Microstructural characterization of abnormal grain growth behavior of Al-doped ZnO in thin film solar cells

The microstructure of abnormal grains in Al-doped ZnO (AZO) films using a seed layer was examined with the aim of enhancing the performance of transparent conducting oxides in thin film solar cells. The AZO films were prepared by DC-pulsed magnetron sputtering using a two-step process. The seed layer was deposited on a glass substrate under an Ar/O2 atmosphere with a gas flow ratio of 24/1, and the bulk layer was formed on the seed layer in pure Ar. The microstructural properties of the AZO films were investigated by transmission electron microscopy (TEM), fast Fourier transform simulation, and strain analysis with high-resolution TEM images. The TEM revealed peculiar microstructures on the AZO film showing abnormal grains and a rough surface. The AZO seed layer was formed at a thickness of 50 nm, and two types of grains with columnar and hillock structures were formed on the seed layer. Elemental analysis using energy dispersive X-ray measurement showed that the abnormal grains contained more metallic elements (Al and Zn) than the normal region with columnar structure in the AZO film. The analysis of fast Fourier transform images of each region showed that the crystal structures of the normal and abnormal grains were the same, whereas the abnormal grain had a different growth orientation from that of the normal region. Copyright (C) 2012 John Wiley & Sons, Ltd.