Design,Synthesis And Characterizations Of Novel Organic Small Molecules For Field-Effect Transistors

Organic field-effect transistors have drawn much attention owing to their unique and superior properties over inorganic field-effect transistors.For example,(1)the wide range of organic semiconductor materials,(2)ease of processing,and(3)potential for large area flexible devices.For these reasons,organic semiconductor materials have been extensively studied over the years as a core component of organic field-effect transistors,and a large number of organic semiconductors with many advantages have been designed and synthesized.Compared with polymeric materials,the physicochemical properties of small molecules can be effectively modulated by changing the molecular structure and adjusting the functional groups;small molecules have a constant molecular weight,as well as various methods of purification;the intermolecular interactions are tight and easy to form regular ordered solid stacked,single crystals can be easily formed,which facilitate the study of structure-property relationships.For the above reasons,organic small molecular field-effect transistors have been developed rapidly in recent years.High mobility and great stability in ambient conditions are two prerequisites of organic semiconductors for high-performance OFETs.The stability of organic semiconductor materials has an important impact on the practical applications of organic field-effect transistors in organic circuits.Many researches on adjusting molecular structures,packing motifs,energy levels and gaps to improve the mobility have been reported.However,there are limited studies on the stability of thermal and vacuum the OFETs.Our main focus in this dissertation is to design and synthesize novel organic semiconductors with high stability for field-effect transistors.The main contents are summarized as follows:(1)2,6-di-(4-cyclohexylphenyl)anthracene(DcHPA)was designed and synthesized,and corresponding thin films and single crystals devices were fabricated.The thermal stability of DcHPA and 2,6-di(4-n-hexylphenyl)anthracene(DnHPA)thin films OFETs was studied.It was found that DnHPA showed sublimation temperature around310~oC.Corresponding DcHPA with similar molecular weight was found with sublimation temperature of 360~oC,which indicated that DcHPA had higher thermal stability.DcHPA and DnHPA thin film transistors with bottom-gate top-contact structure were fabricated,respectively.The measurements showed that the mobility of DnHPA devices decreased by 50%when heated to 80~oC,and had almost lost field-effect performance when heated to 120~oC.By contrast,the change in mobility of DcHPA devices was as small as 8%when heated to 80~oC,and the devicess still maintained half of its original value up to about 150~oC,indicating that DcHPA thin film devices have higher thermal stability.The method of physical vapor transportion(PVT)was taken to obtain DcHPA and DnHPA single crystals,and corresponding field-effect transistors were fabricated.DnHPA single crystal devices exhibited an average mobility of 1.30 cm~2/(V?s),and DcHPA single crystal devices exhibited an average mobility of 1.98 cm~2/(V?s)with the maximum mobility up to 3.16 cm~2/(V?s),which indicated that the mobility of DcHPA single crystal devices was higher than DnHPA.Which indicates that,compared with n-hexyl substitution,the cyclohexyl substitution could improve the material's thermal stability without sacrificing its mobility.(2)Two novel asymmetrical molecules were designed and synthesized,namely TBTBT1 and TBTBT2,for thin films and single crystals field-effect transistors.The stability under vaccum of TBTBT1 and TBTBT2 was improved compared to BTBT.Uv-vis spectrum and cyclic voltammetry measurements showed that TBTBT1 and TBTBT2 have the similar energy gap with BTBT,indicating the good stability.TGA showed that decomposition temperature of TBTBT1and TBTBT2 were 214~oC and 215~oC,respectively.Single crystal analysis exhibited that TBTBT1 adopts an arrangement of herringbone stacking with two molecules tilted in the same direction and pairs of molecules arranged in the herringbone pattern.And the intermolecular distance between parallel adjacent molecules is about 3.45?with large?-overlap,and the herringbone angle was 83.86°.The influence of different dielectrics on TBTBT1 and TBTBT2 thin films was investigated.The results showed that morphology of thin films grown on OTS-treated substrate was better than that on bare substrate.The mobility was 0.05 cm~2/(V?s)that was two orders of magnitude higher than that of the devices fabricated on bare silicon wafer.OFETs based on single crystals were also investigated.Single crystals of TBTBT1 were obtained by drop-casting method,and the corresponding devices fabricated by gold-layer sticking technique exhibited mobility of0.30 cm~2/(V?s).TBTBT1 and TBTBT2 single crystals were fabricated by PVT simultaneously,and the devices of TBTBT1 and TBTBT2 exhibited mobility of 0.25 cm~2/(V?s)and 0.33 cm~2/(V?s),respectively.The performance could be further improved by improving the quality of single crystals.Test on the stability under vacuum showed that the stability of TBTBT1and TBTBT2 have been significantly improved compared with BTBT.