The soft and ionic crystal structure of perovskites possesses many new and unexpected properties. In contrast to conventional semiconductors they exhibit comparatively low energy phonon modes and a small elastic constant. A direct consequence of the soft ionic lattice is the significant coupling of charge carriers to the ions in the lattice. Thus, the microscopic description of the electron-phonon coupling in these materials is nontrivial, essentially because it is mediated by a large anharmonicity and dynamic disorder, which requires the introduction of polarons – a quasiparticle representing charge carriers coupled to the lattice vibrations. Bearing in mind the soft, ionic, and hybrid lattice of perovskites it is expected that a charge carrier moving through the lattice will interact with surrounding ions. The interaction between carriers and lattice polarization leads to notable effects, such as an increase in the effective mass of charge carriers and a modification of the electron-hole interaction potential which is no longer a pure Coulomb interaction. This carriers-lattice interaction implies that in polar crystals in principle we should talk about polarons (exciton-polaron) i.e. charge carrier (electron-hole pairs) “dressed” in a phonon cloud. These polaronic excitations are characteristic of organic and ionic semiconductors.
We aim to understand polarons in 2D layered perovskites by exploring the relationship between hybrid structures and polaron formation and controlling polaron states to tailor electronic properties. While the significance of polarons in 2D perovskites is increasingly recognized, a detailed understanding remains lacking, limiting both fundamental insights and the ability to tune polaronic states for enhanced optoelectronic properties such as charge carrier thermalization and effective mass. Our research seeks to provide direct evidence for the control of polarons in hybrid 2D metal-halide perovskites, advancing both fundamental knowledge and the potential for future applications.
Figure (a) Cartoon showing a formation of a polaron – a quasi-particle resulting from the interaction between a charge carrier (electron or hole) and surrounding crystal lattice. The charge carrier displaces ions from their equilibrium positions either drawing atoms towards itself or pushing them away. The net effect can be a particle with an effective mass that is different from that of the free electron/hole. (b) Structure of the 2D perovskite (PEA)2PbI4. The structure of 2D layered perovskites consists of thin slabs of lead(tin)-halide octahedra, separated by large organic molecules (spacers).