Chinese solar manufacturer Longi has certified a two-terminal tandem perovskite silicon solar cell prototype at 34.6 per cent efficiency, a figure that pushes past a theoretical ceiling long treated as a hard limit for single-junction silicon cells.According to PV Magazine, which reported the announcement directly, the company explained how the cell works. According to the U.S. Department of Energy, the 33.7 per cent figure refers to the Shockley-Queisser limit, the maximum theoretical efficiency a single-junction silicon solar cell can achieve.Longi's 34.6 percent result gets past that ceiling not by breaking physics but by stacking a second material, a perovskite crystal layer, directly on top of a silicon cell, letting the two layers absorb different parts of the light spectrum together rather than relying on silicon alone. PV Magazine says LONGi’s 34.6% result set a new world record for a silicon-perovskite tandem solar cell, but it does not say this was the first certified efficiency to exceed the single-junction Shockley-Queisser limit for a double-junction tandem solar cell.How Longi's research team actually pulled this offAccording to the paper published in the journal Nature and reported by PV Magazine, Longi's team achieved the result using what they call a bilayer interface passivation strategy, adding a thin layer of lithium fluoride along with short-chain ethylenediammonium diiodide molecules to the point where the perovskite and silicon layers meet.
A photovoltaic roof installation filling the entire roof. Credit – Wikimedia Commons
This combination reduces energy losses that normally occur at that junction, allowing more of the electrical charge generated by sunlight actually to be collected rather than lost as heat. The team also used an asymmetric textured silicon surface, with the side facing the perovskite layer finely textured to promote uniform bonding, and a different texture on the opposite side to improve infrared light capture. The US Department of Energy's National Renewable Energy Laboratory independently confirmed Longi's earlier December 2023 version of this cell at 33.9 per cent efficiency, according to PV Magazine, lending third-party credibility to the lab's results before this latest 34.6 per cent figure was announced.A lab record, and the real gap before it reaches your rooftopIt is worth being precise about what this 34.6 per cent figure actually represents, since lab records and commercial products are not the same thing. According to PV Magazine's report, that number applies specifically to a small prototype device, not a market-ready solar panel. Longi's commercial-sized tandem cells, built at a size actually usable in mass production, were separately certified at 30.1 per cent, and the company's first square-metre-scale tandem module, closer to what would eventually go into a real installation, reached 25.8 per cent. Both of those figures are still a meaningful improvement over conventional single-junction silicon panels, which typically top out in the low twenty per cent range, but the gap between a record-setting lab prototype and an actual mass-produced panel remains real. Whether tandem perovskite silicon cells reach ordinary rooftops at anything close to 34.6 per cent efficiency will depend on how well this laboratory result can survive the much harder process of scaling up to full-sized, durable, mass-manufactured panels.