**1. Introduction**

28 Will-be-set-by-IN-TECH

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Quantum-dot solar cells have attracted much attention because of their potential to increase conversion efficiency of solar photo conversion up to almost 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higer photocurrents (Nozik, 2002). Specifically, the optical-absorption edge of a semiconductor nanocrystal is often shifted due to the quantum-size effect. The optical band gap can then be tuned to the effective energy region for absorbing maximum intensity of the solar radiation spectrum (Landsberg et al., 1993; Kolodinski et al., 1993). Furthermore, quantum dots produce multiple electron-hole pairs per -photon through impact ionization, whereas bulk semiconductor produces one electron-hole pair per -photon.

Wide gap semiconductor sensitized by semiconductor nanocrystal is candidate material for such use. The wide gap materials such as TiO2 can only absorb the ultraviolet part of the solar radiation spectrum. Hence, the semiconductor nanocrystal supports absorbing visible (vis)- and near-infrared (NIR) -light. Up to now, various nanocrystalline materials [InP (Zaban et al., 1998), CdSe (Liu & Kamat, 1994), CdS (Weller, 1991; Zhu et al., 2010), PbS (Hoyer & Könenkamp, 1995), and Ge (Chatterjee et al., 2006)] have been investigated, for instance, as the sensitizer for TiO2. Alternatively, a wide-gap semiconductor ZnO is also investigated, since the band gap and the energetic position of the valence band maximum and conduction band minimum of ZnO are very close to that of TiO2 (Yang et al., 2009). Most of these composite materials were synthesized through chemical techniques, however, physical deposition, such as sputtering, is also useful. In addition, package synthesis of the composite thin film is favorable for low cost product of solar cell.

In this chapter, Ge/TiO2 and PbSe/ZnSe composite thin film are presented, and they were prepared through rf sputtering and hot wall deposition (HWD), with multiple resources for simultaneous deposition. The package synthesis needs the specific material design for each of the preparation techniques. In the rf sputtering, the substances for nanocrystal and matrix are appropriately selected according to the difference in heat of formation (Ohnuma et al., 1996). Specifically, Ge nanocrystals are thermodynamically stable in a TiO2 matrix, since Ti is oxidized more prominently than Ge along the fact that the heat of formation of GeO2 is greater than those of TiO2 (Kubachevski & Alcock, 1979). Larger difference in the heat of formation [e.g., Ge/Al-O (Abe et al., 2008a)] can provide thermodynamically more stable nanocrystal. Hence, the crystalline Ge was homogeneously embedded in amorphous Al oxide matrix, and evaluated unevenness of the granule size was ranged from 2 to 3nm, according to high resolution electron microscopy (HREM). In the HWD, on the other hand,

One-Step Physical Synthesis of Composite Thin Film 151

<sup>2000</sup> (a)

Ge(311)

(211)

(105)

(211)

(105)

(220)

Ge(220)

Ge(111)

(110)

(101)

20 30 40 50 60

2 / deg

<sup>2400</sup> (b)

20 30 40 50 60

2 / deg

0

0.1

0.2

0.3

O2 (%) 0.4

(112) (004)

(103)

Fig. 2.1. (a) XRD patterns of Ge/TiO2 composite films versus Ge concentrations. () indicates anatase structure, and (○), rutile structure. (b) Same patterns versus

additional oxygen ratio in argon. () indicates anatase structure, and (○), rutile structure

Figure 2-1(b) depicts the XRD pattern of Ge/TiO2 thin films as a function of the additional oxygen ratio in argon. In this case, the oxygen ratio is varied from 0 to 0.4%, and the number of Ge chips is kept constant at 2. When the ratio is increased to 0.1%, the (004) Bragg reflection becomes more prominent as seen in the figure. A further increase of the oxygen ratio then indicates weakness. An anatase-dominant structure with strong intensity at (004) reflection is thus observed at an oxygen ratio of 0.1%. We cannot observe an XRD peak of Ge in the pattern within the precision of our experiment technique, possibly due to the relatively low Ge concentration of 5.8at.%. This c-axis growth behavior in an anatase-dominant structure seems to be unique even though the composite film is deposited on a glass substrate. Thus, the crystal structure of TiO2 matrix is found to be changed with respect to the Ge number and the oxygen ratio as illustrated in Figs. 2-1(a)

(112)

(004)

(101)

(103)

(111)

(210)

(200)

0

0

400

800

(101)

1200

1600

Intensity (arb. unit)

(b) (after Abe et al., 2008b).

and 2-1(b).

2000

500

1000

Intensity (arb. unit)

1500

E D

C

B

A

the substances for nanocrystal and matrix are also selected following thermodynamic insolubility. The HWD technique, which is a kind of thermal evaporation, causes unintentional increase of the substrate-temperature due to the thermal irradiation. Hence, simultaneous HWD evaporation from multiple resources often produces solid solution [e.g., Pb1-*x*Ca*x*S (Abe & Masumoto, 1999)]. Hence, package synthesis of the composite thin film needs insolubility material system. The bulk PbSe-ZnSe system, for instance, is found to phase-separate at thermal equilibrium state (Oleinik et al., 1982). It is therefore expected that PbSe nanocrystals phase-separate from the ZnSe matrix in spite of the simultaneous evaporation from PbSe- and ZnSe-resource.

Accordingly, the two thermodynamic material-designs, heat of formation for rf sputtering and insolubility system for HWD, are employed here for package synthesis of composite thin film. This chapter focuses on one-step physical synthesis of Ge/TiO2 composite thin films by rf sputtering and PbSe/ZnSe composite thin films by HWD, as candidate materials for quantum dot solar cell.
