HKUST Develops Novel "Molecular Velcro" to Boost Efficiency and Stability of Perovskite Solar Cells

Researchers from the School of Engineering at The Hong Kong University of Science and Technology (HKUST) have developed a robust coating layer that significantly enhances the durability of perovskite solar cells. In tests simulating intense midday sunlight at 85°C, these solar cells retained over 95% of their initial efficiency after more than 1,100 hours of continuous operation. This breakthrough demonstrates the real-world application of perovskite cells in outdoor environments, paving the way for durable, high-efficiency, and low-cost solar technology.

Perovskite solar cells are widely recognized for their high efficiency and low manufacturing costs, but their limited long-term stability has remained a major barrier to widespread adoption. A common strategy to address this challenge is to coat three-dimensional (3D) perovskite absorbers with a thin low-dimensional (LD) perovskite layer to passivate surface defects and boost voltage. However, conventional LD layers, typically composed of monovalent ammonium salts, bind weakly to the perovskite lattice and degrade under heat and illumination, leading to rapid performance decline.

To address this, Dr. CHANG Xiao-Ming, Postdoctoral Fellow of the Department of Electronics and Computer Engineering at HKUST, pioneered a materials engineering approach using multivalent amidinium ligands. These ligands anchor to the perovskite surface at multiple points via two nitrogen sites in their headgroup, functioning like “Molecular Velcro”, such a “multi-point grip” ensures that the LD layer remains stable under operating conditions.

Dr. Chang explained, “Traditional ammonium-halide molecules diffuse into the perovskite bulk at high temperatures, causing breakdown or reaction with the organic ion formamidinium. This weakens the protective LD layer and accelerates degradation. In contrast, our multivalent amidinium ligands have a flat molecular shape and a resonance-stabilized charge distribution, forming stronger hydrogen bonds with halide ions and resist degradation.

Co-author Prof. LIN Yen-Hung, Assistant Professor of the Department of Electronic and Computer Engineering at HKUST, remarked, “Using operando hyperspectral imaging, a specialized technique supported by the HKUST Equipment Fund under the Vice-President for Research and Development Office, we mapped pixel-by-pixel under open-circuit, maximum-power-point, and short-circuit operation. In the accelerated ageing test, devices with our ‘Molecular Velcro’ interface show almost unchanged photoluminescence patterns and spectra, indicating a stable interface and intact perovskite layer.”

A key breakthrough lies in the ability to fine-tune the basicity of a nitrogen atom within the pyridine group. The research team discovered that, in LD structures, amidinium ligands disrupt the fully 3D crystal network, enabling metal-halide octahedra to form either one-dimensional (1D) chains or two-dimensional (2D) sheets. By adjusting ligand basicity and the molecular conformation, they successfully transitioned the surface perovskite from a 1D, chain-like stacking arrangement to a hydrogen bonds 2D sheet-like network, creating a continuous and uniform protective layer.

In inverted perovskite devices, this 3D/2D interface strategy delivers a certified steady-state power conversion efficiency of 25.4% on cells of about 1.1 cm² and 24.2% on 6.8 cm² mini-modules.

To assess the durability of these devices, the team adhered to the International Summit on Organic Photovoltaic Stability (ISOS) protocol, which is widely used to compare perovskite solar cell lifetimes. During the ISOS-L-2 test, encapsulated cells operated continuously at their optimum operating point under 1-sun-equivalent illumination (light intensity similar to bright midday sunlight) at 85°C in air. Remarkably, the devices kept more than 95% of their initial efficiency after 1,100 hours of operation, demonstrating the robustness of the new interface design.

Prof. Lin said, “To the best of our knowledge, this represents the highest certified stabilized efficiency reported in a peer-reviewed publication for inverted perovskite solar cells with an active area of around 1 cm².”

This breakthrough was published in Science in a paper titled “Multivalent ligands regulate dimensional engineering for inverted perovskite solar modules.”

The study was conducted in collaboration with King Abdullah University of Science and Technology, The Chinese University of Hong Kong, Shenzhen, Shaanxi Normal University, Korea University, the National University of Singapore, the National Technical University of Athens, and the University of Manchester. Contributors from HKUST included Prof. Lin’s research group, and Dr. Fion YEUNG Sze-Yan, Senior Manager at the State Key Laboratory of Displays and Opto-Electronics.