Precision Doping of Iodine for Highly Conductive Copper(I) Iodide Suitable for the Spray-Printable Thermoelectric Power Generators

A highly conductive copper iodide (CuI) film with excellent thermoelectric (TE) performance was simply prepared by using the solution-processable spray printing technique for the fabrication of transparent and flexible TE generators (TEGs). Compensating for sublimation- or evaporation-induced iodine loss during the spray printing process, the addition of a small amount (1 at. %) of iodine (I2) into the CuI precursor solution was dramatically effective for obtaining CuI films showing the excellent electrical conductivity of 207.6 S·cm–1 and power factor of 673.3 μW·m–1·K–2, resulting in a high zT value of 0.31. Importantly, added I2 played a critical role in increasing the electrical conductivity by providing iodide to improve the Cu to I stoichiometry in the CuI films and improve crystallite growth. Using 1 at. % I2 doping, two different flexible TEGs (one coiled and one foldable) were successfully fabricated that showed a high output power density of 236.5 μW·cm–2 at a temperature gradient of 22 °C and high sensitivity to small temperature changes induced by a halogen lamp, respectively. This efficient and facile approach to fabricating solution-processable transparent conductors with high TE performance suggests potential applicability to the development of energy-harvesting windows or screens.