New Dibenzothiophene-Containing Donor−Acceptor Polyimides for High-Performance Memory Device Applications

We report the synthesis and characterization of two novel polyimides (PIs), PI(2,8-APDBT-6FDA) and PI(3,7-APDBT-6FDA), consisting of alternating electron-donating 2,8- or 3,7-phenylenesulfanyl-substituted dibenzothiophene and electron-accepting phthalimide moieties for high-performance memory device applications. The optical band gaps of PI(2,8-APDBT-6FDA) and PI(3,7-APDBT-6FDA) films were 3.40 and 3.31 eV, respectively, indicating the significance of the linkage position. The device with the ITO/PIs/Al configuration showed the multimemory characteristics changing from high-conductance ohmic current flow to nonvolatile negative differential resistance (NDR), dynamic random access memory (DRAM), and insulator, with the corresponding film thickness of 12, 20, 25, and 45 nm, respectively. Both PIs exhibited similar memory characteristics, but PI(3,7-APDBT-6FDA) with a lower threshold voltage due to its high-lying HOMO energy level. The 20 nm PIs device exhibited the nonvolatile behavior with the NDR region and trilevel programming for the electrical stability of at least 104 s, resulting from the diffusion of Al atoms into the PIs layer. The 25 nm PIs device showed the reproducible DRAM characteristic with the high ON/OFF current ratio more than 108. The weak instantaneous dipole moment led to the unstable induced charge-transfer complex, which was confirmed by density functional theory. The experimental results suggested that the tunable switching behavior could be achieved through the appropriate design of the donor−acceptor PIs structure and controllable thickness of the active memory layer.