*4.2.2. Alternatives to ITO*

**Figure 7.** The plot of resistivity improvement versus time for various TCOs.

by various solution-based deposition processes is given in **Table 3**.

**Table 3.** Properties of ITO films prepared by various solution-deposition processes.

 **(Ω)** *t* **(nm)** *T* **(%) ΦH (10−2 Ω−1) Deposition Ref**

45.7 47.5 144 85.3 0.420 Electrospray [79] 42.3 188 225 80 — Dip coating [76] 2.5 42 60 ∼85 0.469 Dip coating [81] 2.1 7.1 295 ∼78 1.171 Dip coating [82] 310 — 75 >90 — Spin coating [80] 0.52 356 146 93 0.136 Spin coating [74] 4.2 81 70 — — Spin coating [77] 7.2 30 241 90.2 1.191 Spin coating [72]

ITO is the most widely used TCOs because of its two key characteristics: conductivity and optical transparency. Conventionally, ITO films are produced by vacuum-based deposition techniques. The best resistivity of ITO film deposited by sputtering was reported as low as 1.2 × 10−4 Ω cm [70]. As an alternative to vacuum-based deposition, recently much efforts have been focused on production of solution-based deposition for high-performance ITO films using various approaches such as nanomaterials-based [71–75], aqueous metal salts solution [76], combustion synthesis [77, 78], and advanced annealing techniques (microwave annealing [79] and ultraviolet laser annealing [80]. A summary of properties of ITO films prepared

**ρ (10−4 Ω cm)** *R***<sup>s</sup>**

**4.2. N-type conductive oxides**

*4.2.1. Indium-tin-oxide (ITO)*

88 Green Electronics

There are increasing emphases on not only developing a TCO with higher conductivity but also with a material other than ITO. Two major concerns with ITO are the cost and the potentially limited resource.

Current candidate materials for a higher conductivity TCO are summarized in **Table 4**, which include the conventional binary oxides of CdO, SnO<sup>2</sup> , In<sup>2</sup> O3 , and ZnO, with alternative dopants, and combinations of these binaries, plus Ga2O3, in ternary, quaternary, and even quinary combination of these five oxides.

As a promising binary TCO material, ZnO is considered the prime candidate for a higher conductivity and to be low cost. However, ZnO is more sensitive to oxygen than ITO, and process control is more difficult. Work continues on improving ZnO with alternative dopants, especially Al (AZO) and Ga (GZO). As for sol-gel AZO film, the lowest reported resistivity is 1.5 × 10−4 Ω cm [83], which is close to that of film prepared by sputtering [84].

**Figure 8.** (a) Cross-sectional structure and (b) transfer curves of ZIZO-TFT using SCS-ITO as S/D electrodes.


below 300°C. We then used lanthanide elements (Ln, except Ce) to stabilize the amorphous phase. As a result, we found a series of solution-processable amorphous p-type conductive Ln-M-O (a-Ln-M-O, where M = Ru, Ir, and Ln is lanthanide elements except Ce) having low resistivity (10−3–10−2 Ω cm) (**Figure 9**). The resistivity increases with increasing Ln atomic number (Z) and changes evidently faster for Z > 64 (Gd), which may be more related to the influence of 4f electrons (4f subshell is more than half-filled at Z > 64) than to the decreasing size of Ln ions. These oxides are thermally stable to a high degree, being amorphous up to 800°C, and processable below 400°C. These oxides have three pronounced features: first, their

2*g*

Ru<sup>6</sup>

tively different from the closed-shell or pseudo-closed-shell configurations in other p-type

Second, the conduction of amorphous La-Ru-O (semiconducting) is completely different from

resistivity of solution-processed a-La-Ru-O is lower than that of the sputtered sample. Hence, detailed understanding of the electronic structure of these materials and the mechanism underlying processing-composition-conduction correlation is of fundamental importance for

The unique properties of amorphous p-type oxides have high potential for use in printed electronics, e.g., as gate electrode in n-type oxide TFTs [33, 94], and may be modified to use as p-channel in TFTs or as an active layer in solar cells. The narrow band-gap is advantageous for light absorbing in solar cells. The electronic configurations of these amorphous oxides are apparently not analogous to those of known p-type oxides. This deserves theoretical investi-

of A element. These compounds are semiconducting for A = Y and the lanthanides Pr-Lu, and weakly metallic for A = Bi and Pb. The Ln of the aforementioned oxides was thus replaced with Bi and Pb, leading to low-resistivity amorphous oxides produced at low

O11 and La<sup>4</sup>

in SnO, Rh *t*

<sup>4</sup> in Ru 4d and *t*

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

2*g*

2*g*

<sup>6</sup> in Zn-Rh-O, and Co *t*

Ru<sup>2</sup> O7

5 in Ir 5d), which is distinc-

2*g*

, is sensitive to the type

O19, which are both metallic). Third, the

http://dx.doi.org/10.5772/intechopen.75610

<sup>6</sup> in Zn-Co-O).

91

valence electrons have open-shell configurations (*t*

in Cu oxides, Sn 5s<sup>2</sup>

Ru<sup>3</sup>

gation and suggests that more p-type amorphous oxides are ahead of us.

It is known that the conduction of ruthenium pyrochlore, A<sup>2</sup>

**Figure 9.** (a) XRD patterns and (b) resistivity of LnRuO films.

oxides (e.g., Cu 3d10s<sup>o</sup>

future research.

that of crystalline phases (e.g., La<sup>3</sup>

**Table 4.** Conductive oxides based on multinary metal oxide systems other than ITO.
