Aqueous Synthesis of Ultrastable Dual-Color-Emitting Lead-Free Double-Perovskite Cs₂SnI₆ with a Wide Emission Span Enabled by the Size Effect

Lead-free perovskites are promising materials for the functionalities of optoelectronic devices and systems. In this work, we develop a green-route approach for the synthesis of lead-free double-perovskite Cs2SnI6 powder and nanocrystals (NCs). An environmentally friendly aqueous solution is used to prepare the Cs2SnI6 powder at room temperature, and ultrasonicating the Cs2SnI6 powder in hexane yields the Cs2SnI6 NCs. The photoluminescence (PL) analysis reveals band gaps of 1.3 and 2.8 eV for the Cs2SnI6 powder and NCs, respectively. The Cs2SnI6 powder exhibits a remarkable photothermal effect driven by a laser of 785 nm with electric voltage ranging from 0.31 to 0.36 V, and the PL peak experiences a blueshift of 940 to 934 nm. Increasing temperature from 30 to 90 °C causes a blueshift of the PL peak of the Cs2SnI6 powder with an apparent energy barrier of 1.298 eV for thermal quenching. The Cs2SnI6 NCs exhibit a PL peak centered at 440 nm and a photoluminescence quantum yield of ∼7% at room temperature and have a long-term stability over 50 days. The anomalous behavior of the dual-color emission of the perovskite Cs2SnI6 is likely attributed to the size dependence of the dielectric constant.