Design, synthesis, and characterization of n-type and ambipolar small molecules as air-stable and solution-processable semiconductors in ofets
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The design and development of novel ambipolar and n-channel semiconductors is very crucial to advance various optoelectronic technologies including organic fieldeffect transistors (OFETs) and complementary (CMOS) integrated circuits. Although numerous ambipolar and n-channel polymers have been realized to date, small molecules have been unable to provide high device performance in combination with ambient-stability and solution-processibility. In the first part of this thesis, two novel small molecules, 2OD-TTIFDK and 2ODTTIFDM, were designed, synthesized and characterized in order to achieve ultralow band-gap (1.21-1.65 eV) semiconductors with sufficiently balanced molecular energetics for ambipolarity. Bottom-gate/top-contact OFETs fabricated via solutionshearing of 2OD-TTIFDM yield perfectly ambient stable ambipolar devices with reasonably balanced electron and hole mobilities of 0.13 cm2 /V·s and 0.01 cm2 /V·s, respectively with Ion/Ioff ratios of ~103 -104 , and 2OD-TTIFDK-based OFETs exhibit ambipolarity under vacuum with highly balanced (µe/µh ~ 2) electron and hole mobilities of 0.02 cm2 /V·s and 0.01 cm2 /V·s, respectively with Ion/Ioff ratios of ~105 -106 . Furthermore, complementary-like inverter circuits were demonstrated with the current ambipolar semiconductors resulting in high voltage gains of up to 80. Our findings clearly indicate that ambient-stability of ambipolar semiconductors is a function of molecular orbital energetics without being directly related to bulk ?-backbone structure. To the best of our knowledge, considering the processing, charge-transport and inverter ii characteristics, the current semiconductors stand out among the best performing ambipolar small molecules in the OFET and CMOS-like circuit literature. Our results provide an efficient approach in designing ultralow band-gap ambipolar small molecules with good solution-processibility and ambient-stability for various optoelectronic technologies including CMOS-like integrated circuits. In the second part of this thesis, a new solution-processable and air-stable liquidcrystalline n-channel organic semiconductor (?,?-2OD-TIFDMT) was designed, synthesized, and characterized. The new semiconductor exhibits a low LUMO energy level (-4.19 eV) and a narrow optical band gap (1.35 eV). Typical pseudo focal-conic fan-shaped texture of a hexagonal columnar liquid crystalline (LC) phase was observed over a wide temperature range from melting point at 139 °C to isotropic transition point at 232 °C. The semiconductor thin-films prepared by spin-coating ?,?-2OD-TIFDMT shows the formation of large (~0.5-1 µm sizes) and highly crystalline plate-like grains with good interconnectivity. The molecules were found to adopt edge-on orientation on the dielectric surface resulting in favorable charge-transporting networks of ?-? stacking along the dielectric-semiconductor interface. Top-contact/bottom-gate organic fieldeffect transistors fabricated by using the spin-coated semiconductor films, which were annealed at a low temperature (Tannealing = 50 °C), have yielded good electron mobilities as high as 0.11 cm2 /V·s and high Ion/Ioff ratios of 107 -108 with excellent ambient stability. This indicates two orders of magnitude (100×) enhancement in OFET mobility when compared with a low-temperature annealed well-known semiconductor, ß-DDTIFDMT. Side-chain engineering in the new semiconductor structure offers great advantage for the D-A-D ?-core co-planarity while maintaining a good solubility in organic solvents, and leads to favorable optoelectronic and physicochemical characteristics for better OFET performance. Thermal annealing at LC phase results in significant deterioration in charge-transport with much lower (10,000×) electron mobility. These remarkable findings demonstrate that this new small molecule is a promising semiconductor material for the development of n-channel OFETs on flexible plastic substrates and LC-state annealing in columnar liquid crystals can be deteriorating for transistor-type charge transport.