(581d) Nanoconfinement to Stabilize Light Harvesting Metal-Halide Perovskites

Metal-halide perovskites have emerged as frontrunners in the field of emerging solution-processable photovoltaics, with solar conversion efficiencies of MHP-based solar cells recently exceeding 25%. A critical obstacle facing the commercialization of these compounds is their sensitivity to humidity-induced degradation and temperature-induced polymorph transitions. In this work, we present nanoconfinement as a generalizable strategy to impart unprecedented air and thermal stability to MHPs. Specifically, MHPs, including MAPbI₃ and CsPbI₃, were crystallized in the cylindrical nanopores of anodized aluminum oxide templates and their properties studied via x-ray diffraction, temperature-dependent photoluminescence spectroscopy, and low-frequency Raman spectroscopy. We discovered that nanoconfinement affects the relative Gibbs free energies of solid-state polymorphs by increasing the surface free energy contribution to the total Gibbs free energy compared to the volume free energy contribution [1,2]. By shifting thermodynamic phase transition temperatures and kinetically trapping crystals within nanoconfined environments, the active phase of CsPbI₃ was stabilized to temperatures as low as 4 K, enabling the extraction of longitudinal optical phonon energies through temperature-dependent photoluminescent experiments [1]. The stability of nanoconfined MHPs upon storage in air was also dramatically improved, with no degradation observed after at least 2 years [3,4]. Most recently, we combined the use of nanoconfinement and a below bandgap 976 nm laser source to study phonon modes in stabilized MHPs via low-frequency Raman spectroscopy measurements. Through systematic replacement of I^- with Br^- or Cs⁺ with methylammonium (MA⁺) in nanoconfined CsPbI₃ crystals, we discovered that the energy of these phonon modes depends primarily on the lattice dimensions, not the identity of the ions themselves. Given that electron-phonon coupling is thought to be a limiting factor in the optoelectronic performance of metal-halide perovskite active layer, an accurate understanding of the nature of these interactions is critical.

1. Kong, X., Shayan, K., Hua, S., Strauf, S., and Lee, S. S. “Complete Suppression of Detrimental Polymorph Transitions in All-Inorganic Perovskites via Nanoconfinement” ACS Applied Energy Materials 2, (2019): 2948–2955.
2. Kong, X., Zong, K., and Lee, S. S. “Nanoconfining Optoelectronic Materials for Enhanced Performance and Stability” Chemistry of Materials 31, (2019): 4953–4970.
3. Lee, S., Feldman, J., and Lee, S. S. “Nanoconfined Crystallization of MAPbI3 to Probe Crystal Evolution and Stability” Crystal Growth & Design 16, (2016): 4744–4751.
4. Kong, X., Shayan, K., Lee, S., Ribeiro, C., Strauf, S., and Lee, S. S. “Remarkable Long-Term Stability of Nanoconfined Metal-Halide Perovskite Crystals against Degradation and Polymorph Transitions” Nanoscale 10, (2018): 8320–8328.