Effects Of Polymer/Nanoparticle Interface On The Resistive Switching Of Its Composite Thin Films

Generally, introducing nanoparticles (NPs) to organic layer can improve the electrical switching of the organic devices. However, lots of problems still need to be resolved. Firstly, the reliability and reproducibility in multiple writing-erasing processes are poor. Secondly, there are still no effective observation and analysis means to study the film microstructure, especially the polymer/NPs interfaces. Currently, though some theories such as capture and release of charge carriers, charge transfer, molecular conformation change have been proposed to explain the switching, the switching mechanisms are still under discussion.To understand the effects of film microstructure on resistive switching, this paper mainly studied the influences of polymer properties, NPs properties, and embedding polymers/NPs thin films with small donor or acceptor molecules on the electrical properties of composite films. The test and analyses for the samples were as follows. Firstly, measurement of surface topography and current-voltage curves were carried out, and the corresponding ON/OFF ratio and threshold voltage were summarized. Then, based on the transport theory model, namely, the Ohmic conduction, Space Charge Limited Current, and Schottky Emission, the current-voltage curves were analyzed. Finally, theoretical calculations based on first principles and energy level diagram were introduced to study the interfacial effects of polymer/NPs on the resistive switching. The contents and conclusions are as the following:Firstly, different polymer/NPs interfaces were built by embedding graphene cracks in Polymethylmethacrylate (PMMA), Polyvinylcarbazole (PVK), Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadia-zol-4,8-diyl)] (PFBT) films to explore the effects of polymer properties on the resistive switching. The results show that the electrical switching depends largely on polymer resistance. After introducing graphene cracks to insulative PMMA films, the effects of charge carriers inspired by graphene is greater than that of the energy traps introduced by graphene, which increases the OFF state current and further decreases the ON/OFF ratio. For conductive PVK and PFBT, the effects of energy traps are larger, which results in improved switching performance. Additionally, owing to the asymmetric electrical surface potential of the alkane chains and the conjugated backbone of PFBT, more charge carriers can be trapped at the conjugate backbone, which limits the OFF state current and improves the ON/OFF ratio.Secondly, P-and N-type metal oxide NPs (NiO and ZrO2 and ZrO2 NPs doped with La were embedded in PVK and PFBT, respectively, to change polymer/NPs interface and further study their effects on the switching. The results show that the influences of different NPs on the polymer film depend largely on polymer properties. For PVK, when the concentration of embedding ZrO2 or NiO NPs is small, they can both increase the ON/OFF ratio, and the effects of ZrO2 NPs are more significant. In case of PFBT, due to the large gap caused by its side chains, the small-size NPs have little effects on film properties. The switching mechanism of the composite devices is capture and release of charge carriers. The trap effect is proportional to the the ON/OFF ratio, threshold voltage, slope and transformation voltage of log-log curves of current-voltage. In addition, the doped La can decrease the energy difference between ZrO2 NPs and PVK, which weakens the trap effects, and the La-inspired carrier increases the OFF state current, resulting in decreased ON/OFF ratio.Finally, small donor and acceptor organic molecules, namely, carbazole and phenazine were introduced to adjust the PVK/Au NPs interfaces and their electrical switching, respectively. In the first scanning, the ON/OFF ratio was large, while in subsequent voltage sweeping, the OFF state current increased and the corresponding ratio was small. The improved ON/OFF ratio may result from the enhanced trap effects caused by carbazole and phenazine. However, the charge carriers trapped in the first scanning can only be partly released, which results in weaker electrical switching in the following sweeping. In addition, the effects of carbazole and phenazine on the switching of the PVK:Au NPs thin films are the same, and the switching behaviors show no dependence on molecular concentration. This may be due to the charge carriers inspired by carbazole and phenazine, which also explains the transfer from Ohmic charge transport to Space Charge Limited Current mechanism.