Low-dimensional Ruddlesden-Popper (RP) structures are becoming ubiquitous in the field of halide perovskites optoelectronics. These materials are tunable between 2D (n = 1) and quasi-2D (1 < n < inf.) phases which determine their optical (bandgap), electronic (binding energy, conductivity) and chemical properties (hydrophobicity) [1]. The tunability has enabled their use as the active layer for light absorption, detection and emission, or at interfaces for efficient charge transport and improved stability. However, solution-processing of such films is challenging since interactions between precursors (organic and metal cations, halide anions) and solvents (such as DMF, DMSO) cause competing crystallization reactions that undermine dimensional purity and impact the optoelectronic properties and device stability [2]. We developed a co-evaporation method to deposit RP films with high control on crystallization by eliminating these precursor-solvent interactions [3,4]. Here, we deposit prototypical 2D (PEA₂PbI₄) and quasi-2D (PEA₂FAPb₂I₇) thin films using phenethylammonium (PEA+) as the organic spacer cation and formamidinium (FA+) as the organic cation for the quasi-2D phase and study the interactions that determine film formation.

The formation of the bare 2D layer (PEA₂PbI₄) and the development of an interface on top of a 3D perovskite film was studied using X-ray diffraction, X-ray photoelectron spectroscopy, electron microscopy and photoluminescence microscopy. The co-evaporated 2D structures were then used as interfacial layers at the hole-transporting interface to yield approx. 22% efficient perovskite solar cells, driven by improvements in the open-circuit voltage and fill factor [4]. Notably, this performance gain is observed over a large RP thickness range, owing to the improved control on RP phase through co-evaporation.

We then studied the crystallization of quasi-2D structures (PEA₂FAPb₂I₇) and observed the impact of phosphonic acid substrate modification using density functional theory calculations, synchrotron-based X-ray scattering and ultrafast pump-probe spectroscopy [5]. Here, favorable interactions between unbound solution-processed  propylphosphonic acid (PPAc) and PEA⁺ increases the relative uptake of PEA⁺ compared to FA⁺, favoring the formation of the 2D phase over the quasi-2D phase. This allows the formation of heterostructures between 2D and quasi-2D phases, controlled by the PPAc concentration and the co-evaporated film thickness.

Taken together, our work demonstrates a new synthesis method to overcome a critical challenge in coating RP perovskites with high phase purity. We demonstrate the applicability of this approach in solar cells as well as present a new way to synthesize controlled RP heterostructures. This work also complements ongoing research in the field of halide perovskites on vapor-based coating methods and presents a scalable method to coat bulk films as well as interfacial layers for high-performance devices.