For n-Poly contact formation, the metallization paste forms direct contact through silver precipitates and tunneling with silver crystallites in the glass layer. However, for TBC cells, particularly on the 300 nm thick p-Poly side, silver ions are difficult to reduce to form silver precipitates in the glass layer. Additionally, the use of aluminum in the paste is challenging due to its tendency to form large silver-aluminum spikes, which raise J0,m. Moreover, the LEF process has not yet been adopted for p-Poly contact formation, preventing the formation of direct contact. In contrast, the n-Poly contact formation mechanism is largely similar to that of TOPCon. Unlike the textured wafer surface of TOPCon, the polished rear surface of the TBC cell, which has a higher Voc, creates difficulties for the glass frit to adequately soak the surface. This results in random etching on the wafer surface and reduces the chances of forming silver crystallites and silver precipitation. These challenges are particularly prevalent on the p-Poly side, added Guo.
To overcome these metallization challenges on the p-Poly side of the TBC cell, Guo suggested that paste makers should optimize the glass frit and silver powder to improve contact. This can be achieved by optimizing the floating behavior of the glass frit to better soak the flat surface. Additionally, the optimized glass frit design should melt enough silver to form silver ions and precipitates, which can help avoid high J0,m, and increase contact resistance by controlling the size and number of silver precipitates.
Furthermore, the trend of thinning polysilicon layers in both the p-Poly (>200 nm) and n-Poly (250-300 nm) areas, required for cost optimization and higher bifaciality, faces challenges – particularly in the p-Poly area – due to increased contact resistance. Therefore, to reduce p-Poly thickness, fine-tuning the metallization paste composition is essential. For the thinner n-Poly layer, paste optimization involves balancing Voc and fill factor (FF).
Regarding the co-firing of n-Poly and p-Poly pastes at the same temperature, and the limitations of the p-Poly paste in forming a good contact, Guo suggested optimizing the p-Poly paste recipe and firing temperature, followed by optimizing the n-Poly paste for the firing process. Finger morphology, which directly impacts contact area and gridline resistance, can be controlled by adjusting silver powder size, sintering characteristics, and compatibility with the glass frit. This makes the optimization of the p-Poly paste’s organic and silver content key to achieving high-efficiency and low-cost TBC cells, as noted by Guo.
Reduction of Ag Consumption
To reduce silver (Ag) consumption for cost-optimized TBC cell metallization, the industry is exploring multiple approaches, including fine-line printing, lower silver-content pastes, and base metal pastes.
Paste Solutions
DKEM’s paste solutions for TBC cell metallization include the DK95H series for n-Poly paste, the DK73K series for p-Poly paste, and the DK82B series for busbar paste. paste.
