Controlling Phase Purity of Co-Evaporated Lower Dimensional Ruddlesden-Popper Films Using Surface Functionalization

High quality Ruddlesden-Popper lower-dimensional perovskite-derived phases (nominally R₂A_n-1Pb_nI_3n+1 where R is an organic spacer cation and A is a monovalent small organic cation) are an attractive materials platform for optoelectronic devices such as LEDs, lasers, photodetectors and solar cells. Changing the number of conjoined lead halide octahedral sheets (represented by the n value) between organic spacer cations can tune the optical bandgap, quantum confinement and exciton binding. This furthermore impacts defect formation energy and ion migration rates, affecting device performance and stability. However, solution-based thin film processing methods typically yield heterogeneous quasi-2D films consisting of several n-value phases with different optical bandgaps, and present a very narrow processing window for the deposition of high quality quasi-2D films. This results from solubility differences among precursor salts, leading to heterogeneous crystallization of different n-value phases and the formation of phase gradients.<br/>In this work, we demonstrate thermal co-evaporation deposition routes for the development of phenethylammonium (PEA)-based Ruddlesden-Popper quasi-2D structures. The method is solvent-free and compatible with industrial processes used in the fabrication of optoelectronic devices. Using synchrotron-based structural characterization, we show that the elimination of precursor-solvent interactions can yield higher phase-purity and eliminate phase gradients across the thickness of the film. However, the crystallization of minority secondary phase, as probed by ultrafast transient absorption spectroscopy, limits exciton lifetime due to charge-carrier quenching by lower-bandgap states. We use phosphonic acid surface functionalization to drive the growth of the Ruddlesden-Popper phase and suppresses secondary phase crystallization. Using X-ray photoelectron spectroscopy, we study the diffusion of phosphonic acid molecules and their role in the crystallization process. This templated-growth results in an increase of the exciton lifetime from 4 ps to 220 ps from the quasi-2D phase. Finally, using in-situ synchrotron-based structural characterization, we study the impact of phase-purity on thin film stability under high humidity conditions.