Organic Thin-film Transistor Techniques For Biological/Chemical Sensing Applications

The solution processed organic thin-film transistor(OTFT)is regarded as one of the most promising technology platform for developing ubiquitous sensors,for the its potential of being manufactured by low cost high throughput printing,compatibility with arbitrary substrates(plastic,paper and fabric),superior mechanical flexibility,and convenience of being functionalized for multi-physical or biological/chemical sensing.For the envisioned applications,a series of functions required to be performed in a severely power constrained system,however,cannot be realized by the OFET technology alone.A hybrid technology is thus necessary to combine the additive printing of the OTFT-based sensing front end with a fixed low voltage silicon chips for the custom design,which has attracted more and more attentions over the last decade.Therefore,low-voltage operation and excellent operational stability becomes increasingly important,which,however,is challenging to be achieved,especially when the devices need to be manufactured by all solution or printing processes to make a commercially viable technology.This thesis is focused on addressing these key issues with the main contributions given as follows:1.A facile inkjet printing method developed to obtain uniform fine silver(Ag)electrodes is studied to meet the strict requirements of the source/drain and gate electrodes in all solution processed OTFTs.Based on the dependence of electrode width on printing drop spacing and substrate surface energy,by modulating the polymer gate dielectric's surface with self-assembly monolayers(SAMs)and optimizing the printing conditions,narrow electrodes(about 35 ?m)and relatively short channel(about 15 ?m)are achieved.The electrode's surface profile,roughness,conductivity and work function are comprehensively characterized.The fabricated all solution processed OTFTs incorporating the inkjet printing Ag electrodes present negligible hysteresis,high mobility,low leakage and low contact resistance.Moreover,by selectively controlling the surface wettability of the dielectric layer using UVO treatment,self-assembled organic semiconductor islands are also formed.This work would provide a reliable electrode implement route for realizing printable OTFTs.2.Optimized strategies for reducing the operation voltage of both small molecule and polymer solution-processed OTFTs are proposed,respectively,and the individual benefits are comprehensively revealed.1)By using a micrometer thick SU8 gate dielectric layer,commercially available small molecule semiconductor(TIPS-pentacene),and the developed inkjet printed silver(Ag)electrode materials,the effectiveness of reducing the channel subgap density of states for developing low-voltage printable OTFTs with commercially competitive manufacturing processes is conclusively proven.The benefit of this device structure on improving the power efficiency is also revealed.The micrometer thick dielectric layer results in the smallest gate capacitance(Cdiel)among all the reported low-voltage OTFTs.Being switched between ON and OFF states with a similar low voltage but at a much smaller Cdiel,the device presents the best reported power efficiency.Moreover,logic circuits are shown to be able to run faster but consume significantly less power than circuits using the conventional low-voltage OTFTs with only enlarging Cdiel.2)High-k/low-k bilayer polymer gate dielectric(P(VDF-Tr FE-CFE)/CYTOP)is introduced to fabricate low voltage solution processed polymer(C16IDT-BT)OTFTs for the first time.The P(VDF-Tr FE-CFE)is to enlarge the gate dielectric capacitance for low operation voltage.The thin,low polar CYTOP layer adjacent to the semiconductor is proved to be able to screen the dipole effects and thus minimize the energetic disorder and carrier localization to remain high mobility and also reduce hysteresis.This strategy is proven to be very useful for achieving low voltage polymer OTFTs with high performance.3.The bias stressing stabilities of the developed low-voltage OTFTs are estimated in details with the mechanism for improving stability performance further illustrated.1)It is found that mositure in air can be absorbed into the SU8 dielectric for unencapsulated low-voltage TIPS-pentacene OTFT with a channel of small Nsub and a thick gate dielectric,degrading the performance and causing stability issue.While in the low-voltage OTFT encapsulated by a CYTOP layer,both the high quality semiconductor/dielectric interface and the low gate field(< 0.05 MV/cm)contributes to reduce the probability of charge trapping into localized states,resulting in excellent operational stability.2)The operational stability under bias stress for low-voltage OTFTs based on the high-k relaxor ferroelectric polymer poly(vinylidene fluoridetrifluoroethylene-chlorofloroethylene)(P(VDF-Tr FE-CFE))is studied for the first time.The physical mechanism for stability issue is discussed and a dielectric solution for addressing it is proposed.Unlike the normally observed negative threshold voltage(Vth)shift in a p-type OTFT operated in inert environment due to the trapping of carriers from the gate bias induced conduction channel into less mobile localized states,Vth of the low-voltage OTFT using the P(VDF-Tr FE-CFE)high-k gate dielectric device shifts toward the positive direction under negative gate bias stress(NBS).The physical mechanism is considered to be that the dipole electric field from remnant polarization in P(VDF-Tr FE-CFE)induces additional mobile charges into the channel.By adding a thin CYTOP layer between the P(VDF-Tr FE-CFE)layer and the semiconductor layer,the two effects of charge trapping and remnant polarization under gate bias are found to be neutralized with each other,resulting in low voltage OTFTs of negligible hysteresis and excellent NBS stability.This work would provide clearer theoretical foundations and reliable dielectric solutions for developing low voltage and stable OTFTs by using the high-k relaxor ferroelectric.4.With the low voltage operation behavior and excellent operational stability of the developed all solution processsed OTFT device,an integrated p H sensor tag is demonstrated as a proof of concept.It comprises an extended gate OTFT for sensing and another OTFT as the load OTFT to convert the current signal to a voltage output for easy design of the subsequent readout circuit.The sensor tag is operated in a 3.3 V battery powered electronic system for reliable p H sensing.Such a hybrid system combines the advantages of fully printable OTFTs in low cost and scalability on large area and flexible surfaces,and those of conventional Si-based electronics in high performance and high integration density for complex and accurate signal processing.This work would be able to provide a commercially competitive technology platform for ubiquitous biological/chemical sensing applications.This thesis presents the device design route for achieving low operation voltage and high stability OTFTs by solution processes,providing very useful theoretical foundations for the practical application of OTFT biological/chemical sensors.