Development and evaluation of bend‐testing techniques for flexible‐display applications

Abstract— Two different approaches to automated bend testing of flexible substrates for display applications were implemented and characterized: a conventional collapsing radius geometry and a novel technique called the “X‐Y‐θ” geometry. Indium tin oxide (ITO) coated polymer substrates were used to compare the performance of the two automated systems by in‐situ electrical‐resistance measurements. Manual bending on fixed‐diameter mandrels was used to help interpret the results. The advantages and drawbacks of the two systems for providing information of practical use to flexible display R&D are discussed.     

Low‐cost manufacturing of patterned films with nano‐precision for display applications

Abstract— Much attention has been given to methodologies for patterning optical films with nanoscale precision at the scale and economics required by the flat‐panel‐display industry. By using Fluorocur® mold materials, the PRINT® (Pattern Replication in Nonwetting Template) technology enables low‐cost manufacturing of precise microscale and nanoscale features with single‐nanometer precision from virtually any material.     

Highly conductive SnO2 thin films for flat‐panel displays

Abstract— SnO₂ is considered to be a promising alternative material for indium tin oxide (ITO), which is used for thin‐film transparent electrodes in flat‐panel displays (FPDs) and is facing a serious indium depletion problem. However, annealing processes in the manufacture of plasma‐display panels (PDPs), which are major FPDs, cause high resistivity in SnO₂ films. To obtain lower resistivity after the annealing processes, the relationship between deposition conditions and resistivity and the influences of annealing on resistivity, both theoretically and experimentally, were investigated. As a solution, a method involving the formation of a coating of SiO₂ on SnO₂ is proposed, and a SnO₂ resistivity as low as 6.60 × 10^?5 Ω‐m was obtained after annealing.     

Letter: Solution‐processed flexible zinc‐tin oxide thin‐film transistors on ultra‐thin polyimide substrates

In this letter, solution‐processed flexible zinc‐tin oxide (Z_0.35T_0.65O_1.7) thin‐film transistors with electrochemically oxidized gate insulators (AlOx:Nd) fabricated on ultra‐thin (30 µm) polyimide substrates are presented. The AlOx:Nd insulators exhibited wonderful stability under bending and excellent insulating properties with low leakage current, high dielectric constant, and high breakdown field. The device exhibited a mobility of 3.9 cm²/V · s after annealing at 300 °C. In addition, the flexible device was able to maintain the electricity performance under various degrees of bending, which was attributed to the ultra‐thin polyimide substrate.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

Single‐source dual‐layer amorphous IGZO thin‐film transistors for display and circuit applications

In this study, the authors report on high‐quality amorphous indium–gallium–zinc oxide thin‐film transistors (TFTs) based on a single‐source dual‐layer concept processed at temperatures down to 150°C. The dual‐layer concept allows the precise control of local charge carrier densities by varying the O₂/Ar gas ratio during sputtering for the bottom and top layers. Therefore, extensive annealing steps after the deposition can be avoided. In addition, the dual‐layer concept is more robust against variation of the oxygen flow in the deposition chamber. The charge carrier density in the TFT channel is namely adjusted by varying the thickness of the two layers whereby the oxygen concentration during deposition is switched only between no oxygen for the bottom layer and very high concentration for the top layer. The dual‐layer TFTs are more stable under bias conditions in comparison with single‐layer TFTs processed at low temperatures. Finally, the applicability of this dual‐layer concept in logic circuitry such as 19‐stage ring oscillators and a TFT backplane on polyethylene naphthalate foil containing a quarter video graphics array active‐matrix organic light‐emitting diode display demonstrator is proven.     

Temporal signatures of resistivity in bending of indium tin oxide‐coated flexible transparent conductive films for flexible electronics: Influence of coating thickness and bending radius

The study is focused on the fundamental understanding of behaviors of polymer films coated with indium tin oxide (ITO) of varying thicknesses and thus various conductivities/transparencies in repeated bending by tracking the electrical resistivity real‐time using a specially designed multi‐purpose flexing system. The results show that temporal increases of resistance provide important clues as to the initiation and progress of failure. In tension, the resistance typically remains flat unless a critical minimum radius of curvature is breached that leads to progressive cracking of ITO coating bringing rapid rise of resistance. This critical minimum radius of curvature increases with the increase of ITO coating thickness making the higher conductivity films more susceptible to damage. In compression mode, similar temporal signature can be found when bent to a curvature below a critical minimum radius. When cracks form in both modes, the resistance signature changes to one of oscillation and the high and low values observed at each cycle progressively increase with more cycles leading ultimately to catastrophic failure.     

Characteristics and stability of improved re‐crystallized metal‐induced laterally crystallized polycrystalline‐silicon thin‐film transistors for display applications

Abstract— The performance of high‐temperature re‐crystallized (RC) metal‐induced laterally crystallized (MILC) polycrystalline‐silicon (poly‐Si) thin‐film transistors (TFT) have been improved by (1) patterning the active islands before MILC, (2) removing nickel‐containing residues using acid cleaning, (3) using heavily boron‐doped poly‐Si gates to achieve threshold voltage symmetry, and (4) double‐implanting n‐type source/drain junctions. A 30‐MHz driver circuit based on this improved technology was demonstrated. The reliability of optimized RC‐MILC poly‐Si TFTs has not been adversely affected by residual nickel‐containing contaminants in the TFT channel regions.     

A design method for flicker‐free liquid crystal display panels based on indium‐gallium‐zinc‐oxide thin‐film transistors

This paper proposes a design method to reduce the flicker of liquid crystal display panels based on indium‐gallium‐zinc‐oxide (IGZO) thin‐film transistors (TFTs). The proposed design method employs a human factor model to convert the flicker measured at low frame frequency (F _FRAME) to a modification value of the measured flicker (MVMF ) having a frequency sensitivity of flicker, which can distinguish between no blinking and weak blinking. To investigate the causes and characteristics of flicker, the frequency component and increase factor of flicker are analyzed using the checkerboard and solid images. The increase factor in flicker is examined using IGZO TFTs with different antenna ratios (AR s) that cause the variation in threshold voltage of IGZO TFT. To verify the proposed design method, two test panels are implemented with asymmetric and symmetric AR s. The MVMF s of the 15 Hz component at a low F _FRAME of 30 Hz show that the solid image with a symmetric AR has an MVMF of ?62.9 dB, which is improved by 24.3 dB compared to that with an asymmetric AR . Therefore, the proposed method is applicable for a flicker‐free liquid crystal display panels at a low F _FRAME.     

An averaging pixel structure using microcrystalline‐silicon films prepared at high temperature for AMOLED displays

Abstract— A new voltage‐addressed pixel using a multiple drive distribution has been developed to improve, in a simple way, the brightness uniformity of active‐matrix organic light‐emitting‐diode (AMOLED) displays. Moreover, circuits were realized using microcrystalline‐silicon (μc‐Si) films prepared at 600°C using a standard low‐pressure CVD system. The developed p‐channel TFTs exhibit a field‐effect mobility close to 6 cm²/V‐sec. The experimental results show that the proposed spatial distribution of driving TFTs improves the uniformity of current levels, in contrast to the conventional two‐TFT pixel structure. Backplane performances have been compared using circuits based on μc‐Si and furnace‐annealed polysilicon materials. Finally, this technology has been used to make an AMOLED demonstration unit using a top‐emission OLED structure. Thus, by combining both an μc‐Si active‐layer and a current‐averaging driver, an unsophisticated solution is provided to solve the inter‐pixel non‐uniformity issue.     

In‐pixel temperature sensor for high‐luminance active matrix micro‐light‐emitting diode display using low‐temperature polycrystalline silicon and oxide thin‐film‐transistors

We propose an in‐pixel temperature sensor using low‐temperature polycrystalline silicon and oxide (LTPO) thin‐film transistor (TFTs) for high‐luminance active matrix (AM) micro‐light‐emitting diode (LED) displays. By taking advantage of the different off‐current characteristics of p‐type LTPS TFTs and n‐type a‐IGZO TFTs under temperature change, we designed and fabricated a temperature sensor consists of only LTPO TFTs without additional sensing component or material. The fabricated sensor exhibits excellent temperature sensitivity of up to 71.8 mV/°C. In addition, a 64 × 64 temperature sensor array with 3T sensing pixel and integrated gate driver has also been fabricated, which demonstrates potential approach for maxing out the performance of high‐luminance AM micro‐LED display with real‐time in‐pixel temperature monitoring.