Two-Dimensional Perovskites with Maximum Symmetry Enable Exciton Diffusion Length Exceeding 2 Micrometers

Realizing semiconductors with high symmetry has been a virtue of inorganic materials and has resulted in novel physical behaviors such as thermoelectric, high harmonic generation, and topological materials. In contrast, hybrid (organic or inorganic) crystals such as two-dimensional halide perovskites exhibit lower symmetry with in-plane or out-of-plane octahedral distortions. Here we report for the first time, a new series of layered two-dimensional perovskite that exhibit the highest theoretically predicted symmetry with a tetragonal P4/mmm space group. These Dion−Jacobson (DJ) phase 2D perovskites were synthesized by using a modified approach to control the kinetics, which enabled the incorporation of formamidinium (FA) as cage cation and exhibit no octahedral distortions octahedron in the average structure at ambient conditions. The FA-based 2D perovskites exhibit short interlayer distance (4 Å), which results in systematically lower bandgaps among reported 2D perovskites (~ 1.7 to 1.8 eV), making them ideal for Si/perovskite or perovskite/perovskite multijunction solar cells. The distortion-free structure results in an exciton diffusion length of 2.5 µm, in contrast to 100-200 nm, reported for other 2D perovskites.