Electrically bistable digital memory behaviors of thin films of polyimides based on conjugated bis(triphenylamine) derivatives

Three functional polyimides (PIs) bearing conjugated bis(triphenylamine) (2TPA) derivatives with electron-donating and accepting groups were synthesized with reasonably high molecular weights. The PIs exhibited high thermal and dimensional stabilities and furthermore produced high-quality nanoscale thin films via conventional solution coating process. All of the PIs in the films were found to be amorphous, but they were oriented somewhat preferentially in the film plane, rather than randomly. Their film densities and interchain distances were measured, and the optical and electrochemical properties were determined. All of the PIs in the devices with aluminum top and bottom electrodes initially revealed a high resistance (OFF-state). However, under positive and negative voltage sweeps, the PIs demonstrated volatile or nonvolatile digital memory behavior, depending on the substituents of the 2TPA unit. The 2TPA-based PI, as well as the PI bearing 2TPA with electron-donating methoxy substituents showed unipolar write-once-read-many-times (WORM) memory behavior, whereas the 2TPA-based PI containing electron-accepting cyano groups exhibited unipolar dynamic random access memory (DRAM) behavior. All of the PI films revealed excellent retention abilities in both the OFF- and ON-state, even under ambient air conditions. Moreover, they all revealed high ON/OFF current ratios (10⁶-10¹⁰). All of the memory behaviors were found to be governed by a mechanism involving trap-limited space-charge limited conduction and local filament formation. Such memory behaviors were further investigated in detail with taking into consideration the PI components' chemical nature and molecular orbital levels, possible trapping sites, substituents' effect, and the metal electrodes' work function. Overall, this study demonstrated that the thermally, dimensionally stable PIs are highly suitable for the low-cost mass production of high performance, polarity-free digital memory devices that can be operated with very low power consumption. Moreover, the memory mode can be tuned by changing the substituent in the 2TPA unit.