Synthesis And Characterization Of Ambipolar And N-Type Organic Semiconductors

Organic field-effect transistors (OFETs) have attracted much attention in recent years owing to the advantages such as mechanical flexibility and potential low-cost large-area fabrication, leading to possible applications in future electronic devices. The performances of OFETs are largely dependent on the characteristics of the organic semiconductors used as the active layer in the devices. Great numbers of excellent p-type organic semiconductor materials have been obtained and applied in organic electronic devices. However, in contrast to the large number of p-type materials reported in literature, much fewer ambipolar or air-stable n-type organic semiconductors have been studied.In this thesis, we conducted an investigation on developing new ambipolar and air-stable n-type organic semiconductors. We designed and synthesized one series of ambipolar semiconductors and three series of n-type semiconductors. The physical and chemical properties of these new materials have been studied. The main content of this thesis includes the following parts:1. A series of s-indaceno [l,2-b:5,6-b']dithiophene-4,9-dione derivatives with different n-alkyl groups have been synthesized. The ambipolar transport properities of these compounds have been studied by experimental and theoretical investigations. This work indicated that unipolar organic semiconductors may be converted into ambipolar materials by changing the packing motif of the conjugated cores and the thin-film microstructures. The optical and electrochemical measurements suggest that the three organic semiconductors have very similar energy levels; however, their OFETs exhibit dramatically different transport characteristics. Transistors based on compound la or lc show ambipolar transport properties, while those based on compound lb show p-type unipolar behavior. Specifically, compound lc is characterized as a good ambipolar semiconductor with the highest electron mobility of 0.22 cm2 V-1 s-1 and the highest hole mobility of 0.03 cm2 V-1 s-1. Complementary metal oxide semiconductor (CMOS) inverters incorporated with compound lc show sharp inversions with high gains above 50. Theoretical investigations reveal that the drastic difference in the transport properties of the three materials is due to the difference in their molecular packing and film microstructures.2. We designed and synthesized a new seires of n-type semiconductors based on s-indaceno [1,2-b:5,6-b'] dithiophene-4,9-diylidene) dimalononitrile framework with differernt alkyl groups. The OFETs devices could be fabricated by vacuum deposition owing to the good thermal stability of the materials. The highest electron mobility of the three materials is 0.14 cm2V-1s-1. The devices show similar properties by using different metal electrode (Au or Ag). The performance of OFETs could be improved by vacuum sublimation purification of the materials and increasing the substrate temperature. These materials have the similar energy levels with C60, therefore could be applied in organic solar cells as accepter.3. We designed and synthesized three derivatives based on thieno [3,4-c]pyrrole-4,6-dione (TPD) structure, and studied their FET and nonlinear optical properites. The OFET devices could be fabricated by spin-coating method followed by annealing at different temperatures. Dicyanomethylene-substituted TPD-containing quinoidal molecule (TPD-T-CN) is an air-stable n-type organic semiconductor, while the different end-substituted compounds (TPD-2TT-H/C6) are p-type materials. All the three materials have nonlinear optical properties, and TPD-T-CN shows the best performance.4. A new six nitrogen-containing dihydroazapentacene (DHHAP) has been synthesized through a consise synthetic method. The tautomerism and pH response properties of DHHAP have been studied. The studies by UV-vis and NMR spectroscopy expounded a uniqued tautomerism process. Theoretical investigations of the aromaticity indicate the DHHAP and derivatives are'globally aromatic' molecules. In addition, the nonlinear optical properties of DHHAP were also investigated.5. Small conjugated molecules containing different hetero-atoms have been designed and synthesized, including oligothiophene with carbonyl, five-ring fused compounds with N, S atoms, and N, O doped heteropentacene. The basic properties of those molecules have been studied. We expect these organic materials could show ambipolar or n-type transport properties and be applied in OFETs.