Influence And Regulation Of Electrode Contact On Electrical Properties Of Bottom-Contact DNTT Thin-film Transistors

Organic thin film transistors(OTFTs)are one of the fundamental units of flexible electronic devices,which show a great application potential in wearable smart sensors,flexible displays and logic circuits,etc.The integration of OTFTs into circuits is often based on coplanar geometry(bottom gate bottom contact,BC),because damages to the deposited organic semiconductors can be minimized by reducing the following processing steps.However,the BC OTFTs usually exhibit inferior field-effect performance compared with staggered(bottom gate top contact,TC)devices,because BC OTFTs suffer from a tiny fraction of the gate-field-induced carrier injection.Besides,the thin film in BC devices normally exhibit a discontinuous coverage,particularly at the contact edges.Therefore,the contact resistances(R_C)for the BC OTFTs are considerably large.Until now,researchers have adopted various attempts such as self-assembled monolayers(SAMs),doping,inserting charge injection layer,to solve the poor contact problems of BC OTFTs.Most of the approaches mainly focused on reducing the injection resistances(R_inj)at the electrode/organic semiconductor interface.In contrast,the investigations have been rarely reported on optimizing the access resistances(R_acc),which originate from the resistance of transporting carriers to the channel after the carriers are injected.Furthermore,there still remains theoretically blank about the regulating mechanism of the R_acc,which is a critical obstacle to further reduce the total R_C.Here,aiming at the bottleneck problem for optimizing contact injection,we analyzed the origin of the R_C in BC OTFTs in depth,and implemented a two-step strategy.Firstly,we utilized PFBT modification to reduce the R_inj.On this basis,we have developed a powerful strategy,blurred electrodes,to effectively reduce the R_acc of BC OTFTs,and also filled the gap in the studies of tuning mechanism for the R_acc.The main results are as follow:1.We utilized PFBT SAMs to functionalize Au electrodes for purpose of reducing the R_inj.We firstly explored the impacts of the modification time on the device performance.We found that the performance could reach a stable status when the modification time was over2 minutes,and the performance was much superior to the devices with pristine electrodes,which suggested that the modification method effectively reduced the R_inj.To investigate the optimizing mechanism of PFBT modification,we utilized UPS to characterize the surface of Au electrodes,and found that the modification could increase the work function of Au electrodes,thus reducing the hole injection barrier.AFM measurements also indicated that this modification improved the morphology of deposited DNTT thin film on the electrodes,and increased the grain size.Moreover,to explore the feasibility of electrode functionalization in the large-scale device fabrication based on photolithography,we utilized the functionalized electrodes to fabricate DNTT TFTs arrays.The mobilities of all the devices in arrays were higher than 1.40 cm²V^-1s^-1,and showed a high uniformity.2.Aiming at achieving the modulation of the R_acc,we developed a powerful strategy,blurred electrodes,and investigate the impacts of this strategy on the field-effect performance of bottom-contact DNTT TFTs.Through characterizing morphology and crystallinity of DNTT thin film,we found that this strategy could not only continuously tune the organic molecular packing and thin-film growth along and across the contact edge,but also alleviated the steep step formed by the electrodes,promoting a continuous coverage of post-deposited organic semiconductor layer at the edge.On the basis of this strategy,we successfully fabricated high-performance BC DNTT TFTs,with highest field-effect mobility up to 6.08 cm²V^-1s^-1,which is far superior to the mobility of coplanar DNTT TFTs previously reported.Additionally,our devices exhibited a relatively high operation and ambient stability.Moreover,the R_C of BC devices was low to 2.32 k?cm,almost comparable to the TC devices fabricated simultaneously.These results suggested that our strategy,blurred electrodes,could effectively reduced the R_acc.3.We have analyzed and investigated the charge injection mechanism of the DNTT TFTs utilized the blurred electrodes.Combined with SKPM measurements on the DNTT thin film,we found a gradual transition of surface potential on the thin film across the contact edge,indicating that continuously distributed gap states induced by the gradually changed orientation of organic molecules existed.The continuous states inside the gap provide an access pathway,where carriers can hop through these states to transporting channels,thus significantly promoting the edge charge injection and access across the contact interface.The design idea of blurred electrodes filled the gap in the studies of tuning mechanism for the R_acc,and thus addressing the crucial impediments for further reducing the R_C in BC OTFTs.