Design,Synthesis Of N-heteropentacene And Their Field-effect Transport Performances

In this thesis, we designed and synthesized a series of heteroacene derivatives. Their field-effect transistors (FETs) transport and nonlinear optical properties were characterized. A combined experimental and theoretical study was carried out to explore the relationships with molecular structures and optical or electrical properties of N-heteropentacene. We tuned the charge transport properties and nonlinear optical properties of heteropentacenes by chemically modified molecular structures. The finding of this work provides a general guidance to ensure the organic materials, which can be well applied for FET and NLO.The thesis contains the following seven parts:In chapter 1, we provided a review of organic field-effect transistors (FETs) materials. A literature survey is given on the recent developments and classification on organic semiconductors, and the techniques and principles associated with the fabrication of OFET devices using these materials have been discussed. This work aims to design and synthesis organic materials showing excellent photoelectric properties, and tune their OFET properties by chemical modification of molecular structures.In chapter 2, we described the design and synthesis of a new series of N-heteropentacene derivatives as organic semiconductors for OFETs. We designed and synthesized a new series of N-heteropentacene derivatives (4F2NTIPSP,4C12NTIPSP,4F2NTESP, and 4C12NTESP), their optical, electrochemical, thermal, molecular orbital energy and single-crystal properties were characterized. We optimized the preparation conditions of thin film OFET at different Tsub, different metal electrodes, different W/L. Meanwhile, molecular orientation and thin film morphology of these new materials were characterized by thin film XRD patterns and AFM images. The results show:4F2NTIPSP and 4C12NTIPSP exhibited excellent charge transport performance (electron mobility is higher the hole mobility) and possessed a good molecular orientation and thin film morphology for charge transport when Tsub=60℃, WL= 20, Au as source/drain electrodes. TIPS-substituted 4F2NTP and 4C12NTP have the same preparation condition of devices for OFET with former N-heteropentacene.4F2NTP and 4C12NTP showed good ambipolar transporting performance (high electron and low hole mobility,4F2NTP μh=0.14 cm2/V·s and μe=0.57 cm2/V·s,4C12NTP μh=0.07cm2/V·s and μe=0.83 cm2/V·s). The N-heteropentacene would provide an important molecular model for further theoretical study ambipolar transport performance of organic semiconductors.In chapter 3, we employed a series of N-heteropentacenes as molecular model to explore primary influence factors of ambipolar transport performance. A combined experimental and theoretical study was carried out to investigate the ambipolar charge transport process and charge injection process in organic field-effect transistors (OFETs) using a series of N-heteropentacenes (TP,1NTP,2NTP,4F1NTP,4C11NTP,4F2NTP,4C12NTP) as molecular model. The introduction of nitrogen atoms in the core and halogen atoms around the periphery of the pentacene framework can efficiently tune the frontier molecular orbital energy levels (HOMOs and LUMOs) of the N-heteropentacenes, and tune the carrier mobility and threshold voltage of OFET. By lowing their HOMO and LUMO energy levels, the OFETs of these molecules exhibited a transition from hole-dominant bipolar, to balanced ambipolar and to electron-dominant bipolar transport characteristics. Meanwhile, with the lowering of the frontier molecular orbital energy levels, the transistors also exhibited a decrease of the electron threshold voltage and an increase of the hole threshold voltage. Charge transport process was simulated based on Marcus theory and first principle molecular dynamics. This study revealed:(1) for the given device structure, the ratio of electron and hole mobilities of the ambipolar OFETs was strongly affected by the charge injection barrier. (2) An ideal ambipolar organic semiconductors should meet a relatively small LUMO-HOMO gap, moreover, the LUMO and HOMO should be at appropriate positions and molecular packing in thin film should favor both hole and electron transport. (3) Our work also indicates that the theoretical simulation based solely on the electron hopping process are not sufficient to describe the device performance of ambipolar OFETs and further developments of theoretical toolboxes to include the charge injection process are necessary for studying ambipolar organic electronics.In chapter 4, we investigated nonlinear optical properties of a series of N-heteropentacenes derivatives. By using Z-scan technique, we studied nonlinear optical properties of a series of N-heteropentacenes (TP,1NTP,2NTP,4F1NTP,4C11NTP,4F2NTP,4C12NTP) in solution and thin film. Using the modified time-resolved pump-probe technique and nanosecond flash photolysis method we explored and discussed nonlinear optical mechanism. This study revealed: (1) All the N-heteropentacenes showed excellent optical power properties in solution and thin film, and got high by theoretically fit. The nonlinear absorption coefficient β of 4C11NTP and 4C12NTP was one order of magnitude higher than that of C60. (2) We tuned the NLO performance by chemically modified molecular structures. (3) Picosecond pump-probe spectroscopy and nanosecond transient absorption spectroscopy reveals that the N-heteropentacenes domains long excited-state lifetimes to an excited state absorption (ESA) mechanism. The finding of this work provides a general guidance to look for excellent optical power limiting materials from the organic semiconductor material for OFET.In chapter 5, we introduced heteroatom into the acene framework to tune their field-effect transistors (FETs) transport properties. We lowered the frontier molecular orbital energy levels (HOMOs and LUMOs) of organic molecule by introducing heteroatoms (N, S, O) into the acene framework to improve stability of the molecule, and improve solubility by introducing long-alkyl chain or TIPS groups. Through above method we obtained highly stable, soluble heteroacene derivative (ADT, DQ and TIPS -DQ, and 2NThP and 1NThP). We hope these materials will possess excellent charge transport performance for OFET and giant nonlinear optical response for NLO.In chapter 6, we tried to synthesize air-stable n-type organic simconductors based on pentacene skeleton. We have tried expanding single benzene bearing 2-(1,3-dithiol-2-ylidene)malonitrile, and synthesize n-type charge transport performance (μe=6.6×10-3 cm2/V·s,Vth=14V, Ion/Ioff=4×102) base on small organic molecules to confirm the reaction occurred. The charge distribution of the atoms was calculated by Gaussian 09 program. We can expand five benzene base on pentacene bearing 2-(1,3-dithiol-2-ylidene)malonitrile to increase conjugation and lower molecular orbital energy for high-performance, ambient-stable n-type organic field-effect transistor.In chapter 7, we prospect the FET and NLO properties based on organic semiconductor materials.