Two-dimensional or passivation treatment: the effect of hexylammonium post deposition treatment on 3D halide perovskite-based solar cells

Much effort has been made to push the power conversion efficiency of perovskite solar cells (PSCs) towards the theoretical limit. Recent studies have shown that post deposition treatment of barrier molecules that might form 2-dimensional (2D) perovskite on top of already crystalized 3D perovskite can improve the device performance and enhance stability. Still, it is debatable whether a 2D perovskite can indeed be formed on top and if so, whether it significantly contributes to photovoltaic (PV) performance. In this work we focused on the effect of hexylammonium iodide (HAI), which functions as a barrier molecule, post deposition treatment on top of already crystalized Cs(0.2)FA(0.8)Pb(I0.8Br0.2)(3) 3D perovskite. The HAI treated devices showed mainly an increase in the open circuit voltage and in fill factor compare to the non-treated devices. Since a clear improvement in the PV parameters of the treated devices was observed, we wanted to determine whether HAI treatment results in a 2D perovskite layer or serves as a passivation layer on top of the 3D perovskite. In order to address this question, a series of optical and physical characterizations at the nanometric HAI/perovskite interface were performed. A high-resolution transmission electron microscopic image of the interface, including the Fourier transform, shows a different crystallographic structure. In addition, X-ray photoelectron spectroscopy (XPS), combined with an etching process using argon (Ar) gas, was used to determine the chemical composition of this nanometric interface. There was no evidence of a significant and continuous 2D structure due to the HAI treatment. Most of the HAI can be found on top of the perovskite surface, serving as a passivation layer; this reduces the recombination rate and enhances the photoluminescence quenching. Understanding how the interface of the perovskite/R-NH3 molecules can be treated is a key to enhance further the performance of these perovskite solar cells.