A somewhat revolutionary improvement that played a big role in the majority of companies claiming to have achieved the 26% efficiency level for TOPCon cells is laser-assisted contact optimization. At its core, the process uses laser power to form the front contacts of TOPCon solar cells. To provide some background, the emitter in a TOPCon cell is p+, which leads to the silver ion getting suppressed. The current practice to overcome this limitation is to use a silver-aluminum paste on the front side. Though the silver-aluminum paste solution improves contact resistivity, it sometimes results in higher recombination. This can be explained due to the formation of silver-aluminum spikes at a micron-scale, which damages junctions.
Employing a laser for contact formation can overcome this limitation. Here, a bias voltage and strong light injection with high current density diffuse the silver paste into silicon to form an ohmic contact. The process essentially decouples the metal-contact recombination and contact resistivity. The firing process controls J₀.m while the laser process enables good contact formation, which together mitigates the bottleneck of using Ag-Al metallization paste in p+ emitter cells. The LEF process pushes the balance between the Voc and FF higher.
Cell Engineering (CE), a company specializing in solar cell technologies, began developing this process called Laser Enhanced Contact Optimization (LECO) in 2017 in cooperation with Qcells. This approach has improved the efficiency of TOPCon as well as PERC cells by 0.2% to 0.5% absolute – a game changer, especially for TOPCon technology. In October 2022, Qcells took a significant step by acquiring Cell Engineering. As a result, Qcells now holds 100% ownership of CE and the intellectual property rights associated with LECO technology.
On the other hand, almost every Chinese PV manufacturer involved in TOPCon has also developed a similar process under a wide variety of abbreviations. As to the questions of intellectual property (IP), while choosing to remain anonymous, an executive working very closely with the technology and its active development told TaiyangNews that most companies have developed proprietary approaches on similar grounds, but they are not the same.
Leading paste makers such as DKEM are offering pastes specifically designed for these applications. The paste with a superior chemistry of organic and inorganic compounds and their mix facilitates direct contact with the silicon, which improves the conductivity and contact quality between the silver paste and the silicon, according to DKEM, as elaborated in DKEM’s presentation at the TaiyangNews High Efficiency Solar Technologies Conference 2023 (watch here). The paste also showed better printability; while silver-aluminum pastes pose challenges for screen openings below 13 µm, DKEM’s special paste for the laser process has shown relatively better printability in a narrower screen opening of 11.5 µm already in 2023. Not only has DKEM successfully tested the reliability of this front contacting paste, but it is also offering the complete package, including the paste for rear-side and busbars, which also require considerable optimization.
Furthermore, Solamet also emphasized that fine-line printing and low solid content rear-side pastes are the 2 mature and effective solutions for cost reduction in TOPCon, as these approaches reduce the use of silver (Ag). At the TaiyangNews High Efficiency Solar Technologies Conference 2024 [link?], Solamet emphasized that silver particles – a key component of paste composition along with Ag powder, glass frit, binder, and solvent – play a critical role in determining printability (see Pushing The Efficiency Limit: Solamet’s Perspective).
Solamet highlighted that fine-tuning the surface morphology of Ag powder particles can significantly enhance paste performance and uniformity. In addition, optimizing the ratio of different Ag particle sizes and carefully balancing solid content, viscosity, and sintering density all contribute to improved fine-line printability and lower grid line resistance. In-house test results with optimized paste formulation resulted in a uniform grid morphology with screen openings down to 9 µm.
This text is an excerpt from the TaiyangNews Cell & Module Technology Trends 2025 report, which can be downloaded for free here.
