Thermal Stability of the Black Perovskite Phase in Cesium Lead Iodide Nanocrystals Under Humid Conditions

Abstract

The equilibrium phase of cesium lead iodide (CsPbI3) at room temperature is yellow and optically inactive due to its indirect band gap. The metastable black phase of CsPbI3 on the other hand exhibits optical properties that are suitable for photovoltaic and light-emitting devices. Here, we examine the stability of the black phase of ligand-stabilized CsPbI3 nanocrystals heated in humid air. Water vapor is known to catalyze the transition of CsPbI3 from the black phase to the yellow phase. Uniform nanocrystals with cube shape were synthesized with capping ligand mixtures of oleylamine and oleic acid or diisooctylphosphinic acid, assembled into superlattices with preferred crystal orientation, and studied using grazing incidence small- and wide-angle X-ray scattering with in situ heating. The black-phase nanocrystals are found to inhabit the γ-orthorhombic phase and do not revert to the equilibrium yellow δ-orthorhombic phase until reaching a relatively high temperature, between 170 and 200 °C, coinciding with superlattice degradation.

The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals

“…Spectral signatures of the NCs' luminescence. The CsPbI 3 NCs of this study are synthesized with an orthorhombic γ-phase crystal structure 24,25 (Fig. 1a).…”

Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy

“…MHP NCs can assemble into microparticles and superlattices in dispersions or upon deposition. 42,233,[261][262][263][264][265][266][267][268][269] Perovskite NC lms are promising candidates for optoelectronic devices, but their limited ambient stability and charge transport across ligand-NC interfaces remains a challenge. 15,51,257,259,270,271 The small size of MHP NCs can also induce strain and modify surface energies, altering the energetics of particular phases (e.g.…”

Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices: Effects on Photoluminescence and Energy Transfer

“…Regardless of the synthetic methods and surface passivations of CsPbX3 nanocrystal building blocks, their dynamic chemistry makes nanocrystal assemblies metastable, as has been documented in reports of coalescence and halide expulsion under external stimuli such as pressure, 32 heating [33][34][35] or illumination. 36 Much less is known about the nanocrystal stability in superlattices under common sample handling conditions (e.g., under vacuum or in the air) and its effect on the optical properties of individual superlattices.…”