Dibenzo[a,h]anthracene Liquid Crystal Semiconductors And Their Thin Film Transistor Application

Among organic semiconductor materials,liquid crystal semiconductor has attracted wide attention because of the characteristics their anisotropic and orderly arrangement of crystal molecules and the advantages of liquid fluidity.Small molecule liquid crystal semiconductor materials can be obtained by solution,using the excellent self-assembly advantages of molecules to form two-dimensional large area ordered arrangement,so as to obtain high performance field effect transistor.It is easy to make materials to possess the characteristic of liquid crystal through selecting the rigid aromatic thick ring system with high solubility and crystal properties and introducing substituents at both ends of the molecular central nuclear axis.Based on this,we designed Dibenzo[a,h]anthracenes（DBA）aromatic nuclei which is the core of the band center,the bandπ-conjugated molecules,comparing to linear or quasi-linear molecules,are favorable for the generation of rotation and translation radius,which effectively limits the movement of molecules in the solid state,while giving the device high mobility and stability.Based on DBA kernel,this paper introduces substituents to control its performance.according to the type of substituents,it is mainly summarized into two parts:Design,synthesis and thin-film transistor application of a dialkyl chain symmetrically substituted DBA（R-DBA-R）.The same straight-chain substituents(R=C₈H_17,C₁₂H₂₅)and cyclic alkyl chains were introduced at the DBA ends of the central kernel.Firstly,the symmetric R-DBA-R material was synthesized by a simple synthesis procedure,and its structure was determined by ¹HNMR and HRMS.The basic optical properties of R-DBA-R materials,including UV-visible spectrometer and fluorescence spectrum,were tested.We found that R-DBA-R materials have similar optical properties and good solution stability.We used thermogravimetric analyzer to test the ground temperature under the condition of 5%weight loss of the material.We found that the cyclohexyl substituents had higher thermal stability than the straight chain substituents.The thermal transition behavior of R-DBA-R materials was analyzed by combining polarizing microscope and DSC curve,and it was found that all materials have liquid crystal phase.Using BCB/Si O₂substrate,the uniform film morphology was obtained by adjusting the experimental conditions of scraping,and the corresponding transistor devices were assembled by vacuum evaporation with the highest mobility of2.97 cm²V^-1s^-1.In addition,the scraped films were characterized by AFM and find that R-DBA-R materials were grown in a single molecule arrangementDesign,synthesis and thin-film transistor applications of asymmetrically substituted DBAs（Ph-DBA-R）.Adopting Asymmetric synthesis strategy,benzene ring and alkyl chain(R=C₈H₁₇,C₁₀H₂₁,C₁₂H₂₅)were introduced at one end of the DBA central nucleus and named Ph-DBA-R.By a relatively simple synthesis route,the target product Ph-DBA-R is obtained.We characterized the basic optical properties of Ph-DBA-R,including UV-visible absorption light spectrum and fluorescence spectrum,similar to the product Ph-DBA-R,in addition,compared with the symmetric substituent,the introduction of phenyl group has a greater effect on the position of absorption peak.The thermogravimetric and DSC curves of the materials were tested,and it was found that the Ph-DBA-R of the materials had high order Sm E phase,which may be because the introduction of phenyl is more conducive to the formation of high order liquid crystal phase.The active layer was prepared by scraping coating method,and the organic thin film transistor device was assembled.The effects of different dielectric layers（BCB andβ-PTS）on the performance of the device were compared.Through effective regulation,Ph-DBA-C8 exhibited the highest mobility of 11.8 cm²V^-1s^-1 inβ-PTS/Si O₂ substrate.The average mobility of all devices exceeds 2.8 cm²V^-1s^-1.Our material has good thermal stability and still has high mobility(1 cm²V^-1s^-1)at 160℃.The scraped film was characterized by microscopy,AFM and XRD,it is found that Ph-DBA-R is grown in bimolecular form,which is conducive to the transfer of interlayer carriers,and it’s the reason for the high mobility of transistor devices.