AMOLED Research Articles

47-4:Invited Paper: Flexible AMOLED based on Oxide TFT with High Mobility

We report a flexible AMOLED display driven by oxide thin film transistors (TFTs) based on a polyimide (PI) substrate. In the oxide TFT, a triple-stacked structure of active layer is employed to improve the mobility and stability. The fabricated TFT exhibit a high field-effect mobility of 38.8 cm2 V−1 s−1, a subthreshold swing of 0.15 V/dec, and an Ion/Ioff ratio of 109. Meanwhile, Al2O3 film prepared by the method of atomic layer deposition (ALD) is used as the encapsulation of AMOLED display, which demonstrating a superior water vapor transmission rate (WVTR) as low as 5.43×10−5 g/m2/day estimated by Calcium (Ca) corrosion method at 40 °C/100%.

P-37: A High Accuracy Current Comparison Scheme for External Compensation Circuit of AMOLED Displays

A current comparison scheme for external compensation circuit of AMOLED displays is proposed. In this scheme, a current comparator is designed to sense the difference between the reference current and the driving current of pixels and to output a corresponding digital signal to the compensation block. For the current comparator, a sense amplifier is used to amplify the current difference, which can effectively increase the accuracy of the current comparator. Simulation results show that the accuracy of the proposed current comparator is 1nA and the sensing error is lower than 0.224 %, when the reference current is 500 nA.

78-3:Distinguished Paper: Flexible AMOLED Displays with Bending Interactive Interface

The human-machine interaction provides effective operation and control of the machine on the users' end, and feedback from the machine. Flexible display prevails over the conventional flat display by its ability to change the display geometric appearance, and can provide alternative user-display interactions. In this paper, a bendable phone technology with flexible AMOLED display and a novel bending interactive interface has been disclosed. The bending interface provides a more intuitive operation for users

P-50: Application of High Temperature Debonding Layer in the Fabrication of Flexible AMOLED Displays

Flexible AMOLED display has received great attention in recent years. In this paper, we provided a method of fabricating flexible AMOLED display by using a nanocomposite material as a debonding layer (DBL). This material provided flexible substrate with good debonding capability even after 450°C low-temperature poly-silicon (LTPS) process. After array and OLED process, the panels were released by the mechanical roller debondor(MRD). The effect of temperature on debonding force of flexible substrate was investigated. Besides, electrical characteristics of thin-film transistors (TFT) and pitch variation after separation were also studied. Finally, the fixed curvature of Flexible AMOLED display was showed.

Building the Entertainment Machine: Charles L. Mee's Time to Burn and the Performance of Postindustrial Decay in Chicago

During his first mayoral campaign in January 1989, Richard M. Daley insisted that “Everybody talks about bringing manufacturing back. There aren't going to be any more soap factories on Clybourn Avenue[.] … The city is changing. You're not going to bring manufacturing back.” Although this was a controversial statement at the time and Mayor Daley later embraced promanufacturing policies, it reflected an awareness of a fundamental economic shift in Chicago. By the late 1980s, the city had lost over half of its post–World War II manufacturing jobs, and companies were continuing to leave the city for more space, lower taxes, and a less expensive labor force. In fact, only months after Daley's comments, Procter & Gamble announced that it would close its fifty-nine-year-old soap plant at 1232 West North Avenue on the North Branch of the Chicago River, eliminating 275 manufacturing jobs in the process. Deindustrialization was under way, causing anxiety for politicians and pain for factory workers, but a new economy that was focused on real estate, finance, and culture was emerging in Chicago.

Improvement of Electron Injection by a Ramp-Shape N-Doped Architecture for High-Efficiency Organic Light Emitting Diodes

Over the past decades, organic light-emitting diodes (OLEDs) have drawn a lot of attention due to its numerous advantages over light-emitting diodes (LEDs) and liquid crystal display (LCDs). These characteristics make OLED become one of the most promising candidates for commercial applications such as solid state lighting and active matrix organic light-emitting diode (AMOLED) display. However, device properties such as lifetime, efficiency, power consumption are still the main issues needed to overcome for their practical uses. In order to fulfill a highly power-efficient light source or display with OLEDs, the device energy loss during electron–photon conversion should be low. Recent years, there are plenty of approaches proposed to improve the electron or hole injection from the electrodes and reduce the ohmic losses. The most typical way is inserting a chemically p- or n-doped layer between the hole transporting layer (HTL) and anodes or between the electron transporting layer (ETL) and the cathodes, respectively. In our work, we report OLED with a novel n-doped ramp-shape electron injection structure composed of rubidium carbonate (Rb2CO3) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) which is prepared by thermal co-evaporation. We increase the doping concentration from 17.5 mol% to 84 mol% while depositing the electron injection materials approaching the cathode. By increasing the doping concentration, the activation energy of the material decreases as the Fermi level shifting toward the lowest unoccupied molecular orbital (LUMO). Under thermal equilibrium conditions, a ramp-shape architecture is formed and gradually distribute the energy barrier from cathode to electron transport layer in the device therefore achieve significant electron injection improvement. As a result, a OLED with the ramp-shape electron injection structure ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (14nm, 17.5~84 mol%)/LiF (1nm)/Al (140nm) accomplished maximum luminance of 33370 cd/m2 and current efficiency of 5.292 cd/A. By contrast, the device without the ramp-shape architecture (the normal device) ITO/NPB (40nm)/Alq3 (40nm)/LiF (1nm)/Al (140nm) only achieved a maximum luminance of 17030 cd/m2 and current efficiency of 3.41 cd/A. The total improvement (55% by current efficiency and 95% by luminance) made by adding ramp-shape structure to device indicated a remarkable electron injection facilitation and its performance is much more efficient than the device with typical electron injection layer ITO/NPB (40nm)/Alq3 (40nm)/Rb2CO3:MADN (5nm, 64 mol%)/LiF (1nm)/Al (140nm). Figure 1

A Hybrid AMOLED Driver IC for Real-Time TFT Nonuniformity Compensation

An active matrix organic light emitting diode (AMOLED) display driver IC, enabling real-time thin-film transistor (TFT) nonuniformity compensation, is presented with a hybrid driving method to satisfy fast driving speed, high TFT current accuracy, and a high aperture ratio. The proposed hybrid column-driver IC drives a mobile UHD $\left({{3840} \times {2160}} \right)$ AMOLED panel, with one horizontal time of ${7}.{7}\,{\upmu \text{s}}$ at a scan frequency of 60 Hz, simultaneously senses the TFT current for back-end TFT variation compensation. Due to external compensation, a simple 3T1C pixel circuit is employed in each pixel. Accurate current sensing and high panel noise immunity is guaranteed by a proposed current-sensing circuit. By reusing the hybrid column-driver circuitries, the driver embodies an 8 bit current-mode ADC to measure OLED $V$ – $I$ transfer characteristic for OLED luminance-degradation compensation. Measurement results show that the hybrid driving method reduces the maximum current error between two emulated TFTs with a 60 mV threshold voltage difference under 1 gray-level error of 0.94 gray level (37 nA) in 8 bit gray scales from 12.82 gray level (501 nA). The circuit-reused current-mode ADC achieves 0.56 LSB DNL and 0.75 LSB INL.

Bulk‐Accumulation Oxide‐TFT Backplane Technology for Flexible and Rollable AMOLED Displays: Part I

In the first of a two‐part series on a new backplane technology for flexible and rollable AMOLED displays, the author reviews a bulk‐accumulation (BA) amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) thin‐film transistor (TFT) with 3–5 times the drain current of a comparable conventional single‐gate TFT. The advantages of BA TFTs include excellent performance from circuits such as ring oscillators and gate drivers and also higher robustness under mechanical bending.

Visual fatigue occurrence time when using hand-held intelligent devices

Because the mobile phone becomes cheaper and cheaper, the Smartphone will be popular in our life in recent years. Smartphone display technologies have two types, including TFT-LCD (thin film transistor-liquid crystal display) and AMOLED (active matrix organic light-emitting diode). Little studies investigated time to visual fatigue for watching these two display types. Therefore, this study expects to find time to fatigue while using these two display types in order to prevent users from accumulative visual fatigue after prolonged viewing the small size screens. This study recruited 19 young volunteers, 10 males and 9 females, for the experiment. TFT-LCD and AOMLED of display panels were used for test. Another independent variable was the visual stimulation scenarios, including static and dynamic stimulation scenarios. The static stimulation scenario was to read a novel about swordsman on a mobile phone screen. The dynamic stimulation scenario was to play “Jewel Block” on a mobile phone. Objective eyestrain occurrence time and the subjective maximum time to fatigue were measured here for prolonged viewing the mobile phone screen. The results indicated that the time to fatigue from TFT-LCD was significantly longer than that of AMOLED. Dynamic stimulation scenario caused significantly longer time to fatigue than static one. The findings of this study are very useful to develop an ambient intelligent system being able to monitor the time spent for watching from the screen and to intelligently remind the user to take a break at the right time.

High-SNR Capacitive Multi-Touch Sensing Technique for AMOLED Display Panels

A mutual-capacitive multi-touch sensing technique is presented, especially for ultra-thin active-matrix organic light-emitting diode (OLED) display panels. The proposed scheme recognizes touch positions by detecting LC oscillation frequency shift. The signal feedback to the common cathode electrode of OLEDs cancels out the effects of parasitic coupling capacitances. In addition, the ground-floating-based scanning method enables a multi-touch function. The proposed touch sensor was implemented in a 0.35-μm CMOS process. Simulations were performed in model of 3.97-in on-cell-type TSP with various noise signals gathered by actual measurements. The proposed sensing technique achieves 68.9- and 61.6-dB signal-to-noise ratios with 6- and 2-mm diameter metal pillars, respectively.

Research progress on oxide-based thin film transisitors

Oxide semiconductor is regarded as one of most suitable active materials of thin-film transistors (TFTs) for driving organic light-emitting diodes because of its advantages of high mobility, low-temperature processing, good electrical uniformity, visible-light transparency, and low cost. Currently oxide TFTs have been successfully applied to the backplanes of the flat-panel displays. This review gives a comprehensive understanding of the development process of oxide TFTs. In the present article, we review the major trend in the field of oxide TFTs. First, the questions of how to achieve high-mobility and high-stability oxide semiconductors are introduced, and the carrier transport mechanism is also addressed. Next, the device structures and the fabrication processes of the oxide TFTs are introduced. The electrical instability of the oxide TFTs is also discussed, which is critical for their applications in backplanes of the flat-panel displays. Especially, the mechanism of the threshold voltage instability of the oxide TFTs under negative bias illuminant stress is discussed in detail. Finally, the applications of oxide TFTs in flat-panel displays, such as active matrix organic light-emitting diodes and flexible displays, are addressed.

Novel Pixel Circuit with Compensation for Normally-Off/On a-IGZO TFTs and OLED Luminance Degradation

This letter presents a new pixel circuit reaching simultaneous compensation for a threshold voltage ( $V_{{\rm{TH}}}$ ) shift of normally- off and normally- on a-IGZO TFTs, as well as luminance drop of an OLED device for ultrahigh-definition active-matrix organic light-emitting diode displays. Simulation results demonstrate the successful detection of a 1 V shift of threshold voltage despite normally- off and normally- on characteristics by the novel source-follower structure. Furthermore, the interdependence between luminance and threshold voltage shift of the OLED device is measured. Based on the simulations using the measured interdependence, the luminance drop caused by OLED aging is suppressed below 5% at different data voltages.

New a-IGZO Pixel Circuit Composed of Three Transistors and One Capacitor for Use in High-Speed-Scan AMOLED Displays

An amorphous indium-gallium-zinc-oxide (a-IGZO) pixel circuit with three transistors and one capacitor is proposed for use in high-speed-scan active-matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit can compensate for the threshold voltage shifts in both normally-off and normally-on a-IGZO TFTs. Based on the simulation results with ±1 V threshold voltage shifts of the driving TFT, the relative error rates of OLED currents are less than 4.46% for the entire range of data voltages (-9.9 V ~ 5 V).

Thermal Modeling of Organic Light-Emitting Diode Display Panels

Power densities required to operate active-matrix organic-light-emitting diode (AMOLED) based displays for high luminance applications, lead to temperature rise due to self heating. Temperature rise leads to significant degradation and consequent reduction in life time. In this work numerical techniques based computational fluid dynamics (CFD) is used to determine the temperature rise and its distribution for an AMOLED based display for a given power density and size. Passive cooling option in form of protruded rectangular fins is implemented to reduce the display temperature.

Characterization and compensation of OLED aging in a digital AMOLED system

Organic light-emitting diode (OLED) aging is the root cause for image sticking artifact and considered as the toughest problem besides the low yield problem of active-matrix organic light-emitting diode (AMOLED) displays. Digital driving can eliminate Mura artifact and allow a similar yield like LCD. However, it is more prone to OLED aging than analog driving, so that the lifetime will become shorter. In this paper, we pursue the approach to measure the pixel current and compensate OLED I–V drift. Information gained from electrical measurements during the lifetime of the display may be correlated to electro-optical drift, particularly the current efficiency. The aging model has to consider the dependence of I–V drift and efficiency loss on the operation point/voltage for each subframe. Specific compensation algorithms have been developed. Two AMOLED prototypes (1.5 and 2.8 in.) were validated. Burned-in pattern can be compensated, so the concept has been proven as effective. With the method described in this paper, digital AMOLED may reach a similar and even significantly higher lifetime than an analog AMOLED.

Poly‐Si TFTs with bottom‐gate structure using excimer laser crystallisation for AMOLED displays

An n-type polycrystalline silicon thin-film transistor (poly-Si TFT) with a bottom-gate structure using excimer laser annealing (ELA) for active matrix organic light-emitting diode displays is proposed. A problem with bottom-gate poly-Si TFTs (BGPs) using ELA, namely, the disconnection of poly-Si at the boundary of the gate metal during the ELA crystallisation, was solved by developing a novel process to control the slope of the gate metal. We realised ELA BGPs with pure-Mo gate having a thickness of more than 150 nm. The BGPs have better breakdown voltage characteristics compared with the conventional top-gate poly-Si TFTs because of its flat channel region and homogeneous electric field distribution in the gate insulator.

A-InGaZnO Active-Matrix Organic LED Pixel Periodically Detecting Thin-Film Transistor Threshold Voltage Once for Multiple Frames

This letter proposes a new voltage-programmed amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) pixel circuit for active-matrix organic light-emitting diode (AMOLED) displays. The proposed circuit detects the $\text{V}_{\mathrm {TH}}$ of the driving TFT only in a periodically executed compensation frame, allowing the other frames to perform only data input and emission for high-speed applications. An HSPICE model is also established based on the measured electrical characteristics of a fabricated a-IGZO TFT. The simulation results reveal that the proposed circuit can compensate for the degradation of the driving TFT and the OLED without external circuits. Moreover, the current error rates are <6.32%, so the proposed circuit is effective for use in AMOLED displays.

Thin-Film Silicon Heterojunction FETs for Large Area and Flexible Electronics: Design Parameters and Reliability

The design parameters and reliability of thin-film heterojunction FET (HJFET) devices comprised of hydrogenated amorphous Si (a-Si:H) gate-stacks on crystalline Si (c-Si) substrates are discussed. It is shown that the pinchoff voltage of the HJFET can be adjusted over a wide range of voltages, the gate leakage can be reduced both by improving the a-Si:H growth conditions and reducing the gate area, and the gate capacitance can be adjusted by proper layout design. In addition, high-voltage bias effects including breakdown are investigated, and the pinchoff voltage stability is evaluated by accelerated testing and long-term measurements. It is found that even though the HJFET is intended for low operation voltages, it performs well at relatively high voltages. It is also found that the HJFET is a highly stable device and, therefore, promising for demanding applications, such as active-matrix organic light-emitting diode displays. In addition, due to a low process temperature limited to 200 °C, the HJFET is promising to enable the use of low cost and flexible substrates.

Analysis of Key Factors for High Yield AMOLED Display

To figure out the key factors of the reliability of AMOLED displays, we analyzed the luminance degradation based on a large-scale simulation. The initial non-uniformity and aging parameters were incorporated into the luminance decay model. For each set of parameters, we simulated the luminance degradation of 10,000 panels for 100,000 operating hours and analyzed by two criteria; contrast deviation and luminance variation of panels to determine failures. Stability of TFTs is the most significant parameter for realizing highly reliable AMOLED display. Stability of OLEDs is also potential factor when which of TFTs is sufficient.

Image quality affected by diffraction of aperture structure arrangement in transparent active-matrix organic light-emitting diode displays.

Transparent display is one of the main technologies in next-generation displays, especially for augmented reality applications. An aperture structure is attached on each display pixel to partition them into transparent and black regions. However, diffraction blurs caused by the aperture structure typically degrade the transparent image when the light from a background object passes through finite aperture window. In this paper, the diffraction effect of an active-matrix organic light-emitting diode display (AMOLED) is studied. Several aperture structures have been proposed and implemented. Based on theoretical analysis and simulation, the appropriate aperture structure will effectively reduce the blur. The analysis data are also consistent with the experimental results. Compared with the various transparent aperture structure on AMOLED, diffraction width (zero energy position of diffraction pattern) of the optimize aperture structure can be reduced 63% and 31% in the x and y directions in CASE 3. Associated with a lenticular lens on the aperture structure, the improvement could reach to 77% and 54% of diffraction width in the x and y directions. Modulation transfer function and practical images are provided to evaluate the improvement of image blurs.