The race between complicated multiple cation/anion compositions and stabilization of FAPbI(3) for halide perovskite solar cells

Compositional modifications and passivating additives have been key enablers to achieve operationally stable halide perovskite devices with excellent optoelectronic properties. The thermal and structural instability of the most desirable single-cation metal halide perovskites motivates the use of multiple cation and mixed-halide compositions that indeed lead to superior optoelectronic and photovoltaic properties over the course of the development. The application of multiple cation/anion and additive-based alloyed perovskites could, however, be hindered by the formation of non-perovskite phases and bandgap increase. Recent studies have shown that exceptional performance can be achieved using simpler compositions, such as FAPbI(3), with appropriate passivation methods. In this perspective, we will present the current status of multiple cation/anion perovskites and discuss the role of common monovalent cations such as FA, MA, Cs, Rb, and K on the stability, optoelectronic properties, and charge transport behavior of lead halide perovskites. We further present the common stabilization and passivation strategies for phase-pure FAPbI(3). We highlight the key role of solid-state NMR in determining the atomic-level mechanism of action of the various dopants and passivation agents. We then summarize the current understanding of the benefits and drawbacks of the cation alloying approach relative to the passivated phase-pure FAPbI(3). Finally, we discuss the perspective for future research directions to achieve stable perovskite solar cells that approach the theoretical limit.