Halide Choice is Critical for The Formation of 2D/3D Perovskites and Their Performance in Solar Cells

2D/3D perovskite solar cells (PSCs) hold great promise for photovoltaics applications.¹ The key to the development of stable and efficient devices is the synergy between the higher ambient stability of 2D phases and the outstanding electrical and optical properties of 3D halide perovskites. These materials also have high versatility in terms of composition, allowing for different material design and engineering strategies.<br/>The vast list of molecules (spacers cations) that can be used for the synthesis of the 2D phases also poses a challenge to the development of these materials.² Each of these spacer cation presents a different reactivity with the underlying 3D perovskite, encouraging trial-and-error approaches rather than a rational design of stable 2D/3D interfaces.³ For example, the choice of the halide anion, both in the spacer cation and in the 3D perovskite, greatly influences materials’ photovoltaic and device properties.⁴ To provide more insights into the formation and ionic dynamics of these complex interfaces, we examined the impact of halide choice (I and Br) in 3D perovskites treated with phenylethylammonium salts (PEAI and PEABr, respectively).<br/>We evaluated the formation of layered perovskites using PEAI and PEABr salts on top of 3D perovskites with pure iodine and mixed halides, Cs_0.17FA_0.83PbI₃ and Cs_0.17FA_0.83PbI_2.5Br_0.5, respectively, before, during, and after annealing at 65 °C for 30 minutes. Through various spectroscopic techniques, we revealed that PEABr reacts less readily with the 3D film compared to PEAI to form the 2D phases. As a consequence of this lower reactivity, PEABr can diffuse through the grain boundaries as an ionic pair, and this behavior was revealed by X-ray photoelectron spectroscopy (XPS). Also, the mixed-halide 3D perovskite is less reactive than its pure-iodine counterpart. The lower reactivity of PEABr may be attributed to the higher lattice energy of ammonium bromides in comparison to ammonium iodides,⁵ and similar reasoning applies to the mixed-halide 3D perovskite. Consequently, PEAI presents higher passivation ability than PEABr, and the best device performance and reproducibility were achieved with PEAI-treated Cs_0.17FA_0.83PbI₃. Our work provides valuable insights into the fundamental properties and formation mechanisms of 2D/3D perovskites, which is crucial for advancing these photovoltaic devices.