Solamet highlighted the potential efficiency improvement routes for TOPCon cells, noting that each pathway requires enhanced performance from the metallization paste
Fine-line screen printing, which addresses optical losses and current transport cell efficiency losses, requires a metallization paste with good printability and low line resistance
Multiple TOPCon cell efficiency improvement routes, such as FS Poly finger, boron diffusion with high Rsheet, and rear-side poly optimization, demand metallization paste that can balance between Ag-Si contact and surface recombination along the cell surface
Passivated edge technology (PET) requires improved thermal stability of TOPCon cell
The ongoing shift toward high-efficiency cell structures, driven by the pursuit of greater efficiency and cost optimization, consistently challenges metallization pastes to meet the required cell parameters. The TOPCon cell structure, widely adopted by leading global cell manufacturers, has undergone continuous technological advancements in mass production since 2021. Initially, the TOPCon cell featured a homogeneous p+ emitter metallized with Ag-Al paste, with mass-scale efficiency ranging from 24.5% to 25%. By the end of Q3 2023, this progressed to a more efficient TOPCon cell, achieving up to 25.3% efficiency with the introduction of laser doping selective emitter (LDSE) technology. Furthermore, the adoption of the laser-enhanced contact optimization (LECO) process, combined with selective emitter (SE) technology, boosted mass-scale TOPCon cell efficiency to 26% by the end of Q1 2024. More recently, replacing SE technology with the LECO metallization process alone has pushed efficiency beyond 26%. Metallization paste makers face several challenges with various cell efficiency improvement technologies in their pursuit of even higher efficiencies.
At the recent TaiyangNews High-Efficiency Solar Technologies 2024 virtual conference, Zinan Zhang, Technical Director at Solamet, provided a brief overview of the technological advancements needed in TOPCon cell metallization pastes to align with various efficiency improvement pathways (see Solamet presentation here). Jiangsu Solamet Electronic Materials Co. Ltd. acquired the DuPont Solamet metallization business in 2021 and is headquartered in Shanghai and Huzhou-Zhejiang, China. It also has manufacturing and research and development (R&D) centers in China and Taiwan.
Zhang highlighted the potential efficiency improvement routes for TOPCon cells, including fine-line screen printing, front-side poly finger, boron diffusion with high sheet resistance (Rsheet), passivated edge technology (PET), and rear-side poly optimization. Each of these pathways addresses different efficiency loss mechanisms – optical losses, electrical losses, surface recombination, and current transport.
Fine-line screen printing, which addresses optical losses and current transport, requires a metallization paste with good printability and low line resistance. The company emphasized that the printability of a metallization paste, in terms of morphology and paste transfer, can be characterized by electroluminescence (EL) performance and the resistance of grid lines (RGL) in sample cells. Solamet shared in-house test results comparing 3 paste samples with different screen openings, ranging from 14 µm to 6 µm.
The SYS 3 sample showed low RGL and a very uniform grid line morphology when printed using a screen opening smaller than 9 µm, compared to SYS 1 and SYS 2. However, to assess the printability of the metallization paste using ultra-fine line screen printing (with screen openings smaller than 7 µm), Solamet's testing showed that silver particles – part of the paste composition, including Ag powder, glass frit, binder, and solvent – significantly impact printability. Solamet emphasized that fine-tuning the surface morphology of Ag powder particles, particularly by controlling the D100 particle distribution, can improve paste performance and uniformity. Additionally, optimizing the ratio of different Ag particle sizes, removing D100 particles, and balancing solid content, viscosity, and sintering density contributes to improved fine-line printability and reduced grid line resistance.
The demand for metallization pastes that maintain a balance between Ag-Si contact and metal recombination at the surface of the cell is driven by multiple efficiency improvement routes, such as FS Poly finger, boron diffusion with high Rsheet, and rear-side poly optimization. The company noted that metallizing a high Rsheet p+ emitter in TOPCon cells with LECO and special metallization pastes shows limitations when sheet resistance exceeds 400 Ohms. Zhang acknowledged the need for further investigation, as these metallization pastes are reaching their contact bottleneck. Additionally, the front-side poly finger TOPCon cell structure, which carries a potential risk of parasitic absorption, could damage the front-side poly-Si layer due to large contact points generated during the LECO process.
Furthermore, passivated edge technology (PET), which involves passivating the cleaved edges of cells with a thin film of Al2O3 followed by an annealing process at 250°C for 30 minutes, requires improved thermal stability in TOPCon cells metallized with LECO paste. However, there are several approaches to reduce silver consumption in TOPCon metallization, including fine-line printing, reducing Ag content in rear-side paste, and adopting busbarless (0BB) metallization, added the company.