New random copolymers with pendant carbazole donor and 1,3,4-oxadiazole acceptor for high performance memory device applications

Abstract

New non-conjugated random copolymers containing pendent electron-donating 9-(4-vinylphenyl)carbazole (VPK) and electron-accepting 2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (OXD) or 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD) were successfully synthesized by nitroxide-mediated free radical polymerization (NMRP) method. The prepared random copolymers are denoted as P(VPK_xOXD_y) or P(VPK_xBOXD_y) with three different electron donor/acceptor (x/y) ratios of 8/2, 5/5, and 2/8. The electrical switching behavior based on the ITO/polymer/Al device configuration could be tuned through the donor/acceptor ratio or acceptor trapping ability. Both experimental and theoretical results indicated that the charge transfer between the pendant donor and acceptor was relatively weak without significant orbital hybridization. In addition, the low-lying HOMO energy level of OXD or BOXD units as compared to VPK units created the trapping environment. Therefore, distinct electrical current–voltage (I–V) characteristics changed between the diode, the volatile memory, and the insulator depending on the relative donor/acceptor ratios of 10/0, 8/2, and (5/5, 2/8 and 0/10), respectively. The memory device based on P(VPK₈OXD₂) or P(VPK₈BOXD₂) copolymers exhibited volatile static random access memory (SRAM) behavior with an ON/OFF current ratio of approximately 10⁴–10⁵, up to 10⁷ read pulses, and retention time of more than 1 h. The unstable ON state in the device was due to the shallow trapped holes with spontaneously back transferring of charge carriers when the electric field was removed and thus exhibited a volatile nature. The slightly lower HOMO level of OXD moieties than that of BOXD led to the P(VPK₈OXD₂) device storing the charge for a longer period of time. The present study suggested the high performance polymer memory devices could be achieved by changing the donor/acceptor ratio or chemical structure.