Novel boron-nitrogen coordinated polymers as next-generation n-type thermoelectric materials

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Despite the high interest in new, greener energy sources, we are not using our current energy sources efficiently: the amount of waste heat generated in the UK in 2019 equals approximately 10% of the national electricity demand in the same year. In addition to finding new energy sources, recycling unused energy, especially waste heat which is attributed to the majority of the energy loss, is also vital for a more sustainable world in the future. Thermoelectric generators can capture the waste heat and convert it to electricity, hence their importance in contributing to sustainability has been highlighted in recent years. Organic thermoelectric generators, which are built up with organic polymeric p-type and n-type thermoelectric material couples, are more favoured over conventional heavy-metal materials concerning sustainability. Although many good p-type thermoelectric materials are established, a lack of good n-type materials lags the development of organic thermoelectric generators. This is because good n-type materials require low-lying HOMO/LUMO energies and narrow band gaps for effective charge transfers and decent air stabilities. So far only a few groups of compounds meet the criteria. This project will work on a new group of n-type organic thermoelectric materials: boron-nitrogen coordinated polymers. The B-N coordination unit has been applied in several families of electronic materials as an effective way to boost up the electron affinities but has almost not been exploited in the thermoelectric field. Introducing the B-N coordination unit to organic thermoelectric materials may allow us to discover more high-performance n-type thermoelectric materials.This project aims to develop conjugated polymers bearing boron units within the structures that are either novel or yet to be studied as n-type thermoelectric materials. In general, the identified targets will be synthesised, characterised and made into thin films. Intrinsic thermoelectric properties will be measured to determine if targets display good n-type performance. When possible, the developed materials will be paired up with commercially available, high-performance p-type thermoelectric materials, such as PEDOT:PSS, to build proof-of-concept thermoelectric generators to showcase energy harvesting abilities. Approaches towards the targets will be the following four routes, with target novelties and technical difficulties gradually increasing from Route 1 to Route 4:Route 1: Thermoelectric performance of known boron-nitrogen based polymers. Initially published n-type B-N coordinated polymers that have been exploited in fields other than thermoelectric materials but show promising properties (e.g. low-lying frontier orbitals, narrow band gaps) that fit the requirements for good n-type thermoelectric materials will be trialled.Route 2: B-N decoration on known n-type thermoelectric polymers. This route will focus on known examples of n-type thermoelectric polymers and discover the extent of enhancement on the n-type thermoelectric performance by incorporating B-N coordination units into the structures.Route 3: Optimisation on developed targets. Optimisation methods like adding electron-withdrawing groups and extending the length of sidechains are commonly used on n-type thermoelectric materials. Therefore, this route will further optimise the compounds developed from Route 1 and 2 to maximise their performance. Computational calculations (e.g. DFT) will evaluate the design, particularly the HOMO/LUMO levels, before targets are synthesised.Route 4: Polymers bearing other coordination units. In addition to B-N coordination, other types of coordination units, such as B-O, might do a better job than B-N in n-type thermoelectric materials. Likewise, calculations in silico will be done as a proof of concept before synthesis.