Phase Segregation in Cs-, Rb- and K-Doped Mixed-Cation (MA)_x(FA)_1–xPbI₃ Hybrid Perovskites from Solid-State NMR

Abstract

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use 133Cs, 87Rb, 39K, 13C, and 14N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability.

Supramolecular Engineering for Formamidinium‐Based Layered 2D Perovskite Solar Cells: Structural Complexity and Dynamics Revealed by Solid‐State NMR Spectroscopy

“…We have previously shown that solid-state NMR is well suited to probe the atomic-level microstructure of multicomponent lead halide perovskites, [26][27][28][29] yet this approach has not been systematically employed to analyze the structural features of layered 2D perovskites so far. We have previously shown that solid-state NMR is well suited to probe the atomic-level microstructure of multicomponent lead halide perovskites, [26][27][28][29] yet this approach has not been systematically employed to analyze the structural features of layered 2D perovskites so far.…”

“…[16,26,27] Since the A/FA and A′/FA phases contain a 3D perovskite phase, 14 N MAS NMR can yield information on whether or not this phase is microscopically modified by the presence of the spacer. In the case of 3D perovskites, 14 N MAS NMR spectra feature a spinning sideband (SSB) pattern that corresponds to the central cation reorienting on the picosecond timescale within the cubooctahedral cavity.…”

Halide perovskites: current issues and new strategies to push material and device stability

“…Being too small to be inserted in the perovskite framework, it has been suggested that they can sit at the interstitial sites of the perovskite lattice, inhibiting iodine vacancies migration [26] (see section 3). Discussion on this is still open: recent solid state NMR studies have proven that neither Rb + nor K + can enter the perovskite lattice [27]: they rather segregate into phases such as δ-Cs 0.5 Rb 0.5 PbI 3 . On the other side, Li et al [28] showed that small cations such as Li + and Na + can migrate through the perovskite, possibly influencing the long term stability of the devices.…”

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field‐Effect Transistors

“…The MHP crystalline structure is usually depicted by the general formula ABX 3 , where A and B are cations while X stands for an anion . The A‐site monovalent cations can be organic (e.g., methylammonium (MA) CH 3 NH 3 + , or formamidinium (FA) CH(NH 2 ) 2 + ), inorganic (e.g., Cs + , or Rb + ), or organic–inorganic mixtures . The B‐site divalent metal cations are usually Pb 2+ , Sn 2+ , Ge 2+ , Cu 2+ , or Bi 2+ or their mixtures, whereas X represents a monovalent halide anion (e.g., Cl − , Br − , or I − ).…”