One-Step Physical Synthesis of Composite Thin Film

Abstract

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 & Konenkamp, 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,