Microstructure-modulated conductive filaments in Ruddlesden-Popper perovskite-based memristors and their application in artificial synapses

Low-dimensional, lead-free perovskite-based electronic/optoelectronic devices exhibit the advantages of stability and nontoxicity. However, their electrical performance is often lower than that of their three-dimensional, lead-based counterparts. Hence, understanding the correlations between microstructural features and electrical characteristics of low-dimensional, lead-free perovskite-based devices is essential for enhancing device performance. In this study, a two-dimensional Ruddlesden-Popper type perovskite, phenylethylammonium tin iodide (PEA₂SnI₄), was selected as the active material. Various PEAI:SnI₂ blending ratios and molar concentrations were adopted to fabricate PEA₂SnI₄-based memristors. The absorption spectra of PEA₂SnI₄ thin films from diverse process conditions show significant differences, which are rarely discussed in the literature. With the aid of theoretical calculations, we found that the variations in absorption spectra reflect that the PEA₂SnI₄ specimens with low PEAI loading have short crystallite size perpendicular to the substrate and deficiency of organic components in lattice structures, slowing the formation and disruption of silver (Ag) conductive filaments (CFs) of devices. The PEA₂SnI₄ specimens from high molar concentration possess long and short crystallite sizes perpendicular and parallel to the substrate, respectively, leading to efficient formation and disruption of Ag CFs and a large resistive switching window of devices, but weakening the interaction between adjacent PEA₂SnI₄ layers, causing decreased operational stability of devices. Cross-sectional scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy was employed to confirm Ag CF formation. As synaptic devices, these PEA₂SnI₄-based memristors can perform spike-time- and spike-number-dependent plasticity, microstructure-dependent synaptic characteristics, and opposite synaptic behaviors at high and low resistance states. By engineering microstructural features, low-dimensional, lead-free perovskite-based memristors with improved electrical performance or a variety of synaptic behaviors can be achieved.