Research On The Effect Of Zinc Phthalocyanine Modified Layer On The Performance Of Organic Thin-film Transistor

Compared with amorphous silicon thin film transistor, the potential applications inthe low-cost, large-area electronic devices such as a flexible display, biochemicalsensor and the smart card make organic thin film transistor（OTFT） receive widespreadattention. Wherein the Pentacene OTFT shows attractive advantages, such as: highfield-effect mobility, a low preparation temperature, low cost, etc. Field-effect mobilityof the Pentacene depends not only on the crystal quality of Pentacene, but determinedby the choice of the insulating layer, the interface properties and other factors. Theinterface also includes the organic layer/insulating layer interface and the source（drain）electrode/organic layer interface. In the past few decades, various surface treatmentssuch as O₂plasma, the surface of the self-assembled layer processing methods wereused to modify these two interfaces, For the purpose of improving the performance ofthe OTFT. Changing the interfacial properties by doping is a relatively simple andfeasible method. In this paper, we mainly research the impact of ZnPc doping of on theeffect of OTFT performance.In this thesis, we conducted the following work:Preparation OTFT （Organic Thin Film Transistor） with a bottom-gate top-contactstructure, silicon with silicon nitride on it is used as substrate, Pentacene the organicsemiconductor layer, Au the source/drain electrodes, and we extracting the relevantparameters of the device by applying the inorganic MOSFET parameter extractionmethod.We proposed doping Pentacene with zinc phthalocyanine（ZnPc） which largelyused in organic solar cell and organic light-emitting devices. This material couldmodify the surface morphology of the dopant material. First we prepared a groupdevices with different doping position: undoped device, a bottom-doped（doping in Pentacene layer near the insulating layer in a bottom-gate top-contact structure OTFT）device, and a top-doped（doping in Pentacene layer near the source/drain electrodes in abottom-gate top-contact structure OTFT）device, the doping concentration of thedevices are10%and doping thickness is setted as10nm, We obtained that the mobilityof these three devices are4.12×10^-2cm²/Vs,2.68×10^-3cm²/Vs,4.75×10^-2cm²/Vs, theirthreshold voltages are-2.64V,-7.06V,-1.26V, We concluded that top doping improvethe device performance, while the bottom doping make the performance worse.We analyzed that the performance improvement in a top-doped device is due tothat the top-doped ZnPc makes the roughness of the organic semiconductor layer lower,which to some extent block the deposited source and drain metal penetrate into thePentacene active layer, and reducing the hole injection barrier, Corresponding thedevice performance be enhanced. On the other hand ZnPc material itself impede holetransport, so that the bottom-doped ZnPc degrade device performance.To verify the above conclusion, we do two group experiments. In the firstexperiment we prepared two groups samples on silicon substrate on which a layer ofsilicon nitride is growed, we grow100nm Pentacene layer on the substrate troughvacuum deposition in the first group, we grow90nm Pentacene layer and10nm20%ZnPc doping layer in sequence in the second group. Atomic force microscopy are madein these two groups samples, we gained that the average roughness od the two groupssamples are11.10nm and9.34nm respectively. In the second experiment we make twosingle-carrier devices, the structure of the device1is: ITO substrate/10nm MoO_x/40nmPentacene/10nm MoO_x/Al; device2: ITO substrate/10nm MoO_x/40nm20%ZnPcdoped Pentacene layer/10nm MoO_x/Al, we test their current density versus voltagecurve and gained that the current density of device2is smaller than device1, thecurrent density decreased34%when the voltage is setted to5V, showing that the ZnPcmaterial itself hinder the hole transport.To further validate the conclusion that the ZnPc impede hole transport, We make devices with different bottom doping thickness, doping density is10%, and the dopingthickness are0,1nm,5nm,10nm, respectively, trough test we gained a rapiddeterioration of the device performance with the increase of the doping thickness. Themobility change from4.09×10^-2cm²/Vs to6.61×10^-3cm²/Vs，while the thresholdvoltage increase from-5.49V to-6.82V.To further optimize the device performance, We produce a group device withdifferent ZnPc doping density: make the top doping thickness10nm, and the dopingdensity are setted as5%,10%,20%,40%respectively, we gained the optimum dopingdensity20%when the top ZnPc doping thickness is setted as10nm, whose mobility ofthe saturate zone（Vds=-20V） is5.35×10^-2cm²/Vs, when compared with the undoped onea30%mobility increase is received.