Flexible Active-matrix Organic Light-emitting Diode Research Articles

Analysis of heat diffusion considering driving images on 6-inch flexible AMOLED display

We report the heat diffusion on flexible active-matrix organic light-emitting diode (AMOLED) displays. Two-dimensional heat diffusion is used for the heat conduction and convection analysis, generated on the surface of a flexible AMOLED display. The heat diffusion parameters and the time constant are studied in terms of the driving conditions of a 6-inch flexible AMOLED display. The temperature distribution on the screen is obtained by applying heat diffusion using the finite difference method (FDM) with an FPGA driving board. We can predict temperature more precisely by calculating temperature changes according to the images. The proposed method improves the accuracy of predicting the temperature. The average and standard deviation of the temperature error (TER) are ∼0.5°C and ∼0.5°C for the proposed method, respectively. For the conventional method, the values are ∼4.5°C and ∼0.7°C, respectively, neglecting the increase in temperature based on the images. This method can be used in compensation technology for OLEDs and thin film transistors (TFTs) that require accurate temperature distribution on the screen.

Effect of Interlayer Stress on U Bends and Water Droplet Bends

The interlayer stress (IS) analysis of the flexible active-matrix organic light-emitting diode (AMOLED) panel is critical for decreasing the stress concentration and reducing the risk of overall screen failure. This article relies on the linear elastic models of ABAQUS to simulate finite element analysis of foldable panels. This article discussed three sensitive locations of the flexible AMOLED panel caused by two different bending methods including U bends and water droplet bends. The finite element method for simulating the layer analysis was constructed to analyze the effect of optically clear adhesive (OCA) film thickness on the distribution of stress and strain. The module can be optimized by changing the thickness and shear modulus of OCA. With the decrease of OCA thickness and the increase of stiffness, the maximum stress of the organic light-emitting diode (OLED) layer is decreased by 20% which is 55 Mpa; meanwhile, the IS of the entire module is reduced by about 30%.

Numerical simulation theory and structural optimization of a flexible AMOLED module subjected to the impact of a falling ball

When the flexible active-matrix organic light-emitting diode (AMOLED) module is subjected to external impact, the key display device is easily damaged, so it is necessary to optimize the module structure to improve the impact resistance of the module. The adhesives in the module make the structural optimization of the module extremely complicated. In this paper, the viscoelastic and hyperelastic constitutive models of adhesives are established, and the method of simplifying the simulation model of the impact test is studied. Through the attenuation of the stress wave in the transmission, the reason why the dead pixels appear in the center of the module impacted by the ball is explained. According to the actual result that the area of dark spots does not expand with time, it is inferred that the AMOLED layer is damaged, and the maximum principal strain of that is extracted for analysis. In order to ensure the impact resistance of the module, the thickness of each layer should be increased to a certain extent. Slightly increasing the modulus of the film on the upper side of the AMOLED layer is likely to reduce the impact resistance of the module, whereas greatly increasing the modulus of that can improve the impact resistance of the module. An experimental method for determining the critical height of the falling ball is proposed, and it proves that the simulation theory and the optimization scheme are reasonable.

Mechanical strain and bias-stress compensated, 6T-1C pixel circuit for flexible AMOLED displays

This paper proposes a novel topology for a pixel circuit with a voltage-programmed threshold voltage compensation technique for amorphous indium gallium zinc oxide thin-film transistor (a-IGZO TFT) used in flexible AMOLED (Active-Matrix Organic Light-Emitting Diodes) displays. Simulation of the proposed circuit was done in Cadence Spectre using SPICE models for a-IGZO TFT, and the circuit utilizes six a-IGZO TFTs and one storage capacitor. Moreover, a detailed analytical analysis of the circuit is also done to estimate the error introduced during the emission phase. The simulation results showed that this pixel circuit offers excellent immunity to threshold voltage variation of driving TFT caused by bending and prolonged operation under gate bias. It can compensate for a wide range of VTH variations, i.e., 0 V–2.1 V, from the original 0.7 V of the TFT model, and the maximum percentage error in OLED current obtained was below 1.14%. The circuit proposed also alleviates the effect of electrical instability caused by threshold voltage shifts due to the compressive and tensile strain in the flexible substrate of the AMOLED display. The OLED current error was found to be below 0.0018% for a ±0.3% strain in the driving TFT.

31-Inch 4K Flexible Display Employing Gate Driver With Metal Oxide Thin-Film Transistors

A 31-inch 4K flexible active-matrix organic light-emitting diode (AMOLED) display integrating with gate driver on the array (GOA) technology has been proposed and fabricated on a polyimide substrate. The GOA circuits, composed of two series-connected thin-film transistors (TFTs) and dual low-voltage power structure, are introduced to decrease the leakage current in TFTs. They are arranged symmetrically along the sides of the display. Inkjet printing (IJP) OLED process is used to reduce the number of sub-pixels from four (R/G/B/W) in commercial white OLED process to three (R/G/B) and a high pixel density in the display (144 pixels per inch) is achieved. Moreover, top-gated (TG) amorphous indium-gallium-zinc oxide (IGZO) TFTs with good electrical and mechanical reliability, have been successfully utilized to decrease the capacitance load in the display and improve signal transfer speed in GOA. Finally, this 31-inch flexible AMOLED display was operated for 500h under accelerating test conditions of 60°C and 90% humidity.

Emerging Self-Emissive Technologies for Flexible Displays.

Featuring a combination of ultrathin and lightweight properties, excellent mechanical flexibility, low power-consumption, and widely tunable saturated emission, flexible displays have opened up a new possibility for optoelectronics. The demands for flexible displays are growing on a continual basis due not only to their successful commercialization but, more importantly, their endless possibilities for wearable integrated systems. Up to now, self-emissive technologies for displays, flexible active-matrix organic light-emitting diodes (flex-AMOLED), flexible quantum dot light-emitting diodes (flex-QLEDs), and flexible perovskite light-emitting diodes (flex-PeLEDs) have been widely reported, but despite the significant progress made in these technologies, enormous obstacles and challenges remain for the vision of truly wearable applications, in particular with flex-QLEDs and flex-PeLEDs. Here, a review of the recent progress of all three self-emissive technologies for flexible displays is conducted, including the emissive active materials, device structures and approaches to manufacturing, the flexible substrates, and conductive electrodes, as well as the encapsulation techniques. The fast-paced improvement made to the efficiency of flexible devices in recent years is also summarized. The review concludes by making suggestions on the future development in this area, and is expected to help researchers in gaining a comprehensive understanding about the newly emerging technologies for flexible displays.

64-3:Distinguished Student Paper: Bulk Accumulation Oxide TFTs for Flexible AMOLED Display with High Yield Integrated Gate Driver

A 4-inch flexible active-matrix organic light-emitting diode (AMOLED) display with a Bulk-Accumulation (BA) oxide thin-film transistors (TFT)-backplane and BA oxide TFT-based high yield integrated gate-driver is demonstrated. The gate-driver with 320 stages exhibit excellent driving ability with rising and falling times less than 1 µs — even under tensile strain induced by bending to a radius of 3 mm— which makes it suitable for flexible mobile AMOLED display applications.

Characterizing mechanical behaviors of a flexible AMOLED during the debonding process

The stress distribution of a flexible active-matrix organic light-emitting diode (AMOLED) display during the debonding process was investigated using finite element analysis. During the fabrication of an AMOLED display, an AMOLED with a polyimide (PI) substrate is detached from a glass carrier; this is a critical process and generally results in failure of the AMOLED. To enhance the yielding rate of AMOLEDs, their stress states generated during the debonding process must be reduced. The interfacial fracture behavior between the PI substrate and glass carrier was characterized on the basis of bimaterial fracture mechanics, and the fracture toughness associated with mode mixity determined through peeling tests was considered a criterion for detaching the AMOLED from the glass carrier. The stress distribution of the AMOLED at the inception of debonding crack extension was evaluated according to fracture toughness. In addition, the parameters possibly influencing the stress states of the AMOLED in the debonding process are discussed.

Carbon Nanotube Flexible Electronics

Being mechanically flexible with superior electrical properties, thin-films of single-wall carbon nanotubes are proven to be one of the most promising material platforms for high performance flexible electronics. In this talk, I will discuss our solution-based platform for wafer-scale assembly of high-purity semiconducting carbon nanotube networks, which leads to low-cost fabrication of large quantity of thin-film transistors (TFTs) with excellent yield, uniformity, and respectable performance on flexible substrates. Based on the semiconducting carbon nanotube TFTs, a wide range of macro-scale system-level electronics have been demonstrated including flexible integrated circuits, flexible active-matrix organic light-emitting diode displays, and a user-interactive electronic skin that can simultaneously map and respond to various forms of stimuli. With emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required, our demonstrations present a unique niche for nanotube electronics by taking full advantage of their superior electrical and physical properties. Our work shows carbon nanotubes’ immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent performance.

Fabrication of Flexible Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors by a Chemical Vapor Deposition-Free Process on Polyethylene Napthalate

We have fabricated flexible amorphous Indium-Gallium-Zinc-oxide (IGZO) thin-film transistors (TFTs) with back-channel-etch structure by a chemical vapor deposition-free process. All the processes are performed well below 160°C on a polyethylene napthalate (PEN) substrate with anodic aluminum oxide as gate dielectric, which can be highly immune to the strain failure. The IGZO-TFTs show field-effect mobility of 11.2 cm2/V s, subthreshold swing (SS) of 0.27 V/decade, low off-state current ∼10 fA, low leakage current <1 pA and high ION/IOFF ratio of 109. Meanwhile, the performance of flexible IGZO-TFTs do not deteriorate during the bending with the curvature radius as 10 mm. Using a PEN as substrate, flexible active-matrix organic light-emitting diode (AMOLED) displays driven by the IGZO-TFTs have been demonstrated.

Research progress of active matrix organic light emitting diode (AMOLED) displays driven by metal oxide thin film transistors (TFT)

Metal oxide thin film transistors (MOTFTs) have been attracting great attention, because TFT technology are suitable for the requirement of novel display technology such as high definition, large screen liquid crystal display (LCD) and active matrix organic light emitting diode (AMOLED) display, with their high mobility, good uniformity, simple process, low process temperature, and low cost. In this article, material, device structure, fabricating process and application of MOTFT are introduced, and factors that influence MOTFT performance have been discussed. We developed a new class of metal oxide materials, which has a highest mobility of 35 cm2/Vs, threshold voltage of 1.63 V, current on/off ratio of 109, and sub threshold swing of 0.21 V/decade. Based on our MOTFT panel, we have developed a first 3~5 inch color AMOLED screen, transparent AMOLED, and flexible AMOLED. This novel MOTFT panel has a booming future in novel display such as TFT-LCD and AMOLED.

Flexible high‐resolution full‐color top‐emitting active‐matrix organic light‐emitting diode display

AbstractWe developed a high‐performance 3.4‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display with remarkably high resolution using an oxide semiconductor in a backplane, by applying our transfer technology that utilizes metal separation layers. Using this panel, we also fabricated a prototype of a side‐roll display for mobile uses. In these AMOLED displays, a white OLED combined with a color filter was used in order to achieve remarkably high resolution. For the white OLED, a tandem structure in which a phosphorescent emission unit and a fluorescent emission unit are serially connected with an intermediate layer sandwiched between the emission units was employed. Furthermore, revolutionary technologies that enable a reduction in power consumption in both the phosphorescent and fluorescent emission units were introduced to the white tandem OLED.

Flexible AMOLEDs with low‐temperature‐processed TFT backplane technologies

AbstractDevelopments of backplane technologies, which are one of the challenging topics, toward the realization of flexible active matrix organic light‐emitting diodes (AMOLEDs) are discussed in this paper. Plastic substrates including polyimide are considered as a good candidate for substrates of flexible AMOLEDs. The fabrication process flows based on plastic substrates are explained. Limited by the temperature that plastic substrates can sustain, TFT technologies with maximum processing temperature below 400 °C must be developed. Considering the stringent requirements of AMOLEDs, both oxide thin‐film transistors (TFTs) and ultra‐low‐temperature poly‐silicon TFTs (U‐LTPS TFTs) are investigated. First, oxide TFTs with representative indium gallium zinc oxide channel layer are fabricated on polyimide substrates. The threshold voltage shifts under bias stress and under bending test are small. Thus, a 4.0‐in. flexible AMOLED is demonstrated with indium gallium zinc oxide TFTs, showing good panel performance and flexibility. Further, the oxide TFTs based on indium tin zinc oxide channel layer with high mobility and good stability are discussed. The mobility can be higher than 20 cm2/Vs, and threshold voltage shifts under both voltage stress and current stress are almost negligible, proving the potential of oxide TFT technology. On the other hand, the U‐LTPS TFTs are also developed. It is confirmed that dehydrogenation and dopant activation can be effectively performed at a temperature within 400 °C. The performance of U‐LTPS TFTs on polyimide is compatible to those of TFTs on glass. Also, the performance of devices on polyimide can be kept intact after devices de‐bonded from glass carrier. Finally, a 4.3‐in. flexible AMOLED is also demonstrated with U‐LTPS TFTs.

Transparent organic thin-film transistors based on high quality polycrystalline rubrene film as active layers

Transparent organic thin-film transistors (OTFTs) with high performance are demonstrated by using high quality polycrystalline 5,6,11,12-Tetraphenylnaphthacene (rubrene) as an active layer, which is prepared by weak epitaxy growth (WEG) method. Benefiting from epitaxial relationship is formed between the inducing layer and the rubrene films, highly oriented and continuous organic polycrystalline thin films with large grains were obtained, which enhances the carrier transport in the film plane. The mobility of devices reaches 1.3cm2/Vs, the threshold voltage is lower than −0.9V and the on–off current ratio (Ion/Ioff) is higher than 106 after the photolithography process. Moreover, the array consisting of the transparent thin-film transistors displays a high optical transparency more than 65% in visible light regions. The high-performance transparent OTFTs promote the practical applications for large-area and flexible active-matrix organic light-emitting diodes (AMOLEDs) display.

36.3: Invited Paper: Highly Reliable Flexible Active Matrix OLED Driven by a‐Si TFT Device

AbstractFlexible display is believed as an innovative device which can change our living mode in the future. Recent technology advances show the possibility of building with real flexible display which has small package, low weight and design freedom, and make active matrix organic light emitting diode (AMOLED) attractive in replacing existing glass display. A‐Si TFTs on flexible substrates can be candidate devices of a circuitry of AMOLED with dual driving TFTs structure. By using a‐Si TFTs, we have successful improvements on the lifetime of flexible AMOLED with a new pixel circuit and driving methods for the threshold voltage compensation. In this paper, we will discuss on flexible AMOLED as an emerging display, and will review various technical issues and results which have to be examined.

Transparent Active Matrix Organic Light-Emitting Diode Displays Driven by Nanowire Transistor Circuitry

Optically transparent, mechanically flexible displays are attractive for next-generation visual technologies and portable electronics. In principle, organic light-emitting diodes (OLEDs) satisfy key requirements for this application-transparency, lightweight, flexibility, and low-temperature fabrication. However, to realize transparent, flexible active-matrix OLED (AMOLED) displays requires suitable thin-film transistor (TFT) drive electronics. Nanowire transistors (NWTs) are ideal candidates for this role due to their outstanding electrical characteristics, potential for compact size, fast switching, low-temperature fabrication, and transparency. Here we report the first demonstration of AMOLED displays driven exclusively by NW electronics and show that such displays can be optically transparent. The displays use pixel dimensions suitable for hand-held applications, exhibit 300 cd/m2 brightness, and are fabricated at temperatures suitable for integration on plastic substrates.

Active-matrix OLED on bendable metal foil

This brief reports a flexible active-matrix organic light-emitting diode display based on a poly-Si thin-film transistor (TFT) backplane. The p-channel poly-Si TFTs on metal foil exhibited a maximum field-effect mobility of 86.1 cm/sup 2//Vs, threshold voltage of 3.5 V, gate voltage swing of 0.8 V/dec, and the minimum off current of 10/sup -12/ A//spl mu/m at V/sub ds/=-0.1 V. A 4.1-in active-matrix backplane was fabricated with the poly-Si TFT with a conventional pixel circuit consisting of 2 TFTs and one capacitor. The scan driver circuits with PMOS were integrated on the flexible metal foil. The top emission, organic light emitting display having a brightness of 100 cd/m/sup 2/.

A-Si Amoled Display Backplanes on Flexible Substrates

AbstractIn view of its maturity and low-cost, the amorphous silicon (a-Si) technology is an attractive candidate for active matrix organic light emitting diode (AMOLED) display backplanes on flexible substrates. However, the a-Si material comes with significant intrinsic shortcomings related to speed (mobility) and stability of operation, requiring novel threshold-voltage-shift (ΔVT) compensated thin-film transistor (TFT) pixel circuits and architectures to enable stable OLED operation. But given the dramatic progress in efficiency of OLED materials over recent years, the drive current requirement has been significantly lowered, thus relaxing the constraints on a-Si TFTs. For compatibility to plastic substrates, the a-Si TFT process temperature must be reduced from the conventional 300°C to ∼150°C or below, which tends to compromise the integrity of thin- film materials and device performance. Hence, optimizing the TFT process for high device performance with limited thermal budget is a necessary step towards flexible AMOLEDs with a-Si backplanes. This paper reviews the design and process challenges, and specifically examines the performance of TFTs and ΔVT-compensated integrated pixel driver circuits on plastic substrates with respect to current driving ability and long term stability. More importantly, lifetime tests of circuit degradation behaviour over extended time periods demonstrate highly stable drive currents and its ability to meet commercial standards.

Mechanically strained a-Si:H AMOLED driver circuits

AbstractThis paper examines the variations in performance of amorphous silicon (a-Si:H) thin-film transistor (TFT) pixel driver circuits for active-matrix organic light-emitting diode (AMOLED) displays, which are subject to compressive or tensile mechanical strain. The external strain is induced by bending of the TFT substrate, and is measured by the observed changes in resistance of in-situ strain gauges. Mechanical strain impacts the performance of the circuit in terms of its drive current, which may be attributed to mobility and Fermi energy shifts in the individual TFTs. The effect of strain-induced shifts in the TFTs as a function of strain orientation (longitudinal or transverse) with respect to direction of current flow is also examined. Our measurements show that the variation in the drain current of a longitudinally oriented TFT can be as much as ∼ 1.5% for strains as high as 10−3. Proper layout and circuit design can suppress the effect of strain-induced shifts, and should be taken into consideration when designing stable TFT driver circuits for mechanically flexible AMOLED displays.