Layered Hybrid Formamidinium Lead Iodide Perovskites: Challenges and Opportunities

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, fonnamidinium (FA)) or inorganic (e.g., Cs+), M is a divalent metal ion (e.g., Pb2+ or Sn2+), 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 alpha-FAPbI(3) (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the delta-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 alpha-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 = co (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 alpha-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-thiophenemethylammo-nium, 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