Abstract: In this study,the effect of hydrogen injection on ultrathin（＜10 nm）hydrogenated amorphous silicon（a-Si:H）films by catalytic chemical vapor deposition（Cat-CVD）system is investigated. By applying a proper hydrogen injection process,the hydrogen contents is increased in the a-Si:H film. Simultaneously,the optical bandgaps are broadened is enhanced acwmpanied with microstructural factors being reduced. When the process is applied to i-or n-a-Si:H window layers of n-type rear-emitter SHJ solar cells,passivation qualities are improved apparently. The numerical analysis by AFORS-HET further demonstrates that the improved performance of solar cells originates from the reduced defect densities at the front i-a-Si:H/c-Si interface as well as in the n-a-Si:H bulk.As a consequence of the improved interface quality and a-Si:H bulk properties at the front side,the characteristics of SHJ solar cells is enhanced significantly（Vocfrom 728.4 to 736.1 mV,Jscfrom 37.99 to 38.20 mA/cm~2,FF from 79.67% to 81.07% and Eff from22.04% to 22.79%）.