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Scientists at Longi, the Chinese solar manufacturer, have examined the physical processes underlying edge masking in high-efficiency photovoltaic cells, according to pv magazine. Their findings indicate that the approach curbs edge recombination by limiting the sideways movement of charge carriers.
Edge masking is a lab measurement practice whereby an opaque substance covers a cell’s perimeter so that its edges receive no light, which lowers recombination losses and can lift the conversion efficiency recorded during testing. Genshun Wang, a Longi researcher, told the publication that the work began from a practical question about efficiency measurement. In an earlier investigation of high-efficiency back contact cells, the team identified the wafer edge as a significant route of current loss, and covering the edge while testing raised the measured efficiency to 26.74% across the total area, compared with 27.09% across the designated illumination area.
Wang noted that while edge masking is common in record efficiency measurements, especially for small-area devices, the physical processes at work had remained incompletely understood. Prior studies had regarded the masked region either as geometric shielding or as a parallel dark sink within a lumped equivalent circuit, with scant attention to carrier transport, he said. Shedding light on the mechanism offers design guidance, since it enables the optimal masking width to be quantified, allowing a suitable grid design to yield a greater masking gain.
In a paper titled 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 showed that masking produces a dark, electrically resistive zone that curtails carrier movement toward the cell perimeter. Wang said the study reveals that masking triggers a light-to-dark transition in the masked region, turning it into a resistive dark area that strengthens what the team terms a resistive barrier. This barrier impedes the lateral transport of carriers toward the physical edge, thereby suppressing edge recombination and yielding higher efficiency.
The researchers also explored whether altering the metallization layout could further enhance cell performance. Wang said their model predicts that stripping metallization from the masked area, known as selective metallization, severs the remaining path along the grid and isolates the dark edge load, improving both fill factor and open-circuit voltage.
Wang emphasized, however, that the method is meant for laboratory characterization rather than commercial production. He said selective metallization introduces process complexity, and since the masked area produces no power, the utilization of the total cell area falls. In practice, it is a laboratory test method that, by eliminating the edge contribution, enables other loss factors to be evaluated more reliably.
The analysis determined that masking the cell perimeter lowers edge recombination losses and boosts power conversion efficiency, mainly via a rise in fill factor. The impact was stronger in cells lacking edge passivation, which displayed an efficiency gain of roughly 0.2 percentage points.
The experiments pinpointed an optimal masking width of 5 mm, whereas broader masks of 10 mm and 15 mm lowered efficiency owing to the expanding inactive area and related electrical losses. The researchers also observed that short-circuit current density first rose with masking width, while open-circuit voltage fell.
With Quokka 3 simulations, the team showed that masking produces a transition from an illuminated, conductive region to a dark, resistive area near the cell perimeter. This reinforces a resistive barrier that curbs lateral carrier transport toward the physical edge, thus suppressing recombination losses.
Additional simulations revealed that carriers can still arrive at the masked region via metal grid lines, capping the efficiency gains. To counter this, the researchers suggested selectively removing metallization from the masked area to electrically isolate the edge. The research team included scientists from the Shenzhen Campus of Sun Yat-sen University in China.
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World's largest monocrystalline silicon producer
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Crystal growth equipment and materials
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PV manufacturing equipment
LED driver ICs and chips
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