ORGANIC light-emitting devices (OLEDs) are brightly

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1 JOURNAL OF DISPLAY TECHNOLOGY 1 Using Integrated Optical Feedback to Counter Pixel Aging and Stabilize Light Output of Organic LED Display Technology Jennifer Yu, Jonathan R. Tischler, Charles G. Sodini, Fellow, IEEE, and Vladimir Bulović Abstract We define a metric of useful operating lifetime of an organic light-emitting device (OLED) display and relate it to the commonly measured half-life of constituent OLED pixels. We enumerate sources of OLED operational instability and propose an optical feedback solution in a novel integrated configuration to counter pixel aging and maintain stable light output across all of the pixels of an OLED display. Such optical feedback can correct pixel imperfections in both active matrix and passive matrix OLED displays. As an example, we analyze lifetime data previously published by Kwong et al.. in 2002, and demonstrate that our optical feedback technique could maintain 100 cd/m 2 display light output within a 2% brightness accuracy for more than hours of continuous use for this specific OLED system. From this example we draw conclusions generally applicable to extending stable operating lifetime of other OLED structures. Index Terms Display, half-life, lifetime, optical feedback, organic LED (OLED), reliability, stability. I. INTRODUCTION ORGANIC light-emitting devices (OLEDs) are brightly emissive, efficient, color-tunable, planar light sources, with fast response times, identifying them as a viable contender for a dominant emissive flat-panel display technology. The past 20 years of OLED technology development has demonstrated steady improvement of device efficiency and emissive pixel stability [1], with the external power conversion efficiency of many OLED structures exceeding the efficiency of the best liquid crystal display technologies that presently dominate the flat-panel display market. However, the evolution of OLED displays as the dominant large-area display technology has been hampered by the critical challenge of pixel-to-pixel stability. Individual OLED pixel performance is affected by the quality of environmental encapsulation, operating temperature change, device thickness uniformity, and previous operating conditions, which are all manifested in pixel-to-pixel nonuniformity. If OLEDs are to be used as pixel elements in information displays with hours of stable operating lifetime, we show below that operating half-life,, of individual OLED pixels Manuscript received December 11, 2007; revised February 4, This work was supported in part by the Focus Center for Circuit & System Solutions (C2S2), one of five research centers funded under the Focus Center Research Program, a Semiconductor Research Corporation Program and the Presidential Early Career Award for Scientists and Engineers (PECASE). The authors are with the Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA USA ( jenyu@mit.edu). Color versions of one or more figures are available online at ieee.org. Digital Object Identifier /JDT (measure of time during which the OLED pixel quantum efficiency decays to half of the starting efficiency under constant current driving conditions) has to be extended beyond hours. To date, only a handful of red OLED structures [1] show such long, implying a significant challenge to the viability of OLEDs in long-lived, high quality full-color information displays. In this study we first enumerate sources of OLED instability, and then introduce a simple technological solution to compensate for pixel aging and other instabilities to retain stable uniform light output. Using optical feedback in a novel integrated configuration, we demonstrate an optical compensation scheme that is applicable to correcting pixel non-uniformities in either active matrix or passive matrix pixelated OLED displays. In addition to compensating for OLED non-uniformities due to pixel aging, the technique also corrects for other pixel imperfections, such as variations in pixel-to-pixel driver electronics of active matrix backplanes, and variable resistance of bus lines in passive matrix displays. II. OLED DISPLAY LIFETIME Stability of pixel brightness in information displays is necessitated by the high sensitivity of human vision to brightness variation [2], with an average human eye capable of distinguishing a 2% difference in relative intensity of neighboring pixels [3]. The conventionally stated metric of OLED half-life,, which measures 50% change in OLED pixel brightness at constant current driving conditions, is therefore unsuitable for evaluating viability of particular OLED technology in information displays that contain large numbers of neighboring pixels that degrade different amounts according to usage. Consequently, within the present work, we define the OLED display lifetime as either,or, the time it takes an OLED pixel to degrade to 98%, 97%, or 95% of its initial brightness, respectively, when operated under constant current conditions. Among these, is the most conservative estimate on display lifetime, as it considers the extreme scenario in which one pixel is continually aged to 98% of initial brightness while its neighbor is not operated. Nevertheless, the and times are useful in evaluating viability of individual OLED pixels in displays with simultaneous, although nonuniform, aging of neighboring pixels. Fig. 1 plots the relationship between, and, assuming that the OLED luminescence efficiency degrades as an inverse exponential of the operating time. The relationship between and can be written as X/$ IEEE

2 2 JOURNAL OF DISPLAY TECHNOLOGY Fig. 1. OLED display lifetime, defined as uniformity within 2% (t ), 3% (t ) or 5% (t ) of initial luminance, for a given OLED half-life, t with = 1. with for the single exponential decay and for the stretched exponential decay. The stretched exponential decay has recently been used for accurately fitting the shape of particular OLED degradation curves [4], however the choice of is dependent on a particular device structure. In our analysis, we set in the above expression to compute the lower limit for the required OLED half-life. Therefore, for displays with 2% brightness accuracy over hours of operation, we need to utilize OLED pixels with hours (which we calculated by setting in the above expression). Displays with more relaxed brightness accuracy condition, allowing for 3% or 5% brightness non-uniformity, require OLEDs with hours and hours, respectively. Recently published data on phosphorescent OLEDs shows that has been extended to over hours for yellow green and deep-red devices operated at video brightness [1], [5]. Indeed, the shorter of blue OLEDs, does not meet even this most relaxed operating condition. Also, note that setting necessitates OLEDs with longer. For example, with the green phosphorescent devices analyzed in this study [6], average for accelerated aging conditions is 0.76 which requires hours to maintain 2% brightness accuracy over hours. Even if OLED degradation issues are mitigated through materials and device structure advancements, yielding suitable lifetimes, OLED displays can exhibit additional sources of instability: 1) Current transport and electroluminescence efficiency of an OLED are temperature-dependent, so that pixel brightness is dependent on the pixel temperature [7], [8]. As OLED displays are built on glass substrates, the poor thermal conductance of glass can result in uneven convection cooling of the display plane contributing to temperature gradients across the display surface, which increases with larger display area [9]. 2) OLED efficiency also changes nonlinearly with increasing drive current due to exciton exciton and exciton polaron annihilation, complicating the linear scaling of pixel brightness with current. These effects are most pronounced in phosphorescent OLEDs with long-lived excitons [10]. 3) Thickness uniformity of OLED charge transport and luminescent layers of OLEDs is more difficult to maintain across large area substrates, and can also affect the brightness uniformity of a display [11]. 4) Threshold voltage of thin film transistor OLED pixel drivers have been shown to change with operation, affecting the drive current and brightness of OLED pixels [12]. With these four additional sources of operational instability, we suggest in the present study that use of an optical feedback is likely the only method of simultaneously correcting for all sources of brightness nonuniformities in OLED displays. By intermittently (or continuously) measuring an optical output of each display pixel we can adjust pixel driving conditions and thus maintain uniform brightness across the display. III. OPTICAL FEEDBACK SOLUTION In one proven optical feedback scheme [13], [14], OLED pixels are driven at a high constant current with a photo-transistor circuit monitoring and integrating the light output, which is then compared to the desired output luminance. When the target output luminance is reached, the OLED is turned off. In this scheme the OLED is operated at high brightness for some fraction of the picture frame cycle, which is slowly extended as the OLED pixel ages and loses efficiency. The useful display operating lifetime is reached when an OLED pixel needs to stay turned on for times longer than the entire frame cycle. Implementation of the active matrix backplane for this optical feedback scheme is compact and elegant, as each pixel has self-contained optical monitoring. However, this scheme necessitates OLED pixel operation at high constant currents, an operating regime that reduces the quantum efficiency and power efficiency of OLED pixels, and requires backplanes with transistors capable of supplying the higher currents. Lower power efficiency and higher current operation also raises the fraction of power that contributes towards heating of the display, with higher temperatures resulting in accelerated device degradation [15]. The initial OLED driving conditions and display refresh rate ultimately limit the achievable display lifetime in this feedback scheme. In our proposed optical-feedback technique, the optical output of each OLED pixel is measured by a photodetector, and in response to the measurement the OLED drive current is adjusted to achieve the desired luminescence output [16], [17]. As the pixels age with use, the drive current that is required to produce video brightness increases. The useful display lifetime is, therefore, determined by the ability of an aged OLED pixel to produce video brightness given appropriate drive current and voltage. An additional limit is set by the maximum voltage and current that can be supplied by the drive electronics. In comparison to the optical feedback scheme described in the previous paragraph, our optical compensation method does not overdrive OLED pixels, as the pixels are turned-on during the entire picture-frame cycle. We can project the useful operating lifetime of an OLED display that utilizes our feedback scheme by analyzing OLED efficiency degradation curves at variable drive currents, as done below. A set of data on performance degradation in high quality OLEDs, published by Kwong et al. [6] of Universal Display

3 YU et al.: FEEDBACK TO COUNTER PIXEL AGING AND STABILIZE LIGHT OUTPUT OF OLED DISPLAY TECHNOLOGY 3 Fig. 2. Luminance degradation of three identical phosphorescent OLEDs operated at different initial brightness of 200 cd/m, 500 cd/m, and 1000 cd/m is reproduced from Kwong et al. [7] (data points) and is compared to projections of luminance degradation (solid lines) determined by scaling the luminance output of the device that is operated at L = 1000 cd/m. The projected degradation of a device that is operated at 100 cd/m initial brightness is also shown. Since each of the three devices is operated in a constant current mode, we can integrate the current as a function of time and replot the normalized device efficiency as a function of total charge that passed through the devices, as shown in the inset. Corporation and reproduced in Fig. 2, shows aging of three identical phosphorescent OLED pixels at three different initial brightnesses, projecting of hours when operated at video brightness of 100 cd/m. For this device structure, the efficiency degradation is coulombic and depends on aggregate charge flow through the device [5]. This dependence can be seen by replotting the Kwong et al. data obtained at different current driving conditions (corresponding to different initial brightnesses), as in the inset of Fig. 2. The OLEDs aged at different current drives converge to a single degradation curve when normalized efficiency is plotted against total charge that has passed through the devices. Using this information about device aging, we can predict the change in efficiency of a device with time by tracking the amount of charge that has passed through the device. To illustrate, in Fig. 2 we use the aging data of the device with initial brightness of 1000 cd/m to predict the aging curves for devices whose initial brightness is set as 100, 200, or 500 cd/m, respectively. The plots of predicted aging for 200 or 500 cd/m initial brightness match well with the data obtained from measuring the aging of actual devices at these initial brightnesses. In the same manner, we can simulate a current-correcting optical feedback that would monitor device light output and increase the current through the device whenever the output brightness decreased to 98% of the initial brightness. The increase in current would bring the OLED pixel light output back to the initial brightness condition, thus maintaining OLED brightness within 2% of the initial value. The constant brightness operation of an OLED is simulated in Fig. 3, by extrapolating from the data of Kwong et al. for the pixel with starting luminance of 1000 cd/m. To maintain constant brightness of 100, 200, 500, and 1000 cd/m, respectively, we project gradually increasing the current with time. For example, to maintain 100 cd/m in such constant-brightness mode of operation, the OLED drive current will increase gradually to Fig. 3. Normalized drive current needed to maintain constant brightness with optical feedback derived from the data set of using data on L = 1000 (cd/m ) aged phosphorescent OLEDs [7]. Inset shows corresponding normalized drive voltage for given constant brightness. twice the initial value after hours of OLED operation. The increase in current will be accompanied by a rise in voltage across the device, which is plotted in the inset of Fig. 3 for different constant-brightness conditions and amounts to a 10% rise for hours of operation at 100 cd/m. To operate the same device in constant 100-cd/m brightness mode for hours, the drive current would gradually increase to three-times the initial current with 15% rise in voltage. These rise in voltages are estimated by using the initial current voltage characteristics of the device. However, the current-voltage characteristics also change with time. Consequently, we estimate a maximum of 25% error in the stated projected voltage rise. In contrast, Fig. 2 data shows that OLEDs operated in the constant current mode (with no optical-feedback compensation) would lose 2% of their brightness accuracy after 300 hours of operation, and 5% of their brightness accuracy after 900 hours. From this example, it is evident that large gains in utility of OLED displays can be gained by implementing optical feedback. IV. EDGE-EMITTED CONFIGURATION The proposed optical feedback solution can be realized by growing a transparent organic LED (TOLED) [18], [19] pixel on top of silicon photodetectors integrated into silicon transistor backplane, as we previously suggested [17]. However, the optical signal from each pixel can also be obtained by measuring the light emitted from the edge of the glass substrate on which OLEDs are grown [20], [21] as shown in the inset of Fig. 4. Analysis and measurement of the optical modes of an OLED [22] show that OLED pixel light intensity emitted from the edge of a substrate is comparable to the light intensity emitted through the front plane. Consequently, photodetectors mounted on the edges of the display can measure the light that is normally trapped in the glass substrate and provide intensity measurements to the optical feedback system. The ratio of the edge-emitted to the forward-emitted light is maintained as the device ages since the optical geometry of the OLED pixel is maintained throughout OLED use. The stability of the silicon photodetector exceeds OLED stability due to the atmospheric inertness of the covalently bonded single crystal

4 4 JOURNAL OF DISPLAY TECHNOLOGY Fig. 4. Edge-emitted light captured from an OLED structure of glass, indium tin oxide (ITO), 50-nm thick Tris-(8-hydroxyquinoline)aluminum (Alq ), 50-nm thick Mg:Ag, and 100 nm thick Ag excited by a 408-nm laser is proportional to inverse distance from the detector. Inset shows configuration for utilizing edge-emitted light in optical feedback with a typical OLED structure of ITO anode, hole transport layer (HTL), electron transport layer (ETL), and metal cathode. silicon as compared to van der Waals bonded organic thin film structures comprising OLEDs. Additionally, the current density that passes through the photodetector is significantly smaller than the current density through the OLEDs by at least an order of magnitude, contributing to significantly slower charge-initiated degradation in the photodetector. Use of the edge-mounted optical detectors is applicable in both active and passive matrix OLED display configurations. In addition to compensating for pixel aging, the feedback can also compensate for nonuniformities in pixel driver electronics, and inadvertent variations in resistance of signal bus lines. However, use of edge-mounted detectors is accompanied by more complex addressing and calibration of the pixels. First, the intensity of the detected signal is inversely proportional to the distance away from the pixel. We confirmed this by optically exciting a spot on a glass substrate coated with a luminescent organic thin film, as shown in Fig. 4. The fraction of light collected by the edge-mounted photodetector is a ratio of the detector length over the collection circumference of the pixel radiation. We use this dependency to show the fraction of edge-emitted light that is collected from a fixed detector size along one of the edges of the display in Fig. 5. A change the intensity of the optical signal scales the detected output by the same factor. Implementing the edge-emitted solution depends largely on the sensitivity of the photo detector. For example, a pixel OLED display on a 1-mm thick glass substrate with m pixel size, an edge-detector spanning the display dimension and 1 mm in width would measure 0.45-nA photocurrent due to light captured from a single green OLED pixel (555-nm peak emission) at the center of the display operated at 100 cd/m (where we assumed photodetector sensitivity of 0.3 A/W). This is the lowest amount of signal that we would ever measure as for the red and blue OLEDs more photons are needed to achieve video brightness, contributing to a higher photodetector current. Assuming a dark current density of 1 pa/mm for such edge-detector implies 0.19 na of dark current signal. For lower resolution displays the photodetetector signal increases while the dark current signal is Fig. 5. Fraction of edge emitted light captured from a single pixel on a display with photodetector spanning the left (a) or top (b) edge of the display, and the maximum fraction of edge emitted light captured from a single pixel to a single photodetector that spans any edge of the display (c). reduced, facilitating easier implementation of the proposed feedback scheme. Increasing display size changes pixel size by the square of the scaling factor while the detector area changes linearly by the scaling factor. Therefore, detected signal increases by the scale factor squared while the dark current increases only by the scale factor. Increasing display size can help mitigate the challenges of sensor sensitivity and feedback. Also, a single edge-mounted detector is capable of measuring output of only a single pixel at a time, so that calibration of a one-million-pixel display requires turning-on-then-off all of the pixels in sequence. Typical response of an OLED pixel is limited by their RC-time constant, which is on the order of a few s [23]. Consequently, by allotting 10 s for each pixel, the display calibration would require 10 s. To increase the speed of calibration, multiple photodetectors can be mounted around the display perimeter, allowing multiple pixels to be turned on and measured simultaneously, thereby reducing the calibration time in proportion to the number of mounted photodetectors [21]. For example, start with two photodetectors on the top and bottom edge of the display. The amount of light that reaches each detector from each pixel is a known quantity, dependent on pixel location relative

5 YU et al.: FEEDBACK TO COUNTER PIXEL AGING AND STABILIZE LIGHT OUTPUT OF OLED DISPLAY TECHNOLOGY 5 to the detector and the geometry of the display. As long as the ratio of the contribution of one pixel to the other is unique for each detector, the brightness of each individual pixel can be determined. This process can be expanded to multiple detectors, enabling simultaneous calibration of the same number of pixels as detectors. Alternatively, the calibration can be accomplished any time the display is instructed to turn off, for example, when the laptop screen is placed in the closed/stored position. V. CONCLUSION In this work, we emphasize that the lifetime for an OLED display requires a different metric than the typically measured OLED half-life. We estimate that if constant-current-driven OLEDs are to be used in a display with 2% brightness accuracy over hours of operation, then the OLED half-life has to be extended beyond hours. Additionally, other sources of operational instability such temperature-dependent current voltage characteristics, nonlinear drive-current dependent device efficiencies, and variation in thicknesses and threshold voltage can compromise the uniformity of an OLED display. We suggest that an optical feedback is likely the only solution to stabilize the light output of an OLED display. By using a drive-current-correcting optical-feedback scheme, we expect more than a tenfold display lifetime improvement, while aging individual OLED pixels to the point of twofold or threefold decrease in their luminescence efficiency. Using published data on phosphorescent OLEDs, we conclude that the operating lifetime of an OLED display can be extended close to the OLED half-life using a current-correcting optical feedback with less than a threefold increase in drive current. ACKNOWLEDGMENT One of the authors, J. R. Tischler, thanks Y. Bogart and J. Kymissis for helpful discussion. REFERENCES [1] M. S. 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Bassler, Electron mobility in tris(8-hydroxy-quinoline) aluminum thin films determined via transient electroluminescence from singleand multilayer organic light-emitting diodes, J. Appl. Phys., vol. 89, no. 7, pp , Apr Jennifer Yu received the B.S. and M.Eng. degrees in electrical engineering and computer science from the Massachusetts Institute of Technology (MIT) in Cambridge, MA, in 2002 and 2004, respectively, and is currently working toward the Ph.D. degree in the Department of Electrical Engineering and Computer Science. Jonathan R. Tischler received the B. A. degree in physics and minor in mathematics from the University of Pennsylvania, Philadelphia, in 1999, and the S.M. and Ph.D. degrees in electrical engineering and computer science from the Massachusetts Institute of Technology (MIT), Cambridge, in 2003 and 2007, respectively. He is currently conducting research as a post-doctoral fellow in the Department of Electrical Engineering and Computer Science at MIT.

6 6 JOURNAL OF DISPLAY TECHNOLOGY Charles G. Sodini (S 80 M 82 SM 90 F 94) received the B.S.E.E. degree from Purdue University, West Lafayette, IN, in 1974, and the M.S.E.E. and the Ph.D. degrees from the University of California at Berkeley in 1981 and 1982, respectively. He was a Member of the Technical Staff at Hewlett-Packard Laboratories from 1974 to 1982, where he worked on the design of MOS memory and later, on the development of MOS devices with very thin gate dielectrics. He joined the faculty of the Massachusetts Institute of Technology, Cambridge, in 1983, where he is currently a Professor in the Department of Electrical Engineering and Computer Science. He was the Associate Director of MIT s Microsystems Technology Laboratories from 1989 to His research interests are focused on integrated circuit and system design with emphasis on analog, RF and memory circuits and systems. Along with Prof. Roger T. Howe, he is a coauthor of an undergraduate text on integrated circuits and devices entitled Microelectronics: An Integrated Approach (Prentice Hall, 1997). He also studied the Hong Kong electronics industry and co-authored a chapter with Prof. Rafael Reif in a recent book entitled Made in Hong Kong (Oxford University Press, 1997). Dr. Sodini held the Analog Devices Career Development Professorship of MIT s Department of Electrical Engineering and Computer Science, and was awarded the IBM Faculty Development Award from 1985 to He has served on a variety of IEEE conference committees, including the International Electron Device Meeting, where he was the 1989 General Chairman. He was the 1992 Technical Program Co-Chairman and the Co-Chairman of the Symposium on VLSI Circuits. He served on the Electron Device Society Administrative Committee from 1988 to He is the past president of the IEEE Solid-State Circuits Society and a member of its Administrative Committee. Vladimir Bulović received the M.S. degree in electrical engineering from Columbia University, New York, NY, in 1993 and the Ph.D. from Princeton University, Princeton, NJ, in He is the KDD Career Development Associate Professor of Electrical Engineering at the Massachusetts Institute of Technology (MIT), Cambridge. At MIT, his research interests include studies of physical properties of organic and organic/inorganic nanocrystal composite thin films and structures, and development of novel optoelectronic organic and hybrid nano-scale devices. He authored over 60 published research articles and is inventor of more than 40 U.S. patents in areas of organic and nanostructured light emitting diodes, lasers, photovoltaics, photodetectors, and programmable memories, most of which have been licensed and utilized by both start-up and multinational companies. Prior to joining MIT, he was a Senior Scientist and Project Head of Strategic Technology Development at Universal Display Corporation (UDC) where he developed applications of organic materials to light emitting and photosensitive devices. Prof. Bulovic is a recipient of the U.S. Presidential Early Carrier Award for Scientist and Engineers, the National Science Foundation Career Award, and was named to Technology Review TR100 List.