AMOLED Research Articles

Recent research on the status and advances of OLED

Organic Light-Emitting Diodes (OLEDs) have already been a prominent display technology in various applications for many decades, ranging from smartphones and televisions to wearable devices and lighting. It is important to understand the current state of OLED research in order to assess its progress, identify challenges, and explore more potential ways for further development. In this paper, the significance and widespread adoption of OLED technology are briefly introduced, and its advantages over traditional display technologies are highlighted. The paper then provides a concise overview of the major components and working principles of OLEDs without delving into basic notions. The paper discusses both small molecule OLEDs (SMOLEDs) and polymer OLEDs (POLEDs), emphasizing their unique structure, characteristics and properties. Different strategies employed to enhance OLED performance is investigated, including the regulation of band gaps that influence the efficiency and brightness. This paper also focuses on the evaluation of device reliability and stability aspects. The techniques and materials employed in studies related to OLED development are also discussed with many searched experimental data and ongoing research activities. Furthermore, the research content covered the aspects of recent commercial advancements in OLED technology, such as Active matrix OLEDs (AMOLED) and Passive matrix (PMOLED). The review provides a guideline for researchers, industry professionals, and enthusiasts to better understand the OLED technology into its details, potential, and the areas that require further exploration.

Nonvolatile Memory Organic Light-Emitting Transistors.

In the field of active-matrix organic light emitting display (AMOLED), large-size and ultra-high-definition AMOLED applications have escalated the demand for the integration density of driver chips. However, as Moore's Law approaches the limit, the traditional technology of improving integration density that relies on scaling down device dimension is facing a huge challenge. Thus, developing a multifunctional and highly integrated device is a promising route for improving the integration density of pixel circuits. Here, a novel nonvolatile memory ferroelectric organic light-emitting transistor (Fe-OLET) device which integrates the switching capability, light-emitting capability and nonvolatile memory function into a single device is reported. The nonvolatile memory function of Fe-OLET is achieved through the remnant polarization property of ferroelectric polymer, enabling the device to maintain light emission at zero gate bias. The reliable nonvolatile memory operations are also demonstrated. The proof-of-concept device optimized through interfacial modification approach exhibits 20 times improved field-effect mobility and five times increased luminance. The integration of nonvolatile memory, switching and light-emitting capabilities within Fe-OLET provides a promising internal-storage-driving paradigm, thus creating a new pathway for deploying storage capacitor-free circuitry to improve the pixel aperture ratio and the integration density of circuits toward the on-chip advanced display applications.

An Active-Matrix Organic Light-Emitting Diode Pixel Circuit Featuring Mobility Compensation for Portable Applications.

A 6T1C pixel circuit based on low-temperature polycrystalline oxide (LTPO) technology for portable active-matrix organic light-emitting diode (AMOLED) display applications is proposed in this paper. For superior high-end portable applications including 4K high resolution and high PPI (pixels per inch), the proposed pixel circuit employs a single storage capacitor and signal sharing switch-control design and provides low-voltage driving and immunity to the IR-drop issue and OLED degradation. Furthermore, the threshold voltage and mobility-compensating capabilities are improved by both compensation mechanisms, which are based on a negative feedback system, and mobility-related compensation parameters. Simulation results reveal that threshold voltage variations of ±0.33 V in the driving thin-film transistors can be well sensed and compensated while the maximum OLED current shift is 4.25%. The maximum variation in OLED currents within all gray levels is only 1.05% with mobility variations of ±30%. As a result, the proposed 6T1C pixel circuit is a good candidate for portable AMOLED display usage.

Opportunities for cost-effective manufacturing of fully printed high performance displays enabled by vertical light-emitting transistor pixels

With the vertical organic light-emitting transistor (VOLET), we introduce a promising solution that could significantly benefit the manufacturing of displays, accelerating the wide adoption of flexible and printed electronics. The VOLET—like conventional, lateral channel, organic thin film transistors—is compatible with a variety of printing techniques as well as flexible substrates and low-temperature processing. In combination these devices will enable a more cost-effective approach to mass-production that can dramatically extend the market potential of active-matrix organic light-emitting diode (AMOLED) displays. In this paper we discuss the prospects that AMOLED presents for the future of the display market, with a focus on the innovative VOLET device architecture. We assess how the integration of this device into active-matrix displays can contribute to the long range sustained competitiveness of AMOLED technology. We review recent progress in mass production techniques for printed electronics, with a particular emphasis on large-scale carbon nanotube material deposition. Finally, we explore the prospects for fully printed active-matrix light-emitting displays, including a review of high-performance printed components whose integration could facilitate the mass production of low-cost, high-performance, VOLET based AMOLEDs.

Reducing Leakage Current Using LTPS-TFT Pixel Circuit in AMOLED Smartwatch Displays

This work proposes a new pixel circuit that is operated at a low frame rate using low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) for use in active-matrix organic light-emitting diode (AMOLED) smartwatch displays. The proposed circuit uses a new leakage-prevention method (LPM) to balance the off currents at the gate node of the driving TFTs, eliminating the drop of the driving voltage. Uniform OLED currents are thus generated throughout the extended emission period. Also, the OLEDs are all turned off except in the emission stage to suppress image flicker. To confirm the feasibility of the proposed pixel circuit, a 1.41-inch panel, based on the proposed circuit, is fabricated. Experimental results demonstrate that diminishing the off currents maintains the driving capability of the pixel circuit to stabilize the luminance of the fabricated panel. The exhibitions of red, green, blue, and white images at frame rates from 60 Hz to 15 Hz are uniform, verifying that the proposed pixel circuit with LPM has high potential for use in AMOLED smartwatch displays with a low frame rate.

Carrier Transport Regulation of Pixel Graphene Transparent Electrodes for Active-Matrix Organic Light-Emitting Diode Display.

Integrating a graphene transparent electrode (TE) matrix with driving circuits is essential for the practical use of graphene in optoelectronics such as active-matrix organic light-emitting diode (OLED) display, however it is disabled by the transport of carriers between graphene pixels after deposition of a semiconductor functional layer caused by the atomic thickness of graphene. Here, the carrier transport regulation of a graphene TE matrix by using an insulating polyethyleneimine (PEIE) layer isreported. The PEIE forms an ultrathin uniform film (≤10nm) to fill the gap of the graphene matrix, blocking horizontal electron transport between graphene pixels. Meanwhile, it can reduce the work function of graphene, improving the vertical electron injection through electron tunneling. This enables the fabrication of inverted OLED pixels with record high current and power efficiencies of 90.7cd A-1 and 89.1lm W-1 , respectively. By integrating these inverted OLED pixels with a carbon nanotube-based thin-film transistor (CNT-TFT)-driven circuit, an inch-size flexible active-matrix OLED display isdemonstrated, in which all OLED pixels are independently controlled by CNT-TFTs. This research paves a way for the application of graphene-like atomically thin TE pixels in flexible optoelectronics such as displays, smart wearables, and free-form surface lighting.

P‐156: Late‐News Poster: Effect of Light Emission Delay on AMOLED Panel

The Active Matrix Organic Light Emitting Diode (AMOLED) display causes the current deviation between pixels due to the dispersion based on device characteristics such as Threshold voltage (Vth) of Thin Film Transistors (TFT). This dispersion is compensated with pixel circuits inside the AMOLED panel, but the compensation time gradually decreases because of the performance improvement of AMOLED display such as high resolution and high‐speed driving, and as a result, it is difficult to solve the problem of the current deviation at low luminance which is using low current. The deviation of charging time for the capacitance of OLED (OLED Cap) by the current deviation based on device characteristic causes the light emission delay deviation, which in turn causes Mura on the panel. In this paper, we analyze the effect of the light emission delay deviation on Mura of AMOLED panel.

Fine-Tunable Emission Pulse Generation Circuit Based on p-Type Low-Temperature Poly-Si Thin-Film Transistors for Active Matrix Organic Light-Emitting Diode Displays

Active matrix organic light-emitting diode (AMOLED) displays have deployed the compensation techniques to cope with the luminance non-uniformity issues caused by variations on electrical characteristics of thin-film transistors (TFTs). While some compensation circuits require control signals of longer pulse widths than a line time, the luminance control as well as the higher bit depth representation have been also accomplished by adjusting the pulse widths. A proposed EM pulse generation circuit consists of 11 p-type low-temperature poly-Si (LTPS) TFTs and a one coupling capacitor. While previous tunable circuits could address pulse widths of either even or odd multiples of a line time, the proposed circuit can generate any multiples by changing phases of additional clock signals. In particular, the internal inverter is implemented with a load connected to the output of a previous stage’s inverter to reduce the power consumption over pulse widths. The coupling capacitor is also connected between two adjacent stages through one TFT, eliminating coupling noises on the output pulses. The proposed EM circuit is simulated at a line time of 3.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> s for a 120 Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{3840}$</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{2160}$</tex-math> </inline-formula> display. The results ensure that pulse widths from three to 2160 lines are generated successfully without coupling noises and the small variation on power consumption from 1.23 to 0.33 mW is achieved at 30 stages for the whole range of pulse widths, compared to the large range from 1.26 to 76.91 mW with the inverter of a diode-connected load.

P‐126: An Introduction to GGRB Virtual Pixel Technology

The constructions of the GGRB virtual pixel and the effect on the display performance are introduced by comparing with that of the Diamond RGBG virtual pixel, a typical virtual pixel technology applied to active matrix organic light‐emitting diode (AMOLED) display. Because of the characteristic structure of GGRB sub‐ pixel arrangement, the occurrence of perceptible artificial color discontinuity is reduced in the GGRB virtual pixel display. The distribution uniformity of pixel luminance centroids is important for the display of pattern details. In the GGRB virtual pixel display, the distribution uniformity of pixel luminance centroids is achieved via a non‐uniformly distributed green sub‐pixel array of which the luminance profile is properly modulated by the red and blue sub‐pixels with lower spatial frequency distribution. The results of subjective evaluation and comparison suggest that the performance improvement of the GGRB virtual pixel display is consistent with the conceptual expectation. Some of the essential characteristics is superior to those of the reference technology.

P‐11: A Novel AMOLED Pixel Circuit using Double‐Gate LTPO TFTs for Variable Refresh Rate with Low Power Consumption

This paper proposes an Active Matrix Organic Light Emitting Diode (AMOLED) pixel circuit for Variable Refresh Rate (VRR) driving from 1Hz to 240Hz. The circuit is based on a double‐gate Low Temperature Polycrystalline Silicon and Oxide (LTPO) Thin Film Transistor (TFT) backplane. For low frame rate driving we used oxide TFTs as the Switching (SW) TFTs. Moreover, we applied double‐gate Low Temperature Polycrystalline Silicon (LTPS) process to the Driving (DR) TFT for compensation of the Threshold Voltage (Vth) by controlling the bottom gate bias at a high refresh rate. In this paper, operation principle and simulation results will be illustrated to demonstrate the performance of the proposed circuit.

7‐1: A New a‐IGZO TFT Pixel Circuit for High‐Resolution Mobile AMOLED Display with Highly Stable at Low Gray Levels

This paper proposes a highly stable amorphous indium‐gallium zinc‐oxide (a‐IGZO) thin‐film transistor (TFT) pixel circuit for the active‐matrix organic light‐emitting diode (AMOLED) displays, which can compensate for the non‐uniform threshold voltage (VTH) of the TFT at a low gray level. The stable operation was achieved by minimizing the operation number of the switch connected to the VTH storage capacitor after the VTH compensation stage. The HSPICE simulation verified that the relative current error rate is within only ‐15.7% at 567 pA (13 gray level) against the VTH variations.

24‐4: Invited Paper: A study of flickering in the low‐frame‐rate driven AMOLED

The flickering of low‐temperature polycrystalline silicon and metal‐oxide (LTPO) active‐matrix organic light‐emitting diode (AMOLED) display under low‐refresh‐rate driving condition can be detected even though there is no critical leakage current path from the storage capacitor. The flickering is mainly due to the brightness increase during data holding period. The effective channel length of driving thin‐film transistor (DTFT) would be reduced by electron trapping into the gate insulator of DTFT which operates under saturation mode. This study presents not only the physical model of electron trapping but also a novel driving method to release the trapped electrons. The experimental results show that that the brightness variation can be reduced from 3.7 % to 1.4 % at the 63rd gray level by the novel driving method.

25‐2: Subpixel Optimization in AMOLED Displays by Self‐assembled Patterning of a Secondary Cathode

AMOLED display performance has been highly optimized around the blue subpixel due to the weaker performance of blue emitters than that of red and green emitters. A subpixel optimization approach was demonstrated using the Selfassembled patterning of a secondary cathode. With the introduction of the secondary cathode for the red subpixel, the red device performance can be significantly improved without sacrificing the optimal performance of the blue subpixel. Moreover, this design can also dramatically reduce the sheet resistance which is critical for middle size top‐mission AMOLED panels for applications like tablets and laptops.

15‐2: A Novel Compensation Algorithm for Multi‐Frame‐Drop on AMOLED Display

In this paper, a novel compensation algorithm is proposed for improving multi‐frame drop (MFD) on AMOLED display. The MFD phenomenon occurs as the image switching or sliding when the display with extremely low brightness, resulting in the color shift and distortion at the edge of moving objects. Because the current is too small and the emission time is too short at extremely low brightness, more than one frame is needed to alleviate the impact of the parasitic capacitance between the gate and drain node of driving TFT (DTFT), and the gradual changing of drain voltage (VD) causes the DTFT to move from the linear to saturation region, which also aggravates this phenomenon. This paper analyzes the MFD phenomenon based on the existing 7T1C pixel circuits, records the grayscale changes of multiple frames, and gives the different offsets at dropped frames. By experiment results, this method is not changing the pixel circuit, such as the 8T1C, which could compensate for the dropped frames for reducing MFD issues and is easily implemented in the display driver IC for many different OLED panels.

32‐1: Invited Paper: A Mask‐Reduction Process With Innovative Undercut for Large Size AMOLED Display

We propose a mask reduction process using triple‐metal TFT source‐drain layers by removing the protective pad layer. In this process flow, an innovative undercut structure is fabricated, which could successfully contact the auxiliary electrodes with topemission OLED cathode to reduce IR‐drop in large size AMOLED display.

A novel LTPO AMOLED pixel circuit and driving scheme for variable refresh rate

This paper proposes a novel pixel circuit and driving scheme that adopts low-temperature polycrystalline silicon and oxide thin-film transistors (LTPO TFTs) for mobile devices using active-matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit and driving scheme provide uniform luminance and render flicker invisible at variable refresh rates (VRRs) from 1 to 120 Hz. Using the proposed pixel circuit with extended compensation time (tCOMP) improves the luminance uniformity at a high-frame rate. The proposed driving scheme applies a voltage to the driving TFTs (D-TFTs) higher than the programmed data voltage during the skip frame. This reduces the flicker caused by the hysteresis of D-TFTs during low-frame rate driving. A 6.0-inch quad high-definition (QHD) LTPO-based AMOLED display was fabricated using the new pixel circuit and driving scheme. Experimental results of the proposed pixel circuit show that the standard deviation of luminance was reduced from 0.056 to 0.008 by extending tCOMP from 2 to 8 µs. The flicker level was −51 dB, so there was no visual artifact during 1 Hz driving. A flicker-free LTPO-based AMOLED display with low power consumption is possible; driving can proceed in 1–120 Hz range.

High‐performance, coplanar amorphous InGaZnO thin‐film transistors by spray pyrolysis on polyimide substrate for low‐cost manufacturing of foldable active‐matrix organic light emitting diode display

AbstractWe report a method for low‐cost deposition of bubble‐free, high‐quality amorphous InGaZnO (a‐IGZO) on polyimide (PI) substrate for foldable active‐matrix organic light emitting diode (AMOLED) display by spray pyrolysis. The coplanar thin‐film transistor (TFT) with spray‐pyrolyzed (SP) a‐IGZO on PI substrate exhibits threshold voltage (VTH) of −0.8 V, saturation mobility of 40.95 cm2 V−1 s−1, and subthreshold swing of 0.18 V per decade with on/off drain current ratio of ~108. The high mobility TFT could be achieved using high substrate temperature (350°C) and low contact resistance by NF3 plasma on the SP a‐IGZO. The TFT shows a ΔVTH of −0.4 V when it is bent on a cylinder of 1‐mm radius. The TFT shows the ΔVTH of +0.05 V for positive bias temperature stress at +20 V at 60°C for 1 h. The ring oscillator made of a‐IGZO TFTs exhibits an oscillation frequency of 2.38 MHz at VDD of 10 V with a propagation delay of 9.13 ns per stage. Therefore, the a‐IGZO TFT by spray pyrolysis could be used for low‐cost, high‐performance, and flexible display TFT backplane.

A Pixel Circuit for Compensating Electrical Characteristics Variation and OLED Degradation

In recent years, the active-matrix organic light-emitting diode (AMOLED) displays have been greatly required. A voltage compensation pixel circuit based on an amorphous indium gallium zinc oxide thin-film transistor is presented for AMOLED displays. The circuit is composed of five transistors–two capacitors (5T2C) in combination with an OLED. In the circuit, the threshold voltages of both the transistor and the OLED are extracted simultaneously in the threshold voltage extraction stage, and the mobility-related discharge voltage is generated in the data input stage. The circuit not only can compensate the electrical characteristics variation, i.e., the threshold voltage variation and mobility variation, but also can compensate the OLED degradation. Furthermore, the circuit can prevent the OLED flicker, and can achieve the wide data voltage range. The circuit simulation results show that the OLED current error rates (CERs) are lower than 3.89% when the transistor’s threshold voltage variation is ±0.5V, lower than 3.49% when the mobility variation is ±30%.

P‐5.1: De‐Mura Processing with a High Performance for Low Grayscales in AMOLED Display

Mura artifacts reduction (De‐Mura) processing, with a high performance for low grayscales, is developed and integrated in display drivers for active‐matrix organic light‐emitting diode (AMOLED) display. The proposed De‐Mura processing, designed to compensate an AMOLED panel's luminance Mura (L‐Mura) and color Mura (C‐Mura), shows excellent display performance, and the major display properties include a diminished luminance non‐uniformity under low grayscales (&lt;= Gray 16) and low luminous brightness (&lt;= 2 cd/m 2 ), and an improved visually imperceptible color non‐uniformity. Our works reveals an effective solution to work around the inevitable difficulties of optical measurement of AMOLED panels under low luminous brightness.

High Mobility IZTO Thin-Film Transistors Based on Spinel Phase Formation at Low Temperature through a Catalytic Chemical Reaction.

In this paper, In0.22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) films with a single spinel phase are successfully grown at the low temperature of 300 °C through careful cation composition design and a catalytic chemical reaction. Thin-film transistors (TFTs) with amorphous In0 .22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) channel layers have a reasonable mobility of 41.0 cm2 V-1 s-1 due to the synergic intercalation of In and Sn ions. In contrast, TFTs with polycrystalline spinel In0 .22 Znδ Sn0.78- δ O1.89- δ (δ = 0.55) channel layers, achieved through a metal-induced crystallization at 300 °C, exhibit a remarkably high field-effect mobility of ≈83.2 cm2 V-1 s-1 and excellent stability against external gate bias stress, which is attributed to the uniform formation of the highly ordered spinel phase. The relationships between cation composition, microstructure, and performance for the In2 O3 -ZnO-SnO2 ternary component system are investigated rigorously to attain in-depth understanding of the roles of various crystalline phases, including spinel Zn2- y Sn1- y In2 y O4 (y = 0.45), bixbyite In2-2 x Znx Inx O4 (x = 0.4), rutile SnO2 , and a homologous compound of compound (ZnO)k (In2 O3 ) (k = 5). This work concludes that the cubic spinel phase of Zn2- y Sn1- y In2 y O4 (y = 0.45) film is a strong contender as a substitute for semiconducting polysilicon as a backplane channel ingredient for mobile active-matrix organic light-emitting diode displays.