Nonvolatile memory devices based on polyimides bearing noncoplanar twisted biphenyl units containing carbazole and triphenylamine side-chain groups

Two novel polyimides, PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA), consisting of alternating electron-donating 2,2′-bis[4-(9H-carbazol-9-yl)phenyl]- or 2,2′-bis[4-(diphenylamino)phenyl]-substituted biphenyl moieties and electron-accepting phthalimide moieties were synthesized and characterized. These polyimides are thermally stable with 5% weight loss over 500 °C and the glass transition temperatures of the polyimides were found to be 293 °C. The optical band gaps of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) were 3.42 and 3.30 eV, respectively, indicating the significance of the linkage groups. The estimated energy levels (HOMO, LUMO) of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) were (−5.51, −2.10) and (−5.22, −2.02) eV, respectively. Resistive switching devices with the configuration of Al/polymer/ITO were constructed from these polyimides by using the conventional solution coating process. The as-fabricated PI(CzBD-BTFBPDA) film exhibited a nonvolatile bipolar write-once–read-many times (WORM) memory character, whereas devices with the PI(TPABD-BTFBPDA) film showed “write–read–erase” flash type memory capability. The ON/OFF current ratios of the devices were both around 106 in the ambient atmosphere. The mechanisms associated with the memory effect were further elucidated from the density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basis set. The present study suggested that the tunable switching behavior could be achieved through the appropriate design of the donor–acceptor PIs structure to have potential applications for memory devices.