A Novel Buffering Technique for Aqueous Processing

A novel buffering method is presented to improve the stability of zinc oxide processed in aqueous solutions. By buffering the aqueous solution with a suitable quantity of sacrificial zinc species, the dissolution offunctional zinc oxide structures and' the formation of unwanted impurities can be prevented. The method is demonstrated for ZnO films and nanowires processed in aqueous solutions used for the selective etching of mesoporous anodic alu· mina templates and the electrochemical deposition ofCu20. In both cases, improved ZnO stability is observed with the buffering method. ZnO·Cu20 heterojunction solar cells (bilayer and nanowire cells) synthesized using both traditional and buffered deposition methods are characterized by impedance spectroscopy and solar simulation measurements. Buffering the CU20 depo· sition solution is found to reduce unwanted recombination at the heterojunc· tion and improve the photovoltaic performance. heterojunctions such as ZnO-CdSe-CuSCN and ZnO-Cu20 have been employed in inexpensive inorganic solar cells,l10-131 While there has been much effort to synthesize functional ZnO structures by inexpensive, aqueous methods (e.g. electrodeposition, hydrothermal growth), the quality of the interfaces and nanostructures formed by the aqueous techniques has typically been overlooked. Consider the examples in Figure 1. In panel (a) , a heterojunction is formed by electrochemical deposition onto a ZnO film or nanostructure from an aqueous solution, a technique used previously to synthesize photovoltaic devices.llO-131 In panel (b), a surface treatment or coating is applied to a ZnO nanostructure using an aqueous solution. Plank et al. for example,