New Conjugated Polymer Thin-film Transistors And Their Functional Applications

The exploration and research of new materials and corrosponding electronic devices is an important foundation for improving our country's independent innovation,scientific research,and promoting the development of our country's science and technology.Organic electronics benefited from its unique advantages in large area,flexibility,high transmittance,low pollution,low temperature,and low-cost manufacturing.It has recently developed rapidly.New conjugated polymer materials,especially a series of new conjugated polymer semiconductor materials represented by donor-acceptor(D-A)polymers,has attracted widespread attention from researchers.The reported mobility of transistors based on the new D-A polymer has generally exceeded 1 cm²V^-1s^-1,thus exceeding the mobility of amorphous silicon transistors(the highest value is even close to 100 cm²V^-1s^-1);The conjugated polymer transistor also exhibits ultra-high stability,and therefore has practical value.More importantly,these D-A polymer materials have physical properties that are close to amorphous.They can be manufactured by simple solution spin coating,roll-to-roll printing and other low-cost processes to produce high-performance devices.These devices have scalability and self-healing ability.These characteristics make conjugated polymer organic devices expected to replace the current amorphous silicon,polysilicon and even metal oxide devices in many fields.New conjugated polymer thin film transistors(PTFTs)still have the following problems to be solved.For their electrical properties,PTFTs are inferior to the most monocrystalline silicon devices in mobility,on/off ratio,subthreshold swing and gate-drain current stability.For their fabrication technology,PTFTs have no mature and unified fabrication technology to realize their uniformity,high performance and low cost.For their device physics research,the mechanism of most PTFTs is explained simply by the traditional silicon-based device physical model,and the device model is not modified according to the characteristics of PTFTs.For the device application,the integrated degree,power consumption and operating speed of PTFTs are far from reaching the performance factors that can directly replace the traditional silicon-based integrated circuits,it is necessary to develop functional applications which are more suitable for the characteristics of conjugated polymer devices.Therefore,in order to promote the utility of the novel conjugated polymer devices,this paper improves the electrical properties of the devices by improving the fabrication processes of PTFTs.The device physics of PTFTs is deeply studied,and their voltage-current model which is more suitable for PTFTs is proposed,so that the electrical performance parameters of PTFTs can be evaluated more accurately.Finally,based on the unique radiation resistance of conjugated polymer materials and the novel structure of PTFTs,the functional applications of the radiation-resistant PTFTs and conjugated polymer memristors are realized.The main contents and innovations of this paper are as follows:(1)Understanding thickness-dependent electrical characteristics in IDT-BT and DPPT-TT conjugated polymer transistorsketopyrrolopyrrole-thieno[3,2-b]thiophene(DPPT-TT)and indacenodithiophene-co-enzothiadiazole(IDT-BT),two representatives of the new conjugated polymers,were employed for PTFTs to understand the electrical properties of semiconductor films with various thicknesses.The experimental results show that the threshold voltage,the sub-threshold swing and the contact resistance of the conjugated polymer thin-film transistors all show a“V”-like trend with the increase of the thickness of the semiconductor,these parameters reach the optimal value when the thickness of semiconductor layer(T_SC)is close to the height of source and drain electrode(H_C).When T_SC is less than H_C,these staggered PTFTs perform like coplanar PTFTs.Their charge injection is severely limited and the device performance is substantially degraded.When T_SC is higher than the height of H_C,the devices return to normal staggered transistors,where the charge injection is instead limited by access transport with thickening the polymer film.Our results suggest that adjusting Tsc at the critical value of H_C can deliver the best device performance,which is helpful to reach optimum and uniform performance with the least semiconductor utilization.(2)Reliable mobility evaluation of conjugated polymer transistorsAs one of the most critical performance parameters of organic field effect transistors,mobility is usually extracted by traditional methods based on the saturation transfer characteristic curve.However,this model was originally developed for silicon-based field-effect transistors.The model is based on the gradual channel approximation theory of the ohmic contact and the energy band transfer model.In contrast,PTFTs have variable-range charge hopping and obvious non-ohmic contacts.Therefore,the mobility of PTFTs extracted by traditional methods will cause different degrees of deviation,and we need to establish a more reliable method to evaluate the intrinsic carrier mobility of PTFTs.In this paper,a method of Y function mobility extraction(y function method)is proposed to extract the mobility of PTFTs accurately.The Y function method is used to extract and compare the mobility of PTFTs with different channel lengths and contact materials,it is proved that the extraction of mobility by Y function method has a higher linear fitting degree,and the result of the extraction of mobility is closer to the intrinsic field effect mobility of the transistors.The Y function method has the function of contact resistance correction and does not depend on the channel length of the devices.Our results prove that the Y function method can be widely used to extract the intrinsic mobility of PTFTs.(3)Direct patterning on top-gate polymer thin-film transistorsIDT-BT,DPPT-TT,and poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}(N2200)PTFTs with a top-gate structure were patterned by simple oxygen plasma treatment instead of conventional processes such as photolithography and inkjet printing.The ungated layer,which is known to cause leakage current paths,was efficiently removed by oxygen plasma treatment.After patterning,the gate leakage current in PTFTs was suppressed below the order of 10^-9 A,the on/off current ratio of DPPT-TT PTFTs increased from5×10⁴ to 2×10⁷,and the subthreshold swing extracted from N2200 PTFTs decreased from 1.6 to 0.3 V dec^-1.All the measured electrical parameters of the patterned PTFTs were distributed in narrower ranges than those of the corresponding non-patterned devices,indicating the high uniformity and reproducibility of the patterned PTFTs.Our low-cost and simple patterning method can be widely used to improve the electrical performance of PTFTs.(4)Study on total ionizing dose radiation resistance of conjugated polymer thin film transistorsThis work systematically investigated the total ionizing dose(TID)response on PTFTs.Eight different types of conjugated PTFTs with IDT-BT and DPPT-TT as semiconductor layers and PS,PMMA,PTFE and Cytop as dielectric layers were used in total dose irradiation experiments.The results show that the PS-based PTFTs,independent of the semiconductor materials,have the highest radiation resistance to TID response.PS/IDT-BT PTFTs can maintain more than 95%of their electrical performance even after exposure to TID of 7.5 Mrad(Si O₂).The irradiated device has a long shelf lifetime of more than 1000 days.Simple inverter circuits are further fabricated,and can also be tolerant to high TID of 7.5 Mrad.This work demonstrates the possibility of the PTFTs for applications in harsh environments.(5)Conjugated polymer thin film memristor based on asymmetric electrode structureIn this work,a novel transistor structure model of asymmetric electrode is proposed.It is proved theoretically that the model can be used to construct conjugated polymer thin film memristors.The memristors based on IDT-BT,DPPT-TT and N2200conjugated polymer were fabricated by solution method,and their applicability and feasibility of the model were verified experimentally.Furthermore,the properties of non-volatile memory of more than 600 s,flexible memory of 143 curvature curvature,multi-level memory of more than three current orders of magnitude,neural synapse-like behavior simulation in the IDT-BT conjugated polymer film memristor are realized.The functional applications of polymer devices in flexible memory and neural synapse-like behavior simulation are extended.