Study Of Interfacial Effects On The Resistive Switching Of Flexible Polymer Films Embedded With Nanoparticles

With the rapid development of molecular materials and nano technology,molecular films incorporated with nanoparticles(NPs)have become a hot topic in the information storage industry.Flexible organic memory devices have attracted lots of efforts due to their low-cost,simple fabrication,large storage window and high flexibility.In order to improve the switching performance of devices,memory thin films are optimized and modified by great quantities of research groups.However,few literatures about interfacial engineering and switching mechanism in micro-scale were studied.In this work,series of organic/NPs devices with different interfaces were built by engineering the concentration of NPs and changing electrodes,and their bending endurance was tested.The contents and conclusions are as followings:Firstly,hole-transport material PVK and electrical insulating material PMMA were used as polymer matrix.The selected semiconductors were TiO2 NPs and ZnO NPs with the same band gap.Composited the high molecules and NPs then spin coated onto the ITO substrates.The concentrations of NPs were engineered as varing from 0.5%to 1.5%.The results show that the resistive performance largely depends on the concentration.If the concentration is relatively low,the NPs will be very dispersed in insulating layer,which can result in a lower ON/OFF ratio.On the contrary,the NPs clusters will bridge the top and bottom electrodes due to the high concentration.The First Principles was used to calculate the electronic state of PVK and PMMA composite thin films.It is shown that both of the composite units have a deep HOMO and shallow LUMO,which means their band gaps become narrow.So the middle energy band is formed to trap charges in the film.After comparing their electrical switching characteristics,the PVK-1%TiO2 NPs film was chosen for later experiments.Secondly,the bending effects on the resistive performance of pure PVK film and PVK-1%TiO2 NPs film are studied.Bending times changed from 0 to 2000 and the distance after bending varied from 40 mm to 5 mm.The results show that polymers can pile up in the substrate after bending.Cracks appear on ITO electrode due to the tensile stress,which hinders the carrier transportation in the film.Thus,the ON/OFF ratio drops.When the bending times range from 0 to 500,the ON/OFF ratios of the two films falls rapidly.After 200 times bending,crack density becomes saturated and the ON/OFF ratio keeps at a lower level.However,the ON/OFF ratios show a linear decline with the bending distance changes from 40 mm to 5 mm.Finally,the switching performance and bending endurance of devices with different electrodes were discussed.Metal electrodes such as Ag,Al,Cu,ITO and Au were deposited on PET substrates through vacuum evaporation.Then the PVK-1%TiO2 NPs solution was spin coated on the substrate.Their switching performance and bending endurance were compared with as-fabricated ITO devices.Results show that the device with Au electrode has the highest ON/OFF ratio,reaching to 105.But it declines to 102 after 2000 times bending.Cu electrode device has excellent bending performance,which shows in its ON/OFF ratio increases 10 times after bending 100 times.The bending endurance has relationship with the ductility,adhesive and work function of electrodes.The better electrodes' ductility and adhesive ability has,the harder cracks and wrinkles form.A larger band gap between electrode and functional layer makes for the easier charge transfer.Both of the features keep devices an outstanding resistive characteristic after bending.In summary,resistive switching and bending endurance are observed in the devices by engineering their polymer/NPs and electrode/functional layer interfaces.Certain concentration of NPs can increase the ON/OFF ratio and reduce the threshold voltage.Some differences in electrical performance are existed in pure organic film and embedded film due to their various energy band structures.Electrodes with kinds of work functions have influence on their charge transfer between insulating layer.Significant information is provided in this work for developing novel organic flexible devices.