HJT as a technology is more than 2 decades old and has always been an efficiency pioneer. Until very recently, HJT always had an edge over its peers; even now, albeit marginal. But the major bottleneck of the technology has been its higher costs, mainly stemming from higher equipment CapEx and, more importantly, higher manufacturing costs. Metallization is the key contributor to costs, and if narrowing down to the microscale, silver paste is the origin. This is primarily because of the cost associated with specialized low-temperature silver pastes required for HJT. The lower conductivity of these pastes compared to the high-temperature pastes used in PERC and TOPCon necessitates wider fingers or higher paste laydown, increasing silver consumption. Thus, advancements in metallization are crucial for HJT's cost reduction roadmap.
Leading paste supplier Fusion has provided a nice overview on the status of the current metallization scene, recent advancements and outlook in this report. Metallization costs can be reduced in 2 ways: by reducing the paste usage and lowering the silver loading of the paste composition. The reduction in paste consumption can be achieved differently for busbars and fingers, and they are also interlinked.
More Busbars to No Busbars
Busbar optimization is a rather low-hanging fruit in optimizing metallization for HJT. The first line of action related to busbars is to increase the number of busbars. The number of busbars went from 9 to 20 between 2022 and early 2024, as shown in the graph below. This increase helped narrow the screen opening from 50 μm to 27 μm. Toward the end of 2024, Fusion developed an interesting version of silver-coated copper paste for busbars, with 55% silver content, and it quickly entered mass production. On the other hand, most of the leading manufacturers had moved away from using busbars in the metallization layout by then, adopting ZBB technology.
Low on Silver Loading
The cost reduction roadmap for fingers can take 2 parallel paths: reducing the paste laydown and reducing silver loading. An innovative form of the latter approach is using silver-coated copper particles instead of pure silver particles. This technology is not new, and the industry has been reducing the silver content of such pastes steadily. The industry was able to use Fusion’s paste formulation mentioned above and from other leading paste vendors with 50% silver content in 2023, followed by further reductions to 40% in Q2 2024 and 30% in Q4 2024. These pastes are suitable for use in both super MBB (SMBB) and ZBB architectures, and can be applied to both front and rear fingers. More recently, Fusion developed a new formulation with just 20% silver, currently recommended only for rear finger applications. This is mainly to keep performance parity with pure silver pastes within the acceptable deviation. The table below summarizes the performance losses of silver-coated copper along with the reduction of silver content. A point to be noted: the reduction in silver does not directly relate to cost savings, as one also needs to factor in the processing costs of silver-coated copper into the costs of silver-coated copper paste. For example, 50% silver content in a silver-coated copper paste reduces the costs by 35%, and its performance is comparable to pure silver-based pastes. A paste with 20% silver content reduces costs by 50%, maintains good reliability, and keeps efficiency losses within 0.1%.
