Electrodeposition of ZnO Thin Films

Different precursor routes exist to cathodicaUy electrodeposit ZnO from aqueous electrolytes with either O2 [73, 74], NOj [75-77], or H2O2 [78, 79] as oxygen precursors in solution. In general, the reactions can be best described as electrochemi-caUy induced precipitation reactions rather than simple Faradaic reactions to convert soluble, typically ionic, species into solids as is the case for the electrodeposition of metaUic films. The respective precursor is reduced at the cathode accompanied by a pH increase  

This increase in pH leads to a situation in which the solubility product of Zn(OH)2 or ZnO is exceeded and a supersaturation situation is reached [49]. In the presence of appropriate crystaUization seeds ZnO crystals nucleate and grow under very well-controlled conditions dose to equihbrium at a growth rate approximately five times faster on the 0001 plane compared to the 10-10 or 01-10 planes [80]. As a consequence, predpitation on the cathode is observed since here the concen- 

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Recently, the mechanism of cathodic electrodeposition of ZnO thin films from aqueous Zn(N03)2 baths has been studied using a rotating disc electrode [165]. Non-electrochemical methods of preparation of ZnO were also described, but they are not covered in this review. 

Electrodeposition of ZnO thin films can be triggered by reduction of nitrate typically in an aqueous solution of zinc nitrate.62) The overall reaction for deposition of ZnO can be written as follows. 

ZnO thin films doped with In or Al were electrodeposited at a constant potential at 80°C from aqueous solutions of Zn(N03)2 with InCh or A1(N03)3 [162]. 

Karappuchamy etal. [164] have described cathodic electrodeposition of porous ZnO thin film modified with the dye in an aqueous solution of zinc nitrate and riboflavin 5 -phosphate. 

Yoshida, T., Miyamoto, K., Hibi, N., Sugiura, T., Minoura, H., Schlettwein, D., Oekermann, T., Schneider, G., and Wbhrle, D. (1998) Self assembled growth of nanoparticulate porous ZnO thin film modified by 2,9,16,23-tetras ulfophthalocyanatozinc(II) by one-step electrodeposition. Chem. Lett., 7, 599-600. 

Pauporte, T., Yoshida, T., Goux, A., and Lincot, D. (2002) One-step electrodeposition of ZnO/eosinY hybrid thin films from a hydrogen peroxide oxygen precursor. /. Electroanal. Chem., 534, 55. ... 

Cathodic electrodeposition of ZnO/eosin Y hybrid thin films from oxygen-samrated aqueous solution of ZnCh and eosin Y. J. Electrochem. Soc., 150, C608-C615. 

Photoelectrochemical characterisation and optimisation of electrodeposited ZnO thin films sensitised by porphyrins and phthalocyanines. Phys. Chem. Chem. Phys., 33, 3867-3875. 

Nonomura, K., T. Loewenstein, E. Michaelis, D. Wohrle, T. Yoshida, H. Minoura, and D. Schlettwein (2006). Photoelectrochemical characterisation and optimisation of electrodeposited ZnO thin films sensitized by porphyrins and phthalocyanines. Phys.Chem.Chem.Phys. submitted. 

Yoshida, T, T. Oekermann, K. Okabe, D. Schlettwein, K. Funabiki, and H. Minoura (2002). Cathodic electrodeposition of ZnO/eosinY hybrid thin films from dye added zinc nitrate bath and their photoelectrochemical characterizations. Electrochemistry 70, 470 87. 

Nonomura K, Loewenstein T, Michaelis E, Wohrle D, Oekermann T, Yoshida T, Minoura H, Schlettwein D (2006) Photoelectrochemical characterization of electrodeposited ZnO thin films sensitized by porph5oins and phthalocyanines. Phys Chem Chem Phys 8 3867-3875... 

Idowu M, Loewenstein T, Hastall A, Nyokong T, Schlettwein D (2010) Photoelectrochemical characterization of electrodeposited ZnO thin films sensitized by octacarboxymetallophthalocyanine derivatives. J Porph Phthalocyanin 14 142-149 Masilela N, Nombona N, Loewenstein T, Nyokong T, Sehlettwein D (2010) S5nnmetrically and unsymmetrically substituted carboxy phthalocyanines as sensitizers for nanoporous ZnO films. J Porph Phthalocyanin 14 985-992... 

Fig 6.10 SEM photograpns of ZnO (a) and ZnO/TSPcSi (b) thin films electrodeposited on basal-plane of onented pyrolytic graphite (OPG) electrode. 

Electrodeposition of dissolved precursors (especially in aqueous solutions) is a low cost and scalable method which is well suited to the mass production of thin films of metal-oxide semiconductors such as Ti02, Cu20, WO3, and ZnO. Control of temperature, pH and the deposition potential are important because the corresponding electrochemical reactions within the deposition bath mainly depend on these parameters. 

The methods available for preparation of the different layers in thin-film solar cells include physical methods such as vacuum sputtering, vapor-phase deposition, and molecular beam epitaxy as well as chemical methods such as chemical vapor-phase deposition, metal organic vapor-phase epitaxy, chemical bath deposition (CBD), and electrochemical deposition (ED). This chapter explores the potential of electrodeposition as a route to the fabrication of absorber layers such as CdTe, CIGS, and CZTS for thin-film solar cells. Electrochemistry may also be usefiil for the preparation of transparent layers such as ZnO this topic has been reviewed by Pauporte and lincot [13]. 

Figure 4.n Scanning electron microscopy analysis of ZnO/dye hybrid thin films electrodeposited at -0.9V and 70 C for 60 min from a 0.1 M Zn(NOj)2 aqueous solution (a) pure ZnO (b) ZnO/TSPcZn... 

Okabe, K., Yoshida, T., Sugiura, T., and Minoura, H. (2001) Electrodeposition of photoactive ZnO/xanthene dye hybrid thin films. Trans. Mater. Res. 

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