Layered Hybrid Formamidinium Lead Iodide Perovskites: Challenges and Opportunities

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Hybrid halide perovskite materials have become one of the leading candidates for various optoelectronic applications. They are based on organic−inorganic structures defined by the AMX 3 composition, were A is the central cation that can be either organic (e.g., methylammonium, formamidinium (FA)) or inorganic (e.g., Cs + ), M is a divalent metal ion (e.g., Pb 2+ or Sn 2+ ), and X is a halide anion (I − , Br − , or Cl − ). In particular, FAPbI 3 perovskites have shown remarkable optoelectronic properties and thermal stabilities. However, the photoactive α-FAPbI 3 (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the δ-FAPbI 3 (yellow) phase that is not suitable for optoelectronic applications. This has stimulated intense research efforts to stabilize and realize the potential of the α-FAPbI 3 perovskite phase. In addition, hybrid perovskites were proven to be unstable against the external environmental conditions (air and moisture) and under device operating conditions (voltage and light), which is related to various degradation mechanisms. One of the strategies to overcome these instabilities has been based on low-dimensional hybrid perovskite materials, in particular layered two-dimensional (2D) perovskite phases composed of organic layers separating hybrid perovskite slabs, which were found to be more stable toward ambient conditions and ion migration. These materials are mostly based on S x A n−1 Pb n X 3n+1 composition with various mono-(x = 1) or bifunctional (x = 2) organic spacer cations that template hybrid perovskite slabs and commonly form either Ruddlesden−Popper (RP) or Dion−Jacobson (DJ) phases. These materials behave as natural quantum wells since charge carriers are confined to the inorganic slabs, featuring a gradual decrease in the band gap as the number of inorganic layers (n) increases from n = 1 (2D) to n = ∞ (3D). While various layered 2D perovskites have been developed, their FA-based analogues remain under-represented to date. Over the past few years, several research advances enabled the realization of FA-based layered perovskites, which have also demonstrated a unique templating effect in stabilizing the α-FAPbI 3 phase. This, for instance, involved the archetypical n-butylammonium and 2-phenylethylammonium organic spacers as well as guanidinium, 5-ammonium valeric acid, iso-butylammonium, benzylammonium, n-pentylammonium, 2-thiophenemethylammonium, 2-(perfluorophenyl)ethylammonium, 1-adamantylmethanammonium, and 1,4-phenylenedimethanammonium. FAPbBr 3 -based layered perovskites have also demonstrated potential in various optoelectronic applications, yet the opportunities associated with FAPbI 3 -based perovskites have attracted particular attention in photovoltaics, stimulating further developments. This Account provides an overview of some of these recent developments, with a particular focus on FAPbI 3 -based layered perovskites and their utility in photovoltaics, while outlining challenges and op...

Formamidinium-based Ruddlesden–Popper perovskite films fabricated via two-step sequential deposition: quantum well formation, physical properties and film-based solar cells

“…The complex formation mechanism of MA-based perovskite QWs has been well understood; this helps in achieving the desired QW distribution in the film, efficient carrier transport between QWs, and the final photovoltaic outcome. 19 In contrast to these well-understood MA-based layered 2D perovskites, FA-based analogs, which are believed to be an efficient strategy for customizing the energy characteristics toward stabilizing the 3D a-FAPbI 3 phase under harsh environmental and device operating conditions, 20 remain under-represented to date. This is likely due to the phase instability of FAPbI 3 -based systems and the difficulty in obtaining high-quality FA-based layered perovskite films in contrast to MA-based analogs.…”

Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

“…We do not observe other signals for ⟨n⟩ = 3 compositions, which is in accordance with the previous reports suggesting that the formation of 2D FAPbI 3 -based n > 2 phases is rather challenging. 27 In addition, the lack of preferential orientation in the ⟨n⟩ = 1 composition has been evidenced by GIWAXS (Section S5, Figure S6, SI) that display ring-shaped features with uniform angular intensity distribution that is associated with the random orientation of crystallites, whereas there is some preferential out-of-plane orientation for the distinct phase of the ⟨n⟩ = 2 composition. Such differences in preferential orientation between the n phases are not uncommon and have been reported previously.…”

“…We have primarily focused on lower compositional representatives (⟨n⟩ = 1−3) as ⟨n⟩ = 1 compositions are known to form better defined 2D phases, whereas ⟨n⟩ > 1 compositions commonly form mixtures of different ⟨n⟩ 2D/3D phases. 4,5,22,23,27 We analyzed the morphology, structural, and optoelectronic properties by a multi-technique approach including both theoretical and experimental analysis.…”

“…Such differences in preferential orientation between the n phases are not uncommon and have been reported previously. 14,27 Owing to this high degree of long-range disorder, we turn to solid-state NMR to investigate the local structure and atomic-level mixing. 3,13,32−36 We have performed echo-detected 14 N as well as 1 H → 13 C and 1 H → 15 N cross-polarization (CP) magic-angle spinning (MAS) NMR measurements (Figure 4).…”

Stabilization of Metastable Halide Perovskite Lattices in the 2D Limit

“…It is interesting to mention that for the MA-based stable perovskites, the use of long-chain ammonium cation additives in the film deposition leads to the formation of multiple-phase 2D RP perovskites with different values of n and 3D perovskite crystallites. [70][71][72] In some stabilized FAPbI 3 perovskite films, [73] besides the absorption edge from 3D perovskite phase, the absorption spectra of the films exhibit a few additional blueshifted peaks associated with 2D RP perovskites. Controlling phase purity has been a challenge due to the complex interplay between thermodynamics and kinetics, although the mixed phase films could be beneficial for the directional charge/ energy transfer processes.…”