■ The Photovoltaic Research Department of the Korea Institute of Energy Research (KIER) has developed a novel catalytic process that extends the lifespan of perovskite solar cells. This technology is designed to eliminate lithium salts, which contribute to the shortened lifespan of perovskite solar cells, and thus will promote the commercialization of perovskite-based tandem solar cells.

* Perovskite: A promising material for next-generation solar cells based on having the same crystal structure as calcium titanate (CaTiO₃), with efficient light absorption

□ Perovskite is often used as a material for the top subcell of tandem solar cells. In tandem solar cells, different subcells are stacked to maximize absorption efficiency. Perovskite solar cells are divided into regular and inverted architectures according to the stacking order of their constituent layers. The regular architecture has matured over many years as a single-junction device technology and has achieved high efficiency; however, device lifespan is often shortened under high-temperature and high-humidity conditions.

□ Perovskite solar cells include a hole-transport layer that allows the generated electricity to flow out of the cell. In the regular architecture, lithium salts and additives that enhance their solubility are added to the hole-transport layer to improve charge transport efficiency. The challenge is that these additives are prone to evaporation under the actual operating conditions of these devices, including elevated temperatures and high humidity, which causes the formation of undesired gaps between the constituent layers of solar cells. These gaps may result in inter-layer defects and thereby undermine the lifespan of 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. 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 limitations, the KIER research team has developed a novel catalytic and filtration process in which lithium salts are used only to induce the necessary chemical reactions before the lithium ions are extracted together with the catalyst through filtration and subsequently discarded.

□ 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. 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 research team also demonstrated a physical filtration process to remove and discard the used nickel oxide powder after the necessary chemical reactions are complete. During this process, the nickel oxide catalyst with a large particle size, along with the lithium ions attached to its surface, is filtered out. The resulting solution, with the lithium ions removed, can then be used to fabricate a defect-free hole-transport layer that does not contain any additives, residual lithium, or nickel oxide catalyst, all of which could otherwise cause performance degradation.

□ The research team further applied the developed technology to fabricate perovskite solar cells and evaluated their durability under high-temperature (85°C) and high relative humidity (85% RH) conditions. These cells retained 95% of their initial efficiency even after 1,000 hours, underscoring their world-class competitiveness.

□ Furthermore, these cells achieved a power conversion efficiency (PCE) of 25.24%. Surpassing the value of 25.21% achieved using conventional additive-based processes, this result demonstrates that comparable or even higher efficiency can be achieved without additives.

* Power conversion efficiency (PCE): The ratio of the electrical power generated by a solar cell to the total power of the light it receives

■ Se Jin Ahn, Principal Researcher at KIER and the leader of the project, said, "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." The lead researcher added, "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."

□ This joint research project was conducted by the KIER research team in collaboration with a team led by Prof. Dong Suk Kim at UNIST. The major findings were published as a cover article in Advanced Science (IF 14.1), a top-tier international journal in materials science.