Organic semiconductors for organic thermoelectric application

A large proportion of energy generated from the burning of fossil fuels is wasted as heat with as much as 60% of globally produced energy wasted in this manner. Much research has been conducted on the use of thermoelectric generators as a means of harvesting this wasted energy and converting it back into electricity. Thermoelectric energy generation has also been identified as a means of generating power from various sources such as water pipes, windows and in wearable devices using heat from the body. Thermoelectric devices generally require two components, a positive charge carrying (p-type) semiconductor and a negative charge carrying (n-type) semiconductor. Inorganic semiconductors have proved useful for use in generators which are intended to operate at high temperatures however over 50 % of waste heat is low temperature, i.e. less than 250 C. To efficiently recover this low temperature heat, semiconductors which can be easily processed for large area applications are required; conjugated organic polymers have been identified as potential candidates for these low temperature applications since they are made from relatively abundant materials and can be processed from solution allowing for deposition over large areas by printing. Suitable organic n-type materials for thermoelectric applications are currently lacking when compared to available p-type materials and the purpose of this project is to design and synthesise a library of polymers which can be used as the n-type component of thermoelectric devices.

For n-type materials a dopant is required to reduce the charge carrying species by donating electrons, this means that n-type polymers require a high electron affinity (EA) to allow for easy uptake of electrons upon doping. The requirement for a high EA has meant that stable n-type materials have been difficult to produce. In addition to this using an extrinsic molecular species as the dopant can often cause problems with phase separation in processing. Recently however it has been found that covalently bonding the dopant species to the semiconductor is a useful strategy for avoiding diffusion since both dopant and polymer are part of the same molecular species. Good charge mobility along the polymer backbone is also important and can be achieved by a high amount of planarity which should lead to reduced rotational disorder and enhanced wave function delocalization. The aim of this project is to design and synthesise stable polymers with low electron affinities, high backbone planarity and sites which allow for covalent bonding with dopant species. The ease with which these materials can then be solution processed will allow the fabrication of conformal thermoelectric generators and the efficient exploitation of low temperature waste heat as an alternative energy source.