Sputtered hydrogenated amorphous silicon for silicon heterojunction solar cell fabrication

This work shows that RF sputter-deposited hydrogenated amorphous silicon (a-Si:H) films are very effective in passivating silicon surfaces. We have previously found that sputter-deposited 45 nm thick intrinsic a-Si:H provides outstanding surface passivation on n-type silicon, similar to that achieved by 'classic' plasma enhanced chemical vapour deposition [1]. In this paper, we show that p-type silicon surfaces can be well passivated as well, achieving effective carrier lifetimes of 1.1 ms for a 1 Ω.cm ptype wafer, compared to 4.5 ms for a 1.5 Ω.cm n-type sample. Next, on n-type textured surfaces reasonable passivation is also achieved. Post-deposition annealing of our samples shows that sputtered a-Si:H films can perform similarly to PECVD deposited films in terms of thermal stability. Importantly, with stacks of intrinsic and doped (n or p) amorphous silicon effective carrier lifetimes of 1.9 ms and 1.6 ms on 1.5 Ω.cm n-type wafers were obtained for i/n⁺ and i/p⁺ stacks respectively. These results underline the promise of sputter-deposited a-Si:H as an attractive alternative for heterojunction solar cell fabrication. However, dark conductivity measurements show that sputter-deposited doped a-Si:H films feature a relatively low conductivity, so far. We speculate that this may be caused by differences in microstructure compared to PECVD a-Si:H films, as suggested from the extracted optical band gap values for the respective films.