Low-Refractive Index Silica Aerogel as Rear Reflector for Large-Area PERC Solar Cell Applications

Increasing the path length of light in the absorber is one of the most effective optical solutions for dealing with light that is not fully absorbed in a single pass through solar cell devices [1]. The design to improve the path length should also have a high internal reflectance on the absorber surfaces to minimize the energy loss per light-surface interaction [2]. Recent studies show that SiN_x layers with low refractive index (~ 2) are also preferable to increase rear reflection [3] and passivation of the rear surfaces [4] for Si solar cells architectures such as PERC and PERL.<br/>To reduce light absorption in rear metal, inserting a thick non-absorbing reflector layer with a low refractive index n between the absorber and rear metal is very beneficial [5], since a lower n leads to a smaller critical angle at the rear surface and a lower probability that the light will be absorbed by the metal. Passivation is one of the most important parameters for solar cells. SiN_x layer is used for chemical surface and field-effect passivation of PERC solar cells. SiN_x layer on the rear surface is preferably thicker than SiN_x layer on the front surface as an anti-reflective layer to improve the rear reflection on the rear side of the cell by obtaining a film with a lower refractive index [4].<br/>In this work, we studied the effects of a solution-based silica aerogel layer (by sol-gel method), which has a low refractive index and high porosity, inserted between the passivation layer and the rear metal to PERC solar cells. Large area (~156 mmX156 mm) PERC solar cells were fabricated using a thin-SiNx passivation layer (~20 nm), and it was investigated how the optical and thus the cell performance (i.e., J_sc, power conversion efficiency) improves when silicon aerogel is used between the passivation and rear metal. PERC solar cells with two different SiN_x passivation layer thicknesses (20 nm and 120 nm) were used for this study. Since silica aerogel is to be used as the rear reflector, the thickness of the thin SiN_x passivation layer (~20 nm) was chosen so that it would not negatively affect surface passivation [6].<br/>Methyltrimethoxysilane (MTMS), methanol (MeOH), oxalic acid as acid catalyst, and ammonium hydroxide (NH₄OH) as base catalyst were used to synthesize aerogel solutions. The silica aerogel was sprayed on the rear side of the PERC with rear 20nm SiN_x and compared with the PERC without aerogel and the industrial PERC (there is ~120 nm SiN_x passivation layer on the rear side of the cell) with respect to the cell performance.<br/>Transmission (T) and reflection (R) measurements show that the silica aerogel has an optical enhancement in the near-infrared (NIR at ~ 1200 nm wavelength) region. We have an increase of a ~ 25% in T+R spectra when aerogel is deposited on the top of thin-SiNx passivation layer on the rear side of the cell. Also, it was observed that PERC with thin-SiN_x spray-coated with aerogel on the rear side has the same optical performance as the industrial PERC with 120 nm rear SiN_xon the T+R spectra. The optical enhancement in this new PERC design also improves cell performance. We have demonstrated that the improvement in rear reflection increases the short-circuit current by ~1.6 mA/cm² compared between PERC with aerogel and without aerogel.<br/>In conclusion, we have shown that the proposed aerogel layers ultrasonically sprayed on the rear side of the PERC solar cells with a thin-SiN_x passivation layer achieved an efficiency of 19.9%, while the reference cell (PERC with thin-SiN_x and without aerogel) had an efficiency of 18.75%. Moreover, the PERC with aerogel performed as well as the industrial PERC solar cell with 120 nm rear SiN_x (20.1%) over a long period of time (~700 hours).<br/>[1] https://doi.org/10.3390/MA13081860.<br/>[2] https://doi.org/10.1109/T-ED.1986.22753.<br/>[3] https://doi.org/10.1016/j.solmat.2013.09.017.<br/>[4] https://doi.org/10.1002/er.6201.<br/>[5] https://doi.org/10.1038/lsa.2013.62.<br/>[6] https://doi.org/10.1016/j.surfin.2021.101496.