The aluminium back surface field (Al-BSF) solar cell has been the working horse for the photovoltaic industry in the recent decades. However, from 2013 the industry is changing to the so-called PERC (passivated emitter rear contact) structure. The schematics of these two solar cells is shown in Figure 1. The main difference between these two structures is at the rear where the full area aluminium alloyed back contact is replaced by a dielectric passivation layer, a dielectrically displaced mirror, and local aluminium alloyed contacts. This small change significantly increases the efficiency potential of a silicon solar cell as it significantly reduces the recombination of photogenerated electron-hole pairs at the rear surface resulting in a boost in open circuit voltage. In addition, the dielectrically displaced mirror results in a significant increase in the rear internal reflection for near bandgap photons which increases the short circuit current density of the solar cell. The PERC solar cell was first introduced by UNSW in 1983 and it claimed the world-record efficiency in 1989 [1]. The PERC concept was further improved by the usage of locally doped contacts in the UNSW PERL (passivated emitter rear locally diffused) solar cell which achieved the landmark 25% energy conversion efficiency in 1999 (after a recalibration using the revised AM1.5G spectrum) [2].
Figure 1: Schematic of a (top) aluminium back surface field and (bottom) passivated emitter rear contact (PERC) solar cell.
The main challenges for the commercialization of the PERL solar cell were the dielectric passivation, local contact opening, and the local doping. The solution for the local doping was already presented in 1989 by Knobloch et al. who showed that aluminium alloying could be used instead of boron diffusion for the localized contacts, which significantly simplified the processing [3]. Paste manufacturers have developed screen printing paste which is compatible with this process and, consequently, both the front and rear of the PERC solar cell still have screen printed contacts. The development in affordable high power and reliable lasers opened the way to use them for the formation of the local contacts in high-volume manufacturing. With respect to the rear dielectric passivation, the biggest innovation was the development of aluminium oxide at Eindhoven University of Technology and IMEC [4-6]. Aluminum oxide has an intrinsic negative charge density which is very beneficial for the passivation of p-type silicon and does not result in artefacts when using positively charged silicon nitride [7]. The PERC solar cell is predicted to become the dominant solar cell in the industry in the next few years [8]. The process flow for the PERC solar cell is shown in Figure 2 and requires three new steps compared to the Al-BSF solar cell as indicated by the red and purple colors. The dielectric stack at the rear is aluminium oxide capped with silicon nitride and the localised openings are made by a laser. At the end of the solar cell manufacturing, a LID elimination step is employed. Currently, most of the industry has moved to gallium doping which negates the use a separate LID mitigation step.
Figure 2: Process flow for the manufacturing of a PERC solar cell. In comparison to the conventional aluminium back surface field solar cell process flow, an addition dielectric stack is deposited on the rear of of the solar cell and an light induced degradation (LID) elimination step is added.
[1] A. W. Blakers, A. Wang, A. M. Milne, J. H. Zhao, and M. A. Green, “22.8-PERCENT EFFICIENT SILICON SOLAR-CELL,” (in English), Applied Physics Letters, Article vol. 55, no. 13, pp. 1363-1365, Sep 1989. Available: https://doi.org/10.1063/1.101596
[2] J. H. Zhao, A. H. Wang, and M. A. Green, “24.5% efficiency silicon PERT cells on MCZ substrates and 24.7% efficiency PERL cells on FZ substrates,” (in English), Progress in Photovoltaics, vol. 7, pp. 471-474, 1999. Available: https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1099-159X(199911/12)7:6%3C471::AID-PIP298%3E3.0.CO;2-7
[3] J. Knobloch, A. G. Aberle, and B. Voss, “Cost effective processes for silicon solar cells with high performance,” in 9th EU-PVSEC, ed, 1989, pp. 777 – 780. Available: http://publica.fraunhofer.de/documents/PX-8744.html
[4] B. Hoex, S. B. S. Heil, E. Langereis, M. C. M. van de Sanden, and W. M. M. Kessels, “Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2O3,” (in English), Applied Physics Letters, vol. 89, p. 3, 2006. Available: https://doi.org/10.1063/1.2240736
[5] G. Agostinelli, A. Delabie, P. Vitanov, Z. Alexieva, H. F. W. Dekkers, S. De Wolf, and G. Beaucarne, “Very low surface recombination velocities on p-type silicon wafers passivated with a dielectric with fixed negative charge,” (in English), Solar Energy Materials and Solar Cells, vol. 90, p. 3438, 2006. Available: https://doi.org/10.1016/j.solmat.2006.04.014
[6] J. Schmidt, A. Merkle, R. Brendel, B. Hoex, M. C. M. van de Sanden, and W. M. M. Kessels, “Surface passivation of high-efficiency silicon solar cells by atomic-layer-deposited Al2O3,” (in English), Progress in Photovoltaics, vol. 16, pp. 461-466, 2008. Available: https://doi.org/10.1002/pip.823
[7] S. Dauwe, L. Mittelstadt, A. Metz, and R. Hezel, “Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells,” Progress in Photovoltaics, vol. 10, p. 271, 2002. Available: https://doi.org/10.1002/pip.420
[8] International Technology Roadmap for Photovoltaics (ITRPV) 13th Edition, 2022