Longi achieves 34.6% efficiency for two-terminal tandem perovskite solar cell prototype

In a paper published in nature, the Chinese module maker explained that the 33.9%-efficient tandem device it unveiled in December 2023 is based on a bilayer intertwined passivation strategy that combines efficient electron extraction with further suppression of nonradiative recombination. It also revealed that its two-terminal tandem prototype devices achieved an efficiency of 34.6%.

September 17, 2024 Vincent Shaw and Emiliano Bellini

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From pv magazine Global

Chinese solar module manufacturer Longi has revealed it achieved a power conversion efficiency of 34.6% in two-terminal tandem perovskite solar cell prototype devices.

“Currently, the two-terminal tandem prototype devices developed by Longi’s tandem team have been authoritatively certified with a record efficiency of 34.6%,” the manufacturer said, without providing more technical details. “The commercial-sized two-terminal tandem cells developed for mass production (M6) and the world’s first square meter four-terminal tandem modules have been authoritatively certified with efficiencies of 30.1% and 25.8%, respectively. These results demonstrate a significant efficiency advantage over single junction silicon cell technology.”

The announcement was made during the launch of a scientific paper that describes in detail the perovskite-silicon tandem solar cell it unveiled in December 2023. This PV device achieved what remains to date the highest power conversion efficiency ever recorded for this kind of cell.

At the time, the US Department of Energy’s National Renewable Energy Laboratory (NREL) confirmed the results, which represent a world record for this cell typology. The previous record was held by Saudi Arabia’s King Abdullah University of Science and Technology (KAUST), which achieved a 33.7% efficiency for a device with the same configuration in June.

Longi’s result is also the first reported certified efficiency exceeding the single-junction Shockley-Queisser limit of 33.7% for a double-junction tandem solar cell.

The manufacturer’s research team explained that the cell was based on a bilayer interface passivation strategy that reportedly maximizes both electron transport and hole blocking. This is achieved through the incorporation of a thin lithium fluoride (LiF) layer and the deposition of short-chain ethylenediammonium diiodide (EDAI) molecules.

“Thicker LiF layer may help improve the passivation, but come with a considerable undesirable resistive loss,” they said. “Nevertheless, EDAI molecule can chemically passivate the unpassivated areas that are not contacted by the LiF layer, forming nanoscale localized contacts at perovskite/C60 interface, which can provide an optimal tradeoff between passivation and charge extraction.”

The researchers said they were able to achieve “better” structural coupling between the perovskite top cell and the crystalline silicon bottom cell through a patented technology for silicon heterojunction solar cells with an asymmetric textured surface. “The front surface of this silicon cell has a fine textured surface, facilitating the solution-based preparation of perovskite film, while the rear surface of the silicon cell uses a standard large size textured surface to achieve better passivation and infrared spectral response,” they added.

Thanks to the LiF/EDAI bilayer was responsible for increasing the device’s open-circuit voltage and fill factor, which reached values of 1.97 V and 83.0%, respectively, as it suppressed interfacial recombination coupled with more efficient charge extraction at the electron transport layer (ETL) interface.

Longi said its research work has received strong support from Suzhou University, Huaneng Clean Energy Research Institute, and the Hong Kong Polytechnic University (HKPU). It was presented in the study “Perovskite-silicon tandem solar cells with bilayer interface passivation,” published in nature.

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