Model for assessing organic light‐emitting diode display lifetime across applications

A model for assessing organic light‐emitting diode (OLED) display lifetime is developed and discussed for estimating OLED display lifetime in various applications. The lifetime model extends existing stretched exponential models of luminance decay for OLED devices to permit this decay to be estimated as a function of time and current density. This extended model is illustrated within an application to assess the power consumption and luminance decay of diodes within an OLED display. Various metrics of display lifetime are discussed with the aim of developing methods to assess the perceived lifetime of an OLED display to global and local luminance decay mechanisms. Finally, these metrics are applied to illustrate the performance of the model for assessing the impact of an image processing algorithm on OLED display lifetime.     

Practical CNT‐FED structure for high‐luminance character displays

Abstract— A high‐luminance CNT‐FED character display using a simple line rib structure was constructed. The display panel had 48 × 480 dots and the subpixel pitch was 1 mm. The greatest benefit of a display using CNT technology is high luminance performance with low‐power consumption. The luminance of the green‐color dot wasca. 10,000 cd/m² under 1/1 6 duty‐cycle driving at a 6‐kV anode voltage. The high luminance of the display panel can provide good visibility when installed even in outdoor locations, and the power consumption was ca. 4 W at the character displaying module. This, a CNT‐FED for character displays also has potential multifunctionality, which could be battery driven. It should be useful for public displays even under emergency no‐power conditions. In this work, a practical structure and process technologies for making ribs with reasonable cost were developed. The newly introduced 2‐mm‐tall line ribs as spacers were formed by using innovative production processes; i.e., the rib paste was pushed out of a multi‐slit nozzle, and the rib shape was formed by UV‐light irradiation. The developed panel structure and manufacturing processes also had the advantages of size flexibility and high production yield.     

Pixel structures using the negative feedback method for high‐resolution active matrix organic light‐emitting diode displays

Novel two pixel structures are proposed for high‐resolution active matrix organic light‐emitting diode displays. The proposed two pixels (pixel structures A and B) use the negative feedback method for high‐resolution displays that requires to have small‐sized storage capacitance. The proposed pixel structures A and B improve the luminance uniformity by reducing the voltage distortion in the storage capacitor. However, the proposed pixel structure A is vulnerable to the organic light‐emitting diode (OLED) degradation because the anode voltage of the OLED affects the emission current. In order to compensate the OLED degradation, the proposed pixel structure B stores the turn‐on voltage of OLEDs in the storage capacitor. The simulation results show that the emission current error of the proposed pixel structure B is improved by four times in comparison with the proposed pixel structure A when the OLED turn‐on voltage increases by 0.1 V. Also, the emission current error of the proposed pixel structure B when the threshold voltage of driving thin‐film transistors varies from ?2.2 to ?1.8 V is from ?0.69 least significant bit (LSB) to 0.13 LSB, which shows the excellent luminance uniformity. The proposed pixels are designed for 5.5‐in. full high‐definition displays.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Pulse‐width modulation with current uniformization for AM‐OLED micro‐displays on Si LSI chips

We have developed pulse‐width modulation (PWM) with current uniformization for active‐matrix organic light‐emitting diode (AM‐OLED) micro‐displays on Si large‐scale integration (LSI) chips. This driving method can simultaneously solve luminance unevenness and image sticking due to characteristic deviations and degradations of driving transistors and OLEDs. With the use of circuit simulation, it is verified that the PWM with current uniformization (PWM‐CU) can simultaneously achieve precise gray scale and exceedingly improve luminance uniformity. Moreover, an actual panel is designed and fabricated, where the OLEDs are layered on the Si LSI chip. It is found that the luminance uniformity can be improved within 2% to 3%. It is meaningful that the correct images can be displayed using the PWM‐CU for the first time for AM‐OLED micro‐displays on Si LSI chips.     

Fluorescent‐based tandem white OLEDs designed for display and solid‐state‐lighting applications

Abstract— Highly efficient tandem white OLEDs based on fluorescent materials were developed for display and solid‐state‐lighting (SSL) applications. In both cases, the white OLED must have high power efficiency and long lifetime, but there are a number of attributes unique to each application that also must be considered. Tandem OLED technology has been demonstrated as an effective approach to increase luminance, extend OLED lifetime, and allow for use of different emitters in the individual stacks for tuning the emission spectrum to achieve desired performance. Here, examples of bottom‐emission tandem white OLEDs based on small‐molecule fluorescent emitters designed for displays and for SSL applications are reported. A two‐stack tandem white OLED designed for display applications achieved 36.5‐cd/A luminance efficiency, 8500K color temperature, and lifetime estimated to exceed 50,000 hours at 1000 cd/m². This performance is expected to meet the specifications for large AMOLED displays. A two‐stack tandem white OLED designed for SSL applications achieved 20‐lm/W power efficiency, 38‐cd/A luminance efficiency, 3500K color temperature, and lifetime estimated to exceed 140,000 hours at 1000 cd/m². With the use of proven light‐extraction techniques, it is estimated that this tandem device will exceed 40 lm/W with more than 500,000‐hour lifetime, performance that should be sufficient for first‐generation lighting products.     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

Microcavity white‐emitting OLED devices

Abstract— Microcavity designs for OLED devices with an unpatterned white emitter have the potential to provide greater brightness and larger color gamut than non‐microcavity designs while still enabling lower‐cost large‐format manufacturing. In this paper, such microcavity and non‐microcavity designs are compared. Color filters must still be employed to provide an adequate color gamut. Top‐emitter structures have somewhat greater on‐axis luminance and color gamut, but increased angular change, than bottom‐emitter designs. In a single‐stack bottom‐emitter active‐matrix TFT device using an RGBW format, the use of microcavities is estimated to reduce the average power usage by 35% and the peak power by 58%, while increasing the NTSC ratio for color gamut area by about 10%. Angular luminance and color change is likely to be acceptable, especially for hand‐held applications. Tandem devices employing multiple emitter stacks increase the lifetime of OLED devices but require larger driving voltages; for such devices, microcavity structures are useful although the percentage reduction obtained in power usage is not quite as large. Generally, tandem devices with microcavities have a slightly stronger cavity effect yielding slightly larger color gamut, but also greater angular color and luminance shift. Therefore, microcavity architectures are less appealing for tandem devices.     

Novel a‐Si:H TFT pixel circuit for electrically stable top‐anode light‐emitting AMOLEDs

Abstract— A novel pixel circuit for electrically stable AMOLEDs with an a‐Si:H TFT backplane and top‐anode organic light‐emitting diode is reported. The proposed pixel circuit is composed of five a‐Si:H TFTs, and it does not require any complicated drive ICs. The OLED current compensation for drive TFT threshold voltage variation has been verified using SPICE simulations.     

Reducing image sticking in AMOLED displays with time‐ratio gray scale by analog calibration

Abstract— Early loss of image uniformity has been a critical drawback of active‐matrix organic light‐emitting‐diode (AMOLED) displays operated in time‐ratio gray‐scale mode. This problem is addressed with an analog calibration technique which measures the voltage across each OLED for a given current and subsequently controls the supply voltage of pixels and the voltage drop across the driving th in‐film transistor (TFT) of each OLED. The uniformity of test cells, which were aged to produce image sticking in a chessboard pattern, were improved. A measure of image sticking, called the extracted image‐sticking value (EISV), was formulated, which is developed and used for the quantitative evaluation of the calibration method. OLED voltages over a range of about 0.35 V were compensated to produce more uniform OLED currents than those before aging. The variation of luminance associated with image sticking was reduced by about 40% for a full‐white image after between 2 and 10 hours of accelerated aging with a constant voltage of 8 V across an OLED.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (～14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (?10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.     

A pixel structure for simultaneous programming and emission method for shutter‐glasses‐type stereoscopic 3‐D AMOLED displays

Abstract— A pixel structure for shutter‐glasses‐type stereoscopic 3‐D active‐matrix organic light‐emitting‐diode (AMOLED) displays is proposed. The proposed pixel programs data to the pixel during the light‐emission time of an OLED. Because the emission time of the proposed pixel is extended, it is expected that the proposed pixel not only decreases the peak current of the OLED during the emission period but also reduces flicker. Moreover, the aperture ratio of the proposed pixel is 58.69% for a 50‐in. full‐high‐definition (FHD) condition by minimizing the number of thin‐film transistors (TFTs), capacitors, and control signal lines as seven TFTs, two capacitors, two power lines, and four control lines per unit pixel. Simulation results show that the error in the emission current of the proposed pixel is from ?0.82% to +0.90% when the threshold‐voltage variation of the driving TFT is ±1.00 V, and the maximum variation of the emission current is ?1.35% when a voltage drop in the power line is ?0.50 V on a full‐white‐image display.     

A novel gate driver circuit for depletion‐mode a‐IGZO TFTs

In this paper, a novel gate driver circuit, which can achieve high reliability for depletion mode in a‐InGaZnO thin‐film transistors (TFTs), was proposed. To prevent the leakage current paths for Q node effectively, the new driving method was proposed by adopting the negative gate‐to‐source voltage (V_GS) value for pull‐down units. The results showed all the V_OUT voltage waveforms were maintained at VGH voltage despite depletion‐mode operation. The proposed circuit could also obtain stable V_OUT voltage when the threshold voltage for all TFTs was changed from ?6.5 to +11.5 V. Therefore, the circuit can achieve high reliability regardless of threshold voltage value for a‐IGZO TFTs. In addition, the output characteristics and total power consumption were shown for the alternating current (AC)–driven and direct current (DC)–driven methods based on 120‐Hz full‐HD graphics (1920 × 1080) display panel. The results showed that the AC‐driven method could achieve improved V_OUT characteristics compared with DC‐driven method since the leakage current path for Q node can be completely eliminated. Although power consumption of the AC‐driven method can be slightly increased compared with the DC‐driven method for enhancement mode, consumption can be lower when the operation has depletion‐mode characteristics by preventing a leakage current path for pull‐down units. Consequently, the proposed gate driver circuit can overcome the problems caused by the characteristics of a‐IGZO TFTs.     

A new driving method introducing a display period for AMOLEDs

Abstract— We have proposed an Advanced‐Clamped‐Inverter driving method for the fabrication of AMOLEDs, in which one frame period is divided into an addressing period and a display period. This driving method enables the AMOLEDs to produce excellent moving images without motion blur and false pixels, and has peak‐luminance characteristics because of its unique light‐emission scheme of the OLED elements. Good inter‐pixel uniformity was also achieved in a previous clamped‐inverter driving method. We fabricated AMOLEDs and experimentally confirmed their characteristics.     

OLED lifetime issues from a mobile‐phone‐industry point of view

Abstract— Lifetime issues have been a hot topic throughout the history of OLEDs. The rapid development of lifetimes since 2002 has enabled OLED displays to become acceptable for mobile phones. The lifetime requirements of 30,000 hours expressed by the representatives of mobile‐phone‐terminal makers were felt to be unrealistic to be obtained in 2003, since the lifetime of the blue color was below 1000 hours. Today, 5 years later, lifetimes of AMOLED panels are over 50,000 hours. OLED displays are suffering from a burn‐in effect due to limited lifetime. After 2003, it was understood by the panel and terminal makers that instead of lifetime, burn‐in sensitivity became the limiting factor from an AMOLED‐panel usability point of view. The burn‐in effect becomes visible at 2–3% luminance degradation levels between adjacent pixels. To take this effect into account in mobile‐phone applications, the lifetime needs to be increased from 30,000 to 60,000 hours, and suitable algorithms need to be used for the display of the terminal. There is also pressure to double the peak luminance values used in the terminals in order to improve the performance of the screen in outdoor environments. The roles of the material developers, panel makers, and terminal makers are reviewed in this paper from a lifetime perspective.     

A reduced‐voltage differential signaling (RVDS) interface for chip‐on‐glass TFT‐LCD applications

Abstract— Reduced‐voltage differential signaling (RVDS) is a novel interface for TFT‐LCD panels with a chip‐on‐glass (COG) structure, which has a point‐to‐point topology and a voltage mode differential signaling scheme. The voltage‐driving interface scheme has advantages in high‐speed operation owing to its relatively small time constant for the resistive channel condition. And reduced‐voltage signaling can reduce the power consumption of a transmitter. The display source driver IC with an RVDS interface, which is fabricated by using a 0.25‐μm CMOS process with a 2.5‐V logic supply voltage, offers a high data rate up to 500 Mbps, low‐current consumption of 2.2 mA, and good EMI characteristics. Also, an RVDS interface has programmable options that control the bandwidth, system power, and EMI performance. Therefore, the RVDS interface is a competitive solution for low‐power, low‐cost, and slim notebook applications.     

High‐performance white OLEDs with high color‐rendering index for next‐generation solid‐state lighting

Abstract— Several white‐OLED structures with a high color‐rendering index (CRI) were investigated for lighting applications. A two‐unit fluorescent/phosphorescent hybrid white OLED achieved an excellent CRI of 95, high luminous efficacy of 37 lm/W, and long lifetime of over 40,000 hours at 1000 cd/m². White‐OLED lighting panels of 8 × 8 cm for high‐luminance operation were fabricated, and a stable emission at 3000 cd/m² was confirmed. Quite a small variation in chromaticity in a different directions was achieved by using an optimized optical device structure. With a light‐outcoupling substrate, a higher efficacy of 56 lm/W, high CRI of 91, and longer half‐decay lifetime of over 150,000 hours at 1000 cd/m² was achieved. All‐phosphorescent white OLEDs placed on the light‐outcoupling substrate show a high CRI of 85 and higher efficacy of 65 lm/W with a fairly good half‐decay lifetime of over 30,000 hours. With a further voltage reduction and a high‐index spherical extractor, 128 lm/W at 1000 cd/m² has been achieved.     

Compact and power‐saving polysilicon data driver with common‐decoder DA converter (CD‐DAC)

Abstract— A common‐decoder architecture for a data‐driver circuit fabricated by using a polysilicon process has been developed. The architecture achieves a compact circuit and low‐power consumption. In application to an integrated polysilicon data driver for small‐sized displays, this architecture reduces the area of the data driver by removing the vertical bus lines that occupy a large area. It also suppresses the power consumption of the data bus by reducing the number of driven lines in the data bus during word‐to‐word transitions from six to two. By using a conventional 4‐μm design rule, we fabricated an active‐matrix OLED (AMOLED) panel with an integrated six‐bit data‐driver circuit with 384 outputs. The driver circuit had a height of 2.6 mm and a pitch between output lines of 84 μm. The maximum power consumption of the driver was only 5 mW, i.e., 3.8 mW for logic‐data transfer and 1.2 mW for reference‐voltage source. Furthermore, we also fabricated an active‐matrix LCD (AMLCD) panel including driver circuits of the same type as the integrated elements. Six‐bit full‐color images were successfully displayed on both panels.     

A new a‐Si:H TFT pixel circuit employing data‐reflected negative‐bias annealing for a stable and uniform AMOLED

Abstract— A new a‐Si:H pixel circuit to reduce the VTH degradation of driving a‐Si:H thin‐film transistors (TFTs) by data‐reflected negative‐bias annealing (DRNBA) is presented. The new pixel circuit compensates VTH variation induced by non‐uniform degradation of each a‐Si:H pixel due to various electrical stress. The proposed pixel circuit was verified by SPICE simulations. Although the VTH of the driving a‐Si:H TFT varies from 2.5 to 3.0 and 3.5 V, the organic light‐emitting diode (OLED) current changes by only 1.5 and 2.8% in the emission period, respectively. During the negative‐bias annealing period, the negative VGS is applied to the driving TFT by using its own data signal. It is expected that the VTH shift of the driving TFT can be effectively reduced and the VTH shift can be compensated for in our new pixel circuit, which can contribute to a stable and uniform image from an a‐Si:H TFT active‐matrix OLED.