Solution-Processed Low Voltage Organic Thin-Film Transistors And Circuit Integration

Solution-processed organic thin-film transistors?OTFTs?have received worldwide attention,owing to their attractive features of low cost drop-on-demand printable processes,superior intrinsic mechanical flexibility,and sustainable performance improvement.These features of OTFTs make them suitable for being used in applications of sensors,radio frequency identification?RFID?tags,smart memories,bio-medical electronics,flexible display backplanes and wearable devices.During the past few decades,significant efforts have been focused on developing high carrier mobility organic semiconductors and solution-based processes with reported device performance far beyond that of amorphous silicon?a-Si?thin-film transistors.The reported mobility of OTFTs can meet the requirements of most low-cost applications.However,the demonstrated high operation voltage of typically a few tens volts for solution processed OTFTs remains to be the main bottleneck preventing further advances towards the targeted applications.For the actual applications,it is important to reduce the operation voltage of printable OTFTs,by means of device physic analysis,device architecture design,material selection and interface optimization.Apart from decreasing the operation voltage,improving the air stability of OTFTs is also essential for actual applications.This work devotes to solve the key problems mentioned above,namely,reduce the operation voltage and improve the air stability of printable OTFTs.The challenges of further circuit/system level integration for the developed OTFTs will also be investigated.With device simulation and consideration of processes,bottom-gate bottom-contact?BGBC?device structure is used in the further experiments.Subthreshold swing?SS?is very important for the operation voltage of OTFTs,which is determined by the gate dielectric capacitance per unit area?C_i?and effective semiconductor/dielectric interface trap density(N_SS).In the past,significant efforts have been devoted to reducing the operation voltage by increasing the C_i.However,this method was thought to be incompatible with solution processes and materials over large-area and flexible substrates.In this work,6,13-bis?triisopropylsilylethynyl?-pentacene?TIPS-pentacene?blended with polystyrene?PS?was used as the channel layer,the vertical phase separation induced by different solubility parameters of TIPS-pentacene and PS was found to be beneficial for the crystallization of semiconductor.With this approach,a low N_SS is achieved for low voltage operated OTFTs.This approach is more suitable for printable OTFTs since there is less strict requirement on the C_i.With a 407 nm solution processed polyvinyl alcohol?PVA?gate dielectric layer,the device showed a state-of-the-art small SS of 100 mV/decade and a low operation voltage down to 3 V.The compatibility of this approach for low operation voltage was also proved by using different semiconductor deposition processes and different dielectrics.Based on the developed low voltage device architecture,the challenges for circuit integration were also investigated.Threshold voltage(V_th)control using different metal gates?aluminum and gold?was applied to realize high performance dual-V_th low voltage inverter.Afterwards,ink-jet printed?IJP?silver?Ag?electrodes were integrated into the device architecture.All solution processed low voltage OTFT inverter on flexible polyethylene naphthalate?PEN?substrate was demonstrated.Moreover,to improve the air stability of OTFTs,the dielectric layer was proved to be very important,with a proper non-polar hydrophobic polymer dielectric layer poly?vinyl cinnamate??PVC?,very stable electrical properties were achieved for an all solution processed low voltage OTFT even in a bottom gate unencapsulated configuration,with the channel being exposed to the ambient for analyte detection.For the first time,the excellent features of low voltage and stable operation allow the sensor tag made of the OTFT to be incorporated into a self-designed lithium-battery powered electronic system for continuously reliable sensing of ammonia vapor in ambient air.The sensing system consists of the flexible OTFT sensor tag and silicon-IC readout circuit.The power consumption of the OTFT sensor tag is as low as 50 nW.Finally,all IJP low voltage OTFTs were realized.With proper device architecture design,material selection and process optimization,combining the achieved conclusions of low voltage OTFTs above,this work reports the first all IJP low voltage OTFT on flexible PEN substrate,which well-proves the good compatibility between the developed low voltage device architecture and printing processes.