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.