The development of electron deficient materials for organic electronics applications

This thesis reports on the development of electron deficient (n-type) small molecule and polymer semiconductors. Firstly, the synthesis of a new electron deficient 4,5,6-trifluoro-2,1,3-benzothiadiazole (TFBT) end group is presented. Coupling of TFBT to an electron rich indacenodithiophene (IDT) core, through direct arylation conditions, affords a TFBT IDT material which performs as a poorly ambipolar semiconductor in organic field effect transistor (OFET) devices. A range of related TFBT-based materials with expanded IDT or cyclopentadithiophene (CDT) cores were prepared and characterised. A six-fold nucleophilic aromatic substitution reaction with cyanide was developed and applied to all fluorinated materials. This one step modification resulted in the formation of 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT) end group. This modification dramatically changed structural, optoelectronic and semiconducting properties compared to their fluorinated counterparts. This highlights the importance of strong π-acceptors, like cyano groups, in influencing electron accepting properties, compared to inductively withdrawing fluorine atoms. TCNBT-based semiconductors were utilised in a range of applications such as organic field effect transistors (OFETs) and organic photodetectors (OPDs), demonstrating good stability and high electron mobility. The strong electron accepting properties of the TCNBT end group resulted in low band gap materials that strongly absorb in the NIR range and were utilised to afford semi transparent electronic devices. A range of electron deficient donor-acceptor (D-A) type polymers were also synthesised, based on benzothiadiazole, functionalised with a mixture of cyano, nitro and fluoro groups. This study highlights the importance of molecular engineering and how small structural modifications have a great impact on the nature of the resulting semiconductor. More specifically, the effect of fluorine atoms and their influence on backbone planarity is shown to affect charge transport properties. On the other hand, cyano groups cause backbone twisting that is not easily overcome in the solid state and in turn disrupts charge transport in the polymer backbone. When applied to OFET and organic photovoltaic (OPV) devices, these polymers performed differently to the small molecules presented in this thesis, with fully cyanated polymers showing lower electron mobilities compared to the ones containing fluorine atoms. This demonstrates that the impact of cyano groups have a different effect in polymeric compared to small molecule systems.Open Acces

Hybrid Organic-Inorganic Coordination Complexes as Tunable Optical Response Materials.

Novel lead and bismuth dipyrido complexes have been synthesized and characterized by single-crystal X-ray diffraction, which shows their structures to be directed by highly oriented π-stacking of planar fully conjugated organic ligands. Optical band gaps are influenced by the identity of both the organic and inorganic component. Density functional theory calculations show optical excitation leads to exciton separation between inorganic and organic components. Using UV-vis, photoluminescence, and X-ray photoemission spectroscopies, we have determined the materials' frontier energy levels and show their suitability for photovoltaic device fabrication by use of electron- and hole-transport materials such as TiO2 and spiro-OMeTAD respectively. Such organic/inorganic hybrid materials promise greater electronic tunability than the inflexible methylammonium lead iodide structure through variation of both the metal and organic components