High Performance Conjugated Polymer Thin Film Transistors Based On Interlayer Regulation

The development of information technology can not be separated from its basic material carrier transistors.Organic thin film transistors benefit from their rich material sources,low temperature and low-cost technology,compatibility with flexible substrates,and large-area preparation by solution method,in the academic and commercial fields.As the core of conjugated polymer thin film transistors,the focus of research in recent years has been focused on donor-acceptor(D-A)polymers,the strong donor-acceptor charge interaction between the molecules promotes the mobility of the polymer compared with that of the early conjugated polymer.As a new type of D-A conjugated polymer material,DPPT-TT benefits from its excellent planarity,strong electron absorption,excellent chemical stability,and easy formation of strong intermolecular force,it is very suitable for fabrication of transistors.DPPT-TT conjugated polymer in the practical application of transistors,there are still some problems to be solved.At present,there is no unified and mature technology to regulate the mobility of DPPT-TT conjugated polymer thin film transistors.The mobility,threshold voltage,sub-threshold swing,switching ratio,and leakage current control of DPPT-TT conjugated polymer thin film transistors need to be improved.For DPPT-TT conjugated polymer thin film transistors,it is no longer simple to apply the traditional device physical model to focus on carrier transport in the channel,more should be explored on the contact interface between the metal electrode and the semiconductor layer.The quality of the interface between the gold electrode and the semiconductor layer of the DPPT-TT conjugated polymer thin film transistor is directly related to the process of carrier injection and collection.Too large contact resistance on the interface will cause a voltage drop that can not be ignored and weaken the transverse electric field in the channel that drives carrier transport.At the same time,because the Fermi level of gold is lower than the HOMO level of the DPPT-TT conjugated polymer,the higher the Schottky barrier level at the interface between the gold electrode and the DPPT-TT semiconductor layer results in fewer carriers entering the channel through thermal electron emission.Therefore,to improve the performance of DPPT-TT conjugated polymer thin film transistor,the interface between electrode and semiconductor is improved by introducing an interface intermediate layer,the large increase of quasi-ohmic contact,and transistor mobility are realized,and the contact resistance and contact barrier height are evaluated by experimental characterization.The main work content and innovation points of this master's professional dissertation are shown in the following two sections:(1)Regulation of molybdenum trioxide interlayer thickness on electrical properties of DPPT-TT conjugated polymer thin film transistorsThis work is based on the fabricated top-gate-bottom contact interlaced structure device with gold as contact electrode and DPPT-TT conjugated polymer as the semiconductor layer,the interface between the gold electrode and the DPPT-TT semiconductor layer was inserted by using the molybdenum trioxide as the contact interlayer.The interlayer thickness(1 nm,3 nm,5 nm,7 nm,10 nm,16 nm)was controlled,the effect of interlayer thickness on the electrical properties of DPPT-TT conjugated polymer thin film transistor was investigated.The experimental results show that the threshold voltage of DPPT-TT conjugated polymer thin film transistors decreases about 7.5 V,the sub-threshold swing decreases to 4.4 V/dec,and the contact resistance decreases by an order of magnitude compared with the control devices without interlayer,from 8.1×10⁶?to 6.1×10⁵?.The mobility of these three key electrical parameters increased from 0.23 cm²/(V·s)to 0.77 cm²/(V·s)with the increase of the middle layer thickness,these parameters reach the optimal value when the thickness of the middle layer is 5 nm.When the interlayer is very thin,it is difficult to form a continuous interlayer with smooth morphology,which restrains the carrier injection at the electrode/semiconductor interface.The results of KPFM showed that the work function of the composite electrode with a 1 nm interlayer did not match the HOMO of DPPT-TT.When the middle layer is very thick,it is found by AFM that the roughness of the middle layer increases by 4.4 times,and the low conductivity of Mo O₃and the high trap density of Mo O₃ hinder the performance of the device.It is found that the internal Schottky barrier of the device is higher,0.21 e V and0.17 e V,respectively,when the thickness of the middle layer is too small or too large.The height of the potential barrier plays a decisive role in the hot electron emission mechanism,the higher barrier height makes the carrier need more energy to cross the barrier vertex and enter the DPPT-TT semiconductor layer.Because the intrinsic carrier concentration of DPPT-TT conjugated polymer is very low,there is often a very thick depletion layer at the electrode/semiconductor interface,which leads to the difficulty of carrier injection through tunneling.In the device with the 5 nm interlayer,the Schottky barrier height was suppressed to 0.09 e V,and the contact resistance decreased by an order of magnitude compared to the control device,as a result,the efficiency of hot electron emission is greatly improved,the mechanism of carrier tunneling is enhanced,and more carriers are injected into the channel to form strong accumulation and transmission.The above results show that the electrical properties of DPPT-TT conjugated polymer thin film transistors can be effectively improved by inserting a 5 nm molybdenum trioxide interlayer,which can adjust the Schottky barrier height and optimize the contact resistance of the interface.(2)Regulation of electrical properties of DPPT-TT conjugated polymer thin film transistors by Cu O_x interlayer formed by oxygen Plasma treatmentIn this work,the copper(Cu)layer at 5 nm was inserted by thermal evaporation at the Au electrode/DPPT-TT conjugated polymer semiconductor interface,and the copper layer was oxidized by oxygen plasma treatment was carried out at different time lengths,the regulation of Cu O_x interlayer on the electrical properties of DPPT-TT conjugated polymer thin film transistors were studied by evaluating the electrical parameters such as transistor mobility,contact resistance,threshold voltage,and sub-threshold swing.The results show that the middle layer formed by 30 seconds oxygen plasma treatment is most favorable for the increase of mobility,from 0.21 cm²/(V·s)to0.72 cm²/(V·s).X-ray photoelectron spectroscopy(XPS)was used to analyze the upper surface composition of the Cu O_x interlayer.It was found that the increase of Cu O content in the interlayer was the main reason for the performance improvement of the Cu O_x interlayer.Cu O is one of the few P-type oxide semiconductors,the work function of copper oxide at 5.3 e V was well-matched with the HOMO level(5.33 e V)of DPPT-TT conjugated polymer.The height of the Schottky barrier extracted in the variable temperature experiment was effectively reduced to 0.11 e V after 30 seconds of oxygen plasma treatment.The contact resistance decreases from 8.3×10⁶?to 7.7×10⁵?.However,for the interlayer of oxygen plasma treated for a too long time and too short time,the internal Cuprous oxide content increased sharply,and the work function of the Cuprous oxide at 4.6 e V and the HOMO energy level of the DPPT-TT conjugated polymer was quite different,this results in a 0.24 e V Schottky barrier,which inhibits carrier injection via hot electron emission,a thicker depletion layer due to a larger potential barrier,and a more efficient tunneling mechanism.The results show that the height of the Schottky barrier and the contact resistance at the contact interface is optimized after 30 seconds of oxygen plasma treatment,and the electrical properties of the DPPT-TT conjugated polymer thin film transistor are improved.