Organic Field Effect Transistors Properties Of The Dithieno[2,3-b:3’,2’-d]thiophene Derivatives

Organic field-effect transistors (OFETs) have attracted particular attention due to their potentialadvantages of low-cost, flexibility and large-area. For the past seconds of years, progress has been obtainedand their performances have reached and even exceeded their inorganic counterparts (conventional α-H:silicon-based transistors). Unfortunately, their performances are still lower than that of inorganic devicesbased on polysilicon and monocrystal silicon. It is well known that inorganic semiconductors are combinedwith strong covalent bonds. On the contrary, organic semiconductors are coalesced with weakintermolecular interactions, such as Van der Waals interaction, S S interaction, π-π interaction, and C-Hinteraction. The discrepancy of combination mode may be a primary factor for the tremendous differencebetween the mobilities of inorganic and organic semiconductors. There is significant value to investigatethe relationship between the intermolecular interactions and the device performance of organicsemiconductors.In this thesis, four kinds of new organic semiconductors with dithieno[2,3-b:3’,2’-d]thiophene (DTT)units as the building block have been purified by vacuum sublimation. Their thermal, optical andelectrochemical properties, topographical and X-ray diffraction patterns of films, and the single crystalstructures were investigated. At last, charge-carrier transporting properties of these compounds werestudied. The main work of this thesis can be separated into the three parts as following:1. Four compounds with dithieno[2,3-b:3’,2’-d]thiophene (DTT) units as the building block have havebeen purified by vacuum sublimation. The title compounds are2,5-diphenyl-dithieno[2,3-b:3’,2’-d]thiophene (1),2,5-distyryl-dithieno[2,3-b:3’,2’-d]thiophene (2),2,5-di(2-biphenyl)-dithieno[2,3-b:3’,2’-d]thiophene (3) and1,4-di[2-dithieno[2,3-b:3’,2’-d]thiophen-2-yl-vinyl]benzene (4). Thermalproperties of these materials were investigated by TGA and DSC. Their single crystal structures were alsoinvestigated.2. The photophysical and electrochemical properties of1,2,3and4were investigated. Their opticalband gaps Egare2.99eV (1),2.92eV (2),2.48eV (3) and2.25eV (4), respectively. The HOMO energylevels EHOMOare all-4.80eV. The results show that the HOMO energy level is mainly determined by dithieno[2,3-b:3’,2’-d]thiophene (DTT) units. The introduction of the different functional groups can onlychange the optical band gap, but not change the EHOMOlevel. Topographical images of the films were alsoinvestigated. Except the films of1, all present high crystalline and are composed with lamellar crystals athigh Tsub.3. OFET devices of1,2,3and4were fabricated by vacuum deposition and their performance wereinverested by the current-voltage characteristic.1does not have field-effect performance. The devices of2,3and4exhibit p-type operation, which show the mobility as high as2.20cm~2/Vs (2, Tsub=70oC),0.52cm~2/Vs (3, Tosub=70C) and0.16cm~2/Vs (4, Tsub=70oC), respectively. According to the devicesperformance and single crystal structures, it was found that the existing of S-π intermolecular interactionbenefits not only for improving the mobility but also for reducing the threshold voltage (VT), while theexisting of S S intermolecular interaction is not favorable for promoting the mobility.