a current flowing through the organic EL element. Since the luminance of the organic EL element is substantially proportional to the current, intermediate luminance can be easily obtained by analog control of the current.

  On the other hand, as described in Patent Document 1, the voltage driving method is a method of controlling the voltage applied to the organic EL element to emit light or not emit light. Since the relationship between voltage and luminance is non-linear, it is difficult to emit light with intermediate luminance. Therefore, the light-emitting element is turned on and off, and gradation is expressed by light emission time or light emission area.

  The gradation display by the light emission time expresses the gradation by the length of the light emission period and the number of times of light emission in a period for displaying one image, that is, one frame period. One frame period is divided into a plurality of sub-frames, and the lighting and extinguishing of the organic EL elements are controlled for each sub-frame, and the gradation is expressed by the total light emission time. Each subframe is assigned a different constant light emission time. By assigning the light emission time to each subframe, a high gradation can be obtained with a small number of subframe divisions.

  In the method of obtaining a halftone by adjusting the luminance of the organic EL element, it is necessary to analog-control the output voltage or output current of the transistor. If there is a variation in threshold value, the current flowing through the organic EL element fluctuates and the luminance becomes non-uniform, and a smooth gradation cannot be obtained.

  On the other hand, in the halftone display method based on the light emission time, the organic EL element takes two states of on or off. A transistor that applies a voltage to the organic EL element also operates as a switch that takes on and off values. For this reason, even if there is a variation in the threshold voltage of the transistor, it is not affected.

Japanese Patent Application Laid-Open No. 10-2114060

  In the organic EL display device, when the anode and the cathode are short-circuited due to a part of the light-emitting layer being lost, for example, no voltage is applied to the light-emitting layer, and thus no light is emitted. In the current drive method, only a drive current in white display flows between the shorted electrodes, but in the voltage drive method, the voltage of the voltage source is directly applied to the electrodes, so that a large through current is generated in the shorted portion. Flowing. If the through current continues to flow, the organic compound constituting the light-emitting layer around the short-circuited portion deteriorates and the life of the display device is shortened.

  The cause of the occurrence of a short circuit in the manufacturing process is that the light emitting layer is disconnected at the edge portion of the lower electrode, and the lower electrode is exposed. In addition, if a foreign substance adheres to the lower electrode, the light emitting layer is not formed with a sufficient thickness at that portion. In either case, a short circuit occurs when the upper electrode is formed.

  A short circuit occurring during the manufacturing process can be repaired (repaired) by some method. In the repair method by laser irradiation, an organic EL element that has caused a display defect is irradiated with laser, and the electrode near the contact portion connecting the electrode of the thin film transistor and the pixel electrode is cut to darken this pixel. It is. However, the repair by laser irradiation has problems such as that the precision becomes severe as the definition becomes higher, and that the irradiation part is heated during the laser irradiation and the organic compound in the light emitting layer is denatured.

  There is a method in which a large current is passed through the shorted portion to melt the portion so that no current flows. However, even in this case, the denaturation tends to occur around the fused portion.

  If a short circuit occurs during use as a display device, not only will the portion be deteriorated or destroyed due to the through current, but it will also cause the power supply to fail.

  Thus, in a voltage-driven display device, there is a demand for a display device that prevents an excessive current from flowing due to a short circuit between electrodes, and that does not deteriorate peripheral materials or cause a failure in an electric circuit.

In the present invention, a plurality of pixel circuits including a light emitting element having a pair of electrodes and a light emitting layer sandwiched between the pair of electrodes and a switch for connecting one electrode of the light emitting element to a voltage source are arranged. The switch is turned on or off according to a video signal applied to the pixel circuit, and the voltage of the voltage source is applied between the pair of electrodes when the switch is turned on. Is a display device that emits light,
A memory for storing a position of the pixel circuit including the light emitting element in which the pair of electrodes are short-circuited among the plurality of arranged pixel circuits;
And a signal processing unit that reads the output of the memory and supplies a signal for turning off the switch to the pixel circuit including the shorted light emitting element regardless of the video signal.

  By supplying an off signal to the pixel including the shorted light emitting element regardless of the video signal, the switch is always turned off and the power supply voltage is not directly applied to the light emitting element, so that a through current can be prevented from flowing. .

It is a block diagram which shows the structure of the display apparatus which concerns on 1st Example of this invention. It is a figure which shows the detailed concept of a short detection part. It is a flowchart which shows the procedure of a short detection. It is a figure which shows a detection voltage and a short determination criterion. It is a block diagram which shows the structure of the display apparatus which concerns on the 3rd Example of this invention. It is a figure which shows the detailed structure of a leakage current removal part. It is a flowchart which shows the procedure of leakage current removal. It is a figure which shows a pixel circuit. It is sectional drawing of a pixel circuit.

  The present invention is not limited to display devices using organic EL elements, but can be applied to all display devices using elements that emit light when a current flows between a pair of electrodes, such as inorganic EL elements and LEDs. Hereinafter, these elements including the organic EL element are referred to as light emitting elements.

  In the voltage driving method, a switch is arranged between the light emitting element and the voltage source, and the voltage of the voltage source is applied between the electrodes of the light emitting element when the switch is on. When the switch is turned off, both electrodes of the light emitting element are opened, and the voltage applied between the electrodes until then is quickly attenuated by the current flowing inside the light emitting element. ON / OFF of the switch is controlled by binary image data. Transistors are often used as switches. A source terminal and a drain terminal of the transistor are connected to one electrode of the light emitting element and a voltage source, and the transistor is turned on or off by a signal entering the gate terminal.

  A light-emitting element in which both electrodes are short-circuited has extremely low interelectrode resistance compared to a normal pixel. Therefore, when an L level video signal is input and a voltage source voltage is applied, a large through current flows. According to the present invention, a video signal of H level is always given to a pixel whose light emitting element is short-circuited, so that the transistor of the pixel is turned off regardless of the display image so that no through current flows.

  Hereinafter, the present invention will be described in detail with reference to examples, but before that, individual pixels constituting the display device of the present invention will be described.

  FIG. 8 shows a pixel circuit of a typical organic EL display device. The pixel circuit 801 includes transistors 802 and 803, a storage capacitor 804, a light emitting element 805, a signal line 806, a scanning line 807, a voltage source 808, and a ground line 809.

  When the transistor 802 is turned on by a signal on the scan line 807, a video signal input to the signal line 806 is input to the gate of the transistor 803 through the transistor 802.

  A transistor 803 is provided between the anode of the light emitting element 805 and the voltage source 808, and is a switch in which light emission and non-light emission are controlled by turning on and off.

  The video signal is a binary signal composed of an H (high) level and an L (low) level. When the H level is input, the transistor 803 is turned off with a small conductance, and the light emitting element emits light without transmitting a power supply voltage. do not do. When an L-level video signal is input, the transistor 803 is turned on with high conductance, so that most of the voltage of the voltage source 808 is applied to the light-emitting element, and current flows through the light-emitting element to emit light. Occurs. In the pixel circuit of FIG. 8, the H level is a black signal and the L level is a white signal.

  FIG. 9 is a diagram showing a cross section of the pixel circuit. Only the light emitting element 805 and the transistor 803 are shown, and other circuit elements are omitted.

  An active layer 905, a gate electrode 906, a drain electrode 907, and a source electrode 908 of the transistor 803 are formed over the substrate 901 with the insulating layer 902 interposed therebetween. The drain electrode 907 and the source electrode 908 are connected to the active layer 905 through a contact hole opened in the insulating layer 902. Although not shown in FIG. 9, the drain electrode 907 is extended to become a voltage source 808.

  The entire transistor 803 is covered with a planarization layer 903, and an anode 909, a light emitting layer 910, and a cathode 911 of an organic EL element that is a light emitting element 805 are stacked in this order on the planarization layer 903. The anode 909 and the source electrode of the transistor 803 are connected to each other through a contact hole opened in the planarization layer 903. The light emitting element 805 is separated from a light emitting element of an adjacent pixel by a partition wall 912.

  8 and 9 are cross-sectional views of a single pixel circuit, but a large number of the same pixel circuits as those in FIGS. 8 and 9 are arranged on the substrate 901 in rows and columns in a matrix. . The cathode 911 is continuously formed over all the pixels.

Example 1
FIG. 1 is a block diagram showing the configuration of the display device of the first embodiment.

  The video signal Video input from the outside is sent to the display unit 101 through the signal processing unit 106. As will be described below, the signal processing unit 106 always sets the video signal corresponding to the shorted pixel in the video signal Video sent from the outside to the H level.

  A plurality of pixel circuits 801 in FIG. 8 are arranged in a matrix on the display unit 101, and video signals sent from the signal processing unit 106 are sequentially displayed on each pixel. The panel control unit 105 outputs signals indicating the addresses of the pixel circuits arranged in matrix to the display unit 101 and the signal processing unit 106 in accordance with the horizontal synchronization signal HSYNC and the vertical synchronization signal VSYNC coming from the outside. The address of the pixel circuit is a parameter indicating where the pixel circuit is located in the matrix arrangement, and is specified by a row and column number or the like.

  The short detection unit 102 determines whether each light emitting element of the display unit 101 is short-circuited. The address of the pixel determined that the light emitting element is short-circuited is sent to the memory 104 via the memory controller 103 and stored.

  FIG. 2 is a diagram illustrating a detailed structure of the short detection unit 102.

  The light emitting elements of the pixel circuit 801 are connected to the resistor 202 of the short detection unit 102 with the cathodes connected in common. The sum of the currents of all the light emitting elements flows through the resistor 202, and the voltage generated thereby becomes one input of the comparator 201. The other input of the comparator 201 is connected to the output voltage Vr of the fixed power source 203. The output of the comparator 201 is sent to the memory controller 103 and further to the memory 104 via the inverter 204.

  FIG. 3 is a flowchart showing a procedure for determining whether or not there is a short circuit.

  The short detection is performed during a period when the image display on the display unit 101 is not performed.

  When the detection is started, white data is written in one pixel and black data is written in all other pixels in step S1. This data is generated by the signal processing unit 106 which receives the address of the pixel to which the white data is written by the panel control unit 105 and sent to the display unit 101. Next, in step 2, the voltage generated in the resistor 202 by the cathode current of all the pixels is detected, and it is determined whether or not a short circuit has occurred. When it is determined by the output of the comparator 201 that the level of the voltage generated in the resistor 202 indicates a short circuit, the process proceeds to step S3, and the address of the pixel in which the white data is written is stored in the memory 104. If it is determined that the output of the comparator 201 is L and not short-circuited, step S3 is skipped. Next, in step S4, it is determined whether or not all pixels have been detected. If all the pixels have not yet been determined, the address of the pixel of the white data output by the panel control unit 105 is advanced by one, and the process returns to step S1. When the determination of all the pixels is completed, the short detection is finished.

  FIG. 4 shows the voltage level of the resistor 202 with and without a short circuit. When white data is written in a light emitting element in which both electrodes are short-circuited, a current larger than the current flowing in the light-emitting element not short-circuited flows. Since only a small leak current flows through the light emitting element of the pixel in which the black data is written, the sum of all the pixels is smaller than the current of the pixel in which the white data is written. Therefore, the total current flowing in the resistor 202 of the short detection unit may be regarded as a current flowing in the light emitting element of the pixel in which the white data is written, and it can be determined whether or not the short circuit is caused by the magnitude of the current.

  The total current is converted into a voltage by the resistor 202. Vs is a voltage when a pixel to which white data is written is short-circuited, Va is a voltage when a pixel to which white data is written is not short-circuited, and Vcom is a common voltage at the other end of the resistor. Since Vs is higher than Va, the determination reference voltage Vr is set during that time.

  When the pixel is short-circuited and the voltage exceeds the reference voltage Vr, the comparator 201 outputs an H (high) level.

  In a normal pixel, since the voltage of the resistor 202 is equal to or lower than the reference voltage Vr, the output of the comparator 201 becomes L (low) level. The output of the comparator 201 is a determination result of whether or not there is a short circuit. When the output of the comparator 201 is at the H level, and therefore the invar and the output is at the L level, the memory controller 103 sends the address of the pixel in which the white data output by the panel control unit is written to the memory 104 at that time.

  When the short detection is completed, the image display mode is restored. The signal processing unit 106 sequentially writes the video signal Video on the display unit according to the address signal generated by the panel control unit 105. At that time, the position information of the short pixel stored in the memory 104 is read, the video signal is directly written to the pixels other than the short pixel, and the position information of the short pixel stored in the memory 104 is written to the short pixel. Originally write black data. This prevents a through current from flowing through the short pixel.

(Example 2)
A short circuit occurring in the manufacturing process is inspected by the same short detection unit as in the first embodiment before shipment, and the address of the shorted pixel is held in the memory 104. Thereafter, the short detection unit 102 is removed and shipped as a product. The removed short detection unit 102 can be used for short detection of another product. The display device to be shipped has a configuration in which the short detection unit 102 is removed from FIG. 1 and is simplified compared to the first embodiment. Since black data is written in all the short pixels generated during the manufacturing process, it is possible to prevent the deterioration from spreading. Since the memory 104 is only read after being written in the shipping inspection, a read-only memory can be used, and the cost can be reduced.

(Example 3)
Vl in FIG. 4 is a voltage when black data is written in all pixels. Even when the light emitting element is in a black state, a slight leak current flows, and thereby a voltage Vl is generated. Usually, this voltage is very small and does not affect the detection of the current of the pixel to which the white data is written. However, if the leakage current of the light emitting element is large, the level of Vl in FIG. 4 becomes high, so that the difference between Vs and Va becomes relatively small, the determination error of the comparator 201 becomes large, and the short detection accuracy decreases. In this embodiment, the detection accuracy of a short pixel is increased by removing all pixel leakage current.

  FIG. 5 is a block diagram showing the configuration of the short detection unit and the leakage current removal unit of this embodiment. Prior to being connected to the short detection unit 102, the display unit 101 is connected to the leakage current removal unit 300, and all pixel leakage current is detected.

  FIG. 6 is a diagram illustrating a detailed configuration of the leakage current removing unit 300. FIG. 7 shows a flowchart of leakage current removal and short detection according to this embodiment.

  In step S5 and step S6, the switch SW1 is turned off and the switch SW2 is turned on, and the display unit 101 is disconnected from the short detection unit 102 and connected to the leakage current removal unit 300.

  In step S5, black data is set for all pixels. Leakage currents of all the pixels flow through the resistor 201, are converted to a voltage Vl, and are input to the comparator 302. In the comparator 302, the voltage Vl due to the leak current is compared with the other terminal voltage Vcom of the resistor 201. When Vcom is greater than Vl, the comparator output is H, and when Vcom is less than V1, the comparator output is L. A digital output of the comparator 302 is converted into an analog voltage Vda by a D / A (digital / analog) converter 303 controller and a D / A converter 304. In the operational amplifier 305, the output of the D / A converter 304 is connected to the positive input terminal, and the source voltage of the transistor 306 is connected to the negative input terminal. The output of the operational amplifier 305 becomes the gate voltage of the transistor 306, and the gate voltage of the transistor 306 is adjusted so that the current I ′ = Vda / R2 flows through the resistor 307. When Vda is higher than V0, the current flowing through the resistor 201 is reduced by I '. When Vda becomes lower than V0, the original all-pixel leakage current is controlled to increase. V0 is set to a voltage lower than Vcom. By repeating the above operation, the leakage current is removed.

  After the leakage current removal is completed, the switch SW1 is turned on and the SW2 is turned off. The display unit 101 is disconnected from the leakage current removal unit 300 and connected to the short detection unit 102.

  Hereinafter, whether or not there is a short circuit for each pixel is determined in steps S1 to S4. The procedure is the same as in Example 1.

  When the detection of the short pixel is completed, the image display mode is restored.

  The leakage current removal, short pixel detection, and subsequent video signal writing of the present invention are not limited to the methods of the first to third embodiments.

DESCRIPTION OF SYMBOLS 101 Display part 102 Short detection part 103 Memory controller 104 Memory 105 Panel control part 106 Signal processing part 801 Pixel circuit 803 Switch 805 Light emitting element 808 Voltage source

Claims (6)

A plurality of pixel circuits including a light-emitting element having a pair of electrodes and a light-emitting layer sandwiched between the pair of electrodes and a switch for connecting one electrode of the light-emitting element to a voltage source are disposed. Display that is set to ON or OFF according to a video signal applied to the pixel circuit, and that the light emitting element emits light when a voltage of the voltage source is applied between the pair of electrodes when the switch is ON A device,
A memory for storing a position of the pixel circuit including the light emitting element in which the pair of electrodes are short-circuited among the plurality of arranged pixel circuits;
A display device comprising: a signal processing unit that reads an output of the memory and applies a signal for turning off the switch to a pixel circuit including the shorted light emitting element regardless of the video signal.   The display device according to claim 1, further comprising a short detection unit that detects a position of a pixel circuit including the shorted light emitting element.   The short detection unit includes a circuit that detects a difference between a current flowing in the light emitting element in which the pair of electrodes is short-circuited and a current flowing in the light emitting element in which the pair of electrodes is not short-circuited. The display device according to claim 2.   The short detection unit provides a signal for turning on the switch to one of the plurality of pixel circuits, and a current flowing in the light emitting element when a signal for turning off the switch is given to another pixel circuit. The display device according to claim 2, further comprising a circuit that detects a sum.   Detecting a sum of leakage currents flowing through the light emitting elements when a signal for turning off the switch is given to the plurality of pixel circuits, and giving a signal for turning on the switch to one of the plurality of pixel circuits; 5. The display device according to claim 4, further comprising: a leakage current removing unit that subtracts from a sum of currents flowing through the light emitting elements when a signal for turning off the switch is given to another pixel circuit.   5. The display device according to claim 4, wherein a sum of currents flowing through the light emitting elements is extracted by commonly connecting one of the pair of electrodes different from the one electrode.

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