Researchers from Rice University, Northwestern University, City University of New York, University of Rennes (CNRS), University of Lille (CNRS) and University of Nebraska-Lincoln have developed a new family of FA-based two-dimensional metal halide perovskites that come very close to a “perfect” crystal at room temperature.
These hybrid (organic–inorganic) semiconductors are engineered to achieve near-maximal crystallographic symmetry, adopting a tetragonal P4/mmm space group without in-plane or out-of-plane octahedral distortions. In contrast to most 2D perovskites, whose softer lattices tend to distort and lower symmetry, the new materials maintain a highly ordered framework inspired by three-dimensional cubic (α-phase) FAPbI₃ (FA = formamidinium).
A key structural feature of these layered perovskites is their very short interlayer spacing of about 4 Å, which is among the shortest reported for 2D metal halide perovskites and indicates strong coupling between the inorganic layers. At the same time, the optical bandgap lies in the 1.7–1.8 eV range, significantly lower than in many conventional 2D perovskites, allowing the material to absorb a broader portion of the solar spectrum. As more perovskite layers are connected along the out-of-plane direction, the bandgap gradually narrows: microscopic images show the crystals evolving from bright red, needle-like morphologies to darker, more block-like structures as the quantum-well width increases, reflecting the continuous tuning of the absorption edge.
The team achieved this unusually high symmetry and thickness control by carefully tailoring the cage cations, organic spacers and crystallization protocol. Instead of letting crystals fully equilibrate as a solution cools, they removed them at higher temperatures to lock in the desired structure before it could transform into a lower-symmetry phase. Earlier efforts using the chemically stable formamidinium cation could reliably connect only two perovskite layers in this configuration, but the new approach enables three or more layers to be coherently linked. In two in-plane directions, the material looks like a perovskite, while in the third direction three perovskite layers are connected together, giving a multilayered 2D architecture with enhanced thickness and symmetry.
This structural perfection has a direct impact on energy transport. The absence of octahedral distortions and reduction of disorder enable excitons - the bound electron–hole pairs created when the material absorbs light - to propagate over long distances without getting trapped. The measured exciton diffusion length reaches 2.5 µm, and the exciton diffusivity is 4.4 cm² s⁻¹, both about an order of magnitude larger than reported previously for 2D perovskites. These values are on par with those found in monolayer transition metal dichalcogenides, which are widely studied for high-performance optoelectronic applications such as ultrasensitive sensors and integrated electronic circuits. As Rice University's Aditya Mohite notes: “All the light that gets absorbed forms these material excitations called excitons, which can then propagate through the material for more than two micrometers without losing energy. That’s a big deal, because not many materials can really do this.”
The new perovskites were tested in self-powered photodetectors, where the material converts incident light into an electrical signal without an external bias. Devices based on the multilayered FA perovskite showed higher sensitivity and faster response than devices made from a different 2D perovskite, with these advantages becoming more pronounced for thicker films. This behavior is consistent with the long exciton diffusion length, which allows thicker active layers to be used without sacrificing transport to the contacts. The near-ideal bandgap and enhanced stability also make this 2D perovskite particularly attractive for tandem solar cells, where it can serve as a wide-bandgap top absorber paired with silicon or another perovskite or semiconductor. As Rice University's Faiz Mandani explains: “The 2D perovskites we are developing have enhanced stability. And this specific 2D perovskite has a near ideal band gap to pair with silicon or any other perovskite or semiconductor.”
