Paper TF3-MoA7
Impact of Film Thickness and Thermal Treatment on the Excellent Surface Passivation of c-Si by ALD Al₂O₃ for Solar Cell Applications

Monday, November 9, 2009, 4:00 pm, Room B4

The surface passivation of c-Si by atomic layer deposited (ALD) Al₂O₃ has recently gained considerable interest after extremely low surface recombination velocities (< 10 cm/s) have been reported for low resistivity n- and p-type c-Si wafers [1]. The incorporation of an Al₂O₃ film for boron doped emitter passivation led to enhanced efficiencies of 23.2% for n-type c-Si solar cells [2]. From the cumulative research, various questions related to the thermal stability and other processing aspects of the Al₂O₃ films appeared. In this contribution we will show that a decrease of film thickness down to ~5 nm does not compromise the passivation quality, enabling a straightforward reduction of deposition time and providing freedom in the design of optimal front passivation/antireflection schemes. To activate the Al₂O₃ surface passivation a post-deposition anneal is required, but also the thermal budget during the plasma enhanced chemical vapor deposition of an a-SiN_x:H capping layer was found sufficient to activate the passivation. Although an anneal effect can be observed in a large temperature range (~350 - ~600^oC) the optimal post-deposition anneal temperature window was found to be much smaller. Within the optimum temperature range, the largest anneal effect was observed to take place during the first minutes of the process. For the successful implementation of Al₂O₃ in conventional screen printed solar cells, thermal stability is required. In this paper we demonstrate the relative stability of Al₂O₃ and Al₂O₃/a-SiN_x:H passivation/antireflection stacks against an industrial “firing” process reaching temperatures > 800^oC. Although the minority carrier lifetime was found to decrease during the firing process, values in excess of 1 ms were obtained on 2 Ω cm n-Si wafers after firing. These lifetimes suggest that surface recombination will not likely be the efficiency limiting step for solar cells that combine Al₂O₃ passivation and screen printed metallization as recombination in the metalized area will be dominant. The findings in this paper demonstrate the suitability of thin ALD-synthesised Al₂O₃ passivation films for large scale photovoltaic applications.

[1] B. Hoex, J. Schmidt, P. Pohl, M. C. M. van de Sanden, and W. M. M. Kessels, J. Appl. Phys. 104, 044903 2008

[2] J. Bennick, B. Hoex, M.C.M. van de Sanden, W.M.M. Kessels, O. Schultz, S. Glunz, Appl. Phys. Lett. 92, 253504 (2008)