Controllable a-Si:H/c-Si interface passivation by residual SiH4 molecules in H2 plasma

Abstract

Silicon heterojunction (SHJ) solar cell that combines traditional pure H2 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 H2 plasma treatment to stabilize the gas environment in the PECVD chamber. Here, we report the introduction of residual SiH4 molecules in H2 plasma to treat SHJ solar cell devices. In contrast to the traditional H2 plasma treatments, it requires no time interval between the a-Si:H film deposition and H2 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 H2 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 (Jsc) and open-circuit voltage (Voc) of the SHJ solar cell in comparison to the counterpart cell treated by the traditional pure H2 plasma. For instance, when the n+ a-Si:H window layer is as thin as ~ 1.8nm, the power-conversion efficiency skyrockets from 2.35% treated by the traditional pure H2 plasma to 16.09% treated by the H2 plasma containing residual SiH4 molecules. For a thicker n+ a-Si:H window layer of ~ 4.3nm, the efficiency is also enhanced from 20.66% to 22.74%. This finding paves the way for a more efficient H2 plasma treatment in pursuit of an outstanding SHJ solar cell.