Active-matrix Organic Light-emitting Diode Display Research Articles

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm2/V‐sec, an on/off ratio of >108, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.

Pulsewidth Modulation With Current Uniformization for AM-OLEDs

We propose a novel conception of driving methods, pulsewidth modulation with current uniformization, for active-matrix organic light-emitting diode displays (AM-OLEDs). This driving conception can simultaneously achieve precise grayscale and exceedingly improve luminance uniformity. We evaluate a concrete example, in which a pixel circuit includes seven transistors and a driving period consists of addressing-programming and light-emitting periods. The working and the tolerance against characteristic deviations of thin-film transistors and OLEDs are confirmed using circuit simulation and actual equivalent circuits.

Degradation Evaluation of $\alpha$-IGZO TFTs for Application to AM-OLEDs

We have evaluated the characteristic degradation of amorphous-InGaZnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> thin-film transistors (α-IGZO TFTs) to apply to active-matrix organic light-emitting diode displays (AM-OLEDs) using current-voltage (I-V) and capacitance-voltage (C-V) curves and an extraction technique of trap densities. First, the degradation was a parallel shift of the I-V and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curves after a short time, mainly caused by fixed charge injection into the gate insulator when gate voltage stress was applied by supposing switching TFTs in AM-OLEDs. Second, the degradation was a parallel shift of the I -V curve and slope dullness of the C-V curve after a long time, mainly caused by trap generation in the channel layer when drain current stress was applied by supposing driving TFTs in AM-OLEDs. We should note that two different degradation modes occur when α-IGZO TFTs are applied to AM-OLEDs.

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

In this paper, we propose a novel pixel design and driving method for active-matrix organic light-emitting diode (AM-OLED) displays using low-temperature polycrystalline silicon thin-film transistors (LTPS-TFTs). The proposed threshold voltage compensation circuit, which comprised five transistors and two capacitors, has been verified to supply uniform output current by simulation work using the automatic integrated circuit modeling simulation program with integrated circuit emphasis (AIM-SPICE) simulator. The driving scheme of this voltage programming method includes four periods: precharging, compensation, data input, and emission. The simulated results demonstrate excellent properties such as low error rate of OLED anode voltage variation (<1%) and high output current. The proposed pixel circuit shows high immunity to the threshold voltage deviation characteristics of both the driving poly-Si TFT and the OLED.

69.3: Amorphous Oxide TFT Backplane for Large Size AMOLED TVs

AbstractAmorphous oxide thin film transistor (TFT) arrays have been developed as TFT backplanes for large size active‐matrix organic light emitting diode (AMOLED) displays. An amorphous IGZO (Indium Gallium Zinc Oxide) bottom gate TFT with an etch‐stop layer (ESL) delivered excellent electrical performance with field‐ effect mobility of 21 cm2/V‐s, an on/off ratio of &gt;108, and subthreshold slope (SS) of 0.29V/dec. A full color 19‐inch AMOLED display has been developed using the amorphous IGZO TFT backplane.

Innovative Voltage Driving Pixel Circuit Using Organic Thin-Film Transistor for AMOLEDs

In this work, we propose a novel active-matrix organic light-emitting diode displays (AMOLED) pixel circuit based on organic thin-film transistor (OTFT) architecture, which consisted of four switches, one driving transistor, and a capacitor. The pentacene-based OTFT device possesses a field-effect mobility of 0.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V ldr s, a threshold voltage of -1.5 V, subthreshold slope of 1.8 V/decade and an on/off current ratio 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . The resultant voltage-driving pixel circuit, named ldquocomplementary voltage-induced coupling drivingrdquo (CVICD), is different from the current-driving scheme and can appropriately operate at low gray level for the low-mobility OTFT circuitry. The current non-uniformity less than 2.9% is achieved for data voltage ranging from 1 to 17 V by SPICE simulation work. In addition, the new external driving method can effectively reduce the complexity of OLED pixel circuitry.

53.4: Development of 31‐Inch Full‐HD AMOLED TV Using LTPS‐TFT and RGB FMM

AbstractIn this paper, we describe the technologies for implementation of the world's largest Active Matrix Organic Light‐Emitting Diode (AMOLED) TV. Low Temperature Poly‐Silicon (LTPS) method is applied to integrate driver and pixel circuit in panel. Also, we used Fine Metal Mask (FMM) for color patterned R,G,B pixels. for achieving better color gamut, we applied bottom‐emission micro‐cavity structure. As a result, we have fabricated 31‐inch Full‐HD(1920×l080) AMOLED display. The power consumption is less than 40W, which it displays motion picture. We achieved excellent performance like color gamut over 100%, contrast ratio over 1,000,000:1, life time over 35,000hrs, 200cd/m2 white brightness (peak brightness is 550 cd/m2). We now remarkably achieved a slim TV set under 1cm thickness. The module thickness including electronic devices is only 4.3mm with aluminum frame.

External Compensation of Nonuniform Electrical Characteristics of Thin-Film Transistors and Degradation of OLED Devices in AMOLED Displays

The variation of electrical characteristics of polycrystalline-silicon thin-film transistor (TFT) and degradation of organic light-emitting-diode (OLED) device cause nonuniform intensity of luminance and image sticking in active-matrix OLED (AMOLED) displays. An external compensation method that senses and compensates variations of threshold voltage and mobility of TFTs and degradation of OLED device is proposed. The effect of the external compensation method on AMOLED pixel is experimentally verified by measuring the luminance of OLEDs and the electrical characteristics of TFTs in AMOLED pixels.

Compensation Pixel Circuit Using LTPS TFT For AMOLED Displays

We propose a new 4T1C active-matrix organic light-emitting diode displays (AMOLEDs) pixel circuit using LTPS thin-film transistors is verified by SPICE simulation successfully. The pixel design compensates the variation of threshold voltage between 1 ~ -1V with 1.6% current non-uniformity. Besides, a proposed new external driving method and we combine power & data line for driving can reduce the number of switch transistors in a pixel and circuit complexity.

Low-Power Low-Cost Voltage-Programmed a-Si:H AMOLED Display for Portable Devices

This paper presents a driving scheme to achieve highly stable low-power amorphous silicon (a-Si:H) active-matrix organic light-emitting diode (AMOLED) displays. Although the conventional 2-thin-film transistor (TFT) a-Si:H AMOLED display has demonstrated interesting features, including simplicity, it is prone to growing nonuniformity due to the temporal instability of the a-Si:H material. Several compensating techniques have been proposed to control the nonuniformity, but they tend to compromise the key attributes of the simple 2-TFT display such as low power consumption, high yield, high aperture ratio, low implementation cost, and fast programming. For mobile applications which have tight constrains on power consumption, cost, and escalating resolution requirements, we propose a new driving and addressing scheme that not only improves the backplane stability, but also compensates for the OLED luminance degradation while maintaining the attractive features of the simple 2-TFT pixel circuit. The overhead in power consumption and implementation cost is reduced by over 90% compared to existing compensation driving schemes.

Source Driver Channel Reduction Schemes Employing Corresponding Pixel Alignments for Current Programming Active-Matrix Organic Light-Emitting Diode Displays

We propose two types of novel scheme for reducing the number of output channels of driver-integrated circuit (D-IC) for the current programming compensation pixel structures of active-matrix organic light-emitting diodes (AMOLEDs). One is a 2:1 data demultiplexing technique that can reduce the number of output channels of D-IC by half. The proposed second scheme is a vertically aligned red (R), green (G), and blue (B) subpixel scheme instead of a horizontally aligned R–G–B subpixel one, which is regarded as the conventional pixel alignment scheme. We have also successfully implemented these schemes in a 2.4-in.-sized QCIF + (176 × RGB × 220) AMOLED using p-type excimer laser annealing (ELA) low-temperature polycrystalline silicon (LTPS) technology and evaluated key performance characteristics.

Investigation of Pattern-Induced Brightness Non-uniformity in Active-Matrix Organic Light-Emitting Diode Displays

We recently improved the brightness non-uniformity in active-matrix organic light-emitting diode (AMOLED) regarding non-uniform laser energy distribution by applying a “shot mixing” technique in sequential lateral solidification (SLS) method. Although the new SLS technique was employed, another brightness non-uniformity that appeared as oblique lines on AMOLED panels became a crucial issue. In this work, we attempted to investigate the origins of the non-uniformity. Our systematic analysis on the oblique lines revealed that the line type non-uniformity was attributed to both the SLS process and the thin-film transistor (TFT) fabrication processes. In particular, we found that such oblique patterns might be related to moiré patterns that appear when primary grain boundaries aligned in a repetitive pattern is placed over other repetitive TFT patterns such as metal lines. We adopted a method to diminish the moiré pattern type non-uniformity by applying top emission TFT structure.

53.2: Distinguished Paper: Novel Laser Transfer Technology for Manufacturing Large‐Sized OLED Displays

AbstractA novel laser transfer technology has been developed for manufacturing large‐Sized organic light emitting diode (OLED) displays. OLED materials are transferred from glass donors to a substrate by laser‐induced pattern‐wise sublimation (LIPS). RGB pixels are precisely patterned by LIPS. The LIPS OLED device performance has been examined in comparison with conventional evaporated device. Using this technology, we have fabricated a 27.3‐inch active matrix OLED display.

Novel pixel design for high‐resolution AMOLED displays with a shadow mask

Abstract— The bottlenecks in achieving high resolution for active‐matrix OLED (AMOLED) displays based on currently available manufacturing processes were evaluated and compared. The use of a shadow mask has proven to be viable for mass production, but the fabrication of high‐precision shadow masks becomes very difficult when the resolution exceeds 180 ppi. The latest breakthrough in increasing display resolution is presented. Without an increase in cost, the limitations of the conventional shadow‐mask process using novel subpixel designs have been successfully overcome. A high resolution reaching of 270 ppi has been successfully demonstrated on a 3‐in. VGA‐format AMOLED display, fabricated by using a shadow mask with a much lower resolution of 135 ppi. This innovative pixel design opens up the possibilities of manufacturing high‐resolution displays using a relatively low‐resolution shadow mask.

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) and its low device mobility. Although V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> -instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements

Active-matrix OLED on bendable metal foil

This brief reports a flexible active-matrix organic light-emitting diode display based on a poly-Si thin-film transistor (TFT) backplane. The p-channel poly-Si TFTs on metal foil exhibited a maximum field-effect mobility of 86.1 cm/sup 2//Vs, threshold voltage of 3.5 V, gate voltage swing of 0.8 V/dec, and the minimum off current of 10/sup -12/ A//spl mu/m at V/sub ds/=-0.1 V. A 4.1-in active-matrix backplane was fabricated with the poly-Si TFT with a conventional pixel circuit consisting of 2 TFTs and one capacitor. The scan driver circuits with PMOS were integrated on the flexible metal foil. The top emission, organic light emitting display having a brightness of 100 cd/m/sup 2/.

49.1: A 14.1 inch Full Color AMOLED Display with Top Emission Structure and a-Si TFT Backplane

A structure and a design of device were developed to fabricate large-scale active matrix organic light-emitting diode (AMOLED) display with good color purity and high aperture ratio. With these technologies, we developed a full color 14.1 inch WXGA AMOLED display. For the integration of OLED on an active matrix a-Si TFT backplane, an efficient top emission OLED is essential since the TFT circuitry covers a large position of the pixel aperture. These technologies will enable up the OLED applications to larger size displays such as desktop monitors and TVs.

49.4: Two TFT Pixel Circuit with Non-Uniformity Suppress-Function for Voltage Programming Active Matrix OLED Displays

We proposed the world's first two TFT pixel circuit with an inter-pixel non-uniformity suppress-function for the analog voltage programming to assist in the realization of small pixel bottom emission OLED displays. The pixel circuit provides larger OLED fill factor. We fabricated a 2.5 inch 240 (V) × 960 (H) pixel monochrome bottom emission display.

Improvement of Brightness Uniformity by AC Driving Scheme for AMOLED Display

We propose an AC voltage driving scheme which can improve the brightness uniformity of active-matrix organic light-emitting diode (AMOLED) displays. Through the use of a charge feed-through mechanism and an AC driving voltage, the voltage drop caused by the parasitic resistance can be compensated. A 2.2 in AMOLED panel with a resolution of 176/spl times/RGB/spl times/220 has been fabricated by low temperature polycrystalline silicon technology to evaluate the performance of the AC driving scheme. Experimental results show that a brightness uniformity of higher than 91.6%, achieved by the AC driving scheme, in contrast to that of 74% achieved by a DC driving scheme.

Novel Current Driving Circuit for Active Matrix Organic Light Emitting Diode

This paper describes a novel current driving circuit for an active matrix organic light emitting diode (AMOLED). The proposed current driving circuit has a lower power consumption and higher chip density for the AMOLED display compared with the conventional one because all elements operate at a normal voltage and are shielded from the high voltage of the panel. The chip size and power consumption of the current driving circuit for an AMOLED can be improved by about 30 to 40% and 10 to 20%, respectively, compared with the conventional one.