有源矩阵有机电致发光像素电路的研究进展

有源驱动方式的有机发光二极管（AMOLED）较之无源驱动方式易于实现高亮度和高分辨率、功耗更小,更适合大屏幕显示。但传统的两管驱动电路会出现驱动管阈值电压在整个屏幕上分布不均匀,或长时间加偏压后驱动管的阂值电压发生漂移。本文在两管驱动电路的基础上介绍了几种最近提出的补偿电路并描述了它们的改善效果及各自存在的问题。     

Mirrored OLED pixel circuit for threshold voltage and mobility compensation with IGZO TFTs

In this paper, pixel circuit using mirroring structure with Indium–Gallium–Zinc oxide (IGZO) thin film transistors (TFTs) for active matrix organic light emitting diode (AMOLED) display is proposed. This pixel circuit consists of only four TFTs, and one capacitor. Due to the mirroring structure, characteristic of the driving TFT can be precisely sensed by the sensing TFT, which is deployed in a discharging path for gate electrode of the driving TFT. This discharging process is strongly dependent on threshold voltage (V_T) and effective mobility of the sensing TFT. Circuit operating details are discussed, and compensation effects for threshold voltage shift and mobility variations are verified through numerical derivation and SPICE simulations. Furthermore, compared with conventional schematics, the proposed pixel circuit might have much simplified external driving circuits, and it is a promising alternative solution of high performance AMOLED display.     

一种基于低温多晶硅薄膜晶体管的交流AMOLED像素补偿电路

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 tran- sistor （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 Stable Voltage-Programmed Pixel Circuit for a-Si:H AMOLED Displays

Hydrogenated amorphous silicon (a-Si:H) active matrix organic light-emitting diode (AMOLED) displays are attractive given the potentially low manufacturing cost and ultimately low-temperature fabrication enabling using flexible substrates. Although the conventional two thin-film transistor (2-TFT) AMOLED voltage-programmed pixel circuit (VPPC) can provide high resolution and high yield, the 2-TFT VPPC is prone to image retention over time due to shift in the threshold voltage (V_T-shift) of a-Si:H TFTs. This paper presents a new driving scheme that not only preserves the simplicity of the 2-TFT VPPC, but also demonstrates high uniformity. Experimental results indicate that the current drop in the new driving scheme is less than 11% after 15 days of operation whereas it is over 50% for the conventional driving scheme. Moreover, the new driving scheme is less sensitive to temperature variations due to an internal feedback mechanism. After a 70% change in the temperature, the current in the conventional driving scheme increases by as much as 300%. However, the current in the driving scheme presented here is approximately constant     

Enhancement of Brightness Uniformity by a New Voltage-Modulated Pixel Design for AMOLED Displays

This letter presents a new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) displays using low-temperature polycrystalline silicon thin-film transistors (TFTs). The proposed pixel circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation by the source-follower-type connection and the bootstrap. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using the HSPICE software. The novel pixel design has great potential for use in large-size and high-resolution AMOLED displays.     

具有顶部发光结构的AMOLED交流驱动电路

A new voltage programmed pixel circuit with top emission design for active-matrix organic lightemitting diode(AMOLED) displays is presented and verified by HSPICE simulations.The proposed pixel circuit consists of five poly-Si TFTs,and can effectively compensate for the threshold voltage variation of the driving TFT.Meanwhile,the proposed pixel circuit offers an AC driving mode for the OLED by the two adjacent pulse voltage sources,which can suppress the degradation of the OLED.Moreover,a high contrast ratio can be achieved by the proposed pixel circuit since the OLED does not emit any light except for the emission period.     

A New Pixel Circuit Compensating for Brightness Variation in Large Size and High Resolution AMOLED Displays

A new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) display using low-temperature polycrystalline silicon thin-film transistor (LTPS-TFT) is proposed. The new circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using HSPICE software. The simulated error rate of the output current is also discussed in this paper. The novel pixel design has great potential for use in large size and high resolution AMOLED displays.     

一种LTPS-TFT AMOLED像素电路的理论研究

本文对一种LTPS-TFT AMOLED电压型阈值电压(V_(th))补偿像素电路进行了理论研究,分析了影响V_(th)补偿效果的主要因素。电路的补偿效果主要由驱动TFTV_(th)的获取精度和随后的保持精度决定。在V_(th)获取过程中,相关误差主要由驱动TFT转移特性电流对存储电容充电的充电率不足产生;在显示信号与V_(th)叠加过程中,与V_(th)保持节点连接的电容增量等因素会造成V_(th)保持精度的损失。根据分析的结果,本文解释了高分辨率像素电路补偿效果下降的原因。     

A driving scheme for active-matrix organic light-emitting diode displays based on feedback

A scheme of driving active matrix organic light emitting diode (AMOLED) displays with hydrogenated amorphous silicon (a-Si) thin-film transistors (TFTs) is presented. By sending a feedback voltage from each pixel to a column driver during the programming cycle, the driving scheme can compensate for the instability of the TFTs, in particular, the shift in the threshold voltage. Measurement results show no change in the OLED current in the presence of a 1.3-V shift in the threshold voltage. Based on circuit analysis, a simple lead compensator and an accelerating pulse were employed to achieve fast pixel programming for a wide range of OLED currents. Simulation results show a programming time of less than 70 /spl mu/s for OLED currents as low as 50 nA.     

基于低温多晶硅薄膜晶体管的AMOLED交流像素电路

This work presents a new voltage programmed pixel circuit for an active-matrix organic light-emitting diode(AMOLED) display.The proposed pixel circuit consists of six low temperature polycrystalline silicon thinfilm transistors(LTPS TFTs),one storage capacitor,and one OLED,and is verified by simulation work using HSPICE software.Besides effectively compensating for the threshold voltage variation of the driving TFT and OLED,the proposed pixel circuit offers an AC driving mode for the OLED,which can suppress the degradation of the OLED.Moreover,a high contrast ratio can be achieved by the proposed pixel circuit since the OLED does not emit any light except for the emission period.     

一种新型的电压型AMOLED像素补偿电路

针对常用3T1C结构的AMOLED像素补偿电路容易受到阈值电压漂移影响的问题,介绍了一种新型电压型4T2C结构的AMOLED像素补偿电路,阐明了该电路的工作原理,并对OLED的驱动电流作了定性分析。结果显示该驱动电流值只与驱动电压及电源电压有关,从而有效地解决了因阈值电压变化而造成的显示器亮度不均匀的问题。该电路已通过Spectre仿真软件验证了其有效性。     

A Novel Voltage-Feedback Pixel Circuit for AMOLED Displays

This study presents a novel voltage-modulated pixel circuit for active-matrix organic light-emitting diode (AMOLED) consisting of five n-type thin-film transistors (TFTs), one additional control signal, and one storage capacitor. The proposed circuit, which can be implemented in all-n-type and all-p-type low temperature poly-silicon (LTPS) TFT technologies, successfully compensates for threshold voltage deviation of TFTs and facilitates correction of OLED degradation using a voltage feedback method. Simulation and experimental results for all-n-type TFTs indicate that the proposed pixel circuit reduced the nonuniformity brightness problem effectively by compensating for threshold voltage variation in TFTs and reduced the degradation of emission efficiency in OLEDs.     

A novel 3-TFT voltage driving method of compensating VTH shift for a-Si:H TFT and OLED degradation for AMOLED

This work demonstrates the feasibility of a novel pixel circuit by using three a-Si:H TFTs. The proposed circuit can stabilize the OLED current and provide an additional driving current to ameliorate the brightness degradation of the AMOLED. Measurement results indicate that the current degradation of the proposed circuit, caused by V_TH variations, is less than 5% over more than 50,000 s at 60 °C, whereas that of a conventional 2T1C pixel circuit is larger than 34%. Furthermore, to ameliorate the decrease in luminance owing to the OLED degradation, the OLED current can be increased by 10% by analyzing the current degradation and modulating the detected voltage appropriately.     

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.      

A new a-Si:H TFT pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED

We propose a new hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel circuit for an active matrix organic light-emitting diode (AMOLED) employing a voltage programming. The proposed a-Si:H TFT pixel circuit, which consists of five switching TFTs, one driving TFT, and one capacitor, successfully minimizes a decrease of OLED current caused by threshold voltage degradation of a-Si:H TFT and OLED. Our experimental results, based on the bias-temperature stress, exhibit that the output current for OLED is decreased by 7% in the proposed pixel, while it is decreased by 28% in the conventional 2-TFT pixel.     

AMOLED Pixel Circuit With Electronic Compensation of Luminance Degradation

A new voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diodes (AMOLEDs) is presented. In addition to compensating for the shift in threshold voltage of TFTs, the circuit is capable of compensating for OLED luminance degradation by employing the shift in OLED voltage as a feedback of OLED degradation     

A new pixel circuit for driving organic light-emitting diode with low temperature polycrystalline silicon thin-film transistors

A new pixel circuit design for active matrix organic light-emitting diode (AMOLED), based on the low-temperature polycrystalline silicon thin-film transistors (LTPS-TFTs) is proposed and verified by SPICE simulation. Threshold voltage compensation pixel circuit consisting of four n-type TFTs, one p-type TFT, one additional control signal, and one storage capacitor is used to enhance display image quality. The simulation results show that this pixel circuit has high immunity to the variation of poly-Si TFT characteristics.     

A new voltage-modulated AMOLED pixel design compensating for threshold voltage variation in poly-Si TFTs

A new voltage-modulated active-matrix organic light-emitting diode (AMOLED) pixel design, which successfully compensates for the threshold voltage variations in poly-Si thin-film transitors (TFTs), is proposed, and verified by SPICE simulation and experiments. In order to compensate for variations in OLED current, the proposed pixel design employs a new voltage modulation scheme using diode connections.     

Single-Technology-Based Statistical Calibration for High-Performance Active-Matrix Organic LED Displays

Active-matrix organic light-emitting-diode (AMOLED) displays based on amorphous hydrogenated silicon (a-Si:H) thin-film transistors (TFTs) are the state of the art in display technology, owing to the feasilibility of low-cost fabrication and accessability to well-established TFT-LCD fabrication. While the a-Si:H TFT offers excellent matching of device properties over large areas, it suffers from a gate-bias-dependent threshold voltage shift in time, leading to grayscale inaccuracies. In order to counter this problem, many compensation circuits have been designed. The purpose of the compensation circuit is to estimate the threshold voltage shift in driver TFTs and apply a correction so as to maintain a constant brightness. However, all of the compensation circuits designed to date suffer from low spatial and temporal resolution and reliability issues or high cost due to the use of custom-made CMOS technology. In this paper, we focus on building AMOLED display systems solely based on a-Si:H TFT technology along with the use of off-the-shelf CMOS components to lower costs. Furthermore, we achieve high spatial and temporal resolution and high yield with the use of a two-TFT voltage programmed pixel circuit along with a statistical based external calibration circuit.     

A New Poly-Si TFT Current-Mirror Pixel for Active Matrix Organic Light Emitting Diode

A new poly-Si thin-film-transistor (TFT) current-mirror-active-matrix-organic-light-emitting-diode (AMOLED) pixel, which successfully compensates for the variation of the threshold voltage as well as mobility in the excimer laser annealed poly-Si TFT pixel, is designed and fabricated. The OLED current$(I_ OLED)$of the proposed pixel does not depend on the operating temperature. When the temperature of pixel is increased from 27$^circhboxC$to 60$^circhboxC$, the$I_ OLED$of the new pixel circuit composed of four TFTs and one capacitor increases only about 1.5%, while that of a conventional pixel composed of two TFTs and one capacitor increases about 37%. At room temperature, nonuniformity of the$I_ OLED$in the proposed circuit was also considerably suppressed at around 9%. We have successfully fabricated a 1.2-in AMOLED panel$(hbox96 times hbox96 times hboxred green blue)$to evaluate the performance of the proposed pixel. A troublesome residual image caused by the hysteresis phenomenon of the poly-Si TFT was almost eliminated in the proposed AMOLED panel as a result of current programming.