AMOLED Research Articles

A Study of Device Parameters Affecting the Current Error Rate in a Low-Temperature Polycrystalline Silicon Thin-Film Transistor Pixel Circuit for Active-Matrix Organic Light-Emitting Diode Display Applications

In active-matrix organic light-emitting diode (AMOLED) displays, conventional pixel circuits that compensate for the non-uniformity of the threshold voltage (VT) of low-temperature polycrystalline silicon thin-film transistors (TFTs) can hardly compensate for variations in other TFT parameters, such as carrier mobility (μ0), subthreshold swing (SS) and the various effects of parasitic capacitance. In recent high-resolution AMOLED displays, as the current required for OLED pixel driving decreases, the current error rate (CER) caused by the non-uniform TFT parameters increases. In this study, we analyzed the influence of each TFT parameter on the CER using SPICE simulation. Based on our analysis, the origin of the increased CER can be classified into two categories: the charging capability of driving TFT and the capacitive coupling effect of the switching TFT. The SS of the driving TFT and the parasitic capacitance of the switching TFT are major factors that affect the CER in terms of the charging capability and capacitive coupling effect, respectively. Our analysis results can be summarized as follows: The SS value of the driving TFT should be high, and its variation should be small to minimize the CER. The variation in the parasitic capacitance of the switching TFT possibly occurs due to long-term bias conditions, as well as process non-uniformity. Therefore, the stability of TFT should also be confirmed for the prevention of anomalous CER caused by long-term bias stress.

Image quality enhancement in variable refresh rate LTPO-based AMOLED displays using a gate in panel voltage compensation scheme

This study investigated novel driving scheme for low-temperature polycrystalline silicon and oxide (LTPO) active-matrix organic light-emitting diode (AMOLED) displays under variable refresh rate (VRR) driving conditions with aim of reducing the brightness instability (i.e., flicker and luminance variation) for seamless performance. The mechanisms underlying brightness fluctuations in displays were examined, focusing on frequency-dependent differences in gate-inpanel (GIP) operations. Notably, our study revealed additional factors within the GIP operation that cause LTPO flicker and VRR performance, in addition to previously identified factors of thin-film transistor hysteresis and OLED charging. Consequently, variable GIP voltage driving method that reduced these issues and enhanced display performance was proposed. The proposed method could minimize brightness fluctuations to levels imperceptible to users and facilitate performance improvement through driving options in peripheral circuits without modifying panel design and is applicable across various applications utilizing LTPO OLED technology. Its driving scheme was validated through simulation and measurement of luminance variation during frequency transition using 6.0-inch LTPO-applied AMOLED displays. The experimental results indicated a decrease in luminance difference from 14.2% to 1.5%, particularly at extremely low brightness level of 0.2 nit. The luminance distortion decreased from 7 to less than 1, which is a just noticeable color difference (JNCD).

Low Power Emission Pulse Generation Circuit Based on n-Type Amorphous In-Ga-Zn-Oxide Transistors for Active-Matrix Organic Light-Emitting Diode Displays.

This paper presents a low power emission (EM) pulse generation circuit using n-type amorphous In-Ga-Zn-Oxide (a-IGZO) semiconductor thin-film transistors (TFTs). The low power consumption is achieved by avoiding the shoot-through current paths through an optimized inverter circuit. The proposed circuit consists of 12 TFTs and 2 capacitors including 6 TFTs and 1 capacitor for the inverter circuit to control the pulling-down TFTs. In addition, the wider variance range of the threshold voltage (Vth) from -4 V to 2.5 V is covered by additional 6 TFTs for series-connected two transistor (STT) schemes and two low supply voltages to take into account the negative Vth of depletion-mode TFTs. The simulation of 30 EM circuits is conducted over a 6.1-inch active-matrix organic light-emitting diode display of 120 Hz refresh rate and 3840 × 2160 (UHD) resolution. The power consumption of the EM circuit with the proposed inverter is measured at the low values from 0.836 mW to 0.568 mW over pulse widths from 3 to 2157 horizontal times. It is ensured that the proposed circuit achieves the low power consumption regardless of pulse widths.

Low-voltage operatable top-gated organic transistors based on the solution-processed binary polymers dielectric for simplifying the manufacturing flow of arrayed AMOLED

The complex process flow is an important factor that hinders the development of active-matrix organic light-emitting diode (AMOLED) displays. Organic thin-film transistors (OTFTs) are one of the promising candidates as the pixel circuits for AMOLEDs. Both the architecture and the fabrication of OTFTs are crucial elements to determine the process flow and cost of the AMOLEDs. In this Letter, we develop a strategy to significantly simplify the process flow and reduce the cost of AMOLEDs by constructing top-gated OTFTs with a solution-processed vertically phase separated binary polymer dielectric as the pixel circuits. The design on the OTFTs considers both the process flow and the device performance in terms of mobility and operating voltages. The mechanism to improve device performances is discussed. Finally, a 3 × 4 arrayed AMOLED is demonstrated, in which a high mobility over 0.3 cm2/Vs is obtained on the switching and driving OTFTs, and luminance over 300 cd/m2 is achieved on the OLEDs at the supplied low operating voltages of 10 V. This strategy provides a competitive technological route for the manufacturing of AMOLEDs.

Vertically stacked skin-like active-matrix display with ultrahigh aperture ratio

Vertically stacked all-organic active-matrix organic light-emitting diodes are promising candidates for high-quality skin-like displays due to their high aperture ratio, extreme mechanical flexibility, and low-temperature processing ability. However, these displays suffer from process interferences when interconnecting functional layers made of all-organic materials. To overcome this challenge, we present an innovative integration strategy called “discrete preparation-multilayer lamination” based on microelectronic processes. In this strategy, each functional layer was prepared separately on different substrates to avoid chemical and physical damage caused by process interferences. A single interconnect layer was introduced between each vertically stacked functional layer to ensure mechanical compatibility and interconnection. Compared to the previously reported layer-by-layer preparation method, the proposed method eliminates the need for tedious protection via barrier and pixel-defining layer processing steps. Additionally, based on active-matrix display, this strategy allows multiple pixels to collectively display a pattern of “1” with an aperture ratio of 83%. Moreover, the average mobility of full-photolithographic organic thin-film transistors was 1.04 cm2 V−1 s−1, ensuring stable and uniform displays. This strategy forms the basis for the construction of vertically stacked active-matrix displays, which should facilitate the commercial development of skin-like displays in wearable electronics.

Driving scheme for residual image reduction in active-matrix organic light-emitting diodes display

Residual image is a frequent issue in active-matrix organic light-emitting diode (AMOLED) display, due to hysteresis effects of the internal devices. Up to now, some optimized methods have been researched mainly from process perspective. However, the approaches from driving scheme perspective and their electrical mechanism have rarely been demonstrated systematically. In this work, a technical proposal has been proposed to separate the influences of different devices including both thin film transistors (TFTs) and OLEDs furtherly. It was found that the current curve gap was nearly equal to the luminance curve gap and the main factor of residual image was shown to be the hysteresis of TFTs. An equivalent circuit of two thin film transistors and one capacitor (2T1C) was also adopted to substantiate the influences of threshold voltage (Vth) compensation for reducing residual image, and a series of experiments of tuning capacitance were carried out for corresponding compensation improvements. Moreover, other driving scheme optimizations were also studied to reduce residual image furtherly, including increasing the data input time and resetting the driving TFTs. The research opens the possibility of considering reduction of residual image from driving scheme perspective and more systemic analysis from the internal electrical mechanism in AMOLED display.

64‐2: The Effect of Poly Silicon Grain Boundary Reduction on LTPS Devices and Display Effects Applied to Flexible AMOLED

Low‐temperature polycrystalline silicon (LTPS) thin‐film transistor devices can be used as core components of pixel driving circuits for active matrix organic light‐emitting displays （AMOLED）. Poly silicon (P‐Si) film and device performance directly affects the display effect. This paper investigates polysilicon grain boundary reduction for LTPS devices on PI substrates, exploring the effects of grain boundary reduction on LTPS devices and image sticking. Reduction of polysilicon grain boundaries through optimization of a‐Si deposition process, a‐Si two‐step deposition and excimer laser annealing (ELA) doublescan process, ELA double‐scan process reduces grain boundaries by 67 %. We prepared LTPS devices with thinner gate interlayers (GI) based on P‐Si grain boundary reduction, LTPS devices with positive threshold voltage bias and increased mobility, AMOLED displays with thinner GI can have 23% better image sticking.

77‐3: Power‐Efficient and High‐Color‐Gamut RGBY AMOLED Displays

We present a novel four color sub‐pixel active matrix organic light emitting Device (AMOLED) display that uses only three OLED depositions while enabling very high color gamut with greatly reduced power consumption. Achieving 90% of Rec. 2020 color standard, power consumption is reduced by 39% compared to that of a conventional RGB side‐by‐side top emission (TE) pixel architecture.

P‐72: A Bidirectional Gate Driver Circuit with Scan Signal for Low‐Frame‐Rate LTPO AMOLED Displays

This paper presents a bidirectional gate driver circuit based on low‐temperature polycrystalline oxide thin‐film transistors for a low‐frame‐rate AMOLED display. Simulation results indicate the falling and rising times of the output waveform are all below 3.48 μs and 0.56 μs with the threshold voltage variations of TFTs, ensuring the stability of the proposed circuit. The output waveforms are also generated successfully for a frame rate of 1 Hz display panel and have no fluctuation in consecutive frames. Therefore, the proposed circuit is suitable for a low‐frame‐rate AMOLED display.

P‐219: Research on the Direction of Luminance Modulation at Small Angles Based on Micro Lens

The study investigated the variations in luminance of active‐matrix organic light‐emitting diodes (AMOLED) when viewed at small angles, employing different micro lens arrangements. By examining the micro lens array (MLA) structure, we analyzed how luminance behaves at various angles. Concurrently, we explored the alignment of red, green, and blue (R/G/B) pixel apertures with the micro lens placement, aiming to achieve precise luminance modulation for acute viewing angles.Initially, micro lens arrays are crafted by utilizing materials with varying refractive indices above the pixel aperture, ensuring total internal reflection of light at thejunction between materials of high and low refractive indices. This arrangement also alters the emission angle of light. Within this architecture, the critical angle — determined by the disparity in refractive indices — and the variance in the angle at the reflective interface are crucial in influencing light emission reflection.Furthermore, we explored how the spacing of graphic openings with low refractive indices influences the angle of light emission. This spacing affects the reflection angle, as it is not produced by a singular interface but rather by a composite of multiple angles. The size of the pixel apertures and the spacing of these openings cause the point of total internal reflection to vary, altering the deflection angle of light. Thus, the design of pixel openings and the external expansion is crucial to controlling the light emission angle.Lastly, we examined the influence of the reflective interface and the external expansion design on the light output angle. This analysis highlighted the enhancement of light output at narrow angles and provided strategies to minimize luminance fluctuations at such angles, contributing to the development of high‐precision luminance control in AMOLED displays.

80‐4: Highly Reliable, As‐Grown Crystalline InGaZnO TFTs by Spray Pyrolysis for Low‐Cost Manufacturing of High‐Resolution AMOLED Display

We report a low‐cost method for growing highly‐oriented crystalline InGaZnO (C‐IGZO) for a high‐resolution, active‐matrix thin‐film transistor (TFT) backplane for organic light‐emitting diode displays using spray pyrolysis. The self‐aligned, coplanar oxide TFT with an as‐grown C‐IGZO channel layer demonstrates a threshold voltage of‐0.35 V, saturation mobility of 33.99 cm2 V‐1 s‐1 , and subthreshold swing of 0.15 Vdec‐1, with an on/off current ratio exceeding 108 . Moreover, the C‐IGZO TFT with a channel length of 1.16 μm exhibits excellent operational stability under bias temperature stress.

14‐2: Development of High Mobility and Reliability Metal Oxide TFT for 13.2 inch AMOLED Display

The 13.2 inch AMOLED panel with 262 ppi has been fabricated with the high mobility metal oxide TFTs (thin film transistors). The high mobility oxide TFTs have high mobility ~34cm2 /Vs. On Gen. 6 glass, a small Vth variation with channel length 3.5um has been obtained. Moreover, the high mobility oxide TFT shows small Vth shift under positive/negative bias temperature stress (PBTS/NBTS).

55‐1: A Novel Modeling & Compensation Algorithm for Medium‐Term Image Sticking on AMOLED Display

In this paper, we discuss the different type of image sticking(IS) and propose a novel algorithm for improving medium‐term IS on AMOLED display. This method could not only solve the IS effect on traditional display but also the multi‐areas frame rate (MAFR) display.For future OLED power‐saving applications, the application of MAFR becomes increasingly important. This involves categorizing the same display into high and low refresh rate zones based on display usage efficiency. When the display remains static for a period and then switches, the issue of display retention may arise. Our experimental results confirms that IS can be quantified into a model, and the algorithm is used for real‐time compensation to reduce the retention issues.

P‐34: The Effect Analysis of Display Quality According to the AMOLED Pixel Arrangement

In the AMOLED display, many factors affect the user's visual perception, such as color ratio, refresh rate and drive ability of array circuit, pixel density and so on. In the past, the primary idea to improve the precision and smoothness of the display quality is to increase the pixel per inch (PPI) and optimize the sub pixel rendering (SPR) algorithm between pixel units. At present, the PPI has gradually stabilized at 400+ grade for mobile phone display, and the IC driver algorithm corresponding to the pixel arrangement of each panel factory has also stabilized gradually, therefore, more efforts are needed to optimize the pixel arrangement itself to further improve the quality of display panel without taking into account the above two factors. Bear this in mind, we systematically summarized the common pixel patterns to extract the key characteristics of pixel arrangement benefited to the display quality by simulating and analyzing the display effect, and then put forward several suggestions for the subsequent pixel arrangement design.

P‐124: The IR‐DROP Compensation Method of AMOLED Display under Ultra‐High Brightness

This paper proposes a method to enhance the sunlight readability of AMOLED displays under ultra‐high brightness, and simultaneously improves the global IR‐drop (GIR) issue in light‐loading patterns and the local IR‐drop (LIR) issue in heavy‐loading patterns. In the GIR part, color difference occurs in the brightness increase from high to low APL. Extending to the LIR part, it is found that the uniformity does not exhibit a linear decrease from common brightness to ultra‐high brightness. Finally, based on the approach proposed in this paper, the luminance of OPR 5% is increased to from 1500nits to 3323 nits and color difference is improved from 0.0226 to 0.0012. Besides, the uniformity is enhanced from 61.8% to 86.5% under ultra‐high brightness. Additionally, the proposed improvement compensation method was developed and implemented in driver IC.

P‐6.14: Improvement of Image Sticking Performance in AMOLED Display by Extending Scan Pulse Width via a Novel Driving Circuit

P‐6.14: Improvement of Image Sticking Performance in AMOLED Display by Extending Scan Pulse Width via a Novel Driving Circuit

27‐3: Progress in High‐Performance AMOLED Display with ViP™ Technology

ViP™ (Visionox intelligent Pixelization) Technology is a novel RGB pixelization approach without Fine Metal Mask for OLED patterning. ViP™technology was released in May 2023. In this paper, some key features of display performance and mass production outlook of ViP™ Technology will be discussed.

P‐5.7: Examining Pressure‐sensitive Damping Silicone Adhesive with Excellent Cushioning Capabilities for AMOLED Display

P‐5.7: Examining Pressure‐sensitive Damping Silicone Adhesive with Excellent Cushioning Capabilities for AMOLED Display

Effect of sodium gluconate addition on anomalous codeposition of electroplated nickel-iron alloys

Invar nickel-iron alloy has been widely applied in the fields of electronics, sensors, communications and optics, due to its low thermal expansion coefficient, high hardness, high ductility and other characteristics according to product requirements. The application in metal stencil during the manufacturing process of active-matrix organic light-emitting diodes is mainly due to their good electrical conductivity and soft magnetic properties. In this study, sodium gluconate is used as a substitute for boric acid in the acidic electroplating of nickel-iron alloys; the concentration ratio of nickel-iron in the plating bath is 2:1. After adding sodium gluconate, the precipitation of nickel varies significantly. The iron content in the weight percentage of the deposited nickel-iron alloy is obviously larger than nickel, even up to 3 times. The whole deposition process is in the state of anomalous codeposition, and the selectivity ratio reaches 3 or more; the nickel-iron alloy deposited by adding sodium gluconate to the plating bath has a great influence on the material properties of the nickel-iron alloy plating, The nickel-iron alloy deposited at -1.4 V exhibits a hardness of approximately 3.17 GPa and a modulus of elasticity about 37.88 GPa. Notably, the resistance to plastic deformation of the alloy deposited in sodium gluconate plating solution (H3/U2=0.022) is significantly higher than that in boric acid plating solution (H3/U2=0.007). The experimental results also show that it has the best ability to resist plastic deformation.

Tailoring Subthreshold Swing in A-IGZO Thin-Film Transistors for Amoled Displays: Impact of Conversion Mechanism on Peald Deposition Sequences.

Amorphous IGZO (a-IGZO) thin-film transistors (TFTs) are standard backplane electronics to power active-matrix organic light-emitting diode (AMOLED) televisions due to their high carrier mobility and negligible low leakage characteristics. Despite their advantages, limitations in color depth arise from a steep subthreshold swing (SS) (≤ 0.1 V/decade), necessitating costly external compensation for IGZO transistors. For mid-size mobile applications such as OLED tablets and notebooks, it is important to ensure controllable SS value (≥ 0.3 V/decade). In this study, a conversion mechanism during plasma-enhanced atomic layer deposition (PEALD) is proposed as a feasible route to control the SS. When a pulse of a diethylzinc (DEZn) precursor is exposed to the M2O3 (M = In or Ga) surface layer, partial conversion of the underlying M2O3 to ZnO is predicted on the basis of density function theory calculations. Notably, significant distinctions between In-Ga-Zn (Case I) and In-Zn-Ga (Case II) films are observed: Case II exhibits a lower growth rate and larger Ga/In ratio. Case II TFTs with a-IGZO (subcycle ratio of In:Ga:Zn = 3:1:1) show reasonable SS values (313mV decade-1) and high mobility (µFE) of 29.3cm2 Vs-1 (Case I: 84mV decade-1 and 33.4cm2 Vs-1). The rationale for Case II's reasonable SS values is discussed, attributing it to the plausible formation of In-Zn defects, supported by technology computer-aided design (TCAD) simulations.