An international research team has fabricated an inverted perovskite solar cell with an electron transport layer passivated with amidinium ligands instead of ammonia ligands. The device not only performed superiorly compared to reference cells treated with ammonium, but also showed remarkable stability.
An international team of scientists has developed an inverted perovskite solar cell that uses a novel amidinium passivator for the perovskite electron transport layer (ETL).
Inverted perovskite cells have a device structure known as “pin”, where hole-selective contact p is at the bottom of the intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure, but in reverse: a ‘nip’ arrangement. With nip architecture, the solar cell is illuminated via the electron transport layer (ETL) side; in the pin structure it is illuminated by the HTL surface.
“We sought an alternative to ammonium-based coating for perovskite solar cells,” the study’s first author, Yi Yang, said in a statement. “State-of-the-art perovskite solar cells usually have ammonium ligands as a passivation layer, but ammonium tends to break down under thermal stress. We did some chemistry to convert the unstable ammonium into a more stable amidinium.”
The group hypothesized that amidinium ligands, with their resonance-stabilized NH bonds, could outperform conventional ammonium-based ligands in preventing defect formation and improving stability. “It required rigorous design and validation to confirm that these ligands not only provided chemical passivation but also contributed to field effect stabilization. This dual functionality ultimately led to significant improvements in both the efficiency and long-term stability of PSCs, making the confirmation of this hypothesis the most interesting and impactful result of the study,” said co-author Mercouri Kanatzidis pv magazine.
The amidinium bimolecular passivation (ADBP) approach resulted in laboratory devices that showed a tenfold reduction in the equilibrium constant of the ligand deprotonation and a twofold increase in the retention of the photoluminescence quantum yield after aging at 85 C under illumination in air, the researchers said.
The scientists built the cell with a substrate made of fluorine-doped tin oxide (FTO), self-assembled monolayers (SAM), a perovskite absorber, the proposed passivation layer, a buckminster fullerene (C60) ETL, a tin oxide (SnOx) layer and a copper (Cu) metal contact.
The best laboratory-sized device had an external quantum efficiency (EQE) band gap of 1.53 eV and a certified efficiency of 26.3%. The result was certified by the Fujian Metrology Institute, the National PV Industry Measurement and Testing Center in China. A larger device with an active surface area of 1.04 cm2 achieved an efficiency of 25%, the researchers found.
The tests indicated a significant improvement in the average efficiency of the device based on the new passivator compared to control devices. The team attributed this mainly to an increased open-circuit voltage and fill factor.
The team also found that the cell could maintain 90% of its initial efficiency for 1,100 hours of maximum power point operation at 85 C.
“I would like to emphasize that while most research into stable perovskite solar cells focuses on the stability of the active layer – the perovskite itself – we have discovered that the passivation materials, which are often used to improve the efficiency of devices, can also be a source of instability and deserve the necessary attention. Efficiency and stability must go hand in hand,” said co-corresponding author Bin Chen pv magazine.
The team is committed to closing the “stability gap” with conventional silicon PV. “Perovskite-based solar cells have the potential to contribute to the decarbonization of the electricity supply once we finalize their design, achieve the combination of performance and durability, and scale up the devices,” said co-corresponding author and group leader Ted Sargent in a position.
The research is described in detail in “Amidination of ligands for chemical and field effect passivation stabilizes perovskite solar cells”, published in Science. The team included researchers from Northwestern University and the US Department of Energy’s Argonne National Laboratory in the US, Griffith University in Australia and Canada’s University of Toronto.
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