JP2020020995A - Display device - Google Patents

Abstract

To provide a display device including a display unit having a plurality of pixels comprising self-luminous elements, capable of improving degradation in current-voltage characteristics of a pixel by applying a reverse bias voltage to the pixel as well as suppressing deterioration of the pixel due to application of the reverse bias voltage.SOLUTION: A display device includes a display unit 1 having a plurality of pixels comprising self-luminous elements, in which a control unit 4 switches into a first circuit 21 or a second circuit 22 connected to different power supplies to be connected to the pixel so as to apply a normal bias or a reverse bias voltage. In this process, driving conditions under the reverse bias voltage or the normal bias voltage are corrected depending on the deterioration degree of the pixel, while the reverse vias voltage is corrected independently from the correction of the driving conditions under the normal bias. Thus, excessive loading on the pixel during application of the reverse bias voltage is suppressed, and the display device has high reliability.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a display device having a pixel constituted by a self-luminous element.

  2. Description of the Related Art Conventionally, a display device having a plurality of pixels constituted by spontaneous light emitting elements such as an OLED (abbreviation of Organic Light Emitting Diode) and an FED (abbreviation of Field Emission Display) has been known.

  Here, the OLED has a characteristic that the light emission efficiency is reduced in proportion to the height of the light emission luminance and the length of the driving time, and the light emission luminance is reduced even under the same driving condition. In order to improve the reliability of a display device having an OLED as a pixel, it is important to improve the current-voltage characteristics of the OLED.

  As a method for improving such a decrease in the current-voltage characteristics of the OLED, it is known to apply a drive voltage having a polarity opposite to the polarity of the drive voltage for emitting light to the OLED at regular intervals. Specifically, in the OLED that is a diode, the application of the voltage to emit light is “forward bias” and the reverse is “reverse bias”. When a reverse bias voltage is applied to the OLED at regular intervals, the The deterioration of the current-voltage characteristics is improved.

  In a display device having a pixel constituted by a self-luminous element, the one described in Patent Document 1 is one that adopts the above-described driving method and improves the reduction of the current-voltage characteristic of the OLED. No. The display device described in Patent Literature 1 has a self-luminous element composed of an OLED as a pixel, and improves the current-voltage characteristics of the OLED by applying forward bias and reverse bias voltages to the OLED by AC driving. It is configured to be.

JP 2018-22695 A

  By the way, when applying a reverse bias voltage to the OLED, an effect of improving the current-voltage characteristics of the OLED to recover the OLED (hereinafter, referred to as a “recovery effect”) and an effect of deteriorating due to heat generated by current (hereinafter, “deterioration”) Effect)). As a result of intensive studies, the present inventor has found that the conventional method of applying a reverse bias voltage is insufficient from the viewpoint of driving the OLED for a longer time.

  Specifically, the recovery effect due to the application of the reverse bias voltage decreases as the cumulative light emission time of the OLED increases (in other words, the degree of deterioration of the OLED increases). On the other hand, the deterioration effect increases as the degree of deterioration of the OLED increases. Further, in the OLED, since the current-voltage characteristics gradually decrease as the deterioration proceeds, it is necessary to increase the forward bias voltage by an amount corresponding to the decrease in the current-voltage characteristics in order to maintain the luminance at a predetermined value. .

  On the other hand, when the degree of deterioration of the OLED increases, the deterioration effect due to the reverse bias increases. Therefore, if the voltage of the reverse bias is excessively increased, the deterioration effect becomes larger than the recovery effect. Therefore, from the viewpoint of reliability, it is preferable that the OLED be driven under the condition that the forward bias voltage is gradually increased as the degree of deterioration increases, while the reverse bias voltage does not exceed a predetermined value.

  In the invention described in Patent Document 1, since a forward bias voltage and a reverse bias voltage are applied to the OLED by one AC drive, these voltages are basically the same, and the degree of deterioration of the OLED is more than a predetermined level. Then, the deterioration effect becomes larger than the recovery effect. Therefore, in the display device described in Patent Document 1, as the cumulative light emitting time increases, the OLED is rather deteriorated by application of the reverse bias voltage, which is not preferable from the viewpoint of improving reliability.

  The present invention has been made in view of the above points, and has a pixel configured by a self-luminous element, and obtains a recovery effect of the self-luminous element by applying a reverse bias voltage. It is an object of the present invention to provide a display device in which the deterioration effect of the display device is suppressed as compared with the related art.

  In order to achieve the above object, the display device according to claim 1 includes a display unit (1) including a plurality of pixels each including a self-light-emitting element, and a display device configured to apply a forward bias voltage to the pixels. A driving circuit (2) including a first circuit (21) for emitting light and a second circuit (22) for applying a reverse bias voltage to the pixels, and a degree of deterioration of the plurality of pixels is estimated. A deterioration estimating unit (3), a control unit (4) for controlling the connection destination of the pixel to the first circuit or the second circuit, and a driving condition of the pixel is corrected according to the degree of deterioration estimated by the deterioration estimating unit. And a correction unit (5). In such a configuration, the correction unit changes the forward bias voltage and the reverse bias voltage according to the degree of deterioration of the pixel, and the reverse bias voltage is different from a power supply used for applying the forward bias voltage. Is applied.

  According to the above, a deterioration estimating unit estimates the degree of deterioration of a pixel formed by a self-luminous element, and changes a forward bias voltage and a reverse bias voltage of a pixel by a correction unit according to the degree of deterioration. Display device. In addition, when a reverse bias voltage is applied to the pixel, a power supply different from the power supply used for applying the forward bias voltage is used, so that the reverse bias voltage can be set to an arbitrary value independently of the forward bias voltage. This is a configuration that can be changed to a value. Therefore, it is possible to maintain the reverse bias voltage within a predetermined range while increasing the forward bias voltage in accordance with the degree of deterioration of the pixel. The display device is more suppressed than before.

  In addition, the reference numerals in parentheses attached to the respective components and the like indicate an example of a correspondence relationship between the components and the like and specific components and the like described in the embodiments described later.

FIG. 2 is a block diagram illustrating a configuration of the display device according to the first embodiment. FIG. 2 is a schematic diagram illustrating a circuit configuration of the display device of FIG. 1. 9 is a graph showing a relationship between elapsed time and relative luminance when driving an OLED with a predetermined constant current. FIG. 4 is a diagram illustrating an example of setting of a current-voltage characteristic of an OLED at an initial stage and after deterioration, and a threshold value of a reverse bias voltage.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent are denoted by the same reference numerals and described.

(1st Embodiment)
The display device according to the first embodiment will be described with reference to FIGS. The display device of the present embodiment is applied to, for example, a display device for in-vehicle use, but may be applied to other uses.

  FIG. 2 shows one representative example of a plurality of pixels forming the display unit 1 described later, and omits connection circuits for other pixels, in order to facilitate understanding of the circuit configuration. In FIG. 2, an example of connection to the first circuit 21 by the control unit 4 described later is shown by a solid line, and an example of connection to the second circuit 22 is shown by a broken line.

  The display device according to the present embodiment includes a display unit 1, a drive circuit 2, a deterioration estimation unit 3, a control unit 4, a correction unit 5, a video input unit 6, as shown in FIG. Having. This display device displays an image based on a signal from the image input unit 6 on the display unit 1, and based on an estimation of the degree of deterioration of the pixels constituting the display unit 1 by the deterioration estimation unit 3, 5, the driving conditions of each pixel are corrected, and the control unit 4 controls the driving of each pixel.

  The display unit 1 has a plurality of pixels constituted by self-luminous elements such as OLEDs and FEDs, and displays images and the like by emitting light from these pixels. The display unit 1 includes, for example, a thin film transistor (not shown) as a switching element of each pixel, and may employ an active matrix method that can control on / off of each selected pixel. When the OLED is used as a self-luminous element, the display unit 1 is referred to as an “organic EL display”. The display unit 1 is applied with a voltage from an external power supply (not shown) via the drive circuit 2, and displays various images based on the image signal of the image input unit 6.

  In the present embodiment, an example in which an OLED is used as a self-luminous element will be described. However, as an organic EL display and an OLED, well-known ones can be used. Description is omitted.

  As shown in FIG. 2, the drive circuit 2 includes a first circuit 21 used when applying a forward bias voltage to each pixel to cause each pixel to emit light, and a drive circuit 2 for reducing and improving the current-voltage characteristics of each pixel. And a second circuit 22 used when applying a reverse bias voltage. As shown in FIG. 2, the drive circuit 2 has a configuration in which each pixel is connected to one of the first circuit 21 and the second circuit 22 and can be switched so that either a forward bias voltage or a reverse bias voltage can be applied. Have been.

  In the drive circuit 2, as shown in FIG. 2, the first circuit 21 and the second circuit 22 are connected to different power supplies, that is, a forward bias power supply or a reverse bias power supply, and the forward bias voltage and the reverse bias voltage are changed. It is configured to be applied to each pixel by an independent power supply. This provides a circuit configuration capable of executing drive control such as, for example, increasing a forward bias voltage to a pixel while maintaining a reverse bias voltage to the pixel within a predetermined range.

  The deterioration estimating unit 3 estimates the degree of deterioration of each pixel from, for example, the accumulated time of forward bias voltage application to each pixel. The deterioration estimating unit 3 may be configured as, for example, a control ECU (abbreviation of Electronic Control Unit) including a CPU, a ROM, a RAM, and a non-volatile RAM (not shown), but is not limited thereto, and a known unit may be used. Can be.

  Specifically, for example, a cumulative time (hereinafter, simply referred to as “driving time”) of forward bias voltage application to each pixel is stored in a storage unit (not shown) by a time measurement unit (not shown). The deterioration estimating unit 3 executes, for example, a program for calculating the degree of deterioration stored in the ROM by the CPU, and calculates the degree of deterioration of each pixel using the driving time data of each pixel stored in the storage unit. .

  For the estimation of the degree of deterioration of pixels composed of OLEDs, see, for example, KONICA MINOL-TA TECHNOLOGY REPORT VOL.9 (2012) “Product Development of All-Phosphorescent OLED Lighting Panel” (hereinafter simply referred to as “publicly known documents”) The known life estimation method described can be used. For example, as in the method described in this known document, the OLED degradation is separated into an initial degradation and a subsequent normal degradation, and an acceleration coefficient is calculated based on a measured value of a change with time of the luminance of the OLED, thereby driving the pixel. An estimated life curve showing the relationship between time and luminance change over time is created. The estimated life curve is used as a reference curve, the data of the reference curve is stored in a storage unit, and the accumulated time of each pixel is measured and applied to the reference curve to estimate the degree of deterioration of each pixel. be able to.

  When each pixel is composed of a plurality of sub-pixels having different emission colors, for example, a red sub-pixel, a blue sub-pixel, and a green sub-pixel, the above-described reference curve is created in advance for each of these sub-pixels. By doing so, the degree of deterioration can be estimated for each sub-pixel. Further, the above estimation of the degree of deterioration is merely an example, and another arbitrary method may be adopted.

  The control section 4 controls switching in the drive circuit 2, that is, controls whether the connection destination of each pixel is the first circuit 21 or the second circuit 22. In other words, the control unit 4 switches the connection destination with the pixel to the first circuit 21 or the second circuit 22 as shown by the arrow in FIG. Controls whether to apply a bias voltage. The control unit 4 is configured to function by, for example, reading and executing various data such as a data table and a program stored in a ROM or a RAM by a CPU, but the control unit 4 is not limited to this and may have any other configuration. It may be.

  When causing the pixel to emit light, the control unit 4 connects the pixel to the first circuit 21 so that a forward bias voltage is applied to the pixel. When improving the deterioration of the current-voltage characteristics of the pixel, the control unit 4 connects the pixel and the second circuit 22 such that a reverse bias voltage is applied to the pixel. In addition, the control unit 4 is programmed so as not to connect the pixel to the second circuit 22 for a pixel whose deterioration degree is equal to or higher than a predetermined value among the pixels. This is to prevent a decrease in reliability due to unnecessary reverse bias voltage application to a pixel in which the degradation effect is greater than the recovery effect due to the reverse bias voltage application. The details of the application of the reverse bias voltage to each pixel will be described later.

  The correction unit 5 corrects the driving condition of each pixel with a forward bias based on the degree of deterioration of each pixel by the deterioration estimation unit 3. The correction unit 5 corrects the set value of the voltage or current applied to the pixel in accordance with the degree of deterioration of the pixel estimated by the deterioration estimation unit 3 so that the luminance of the pixel is different from the initial luminance. It plays a role in adjusting the brightness to the same level.

  In addition, based on the degree of deterioration of each pixel by the deterioration estimating unit 3, the correction unit 5 controls each pixel in order to prevent excessive reverse bias voltage from being applied in reducing and improving the current-voltage characteristics of each pixel. To correct the reverse bias voltage. The details will be described later.

  The correction unit 5 is configured to function by, for example, reading and executing various data such as a data table and a program stored in a ROM or a RAM by a CPU, but is not limited thereto. It may be.

  The correction of the driving condition of each pixel with the forward bias and the correction of the reverse bias voltage to each pixel may be performed by one correction unit 5 or different corrections for the forward bias and the reverse bias. It may be executed by the unit 5.

  The video input unit 6 inputs a predetermined video signal corresponding to a video to be displayed on the display unit 1 to the display unit 1. According to the type of video to be displayed on the display unit 1, other various electronic devices (not shown) Connected to parts.

  The above is the configuration of the display device of the present embodiment.

  Next, the application of a reverse bias voltage to each pixel and its effect in the display device of the present embodiment will be described with reference to FIG. The voltage application will be described.

FIG. 3 shows an example of a change with time of the relative luminance of the OLED when the OLED is driven at a constant current under a predetermined temperature environment with a fixed current value at an arbitrary initial luminance (for example, 1000 cd / m ² ). I have. In FIG. 3, a change in the relative luminance of the OLED with time is indicated by a broken line when no reverse bias voltage is applied, and by a dashed line when a conventional reverse bias voltage is applied. The case where the bias voltage is applied is indicated by solid lines.

Here, the “relative luminance” means a ratio of the luminance of the OLED to the initial luminance during a certain elapsed time. In addition, the elapsed time in FIG. 3 is obtained when the OLED is driven at a constant current under a predetermined temperature environment at a luminance higher than a normal use luminance (for example, 200 cd / m ² ), that is, in a situation where the deterioration of the OLED is accelerated. Lighting time. That is, the elapsed time in FIG. 3 is a concept different from the accumulated light emission time when light is emitted at a luminance normally used under a normal environment such as normal temperature.

Hereinafter, for simplicity of description, as shown in FIG. 3, for the sake of convenience, a period in which the elapsed time of the pixel is less than T1 is referred to as “initial”, and a period in which T1 or more and less than T2 is referred to as “middle”. The period exceeding T2 is referred to as “long term”. Note that T1 and T2 are values that vary depending on the initial luminance, current value, environmental temperature, and the like. For example, when the initial luminance is set to 1000 cd / m ² and the environmental temperature is set to 85 ° C. assuming the use in a vehicle, T1 can be about 100 hours, and T2 can be about 1000 hours.

  First, a description will be given of a temporal change in relative luminance when the OLED is driven at a constant current in a method in which a reverse bias voltage is not applied (hereinafter, referred to as a “non-applied method”).

  Here, the “non-applied mode” refers to a driving mode in which the OLED is driven at a constant current and only a forward bias voltage is applied to the OLED. Further, the change with time of the OLED in the non-applied mode shown in FIG. 3 is obtained by fitting to an equation of relative luminance L, which will be described later, based on the actually measured value.

  In this case, as shown in FIG. 3, in the initial OLED, the degree of decrease in relative luminance is greater than when a reverse bias voltage described later is applied, and the relative luminance at the time T1 is 0.9. became. In the middle-stage OLED, the degree of decrease in the relative luminance was smaller than that in the initial stage, and the relative luminance at the time T2 was 0.8. Thereafter, in the long-term OLED, the degree of decrease in the relative luminance was smaller than that in the middle period, and the relative luminance tended to decrease gradually.

  Subsequently, a change over time in the relative luminance of the OLED when a reverse bias voltage is applied at a predetermined interval in the conventional method (hereinafter referred to as “old method voltage application”) will be described.

  The “old-mode voltage application” means that the same reverse bias voltage as the absolute value of the voltage applied in the forward bias driving is applied to the OLED at a constant current driving of the OLED for 1 minute at an interval of 24 hours. The driving situation. In addition, the change with time of the OLED under application of the voltage in the old method shown in FIG. 3 is obtained by fitting to an equation of relative luminance L described later based on the actually measured value.

  In this case, as shown in FIG. 3, in the initial OLED, the degree of decrease in the relative luminance was reduced as compared with the case of the non-applied type, and the relative luminance at the time T1 was 0.98. . This is because carriers such as holes and electrons remaining in the functional layer of the OLED without contributing to light emission are moved and removed by application of a reverse bias voltage, and the deterioration of current-voltage characteristics is improved. it is conceivable that.

  However, in the middle-stage OLED, as shown in FIG. 3, the degree of decrease in the relative luminance is almost the same as the initial time until the predetermined time after the elapsed time T1, and is slightly smaller than the initial time thereafter. Was. As a result, in the OLED, the relative luminance at the elapsed time T2 was 0.8, which was almost the same as that in the non-application type.

  In the long-term OLED, the degree of decrease in the relative luminance was smaller than that in the middle term, but was larger than the long-term decrease in the relative luminance of the OLED in the non-application type. As a result, in the long-term OLED, the relative luminance gradually decreased, and the decrease tended to be larger than that in the non-application type.

  In view of this result, the present inventors presumed as follows. As the deterioration of the OLED progresses, a leak path is gradually generated in the OLED, and the current value of the OLED when a reverse bias voltage is applied increases. As the amount of heat generated by the OLED upon application of the reverse bias voltage increases, the deterioration effect increases, and the recovery effect by applying the reverse bias voltage exceeds the deterioration effect. Was done. As a result, the voltage application in the old method can suppress the deterioration of the OLED characteristics in the initial stage more than in the non-application method, but accelerates the deterioration in the OLED characteristics more than the non-application method in the long term.

  Therefore, the inventor of the present invention, after diligent studies, changed the reverse bias voltage as appropriate according to the degree of deterioration of the OLED, and found that the application of the reverse bias voltage was stopped at a predetermined timing. Thus, the display device according to the present embodiment in which the deterioration effect is suppressed has been achieved.

  Next, a temporal change in the relative luminance of the OLED when a reverse bias voltage is applied in the display device of the present embodiment (hereinafter, referred to as “new-type voltage application”) is described. The change over time of the relative luminance of the OLED under the new system voltage application shown in FIG. 3 was obtained by the same method as the old system voltage application.

  In the voltage application of the new method, during the period from the initial period to the middle of the middle period shown in FIG. 3, the reverse bias voltage is appropriately changed within a range equal to or less than a predetermined value according to the degree of deterioration of the OLED. Is a driving state in which no reverse bias voltage is applied.

  Specifically, in the voltage application of the new method, the voltage is applied to the OLED for 1 minute at an interval of 24 hours while appropriately changing the reverse bias voltage, and the upper limit value of the current and the voltage of the reverse bias voltage are applied. A threshold was set, and the reverse bias voltage was set to be equal to or lower than the threshold. Details of the upper limit value of the current and the threshold value of the voltage will be described later.

  As shown in FIG. 3, when the voltage is applied in the new method, the degree of decrease in the relative luminance of the initial OLED is smaller than that in the case where the voltage is not applied, and the relative luminance at the time T1 is 0. 98.

  In the middle-stage OLED, the degree of decrease in relative luminance was smaller than in the initial stage, and was slightly larger than the middle stage in the non-applied system, but was significantly improved compared to the middle stage in the old system in which voltage was applied. Then, the relative luminance at the elapsed time T2 was 0.83, and the degree of decrease in the relative luminance was smaller than in the voltage application in the non-applied system and the old system. After that, in the long-term OLED, the degree of decrease in the relative luminance is smaller than that in the middle period, and the relative luminance tends to gradually decrease. And the relative luminance was high.

  The result is that, by controlling the reverse bias voltage and its application independently of the forward bias voltage application, while obtaining the recovery effect by applying the reverse bias voltage, the deterioration effect is suppressed, and the OLED can be used for a longer time. It shows that it can be driven.

  Next, the upper limit value of the current and the threshold value of the voltage in the new-type voltage application will be described with reference to FIG. In FIG. 4, the current-voltage characteristics of the initial OLED are indicated by solid lines, and the current-voltage characteristics of the deteriorated OLED are indicated by broken lines.

  First, a description will be given of a change in the current-voltage characteristics of the OLED with the progress of the deterioration of the OLED.

  When a reverse bias voltage (negative voltage) is applied, since the OLED is a diode, as shown in FIG. 4, a current does not initially generate much, and the current density is small. As a result, the current density when a reverse bias voltage is applied increases.

  On the other hand, when a forward bias voltage (positive voltage) is applied, the OLED does not generate much current from 0 V to the light emission start voltage (about 4 V in the example of FIG. 4) and has a small current density. If it exceeds, the resistance value of the light emitting layer decreases, and the current density increases as the voltage increases. As the deterioration progresses, the OLED has almost the same current density from 0 V to the light emission start voltage as the initial state, but has a smaller current density at a voltage exceeding the light emission start voltage, and as shown in FIG. Side.

  Next, the upper limit value of the current and the threshold value of the voltage in the voltage application in the new method will be described.

  In order to prevent an excessive reverse bias voltage from being applied, the upper limit of the current density and the voltage threshold when the reverse bias voltage was applied were set. Specifically, the current density of the OLED when the forward bias light emission start voltage was applied was set as the upper limit of the current density when the reverse bias voltage was applied.

  This is because, when a current density exceeding the current density at the start of light emission is generated in the OLED due to the application of the reverse bias voltage, this is caused by the current in the leak path caused by the progress of deterioration. It is considered that it does not contribute to the improvement. In other words, the current density exceeding the current density at the start of light emission (hereinafter referred to as “excessive current density”) contributes to the removal of unnecessary carriers in the functional layer of the OLED, which is the original purpose of reverse bias voltage application. This is because it is considered that it merely causes heat generation.

  Further, in the initial OLED, the negative voltage, which is the upper limit of the current density, was set as a threshold, and the reverse bias voltage was set to be equal to or lower than the threshold. For example, in the example shown in FIG. 4, the current density at the light emission start voltage of about 4 V is set as the upper limit value of the current density when the reverse bias voltage is applied. About -7 V was set as the threshold. Then, the reverse bias voltage is set to be within a range equal to or less than the threshold value (in the example of FIG. 4, within a range of 0 V to −7 V). Thereby, when applying a reverse bias voltage, the effect of heat generation due to excessive current density on the OLED can be reduced, and the deterioration effect due to the application of the reverse bias voltage can be suppressed.

  In addition, in the display device of the present embodiment, even if the reverse bias voltage is a threshold, if the current density exceeds the upper limit of the above-described current density due to the progress of OLED deterioration, the reverse bias voltage is applied to the OLED (pixel). The application is not performed. Accordingly, in a situation where the degradation effect is larger than the recovery effect of the OLED by the application of the reverse bias voltage, the unnecessary reverse bias voltage is not applied, thereby preventing the OLED from deteriorating and promoting the OLED. Can be time driven.

Specifically, in the initial current density in the light emission start voltage of the OLED is 0.1 mA / cm ^2, a case where the voltage to be 0.1 mA / cm ² by applying a voltage of a reverse bias is -7V Examples explain.

In the case of this example, the upper limit value of the current density when applying the reverse bias voltage is 0.1 mA / cm ² , and the threshold value of the reverse bias voltage is −7V. At the initial stage, for example, when the reverse bias voltage is −4 V, the current density at this time is about 0.001 mA / cm ² , and almost no current is generated in the OLED. As the deterioration of the OLED progresses, the current density at the time of applying the reverse bias voltage gradually increases, and accordingly, the reverse bias voltage gradually approaches the threshold value, for example, -5V and -6V. At this time, the reverse bias voltage has a threshold value (-7 V in this example) as an upper limit, is not set to a negative voltage exceeding -7 V, and is fixed at -7 V after a predetermined cumulative light emission time. You.

  Then, even when the reverse bias voltage is raised to the threshold, if the current density at that time is equal to or lower than the upper limit, the application of the reverse bias voltage at the threshold is continued. On the other hand, in the application of the reverse bias voltage at the threshold value, if the current density at that time exceeds the upper limit, the subsequent reverse bias voltage application is not performed.

  The driving of the OLED with the forward bias is independently performed by a power supply different from the power supply used for applying the reverse bias voltage, and is continued even after the reverse bias voltage application is stopped.

  Next, an example of the correction of the driving condition with the forward bias by the correction unit 5 will be described.

  In order to maintain the same luminance as the initial luminance for each of the pixels constituting the display unit 1, the correction unit 5 corrects the driving condition with each forward bias according to the degree of deterioration of each pixel. At this time, the correction unit 5 performs the correction based on the data of the current-voltage characteristic of the OLED after the improvement by the voltage application in the new method.

  Specifically, for example, various coefficients a1, b1, a2, and b2 in a formula (1) shown below, that is, a calculation formula of a relative luminance L based on a component of initial deterioration and a component of normal deterioration, are applied with a reverse bias voltage. Will update later. Then, based on the calculation formula of the relative luminance L after the update, the driving condition in the forward bias for making the pixel after the application of the reverse bias voltage the luminance substantially equal to the initial luminance is calculated, and the driving condition is calculated. The pixel is caused to emit light. Accordingly, the driving conditions under the forward bias are set while reflecting the recovery of the current-voltage characteristics due to the application of the reverse bias voltage, so that the pixels are prevented from emitting light due to excessive current driving, and the reliability of each pixel is reduced. And the reliability of the display unit 1 is improved.

L = a1 × exp (−t × b1) + a2 × exp (−t × b2) (1)
The first term on the right side of the equation (1) corresponds to the component of the initial deterioration, and the second term on the right side of the equation (1) corresponds to the component of the normal deterioration after the initial deterioration. Further, coefficients a1 and b1 are coefficients related to initial deterioration, coefficients a2 and b2 are coefficients related to normal deterioration, and t is an accumulated light emission time. Further, regarding the calculation of various coefficients, the values of the current and the voltage of the OLED after the recovery by the application of the reverse bias voltage can be calculated by using the expression (1) of the relative luminance L. Further, in the equation (1), various coefficients are simply described for easy understanding. However, similarly to the method described in the known literature, the activation energy E _{a, inintial} of the initial deterioration component and the activation energy of the normal deterioration component are used. It is preferable to use a function of the energy E _{a, normal} , the absolute temperature T, and the Boltzmann coefficient k. As a result, the relative luminance L can be calculated more accurately, and the accuracy of correction is improved.

  When a pixel is composed of a plurality of sub-pixels having different emission colors (for example, three sub-pixels of red, green, and blue), correction is executed for each emission color. The above-described method of calculating the coefficient is merely an example, and another known method may be employed.

  Next, an example of the timing of applying a reverse bias voltage will be described.

  First, when the frame rate of the OLED drive with the forward bias is high, for example, a reverse bias voltage can be applied between the frame rates of the forward bias, that is, between the frames to emit light. Thus, while the image is displayed on the display unit 1, the reverse bias voltage is applied without causing a sense of visual discomfort to humans due to the OLED not emitting light when the reverse bias voltage is applied. Can be.

  Here, “high frame rate” means that the number of times of switching between timing (light emission) and timing (non-light emission) of forward bias current drive to the OLED per unit time is large, and It means that the moment of light emission cannot be confirmed by the naked eye of a person. Further, “the frame rate is low”, which will be described later, indicates that the number of times of the above-described switching per unit time is small, and the moment when the OLED does not emit light can be confirmed by the naked eye.

  On the other hand, when the frame rate of the OLED drive in the forward bias is low, the pixels that do not emit light, that is, the pixels that are not used for displaying an image, Is applied. Thus, as in the case where the frame rate is high, it is possible to apply a reverse bias voltage while displaying an image on the display unit 1 and without causing a sense of discomfort to human eyes.

  In this case, if necessary, a wobbling process for changing the display position of the image on the display unit 1 is performed, and the pixels that need to apply a reverse bias voltage intentionally are set to the non-emission state, and then the reverse bias is applied. May be applied.

  In addition, when the reverse bias voltage cannot be applied when the frame rate is low, for example, when the use of the display device of the present embodiment is started or when the use of the display device is terminated, the reverse bias voltage is not applied. The application may be performed. For example, in the case of in-vehicle use, the former reverse bias voltage application is performed when the ignition power supply is turned on, and the latter reverse bias voltage application is performed when the ignition power supply is turned off. And the like.

  According to the present embodiment, when a reverse bias voltage is applied to each pixel of the display unit 1 at a predetermined interval, the reverse bias voltage is independently and independently of the forward bias voltage according to the degree of deterioration of the pixel. The configuration can be adjusted so that both the improvement of the deterioration of the current-voltage characteristics of each pixel and the suppression of the deterioration can be achieved. Further, when the degree of deterioration of a certain pixel becomes equal to or more than a predetermined value, by stopping the application of the reverse bias voltage, it is possible to suppress the deterioration due to the application of the unnecessary reverse bias voltage. Becomes

(Other embodiments)
Note that the display device shown in each of the above-described embodiments is an example of the display device of the present invention, and is not limited to each of the above-described embodiments, and is not limited to the scope described in the claims. Can be changed as appropriate.

  (1) For example, in the first embodiment, when the current density is equal to or higher than the upper limit value of the reverse bias voltage, an example in which the reverse bias voltage is not applied thereafter has been described. The application of the reverse bias voltage may be stopped. Also, an example has been described in which the upper limit of the current density at the time of applying the reverse bias voltage is the current density at the light emission start voltage in the forward bias, but a value lower than this current density may be set as the upper limit value.

  (2) In the first embodiment, when a pixel is composed of a plurality of sub-pixels having different emission colors, all of these sub-pixels are driven under the new reverse bias voltage application and forward bias driving conditions. The example of performing the correction has been described. However, reverse bias voltage application and correction may be performed only for some of the plurality of sub-pixels.

  For example, among a plurality of sub-pixels, a reverse bias voltage is applied and corrected for a sub-pixel having a large relative luminance decrease (for example, green and blue sub-pixels). On the other hand, reverse bias voltage application and correction are not performed on a pixel having a small degree of relative luminance decrease (for example, a red sub-pixel). As described above, the sub-pixel for which the reverse bias voltage is applied and the correction is performed may be selected.

REFERENCE SIGNS LIST 1 display unit 2 drive circuit 21 first circuit 22 second circuit 3 deterioration estimation unit 4 control unit 5 correction unit 6 video input unit

Claims (5)

A display unit (1) having a plurality of pixels constituted by self-luminous elements;
A first circuit (21) for applying a forward bias voltage to the pixel to cause the pixel to emit light and a second circuit (22) for applying a reverse bias voltage to the pixel. A driving circuit (2);
A deterioration estimating unit (3) for estimating the degree of deterioration of the plurality of pixels;
A control unit (4) for controlling a connection destination of the pixel to the first circuit or the second circuit;
A correction unit (5) that corrects the driving condition of the pixel according to the degree of deterioration estimated by the deterioration estimation unit;
With
The correction unit changes the forward bias voltage and the reverse bias voltage according to the degree of deterioration of the pixel,
The display device, wherein the reverse bias voltage is applied by a power supply different from a power supply used for applying the forward bias voltage.   The display device according to claim 1, wherein the reverse bias voltage is equal to or less than a predetermined threshold.   The reverse bias voltage is such that when the reverse bias voltage is applied to the pixel, the current density of the pixel is the forward bias voltage, and the current of the pixel at a light emission start voltage at which the pixel starts emitting light. The display device according to claim 2, wherein the display device is set to have a density equal to or lower than the density.   The control unit may include, for a plurality of the pixels, a current density when the reverse bias voltage having the predetermined threshold is applied exceeds the current density at the light emission start voltage, and the first circuit The display device according to claim 3, wherein the switching from the first circuit to the second circuit is not performed.   The device according to claim 1, wherein the correction unit corrects a driving condition for causing the pixel to emit light based on a current-voltage characteristic of the pixel after the reverse bias voltage is applied. The display device according to claim 1.

Images