Exploring Doping Mechanisms and Modulating Carrier Concentration in Copper Iodide: Applications in Thermoelectric Materials

Abstract Due to its small hole‐effective mass, flexibility, and transparency, copper iodide (CuI) has emerged as a promising p ‐type alternative to the predominantly used n ‐type metal oxide semiconductors. However, the lack of effective doping methods hinders the utility of CuI in various applications. Sulfur (S)‐doping through liquid iodination is previously reported to significantly enhance electrical conductivity up to 511 S cm −1 . In this paper, the underlying doping mechanism with various S‐dopants is explored, and suggested a method for controlling electrical conductivity, which is important to various applications, especially thermoelectric (TE) materials. Subsequently, electric and TE properties are systematically controlled by adjusting the carrier concentration from 3.0 × 10 19 to 4.5 × 10 20 cm −3 , and accurately measured thermal conductivity with respect to carrier concentration and film thickness. Sulfur‐doped CuI (CuI:S) thin films exhibited a maximum power factor of 5.76 µW cm −1 K −2 at a carrier concentration of 1.3 × 10 20 cm −3 , and a TE figure of merit (ZT) of 0.25. Furthermore, a transparent and flexible TE power generator is developed, with an impressive output power density of 43 nW cm −2 at a temperature differential of 30 K. Mechanical durability tests validated the potential of CuI:S films in transparent and flexible TE applications.