Hydrogenated amorphous si deposition for high efficiency a-Si/c-Si heterojunction solar cells

Abstract

We study the differences in hydrogenated amorphous Si (a-Si:H) depositions between Hot-Wire Chemical Vapor Deposition (HWCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD) for high efficiency a-Si/c-Si heterojunction (HJ) solar cells. In HWCVD, process gases such as silane decompose from the high-temperature hot filament. The resulting deposition is thought to be gentle due to the lack of ion bombardment that may cause damage to c-Si surface. In PECVD, process gases decompose from a high frequency electric field and ion bombardment is expected during the a-Si:H deposition. We found that the initial minority carrier lifetime of a-Si:H passivated high-quality n-type wafer was higher (about a ms) with the HWCVD process, and the final minority carrier lifetime (after 250°C annealing) was higher (over a few ms) with the PECVD process. These findings suggest that the damage from the ion bombarding in PECVD is not as detrimental as we expected; or if there is damage, it can be repaired by the annealing. We also speculate that the lack of further increase of the lifetime after annealing with HWCVD intrinsic a-Si:H layer can be related to the direct substrate heating from the hot filament during the deposition. A high substrate temperature will promote epi-Si growth and drive hydrogen out of the a-Si/c-Si interface to decrease the quality of surface passivation. To reduce the heating effect, a shutter and a low filament temperature are preferred. With the optimized process, we were able to fabricate HJ solar cells with high open circuit voltage of 714 mV and efficiency greater than 19% on an un-textured n-type wafer using the PECVD process, and independently confirm best efficiency of 19.7% on textured n-type wafer with the HWCVD process.