Growth Of 2D Organic Molecular Crystals On Liquid/Liquid Interface And Their Application In High-Performance Organic Field-Effect Transistors

Two-dimensional organic molecular crystals are considered as ideal materials for constructing high-performance organic field-effect transistors,owing to their outstanding charge transport efficiency.However,the size of 2D molecular crystals obtained by the currently developed preparation methods is generally less than 12 mm2,and an additional transfer process is required to move the crystals to the target substrates,which can easily cause contamination and damage to the crystals,seriously hindering the practical application of crystals.Aim at the above problems,we have carried out a series of works and achieved the following innovative results:1.Growth of 2D Organic Molecular Crystals on target substrates by Surficial Marangoni flow-induced and their application research in Organic Field-Effect TransistorsIn this work,toluene and N,N-Dimethylformamide(DMF)are used as good and poor solvents for 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene(diF-TES-ADT)semiconductor materials.In combination with the drop-casting method,a surficial Marangoni flow-induced growth method is developed.Using this method,the ultrathin molecular crystals with millimeter size(1 × 1.1 mm2),thickness of approximately 4.83 nm,and high crystal quality are successfully prepared on various target substrates.The differences of evaporation rate and local surface tension gradient between toluene and DMF induce a vertical convection flow of mixed solvent,to significantly improve the spreading coefficient in the Marangoni flow.The crystallizati-on field of diF-TES-ADT is limited to the two-phase interface between toluene and DMF,and the preparation of low-layer(3-4 layers)diF-TES-ADT molecular crystals is achieved in a dozen seconds,without an additional transfer process.When the volume concentration of DMF in the mixed solvent is controlled between 7.7 vol%and 10 vol%,ultra-thin diF-TES-ADT molecular crystals with the best surface morphology can be obtained.High-resolution atomic force microscope,transmission electron microscope,and two-dimensional grazing incidence X-ray diffraction and so on fully prove that the ultra-thin molecular crystals have a long-range ordered molecular accumulation mode.The organic field-effect transistors are constructed directly based on this diF-TES-ADT molecular crystals.The maximum hole mobility is as high as 3.18 cm2 V-1 s-1,the threshold voltage is as low as-7 V,the current on/off ratio is about 4.2 × 104,and an average hole mobility of 30 devices on the same substrate can reach 2.15 cm2 V-1 s-1.This method of preparation of 2D organic molecular crystals by surficial Marangoni flow-induced has no substrate dependency,and has universal compatibility for small organic molecules such as p-type 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene(C10-BTBT)and n-type N,N-Dioctyl-3,4,9,10-perylenedicarboximide(PTCDI-C8).It shows that this method has great development potential in the flexible construction of functional organic electronic devices based on 2D organic molecular crystals.2.Growth of C10-BTBT 2D molecular crystals by water surficial directional-induced and their application research in High-Performance Organic Field-Effect TransistorsIn this work,a water surficial directional-induced crystallization method is developed by dragging a mixed solution of organic small molecules and polymer Polystyrene(PS)on the water surface,and successfully prepared the high-quality C10-BTBT molecular crystals with centimeter size(1.2 × 7.5 cm2),thickness of approximately 6.23 nm and a surficial continuous smooth.This method uses surface te-nsion gradient between toluene and water to significantly improve the spreading coefficient in the Marangoni flow.The crystallization field of small molecular materials C10-BTBT is limited to the two-phase interface between toluene and water,and the controllable preparation of large-area C10-BTBT molecular crystals is achieved with the drag-coating speed of 100?m s-1 under the connection of polymers.Then the crystals are transferred to the target substrate for 60? annealing treatment,and finally obtain low-layer C10-BTBT molecular crystals with 100%substrate coverage.When the mass ratio of C10-BTBT to PS in the mixed solute is 1:3.5,ultra-thin C10-BTBT molecular crystals with the best surface morphology can be obtained.X-ray diffraction,high-resolution atomic force microscope and transmission electron microscope and so on fully demonstrate the high crystal quality of this ultra-thin molecular crystals.The organic field-effect transistors are constructed based on the C10-BTBT molecular crystals.The maximum hole mobility is as high as 7.58 cm2 V-1 s-1,the threshold voltage is as low as-5 V,the current on/off ratio is over 107,and an average hole mobility of 30 devices on the same substrate can reach 6.21 cm2 V-1 s-1.Besides,this work also studies the effect of organic solvent DMF on the crystallization behavior of C10-BTBT molecules and the effect of 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquino-dimethane(F4-TCNQ)materials on the electrical properties of organic semiconductor devices.It can be seen that this method has good application prospects in the controlled preparation of large-area,high-quality,ultra-thin 2D organic molecular crystals.