HJT manufacturers have moved from high busbar counts to zero busbar (ZBB) layouts and are adopting silver-coated copper pastes to reduce silver usage
Finger optimization using narrower screen openings and steel stencil screens has reduced finger widths to around 10 μm, lowering paste consumption
Despite lower silver content, efficiency losses remain within 0.1%, and silver-coated copper pastes are proving viable for both SMBB and ZBB configurations
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%.
Low Laydown
The finger provides yet another avenue for optimization, which is reducing the width, and is directly proportional to paste usage and costs. According to Fusion, between 2022 and 2024, silver consumption in HJT cells remained higher than in TOPCon. Between 2022 and 2024, screen opening widths were reduced from 22 μm to 14 μm. As a result, silver consumption decreased from 21 mg/W to 11 mg/W, though still slightly above TOPCon levels. Risen puts silver consumption at 10 mg/W in 2023, also using silver-coated copper.
In order to reduce the finger width further, which is necessary to reduce metallization costs, the HJT segment is moving away from mesh screens to stencil screens based on either nickel or steel. Since these screens are free of crossover wires of mesh, they facilitate realizing fingers with higher aspect ratios, meaning lowering finger widths. According to Fusion, using these screens has enabled manufacturers to reduce screen openings to around 10 μm, with a few going even further. Risen emphasizes that it has achieved a finger width of 24 μm and an aspect ratio of 46% with its knotless steel screens. Fusion also attained a 46% ratio and about a 27 μm finger thickness with its steel screen. When combined with ZBB technology and the use of silver-coated copper pastes with just 30% silver content, silver consumption can be brought down to as low as 6 mg/W. At this level, silver usage is already below that of TOPCon, and Risen also put a cost tag of 4.4 to 4.8 RMB cents/W.
With ongoing advancements, including further narrowing screen openings to around 10 μm or even below, and the adoption of steel plate screens, some customers have already achieved additional reductions.
This text is an excerpt from the TaiyangNews Cell & Module Technology Trends 2025 report, which can be downloaded for free here.