Effect of post-PECVD photo-assisted anneal on multicrystalline silicon solar cells

Silicon nitride (SiN_x) films deposited by plasma enhanced chemical vapor deposition (PECVD) contain large amount of atomic hydrogen which can be driven into bulk silicon by post-PECVD anneal. The objective of this paper is to understand and quantify the effects of the anneal on multicrystalline silicon (mc-Si) solar cells. Detailed cell analysis and model calculations are performed to assess the impact of the anneal on mc-Si cells. Simple n⁺-p-p⁺ solar cells with PECVD SiN_x/SiO₂ antireflection (AR) coating are annealed in the temperature range of 350°C to 700°C. The efficiency of the cells made on EFG silicon increases by 45% due to the AR coating and then additional 25% due to the anneal. A trade off between short and long wavelength response is found during the anneal. Low temperature anneal increases the short wavelength response, while high temperature anneal improves the long wavelength response at the expense of the short wavelength response. It is shown that the increase in short wavelength response is due to improved surface passivation, and the decrease in short wavelength response after high temperature anneals is the result of the increase in short wavelength absorption in the SiN_x film. Higher quality HEM silicon cells do not show appreciable increase in the long wavelength response at higher anneal temperatures. Thus post-PECVD low temperature anneal helps all mc-Si cells, but the effect of high temperature anneal is material specific. Cells made from materials which do not respond to hydrogenation can degrade at high temperature anneal