|dc.description.abstract||Fused conjugated ring compounds attracted considerable interest in organic electronics. Among them pentacene based small molecules have been studied in organic electronics due to their excellent electronic and optical properties and owing to its high OTFT performance. The high mobility of pentacene is due to high order in the crystalline film. However, pentacene has the disadvantage of being easily undergo photo-oxidation in air due to its high lying HOMO and narrow band gap. Compared with pentacene, fused thiophene based conjugated small molecules are emerged as a potential organic semiconducting materials due to their excellent optoelectronic properties and relatively higher ambient stability originating from larger band gaps in the crystal structures, their rigid and flat -conjugated frameworks make them the most versatile and effective molecular scaffolds for organic functional materials. Additionally fused thiophenes possesses extensive molecular conjugation and strong intermolecular S•••S interactions promotes close molecular packing lead to higher charge carrier mobilities.
In my research, we have designed and synthesized fused thiophene based organic semiconductors using Stille and Suzuki cross-coupling reactions in various architectures for organic thin film transistors (OTFT), organic photovoltaics (OPV) and two photon absorption (2PA). Also, includes the comparative study of the optical, electrochemical and thermal properties of these organic semiconductors. OTFT, OPV and 2PA characterizations of these compounds suggested that fused thiophene based organic semiconductors are promising new class of organic materials for organic electronics. Electrochemical properties were investigated using differential pulse voltammetry, optical properties were examined by UV-visible absorption spectroscopy and thermal stability was observed through the thermogravimetric analysis (TGA) curves. Thin film microstructure and film morphology were examined by X-ray diffraction, atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and grazing incidence wide-angle X-ray scattering (GIWAXS) to understand the relationships between molecular structure, film morphology/crystallinity, and device performance. The molecular structures of organic semiconductors and their molecular packing properties were determined by single-crystal X-ray diffraction.
Asymmetric phenyl and perfluorophenyl end-functionalized dithienothiophene (DTT) and bisdithienothiophene (BDTT)-based fused-thiophene derivatives gave high p-channel mobilities of 0.74 and 0.73 cm2V-1s-1, respectively for single-crystal transistors and exhibited decent p-channel characteristics with a carrier mobility as high as 0.15 and 0.20 cm2/Vs respectively for organic thin-film transistor. Thio-alkyl (SR) substituted bithiophene (BST)-based semiconductors, end-capped with dithienothiophen-2-yl (DTT) group derivatives exhibits excellent p-type OFETs performance with mobilities up to 1.7 cm2 V-1 s-1 for organic field effect transistors (OFETs). Fused-thiophene (TTA, DTT, TT) semiconductors, end-capped with diperfluorophenylthien-2-yl (DFPT) groups (DFPT-TTA, DFPT-DTT, and DFPT-TT) are synthesized and characterized for organic thin-film transistors (OTFTs), DFPT-TTA and DFPT-TT exhibit good n-type TFT performance, with mobilities up to 0.43 cm2V−1s−1 and 0.33 cm2V−1s−1, respectively. Diketopyrrolopyrrole (DPP) and fused tetrathienoacene (TTA) frameworks are synthesized and characterized for OTFT and organic photovoltaics (OPV) exhibit hole mobilities approaching 0.1 cm2V-1s-1 and OPV efficiencies (PCE) > 4% for DDPP-TTAR. Organic photovoltaic cells based on Bis4T-DPP:P3HT:PC71BM blends achieve power conversion efficiencies (PCE) of 4.87%. Diketopyrrolopyrrole (DPP)-containing quinoidal small molecules for n-type organic semiconductors in thin film transistors (TFTs) exhibits maximum electron mobility up to 0.55 cm2V−1s−1 by solution process. Fused-thiophene-based chromophores are developed and characterized for a two-photon absorption (2PA) study exhibits the strongest and the most widely dispersed 2PA cross-section value of up to 3000 GM for DTT based chromophore.