Longi has published the first technical breakdown of its record-setting 27.81 percent efficient hybrid interdigitated back-contact solar cell, expanding on the announcement it made earlier this year. The company originally revealed the achievement in April, saying only that the device had been certified by ISFH in Germany. Now, newly released details in a paper described by pv magazine show how Longi reached the highest efficiency ever recorded for a silicon cell.
The device uses a passivated tunneling contact structure paired with hybrid back contacts that combine both n-type and p-type regions. The team built the cell on a high-resistivity half-cut M10 wafer and applied edge passivation through a high–low temperature fabrication method known as iPET. This process reduces dopant diffusion while sealing the wafer edges during production.
To reduce recombination on the front surface, the researchers used a multilayer stack of aluminum oxide and silicon nitride over a textured surface, along with an amorphous silicon layer thickened to fully protect sidewalls. Indium tin oxide was added to improve lateral transport, and phosphorus doping in the n-type polysilicon layer was reduced by an order of magnitude. The design also relies on deep 8 micrometer metal trenches with selective ITO removal to separate the n-type and p-type contacts.
A key step was laser-induced crystallization of the amorphous silicon, performed with a green nanosecond laser to maintain edge passivation while lowering contact resistance. The team notes that tuning the thickness and optical properties of the a-Si layer was essential to balance passivation with conductivity.
The record efficiency was measured on a 133.63 cm² active area, producing a short-circuit current of 5698 mA, an open-circuit voltage of 744.9 mV, and a fill factor of 87.55 percent. The low ideality factor, measured below 1 at maximum power point, reflects how closely the device behaves like an ideal diode and is one of the contributors to its unusually high fill factor.
Longi believes the methods demonstrated here can be scaled into future heterojunction production lines, though the company says the p-type contact still shows higher resistive losses than the n-type side, leaving room for further refinement.