The microstructure of the intrinsic amorphous silicon oxide (i-a-SiOx:H) thin films is the key for crystalline silicon surface passivation in high-performance HIT solar cells. Understanding and subsequently optimizing the i-a-SiOx:H microstructure is essential in improving defect passivation and carrier transportation in a-SiOx:H films. In this work, we investigate the relationship between the bulk microstructure of i-a-SiOx:H thin films at different oxygen content and the passivation quality of the silicon surface. The results revealed that, as the oxygen content increases, the dominating growth mechanism of i-a-SiOx:H films changes. Consequently, the component contents of i-a-SiOx:H thin films were varied and the microstructure showed a first improved and then deteriorated trend with increasing oxygen content. A compact, less-defective and ordered microstructure of the a-SiOx:H film can only be obtained when there comes a tradeoff between Si-O and Si H(Si-3) bonding formation. Correspondingly, the improved defect passivation and the enhanced carrier transportation in a-SiOx:H films lead to the increase of Voc, FF and QE of HIT solar cells, all of which are key solar cell performance parameters. (C) 2017 Elsevier Ltd. All rights reserved.