| Benefiting from the advantages of organic field-effect transistors（OFETs）,such as simple preparation process,flexibility and light-weight,OFETs have shown promising applications in smart display,logic circuits and wearable sensors.With great process in materials synthesis technology recently,the performances of OFETs have been significantly improved and especially,the field-effect mobility of OTEFs is comparable to or even better than that of traditional silicon-based transistors.However,from the perspective of device engineering,the issues in carrier injection and transport restrict the further development of OFETs.In OFETs,the electrode/organic semiconductor interface determines the carrier injection,and the organic semiconductor/dielectric layer interface affects the carrier transport.Therefore,it is pivotal for improving the performance of OFETs by ameliorating the interfacial contact quality of electrode/organic semiconductors and organic semiconductors/dielectrics through interface modification technology.In semiconductor industry,the interface modification materials,such as self-assembled monolayer molecules,organic contact dopants and transition metal oxides,are commoly used for improving the quality of interfaces and thereby advancing the device performances.However,self-assembled monolayers,contact dopants and transition metal oxides might induce the interfacial dipole and impurity doping effects,which leads to the device performance degradation.In addition,current interface modification methods focus on how to improve device performances by increasing the injection of majority carriers.As for the device performance degradation caused by minority carrier injection and the associated carrier dynamics is lacked.Therefore,it is important to investigate the interfacial modification methods to improve the interfacial contact quality of OFETs and meanwhile thoroughly study the mechanisms of the influence of minority carrier injection on the operational performances of OFETs.In view of the above problems,this thesis chose copper phthalocyanine as the model material for fabricating the OFET devices because of its common use in industry and better physiochemical stability.Based on device engineering,the influence of minority carrier injection on the device performances of OFETs was comprehensively analyzed.Furthermore,the functions and mechanisms of interface modification methods on improving the device performances were discussed.The research contents of this thesis mainly include the following aspects:（1）The mechanisms on device operational bias-stress instability of OFETs were studied.It was revealed that the minority carriers injected from the source-drain electrodes into the OFET channel results in carrier trapping,recombination,and neutralization both at organic semiconductor/dielectric interface and in organic semiconductor active layer,which persistently reduces the number of majority carriers in the channel and also creates the trap states for carriers,and finally,it leads to the bias-stress instability of OFETs such as the threshold voltage shift and persistent decrease in source-drain current.Based on the researches on the mechanisms of bias-stress instability of OFETs,this thesis proposed a strategy to improve p-type bias-stress stability of OFETs by constructing double interface contact source-drain electrodes based on a metal electrode energy level modification layer（ELML）.Several organic small molecules with high lowest unoccupied molecular orbital（LUMO）energy levels were selected as ELMLs.The energy-level offset between the Fermi level of the drain electrode and the LUMOs of the ELMLs was shown to construct an electron injection barrier for suppressing electron injection from the drain electrode into the channel.As the carrier trapping,neutralization and recombination processes related to electron injection were refrained.After adopted the small molecule ELML,the bias-stress stability of OFETs was improved 6 times,and the drift of threshold voltage was reduced by a factor of 18,and the turn-on voltage was as low as 0.82 V.Besides,a simulation was performed for analyzing the carrier dynamics in OFETs.It is found that the electron injection obeys the Fowler-Nordheim tunneling theory,e.g.,the higher LUMO level of organic ELML materials is,the higher electron tunneling injection barrier and the better bias-stress stability of p-type OFETs could be obtained.（2）It is usually difficult to form the large interfacial barrier based on organic small molecule materials because there are few differences on LUMO levels of different organic small molecules.Therefore,molybdenum trioxide（Mo O3）with high work function was adopted to modify the metal source-drain electrodes for forming a larger interface energy barrier by cooperating with organic small molecules.By constructing the metal/Mo O3/organic multilayered interface contact（MIC）electrode,the electron injection barrier increased and then the bias-stress stability of p-type OFETs was also improved.By using the MIC for fabricating OFETs,the current decay ratio of the device was decreased by 40%compared with only small molecule ELML,and the contact resistance was reduced 3 times,and at the same time,the better operational properties of OFETs could be obtained synchronously.Besides,the dynamical processes of Mo O3diffusion in organic materials were studied,and the parameters related to physics of diffusion and device bias-stress stability were obtained quantitatively.Based on materials physical parameters such as diffusion coefficient,molecular weight and energy level structure,a comprehensive selection rule for constructing MIC electrode structure was proposed.The results provide a new way for solving the contact stability problem in organic electronic devices.（3）In semiconductor industry,the pretreatment of silica substrate is an extremely important process during the electronic device fabrication.The pretreatment can significantly affect the carrier process in OFETs,which determines the device performances.Currently,there are many debates in industry and in academia regarding to the pretreatment of silica substrate including methods and procedures.This thesis systematically investigated the effects of different substrate pretreatment methods on the performances of OFETs.It was found that the difference in device mobility was as high as 400%,and the difference in threshold voltage was as high as 300%,and the difference in current decay ratio was as high as 80%for different cleaning methods.Theoretically,the effects of different pretreatment processes and procedures on the states of surfaces of Si/Si O2 were studied,and the physical mechanisms related to the Si/Si O2 pretreatment and the device operational properties were revealed as well.It is found that the deionized water clean and UV-Ozone treatment can passivate the defect state of Si O2 surface during the process of substrate pretreatment,which are essential steps in the cleaning process.The obtained results presented some referential data for the fabrication of semiconductor devices in industry and academia. |
