Large Area and Flexible Electronics

Mario Caironi, Yong-Young Noh

• Publisher: Wiley-VCH
• Year: 2015
• Print ISBN: 9783527336395
• eBook ISBN: 9783527679997
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

From materials to applications, this ready reference covers the entire value chain from fundamentals via processing right up to devices, presenting different approaches to large-area electronics, thus enabling readers to compare materials, properties and performance.
Divided into two parts, the first focuses on the materials used for the electronic functionality, covering organic and inorganic semiconductors, including vacuum and solution-processed metal-oxide semiconductors, nanomembranes and nanocrystals, as well as conductors and insulators. The second part reviews the devices and applications of large-area electronics, including flexible and ultra-high-resolution displays, light-emitting transistors, organic and inorganic photovoltaics, large-area imagers and sensors, non-volatile memories and radio-frequency identification tags.
With its academic and industrial viewpoints, this volume provides in-depth knowledge for experienced researchers while also serving as a first-stop resource for those entering the field.

Part I
Materials

1
Polymeric and Small-Molecule Semiconductors for Organic Field-Effect Transistors

Hakan Usta and Antonio Facchetti

1.1 Introduction

π-Conjugated small-molecule and polymer semiconductors are of interest because of their unique optical and electrical properties which will enable the fabrication of new optoelectronic devices having unique functionalities [1–3]. Besides the discovery of new materials, the development of organic-semiconductor-based optoelectronics requires a much better understanding of the electronic structure, charge-transport properties, as well as light–molecule/polymer and charge–charge interactions in the corresponding thin films [4]. Although these aspects are fundamental for the optimization of these materials, the goal in this chapter is to review the very recent achievements in the development of molecular and polymeric semiconductors for charge transport in thin-film transistors (TFTs). In particular, we will first introduce the basic concepts of organic semiconductor structure and organic thin-film transistor (OTFT) operation and then focus on initial studies and very recent works. Excellent review articles are available in the literature for the intermediate period [5, 6].

1.2 Organic Semiconductor Structural Design

Organic semiconductors for OTFTs must possess two essential structural features for their successful implementation in printed electronics (Figure 1.1) [7]. The first is a π-conjugated core/chain composed of linked unsaturated units. The extended π orbitals enable achieving the characteristic charge-transport and optical properties [8]. The second is core functionalization with solubilizing substituents, which is essential for inexpensive manufacture by solution methods as well as for enhancing solid-state core interactions [9]. This latter feature was not met in the initial studies, as most of the OTFTs were fabricated with the semiconductor film deposited by vacuum sublimation. Among the most common unsaturated units used for core construction, there are mono(poly)cyclic aromatic hydrocarbons, heterocycles, benzofused systems, and simple olefinic and acetylenic groups. The extent of conjugation/interaction between these units determines the semiconductor solution/solid state electronic structure, which in turn controls the key molecular/polymeric properties such as optical absorption/emission, redox characteristics, and frontier molecular orbital energy levels. Other important architecture parameters relevant to polymers are the molecular weight (M_w) and the polydispersity (PD) index because they influence the solubility, solution aggregation, formulation rheology, and thin-film formation and morphology for both pristine and blended materials. Because when going from low (oligomers) to high (polymer) molecular weights the electronic structure, thermal properties, and microstructure of polymers generally vary considerably, it is important to achieve an M_w/PD regime where a certain property stabilizes so that greater reproducibility of that polymer property can be achieved from batch to batch. This value is likely to be strongly dependent on the polymer structure, but for most soluble thiophene-based polymers, a number-averaged molecular weight value of about 20–30 kDa and a PD of 1.2–1.8 are reasonable for these threshold values [10].

Figure 1.1 Schematic representation of a π-conjugated unit, molecular topologies of the Frontier molecular orbital (FMO), and structural connectivity affording conducting/motifs, twisted/planar connections, and intermolecular interactions.

There are several advantages in using polymeric versus molecular π-conjugated semiconductors. Thin films of polymeric materials are generally very smooth and uniform, enabling a great control over a large range of the film structural and morphological characteristics. Printing requires great control of the solution rheological properties, which can be tuned efficiently for polymeric materials. Polymer crystalline domains are typically much smaller than the length scale of several optoelectronic devices, resulting in isotropic transport characteristics. This results in low device-to-device performance variability, which is particularly important for TFT integration into circuits. Furthermore, the fabrication of multilayers from solution deposition processes requires that each stacked layer is inert to the solvents and processing temperatures that it is subsequently exposed to during device manufacture. The reduced solubility parameter window of polymers and their large bulk viscosity typically increase the options to find orthogonal solvents for solution deposition on top of polymer layers, thus expanding the choice of materials that can be used in devices. Finally, because polymers do not vaporize before decomposition and thus have negligible vapor pressure, they are not susceptible to interlayer diffusion during the typical thermal cycles during device fabrication, and typically exhibit robust mechanical properties, making nanometer-thick semiconductor films potentially compatible with roll-to-roll fabrication on flexible substrates. However, during recent years, several new approaches have been developed to improve small-molecule processability from solutions, including the use of spin coating, slot dye coating, and blade coating.

1.3 Thin-Film Transistor Applications

OTFTs are a low-cost technology alternative to amorphous hydrogenated silicon transistors for applications in large-area OTFT-based arrays, for example, backplane/driver circuits for active matrix displays, where high transistor density and switching speeds are not necessary. They may also be attractive for applications in low-end microelectronics (e.g., radio frequency identification tags and sensors), where the high cost of packaging conventional Si circuits is prohibitive for everyday items [11]. The advantages of OTFTs stems from the potential lower manufacturing costs and reduced capital inve...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• List of Contributors
• Overview
• Part I: Materials
• Part II: Devices and Applications
• Index
• EULA