**4.3. P-type conductive oxides**

Highly conductive p-type oxides serve as critical components for various technological developments such as efficient charge injection layers for light-emitting devices [85], solar cells with better band-matching current collectors [86, 87], invisible circuits, and applications in near-infrared optoelectronics where n-type TCOs provide poor optical transmission. In contrast to widespread use of n-type TCOs such as ITO, p-type TCOs have not been commercialized yet due to their significantly low-carrier mobility and electrical conductivity. In fact, research toward p-type amorphous TCOs is highly challenging even by using vacuum deposition techniques, which result in very limited materials: Zn-Rh-O (resistivity 0.5 Ω cm) and Zn-Co-O (0.05 Ω cm). The difficulties originated from electronic structure of p-type TCOs, which exhibits the strong localization of the upper edge of the valence band to oxide ions.

The Hosono group has initiated breakthrough research toward p-type oxides. They found a series of transparent p-type Cu oxides with delafossite structure such as CuGaO<sup>2</sup> and CuAlO<sup>2</sup> [41, 88–90], and amorphous Zn-Rh-O, Zn-Co-O systems [41, 88, 91]. However, most of these p-type TCOs were produced by vacuum-based deposition techniques. Solution-processed p-type conductive oxides have remained very challenging.

Recently, misfit-layered Ca<sup>3</sup> Co<sup>4</sup> O9 thin film synthesized through solution processing is shown to be high-performing p-type TCOs [92]. The synthesis method consists of sol-gel chemistry, spin coating, and heat treatment at 650°C. A resistivity and visible range transparency of 57 mΩ cm and 67%, respectively, were obtained. However, the required high-annealing temperature over 600°C may limit its practical uses.

In order to produce conductive amorphous p-type oxides from solution at low temperature, we focused on Ru oxides [93]. Crystalline RuO<sup>2</sup> is a well-known electrode material that exhibits metallic conduction. Initially, we found that the amorphous phase of RuO<sup>2</sup> is unstable, and only crystalline phase can be obtained through solution processing even at a low temperature 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 valence electrons have open-shell configurations (*t* 2*g* <sup>4</sup> in Ru 4d and *t* 2*g* 5 in Ir 5d), which is distinctively different from the closed-shell or pseudo-closed-shell configurations in other p-type oxides (e.g., Cu 3d10s<sup>o</sup> in Cu oxides, Sn 5s<sup>2</sup> in SnO, Rh *t* 2*g* <sup>6</sup> in Zn-Rh-O, and Co *t* 2*g* <sup>6</sup> in Zn-Co-O). Second, the conduction of amorphous La-Ru-O (semiconducting) is completely different from that of crystalline phases (e.g., La<sup>3</sup> Ru<sup>3</sup> O11 and La<sup>4</sup> Ru<sup>6</sup> O19, which are both metallic). Third, the 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 future research.

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 investigation 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> Ru<sup>2</sup> O7 , is sensitive to the type 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

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

**4.3. P-type conductive oxides**

**Material Dopant or compound**

SnO<sup>2</sup> Sb, F, As, Nb, Ta

CdO In, Sn ZnO-SnO<sup>2</sup> Zn<sup>2</sup>

Ga-In-O Sn, Ge

O6 Y Cd-In-Sn-O CdIn<sup>2</sup>

O3 Zn<sup>2</sup>

O3 Sn, Ge, Mo, F, Ti, Zr, Hf, Nb, Ta, W, Te, Mg

, ZnSnO<sup>3</sup>

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

ZnO Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, Hf

SnO<sup>4</sup>

O4 -Cd<sup>2</sup> SnO<sup>4</sup>

In2 O5 , Zn<sup>3</sup> In2 O6

In2

90 Green Electronics

ZnO-In<sup>2</sup>

CdSb<sup>2</sup>

to oxide ions.

Recently, misfit-layered Ca<sup>3</sup>

Highly conductive p-type oxides serve as critical components for various technological developments such as efficient charge injection layers for light-emitting devices [85], solar cells with better band-matching current collectors [86, 87], invisible circuits, and applications in near-infrared optoelectronics where n-type TCOs provide poor optical transmission. In contrast to widespread use of n-type TCOs such as ITO, p-type TCOs have not been commercialized yet due to their significantly low-carrier mobility and electrical conductivity. In fact, research toward p-type amorphous TCOs is highly challenging even by using vacuum deposition techniques, which result in very limited materials: Zn-Rh-O (resistivity 0.5 Ω cm) and Zn-Co-O (0.05 Ω cm). The difficulties originated from electronic structure of p-type TCOs, which exhibits the strong localization of the upper edge of the valence band

The Hosono group has initiated breakthrough research toward p-type oxides. They found a

[41, 88–90], and amorphous Zn-Rh-O, Zn-Co-O systems [41, 88, 91]. However, most of these p-type TCOs were produced by vacuum-based deposition techniques. Solution-processed

to be high-performing p-type TCOs [92]. The synthesis method consists of sol-gel chemistry, spin coating, and heat treatment at 650°C. A resistivity and visible range transparency of 57 mΩ cm and 67%, respectively, were obtained. However, the required high-annealing tem-

In order to produce conductive amorphous p-type oxides from solution at low temperature,

only crystalline phase can be obtained through solution processing even at a low temperature

thin film synthesized through solution processing is shown

is a well-known electrode material that exhib-

and CuAlO<sup>2</sup>

is unstable, and

series of transparent p-type Cu oxides with delafossite structure such as CuGaO<sup>2</sup>

its metallic conduction. Initially, we found that the amorphous phase of RuO<sup>2</sup>

p-type conductive oxides have remained very challenging.

Co<sup>4</sup> O9

perature over 600°C may limit its practical uses.

we focused on Ru oxides [93]. Crystalline RuO<sup>2</sup>


**5.2. Electrophoretic display driven by all solution process active-matrix oxide TFTs**

had a double-layer structure comprising ITO and RuO<sup>2</sup>

**Figure 10.** Transfer curves of low-temperature all solution-processed oxide TFT.

**5.3. Direct printing of oxide TFT by nano-rheology printing**

a *SS*-factor of 1.09 V/decade, a *Vth* of 3.06 V, and an "on/off" ratio of 10<sup>5</sup>

other layers such as GE, S/D, CS, and PV were fabricated by other techniques.

comprised RuO<sup>2</sup>

con dioxide (SQ) and RuO<sup>2</sup>

Precursor solutions and a solution process were developed for the fabrication of active-matrix amorphous oxide-TFTs [102]. The gate lines (GE), gate insulator (GI), and channel layer (CH)

(PSZ) film was used for the channel stopper (CS) layer. The source and drain (S/D) electrode

(PE), respectively (**Figure 11(a)**). Details of the precursor solutions and annealing conditions for the film formations are summarized in **Table 6**. The TFTs exhibited a *μ* of 2.68 cm<sup>2</sup> V−1 s−1,

plays (EPDs) with a resolution of 101.6 ppi were successfully fabricated using the all solutionprocessed active-matrix TFTs (**Figure 11(b** and **c)**). Bi-stable black/white images were retained after cutting of the power supply and video signal in these TFT-EPDs (**Figure 11(d)**). It is considered that low-cost electronic paper can be realized using this technology in the near future.

Among various solution-based approaches, a direct printing is a promising low-cost technique for fabricating oxide TFTs. The printing technique offers several advantages in manufacturing electronics such as a direct writing of materials, reduction of materials waste, and reproducibility with high resolution, which are not affordable from other solution-based approaches [103–105]. While many printed organic TFTs have been reported, relatively fewer studies, associated with directly printed oxide TFTs, have been pursued [17, 33, 106]. Furthermore, the printing has mainly been applied for metal-oxide semiconductor as the channel layer, while

Recently, a new printing method, so-called nano-rheology printing (nRP), for metal oxide patterns within sub-micrometer range, was introduced [33]. The nRP method, which is a type of direct thermal imprinting, uses viscoelastic transformation like glass transition of oxide

, LZO, and ZIZO films, respectively. The polysilazane-based silicon dioxide

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

films were used for the passivation layer (PV) and pixel electrodes

films. The silsesquioxane-based sili-

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

93

. Electrophoretic dis-

**Table 5.** Electro-optical characteristics of some solution-processed p-type conductive oxides.

temperatures [95]. The lowest necessary temperatures for a-BiRuO, a-PbRuO, and a-BiIrO were 240, 290, and 350°C, respectively, resulting in RT DC resistivities of 3.8, 1.7, and 3.8 mΩ cm, respectively. The resistivity of a-BiRuO film has nearly reached the value of crystalline bulk Bi<sup>3</sup> Ru<sup>3</sup> O11 (1.4 mΩ cm), which suggests that the films are of high quality. In p-type oxides, the low-resistivity values of these amorphous oxides are matched only by epitaxial LaCuOSe:Mg annealed at a high temperature of 1000°C (1.1 mΩ cm). A summary of electro-optical characteristics of some solution-processed p-type conductive oxides is given in **Table 5**.
