It is common knowledge in the PV industry that the stability of perovskite cells has been a major challenge for their commercial adoption. Over the past years, extensive research has aimed to improve the durability of these cells. Numerous modifications at both the molecular level and in fabrication methods have been published by research institutes worldwide.
By nature, the perovskite structure becomes unstable when exposed to moisture, temperature, or even prolonged light – the very energy it's designed to convert into electricity. The conventional, high-efficiency perovskite cells use 3-dimensional (3D) atomic networks of metal halide structures. These 3D structures contain lattice ‘cages’ with organic cations, shown as green dots in schematic diagrams (on the bottom left image). Common cations include methylammonium (MA⁺) or formamidinium (FA⁺).
Perovskites offer low-cost, high-efficiency, and lightweight alternatives to traditional silicon solar cells. However, they tend to degrade under heat, moisture, and light exposure, largely due to their salt-like ionic structure.
A research team at Brookhaven National Laboratory and Cornell Center for Materials Research, supported by the National Science Foundation, redesigned the perovskite structure to improve its stability under simultaneous light, heat, and humidity.
The team claims to have used a 2-dimensional perovskite layer on top of the 3D structure (a 2D-on-3D heterostructure), which acts as a protective, weather-resistant coating. However, this approach is complex, as the lattice structure of the 2D layer must closely match that of the 3D layer. Previous researchers in this stream had tried using MA⁺ as the cage cation, but due to its poor sunlight stability, such cells lasted only a few hundred hours before degrading. FA⁺, while more stable, posed challenges due to its larger size, which caused strain in the crystal structure and an unstable 2D protective layer.
To address this, the team focused on identifying a cation and ligand combination that would create a lattice match between the 2D and 3D layers. They used ligands aligned with FA⁺ and the surrounding crystal lattice to form 2D perovskites with optimized thickness, stability, and conductivity.
“The basic idea is that a ligand in a 2D perovskite tries to shrink the lattice, while the FA cage cation works to make it bigger and you have these two opposing forces at play. We selected a ligand that doesn’t try to compress the cage too much, allowing it to expand a little and make room for the larger FA cation to fit inside,” said lead author Shripathi Ramakrishnan.
The team highlights that it has synthesized the 2D perovskite layer using FA⁺ as the cage cation and applied it as a protective coat over the 3D perovskite. Structural and optical characterization via synchrotron X-ray diffraction and confocal photoluminescence mapping demonstrated improved stability compared to MA-based structures. The resulting solar cell reached 25.3% conversion efficiency with only a 5% drop in performance after nearly 50 days of continuous testing under light and heat.
