Substituent and Charge Transfer Effects on Memory Behavior of the Ambipolar Poly(triphenylamine)s

A series of poly(triphneylamine)s (CN-PTPA, 2CN-PTPA, 3CN-PTPA, and NO2-PTPA) with pendent acceptors (cyano, dicyanovinyl, tricyanovinyl, and nitro) have been readily synthesized by oxidative coupling polymerization using FeCl3 as oxidant. The tunable memory properties of the ITO/polymer/Al sandwiched memory devices including DRAM, SRAM, and WORM could be achieved by introducing substituent acceptors with different extent of electronic delocalization and electron-withdrawing intensity into the poly(triphenylamine)s. The highly fluorescent CN-PTPA exhibited volatile DRAM memory characteristic due to the large band gap and weak intramolecular charge transfer capability. 2CN-PTPA and 3CN-PTPA showed volatile SRAM memory property with retention time of 5 and 14 min, respectively, depending on electron-withdrawing capability of the acceptors. Furthermore, NO2-PTPA afforded nonvolatile WORM memory behavior attributed to the charge could be trapped into the nonconjugated nitro group even though the dipole moment and electron-withdrawing capability of nitro group were weaker than cyanovinyl groups. Moreover, except NO2-PTPA, all the devices derived from cyano-containing ambipolar polymers including CN-PTPA, 2CN-PTPA, and 3CN-PTPA could be switched to the ON state and exhibited WORM memory behavior in positive unipolar I-V switching. This phenomenon indicated that the Al atoms preferentially interact with poly(triphneylamine)s containing cyano than nitro substituents.