Narrow‐Linewidth Emission and Weak Exciton‐Phonon Coupling in 2D Layered Germanium Halide Perovskites

The photophysical properties of low‐dimensional metal‐halide semiconductors and their tunability make them promising candidates for light‐absorbing and emitting applications. Yet, the germanium‐based halide perovskites to date lack desirable light‐emitting properties, with so far only very broad, weak, and unstructured photoluminescence (PL) reported due to significant octahedral distortion. Here, the photophysical properties of the 2D layered Ruddlesden‐Popper semiconductors (4F‐PMA) 2 GeI 4 and (4F‐PMA) 2 PbI 4 (4F‐PMA: 4‐F‐phenylmethylammonium) are characterized and compared. Using a combination of single‐crystal X‐ray diffraction, variable temperature time‐resolved PL, and density functional theory, structure‐property relations are correlated. Specifically, the results indicate that (4F‐PMA) 2 PbI 4 features stronger coupling to longitudinal optical (LO) phonons, assisting emission from a broad bound‐exciton state due to a soft, deformable lattice. In contrast, (4F‐PMA) 2 GeI 4 , benefitting from intermolecular bonding to scaffold a rigid octahedral structure, shows weaker LO‐phonon coupling, resulting in the longest PL lifetime and most narrow linewidth (≈120 meV linewidth at 2 K) reported for a Ge‐halide perovskite yet, without the occurrence of any additional bound‐state emission at low temperatures. These results highlight the potential of germanium halide perovskite materials for optoelectronic applications.