Resistive random-access memory (RRAM) is a kind of highly promising non-volatile memory technology. Recently, halide perovskites have aroused attention worldwide because of their outstanding resistive switching performance and ease of fabrication. The advantages of the halide perovskite devices include high ON/OFF ratio and low operation voltage, enabling excellent device performance with low power consumption. Currently, the most widely studied halide perovskites contain lead, which is a toxic element that may incur serious environmental problems and significant harm to human health. In order to address these issues, there is a pressing need to develop lead-free halide perovskites and their derivatives possessing comparable functional properties to their lead-based counterparts.
Bismuth-based halide perovskites have emerged as a promising lead-free alternative for applications in RRAM. A great advantage of bismuth-based halide perovskites lies in their high solubility for various elements, thus offering the possibility of the formation of modified compositions to tailor the resistive switching behaviours including ON/OFF ratio, endurance and retention. Cs3Bi2I9 and MA3Bi2I9 (MA = methylammonium) are two common lead-free perovskite halides that have been widely studied for RRAM. However, doping in Cs3Bi2I9 and MA3Bi2I9 is normally conducted on a single chemical site (either A-site or X site) and the impact of co-doping on their resistive switching properties remains less explored. In this project, thin films of several co-doped compositions namely MA2CsBi2BrxI9-x (x=2, 3, 4, 5, 6, 7, 8) were prepared to investigate the double doping (Cs on A-site, Br on X-site) effects on their structural, morphological and electrical properties. In addition, the effect of different top electrodes (Ag and Au) on the electrical performance of the MA2CsBi2BrxI9-x thin films was also studied. It was found that more uniform and denser thin films could be obtained with an increase in Br content. Among the several compositions under investigation, MA2CsBi2Br8I-based thin film with Au top electrodes exhibited typical resistive switching behaviour and an interface-type conduction mechanism. When the perovskites layer was covered by Ag top electrodes, the distinct resistive switching behaviour could be observed with the increase of I content, which could be attributed to the redox reaction of Ag electrodes and iodide ions at the interface between electrodes and the active layer. Compared to other compositions, MA2CsBi2Br2I7-based thin film with Ag electrodes exhibited an outstanding ON/OFF ratio of around 105.
Since the MA2CsBi2Br8I perovskite had good endurance and full-coverage surface, the MA2CsBi2Br8I perovskite was employed for further study. Au/MA2CsBi2Br8I/ITO devices with different thicknesses (290 nm, 307 nm, 341 nm and 435 nm) showed stable bipolar resistive switching behaviours. With the increasing thickness, the SET electric field remains around 6.5 V/μm, which is nearly independent of film thickness. When the thickness of the MA2CsBi2Br8I perovskite layer increased from 136 nm to 307 nm, the device demonstrated better stability over 100 cycles and a higher ON/OFF ratio (~10) at a low reading voltage of 0.27 V.