Research On The Interface And Stability Of Organic Field-Effect Transistor

As one of the most important elements of organic electronics,organic field-effect transistors(OFETs)have great advantages including lightweight,intrinsic flexibility,extensive materials source,solution process,and low cost.OFETs attract worldwide attention due to their high potential in radio frequency identification(RFID),driven circuits,and sensors,etc.However,there remain no commercial OFETs-based products.One reason is assigned to their low stability.Although many organic semiconductors are chemically stable,the electrical performance of their OFETs is susceptible to environmental factors(such as water,oxygen,and light)and their aggregation state change with performance degradation over time.The charge accumulation and transport process both occur at the interfaces in OFETs.In order to solve the stability problems,the chemical/physical effects at interfaces must be clarified.Therefore,this dissertation aims to the interface effect of OFETs,then discloses the instability mechanism,and proposes the strategies to improve the device stability.The main contents of this dissertation are as below:(1)Device stability depends on the interface stress of organic semiconductor/dielectric interface.To improve the compatibility between organic semiconductor and dielectric layer,the self-assembly monolayer(SAM)of small molecules is usually grafted on the surface of the dielectric layer.During the long-term storage,the molecular ordering degree of organic semiconductors usually undergoes a dramatic attenuation.Hence,the electrical performance of OFETs drops significantly.In chapter 3,we use the sumfrequency generation spectroscopy technique to monitor the molecular conformation change at the organic semiconductor/dielectric interface and find that the formation and release of interface stress derived from the molecular conformation evolution of SAM will lead to the disordering of organic semiconductor molecules.This work discloses a novel physical instability mechanism of organic semiconductor and breakthroughs the traditional understanding about the organic semiconductor instability derived from the chemical composition degradation,which paves a new avenue to improve the stability of OFETs.(2)Device stability depends on the aggregation state of organic semiconductors.The aggregation state stability of the organic semiconductor layer severely influences the device stability.In chapter 4,we find that the thickness of the organic semiconductor layer has a significant influence on the aggregation state stability.When the thickness of organic semiconductor layer reaches one or two hundred nanometers,its aggregation state and electrical performance can remain ultra-long lifetime up to five years,which is far longer than the previously reported lifetimes in literature.After exploring the stability mechanism of thick film,we find that the thick film can effectively decrease the free energy of organic semiconductor and the interface stress at organic semiconductor/dielectric interface,and thus reduce the dewetting possibility.Although the thick film has high aggregation state stability,it introduces a large contact resistance in the top-contact devices.Intriguingly,we find that the gold nanoclusters formed during the thermal deposition process gradually penetrate the organic semiconductor layer,which effectively improves the contact quality and reduces the energy barrier at the organic semiconductor/dielectric interface.(3)Photo stability of organic field-effect transistors is focused.Some important application scenarios of OFETs include the driven circuit of OLEDs and the sensor units in the wearable devices require to work under light irradiation.However,most organic semiconductors have a relatively strong photoelectric response.In chapter 5,we propose a kind of new strategy to increase the recombination possibility of photo-generated excitons by adopting polymer poly(acrylic acid)as the dielectric layer.Under the irradiation,the photogenerated excitons(namely electron-hole couple)in organic semiconductor will be quenched by the exciton recombination sites(the-COOH groups on the molecular chains of poly(acrylic acid)).The photo-generated holes can not be separated and involved in the carrier transport in the conductive channel.