Synthetic Control of Water-Stable Hybrid Perovskitoid Semiconductors

Abstract

Hybrid metal-halide perovskites and their derived materials have emerged as the next-generation semiconductors with a wide range of applications, including photovoltaics, light-emitting devices, and other optoelectronics. Over the past decade, numerous single-crystalline perovskite derivatives have been synthesized and developed. However, the synthetic methods for these derivatives mainly rely on acidic crystallization conditions. This approach leads to crystals comprising metal halide building blocks, which show problematic stability when directly exposed to water. In this study, a methodology is developed for synthesizing hybrid metal-halide compounds using lead iodide and the zwitterionic bifunctional molecule cysteamine (CYS), to form various perovskitoid structures under a broad pH range. Interestingly, the different pH conditions alter the coordination environment of lead halides, leading to lead-sulfide and lead-nitride covalent bond formation. This modification significantly enhances their stability when in direct contact with water, lasting for months. Photoluminescence measurements and first principal density functional theory (DFT) calculations reveal that the perovskitoids synthesized under basic and acidic pH conditions exhibit a direct bandgap nature, while those synthesized under neutral conditions display an indirect bandgap. This approach opens new avenues for manipulating synthetic methods to develop water-stable hybrid semiconductors suitable for a wide range of applications, such as solid-state light emitters.

Keywords: halide perovskites; light emission; perovskitoids; single crystals; solid state semiconductor.

Figure 1

Proposed reaction mechanisms of the protonation process of cysteamine under various pH conditions. a) cysteamine in solid form, b) the form of cysteamine under acidic conditions, c) the zwitterionic form of cysteamine under neutral (slightly basic) conditions, d) the form of cysteamine under basic conditions, e) the hydrogen bonding between Pb and I in acidic condition, f) the coordination Pb‐S in neutral condition, and g) the coordination of Pb‐S and Pb‐N in basic condition.

Figure 2

Crystal structures that synthesized at different pH conditions. a) crystal structure of (^+/0CYS‐CYS^+/0)PbI₄ in acidic condition (pH −1.3). b) crystal structure of (CYS^+/−)PbI₂ in neutral condition (pH 7.7). c) Crystal structure of (CYS^0/−)PbI in basic condition (pH 13.2). d) corner‐sharing octahedra connectivity in acidic condition. e) edge‐sharing octahedra connectivity in neutral conditions f) edge‐sharing octahedra connectivity in basic conditions. Raman spectra of g) (^+/0CYS‐CYS^+/0)PbI₄ in acidic condition (pH −1.3), h) (CYS^+/−)PbI₂ in neutral condition (pH 7.7) and i) (CYS^0/−)PbI in basic condition (pH 13.2).

Figure 3

The structural stability of CYS perovskitoids. a) PXRD patterns of basic crystal and the stability check in water. b) PXRD patterns of neutral crystal and the stability check in water. c) PXRD stability test of basic and neutral crystals under a strong base (0.1m KOH (aq)) solution. d) PXRD patterns of acidic crystal after exposure to the water.

Figure 4

Optical properties of CYS perovskitoid single crystals. a) Diffuse reflectance spectra of three perovskitoids. b) Tauc plots of perovskitoids. c) PL spectra of perovskitoids at room temperature. Time‐resolved PL of d) basic e) neutral f) acidic crystals, with inset photographs showing the respective luminescent crystals under UV illumination. 2D temperature‐dependent PL spectra of g) basic h) neutral i) acidic crystals.

Figure 5

Electronic band structures from a) basic crystal, b) neutral crystal, and c) acidic crystal with corresponding bandgap energy.

Figure 6

a) Thin film of neutral crystal/PS composite. Emission of neutral crystal/PS composite thin film under UV light (365 nm) b) in the air, and c) in the water.