LONGi has investigated the physical mechanisms behind edge masking in high efficiency solar cells, finding that the technique suppresses edge recombination by restricting lateral carrier transport rather than simply shielding the cell perimeter from light.
Edge masking is a laboratory measurement method in which the outer edge of a solar cell is covered with an opaque material to prevent illumination of that area. This reduces recombination losses at the perimeter and can increase the conversion efficiency recorded during testing.
In a paper titled “The influence of edge masking on the optoelectronic performance in back contact silicon solar cells,” LONGi researchers used carrier dynamics simulations and equivalent circuit modelling to explain why masking improves performance.
The study found that masking causes a transition from a light exposed to a dark operating state at the cell edge. The masked region becomes electrically resistive and acts as a barrier that impedes lateral carrier diffusion toward the physical edge, thereby suppressing recombination.
This mechanism underpinned an experimental efficiency of 26.85% in heterojunction back contact solar cells, achieved without any edge passivation. The researchers noted that the benefit is greater in cells with poor edge passivation, where losses at laser cut edges are typically more severe.
Edge recombination remains a major constraint for crystalline silicon solar cells as the technology approaches its theoretical efficiency limit of 29.4%. Cut cells are particularly affected because laser cleaving creates dangling bonds and lattice defects along their edges.
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Conventional responses include passivation layers such as AlOx and TiO2, as well as newer liquid phase approaches using Nafion based organic polymer solutions. However, achieving passivation quality on rough cut facets comparable to that of the central cell area remains difficult.
The LONGi team said its findings provide a theoretical framework for edge masking in solar cell testing and offer structural guidance for maximising its benefit. The model predicts that removing metallisation in the masked region can fully isolate the dark edge load, lifting simulated efficiency to 27.39%.
The research could inform future cell designs and test protocols for high efficiency back contact silicon technologies, particularly as manufacturers seek to demonstrate performance closer to the theoretical limits of crystalline silicon.
Author: Bryan Groenendaal
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