In this work, effects of a hydrogen dilution ratio on the intrinsic amorphous hydrogenated silicon (i-a-Si:H) film of heterojunction silicon-based (HJS) solar cells were systematically studied. Long lifetime samples were obtained for R <= 5, indicating a good a-Si:H/c-Si interface. The dark conductivity was drastically decreased for R >= 2, indicating a good film quality. Consequently, an optimized power conversion efficiency of the HJS solar cells was obtained at a moderate R between 2 and 5. In contrast to the previous emphasis on long lifetime, the results indicate that both the interface and film qualities are correlated to the hydrogen dilution, which are important to achieve high-efficiency HJS solar cells. We show that the most optimized HJS solar cell exhibits a marked efficiency of 17.27%. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3607981] All rights reserved.