CuI: A Promising Halide for Thermoelectric Applications below 373 K

Highly transparent p-type γ-CuI thin films with enhanced thermoelectric (TE) properties were produced by the pulsed laser deposition (PLD) technique. The film composed of fine nanoparticles was found to show transparency close to 90% and excellent TE performance at 300–360 K. Postannealing at three different temperatures in the range of 373–573 K in a vacuum was found to diminish the deviation from the stoichiometric composition, namely, δ in Cu1−δI, possibly due to iodine evaporation. The carrier (hole) concentration decreased and the Hall mobility increased on increasing the postannealing temperature, i.e., by decreasing δ. The film postannealed at 373 K showed the best TE performance with a high electrical conductivity of ∼14 000 S/m, a large Seebeck coefficient of ∼350 μV/K, and a high power factor of ∼1600 μW/(m K2) at 300 K. The degeneracy of the heavy- and light-hole bands of γ-CuI could enhance the Seebeck coefficient through enhancing the effective mass of holes and decreasing the carrier concentration, while electrical conductivity was only slightly decreased. Thermal conductivity of the γ-CuI thin films was verified to be as low as 0.77–0.83 W/(m K) at 300 K. The present study firmly demonstrates the high potential of the γ-CuI thin film as a p-type TE material to be paired with an n-type material for flexible TE devices and self-powered electronic systems operating at 300–360 K.