Tunable Switching Characteristics of Low Operating Voltage Organic Bistable Memory Devices Based on Gold Nanoparticles and Copper Phthalocyanine Thin Films

Abstract

The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, using copper phthalocyanine and gold nanoparticle thin films, was investigated using Au, Al, and Hg electrodes. While the ON/OFF ratio higher than 105 was achieved for all the devices, the nature of the memory behavior was found to be dependent on the top electrodes. Thermally evaporated Au and Al electrodes resulted in write-once–read-many times (WORM) behavior, whereas Hg drop soft contact led to write–read–erase–read (rewritable) characteristics, with the device retaining the ON state in the former and returning to the OFF state in the latter. The switching voltage was found to be influenced by the top electrode with the devices switching to the ON state at around 2 V for Hg and close to 1 V for Au and Al electrodes. Additionally, though the ON state conduction mechanism was dominated by Fowler–Nordheim (FN) tunneling through AuNP trap states in all the devices, the dynamics of switching was found to be dependent on the top electrode, showing abrupt switching to the ON state for Au and Al electrodes. In contrast, a gradual increase in current at the onset of FN tunneling before switching was observed for devices with Hg electrodes. Such a significant influence of the top electrodes was mainly attributed to the difference in injection barriers between the top electrode/active layer and gold nanoparticle/active layer junctions. Devices exhibit rewritable behavior when the former is higher than the latter, while they change to WORM behavior when the two are equal. The study shows that the same device structure can be tuned to exhibit WORM or rewritable memory behavior by employing the top electrode with the work function in suitable combination with that of the nanoparticles forming trap states in the bulk of the film.