Room-Temperature Amplified Spontaneous Emission in Lead-Free Cesium Germanium Iodide Perovskite

Lead halide perovskites (LHP), due to the ease of synthesis and great optoelectronic properties, have received significant attention for various applications including emitters in LEDs and semiconductor lasers. Unfortunately, the toxicity of lead is a drawback for the commercialization of such devices, therefore, a lead-free perovskite is needed. One of the most promising approaches is to replace lead (Pb) cation with the less toxic tin (Sn). However, these materials' performances are still inferior to those of lead counterparts. In fact, amplified spontaneous emission (ASE), a fundamental prerequisite for lasing application, has often been observed at cryogenic temperatures. Therefore, is still of fundamental importance to find efficient and stable nontoxic alternatives to LHP. Another emerging element that belongs to the same group as Pb and Sn is germanium (Ge). However, an extensive understanding of the optical properties of this class of materials is still missing.<br/>In this work, we present an in-depth investigation of the structure and the optical properties of CsGeI₃. We structurally characterized solution-processed CsGeI₃ powder using X-ray total scattering and computational methods. The photoluminescence of this sample at high excitation fluence (4 mJ/cm²) and low temperature (200K) shows a sharp ASE peak in the NIR (815nm). Furthermore, we propose a one-step deposition method of CsGeI₃ perovskite thin film maintaining the same crystalline structure. This sample shows much better performances. At room temperature, the ASE peak appears at 748nm with a notably low threshold (75 μJ/cm²), comparable to the well-studied and optimized LHPs. Our work represents the first report of ASE in a germanium-based perovskite, and the low threshold at room temperature makes this material a really interesting candidate for lasing application paving the way to a new class of lead-free perovskite gain material.