Silicon heterojunction (SHJ) solar cell that combines traditional pure H-2 plasma treatments has been frequently reported in the literature. However, this method requires an individual gas blending step between a-Si:H film deposition and post H-2 plasma treatment to stabilize the gas environment in the PECVD chamber. Here, we report the introduction of residual SiH4 molecules in H-2 plasma to treat SHJ solar cell devices. In contrast to the traditional H-2 plasma treatments, it requires no time interval between the a-Si:H film deposition and H-2 plasma treatment, i.e., we merely closed the SiH4 inlet after the a-Si:H deposition. In the meantime, all other PECVD parameters were kept unchanged. Taking advantage of the decreasing SiH4 density during the H-2 plasma process, a dense silicon layer was grown onto the top layer of the as-deposited a-Si:H film, which inhibited free H atoms effusing out of the low-mass-density a-Si:H network. The better a-Si:H/c-Si interface passivation results in improvements to both the short-circuit current density (J(sc)) and open-circuit voltage (V-oc) of the SHJ solar cell in comparison to the counterpart cell treated by the traditional pure H-2 plasma. For instance, when the n(+) a-Si:H window layer is as thin as similar to 1.8 nm, the power-conversion efficiency skyrockets from 2.35% treated by the traditional pure H-2 plasma to 16.09% treated by the H-2 plasma containing residual SiH4 molecules. For a thicker n(+) a-Si:H window layer of similar to 4.3 nm, the efficiency is also enhanced from 20.66% to 22.74%. This finding paves the way for a more efficient H-2 plasma treatment in pursuit of an outstanding SHJ solar cell.