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 SiN_x layer stacked on Al₂O₃, and the low refractive index of SiN_x 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 SiN_x 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 SiN_x, 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/SiN_x 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 SiN_x 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 SiN_x 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 SiN_x 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).