Synthesis and study of novel small organic molecules with optoelectronic and energy storage applications

Abstract

This thesis describes the synthesis and characterisation of novel small organic molecules with potential optoelectronic and energy storage applications. The first chapter presents an introduction about organic semiconductors, their working principle and the common types of technological applications to be utilized in. Examples of some of the best performing materials for each technology are also described. The second chapter describes the synthesis of new thiophene containing small molecules as potential active materials for organic photovoltaics (OPVs). 3,4-Ethylenedioxythiophene (EDOT) is a thiophene derivative with promising optical and electrochemical properties that show potential application as a p-type material in OPVs. The chapter reports the synthesis and the performance of the first generation of EDOT p-type derivatives, SAM-72 and SAM-80 with a FF of ⁓60% and PCEs over 2%. An attempt to synthesise a second generation of these materials aiming to improve the PCE of the first generation is also described in this chapter. The synthesis of a class of thieno[3,4-c]pyrrole-4,6-dione (TPD) based materials that show potential application as p-type for OPVs are also reported along with their optical and electrochemical properties. The third chapter describes the synthesis and characterization of tetrathiafulvalene (TTF) based organic small molecules for OPV and ORFB applications. Among these, two molecules containing TTF fused with benzothiadiazole moiety as p-type materials for OPVs are described. The materials showed instability very likely to be caused by light exposure which prevent further studies on OPV devices. The chapter also describes the synthesis and characterization of a novel TTF-based small molecule for aqueous organic redox flow batteries (ORFBs). The fourth chapter reports the synthesis of small molecules based on the triphenylamine (TPA) unit as potential hole transporting materials (HTMs) for perovskite and solid-state dye-sensitized solar cells (ss-DSSCs). The materials combine TPA unit with either ferrocene (Fc) or TTF moieties due to their promising redox properties. Among these materials, compound 191 that was fabricated for both perovskite and ss-DSSCs and obtained PCE of over 6% and 0.158% for both applications, respectively. The chapter also reports the synthesis and the characterization of TPA-based small molecules as a potential p-type for OPVs. The synthesis of a cross-linkable TPA-based small molecule functionalized with styrene unit is also described, along with its optical and electrochemical properties. Finally, the last chapter describes the synthesis of several violanthrone derivatives in order to explore their potential as n-type materials for OPVs. Three simple small molecules were synthesised, and their optical and electrochemical properties are studied. Among these materials, compound 222 that was utilized as n-type active material for OPVs and showed a low PCE of less than 0.1%. The low efficiency was attributed to the low solubility of the material hindering its fabrication with p-type materials of appropriate energy level alignment. Solubility optimization of the material was reported along with the fabrication of 231 derivative for organic field-effect transistors (OFETs). Surprisingly, the material showed a preference for p-type charge conductivity which might explain the poor performance in OPVs.