Spray-Deposited Al2O3 for Rear Passivation and Optical Trapping in Silicon Solar Cells

Abstract

Si passivated emitter rear contact (PERC) are expected to become the dominant cell technology in the near future [1]. In PERC cells, a thin layer of Al2O3 is deposited on the rear of the Si cell to reduce rear surface recombination on p-type Si by chemical and field-effect passivation [2]. Optically the Al2O3 film is supported by a thicker SiNx layer stacked on Al2O3, and the low refractive index of SiNx improves light trapping in the Si cell by rear reflection [3]. In commercial production, the Al2O3 film is prepared by atomic layer deposition (ALD). ALD is a slow process and the precursor for Al2O3, Al2(CH3)6, is expansive and pyrophoric. For these reasons the thickness of the Al2O3 film in PERC cells is limited to ~10 nm and the 80-nm SiNx layer is deposited on Al2O3 by plasma-enhanced chemical vapor deposition (PECVD). It is noted that the refractive index of Al2O3 is lower than that of SiNx, 1.6 vs. 2.0 [4,5], so a thicker Al2O3 film on the rear of the PERC cell should provide better rear passivation and optical trapping than an Al2O3/SiNx stack. This requires a low-cost process to deposit thicker Al2O3 on Si. In this talk, we report the suitability of spray-deposited Al2O3 for rear passivation and optical trapping in Si PERC cells. Electrical, optical and structural properties of spray-deposited Al2O3 are investigated and compared to the industrial standard ALD Al2O3/PECVD SiNx stack. It was found that spray-deposited Al2O3 has a negative charge density of 3.19×1012 cm–2 for an 80-nm film, indicating that spray-deposited Al2O3 can serve as the passivation layer. Optical properties of spray-deposited Al2O3 are identical to the ALD Al2O3/PECVD SiNx stack, suggesting that spray-deposited Al2O3 can also serve as the optical trapping layer. It was also found that spray-deposited Al2O3 is crack and pore free, and its surface roughness has a root-mean-square value of 0.42 nm for an 80-nm film. Spray-deposited Al2O3 is amorphous and its composition is slightly Al rich. The resistivity and breakdown field of 80-nm spray-deposited Al2O3 are ~1014 Ω-cm and 3.28 MV/cm, respectively. These properties suggest that spray-deposited Al2O3 is a promising candidate to replace the ALD Al2O3/PECVD SiNx stack in Si PERC cells. [1] C. Roselund, International Technology Roadmap for Photovoltaic (ITRPV), 7, 21, (2016). [2] N. Batra, J. Gope, R. Singh, J. Panigrahi, S. Tyagi, P. Pathi, S. K. Srivastava, C. M. S. Rauthan, P. K. Singh, Phys. Chem. Chem. Phys., 16(39), 21804 (2014). [3] B. Vermang, H. Goverde, L. Tous, A. Lorenz, P. Choulat, J. Horzel, J. John, J. Poortmans, R. Mertens, Prog. Photovolt. Res. Appl., 20(3), 269 (2012). [4] M. D. Groner, F. H. Fabreguette, J. W. Elam, and S. M. George, Chem. Mater., 16(4), 639 (2004). [5] J. Schmidt and M. Kerr, Sol. Energy Mater. Sol. Cells, 65(1-4), 585 (2001).