**1. General introduction**

Oxide materials have become high-tech functional materials beyond their traditional role as dielectrics. They show a rich variety of complex emergent behaviors, such as memristive effect [1], catalytic activity [2], and complex multiferroic effects [3]. The discovery of new metal oxides with interesting and useful properties continues to drive much research in chemistry, physics, and materials science. The physicochemical properties of oxides can be tuned through variation of factors such as composition, temperature, pressure, strain, external fields, defects, film orientation, and nanoparticle size [4–6]. Structure-property detailed

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

analysis and understanding of the physicochemical properties of the oxides are prerequisites to improve their properties and to spur development of new oxide materials.

tunneling for very thin films. In this regard, it is useful to consider the important film features and performance metrics that will produce an optimal gate dielectric. The following are general requirements for a good dielectric film: large relative dielectric constant (>10) and small

breakdown field (>4 MV/cm) to preserve the device function as shown in **Figure 1(a)**. In addition, high-mechanical strength, low-thermal expansion, low-water adsorption quality, and high-chemical inertness properties are highly desired. It is worth noting that the transistor mobility strongly depends on the quality of semiconductor/dielectric interface. A dielectric with a rough surface would result in irregular semiconductor/dielectric interface, impeding the flow of charges through the semiconductor. Thus, atomically flat dielectric film surface is essentially required. To achieve the necessary leakage current and breakdown field, films must be as dense as possible and exhibit no pores or cracks. Both from the perspective of surface smoothness and the need for high-breakdown field and low-leakage current, films with amorphous structure are generally preferred for the fabrication of gate dielectric layers. Because of the challenges in producing such insulators through solution methods, most solution-processed oxide TFTs [11–17] have been fabricated by using binary oxide gate insulators formed through vacuum-based depositions. Although binary oxides will continue to be used for TFT gate dielectric applications, they do not represent an optimal approach to realizing high-performance devices. Generally, binary oxides tend to crystalize [18–20] at relatively low-process temperatures, resulting in enhanced impurity interdiffusion and high-leakage currents due to formation of grain boundaries. Two most important prerequisites of oxide

**Figure 1.** (a) General requirements of a dielectric layer and (b) major designs of device-quality dielectric layer.

at 1 MV/cm), low-dielectric loss (tanσ < 0.01), and large

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Low-Temperature Solution-Processable Functional Oxide Materials for Printed Electronics

leakage current density (<10 nA/cm<sup>2</sup>

Among the various process technologies, solution process has many advantages, not only simple and low-cost process but also homogeneity and excellent composition control and high throughput [7]. Oxide solutions are generally synthesized using functional metal precursors in solvents and deposited on substrates by various coating methods. The coated oxide gels were pre-annealed to remove the solvents and post-annealed to develop active layers. Design of metal oxide precursor solutions (metal composition, metal precursor, chelating agents, etc.), treatment of intermediate oxide gels, and annealing techniques are of paramount importance for controlling and improving structural and opto-electronic properties of final oxides.

In this chapter, we review the progress in solution-processed functional oxide thin films produced at a low temperature and highlight the critical challenges for the fundamental understanding and practical implementation of complex oxides in devices.
