Polymers for flexible displays: From material selection to device applications

This article reviews several classes of inorganic semiconductor materials that can be used to form high-performance thin-film transistors (TFTs) for large area, flexible electronics. Examples ranging from thin films of various forms of silicon to nanoparticles and nanowires of compound semiconductors are presented, with an emphasis on methods of depositing and integrating thin films of these materials into devices. Performance characteristics, including both electrical and mechanical behavior, for isolated transistors as well as circuits with various levels of complexity are reviewed. Collectively, the results suggest that flexible or printable inorganic materials may be attractive for a range of applications not only in flexible but also in large-area electronics, from existing devices such as flat-panel displays to more challenging (in terms of both cost and performance requirements) systems such as large area radiofrequency communication devices, structural health monitors, and conformal X-ray imagers.

Inorganic semiconductors for flexible electronics

This article reviews the properties, fabrication and assembly of inorganic semiconductor materials that can be used as active building blocks to form high-performance transistors and circuits for flexible and bendable large-area electronics. Obtaining high performance on low temperature polymeric substrates represents a technical challenge for macroelectronics. Therefore, the fabrication of high quality inorganic materials in the form of wires, ribbons, membranes, sheets, and bars formed by bottom-up and top-down approaches, and the assembly strategies used to deposit these thin films onto plastic substrates will be emphasized. Substantial progress has been made in creating inorganic semiconducting materials that are stretchable and bendable, and the description of the mechanics of these form factors will be presented, including circuits in three-dimensional layouts. Finally, future directions and promising areas of research will be described.

Semiconductor Wires and Ribbons for High- Performance Flexible Electronics

The emergence of smartphones and televisions with curved displays indicates that flexible organic light-emitting diodes (OLEDs) are marching steadily toward commercialization. Regardless of the significant step forward from laboratory to industry, flexible OLEDs are still facing enormous obstacles and challenges in realizing truly wearable, deformable, printable and low-cost electronic products for the future. How to develop highly flexible electrodes, optimize the device efficiency and greatly extend the operation lifetime are among the most important issues to be solved. In this regard, this review summarizes the recent achievements in flexible transparent conductive electrodes, device fabrication as well as light extraction technologies. Furthermore, the device operation stability is discussed with a focus on device degradation mechanisms and encapsulation methods. Some future prospects on new opportunities and research challenges in flexible OLEDs are also covered.

Recent advances in flexible organic light-emitting diodes

The increasing interest in flexible electronics and flexible displays raises questions regarding the inherent mechanical properties of the electronic materials used. Here, the mechanical behavior of thin-film transistors used in active-matrix displays is considered. The change of electrical performance of thin-film semiconductor materials under mechanical stress is studied, including amorphous oxide semiconductors. This study comprises an experimental part, in which transistor structures are characterized under different mechanical loads, as well as a theoretical part, in which the changes in energy band structures in the presence of stress and strain are investigated. The performance of amorphous oxide semiconductors are compared to reported results on organic semiconductors and covalent semiconductors, i.e., amorphous silicon and polysilicon. In order to compare the semiconductor materials, it is required to include the influence of the other transistor layers on the strain profile. The bending limits are investigated, and shown to be due to failures in the gate dielectric and/or the contacts. Design rules are proposed to minimize strain in transistor stacks and in transistor arrays. Finally, an overview of the present and future applications of flexible thin-film transistors is given, and the suitability of the different material classes for those applications is assessed.

Mechanical and Electronic Properties of Thin-Film Transistors on Plastic, and Their Integration in Flexible Electronic Applications.

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/.

Active-matrix OLED on bendable metal foil

We have developed a 4.1 inch AMOLED display with top emission structure on stainless steel foil. The p-channel TFTs on metal foil exhibited the field-effect mobility of 75.1 cm2/Vs, threshold voltage of −3.9V, and subthreshold swing of 0.9V/dec. Active-matrix back planes were fabricated with the poly-Si TFT with a conventional pixel circuit consisting of 2 TFTs and 1 cap. The scan driver circuits with PMOS were integrated on the metal foil.

54.4: 4.1 inch Top-Emission AMOLED on Flexible Metal Foil

Giant-grain silicon (GGS) and its application to stable thin-film transistor

We have studied the mechanical stability of poly-Si thin-film transistor on 50 μm-thick flexible metal foil with a field-effect mobility of 81.2 cm 2 /V s, a threshold voltage of −2.4 V, and an on/off current ratio of 10 6 . We have measured the electrical properties under various compressive and tensile strains by changing the bending radius of the base metal foil. We have found that the TFT is stable until the bending radius of 50 mm which corresponds to the strain of ∼1.4%.

Mechanical stability of poly-Si TFT on metal foil

In this letter, we have studied the inverted staggered thin-film transistor (TFT) using a spin-on-glass (SOG) gate insulator and a low-temperature polycrystalline silicon (poly-Si) by Ni-mediated crystallization of amorphous silicon. The p-channel poly-Si TFT exhibited a field-effect mobility of 48.2 cm2/V ldr s, a threshold voltage of -4.2 V, a gate-voltage swing of 1.2 V/dec, and a minimum off-current of < 4 times 10-13A/ mum at Vds = -0.1 V. Therefore, the gate planarization technology by SOG can be applicable to low-cost large-area poly-Si active-matrix displays.

Jun Hyuk Cheon

Papers

Active-matrix OLED on bendable metal foil

54.4: 4.1 inch Top-Emission AMOLED on Flexible Metal Foil

Giant-grain silicon (GGS) and its application to stable thin-film transistor

Mechanical stability of poly-Si TFT on metal foil

Inverted Staggered Poly-Si Thin-Film Transistor With Planarized SOG Gate Insulator