Chinese researchers show edge masking boosts solar cell efficiency by reducing recombination losses, with simulations suggesting further gains through selective metallization.
Edge masking improves solar cell efficiency
Researchers from the Chinese solar panel manufacturer Longi have demonstrated that edge masking improves the measured efficiency of heterojunction solar cells with rear contacts by limiting charge carrier recombination at the edges. Simulations suggest that selective metallization could further increase efficiency, from 26.85% to a theoretical maximum of 27.39%.How edge masking works
Edge masking is a laboratory measurement technique that involves covering the perimeter of a solar cell with an opaque material to prevent illumination of its edges. This method reduces recombination losses and can thus increase the measured conversion efficiency by limiting lateral transport of charge carriers.Practical motivation for the study
Genshun Wang, a researcher at Longi, explained that their starting point was a practical question regarding efficiency measurement. In a previous study on high-efficiency rear-contact cells, they found that the edge of the wafer constituted an important current loss pathway. Edge masking during testing significantly increased the measured efficiency: 26.74% on the total surface, compared to 27.09% on the defined illuminated area.Understanding the physical mechanisms
The scientist noted that while edge masking is widely used to measure record efficiencies of solar cells, particularly for small-area devices, its underlying physical mechanisms remain poorly understood. Previous studies considered the masked area as a simple geometric shield or as an obscure loss sink in an equivalent electrical circuit, without really examining the charge carrier transport mechanisms.Masking creates a resistive zone
In their article titled "The influence of edge masking on the optoelectronic performance of silicon rear-contact solar cells," published in the journal Solar Energy Materials and Solar Cells, the researchers show that masking creates a dark and resistive zone that limits the movement of charge carriers toward the cell's perimeter. This transition between an illuminated zone and a dark zone reinforces what they call a "resistive barrier."Modifying metallization for better performance
The researchers also studied whether modifying the metallization configuration could further improve cell performance. Their model predicts that removing metallization in the masked area, that is, selective metallization, reduces the residual transport path along the grid lines and isolates the parasitic charge from the dark edge, which improves both the fill factor and the open-circuit voltage.Limitations of the approach
However, the researcher notes that this approach is intended for laboratory characterization, not commercial production. It should be emphasized that this is not a production technology. Selective metallization complicates the process, while the masked area produces no power, which reduces the total surface utilization of the cell. In practice, it is a laboratory testing method: by eliminating the contribution of the edges, it allows more reliable evaluation of other loss sources.Optimal masking width
The researchers' analysis shows that masking the cell's perimeter reduces recombination losses at the edges and improves solar energy conversion efficiency, mainly through an increase in the fill factor. The effect is more pronounced on cells without edge passivation, for which the efficiency gain reaches approximately 0.2 percentage points. The experiments identified an optimal masking width of 5 mm.Simulations reveal key insights
Using simulations conducted with the Quokka 3 software, the team showed that masking creates a transition between an illuminated and conductive zone and a dark and resistive zone near the cell's perimeter. This transition reinforces a resistive barrier that limits the lateral transport of charge carriers toward the physical edge, thereby reducing recombination losses. Complementary simulations revealed that charge carriers can still reach the masked area by following the metallic lines of the contact grid, which limits efficiency gains.
