Surface Engineering For Organic Multilevel Memory

With the rapid development of modern information technology,traditional binary memory techniques are unable to meet the requirements of information storage in modern society,urging the exploration of new information storage techniques.In 2010,our group first reported the ternary memory behavior based on small organic molecules,creating the precedent of multilevel storage.Since then,a series of organic molecules performing memory behavior have been synthesized through adjusting the conjugated structure,planarity and electron-donor/acceptor groups of molecules,which greatly promoted the development of organic memory technology.More and more materials are considered as active materials for the multilevel resistive random access memory(RRAM)and perform good multilevel memory behaviors.In order to approach industrial applications,RRAM devices must have good reproducibility,including a high multilevel device yield and a relatively narrow range of threshold voltage distribution.However,most of the research work has not reported the statistical results based on a large number of devices testing.The recent statistical results of our research group show that although the molecular structure regulation can achieve the enhancement of the multilevel device yield and the optimization of the threshold voltage distribution,the multilevel device yield of organic memory devices is still generally low(usually no more than 50%).Based on this situation,we need to look for a new strategy to improve the reproducibility of the devices.In the field of organic light-emitting diodes(OLED)and organic photovoltaic(OPVs),researchers generally believe that the matching of surface energy and work function between electrode and organic layer is the key factor affecting device performance.Therefore,the surface modification of the substrate materials,especially the ITO glass,is widely used.This paper will introduce surface modification technology into the organic memory field.Using an organic molecule,which performed multilevel memory behavior already,modify the surface of the ITO substrate to affect the accumulation of the molecules and further improve the multilevel device yield of the memory devices.The main research work developed from the following aspects:(1)A square acid molecule was synthesized and fabricated into memory devices.The first time,we used phosphonic acid to modify the surface of ITO substrates and investigated the influence on the memory performance after modification.The results show that the ternary device yield of the modified devices was increased to 82%,and the stability was greatly improved.(2)The ITO substrates were modified by ethyltrichlorosilane,butyltrichlorosilane,hexyltrichlorosilane and octyltrichlorosilane,respectively.Then the organic molecules and top electrodes were evaporated on the modified substrates,respectively.Investigate the effect of aging on the surface morphology and crystallinity of active layer.How the alkyl chain length of alkyltrichlorosilane effect the performance of the devices were also studied.The results show that aging can affect the surface morphology and crystal shape of the active layer,but the change of the device performance is independent of the length of the alkyl chain.(3)PEDOT:PSS layers with different thickness were prepared through different spincoating speed and fabricated into memory devices.The testing results show that the modified devices with thickness of 100 nm(prepared at 1000 rpm)achieve quaternary memory switching with the highest yield(～41%).This is the first time to realize the quaternary though surface engineering.