Synthesis Of N-Heteropentacenes For High-Performance Ambipolar Organic Semicoductors

Ever since the first organic field-effect transistor (OFETs) was reported, interest in this field has risen steadily for both technological and scientific considerations. The majority of the available organic semiconductors are unipolar, ie. they exhibit only hole-transport (p-type) or electron-transport (n-type) property; and few organic semiconductors have been found to exhibit ambipolar characteristics. Since both electron and hole transports can take place within the ambipolar materials, there is no need to selectively deposit n-and p-channel materials by advanced patterning techniques, as a result, the device fabrication can be significantly simplified. Among the few reported ambipolar organic semiconductors, very few show high and balanced hole and electron mobilities.In this thesis, we explore a new strategy of design high performance ambipolar organic semiconductors based on azapentacene framework. By tuning their energy levels and packing arrangement in solid state, we successfully obtained several organic semiconductors with high and balanced hole and electron mobilities. DFT calculation has been employed to understand the effect of molecular structure on the carrier injection barriers and reorganization energy, which factors strongly affect charge mobilities within the devices. The finding of this work provides a general guideline for designing balanced high performance ambipolar organic semiconductors based on N-heteropentacenes.The thesis contains the following parts:In chapter 1, a literature survey is given on the recent developments on organic semiconductors. Current research status on the p-type, n-type and ambipolar organic materials are reviewed, the techniques and principles associated with the fabrication of OFET devices using these materials have been discussed.In chapter 2, we reported the design and synthesis of a new series of soluble, stable N-heteropentacenes. The solubility, thermal and photooxidative stabilities, optical and electrochemical properties and single-crystal arrangements of these new materials were characterized, allowing an understanding to the relationships between molecular structure and various physical properties to be established. The introduction of N-atoms is an efficient mean to tune the HOMO and LUMO energy level. TIPS-groups in the backbone not only imparted solubility but also enhancedπ-stacking in the solid state.In chapter 3, The OFET devices of the newly synthesized N-heteropenacenes were fabricated and their charge transport properties were studied. When using Au, Ag, Cu and Al as source/drain electrodes, all the new materials exhibit ambipolar transport properties and some of them show very high charge carrier mobility. When using Au as electrode material, compounds ADPD,4ClAzaPen and 4FAzaPen show high and balanced hole and electron carrier mobilities above 0.20 cm2 V-1 s-1. The AzaPen gave a hole mobility as high as 0.79 cm2 V-1 s-1 when using Ag as electrodes.In chapter 4, the key factors affecting the charge carrier properties of organic semiconductors were studied by DFT-based theoretical investigations, which help to establish the basic principles for designing ambipolar materials. Introduction of N atoms and halogen atoms to the pentacene framework would decrease the LUMO and HOMO energy levels, but increase the reorganization energy (λ), ionization energy (IE) and electron affinity (EA).In chapter 5, two soluble hexaethynyl-pentacene molecules were designed and synthesized. Introduction of six ethynyl-groups in the backbone of pentacene dramatically enlarge the conjugated systems, resulted in narrow band gaps in solution (1.65 eV). The crystal properties of the compounds were studied and the potential applications in single-crystal FET devices were explored.