Fabrication Of Large-area Crystalline Thin Films Of Soluble Acene For Organic Field-effect Transistors

Organic field-effect transistors(OFETs)have shown great potential use in OLED,sensors,RFID,and flexible displays,owing to their compatibility with low-cost solution processing technologies.Thickness and crystallographic orientation of the organic semiconductor(OSC)thin films are crucial to the OFET performance.Ultrathin OSC films can effectively reduce contact resistance and improve device mobility.Oriented OSC thin films can enhance carrier transport to further improve OFET performance.However,high throughput and fast deposition of highly oriented OSC thin films over large-area substrates remain significant challenges.Aim at these challenges,we have carried out a series of works in this thesis:1.Fast deposition of ultrathin,highly crystalline organic semiconductor films for high-performance transistorsIn this work,m-xylene and o-dichlorobenzene are used as a mixed solvent for bis(triethylsilylethynyl)anthradithiophene(Dif-TES-ADT)semiconducting material.Using the mixed solvent strategy,wafer-scale(1900 mm2)and pinhole-free ultrathin(4.62 nm)Dif-TES-ADT crystalline films are successfully fabricated at a fast blade-coating speed.The mixed solvent system can significantly enhance mass transport of molecules towards the contact line by the Marangoni flow,which allows us to fabricate continuous and smooth ultrathin Dif-TES-ADT films at a fast speed of 1 mm s-1.2D-grazing incidence X-ray diffraction,polarized UV-vis absorption,and high-resolution atomic force microscopy verify that the ultrathin film has very high crystallinity and highly ordered molecule packing.OFETs based on the ultrathin Dif-TES-ADT crystalline film exhibits an increased mobility of 5.54 cm2 V-1 s-1,which is close to that of the Dif-TES-ADT single crystal-based devices and higher than that of most Dif-TES-ADT film-based devices.The average mobility of 40 devices on the same substrate is 3,12 cm2 V-1 s-1.Benefiting from the ultrathin nature,transparent and flexible OFETs based on the ultrathin Dif-TESADT film are demonstrated.The flexible OFETs show a high saturation mobility of 2.64 cm2 V-1 s-1.Moreover,the flexible devices can stand a small bending radius down to 6 mm,exhibiting good bending stability.This mixed solvent strategy also shows good universality,and can be applicable to a host of organic small-molecule semiconductors for the growth of ultrathin crystalline films at a fast speed,such as p-type 6,13-bis(triisopropyl-silylethynyl)pentacene(TIP S-PEN),bis(triethylsilylethynyl)anthradithiophene(Dif-TES-ADT),2,6-Bis(4-hexylphenyl)anthracene(C6-DPA),and n-type tetrachlorotetraazapentene(4Cl-TAP),N,N'-Dioctyl-3,4,9,10-perylenedicarboximide(PTCDI-C8),6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14-tetraazapentacene(TIPS-TAP).This finding should open a new route to achieve ultrathin OSC crystalline film-based high-performance,flexible,and transparent electronics.2.Fabrication of Dif-TES-ADT thin films with aligned single crystals for high-performance transistorsIn this work,Dif-TES-ADT thin films with highly oriented single crystals are successfully fabricated via blade coating of Dif-TES-ADT solution on the patterned substrate with alternating hydrophilic/hydrophobic regions.The periodic hydrophilic/hydrophobic template is composed of hydrophobic FDTS stripes with 2 ?m width and hydrophilic BCB stripes with 2 ?m width.The narrow hydrophilic/hydrophobic regions produce a geometrical confinement effect to the organic solution,which can significantly reduce the nucleation density of organic crystals at the meniscus front.Meanwhile,the blade-coating process ensures consistent molecular packing in the organic crystals and thus improve the crystallographic orientation of organic single crystals.The growth direction of the Dif-TES-ADT crystals is determined to be[010]from transmission electron microscopy results.Grazing incident X-ray diffraction,high-resolution atomic force microscopy,and polarized UV-vis absorption prove that the obtained Dif-TES-ADT single crystal arrays have highly crystalline quality and high molecule packing order.Dif-TES-ADT single crystal array-based on OFETs shows a low threshold voltage of-3 V,a high on/off current ratio of about 107,and an excellent mobility of 2.89 cm2 V-1 s-1,which is higher than that of the Dif-TES-ADT polycrystalline thin film-based devices.The resulting Dif-TES-ADT single crystal arrays possess highly uniform crystal quality and electronic properties,and 30 devices on the same substrate show a high average mobility of 2.11 cm2 V-1 s-1,and a small standard deviation of 0.48 cm2 V-1 s-1.This method for the fabrication of highly aligned organic single crystals with homogeneous crystal and electrical properties opens opportunities for practical applications of organic single crystals in integrated electronic and optoelectronic devices.