Study On Resistive Switching Of Flexible Polymer Films Embedded With Nanoparticles Fabricated By Vacuum Spray Method

With the rapid development of flexible memory devices,polymer thin films embedded with nanoparticles(NPs)have attracted lots of eyes due to its low cost,simple fabrication process,large storage window and high flexibility.In order to improve the switching performance of devices,memory device materials are optimized and modified by great quantities of research groups.It is well known that the interfaces in the memory devices,including polymer/NPs interfaces and the electrode/active layer interfaces play really important roles in the resistive switching performance.However,the interfacial effects of polymer/NPs layer on switching are still under debate,especially under bending conditions.Moreover,the composite films are mostly fabricated by spin-coating,which is difficult to avoid the influences of solvent,oxygen and dust.In this paper,NP concentration-and bending-degree-dependent switching properties measurements are performed and the electrode/active layer interface is enginered by altering the electrode materials to gain greater insight into the interfacial effects on resistive switching of flexible polymer films embedded with TiO2 NPs.Firstly,we fabricate memory layers using TiO2 NPs embedded in polymer materials with different electrical properties,including electron-transporting PFBT(poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadia-zol-4,8-diyl)]),hole-transporting P3HT(Poly(3-hexylthiophene-2,5-diyl)by vacuum spray method to investigate how the polymer/NPs interfaces affect the switching performance.The results show that,with the NPs concentration increased from 0.5%to 1.5%,the charge transport mechanism transformed from SLCL to TCLC and finally Ohmic conduction.The NP dispersion is a critical factor affecting the electrical andb mechanical properties of organic-inorganic composites.A homogeneous distribution of NPs benefits the interfacial contact and result in improved flexural strength and carrier transport.When the NPs concentration is low,the polymer/NPs the portion of interfaces is not enough to affect the charge transport.The ON/OFF ratio shows enhancement with increasing NPs concentration.The optimized NPs concentration is 0.1%with ON/OFF ratio of 4×103 for PFBT sample,and 0.75%with ON/OFF ratio of 1.2×103 for P3HT sample.However,the switching performanc ebecomes saturated and even poor when the NPs concentration is too high,which may be due to the easier carrier transport across the densely neighboring NPs.The two polymer materials shows similar tends with NPs concentation increases.The First Principles results show that PFBT molecule may donate electrons to TiO2 NPs,thus the charge carriers are easier to be captured,which may explain the better switching performance of PFBT samples.Secondly,to investigate the bending effect on the interfaces and switching performance,samples embedded with optimized NPs concentration are bent with bending distance from 5mm to 20mm.The results show that,with the bending distance increased,the number of deep traps in the active layer become decreased,and that of shallow traps increased,resulting in the ON/OFF ratio decreased by three orders of magnitude.At low bias,the shallow traps are almost unoccupied,which can capture nearly all of the injected electrons above the threshold voltage,which explains the largely decreased ON/OFF ratio.For unbent samples,the polymer/NPs interfacial structures may facilitate the charge trapping and detrapping processes.However,upon bending,the interfacial micro cracks may act as blocking layers and thus deteriorate the switching performance.The FEA results are qualitatively consistent with our experimental observations.Finally,the effect of electrode/active layer interface on resistive switching and bending endurance of PFBT:TiO2 NPs thin films is discussed.The Cu electrode was deposited by vacuum evaporation system,and the PFBT:TiO2 NPs thin films were fabricated by vacuum spray method.Compared with the ITO devices,the switching performance was largely changed.The thin film with Cu electrode exhibited unipolar switching behavior,while the ITO samples show bipolar switching.The bending endurance was remarkably enhanced.The switching performance of ITO device showed serious degradation with bending times of 1000,while the switching behavior remains with 8000 times of bending in the Cu samples.The Cu samples with 2000,4000 and 8000 bending times show SCLC,TE and PF conduction,respectively.Due to the chemical characteristics of Cu,the switching mechanism was also changed from charge trapping/detrapping toTE and PF emission.Our study may provide useful information for understanding the underlying physics of polymer nanocomposite memory devices for the application to future flexible electronics.