Understanding size dependence of phase stability and band gap in CsPbI3 perovskite nanocrystals

Abstract

Inorganic halide perovskites CsPbX3 (X = Cl, Br, I) have been widely studied as colloidal quantum dots for their excellent optoelectronic properties. Not only is the long-term stability of these materials improved via nanostructuring, their optical bandgaps are also tunable by the nanocrystal (NC) size. However, theoretical understanding of the impact of the NC size on the phase stability and bandgap is still lacking. In this work, the relative phase stability of CsPbI3 as a function of the crystal size and the chemical potential is investigated by density functional theory. The optically active phases (α- and γ-phase) are found to be thermodynamically stabilized against the yellow δ-phase by reducing the size of the NC below 5.6 nm in a CsI-rich environment. We developed a more accurate quantum confinement model to predict the change in bandgaps at the sub–10 nm regime by including a finite-well effect. These predictions have important implications for synthesizing ever more stable perovskite NCs and bandgap engineering.

Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3

“…When PNCs display sizes near to bulk particles, EPNC is dictated by Ebulk while Esurf is almost negligible. [41][42] However, by increasing the surface area/volume ratio during the formation of PNCs, the number of atoms on the material surface increases and Esurf governs EPNC. [43][44] Esurf of α-and γ-CsPbI3, estimated by DFT calculations, are 1.2 and 4.0 meVÅ -2 , respectively, while δ-CsPbI3 presents a larger value around 7.5 meVÅ -2 .…”

“…Figure 2. a) Energy formation of the α-, γ-and δ-phases of CsPbI3 PNCs by varying the NC size. Reproduced with permission from ref41 . Copyright 2020 Royal Chemistry Society.…”

Structural Properties and Stability of Inorganic CsPbI3 Perovskites

“…Calculation results also indicate a Cs‐rich film presents improved perovskite‐phase stability (Figure 6c). [ 85 ] The superior stability of Cs‐rich perovskite film has been also demonstrated both in solution process and coevaporation process (Figure 6d). [ 86,87 ]…”

“…b,c) Reproduced with permission. [ 85 ] Copyright 2020, American Institute of Physics. d) [Cs]/[Pb] atomic ratio as a function of position measured at the phase boundary.…”

“…Estimated by density functional theory (DFT) calculations, α‐ and γ‐CsPbI 3 present a smaller E surf values of 1.2 and 4.0 meVÅ −2 , respectively, whereas this value of δ‐CsPbI 3 is around 7.5 meVÅ −2 . [ 85 ] At the bulk limit, the total energy converges to the formation energies of the bulk (α > γ > δ), as the surface energy is negligible for large NCs. As the diameter decreases, the total energy starts to increase as surfaces cost energy to form.…”