Abstract
Two-dimensional (2D) perovskites are attracting renewed interest for coherent emission. While excellent results have been achieved with tin-based materials, the real applicability of lead perovskites remains controversial due to limited reproducibility. Critically, there is no fundamental explanation so far to provide an educated prediction about their lasing properties. In this work, we compare the 2D perovskites PEA(2)PbI(4), PEA(2)SnI(4), and their mixed compositions. Photophysical characterization and solid-state nuclear magnetic resonance (ssNMR) spectroscopy reveal that molecular motions of the organic cation and local lattice disorder induced by metal cation mixing critically impact the amplified spontaneous emission (ASE) properties of the material. We show that ASE can be achieved for both perovskites at 78 K employing short pump pulse width (fs) and near-band gap excitation. However, stable operation at room temperature is achieved only in PEA(2)SnI(4), thanks to the lower Auger recombination rate, its peculiar lengthening of the excited state lifetime at higher temperatures and lower impact of trap-mediated recombination compared to PEA(2)PbI(4). Our work highlights the importance of defect control and crystal engineering strategies to enhance the structural rigidity and improve the optoelectronic properties of this class of soft semiconductors.