Complete Suppression of Detrimental Polymorph Transitions in All-Inorganic Perovskites via Nanoconfinement

Abstract

Reducing the size of metal halide perovskite crystals to the nanoscale has been demonstrated to stabilize high-performance metastable polymorphs at room temperature. Cesium lead iodide (CsPbI3), for example, typically exists as the insulating δ-phase at room temperature but can adopt the narrow-bandgap γ-phase when the crystal size is reduced to the nanometer length scale. Here we advance a fundamental understanding of the role of nanoconfinement in CsPbI3 polymorph stabilization through a combination of X-ray diffraction and temperature-dependent photoluminescence. Using a wet annealing method to directly form γ-CsPbI3 from solution in the cylindrical nanopores of anodized aluminum oxide, we discovered that nanoconfinement lowers the δ−γ solid-state phase transition temperature from 448 K in the bulk to 370 K. Once formed, nanoconfined γ-CsPbI3 crystals were found to be stable across the temperature range of 4–610 K and upon an unprecedented one year of air exposure at room temperature. Taking advantage of the nanoconfinement-induced suppression of phase transitions, we report for the first time a detailed analysis of electron–phonon interactions in γ-CsPbI3 via temperature-dependent photoluminescent measurements. In-depth analysis of the temperature-dependent peak broadening revealed electron–phonon interactions to be dominated by Fröhlich scattering, similar to that observed in inorganic–organic hybrid perovskite systems. Photoluminescence mapping further confirmed that nanoconfined γ-CsPbI3 crystals exhibit spatial uniformity on the tens of micrometers length scale, suggesting nanoconfinement as a promising strategy to form stable, high-performance perovskite films from solution for light-emitting and light-harvesting applications.

Polymorphism in metal halide perovskites

“…By tuning the processing conditions, we discovered that -CsPbI 3 could be stabilized in AAO pores as large as 250 nm. 152 Using wet annealing method in which spun cast samples with solvent remained were immediately annealed at a minimum temperature 373 K, -CsPbI 3 was formed within the AAO nanopores. Once formed, nanoconfined -CsPbI 3 was found to be stable in a range of 4 -610 K and for a period of at least 120 days of storage in air.…”

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

“…[19][20][21] In addition to their tunability, NC perovskites also exhibit enhanced phase stability compared to their bulk counterparts as the NC size decreases. 11,12,22 For example, while bulk α-CsPbI 3 can only be stablized at temperatures greater than 600 K, its NCs can be synthesized at room temperature. 11,12,19 Similar trends with NC a) Electronic mail: lztan@lbl.gov size have also been observed in materials classes other than halide perovskites, such as metal nanocrystals and ferroelectric BaTiO 3 .…”

Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3

“…Thus, the generation of the non-perovskite δyellow phase structure, that requires an expansion in the unit cell volume, is thermodynamically avoided under nanoconfinement, as the energy barrier to promote the phase transformation increases (Figure 2b). 40,52 Nonetheless, the reversible phase transitions between α-, β-and γ-black phases that occurs into the confined PNCs, due to the slight distortion of the lattice, is less sensitive to the reduction of particle size. These reasons explain the long-time black phase stability of CsPbI3 and FAPbI3 PNCs both in colloidal solution and in thin film, 20,38,44 being photoactive for months even under air conditions, see Figure 2c,d.…”

“…Therefore, the metastable black phases are stabilized at room temperature, keeping their crystal structure even under their decomposition temperature. 40 For instance, Zhao et al estimated by DFT calculations that γ-CsPbI3 black phase is preferred over δ-CsPbI3 (Figure 3) by extending the surface area greater than 8.6 x 10 3…”

“…b) Activation energy barrier for the αto-δ-phase transition for nanoconfined CsPbI3 NCs and bulk crystals. Reproduced with permission from ref 40. .…”