Resolving the microstructure of aluminum-doped zinc oxide thin films grown on different silicon heterojunction solar cell structures by advanced transmission electron microscopy

Abstract

Advanced microscopy techniques have been employed to resolve the microstructure of transparent conductive oxide (TCO) contacts in silicon heterojunction solar cells. Aluminum-doped zinc oxide (AZO) stands out as a TCO material because of its low cost, abundance, and good optoelectrical properties. The polycrystalline AZO thin films have yielded promising results in solar cell design. However, understanding the nanostructure of AZO thin-film materials is vital for enhancing the cell performance by focusing on the formation of large grains and their influence on the charge-carrier mobility of the film. Therefore, we employed high-resolution transmission electron microscopy (HRTEM) and precession-assisted four-dimensional scanning transmission electron microscopy (4D-STEM) with an automated crystal orientation analysis. These techniques can be used to determine the grain sizes of AZO films sputtered on hydrogenated amorphous silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H) layers. Columnar grains in the AZO/a-Si:H film are evident in the grain mapping with diameters greater than 10 nm, whereas in the AZO/nc-Si:H film, the grains begin at diameters less than 10 nm, showing smaller grains near the substrate than at the top of the film. Additionally, the double-layer with indium-thin doped oxide (ITO)/AZO stack started with grain diameters varying from 5 to 90 nm. They exhibit significantly larger and irregular boundaries. Therefore, microstructural characterization showed that larger columnar grains might lead to higher mobility in the AZO layer. This finding indicates that the impact of the ITO seed layer on AZO significantly enhances grain size, improves charge carrier mobility, and overall improves the power conversion efficiency (ƞ) to be 23.6% comparable to those of AZO on a-Si:H and nc-Si:H.