Humidity Stability Of Perovskite Resistive Random Access Memories Modified By Polyvinylamine Copolymers

Resistive random access memory（RRAM）is a kind of non-volatile memory,which bear the advantages of rapid operation speed,low power consumption and high storage density.The RRAM sandwich structure is usually composed of metal/switching layer/metal,among which the material property in the switching layer is the key issue to determine the resistive switching performance and the device stability.Organic-inorganic hybrid perovskite（MAPb I₃）has attracted extensive attention in the fields of solar cells,photodetectors and light-emitting diodes because of high carrier mobility,large light absorption coefficient,simple preparation process and low cost.In recent years,the resistive switching behavior has been found in MAPb I₃.So,the perovskite material is also able to be used as a resistive switching material in RRAM.However,the obstacle in the practical commercial applications is the weak environmental stability of MAPb I₃.In our previous studies,polyvinylammonium（PVAm·HI）was used to control the crystal structure and morphology of MAPb I₃ films.As a result,the stable solar cells and RRAMs were obtained from the PVAm·HI-modified perovskite.In this study,the perfluorinated PVAm·HI copolymer was designed and synthesized to further modify the chemical structure of PVAm·HI,with a main aim to improve the humidity stability of perovskite RRAMs.The specific research contents are as follows:（1）Polyvinylammonium（PVAm·HI）and polyvinylammonium copolymer（P（VAm·HI-co-HDFD））were synthesized.Their chemical composition and structure were determined by FTIR,¹H-NMR,¹⁹F-NMR,EDS and XPS.（2）The perovskite films with different modification contents of P（VAm·HI-co-HDFD）were prepared.The XRD,XPS,SEM and AFM results show that the copolymers can regulate the nucleation and growth of perovskite crystals,resulting in the decrease of crystallinity and grain size.The appropriate amount of copolymer can be well fixed on the grain boundary and surface through the interaction of copolymeric ammoniums with perovskite grains,which will promote the formation of dense,pinhole free and grain-crosslinked perovskite films.The test results of water contact angle（CA）show that the water CA of unmodified perovskite film is 43.7°.With the increase of copolymer content,the water CA gradually increases and finally reaches the highest value of 103.4°,while the highest water CA of PVAm·HI-modified films is only 55.4°.In addition,after the perovskite films were placed into the high humidity environment of 70%RH at 25 ^oC for 2 days,the XRD results confirm that the unmodified perovskite film decomposes seriously.The decomposition degree of PVAm·HI-modified perovskite films is weakened.Especially,the P（VAm·HI-co-HDFD）-modified perovskite films do not decompose.（3）The planar perovskite RRAMs with the structure of Al/MAPb I₃:P（VAm·HI-co-HDFD）/ITO-Glass were prepared.The RRAM devices exhibit the bipolar resistive switching characteristics,low operating voltage（<3 V）,ON/OFF ratio about 10⁴,long data retention time（10⁴ s）,500-cycle SET/RESET endurance,and good long-term stability（without degradation at ambient environment in 60 days）.（4）The fiber-shaped perovskite RRAMs with the structure of Al/MAPb I₃:P（VAm·HI-co-HDFD）/Al were prepared.The RRAM devices also show the bipolar resistive switching characteristics,low operating voltage（<3 V）,high ON/OFF ratio（10⁸）,long data retention time（10⁴ s）and 1000-cycle SET/RESET endurance.In addition,the RRAM devices can also exhibit the stable resistive switching behavior even at 180°bending angle,indicating the good flexibility.By changing the compliance current and the integrated structure,the potential of multilevel and high-density storage is demonstrated in the copolymer-modified RRAM devices.Finally,the copolymer-modified RRAM devices were placed in a high humidity environment of 70%RH at 25 ^oC to verify their humidity stability.The results showed that the resistive switching performance of unmodified device almost disappears in 1day,while the copolymer-modified device still shows the original resistive switching performance in 3 days.