Removing the selective emitter becomes feasible with LECO, enabling lower recombination and reduced costs, supported by paste formulations
Fine-line printing is progressing rapidly, with screen openings now below 10 µm and optimized silver powder morphology enabling low-resistance gridlines
Rear-side metallization also benefits from LECO, with controlled etching and optimized pastes improving passivation, boosting Voc
After LECO adoption, the next major efficiency gain for TOPCon cells comes from emitter optimization, particularly by eliminating the selective emitter (SE) process to reduce recombination and simplify production. One of the primary strategies involves reducing recombination at the p++ emitter region by eliminating the SE process. This not only enhances efficiency but also reduces production costs. Traditionally, silver-aluminum spikes posed a risk to shallow junctions, making SE removal impractical. However, with the LECO process and paste, contact points are sub-micron in scale, making junction depth less sensitive to damage, according to DKEM. Despite these advancements, non-SE processes initially suffered from Fill Factor (FF) losses. To counteract this, companies like DKEM have developed paste formulations, specifically optimized to enhance contact efficiency in non-SE applications. Paste suppliers have also developed products suitable for lightly doped and shallow emitters.
Fineline printing is yet another notable route of optimization post-LECO adoption. Standard silver aluminum pastes typically use 14.5 μm screen openings, but with new process optimizations, this has been reduced to 13 μm, with some manufacturers experimenting with 11 μm screen openings, explained Fangqing Guo, New Technology Development Director at DKEM,. Reducing aluminum content improves printability while maintaining electrical performance. To further enhance printability, some manufacturers are exploring stencil and metal screen printing techniques, as well as pressure transfer printing (PTP), which offers better line definition and reduced laydown. He provided an update in his talk "From TOPCon to TBC" at the TaiyangNews High Efficiency Solar Technologies Conference in December 2024, stating that screen openings of less than 10 μm have been achieved. Indeed, several leading manufacturers announced that switching to stencil-based printing is a key and the latest development in the TOPCon segment (see Laser-Assisted Metallization Advances For TOPCon Solar Cells).
One of the primary challenges in fine-line printing is ensuring paste printability while achieving low gridline resistance and uniform morphology, noted Solamet. According to Solamet’s study "Innovative Metallization Solutions for n-TOPCon Efficiency and Cost Optimization", which it also presented during the December event, a finger opening of 8 μm kept the gridline resistance at a level that qualifies the paste to be put into mass production of cells. To push the limits, the company also conducted tests for ultra-fine line printing and found that the silver particle size needs to be optimized. Going into the details, the company emphasized that the properties of the silver powder significantly influence printability. Large, irregular particles must be removed, and surface morphology must be fine-tuned to ensure consistent performance. Optimizing the ratio of different silver particle sizes enhances paste performance by improving flow characteristics and reducing resistance.
Reducing finger width not only decreases shading losses but also lowers silver paste laydown, helping reduce overall production costs. Since silver paste is one of the most expensive consumables in solar cell manufacturing, minimizing its use is a key cost-saving strategy. Although lower paste laydown can lead to higher line resistance, this drawback is effectively mitigated by laser-based contact formation, which enables the use of finer fingers without compromising electrical performance.
Metallization on the rear: The LECO process also has a positive impact on rear side contact formation. Rear-side metallization was traditionally considered less influential on efficiency, given its naturally low contact resistivity and recombination. However, LECO opens new avenues for optimization. Paste formulations now focus on etching control to reduce silver precipitation, preserve passivation, and boost Voc – delivering up to 0.5% efficiency gains, according to DKEM. At the same time, LEF enables the use of thinner polysilicon layers without compromising performance, maintaining high Voc and FF even at 20 nm thickness. These rear contact pastes also enable low silver loading of about 87% to 90%. DKEM has also developed special busbar pastes that are compatible with different laser-based metallization technologies.
The text is an edited excerpt from TaiyangNews’ latest Market Survey on Solar Cell Production Equipment 2025, which can be downloaded for free here.
