Researchers from the School of Engineering at the Hong Kong University of Science and Technology have developed a robust coating layer that significantly improves the operational stability of perovskite solar cells while maintaining high efficiency. In accelerated tests simulating intense midday sunlight at 85 degrees Celsius, the coated cells retained more than 95 percent of their initial energy conversion efficiency after more than 1,100 hours of continuous use, demonstrating great potential for outdoor use in real-world conditions.
Perovskite solar cells are known for their high efficiency and relatively low production costs, but their limited long-term stability is a major obstacle to commercialization. A common strategy to address this problem is to coat a three-dimensional perovskite absorber with a thin, low-dimensional perovskite layer that passivates surface defects and improves device voltage. However, conventional low-dimensional layers are typically formed from monovalent ammonium salts that bind weakly to the perovskite lattice and tend to degrade under heat and illumination, causing a rapid loss of performance.
To overcome this limitation, Dr. Chang Xiao Ming, a postdoctoral researcher in the Department of Electronics and Computer Engineering at HKUST, and colleagues developed a new class of multivalent amidinium ligands that act as a molecular Velcro-like interface. These ligands anchor at multiple points to the perovskite surface via two nitrogen sites in their headgroup, creating a multipoint grip that stabilizes the low-dimensional layer under operating conditions. Their planar molecular shape and resonance-stabilized charge distribution enable stronger hydrogen bonding with halide ions and higher resistance to thermal- and light-induced degradation.
Dr. Chang said that at elevated temperatures, traditional ammonium halide molecules can diffuse into the perovskite mass, where they either break down or react with the organic ion formamidinium, undermining the protective function of the low-dimensional layer. In contrast, the multivalent amidinium ligands remain at the interface and maintain the integrity of the surface structure over time. This behavior helps slow down the chemical pathways that normally cause device aging and efficiency losses.
Co-author Prof. Lin Yen Hung, assistant professor at HKUST’s Department of Electronic and Computer Engineering, highlighted the use of operando hyperspectral imaging to monitor device behavior under realistic operating conditions. Using this technique, the team mapped the perovskite layer pixel by pixel under open circuit, maximum power point and short circuit conditions during accelerated aging. Devices incorporating the molecular Velcro interface showed virtually unchanged photoluminescence patterns and spectra, indicating a stable interface and an intact perovskite absorbing layer, even under prolonged strain.
A central aspect of the work is the ability to tune the basicity of a nitrogen atom within a pyridine group in the ligand structure. The researchers discovered that in low-dimensional perovskite structures, amidinium ligands disrupt the fully three-dimensional crystal network and allow metal halide octahedra to reorganize into one-dimensional chains or two-dimensional sheets. By carefully adjusting the ligand basicity and molecular conformation, they converted the surface perovskite from a one-dimensional chain-like stacking motif into a hydrogen-bonded two-dimensional sheet-like network that forms a continuous and uniform protective coating.
Using this three-dimensional to two-dimensional interface engineering strategy in inverted perovskite solar cells, the team achieved a certified steady-state energy conversion efficiency of 25.4 percent on cells with an active area of approximately 1.1 square centimeters. For mini modules with an area of 6.82 square centimeters, the devices achieved an efficiency of 24.2 percent. According to the researchers, these values place their devices among the best performing inverted perovskite solar cells reported to date for similarly active regions.
To systematically assess durability, the team followed the International Summit on Organic Photovoltaic Stability (ISOS) protocol, a widely accepted standard for comparing the lifespan of perovskite solar cells. Under the ISOS L 2 test, encapsulated devices operated continuously at their optimal operating point under one sun-equivalent illumination, corresponding to bright midday sunlight, at 85 C in air. Even under these demanding conditions, the cells with the molecular Velcro interface retained more than 95 percent of their initial efficiency after 1,100 hours, underscoring the robustness of the interface design.
Prof. Lin noted that, to the team’s knowledge, the certified stabilized efficiency they obtained is the highest reported in a peer-reviewed publication for inverted perovskite solar cells with an active area of about 1 square centimeter. The work shows how fine control over molecular-level interactions at the perovskite surface can translate into both record efficiency and significantly improved device lifetime. The findings also suggest a general pathway for developing stable three-dimensional, low-dimensional perovskite heterostructures for future photovoltaic technologies.
The research, published in the journal Science, appears in an article titled Multivalent ligands regulate dimensional engineering for inverted perovskite solar modules. The research involved collaboration with multiple international institutions, including King Abdullah University of Science and Technology, Chinese University of Hong Kong, Shenzhen, Shaanxi Normal University, Korea University, National University of Singapore, National Technical University of Athens and University of Manchester. HKUST’s contributions included the research group of Prof. Lin and Dr. Fion Yeung Sze Yan, senior manager at the State Key Laboratory of Displays and Opto Electronics.
Research report: Multivalent ligands regulate dimensional engineering for inverted perovskite solar panels
