AMOLED Research Articles

Impacts of appearance parameters on perceived image quality for mobile-phone displays

In order to investigate the perceived image quality (IQ) influenced by different image appearance parameters, a psychophysical experiment was carried out on two mobile-phone displays of different technologies, the in-plane switching (IPS) and the active matrix organic light emitting diode (AMOLED). The test images were generated within a wide range of variations by applying various manipulation methods, involving changes in several kinds of image appearance parameters, i.e. lightness, chroma, hue, and spatial frequency. The perceived attributes of naturalness, colourfulness, brightness, contrast, sharpness, clearness, preference, and overall IQ were visually assessed via categorical judgement method for several application types of test images. The impacts of the manipulation methods on individual attributes have been deeply discussed based on the interrelationships of these attributes, which reveal that the significant parameters towards better overall IQ for mobile-phone displays include high lightness, suitable chroma and lightness frequency distribution, and impartial hue, accordingly better image quality will be accomplished after all these parameters have been optimised at the same time. The achievements of this study would provide constructive instructions on achieving excellent display effects for researchers and manufactories of mobile-phone displays.

Pixel structures to compensate nonuniform threshold voltage and mobility of polycrystalline silicon thin-film transistors using subthreshold current for large-size active matrix organic light-emitting diode displays

We propose pixel structures for large-size and high-resolution active matrix organic light-emitting diode (AMOLED) displays using a polycrystalline silicon (poly-Si) thin-film transistor (TFT) backplane. The proposed pixel structures compensate the variations of the threshold voltage and mobility of the driving TFT using the subthreshold current. The simulated results show that the emission current error of the proposed pixel structure B ranges from −2.25 to 2.02 least significant bit (LSB) when the variations of the threshold voltage and mobility of the driving TFT are ±0.5 V and ±10%, respectively.

Laser lift-off systems for flexible-display production

Laser lift-off (LLO) delamination opens the path to polymer-based display backplanes for use in electrophoretic e-readers, AMOLED (Active-Matrix Organic Light Emitting Diode) smartphones, tablets, and OLED-TV. The proper choice of wavelength, optical system, and fluence enables the layer-selective LLO processing of functional thin films not achievable with other radiative or non-radiative heat sources. This becomes increasingly attractive as the feature size and film thickness in microelectronics applications are continuously being downscaled. Excimer lasers provide the 308 nm wavelength and short pulse duration required for highly localized energy coupling. The high output power of excimer lasers enables a large processing footprint and the high-throughput rates needed in mass manufacturing. Thin-film transistor structures fabricated on top of polymer layers spun on glass panels must be delaminated to create thin, lightweight, and rugged flexible-display backplanes. Low-thermal-budget lift-off processing with UV lasers protects the adjacent functional layers. When it comes to large Gen 4.5–Gen 8 substrate panels, which have to be released from polymer thin films, it is most effective to use line beam scanning for LLO within one to three consecutive scans.

Threshold Voltage and IR Drop Compensation of an AMOLED Pixel Circuit Without a $V_{\rm DD}$ Line

This letter proposes a voltage drop by current (IR drop) and threshold voltage compensation pixel circuit, in which the power line is eliminated. The proposed circuit provides a larger fill factor than the conventional one because the circuit does not use a VDD line but instead utilizes the scan line as a power line. The proposed pixel circuit was verified using both circuit analysis and circuit simulations. The proposed circuit could increase the aperture ratio and compensate for the IR drop and threshold voltage variations in active matrix organic light-emitting diode devices, where I and R are current and resistance of the supply power lines. Using the proposed pixel circuit, a large fill factor with both IR drop and threshold voltage compensation was achieved.

Improvement in Brightness Uniformity by Compensating for the Threshold Voltages of Both the Driving Thin-Film Transistor and the Organic Light-Emitting Diode for Active-Matrix Organic Light-Emitting Diode Displays

This paper proposes a novel pixel circuit design and driving method for active-matrix organic light-emitting diode (AM-OLED) displays that use low-temperature polycrystalline-silicon thin-film transistors (LTPS-TFTs) as driving element. The automatic integrated circuit modeling simulation program with integrated circuit emphasis (AIM-SPICE) simulator was used to verify that the proposed pixel circuit, which comprises five transistors and one capacitor, can supply uniform output current. The voltage programming method of the proposed pixel circuit comprises three periods: reset, compensation with data input, and emission periods. The simulated results reflected excellent performance. For instance, whenΔVTH=±0.33 V, the average error rate of the OLED current variation was low (<0.8%), and whenΔVTH_OLED=+0.33 V, the error rate of the OLED current variation was 4.7%. Moreover, when theI×R(current × resistance) drop voltage of a power line was 0.3 V, the error rate of the OLED current variation was 5.8%. The simulated results indicated that the proposed pixel circuit exhibits high immunity to the threshold voltage deviation of both the driving poly-Si TFTs and OLEDs, and simultaneously compensates for theI×Rdrop voltage of a power line.

Fabrication of Flexible Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors by a Chemical Vapor Deposition-Free Process on Polyethylene Napthalate

We have fabricated flexible amorphous Indium-Gallium-Zinc-oxide (IGZO) thin-film transistors (TFTs) with back-channel-etch structure by a chemical vapor deposition-free process. All the processes are performed well below 160°C on a polyethylene napthalate (PEN) substrate with anodic aluminum oxide as gate dielectric, which can be highly immune to the strain failure. The IGZO-TFTs show field-effect mobility of 11.2 cm2/V s, subthreshold swing (SS) of 0.27 V/decade, low off-state current ∼10 fA, low leakage current <1 pA and high ION/IOFF ratio of 109. Meanwhile, the performance of flexible IGZO-TFTs do not deteriorate during the bending with the curvature radius as 10 mm. Using a PEN as substrate, flexible active-matrix organic light-emitting diode (AMOLED) displays driven by the IGZO-TFTs have been demonstrated.

A Novel a-InGaZnO TFT Pixel Circuit for AMOLED Display With the Enhanced Reliability and Aperture Ratio

In this paper, a novel voltage-programmed pixel circuit based on amorphous indium-gallium-zinc-oxide thin film transistor (a-InGaZnO TFT) for active-matrix organic light-emitting diode (AMOLED) displays with the enhanced aperture ratio is proposed. The proposed circuit consists of 5 TFTs and one capacitor. Through extensive simulation and layout works, we verified the proposed circuit compensates for the variation of the threshold voltage in a variety of harsh stress conditions. In addition, by removing one storage capacitor and one signal line, the proposed circuit can increase the aperture ratio as well as decrease the complexity of circuit operation in comparison to the conventional pixel circuit.

A new poly-Si TFT compensation pixel circuit employing AC driving mode for AMOLED displays

This paper presents a new poly-Si pixel circuit employing AC driving mode for active matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit, which consists of one driving thin-film transistor (TFT), three switching TFTs, and one storage capacitor, can effectively compensate for the threshold voltage variation in poly-Si and the OLED degradation. As there is no light emission, except for during the emitting period, and a small number of devices used in the proposed pixel circuit, a high contrast ratio and a high pixel aperture ratio can be easily achieved. Simulation results by SMART-SPICE software show that the non-uniformity of the OLED current for the proposed pixel circuit is significantly decreased (< 10%) with an average value of 2.63%, while that of the conventional 2T1C is 103%. Thus the brightness uniformity of AMOLED displays can be improved by using the proposed pixel circuit.

A Novel Voltage-Programmed Pixel Circuit Utilizing $V_{T}$-Dependent Charge-Transfer to Improve Stability of AMOLED Display

A novel voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays is proposed. The threshold voltage shift (ΔVT) of the drive TFT caused by electrical stress is compensated by an incremental gate-to-source voltage (ΔVGS) generated by utilizing the ΔVT-dependent charge transfer from the drive TFT to a TFT-based metal-insulator-semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of the drive TFT to improve the OLED drive current. The non-ideality of the ΔVT-compensation, TFT overlap capacitance, programming speed, and OLED degradation are discussed. The effectiveness of the proposed pixel circuit is verified by simulation results.

High-Performance Low-Cost Back-Channel-Etch Amorphous Gallium–Indium–Zinc Oxide Thin-Film Transistors by Curing and Passivation of the Damaged Back Channel

High-performance, low-cost amorphous gallium-indium-zinc oxide (a-GIZO) thin-film-transistor (TFT) technology is required for the next generation of active-matrix organic light-emitting diodes. A back-channel-etch structure is the most appropriate device structure for high-performance, low-cost a-GIZO TFT technology. However, channel damage due to source/drain etching and passivation-layer deposition has been a critical issue. To solve this problem, the present work focuses on overall back-channel processes, such as back-channel N2O plasma treatment, SiOx passivation deposition, and final thermal annealing. This work has revealed the dependence of a-GIZO TFT characteristics on the N2O plasma radio-frequency (RF) power and frequency, the SiH4 flow rate in the SiOx deposition process, and the final annealing temperature. On the basis of these results, a high-performance a-GIZO TFT with a field-effect mobility of 35.7 cm(2) V(-1) s(-1), a subthreshold swing of 185 mV dec(-1), a switching ratio exceeding 10(7), and a satisfactory reliability was successfully fabricated. The technology developed in this work can be realized using the existing facilities of active-matrix liquid-crystal display industries.

Research progress of active matrix organic light emitting diode (AMOLED) displays driven by metal oxide thin film transistors (TFT)

Metal oxide thin film transistors (MOTFTs) have been attracting great attention, because TFT technology are suitable for the requirement of novel display technology such as high definition, large screen liquid crystal display (LCD) and active matrix organic light emitting diode (AMOLED) display, with their high mobility, good uniformity, simple process, low process temperature, and low cost. In this article, material, device structure, fabricating process and application of MOTFT are introduced, and factors that influence MOTFT performance have been discussed. We developed a new class of metal oxide materials, which has a highest mobility of 35 cm2/Vs, threshold voltage of 1.63 V, current on/off ratio of 109, and sub threshold swing of 0.21 V/decade. Based on our MOTFT panel, we have developed a first 3~5 inch color AMOLED screen, transparent AMOLED, and flexible AMOLED. This novel MOTFT panel has a booming future in novel display such as TFT-LCD and AMOLED.

High‐mobility self‐aligned top‐gate oxide TFT for high‐resolution AM‐OLED

AbstractHigh‐mobility and highly reliable self‐aligned top‐gate oxide thin‐film transistor (TFTs) were developed using the aluminum reaction method. Al diffusion to the oxide semiconductor and homogenization of the oxygen concentration in the depth direction after annealing were confirmed by laser‐assisted atom probe tomography. The high mobility of the top‐gate TFT with amorphous indium tin zinc oxide channel was demonstrated to be 32 cm2/V s. A 9.9‐in. diagonal qHD active‐matrix organic light‐emitting diode (AM‐OLED) display was fabricated using a five‐mask backplane process to demonstrate an applicable solution for large‐sized and high‐resolution AM‐OLEDs.

LTPS AM-OLED Display Consisting of Two-TFTs/One-Capacitor Pixel Circuits for Producing High-Uniformity Images

A method for generating threshold voltage (Vth) of thin-film transistors (TFTs) is proposed. This method is applied to the simplest pixel circuit, namely, one composed of two TFTs, a drive TFT and an address TFT, and one capacitor [i.e., two TFTs and one capacitor configuration], of active-matrix organic light-emitting diode (AM-OLED) displays. It involves two steps. First, source voltage of the drive TFT is quickly and precisely detected under the condition that carriers, namely holes, in the drive TFT are detrapped. Second, a simple subtraction and bit-shift operation are executed using the detected source voltage of the drive TFT. A prototype 3.8-in 480 × 320-pixel AM-OLED display was developed to confirm that a Vth-distribution image produced by this method corresponds to nonuniform images of AM-OLED displays. Furthermore, the AM-OLED display produced high-uniformity images when the Vth distribution was compensated by the proposed method.

Comparative study of gel-based separated arcdischarge, HiPCO, and CoMoCAT carbon nanotubes for macroelectronic applications

Due to their excellent electrical properties and compatibility with room-temperature deposition/printing processing, high-purity single-walled semiconducting carbon nanotubes hold great potential for macroelectronic applications such as in thin-film transistors and display back-panel electronics. However, the relative advantages and disadvantages of various nanotubes for macroelectronics remains an open issue, despite the great significance. Here in this paper, we report a comparative and systematic study of three kinds of mainstream carbon nanotubes (arc-discharge, HiPCO, CoMoCAT) separated using low-cost gel-based column chromatography for thin-film transistor applications, and high performance transistors-which satisfy the requirements for transistors used in active matrix organic light-emitting diode displays-have been achieved. We observe a trade-off between transistor mobility and on/off ratio depending on the nanotube diameter. While arc-discharge nanotubes with larger diameters lead to high device mobility, HiPCO and CoMoCAT nanotubes with smaller diameters can provide high on/off ratios (> 106) for transistors with comparable dimensions. Furthermore, we have also compared gel-based separated nanotubes with nanotubes separated using the density gradient ultracentrifuge (DGU) method, and find that gel-separated nanotubes can offer purity and thin-film transistor performance as good as DGU-separated nanotubes. Our approach can serve as the critical foundation for future carbon nanotube-based thin-film macroelectronics. Open image in new window

Charge Retention Characteristics of Silicide-Induced Crystallized Polycrystalline Silicon Floating Gate Thin-Film Transistors for Active Matrix Organic Light-Emitting Diode

In this work, non-volatile memory thin-film transistor (NVM-TFT) was fabricated by nickel silicide-induced laterally crystallized (SILC) polycrystalline silicon (poly-Si) as the active layer. The nickel seed silicide-induced crystallized (SIC) poly-Si was used as storage layer which is embedded in the gate insulator. The novel unit pixel of active matrix organic light-emitting diode (AMOLED) using NVM-TFT is proposed and investigated the electrical and optical performance. The threshold voltage shift showed 17.2 V and the high reliability of retention characteristic was demonstrated until 10 years. The retention time can modulate the recharge refresh time of the unit pixel of AMOLED up to 5000 sec.

Flexible high‐resolution full‐color top‐emitting active‐matrix organic light‐emitting diode display

AbstractWe developed a high‐performance 3.4‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display with remarkably high resolution using an oxide semiconductor in a backplane, by applying our transfer technology that utilizes metal separation layers. Using this panel, we also fabricated a prototype of a side‐roll display for mobile uses. In these AMOLED displays, a white OLED combined with a color filter was used in order to achieve remarkably high resolution. For the white OLED, a tandem structure in which a phosphorescent emission unit and a fluorescent emission unit are serially connected with an intermediate layer sandwiched between the emission units was employed. Furthermore, revolutionary technologies that enable a reduction in power consumption in both the phosphorescent and fluorescent emission units were introduced to the white tandem OLED.

A Charge-Pump-Based Current Feedback Method for AMOLED Displays

This work presents a new external driving scheme that uses the current feedback method with a 3T1C pixel circuit. The proposed circuit provides a stable OLED driving current by compensating for the threshold voltage shift of amorphous-silicon thin-film transistors (a-Si TFTs). For fabricated a-Si TFTs and the 0.35 μm CMOS process associated with the proposed driving scheme, measurements reveal that the normalized OLED current degradation is less than 5% after 60 hours of operation at 60°C. Moreover, for a data line load of 1.69 K Ω/110 pF, the relative error rates between data currents and OLED currents are less than 4% within a settling time of 19 μs.

Compensating Pixel Circuit Driving AMOLED Display With a-IGZO TFTs

This letter presents a novel active-matrix organic light-emitting diode (AMOLED) pixel circuit that uses amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with a bottom-gate structure to compensate for the threshold voltage shift of the TFT. An a-IGZO TFT driven AMOLED display (70 × 70 pixels) on a glass substrate is fabricated and its reliability is evaluated under electrical stress.

Paper No 19.3: Back‐Channel‐Etch Process Flow for a‐IGZO TFTs

AbstractIn this study, the authors report high‐quality amorphous Indium‐Gallium‐Zinc‐Oxide (a‐IGZO) thin film transistors (TFTs) fabricated using a new back‐channel‐etch (BCE) process flow on Polyethylene Naphthalate (PEN) foil. The BCE flow allows a better scalability of TFTs for high‐resolution backplanes and related circuits. The maximum processing temperature was limited to less than 165oC in order to ensure good overlay accuracy (&lt; 1µm) on foil. The presented process flow differs from to previously reported by defining the Mo S/D contacts by dry etch prior to a‐IGZO patterning. The TFTs show good electrical performance, including field‐effect mobilities in the range of 15.0cm2/(V.s), sub‐threshold slopes of 0.3V/decade and off‐currents &lt;1.0pA on foil. Finally, the applicability of this new BCE process flow was demonstrated for TFT backplane driving a 32 x 32 active‐matrix organic light‐emitting diode (AMOLED) display.

High-Speed Voltage-Programmed Pixel Circuit for AMOLED Displays Employing Threshold Voltage One-Time Detection Method

This letter presents a high-speed, voltage-programmed pixel circuit for active-matrix organic light-emitting diode displays employing threshold voltage one-time detection method. The proposed pixel circuit that consists of one driving thin-film transistor (TFT), four switching TFTs, two capacitors, and four control signal lines can effectively compensate for the threshold voltage shift of the driving TFT and the OLED degradation. The threshold voltage of one-time detection method is realized by an ingenious application of capacitor coupling effect, and it is well-verified by comparing it with the conventional driving scheme. In this method, data is directly renewed without the periods of initialization and threshold voltage detection from the second-frame beginning. Moreover, this method can also be applied to other compensation pixel circuits.