XI'AN, China - Researchers at Chinese solar manufacturer LONGi have published a study explaining how edge masking, a common laboratory measurement technique, reduces recombination losses in back-contact silicon solar cells. The work, published in Solar Energy Materials and Solar Cells, provides an industry-friendly lesson in carrier physics and clarifies how record-efficiency measurements should be interpreted.
In a solar cell, light-generated electrons and holes must reach the electrical contacts before they recombine. Recombination-the loss of charge carriers before they contribute to current-is a major obstacle to higher efficiency. As crystalline silicon photovoltaics approaches its theoretical limit of about 29.4%, even small loss channels matter.
The outer portion of a cell is particularly subject to exposure in a fabrication process. Laser cutting can create unwanted and dangling bonds as well as lattice imperfections in the process of cutting cells from the particular wafer. These imperfections will result in recombination. The use of passivating layers like AlOx or TiO₂ allows the surfaces to be careful in fabrication, but it is difficult to make the same choice for the fringes given their irregular shape. As a result, the edge can act as the pathway for charge loss in the cases of high-efficiency cells.
Edge masking is routinely used in laboratory efficiency measurements, especially for small-area devices. A mask covers the outer region so that the designated illumination area excludes the edge. Earlier explanations treated the mask mainly as geometric shielding or as a dark parallel load in an equivalent circuit. The LONGi team, led by researcher Genshun Wang, used Quokka 3 simulations and equivalent-circuit modelling to examine the effect at the carrier-transport level.
The study shows that masking creates a light-to-dark transition at the covered perimeter. Without illumination, the masked region becomes electrically resistive. This "resistive barrier" impedes lateral carrier diffusion toward the physical edge and therefore suppresses edge recombination. The mask does more than keep light away from a defective region; it changes the electrical environment so that carriers are less likely to reach the edge and recombine there.
The researchers tested the mechanism in heterojunction back-contact cells fabricated without edge passivation. Under masked measurement, the cells reached an experimental efficiency of 26.85%. The improvement attributed to masking was about 0.2 percentage points, mainly through an increase in fill factor. Fill factor reflects how effectively a cell converts its open-circuit voltage and short-circuit current into power.
The experiments identified an optimum masking width of 5 mm. Wider masks of 10 mm and 15 mm reduced efficiency because the inactive masked area grew and associated electrical losses increased. Short-circuit current density initially rose with masking width, while open-circuit voltage declined-an illustration of the trade-offs involved in measurement geometry.
Simulations also examined whether selectively removing metallization from the masked area would help. In current designs, carriers can still reach the masked region through metal grid lines, limiting the benefit. The model predicts that cutting that conductive path and electrically isolating the dark edge load would raise the simulated efficiency ceiling from 26.85% to 27.39%, with gains in both fill factor and open-circuit voltage.
Wang stressed that edge masking is not a manufacturing technology. Selective metallization would add process complexity, and because the masked area generates no power, total-area utilization decreases. In practice, masking is a laboratory method: by removing the edge contribution, it allows researchers to assess other loss factors more reliably.
For the wider industry, the study provides a clearer physical picture of a technique that has long been used but incompletely explained. As silicon cells move closer to their efficiency limits, understanding where carriers are lost-and how test conditions influence what is measured-becomes increasingly important. The LONGi work, conducted with researchers from the Shenzhen Campus of Sun Yat-sen University, offers a carrier-level explanation of edge masking and a reminder that in high-efficiency photovoltaics, even the edge of a cell carries significant technical meaning.
