Design and optimization of a heat engine based on a porphyrin single-molecule junction with graphene electrodes

A molecular heat engine is proposed based on a porphyrin single-molecule junction. The important thermoelectric transport characteristics, such as electrical conductance, thermopower, output power, and efficiency have been calculated by means of a Landauer-B\"uttiker approach combined with density functional theory. Different ways of engineering the heat engine performance have been investigated. (i) The efficiency of the heat engine can be considerably enhanced in special contact geometry between the molecule and electrodes that manifests a destructive quantum interference effect. (ii) Contrary to the usual length dependence of a single molecular junction, an increase of electrical conductance and thermopower with a number of porphyrin units have been found and used to optimize the molecular heat engine. (iii) The engine performance depends sensitively on the twisting angle between the molecule and graphene electrodes. Thus, a single molecular junction could be a promising candidate for a nanoscale heat engine utilizing quantum effect.