Theoretical Investigation Of The Doping Effect On The Thermoelectric Properties Of N-type Coordination Polymer

The thermoelectric materials,which can interconvert heat and electricity directly,has gained enormous attention due to the growing demand on green energy.Organic materials are becoming emerging thermoelectrics owing to their low thermal conductivity,low toxicity,and low cost.Doping is usually required to improve the figure of merit(z T)of organic thermoelectric materials(OTEs).Depending on the carriers after doping,the OTEs are classified into n-type and p-type materials.Rapid progress has been made in developing p-type OTEs.While efficient n-type materials are rare.The first efficient n-type OTE reported is the potassium-doped poly(nickel-ethylenetetrathiolate)(poly[K_x(Ni-ett)]),synthesized by Zhu et al.in 2012.Understanding the doping effect on charge transport is important for improving the thermoelectric efficiency.However,the investigation might be difficult in experiment,since the intrinsic properties of materials can be concealed by the mophology or other factors.Here,we investigate the thermoelectric properties of poly[K_x(Ni-ett)]based on the density functional theory combined with the Boltzmann transport equation.To evaluate the doping effect,models with different doping levels are constructed by adjusting the molar ratio of dopant to monomers.Our electronic structure calculation reveals a delocalized conduction band(CB)in pristine poly[Ni-ett],which benefits the charge transport.However,the localized polaron is formed after doping due to the charge pinning effect,which reduces the conductivity significantly.Nevertheless,in the lightly-doped poly[K_x(Ni-ett)],the small gap between the Fermi level and the CB allows the electron localized on the electron polaron band(EPB)to be thermally activated to the upper CB,leading to the unusual temperature dependence of Seebeck coefficient.The half-filled EPB dominates the charge transport at low tempertaures,giving rise to the low Seebeck coefficient.At higher temperatures,the CB with few electrons activated to it starts to get involved in the charge transport,causing the suddenly elevated Seebeck coeffcient.While at further higher temperatures,the CB starts to dominate.When the CB is occupied by more and more electrons,the Seebeck coefficient then declines with the temperature.Such nonmonotoic temperature dependence was observed in the experiment,which according to our result is primarily due to the formation of polaron and the participation of CB.This is helpful to the further understanding of doping effects on thermoelectric transport in conducting polymers.