CN110729214B

CN110729214B - Method for determining efficiency degradation of organic light emitting device and display system

Info

Publication number: CN110729214B
Application number: CN201910992468.6A
Authority: CN
Inventors: 戈尔拉玛瑞扎·恰吉
Original assignee: Ignis Innovation Inc
Current assignee: Ignis Innovation Inc
Priority date: 2014-05-23
Filing date: 2015-05-22
Publication date: 2023-11-14
Anticipated expiration: 2035-05-22
Also published as: DE102015209517A1; CN105097872B; CN105097872A; CN110729214A

Abstract

The present application relates to a method of determining efficiency degradation of an organic light emitting device and a display system. Wherein the method comprises the following steps: electrically driving an organic light emitting device of a plurality of organic light emitting devices similar to the selected organic light emitting device according to a plurality of different stress conditions; periodically measuring, for each organic light emitting device, an electrical operating parameter and periodically measuring efficiency degradation using a light sensor on the array-based semiconductor device and in or adjacent to a pixel of a pixel array comprising the organic light emitting device, generating corresponding measurements for different stress conditions; measuring a change in an electrical operating parameter of the selected organic light emitting device based on the electrical operating parameter reference; determining stress conditions of the selected organic light emitting device; and determining an efficiency degradation of the selected organic light emitting device using the measurement results for the different stress conditions, the variation of the electrical operating parameter, and the determined stress conditions.

Description

Method for determining efficiency degradation of organic light emitting device and display system

The application relates to a system and a method for extracting a correlation curve of an organic light emitting device, which are classified as 201510267035.6 patent application with the application number of 2015, 5 and 22.

Technical Field

The present invention relates generally to displays using light emitting devices such as OLEDs, and more particularly to extracting characteristic correlation curves under different stress conditions in such displays in order to compensate for aging of the light emitting devices.

Background

Active Matrix Organic Light Emitting Device (AMOLED) displays offer the advantages of lower power consumption, manufacturing flexibility, and faster refresh rates relative to conventional liquid crystal displays. In contrast to conventional liquid crystal displays, no backlight is present in AMOLED displays, since each pixel is made up of individually emitting different colored OLEDs. The OLED emits light based on the current provided by the driving transistor. The driving transistor is typically a Thin Film Transistor (TFT). The power consumption of each pixel has a direct relationship with the size of the light generated in that pixel.

During operation of the organic light emitting diode device, it suffers from degradation, which results in a decrease in light output at constant current over time. OLED devices also suffer from electrical degradation, which results in a decrease in current at a constant bias voltage over time. These degradations are essentially caused by stresses associated with the magnitude and duration of the applied voltage across the OLED and the current thus generated in the device. Such degradation is compounded by the contribution of environmental factors such as temperature, humidity, or the presence of oxidizing agents over time. The burn-in rate of thin film transistor devices is also dependent on the environment and stress (bias). The pixel is calibrated against the previous stored pixel history data several times to determine the aging effect on the pixel so that the aging of the pixel transistors and the OLED can be properly determined. Thus, accurate aging data is required over the lifetime of the display device.

In one OLED display compensation technique, the aging (and/or uniformity) of the pixel panel is extracted and stored as raw or processed data in a look-up table. The compensation module then uses the stored data to compensate for any shifts in the electrical and optical parameters of the OLED (e.g., shifts in the OLED operating voltage and optical efficiency) and of the backplate (e.g., shifts in the threshold voltage of the TFT), thus modifying the programming voltage of each pixel according to the stored data and video content. The compensation module modifies the bias of the driving TFT in such a way that sufficient current is passed through the OLED to maintain the same brightness level for each gray level. In other words, the proper programming voltage appropriately counteracts the electrical and optical aging of the OLED and the electrical degradation of the TFT.

The electrical parameters of the backplane TFT and OLED devices are continuously monitored and extracted over the lifetime of the display by a measurement circuit based on electrical feedback. Further, an optical aging parameter of the OLED device is estimated from the electrical degradation data of the OLED. However, the effect of optical aging of OLEDs is also dependent on stress conditions on individual pixels, and since stress varies from pixel to pixel, accurate compensation cannot be ensured unless a compensation appropriate for a particular stress level is determined.

Thus, for stress conditions on active pixels, it is desirable to efficiently extract the exact characteristic correlation curves of optical and electrical parameters for compensating for aging effects and other effects. For various stress conditions that active pixels may experience during operation of the display, it is desirable to have various characteristic correlation curves.

Disclosure of Invention

According to one embodiment, a system for determining efficiency degradation of an Organic Light Emitting Device (OLED) in an array-based semiconductor device having an array of pixels, the pixels comprising OLEDs is provided. In the system, determining a relationship between a change in an electrical operating parameter of the OLED and the efficiency degradation of the OLED for at least one stress condition; measuring a change in the electrical operating parameter of the OLED; determining stress conditions of at least one pixel or group of pixels in the semiconductor device; and determining the efficiency degradation of the OLED corresponding to the measured change in the electrical operating parameter of the OLED by using the determined relationship and the determined stress condition.

In one embodiment, the determined relationship is selected using the determined stress condition for determining the efficiency degradation of the OLED corresponding to the measured change in the electrical operating parameter of the OLED (e.g., OLED voltage). The stress condition of the OLED may be determined from a stress history of the OLED (e.g., a moving average of stress conditions experienced by the OLED) or from a rate of change over time of the electrical operating parameter of the OLED that is a function of the stress experienced by the OLED.

Aspects of the present invention will be apparent to those skilled in the art in view of the detailed description of various embodiments with reference to the accompanying drawings, a brief description of which will be given below.

Drawings

The invention may best be understood by referring to the following description taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of an AMOLED display system with compensation control.

Fig. 2 is a circuit diagram of one of the reference pixels in fig. 1 for modifying a characteristic correlation curve based on measurement data.

Fig. 3 is a graph of luminance emitted from an active pixel reflecting different levels of stress conditions over time that may require different compensation.

FIG. 4 is a graph of various characteristic correlation curves and a graph of the results of a technique for determining compensation using predetermined stress conditions.

FIG. 5 is a flow chart of a process for determining and updating a characteristic correlation curve based on a set of reference pixels under predetermined stress conditions.

FIG. 6 is a flow chart of a process for compensating for the programming voltage of an active pixel on a display using a predetermined characteristic correlation curve.

Fig. 7 is a graph of the dependence of OLED efficiency degradation on the change in OLED voltage.

FIG. 8 is a graph of OLED stress history versus stress intensity.

Fig. 9A is a graph of OLED voltage change versus time for different stress conditions.

Fig. 9B is a graph of OLED voltage change rate versus time for different stress conditions.

Fig. 10 is a graph of the rate of change of OLED voltage versus change of OLED voltage under different stress conditions.

Fig. 11 is a flowchart of a process of extracting degradation of OLED efficiency according to a change in OLED parameters such as OLED voltage.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description

Fig. 1 is an electronic display system 100 having an active matrix area or pixel array 102 in which an array of active pixels 104 are arranged in a row and column configuration 102. For ease of illustration, only two rows and two columns are shown. Outside the active matrix area (pixel array 102) is a peripheral area 106, and peripheral circuits for driving and controlling the area of the pixel array 102 are arranged in the peripheral area 106. The peripheral circuitry includes gate or address driver circuitry 108, source or data driver circuitry 110, a controller 112, and an optional voltage source (e.g., EL Vdd) driver 114. The controller 112 controls the gate driver 108, the source driver 110, and the voltage source driver 114. Under the control of the controller 112, the gate driver 108 operates address or select lines SEL [ i ], SEL [ i+1], etc., one in each row of pixels 104 in the pixel array 102. In the pixel sharing configuration described below, the gate or address driver circuit 108 may also optionally operate on global select lines GSEL [ j ] and optionally/GSEL [ j ], the global select lines GSEL [ j ] or/GSEL [ j ] operating on multiple rows of pixels 104 (e.g., every two rows of pixels 104) in the pixel array 102. Under the control of the controller 112, the source driver circuit 110 operates on voltage data lines Vdata [ k ], vdata [ k+1], and the like, one in each column of pixels 104 in the pixel array 102. The voltage data lines carry voltage programming information to each pixel 104 that is indicative of the brightness of each light emitting device in the pixel 104. The storage element (e.g., capacitor) in each pixel 104 stores the voltage programming information until the light emitting device is turned on for a light emitting or driving period. An optional voltage source driver 114, under the control of the controller 112, controls the voltage source (EL Vdd) lines, one in each row of pixels 104 in the pixel array 102. The controller 112 is also connected to a memory 118, the memory 118 being adapted to store various characteristic association curves and aging parameters of the pixels 104 as will be described below. Memory 118 may be one or more of flash memory, SRAM, DRAM, combinations thereof, and/or other memory.

The display system 100 may also include a current source circuit that provides a fixed current on the current bias line. In some configurations, a reference current can be provided to the current source circuit. In such a configuration, the current source control section controls timing of applying the bias current on the current bias line. In a configuration in which the reference current is not supplied to the current source circuit, the current source address driver controls the timing of applying the bias current on the current bias line.

It is known that for each pixel 104 in the display system 100, it is necessary to program it with information representing the brightness of the light emitting devices in that pixel 104. A frame defines a period of time that includes a programming period or phase during which each pixel in the display system 100 is programmed with a programming voltage that is indicative of brightness and a driving or light-emitting period or phase during which each light-emitting device in each pixel is turned on to emit light at a brightness that corresponds to the programming voltage stored in the storage element. Thus, a frame is one of many still images that make up a complete moving picture displayed on the display system 100. There are at least two schemes for programming and driving pixels: line by line or frame by frame. In programming row by row, a row of pixels is programmed and then driven before the next row of pixels is programmed and driven. In frame-by-frame programming, all rows of pixels in display system 100 are first programmed and all frames are driven row-by-row. Either scheme may employ a brief vertical blanking time at the beginning or end of each period where the pixels are not programmed or driven.

Components located outside the pixel array 102 may be arranged in a peripheral region 106 around the pixel array 102, the peripheral region 106 being arranged on the same physical substrate as the pixel array 102. These components include a gate driver 108, a source driver 110, and an optional voltage source driver 114. Alternatively, some components in the peripheral region may be disposed on the same substrate as the pixel array 102, while other components are disposed on different substrates, or all components in the peripheral region may be disposed on different substrates than the pixel array 102. The gate driver 108, the source driver 110, and the voltage source driver 114 together constitute a display driver circuit. The display driver circuitry in some configurations may include the gate driver 108 and the source driver 110 but not the voltage source driver 114.

The display system 100 further includes a current source and read circuit 120, the current source and read circuit 120 reading output data from data output lines VD [ k ], VD [ k+1], etc., one in each column of active pixels 104 in the pixel array 102. A set of optional reference devices (e.g., reference pixels) 130 are fabricated in the peripheral region 106 and are disposed on an edge of the pixel array 102 that is outside of the active pixels 104. The reference pixel 130 may also receive an input signal from the controller 112 and may output a data signal to the current source and read circuit 120. The reference pixel 130 includes a driving transistor and an OLED, but is not part of the pixel array 102 for displaying an image. As will be explained below, different groups of reference pixels 130 are in different stress conditions via different current levels from the current source and the read circuit 120. Since the reference pixel 130 is not part of the pixel array 102 and therefore does not display an image, the reference pixel 130 may provide data representing aging effects under different stress conditions. Although only one row and column of reference pixels 130 is shown in fig. 1, it should be understood that any number of reference pixels may be present. Each reference pixel 130 in the example shown in fig. 1 is fabricated adjacent to a corresponding photosensor 132. The light sensor 132 is used to determine the brightness level emitted by the corresponding reference pixel 130. It should be appreciated that the reference device (e.g., reference pixel 130) may be a stand-alone device rather than being fabricated on a display having active pixels 104.

Fig. 2 shows one example of a driver circuit 200 for one example reference pixel 130 in fig. 1. The driver circuit 200 of the reference pixel 130 includes a drive transistor 202, an Organic Light Emitting Device (OLED) 204, a storage capacitor 206, a select transistor 208, and a monitor transistor 210. The voltage source 212 is connected to the drive transistor 202. As shown in fig. 2, in this example, the driving transistor 202 is a thin film transistor made of amorphous silicon. Select line 214 is connected to select transistor 208 to activate driver circuit 200. Voltage programming input line 216 applies a programming voltage to drive transistor 202. The monitor line 218 monitors the output of the OLED 204 and/or the drive transistor 202. Select line 214 is connected to select transistor 208 and monitor transistor 210. During the read time, select line 214 is pulled high. The program voltage may be applied via program voltage input line 216. The monitor voltage may be read from a monitor line 218 connected to the monitor transistor 210. The signal to select line 214 may be sent in parallel with the pixel programming period.

The reference pixel 130 may be stressed at a certain current level by applying a constant voltage to the programming voltage input line 216. As will be described below, the voltage output measured from the monitor line 218 based on the reference voltage applied to the programming voltage input line 216 allows electrical characteristic data to be determined for the stress conditions applied during the run time of the reference pixel 130. Alternatively, the monitor line 218 and the program voltage input line 216 may be combined into one line (i.e., data/Mon) to perform both programming and monitoring functions through the single line. The output of the light sensor 132 allows the determination of optical property data for stress conditions during run time of the reference pixel 130.

According to one exemplary embodiment, in display system 100 in fig. 1, the brightness of each pixel (or sub-pixel) is adjusted based on the aging of at least one pixel to maintain a substantially uniform display during the operational lifetime of the system (e.g., 75000 hours). Non-limiting examples of display devices that comprise display system 100 include mobile phones, digital cameras, personal Digital Assistants (PDAs), computers, televisions, portable video players, global Positioning Systems (GPS), and the like.

As the OLED material of the active pixel 104 ages, the voltage required to maintain a constant current at a given level in the OLED increases. To compensate for the electrical aging of the OLED, the memory 118 stores a compensation voltage required for maintaining a constant current for each active pixel. It also stores data in the form of characteristic correlation curves for different stress conditions, which is used by the controller 112 to determine compensation voltages to modify the programming voltages for driving each OLED of the active pixels 104 to appropriately display the desired output level of brightness by increasing the current of the OLED and thereby compensating for the optical aging of the OLED. In particular, the memory 118 stores a plurality of predefined characteristic correlation curves or functions that represent degradation of the luminance efficiency of an OLED operating under different predetermined stress conditions. The different predetermined stress conditions generally represent different types of stresses or operating conditions that the active pixel 104 may be subjected to during the lifetime of the pixel. The different stress conditions may include different levels of constant current demand from low to high, constant brightness demand from low to high, or a mixture of more than two stress levels. For example, the stress level may be a stress level at a certain current in a certain percentage of time and another current in another percentage of time. Other stress levels may be specialized stress levels, for example, to represent the level of average streaming video (average streaming video) displayed on the display system 100. Initially, reference electrical and optical characteristics of a reference device, such as reference pixel 130, under different stress conditions are stored in memory 118. In this example, the baseline electrical and optical characteristics of the reference device are measured from the reference device immediately after the reference device is manufactured.

Each such stress condition may be applied to a set of reference pixels (e.g., reference pixel 130) by: maintaining a constant current in the reference pixel 130 for a period of time; maintaining a constant brightness of the reference pixel 130 for a period of time; and/or varying the current in the reference pixel or the brightness of the reference pixel at different predetermined levels and predetermined intervals over a period of time. The current or brightness level generated in the reference pixel 130 may be, for example, a high value, a low value, and/or an average value as desired for a particular application of the display system 100. For example, applications such as computer monitors require high values. Similarly, the period of time that the current or brightness level is generated in the reference pixel may depend on the particular application of the display system 100.

It is contemplated that different predetermined stress conditions are applied to different reference pixels 130 during operation of the display system 100 in order to obtain the same aging effect at each predetermined stress condition. In other words, a first predetermined stress condition is applied to a first set of reference pixels, a second predetermined stress condition is applied to a second set of reference pixels, and so on. In this example, the display system 100 has multiple sets of reference pixels 130 that are stressed under 16 different stress conditions that lie in the range of low to high current values of the pixels. Thus, there are 16 different sets of reference pixels 130 in this example. Of course, a greater or lesser number of stress conditions may be employed, depending on factors such as the desired accuracy of the compensation, the physical space in the peripheral region 106, the amount of processing power available, and the amount of memory used to store the characteristic association curve data.

By subjecting the reference pixel or group of reference pixels to stress conditions continuously, the components of the reference pixels age according to the operating conditions of the stress conditions. When stress conditions are applied to the reference pixels during operation of the system 100, the electrical and optical characteristics of the reference pixels are measured and evaluated to obtain data for determining correction curves that are used to compensate for aging of the active pixels 104 in the array 102. In this example, the optical and electrical properties are measured once per hour for each set of reference pixels 130. Thus, for the measured characteristic of the reference pixel 130, the corresponding characteristic correlation curve is updated. Of course, these measurements may be made for a shorter period of time or for a longer period of time, depending on the accuracy desired for the aging compensation.

In general, the luminance of the OLED 204 has a direct linear relationship with the current applied to the OLED 204. The optical properties of an OLED can be expressed as:

L＝O*I

in this formula, the luminance L is a result of multiplying the coefficient O based on the characteristics of the OLED by the current I. As the OLED 204 ages, the coefficient O decreases, and thus, at a constant current value, the luminance decreases. Thus, the measured luminance at a given current can be used to determine the aging-induced characteristic change of the coefficient O of a particular OLED 204 at a particular time for a predetermined stress condition.

The measured electrical characteristic represents the relationship between the voltage supplied to the drive transistor 202 and the current generated thereby in the OLED 204. For example, a voltage sensor or a thin film transistor such as the monitor transistor 210 in fig. 2 may be utilized to measure the change in voltage required to achieve a constant current level in the OLED of the reference pixel. The required voltage generally increases as the OLED 204 and the drive transistor 202 age. The required voltage has a power law relation with the output current as shown in the following formula.

I＝k*(V-e) ^a

In this formula, the current I is determined by a constant k multiplied by the input voltage V minus a coefficient e, where the coefficient e represents the electrical characteristics of the drive transistor 202. Thus, the voltage has a power law relationship with the current I of the variable a. As transistor 202 ages, the coefficient e increases, thereby requiring a greater voltage to produce the same current. Thus, the current measured from the reference pixel may be used to determine the value of the coefficient e of a particular reference pixel at a particular time for the stress condition applied to the reference pixel.

As described above, the optical characteristic O represents the relationship between the luminance of the OLED 204 of the reference pixel 130 in fig. 2 and the current in the OLED 204 measured by the photosensor 132. The measured electrical property e represents the relationship between the applied voltage and the current resulting therefrom. When a stress condition is applied to the reference pixel, the change in luminance of the reference pixel 130 at a constant current level relative to the baseline optical characteristic may be measured by a light sensor, such as the light sensor 132 in fig. 1. The change in the electrical characteristic from the reference electrical characteristic can be measured from the monitoring line to determine the current output. During operation of the display system 100, stress condition current levels are continuously applied to the reference pixels 130. When a measurement is desired, the stress condition current is removed and the select line 214 is activated. A reference voltage is applied and the resulting brightness level is obtained from the output of the light sensor 132, and the output voltage is measured from the monitor line 218. The data thus obtained is compared with previous optical and electrical data to determine the changes in current output and luminance output caused by aging for a particular stress condition and to update the characteristics of the reference pixel under that stress condition. The characteristic association curve is updated using the updated characteristic data.

Then, a characteristic correlation curve (or function) over time is determined for a predetermined stress condition by using the electrical characteristic and the optical characteristic measured from the reference pixel. The characteristic correlation curve provides a quantifiable relationship between expected electrical aging and optical degradation for a given pixel operating under such stress conditions. More particularly, each point on the characteristic correlation curve determines a correlation between the optical and electrical characteristics of the OLED for a given pixel under the stress condition at a given time that the reference pixel 130 is measured. The controller 112 may then use this characteristic to determine the appropriate compensation voltage for active pixels 104 that have aged under the same stress conditions as those applied to the reference pixels 130. In another example, the baseline optical characteristics may be periodically measured from the base OLED device while the optical characteristics of the OLED of the reference pixel are measured. The base OLED device is not stressed or is stressed at a known and controlled rate. This will eliminate any environmental impact on the reference OLED characteristics.

Each reference pixel 130 of the display system 100 may not have uniform characteristics due to manufacturing processes and other factors known to those skilled in the art, which results in different light emission performance. In one technique, the values of the electrical characteristics and the values of the luminance characteristics obtained by groups of reference pixels under predetermined stress conditions are averaged. A better expression of the effect of stress conditions on the average pixel is obtained by: the set of reference pixels 130 is stressed and a poll averaging (polling averaging) technique is applied to avoid defects, measurement noise, and other problems that may occur during the stressing of the reference pixels. For example, the erroneous values (e.g., erroneous values determined due to noise or failed reference pixels) may be removed by averaging. Such techniques may have predetermined brightness levels and electrical characteristics that must be met before those values are included in the averaging. Additional statistical regression techniques may also be used to provide smaller weights to electrical and optical property values that differ significantly from other measured values for reference pixels under given stress conditions.

In this example, each stress condition is applied to a different set of reference pixels. The optical and electrical properties of the reference pixels are measured and a different property association curve corresponding to each stress condition is determined using a round robin averaging technique and/or a statistical regression technique. The different characteristic correlation curves are stored in the memory 118. Although this example uses a reference device to determine the association curve, the association curve may be determined in other ways, such as from historical data or in ways predetermined by the manufacturer.

During operation of the display system 100, each set of reference pixels 130 may be subjected to a respective stress condition, and the characteristic association curve initially stored in the memory 118 may be updated by the controller 112 to reflect data obtained from the reference pixels 130 subjected to the same external conditions as the active pixels 104. Thus, the characteristic correlation curve of each active pixel 104 may be adjusted based on measurements of the electrical characteristics and brightness characteristics of the reference pixel 130 during operation of the display system 100. Thus, the electrical and brightness characteristics for each stress condition are stored in memory 118 and updated during operation of display system 100. The storage of data may be a piecewise linear model. In this example, such a piecewise linear model has 16 coefficients, which are updated when the voltage and brightness characteristics of the reference pixel 130 are measured. Alternatively, the curves may be determined and updated by using linear regression or by storing the data in a look-up table in the memory 118.

Generating and storing a characteristic correlation curve for each possible stress condition is impractical because a significant amount of resources (e.g., memory storage, processing power, etc.) would be required. The disclosed display system 100 overcomes this limitation by: discrete numbers of characteristic correlation curves under predetermined stress conditions are determined and stored, and those predefined characteristic correlation curves are then combined by using linear or nonlinear algorithms to synthesize a compensation factor for each pixel 104 of the display system 100 according to the particular operating conditions of each pixel. As described above, in this example, there are 16 different ranges of predetermined stress conditions, and thus 16 different characteristic association curves are stored in the memory 118.

For each pixel 104, the display system 100 analyzes the stress conditions being applied to that pixel 104 and determines a compensation factor by using an algorithm and based on the predefined characteristic correlation curve and measured electrical aging of the panel pixels. The display system 100 then provides a voltage to the pixel based on the compensation factor. Accordingly, the controller 112 determines the stress of the particular pixel 104, and determines the two closest predetermined stress conditions for the stress conditions of the particular pixel 104 and the accompanying characteristic data obtained from the reference pixel 130 under these predetermined stress conditions. Thus, the stress condition of the active pixel 104 falls between a low predetermined stress condition and a high predetermined stress condition.

For ease of disclosure, the following examples of linear and nonlinear formulas for combining the characteristic correlation curves are described by two such predefined characteristic correlation curves; however, it should be appreciated that any other number of predefined characteristic association curves may be utilized in the exemplary technique for combining characteristic association curves. The two exemplary characteristic curves include a first characteristic curve determined for high stress conditions and a second characteristic curve determined for low stress conditions.

The ability to use different characteristic correlation curves for different levels can be varied to withstand different predetermined stress conditions than are applied to the reference pixel 130The active pixels 104 of the stress conditions of (a) provide accurate compensation. Fig. 3 is a graph showing different stress conditions of the active pixel 104 over time, showing luminance levels emitted over time. During a first period, the brightness of the active pixel is represented by trace 302, trace 302 showing brightness at 300 and 500 nits (cd/cm) ² ) Between them. Thus, the stress conditions applied to the active pixels during trace 302 are relatively high. In a second period, the brightness of the active pixel is represented by trace 304, trace 304 showing a brightness between 300 and 100 nits. Thus, the stress condition during trace 304 is lower than the stress condition for the first period of time, and the aging effect of the pixel during this period is different than the aging effect under high stress conditions. In a third period, the brightness of the active pixel is represented by trace 306, trace 306 showing a brightness between 100 and 0 nits. The stress condition during the period is lower than the stress condition for the second period. In the fourth time period, the brightness of the active pixel is represented by trace 308, trace 308 showing a return to a higher stress condition based on a higher brightness between 400 and 500 nits.

For a particular stress condition for each active pixel 104, a limited number of reference pixels 130 and a corresponding limited number of stress conditions may require the use of an average or a continuous (moving) average. For each pixel, a particular stress condition may be mapped to a linear combination of characteristic correlation curves from multiple reference pixels 130. The combination of the two characteristic curves under predetermined stress conditions enables an exact compensation of all stress conditions occurring between these stress conditions. For example, two reference characteristic correlation curves under high and low stress conditions can determine a near characteristic correlation curve for an active pixel having a stress condition between the two reference curves. The controller 112 uses a weighted moving average algorithm (weighted moving average algorithm) to combine the first and second reference characteristic correlation curves stored in the memory 118. Stress condition St (t _i ) Can be expressed as:

St(t _i )＝(St(t _i-1 )*k _avg +L(t _i ))/(k _avg +1)

in this formula, st (t _i-1 ) Is the stress condition, k, at the previous time _avg Is a moving average constant. L (t) _i ) Is the measured luminance of the active pixel at that certain time, which can be determined by the following formula:

In the formula, L _peak Is the highest brightness allowed by the design of the display system 100. Variable g (t) _i ) Is the gray scale, g, at the time of measurement _peak Is the highest gray value used (e.g., 255), and γ is the gamma constant. The weighted moving average algorithm of the characteristic correlation curves using predetermined high and low stress conditions can determine the compensation factor K via the following formula _comp ：

K _comp ＝K _high f _high (ΔI)+K _low f _low (ΔI)

In the formula, f _high Is a first function corresponding to a characteristic correlation curve of a high predetermined stress condition, and f _low Is a second function corresponding to a characteristic correlation curve for low predetermined stress conditions. Δi is the change in current in the OLED at a fixed voltage input, which shows the change (electrical degradation) caused by aging effects measured at a specific time. It should be appreciated that the change in current may be replaced by a change in voltage at a fixed current Δv. K (K) _high Is a weighted variable assigned to the characteristic correlation curve of the high stress condition, and K _low Is the weight assigned to the characteristic correlation curve for low stress conditions. The weighting variable K can be determined according to the following formula _high And K _low ：

K _high ＝St(t _i )/L _high

K _low ＝1-K _high

Here, L _high Is the brightness associated with high stress conditions.

Active at any time during operationThe change in voltage or current in the pixel represents an electrical characteristic, while the change in current as part of a function of high or low stress conditions represents an optical characteristic. In this example, the brightness, peak brightness and average compensation factor (function of the difference between the two characteristic correlation curves) K under high stress conditions are _avg Stored in memory 118 for determining a compensation factor for each active pixel. Additional variables are stored in memory 118 including, but not limited to, gray values (e.g., gray value 255) for the maximum brightness allowed by display system 100. In addition, the average compensation factor K can be empirically determined from data obtained during the application of stress conditions to the reference pixels _avg 。

Thus, the relationship between optical degradation and electrical aging of any pixel 104 in the display system 100 may be adjusted to avoid errors associated with differences (diversity) in the characteristic correlation curves caused by different stress conditions. The number of stored characteristic curves can also be minimized to a number that ensures that the averaging technique is sufficiently accurate for the required level of compensation.

Compensation factor K _comp Can be used to compensate for OLED light efficiency aging by adjusting the programming voltage of the active pixels. Another technique for determining an appropriate compensation factor for stress conditions on an active pixel may be referred to as dynamic moving average (dynamic moving averaging). The dynamic moving average technique includes: changing the moving average coefficient K over the lifetime of the display system 100 _avg To compensate for differences between the two characteristic correlation curves under different predetermined stress conditions to prevent distortion of the display output. As the OLED of the active pixel ages, the difference between the two characteristic correlation curves under different stress conditions increases. Thus, K may be increased during the lifetime of display system 100 _avg To avoid sharp transitions between the two curves of the active pixel with stress conditions falling between two predetermined stress conditions. The measured current change Δi can be used to adjust K _avg Values to improve the performance of the algorithm used to determine the compensation factors.

In another technique for improving the performance of the compensation process, known as event-based moving average, the system is reset after each aging stage. This technique further improves the extraction of the characteristic correlation curve for the OLED of each active pixel 104. The display system 100 is reset after each aging stage (or after the user turns the display system 100 on or off). In this example, the compensation factor K is determined by the following formula _comp ：

K _comp ＝K _{comp_evt} +K _high (f _high (ΔI)-f _high (ΔI _evt ))+K _low (f _low (ΔI)-f _low (ΔI _evt ))

In the formula, K _{comp_evt} Is the compensation factor calculated at the previous time, and ΔI _evt Is the change in OLED current during the previous time at a fixed voltage. As with other compensation determination techniques, the change in current may be replaced by a change in OLED voltage at a fixed current.

Fig. 4 is a graph 400 illustrating different characteristic association curves based on different techniques. Graph 400 compares the change in percentage of optical compensation with the change in voltage of the OLED of the active pixel required to produce a given current. As shown in graph 400, at larger changes in voltage used to reflect active pixel aging, the high stress predetermined characteristic association 402 deviates from the low stress predetermined characteristic association 404. The set of points 406 represent correction curves for current compensation of the active pixel determined by a moving average (moving average) technique and at different voltage variations according to the predetermined characteristic correlation curves 402 and 404. As the change in voltage used to reflect aging increases, the transition of the correction curve 406 has a sharp transition between the low stress characteristic correlation curve 404 and the high stress characteristic correlation curve 402. The set of points 408 represents a characteristic association curve determined by a dynamic moving average (dynamic moving averaging) technique. The set of points 410 represent compensation factors determined by event-based moving average (event-based moving averaging) techniques. Based on the OLED characteristics, one of the above techniques may be used to improve the compensation for the degradation of the OLED efficiency.

As described above, the electrical characteristics of the first set of sample pixels are measured. For example, the electrical characteristics of each pixel in the first set of sample pixels may be measured by a Thin Film Transistor (TFT) connected to each pixel. Alternatively, for example, the optical characteristic (e.g., brightness) may be measured by a photosensor provided for each pixel in the first group of sample pixels. The amount of change required for the brightness of each pixel can be extracted from the drift of the voltage of more than one pixel. This may be achieved by a series of calculations for determining the correlation between the drift of the voltage or current supplied to a pixel and/or the brightness of the luminescent material in that pixel.

The above-described method for extracting a characteristic correlation curve to compensate for aging of pixels in an array may be performed by a processing device such as the processing device of controller 112 in fig. 1 or other such device, as will be appreciated by those skilled in the computer, software, and network arts that the above-described processing device may be conveniently implemented by one or more general purpose computer systems, microprocessors, digital signal processors, microcontrollers, application Specific Integrated Circuits (ASICs), programmable Logic Devices (PLDs), field Programmable Logic Devices (FPLDs), field Programmable Gate Arrays (FPGAs), etc. programmed according to the teachings as described and illustrated herein.

In addition, more than two computing systems or devices may be substituted for any of the controllers described herein. Thus, the principles and advantages of distributed processing, such as redundancy, replication, etc., can also be implemented, if desired, to increase the robustness and performance of the controllers described herein.

The operations for compensating for the example characteristic association curve of the aging method may be performed by machine readable instructions. In these examples, the machine-readable instructions comprise algorithms executed by: (a) a processor, (b) a controller, and/or (c) one or more other suitable processing devices. The algorithms may be implemented as software stored on a tangible medium such as flash memory, CD-ROM, floppy disk, hard drive, digital video (versatile) disk (DVD), or other storage devices, but those skilled in the art will readily appreciate that the entire algorithm and/or parts thereof could alternatively be executed by a device other than a processor and/or embodied as firmware or dedicated hardware in a well known manner (e.g., it could be embodied by an Application Specific Integrated Circuit (ASIC), a Programmable Logic Device (PLD), a Field Programmable Logic Device (FPLD), a Field Programmable Gate Array (FPGA), discrete logic, etc.). For example, any or all of the components of the characteristic correlation curve used to compensate for the aging method can be implemented by software, hardware, and/or firmware. Further, some or all of the depicted machine readable instructions may be implemented manually.

Fig. 5 is a flow chart of a process for determining and updating a characteristic association curve of a display system (e.g., display system 100 of fig. 1). The stress conditions are selected to provide a sufficient reference (500) for correlating a range of stress conditions for the active pixel. Then, a set of reference pixels is selected for each stress condition (502). Then, each group of reference pixels corresponding to each stress condition is stressed at that stress condition, and the optical and electrical characteristics of the fiducial are stored (504). The brightness level of each pixel in each group is measured and recorded at periodic intervals (506). The luminance characteristics are then determined by averaging the measured luminance of each pixel in the group of pixels under each stress condition (508). An electrical characteristic of each pixel in each group is determined (510). An average value for each pixel in the group is determined to determine an average electrical characteristic (512). The average luminance characteristics and average electrical characteristics for each group are then used to update the corresponding characteristic correlation curves under predetermined stress conditions (514). Once the correlation curves are determined and updated, the controller may use the updated characteristic correlation curves to compensate for aging effects of active pixels subjected to different stress conditions.

Referring to fig. 6, a flowchart of a process for determining the compensation factor for an active pixel at a given instant using an appropriate predetermined characteristic association curve for the display system 100 as obtained in the process of fig. 5 is shown. The brightness emitted by the active pixel is determined based on the highest brightness and the programming voltage (600). Stress conditions for a particular active pixel are measured based on previous stress conditions, the determined brightness, and an average compensation factor (602). An appropriate predetermined stress characteristic correlation curve is read from memory (604). In this example, the two characteristic correlation curves correspond to predetermined stress conditions, wherein the measured stress conditions of the active pixels fall between these predetermined stress conditions. The controller 112 then determines coefficients by using the current or voltage changes measured from the active pixels and according to each predetermined stress condition (606). The controller then determines the modified coefficients to calculate and add the compensation voltage to the programming voltage of the active pixel (608). The determined stress condition is stored in a memory (610). The controller 112 then stores the new compensation factor, which can then be employed to modify the programming voltage of the active pixel during each frame period after the reference pixel 130 is measured (612).

The OLED efficiency degradation can be calculated based on a correlation curve between the OLED electrical change and the efficiency degradation (e.g., correlation curve in fig. 7). Here, a change in the electrical parameter of the OLED is detected and this value is used to extract the efficiency degradation from the curve. The pixel current may then be adjusted accordingly to compensate for the degradation. The main challenge is that the correlation curve is a function of stress conditions. Therefore, in order to achieve more accurate compensation, one need is to consider the effects of different stress conditions. In one approach, stress conditions for each pixel (pixel group) are used to select among different correlation curves to extract the appropriate efficiency loss for each particular case. Now, a plurality of methods for determining stress conditions will be described.

First, a stress history for each pixel (pixel group) can be created. Briefly, the stress history may be a moving average of stress conditions. To improve the accuracy of the calculation, a weighted stress history may be used. Here, as in the example depicted in fig. 8, the effect of each stress may have a different weight based on stress intensity or period. For example, the impact of low intensity stress is small in the selection of the OLED correlation curve. Thus, a curve having a small weight at a small intensity, such as the curve in fig. 8, may be used. Sub-sampling (sub-sampling) may also be used to calculate stress history to reduce memory transfer activity (memory transfer activity). In one case, it may be assumed that the stress history is low frequency in time. In this case, the pixel condition of each frame need not be sampled. The sampling rate for different applications may be modified based on the content frame rate (content frame rate). Here, only a small number of pixels are selected to obtain an updated stress history during each frame.

In another case, it may be assumed that the stress history is spatially low frequency. In this case, it is not necessary to sample all pixels. Here, the stress history is calculated using a subset of pixels, and then the stress history of all pixels may be calculated using interpolation techniques.

In another case, a low sampling rate in time and a low sampling rate in space may be combined.

In some cases, the memory and computing modules required for stress history may not be included. Here, as shown in fig. 9A and 9B, the rate of change of the OLED electrical parameter may be used to extract the stress condition. FIG. 9A shows ΔV under low, medium and high stress conditions _OLED Time-dependent changes, and 9B shows the dependence of the rate of change on time under the same stress conditions.

As shown in fig. 10, the rate of change of the electrical parameter may be used as an indicator of the stress condition. For example, as shown in fig. 10, the rate of change of the electrical parameter based on the change of the electrical parameter may be modeled or extracted through experiments for different stress conditions. The rate of change may also be used to extract stress conditions based on a comparison of the measured change to the rate of change of the electrical parameter. Here, a function established for the change and the rate of change of the electrical parameter is used. Alternatively, stress conditions, correlation curves and measured variation parameters may be used.

FIG. 11 is a flowchart of a process for compensating for OLED efficiency degradation based on changes in OLED electrical parameters and measurements of the rate of change. In this process, a change in an OLED parameter (e.g., OLED voltage) is extracted in step 1101, and then a rate of change of the OLED parameter is calculated based on the previously extracted value in step 1102. Next, step 1103 uses the change in the parameter and the rate of change to identify a stress condition. Finally, step 1104 calculates efficiency degradation based on the stress conditions, the measured parameters, and the correlation curve.

While particular embodiments, aspects and applications of the present application have been shown and described, it is to be understood that the application is not limited to the precise arrangements and instrumentalities disclosed in the application, and that various modifications, changes and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the application as defined in the appended claims.

Cross reference to related applications

The present application claims priority from U.S. patent application Ser. No. 14/286,711, filed on 5/21/2015, the entire contents of which are hereby incorporated by reference.

Claims

1. A method for determining efficiency degradation of a selected organic light emitting device in an array-based semiconductor device having an array of pixels including the organic light emitting device, the method comprising:

Electrically driving an organic light emitting device of a plurality of organic light emitting devices of the pixel array according to a corresponding plurality of different stress conditions, the plurality of organic light emitting devices being similar to the selected organic light emitting device;

periodically measuring, for each organic light emitting device of the plurality of organic light emitting devices, an electrical operating parameter of the organic light emitting device and periodically measuring an efficiency degradation of the organic light emitting device using a light sensor and an output of the light sensor, generating a measurement corresponding to the organic light emitting device for the different stress conditions, the light sensor being on the array-based semiconductor device and in or adjacent to a pixel of the pixel array comprising the organic light emitting device;

measuring a change in the electrical operating parameter of the selected organic light emitting device based on an electrical operating parameter reference;

determining stress conditions of the selected organic light emitting device; and

the method further includes determining the efficiency degradation of the selected organic light emitting device using the measurement results for the different stress conditions, the measured change in the electrical operating parameter of the selected organic light emitting device, and the determined stress conditions.

2. The method of claim 1, further comprising:

a corresponding correlation curve is determined from the measurements for each of the different stress conditions,

wherein using the measurements for the different stress conditions comprises using the respective correlation curves.

3. The method of claim 1, wherein the electrical operating parameter is an organic light emitting device voltage.

4. The method of claim 1, wherein the stress condition of the selected organic light emitting device is determined based on a stress history of the selected organic light emitting device.

5. The method of claim 4, wherein the stress history is a moving average of stress conditions experienced by the selected organic light emitting device.

6. The method of claim 1, wherein the stress condition of the selected organic light emitting device is determined from a rate of change over time of the electrical operating parameter of the selected organic light emitting device as a function of the stress experienced by the selected organic light emitting device.

7. The method of claim 1, wherein the measurement for each different stress condition represents a relationship between a change in an electrical operating parameter of the corresponding organic light emitting device based on the electrical operating parameter reference and the efficiency degradation of the organic light emitting device.

8. The method of claim 1, wherein the stress condition of the selected organic light emitting device is determined from stress conditions of at least one pixel or group of pixels in the semiconductor device.

9. The method of claim 8, wherein the stress condition of the selected organic light emitting device is determined by interpolation of the stress condition of the at least one pixel or group of pixels.

10. A display system, comprising:

a pixel array on the array-based semiconductor device for displaying an image, each pixel including a driving transistor and an organic light emitting device;

a memory for storing measurements for stress conditions; and

a controller connected to the memory and the array of active pixels, the controller:

periodically measuring, for each organic light emitting device of the plurality of organic light emitting devices, an electrical operating parameter of the organic light emitting device and periodically measuring an efficiency degradation of the organic light emitting device using a light sensor and an output of the light sensor, generating a measurement corresponding to the organic light emitting device for the different stress conditions and storing the measurement into the memory, the light sensor being on the array-based semiconductor device and in or adjacent to a pixel of the pixel array comprising the organic light emitting device;

11. The display system of claim 10, wherein the controller is adapted to:

12. The display system of claim 10, wherein the electrical operating parameter is an organic light emitting device voltage.

13. The display system of claim 10, wherein the stress condition of the selected organic light emitting device is determined based on a stress history of the selected organic light emitting device.

14. The display system of claim 13, wherein the stress history is a moving average of stress conditions experienced by the selected organic light emitting device.

15. The display system of claim 10 wherein the stress condition of the selected organic light emitting device is determined from a rate of change over time of the electrical operating parameter of the selected organic light emitting device as a function of the stress experienced by the selected organic light emitting device.

16. The display system of claim 10, wherein the measurement for each different stress condition represents a relationship between a change in an electrical operating parameter of the corresponding organic light emitting device based on the electrical operating parameter reference and the efficiency degradation of the organic light emitting device.

17. The display system of claim 10 wherein the stress condition of the selected organic light emitting device is determined from stress conditions of at least one pixel or group of pixels in the semiconductor device.

18. The display system of claim 17 wherein the stress condition of the selected organic light emitting device is determined by interpolation of the stress condition of the at least one pixel or group of pixels.