Scientists at the Korea Institute of Energy Research (KIER) have developed a catalytic process that could extend the operational lifespan of perovskite solar cells while maintaining high power conversion efficiencies.
The researchers explained that the hole transport layer (HTL) in perovskite solar cells is typically modified with lithium-based salts and additives that improve their solubility to increase electrical conductivity and facilitate charge transport. However, the volatility of these additives poses a significant challenge to long-term device stability.
Under real-world operating conditions, particularly high temperatures and humidity, these compounds can evaporate or migrate from the HTL, compromising the interfaces between adjacent layers. The resulting voids and structural defects can accelerate device degradation and shorten the operational lifespan of perovskite solar cells.
“Furthermore, at elevated temperatures and high humidity, lithium ions released from lithium salts diffuse into the interior of solar cells in an uncontrolled manner, further shortening their lifespan,” the researchers explained. “However, alternative strategies to achieve the same improvement in efficiency, either by eliminating the use of lithium salts and additives or by replacing them with other agents, have not yet been proposed.”
To address these issues, the researchers developed a catalyst-assisted process in which lithium salts are temporarily used to drive the required chemical reactions before the lithium ions and catalyst are removed from the solution through filtration.
“The key to the developed technology lies in the use of nickel oxide (NiOx) as a catalyst. When added to the hole-transport layer solution, this nickel oxide powder exhibits strong interactions with lithium ions, strongly attracting them,” the academics said. “This mechanism effectively dissociates lithium ions from lithium salts and attaches them to the surface of nickel oxide without using additives, which pose a major challenge in conventional processes.”
The researchers tested the process in solar cells built with a fluorine-doped tin oxide (FTO) substrate, a tin oxide (SnO₂) electron transport layer (ETL), a formamidinium lead iodide (FAPbI₃)-based perovskite absorber, a spiro-OMeTAD HTL, and a gold (Au) electrode.
The champion device achieved a power conversion efficiency of 25.24%, an open-circuit voltage of 1.175 V, a short-circuit current density of 25.83 mA/cm², and a fill factor of 83.21%. It also delivered a stabilized power output of 25.1% under one-sun illumination.
The cells retained more than 90% of their initial efficiency after 500 hours of continuous operation and 95% under accelerated damp-heat testing at 85 C and 85% relative humidity, indicating improved operational stability.
According to the scientists, the process enables oxidation of the hole transport material spiro-OMeTAD in less than five seconds without the conventional additive 4-tert-butylpyridine (tBP). NiOₓ also facilitates the dissolution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) while capturing lithium ions, which are subsequently removed along with the NiOₓ particles through filtration. The process increased the electrical conductivity of the HTL by a factor of 2.5 and produced more uniform films with improved charge extraction.
The new approach was presented in the study “Redox-Active NiOx-Catalyzed Li+ Capture-Extraction Strategy for tBP-Free Spiro-OMeTAD Enables Exceptional Damp-Heat Stability in Perovskite Solar Cells,” published in Advanced Science.
“This research has gone beyond widely accepted ideas and strategies in conventional perovskite solar cell fabrication, proposing a novel catalytic and filtration process in which the catalyst is used only to induce the necessary chemical reactions and then removed, along with the target material, through filtration,” said Se Jin Ahn, principal researcher at KIER and leader of the project. “This technology will mark a significant milestone in addressing persistent issues concerning the lifespan of perovskite tandem solar cells, paving the way for their commercialization.”
In related research, another KIER team recently achieved a record power conversion efficiency of 26.7% for a tandem solar cell combining a perovskite top cell with a copper indium gallium selenide (CIGS) bottom cell. The researchers said the result was confirmed by Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) and included in the Best Research-Cell Efficiency Chart maintained by the US Department of Energy’s National Laboratory of the Rockies.
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