Longi researchers reveals how edge masking reduces recombination losses in solar cells

Researchers at Chinese solar manufacturer Longi have investigated the physical mechanisms behind edge masking in high-efficiency solar cells, finding that the technique suppresses edge recombination by restricting lateral carrier transport.

Edge masking is a laboratory measurement technique in which the perimeter of a solar cell is covered with an opaque material to prevent illumination of its edges, thereby reducing recombination losses and potentially increasing measured conversion efficiency.

“Our starting point was a practical concern about how efficiencies are measured,” Longi researcher Genshun Wang told pv magazine. “In an earlier study on high-efficiency back contact cells, we found that the wafer edge was a major channel of current loss, and that masking the edge during testing appreciably raised the measured efficiency – 26.74% over the total area versus 27.09% over the designated illumination area.”

Wang explained that although edge masking is widely used in record solar cell efficiency measurements, particularly for small-area devices, its underlying physical mechanisms have not been fully understood.

“Earlier work had treated the masked region as geometric shielding, or as a parallel dark sink in a lumped equivalent circuit, with little said at the carrier-transport level,” he said. “Clarifying it provides design guidance, as it allows the optimum masking width to be quantified so that a suitable grid design can deliver a higher masking gain.”

In the paper “The influence of edge masking on the optoelectronic performance in back contact silicon solar cells,” published in Solar Energy Materials and Solar Cells, the researchers demonstrated that masking creates a dark, electrically resistive region that limits carrier movement toward the cell perimeter.

“This study shows that masking drives a light-to-dark transition in the masked region, which becomes a resistive dark area and reinforces what we call a ‘resistive barrier’,” Wang explained. “That barrier hinders the lateral transport of carriers towards the physical edge, thereby suppressing edge recombination and delivering a higher efficiency.”

The researchers also investigated whether changes to the metallization layout could further improve cell performance. “Our model further predicts that removing the metallization in the masked area, i.e. selective metallization, cuts the remaining path along the grid and isolates the dark edge load, with gains in both fill factor and open-circuit voltage,” Wang said.

Wang stressed, however, that the approach is intended for laboratory characterization rather than commercial manufacturing. “It is worth stressing that this is not a production technology,” he said. “Selective metallization adds process complexity, and the masked area generates no power, so the utilisation of the total cell area decreases. In practice it is a laboratory test method: by removing the edge contribution, it allows other loss factors to be assessed more reliably.”

Through their analysis, the researchers found that masking the cell perimeter reduces edge recombination losses and improves power conversion efficiency, primarily through an increase in fill factor. The effect was more pronounced in cells without edge passivation, which showed an efficiency improvement of approximately 0.2 percentage points.

The experiments identified an optimal masking width of 5 mm, while wider masks of 10 mm and 15 mm reduced efficiency because of the growing inactive area and associated electrical losses. The researchers also found that short-circuit current density initially increased with masking width, while open-circuit voltage declined.

Using Quokka 3 simulations, the team demonstrated that masking creates a transition from an illuminated, conductive region to a dark, resistive area near the cell perimeter. This strengthens a resistive barrier that restricts lateral carrier transport toward the physical edge, thereby suppressing recombination losses.

Further simulations showed that carriers can still reach the masked region through metal grid lines, limiting the efficiency gains. To address this, the researchers proposed selectively removing metallization from the masked area to electrically isolate the edge.

The research team included scientists from Shenzhen Campus of Sun Yat-sen University in China.