**6. References**

86 Solar Cells – Thin-Film Technologies

The relatively high chlorine and hydrogen concentrations in the order of 0.5at% lead to outgassing during the recrystallization in completely melting regimes. This effect makes the capping layer arch upwards and widens the voids. Isolated pinholes in the silicon film can be observed. A weak hydrogen chloride peak is detected by mass spectrometry in the base gas atmosphere of the recrystallization chamber. Fig.11 shows an area surrounding a pinhole taken with SEM and the relative element concentrations measured by energy dispersive x-ray analysis (EDX) along the black line. There are no chlorine and hydrogen in

This Chapter descried the influence of the applied EB energy density used for the recrystallization process on the surface morphology of the ploy-silicon film system. At a low EB energy density, the voids were formed in the capping layer and the SiO2 capping layer exhibited a rougher and droplet morphology. With the increase of EB energy density, the capping layer became smooth and the size of the voids decreased. The size and amount of pinholes increased again if the EB energy density was too high. This also led to the formation of larger voids in the capping layer as well as coarser and wider spreading of a

This Chapter also gave the details about the formation of Tungstendisilicide (WSi2). The tungstendisilicide precipitates/silicon eutectic structures were mainly localized in at the tungsten/silicon interface but also at the grain boundaries of the silicon throughout all the EB energy density range, as well as the relationship between energy density and microstructure of WSi2/W areas. Tungstendisilicide forms in its tetragonal by the reaction of tungsten with silicon. WSi2 improves the wetting and adhesion of the silicon melt but the tungsten layer may degrade the electrical properties of the solar absorber. The formation and distribution of the eutectic depended on the crystallization and the growth dynamic of

A tungstendisilicide layer was formed between the tungsten layer and the silicon layer for all EB energy densities used. The higher the applied EB energy density, the thicker the tungstendisilicide layer grows and the thinner the tungsten layer left. It is important to perform the recrystallization process at a moderate energy density to suppress the formation of both WSi2/Si eutectic and pinholes. In addition, there are no chlorine and hydrogen in the area surrounding a pinhole after recrystallization because of outgassing during the

The author would like to thank Prof. J. Müller and Dr. F. Gromball of Technische Universit.t Hamburg-Harburg in Germany for providing experimental conditions and interesting discussion, and also remember Prof. J. Müller with affection for his human and scientific talents. This research was financially supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety under contract #0329571B in collaboration with the Hahn Meitner Institute (HMI), Berlin-Adlershof, Department for Solar Energy Research. The author was financially supported by China Scholarship Council

the tungsten enriched silicon melt. This is a nonequilibrium solidification process.

**3.4 Impurities in the recrystallized silicon film** 

the area surrounding a pinhole in the recrystallized film.

WSi2/Si eutectic crystallite at the grain boundaries.

**4. Summary** 

solidification.

**5. Acknowledgements** 


**5** 

*Sri Lanka* 

Ruwan Palitha Wijesundera

**Electrodeposited Cu2O Thin Films for** 

*Department of Physics, University of Kelaniya, Kelaniya* 

**Fabrication of CuO/Cu2O Heterojunction** 

Solar energy is considered as the most promising alternative energy source to replace environmentally distractive fossil fuel. However, it is a challenging task to develop solar energy converting devices using low cost techniques and environmentally friendly materials. Environmentally friendly cuprous oxide (Cu2O) is being studied as a possible candidate for photovoltaic applications because of highly acceptable electrical and optical properties. Cu2O has a direct band gap of 2 eV (Rakhshani, 1986; Siripala et al., 1996), which lies in the acceptable range of window material for photovoltaic applications. It is a stoichiometry defect type semiconductor having a cubic crystal structure with lattice constant of 4.27 Å (Ghijsen et al., 1988; Wijesundera et al., 2006). The theoretical conversion

Thermal oxidation was a most widely used method for the preparation of Cu2O in the early stage. It gives a low resistive, p-type polycrystalline material with large grains for photovoltaic applications. It was found that Cu2O grown at high temperature has high leakage-current due to the shorting paths created during the formation of the material, and it causes low conversion efficiencies. Therefore it was focused to prepare Cu2O at low temperature, which may provide better characteristics in this regard. Among the various Cu2O deposition techniques (Olsen et al., 1981; Aveline & Bonilla, 1981; Fortin & Masson, 1981; Roos et al., 1983; Sears & Fortin, 1984; Rakhshani, 1986; Rai, 1988; Santra et al., 1992; Musa et al., 1998; Maruyama, 1998; Ivill et al., 2003; Hames & San, 2004; Ogwa et al., 2005), electrodeposition (Siripala & Jayakody, 1986, Siripala et al., 1996; Rakhshani & Varghese, 1987a, 1988b; Mahalingam et al., 2004; Tang et al., 2005; Wijesundera et al., 2006) is an attractive one because of its simplicity, low cost and low-temperature process and on the other hand the composition of the material can be easily adjusted leading to changes in physical properties. Most of the techniques produce p-type conducting thin films. Many theoretical and experimental studies (Guy, 1972; Pollack & Trivich, 1975; Kaufman & Hawkins, 1984; Harukawa et al., 2000; Wright & Nelson, 2002; Paul et al., 2006) have been revealed that the Cu vacancies originate the p-type conductivity. However, electrodeposition (Siripala & Jayakody, 1986, Siripala et al., 1996; Wijesundera et al., 2000; Wijesundera et al., 2006) of Cu2O thin films in a slightly acidic aqueous baths produce n-type conductivity. Further it has been reported that the origin of this n-type behavior is due to oxygen vacancies and*/*or additional copper atoms. Recently, Garutara *et al*. (2006) carried out the photoluminescence (PL) characterisation for the electrodeposited n-type

efficiency limit for Cu2O based solar cells is about 20% [5].

**1. Introduction** 

