Longi reports a 28.29% silicon cell, its third record this year
Victoria Shinder2 min readLongi has announced a power conversion efficiency of 28.29% for a hybrid interdigitated-back-contact (HIBC) silicon solar cell, confirmed by Germany's Institute for Solar Energy Research Hamelin (ISFH). pv magazine reported the result on 24 September. It is a new record for single-junction crystalline silicon and the third Longi has set this year, after 28.04% and then 28.13% in May.
The company did not publish new details with the announcement, but it described the architecture in a scientific paper last November. The cell uses a high-resistivity, half-cut M10 wafer with both n-type and p-type contacts on the rear, combining passivated tunnelling contacts with dielectric passivation layers. Edge passivation is done during fabrication rather than afterwards; an indium tin oxide layer helps carry current sideways; stacked aluminium oxide and silicon nitride coatings suppress recombination at the surface. Deep-trenched metal fingers and selective etching of the ITO keep the two contact polarities from leaking into each other, and a pulsed green nanosecond laser crystallises part of the amorphous silicon to cut contact resistance without spoiling passivation. Longi says the approach could be scaled into heterojunction manufacturing, but that resistive losses in the p-type contact still need to come down.
What it means
The number that matters most in the announcement is not 28.29 but 96.2: Longi's own estimate of how close crystalline silicon now is to its theoretical efficiency limit. Each further tenth of a point will cost more engineering than the last, and the gains between Longi's three records this year - 0.09 and then 0.16 percentage points - are already small. That is why the industry's long-term bets have moved to tandem cells, which stack a second material on top of silicon to get past the single-junction ceiling.
For buyers, a lab cell record changes nothing this quarter. Commercial modules trail record cells by several points, and a back-contact cell in a laboratory says little about yield, cost per watt or degradation on a roof. What it does show is where the leading Chinese manufacturers are placing their process bets: back-contact designs with all the metal on the rear, which also improve appearance and shading tolerance. Watch for whether the laser and edge-passivation steps described in the paper appear in production announcements; that is when a record starts to matter commercially.