Passivation properties of RST silicon ribbon by single SiNx:H or double a-Si:H/SiNx:H layer and solar cells

Abstract

In this work, we have investigated the electronic quality of Silicon Ribbon obtained on a Sacrificial carbon Template (RST) before and after surface passivation by a SiN:H layer or a double a-Si:H/SiN:H layer. We have studied by optical microscope the RST ribbon samples after chemical etching to reveal the defects and grains orientation. The thin layers a- Si:H and/or SiNx:H were deposited on the RST materials in a remote plasma enhanced CVD reactor. The amorphous silicon was deposited at 50°C while the silicon nitride was obtained at 400°C using SiH4 and NH3. The passivation effect was monitored by measuring the minority carrier effective lifetime using the photoconductivity decay set-up. Effective minority carrier lifetime values in the range ~ 6-8 µs were measured on Ribbon samples coated with 70nm thick SiN:H layer while it is about ~4 µs when using first a 10nm thick a-Si:H covered with a 70nm SiN:H layer. The minority carrier diffusion length that can be deduced from such values is in the range 60-80 µm, almost the thickness of the ribbon sheets. Our results indicates that the SiNx:H is very efficient for the surface passivation of the RST ribbon, while the double a-Si/SiN has to be optimized versus the a-Si:H deposition parameters. We have fabricated solar cells on the grown RST sheets. First test cells exhibit efficiencies approaching 10% on 120µm thick ribbons. There are rooms for further improvements by optimizing the process.