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

Abstract

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.

Green-route manufacturing towards future industrialization of metal halide perovskite nanocrystals

“…Similarly, double perovskite Cs 2 SnI 6 NCs were derived from the powder counterpart. 58 These results demonstrate the successful synthesis of NCs by the novel method of post-treatment ligands, which can be attributed to the interaction/bonding between ligands and NCs to enable the attachment of ligands onto NCs. However, the mechanisms of how ligands are attached to NCs remain elusive and need to be explored in the future.…”

“…Tang et al performed a series of aqueous syntheses of double perovskite NCs (DPNCs) via a two-step approach. 57,58 The first step is to synthesize double perovskite powder (DPP) in an aqueous solution by mixing Cs-based and Pb-based precursor solutions, followed by a heating process to evaporate the liquid in as-obtained powder (Fig. 3).…”

“…However, ligands can also be added after the formation of crystals in ultrasonication-assisted methods. 57,58,63,64 This indicates that there exists a subtle interaction between ligands and NCs.…”

“…The PLQY enhancement of PeNCs can be generally attributed to the quantum size (confinement) effect, which occurs when the characteristic size (diameter) of PeNCs is comparable to or smaller than the exciton Bohr radius of the corresponding bulk semiconductor. 22,[74][75][76] Tang et al 58 proposed a critical characteristic size (a c ) to describe a similar phenomenon. If the characteristic size of a crystal is larger than a c , the bandgap of the crystal is not tuneable by changing the crystal size, i.e., the size (confinement) effect will disappear.…”

“…, the size (confinement) effect will disappear. Here, a c is expressed as 58 in which ħ is the reduced Planck constant; m and are the mass and reduced effective mass of an electron, respectively; and k B and T are Boltzmann's constant and the absolute temperature, respectively.…”