Metallization paste supplier Monocrystal from Russia has introduced a new local BSF forming aluminum paste called EFX-9X series. The paste is specially designed for high-efficiency PERC cells featuring a dot laser pattern based rear metallization scheme. As for the associated improvements, Monocrsytal says, the new model realizes a better contact over the previous model, reduces the paste utilization compared to the products in the market and contributes to higher efficiency.
Advantages of dot patterns
Local BSF forming aluminum paste is the most critical within the PERC paste system. A suitable aluminum paste does not react on those areas where it covers the passivation layer, but at the same time it establishes contacts at areas opened with lasers to create a local BSF. The most simple and common practice is to implement a laser pattern in straight lines to realize 30 to 50 mm wide line openings with 1 mm inter-line spacing. According to laser supplier Innolas, the optimal cross-section-equivalent opening ratio for typical PERC cell is about 1%, while the above straight line pattern has an opening ratio in the range of 3 to 6%. Increasing inter-line distance would help to reduce the opening ratio, but such a move also increases the series resistance. A better tradeoff between the series resistance and opening ratio can be attained with a dot pattern. Here, the laser pattern takes the course of the dot pattern by maintaining the same inter dot distance. However, the approach increases the complexity of the contact formation in the co-firing step.
The EFX-9X series was specifically improved in this area of reliable contact formation, making it suitable for dot pattern metallization. According to Monocrystal, the new paste comes with 17% higher local contact efficiency compared to the previous EFX-series pastes. At the same time, the paste also supports low laydown. The paste supplier says, 30% less EFX-9X series paste is required compared to market average. According to Monocrsytal, the superior local contact forming efficiency has a potential to increase the cell efficiency by 0.1% absolute.
In December 2016, Monocrystal already introduced a silver paste called MY-555, which was specially designed for the rear tabbing of PERC cells (see New PERC Metallization Past From Monocrystal).
TaiyangNews has published a report on PERC technology, which also provides background on metallization for PERC cells. The TaiyangNews PERC report can be downloaded free of charge here.