Interplay Among Thermoelectric Properties, Atmospheric Stability, and Electronic Structures in Solution-Deposited Thin Films of P(Na_X[Niett])

Recent progress in organic thermoelectric generators has resulted in high performance through chemical control of the redox levels of organic semiconductors, but the scarcity of good candidates for soluble organic n-type materials limits the use of π-leg structures consisting of complementary elements of p- and n-type materials because of unbalanced transport coefficients that lead to power losses. A sol–gel technique with oxidation is demonstrated to be effective for the fabrication of smooth films of poly(nickel 1,1,2,2-ethenetetrathiolate), P(M_X[Niett]), which is known as an insoluble material. By controlling the annealing temperature, a maximum n-type power factor of 10.1 μW m⁻¹ K⁻² at 100 °C (Seebeck coefficient of −35.4 μV K⁻¹ and electrical conductivity of 80.3 S cm⁻¹) is demonstrated. Chemical and electronic structures of P(M_X[Niett]) thin films are explored based on changes in optical absorption, ultraviolet, and X-ray photoelectron spectroscopies to understand the origin of the high atmospheric stability. Analysis indicates that the stability arises from electronic states of the doped state that satisfy a balance with the redox potential of the air components and that stability under repeated redox reactions is an important factor for obtaining the n-type organic thermoelectric materials that are highly stable in air.