A Luminance Compensation Method Using Optical Sensors with Optimized Memory Size for High Image Quality AMOLED Displays

This paper proposes a luminance compensation method using optical sensors to achieve high luminance uniformity of active matrix organic light-emitting diode (AMOLED) displays. The proposed method compensates for the non-uniformity of luminance by capturing the luminance of entire pixels and extracting the characteristic parameters. Data modulation using the extracted characteristic parameters is performed to improve luminance uniformity. In addition, memory size is optimized by selecting an optimal bit depth of the extracted characteristic parameters according to the trade-off between the required memory size and luminance uniformity. To verify the proposed compensation method with the optimized memory size, a 40-inch 1920x1080 AMOLED display with a target maximum luminance of 350 cd/m(2) is used. The proposed compensation method considering a 4cr range of luminance reduces luminance error from +/- 38.64%, +/- 36.32%, and +/- 43.12% to +/- 2.68%, +/- 2.64%, and +/- 2.76% for red, green, and blue colors, respectively. The optimal bit depth of each characteristic parameter is 6-bit and the total required memory size to achieve high luminance uniformity is 74.6 Mbits

Modeling of OLED degradation for prediction and compensation of AMOLED aging artifacts

Degradation is still the most challenging issue for OLED, which causes the image-sticking artifact on AMOLED displays and limits their lifetime. To overcome the demerit, OLED degradation is modeled in this thesis, and compensation based on the models is applied for AMOLEDs. A data-counting model is firstly developed to quantitatively evaluate the degradation on OLEDs, with consideration of the accumulation stress during operation. An electro-optical model is further built, based on an equivalent circuit. It can simulate the electro-optical characteristic (I-V, Eff-V) and the degradation behaviors in aging process. Besides, the correlation model is aimed to derive the current efficiency decay with measurable electrical values, delivering more dependable results at strongly aged state. The prediction and compensation are implemented based on developed models. The results show that the models exactly predict the efficiency decay during operation. The image-sticking aging artifact on AMOLED can be suppressed by applying compensation, so that the display lifetime is extended.Durch das Einbrennen von Bildern in AMOLED Displays wird deren Lebensdauer verringert; dieser Qualitätsverlust stellt nach wie vor die größte Herausforderung für die OLED Technologie dar. In dieser Thesis wird die Degradation der OLEDs modelliert und eine Kompensierung anhand der Modelle erreicht. Zunächst wurde ein Data-counting Modell entwickelt, um die Degradation von OLEDs unter Berücksichtigung der akkumulierten Belastung während des Betriebs quantitativ zu bewerten. Des Weiteren wurde ein elektro-optisches Modell entwickelt, das auf einem äquivalenten Schaltungsmodell basiert. Es kann die elektro-optischen Eigenschaft (I-V, Eff-V) und das Degradationsverhalten im Alterungsprozess simulieren. Außer den beiden Modellen wird noch ein Korrelationsmodell entwickelt, das darauf abzielt, die Abnahme der Stromeffizienz aus den messbaren elektrischen Werten abzuleiten. Dieses Modell liefert im stark gealterten Zustand zuverlässigere Ergebnisse. Aufbauend auf die entwickelten Modelle wurden die Vorhersage und die Kompensierung implementiert. Die Ergebnisse zeigen, dass die Modelle den Effizienzverlust während des Betriebes genau vorhersagen. Das Einbrennen des Bildes in das AMOLED-Display kann durch das Anwenden der Kompensierung unterdrückt werden, so dass die Lebensdauer des Displays verlängert wird

FINE-GRAINED DYNAMIC VOLTAGE SCALING ON OLED DISPLAY

Organic Light Emitting Diode (OLED) has emerged as a new generation of display techniques for mobile devices. Emitting light with organic fluorescent materials OLED display panels are thinner, brighter, lighter, cheaper and more power efficient, compared to other display technologies such as Liquid Crystal Displays (LCD). In present mobile devices, due to the battery capacity limitation and increasing daily usage, the power efficiency significantly affect the general performance and user experience. However, display panel even built with OLEDs is still the biggest contributor to a mobile device’s total power consumption. In this thesis, a fine-grained dynamic voltage scaling (FDVS) technique is proposed to reduce the OLED display power consumption. In bottom level, based on dynamic voltage scaling (DVS) power optimization, a DVS-friendly AMOLED driver design is proposed to enhance the color accuracy of the OLED pixels under scaled down supply voltage. Correspondingly, the OLED panel is partitioned into multiple display sections and each section’s supply voltage is adaptively adjusted to implement fine-grained DVS with display content. When applied to display image, some optimization algorithm and methods are developed to select suitable scaled voltage and maintain display quality with Structural Similarity Index (SSIM), which is an image distortion evaluation criteria based on human vision system (HVS). Experimental results show that, the FDVS technique can achieve 28.44%~39.24% more power saving on images. Further analysis shows FDVS technology can also effectively reduce the color remapping cost when color compensation is required to improve the image quality of an OLED panel working at a scaled supplied voltage

AMOLED Displays with In-Pixel Photodetector

The focus of this chapter is to consider additional functionalities beyond the regular display function of an active matrix organic light-emitting diode (AMOLED) display. We will discuss how to improve the resolution of the array with OLED lithography pushing to AR/VR standards. Also, the chapter will give an insight into pixel design and layout with a strong focus on high resolution, enabling open areas in pixels for additional functionalities. An example of such additional functionalities would be to include a photodetector in pixel, requiring the need to include in-panel TFT readout at the peripherals of the full-display sensor array for applications such as finger and palmprint sensing

Thin-Film Transistor Integration for Biomedical Imaging and AMOLED Displays

Thin film transistor (TFT) backplanes are being continuously researched for new applications such as active-matrix organic light emitting diode (AMOLED) displays, sensors, and x-ray imagers. However, the circuits implemented in presently available fabrication technologies including poly silicon (poly-Si), hydrogenated amorphous silicon (a-Si:H), and organic semiconductor, are prone to spatial and/or temporal non-uniformities. While current-programmed active matrix (AM) can tolerate mismatches and non-uniformity caused by aging, the long settling time is a significant limitation. Consequently, acceleration schemes are needed and are proposed to reduce the settling time to 20 µs. This technique is used in the development of a pixel circuit and system for biomedical imager and sensor. Here, a metal-insulator-semiconductor (MIS) capacitor is adopted for adjustment and boost of the circuit gain. Thus, the new pixel architecture supports multi-modality imaging for a wide range of applications with various input signal intensities. Also, for applications with lower current levels, a fast current-mode line driver is developed based on positive feedback which controls the effect of the parasitic capacitance. The measured settling time of a conventional current source is around 2 ms for a 100-nA input current and 200-pF parasitic capacitance whereas it is less than 4 μs for the driver presented here. For displays needed in mobile devices such as cell phones and DVD players, another new driving scheme is devised that provides for a high temporal stability, low-power consumption, high tolerance of temperature variations, and high resolution. The performance of the new driving scheme is demonstrated in a 9-inch fabricated display intended for DVD players. Also, a multi-modal imager pixel circuit is developed using this technique to provide for gain-adjustment capability. Here, the readout operation is not destructive, enabling the use of low-cost readout circuitry and noise reduction techniques. In addition, a highly stable and reliable driving scheme, based on step calibration is introduced for high precision displays and imagers. This scheme takes advantage of the slow aging of the electronics in the backplane to simplify the drive electronics. The other attractive features of this newly developed driving scheme are its simplicity, low-power consumption, and fast programming critical for implementation of large-area and high-resolution active matrix arrays for high precision

Low-Power and High-Performance Drivers for OLEDoS Microdisplays

The rapid growth of the microdisplay market, driven by the demand for smartwatches, head-mounted displays in Virtual Reality (VR) and Augmented Reality (AR), and other portable devices, has presented a need to enhance their energy efficiency. This thesis focuses on reducing the power and energy consumption of microdisplays while maintaining display luminance, and image quality; and enhancing key features such as resolution, refresh rate, and color depth. First, a novel driving method and pixel circuit are proposed that reduces the number of subframes in a digitally-driven display. The dual-driver method offers flexibility in different design modes, allowing for the enhancement of various display characteristics. In the low-power mode, the operating frequency is reduced, resulting in decreased dynamic power consumption by the drivers. Experimental results on a proof-of-concept array fabricated using TSMC 65 nm technology demonstrate a significant 39% reduction in power consumption compared to a conventional array. Furthermore, designing the display in other modes yields remarkable improvements, with up to 8.5 times enhancement in refresh rate or resolution. In addition, the high color depth mode presents an opportunity to increase color depth from 8 bits to 14 bits, enhancing the visual experience. Additionally, this thesis investigates power reduction techniques specific to row drivers in microdisplays. Circuit techniques are proposed to recycle energy in the row driver, thereby reducing dynamic power consumption. Measurement results on proof-of-concept arrays implemented in TSMC 65 nm technology reveal substantial reductions of up to 30% in the power consumption of the row driver using different energy recycling techniques. Applying these techniques led to a significant reduction in the dynamic power consumption of the row driver. For instance, employing the direct energy restoration technique resulted in a remarkable decrease of over 45% in the dynamic power consumption of the row driver. Finally, a digital data driver with a data energy recycling feature is presented to further reduce the dynamic power consumption of microdisplays. Measurement results obtained from a proof-of-concept array fabricated using TSMC 65 nm technology demonstrate an average power consumption reduction of 16% in the display’s data driver when subjected to randomly generated test images. This thesis addresses the pressing need for energy-efficient microdisplays, offering innovative driving methods, pixel circuit design, and dynamic power reduction techniques. The proposed solutions provide significant power savings while preserving display quality and enabling enhancements in resolution, refresh rate, and color depth, contributing to extended battery life and improved user experience in portable electronic systems

Wide Bandwidth - High Accuracy Control Loops in the presence of Slow Varying Signals and Applications in Active Matrix Organic Light Emitting Displays and Sensor Arrays

This dissertation deals with the problems of modern active matrix organic light-emitting diode AMOLED display back-plane drivers and sensor arrays. The research described here, aims to classify recently utilized compensation techniques into distinct groups and further pinpoint their advantages and shortcomings. Additionally, a way of describing the loops as mathematical constructs is utilized to derive new circuits from the analog design perspective. A novel principle on display driving is derived by observing those mathematical control loop models and it is analyzed and evaluated as a novel way of pixel driving. Specifically, a new feedback current programming architecture and method is described and validated through experiments, which is compatible with AMOLED displays having the two transistor one capacitor (2T1C) pixel structure. The new pixel programming approach is compatible with all TFT technologies and can compensate for non-uniformities in both threshold voltage and carrier mobility of the pixel OLED drive TFT. Data gathered show that a pixel drive current of 20 nA can be programmed in less than 10usec. This new approach can be implemented within an AMOLED external or integrated display data driver. The method to achieve robustness in the operation of the loop is also presented here, observed through a series of measurements. All the peripheral blocks implementing the design are presented and analyzed through simulations and verified experimentally. Sources of noise are identified and eliminated, while new techniques for better isolation from digital noise are described and tested on a newly fabricated driver. Multiple versions of the new proposed circuit are outlined, simulated, fabricated and measured to evaluate their performance.A novel active matrix array approach suitable for a compact multi-channel gas sensor platform is also described. The proposed active matrix sensor array utilizes an array of P-i-N diodes each connected in series with an Inter-Digitated Electrode (IDE). The functionality of 8x8 and 16x16 sensor arrays measured through external current feedback loops is also presented for the 8x8 arrays and the detection of ammonia (NH3) and chlorine (Cl2) vapor sources is demonstrated

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from 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 drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from 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 drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month

Review of Display Technologies Focusing on Power Consumption

Producción CientíficaThis paper provides an overview of the main manufacturing technologies of displays, focusing on those with low and ultra-low levels of power consumption, which make them suitable for current societal needs. Considering the typified value obtained from the manufacturer’s specifications, four technologies—Liquid Crystal Displays, electronic paper, Organic Light-Emitting Display and Electroluminescent Displays—were selected in a first iteration. For each of them, several features, including size and brightness, were assessed in order to ascertain possible proportional relationships with the rate of consumption. To normalize the comparison between different display types, relative units such as the surface power density and the display frontal intensity efficiency were proposed. Organic light-emitting display had the best results in terms of power density for small display sizes. For larger sizes, it performs less satisfactorily than Liquid Crystal Displays in terms of energy efficiency.Junta de Castilla y León (Programa de apoyo a proyectos de investigación-Ref. VA036U14)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA013A12-2)Ministerio de Economía, Industria y Competitividad (Grant DPI2014-56500-R