Cowen, Lewis Matthew; (2021) N-type organic semiconductors and conductors for plastic electronics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Organic semiconductors have begun to find a place as viable active materials for a new generation of devices. Throughout the development of this relatively new class of materials, it has been apparent that optimising the performance of n-type semiconductors and dopants has been a challenge compared to their p-type counterparts. Many applications in photovoltaics, thermoelectrics, light-emitting diodes and more, require or can benefit from n-type components. To really benefit from the processability and scalability of organic semiconductors in industry it is therefore necessary to develop stable and high-performing n-type materials. This thesis aims to advance the understanding of promising n-type semiconductor and dopant pairs through structural characterisation. A new “self-doping” naphthalene diimide conductor, NDI-OH, was designed and synthesised based on previously made structures which exhibited surprisingly stable radical anions. A full spectroscopic characterisation of the self-dopant revealed that it is the product of hydrolysis after ion-exchange in basic conditions. This was found to have a profound effect on the mechanism of charge transfer between dopant and semiconducting moieties and on the lifetime of charge carriers. It was postulated that the mixture of compounds present in doped films is likely to contribute to charge transport pathways and the electronic conductivity. The interactions between n-type semiconductors and fluoride anions was also explored. Reactions between solvent and fluoride salt were found to have an impact on the efficiency and nature of the subsequent charge transfer on to BDOPV semiconductors. The mechanism of doping relied on factors including the acidity of solvent and the presence of water. Spectroscopic characterisation was carried out in order to determine the active compounds involved in charge transfer between tetrabutylammonium fluoride and BDOPV. Finally, two new n-type polymers, PbTzTT and PbTz3, were designed with the aim of exhibiting a high degree of long-range order in the solid-state. The reactivity of thiazoles and fused thiazolo[5,4-d]thiazole heterocycles was explored at length. The ring-opening of these compounds on metalation and the possibilities of electrophilic substitution were especially of interest. The eventual successful synthesis of the polymer PbTzTT is described via a Stille cross-coupling polymerisation. The PbTz3 macromonomer was also obtained through a double ring-closing condensation. A comparison of the optoelectronic properties of PbTzTT with the structurally analogous p-type polymer PbTTT was also made.

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