Highly reliable a‐IGZO TFTs on a plastic substrate for flexible AMOLED displays

We have successfully reduced threshold voltage shifts of amorphous In–Ga–Zn–O thin‐film transistors (a‐IGZO TFTs) on transparent polyimide films against bias‐temperature stress below 100 mV, which is equivalent to those on glass substrates. This high reliability was achieved by dense IGZO thin films and annealing temperature below 300 °C. We have reduced bulk defects of IGZO thin films and interface defects between gate insulator and IGZO thin film by optimizing deposition conditions of IGZO thin films and annealing conditions. Furthermore, a 3.0‐in. flexible active‐matrix organic light‐emitting diode was demonstrated with the highly reliable a‐IGZO TFT backplane on polyimide film. The polyimide film coating process is compatible with mass‐production lines. We believe that flexible organic light‐emitting diode displays can be mass produced using a‐IGZO TFT backplane on polyimide films.     

Power management unit chip design for automobile active-matrix organic light-emitting diode display module

A power management unit (PMU) chip supplying dual panel supply voltage, which has a low electro-magnetic interference (EMI) characteristic and is favorable for miniaturization, is designed. A two-phase charge pump circuit using external pumping capacitor increases its pumping current and works out the charge-loss problem by using bulk-potential biasing circuit. A low-power start-up circuit is also proposed to reduce the power consumption of the band-gap reference voltage generator. And the ring oscillator used in the ELVSS power circuit is designed with logic devices by supplying the logic power supply to reduce the layout area. The PMU chip is designed with MagnaChip’s 0.25 μm high-voltage process. The driving currents of ELVDD and ELVSS are more than 50 mA when a SPICE simulation is done.     

A novel algorithm for selective current limit used in AMOLED panel

A selective current limit algorithm based on color space conversion is proposed in this paper. There are mainly two advantages for this algorithm. Firstly, only the luminance component Y is decreased, and the color component Cb or Cr is not changed; thus, the color distortion is avoided in the process of current limit. Secondly, the current of pixels with high luminance is limited while that of those pixels with low luminance is not, which means current limit is executed selectively. For the same value of mean square error, the simulation results show that the ratio of pixels with high luminance of the image processed by the proposed algorithm is zero, and that processed by the net power control and the algorithm based on color space conversion is 0.081 and 0.0854, respectively. This algorithm is implemented on a field‐programmable gate array to drive the image display on the AMOLED panel.     

Optical performance characterization of flexible mobile displays

The high interest in the display industry to produce curved or flexible displays for mobile, wearable, and television markets has raised concerns on the performance of displays in their bent state. Flexible display technologies have been developed for this purpose, but their optical characteristics have not been previously reported at various degrees of curvature. The measurement of flexible displays is possible only with special measurement arrangements using common display characterization devices. A conoscope with a working distance of 15 mm was used to demonstrate the curvature dependence of luminance, viewing angle, and color coordinates of the white point of a flexible active‐matrix organic light‐emitting display. The results show that the effect of curvature on the optical characteristics of the display is most evident at the smallest measured bending radii of +/?50 mm and that the effects are most visible when viewing the bent ends of the display, from the central normal vantage point, with uniform curvature along the display.     

Fabrication of 5.8‐in. OTFT‐driven flexible color AMOLED display using dual protection scheme for organic semiconductor patterning

Abstract— A 5.8‐in. wide‐QQVGA flexible color active‐matrix organic light‐emitting‐diode (AMOLED) display consisting of organic thin‐film transistors (OTFTs) and phosphorescent OLEDs was fabricated on a plastic film. To reduce the operating voltage of the OTFTs, Ta₂O₅ with a high dielectric constant was employed as a gate insulator. Pentacene was used for the semiconductor layer of the OTFTs. This layer was patterned by photolithography and dry‐etched using a dual protection layer of poly p‐xylylene and SiO₂ film. Uniform transistor performance was achieved in the OTFT backplane with QQVGA pixels. The RGB emission layers of the pixels were formed by vacuum deposition of phosphorescent small molecules. The resulting display could clearly show color moving images even when it was bent and operated at a low driving voltage (below 15 V).     

Dual‐plate OLED display (DOD) based on amorphous‐silicon (a‐Si) TFT backplane

Abstract— An improved AMOLED with an a‐Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual‐plate OLED display (DOD). While a top‐emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large‐sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large‐sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.     

A fully integrated 1‐in. AMOLED display using current feedback based on a five‐mask LTPS CMOS process

Abstract— In the past, a five‐mask LTPS CMOS process requiring only one single ion‐doping step was used. Based on that process, all necessary components for the realization of a fully integrated AMOLED display using a 3T1C current‐feedback pixel circuit has recently been developed. The integrated data driver is based on a newly developed LTPS operational amplifier, which does not require any compensation for Vth or mobility variations. Only one operational amplifier per column is used to perform digital‐to‐analog conversion as well as current control. In order to achieve high‐precision analog behavior, the operational amplifier is embedded in a switched capacitor network. In addition to circuit verification by simulation and analytic analysis, a 1‐in. fully integrated AMOLED demonstrator was successfully built. To the best of the authors' knowledge, this is the first implementation of a fully integrated AMOLED display with current feedback.     

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.     

High-speed low-power voltage-programmed driving scheme for AMOLED displays

A new voltage-programmed driving scheme named the mixed parallel addressing scheme is presented for AMOLED displays, in which one compensation interval can be divided into the first compensation frame and the consequent N-1 post-compensation frames without periods of initialization and threshold voltage detection. The proposed driving scheme has the advantages of both high speed and low driving power due to the mixture of the pipeline technology and the threshold voltage one-time detection technology. Corresponding to the proposed driving scheme, we also propose a new voltage-programmed compensation pixel circuit, which consists of five TFTs and two capacitors(5T2C). In-Zn-O thin-film transistors(IZO TFTs) are used to build the proposed 5T2C pixel circuit. It is shown that the non-uniformity of the proposed pixel circuit is considerably reduced compared with that of the conventional 2T1C pixel circuit. The number of frames(N) preserved in the proposed driving scheme are measured and can be up to 35 with the variation of the OLED current remaining in an acceptable range. Moreover, the proposed voltage-programmed driving scheme can be more valuable for an AMOLED display with high resolution, and may also be applied to other compensation pixel circuits.     

AMOLED灰阶补偿算法研究

为了解决一种小型AMOLED显示屏由灰阶丢失导致的图像变形问题,提出了一种AMOLED灰阶补偿算法。首先测试不同亮度下AMOLED显示灰阶时亮度变化的数据,然后依据测试数据建立亮度变化与灰阶丢失的映射关系,最后使用非线性灰阶压缩的方法对输入图像进行变换,补偿灰阶丢失引起的图像失真。通过软件仿真和硬件测试表明,该算法可以有效地补偿由于AMOLED亮度变化而无法正常显示的灰阶的问题,使图像能够正常显示。因此该算法可以保证小型AMOLED显示屏在不同亮度下工作时能够正常显示灰阶。