The role of sulfur valency on thermoelectric properties of sulfur ion implanted copper iodide

Copper iodide (CuI) is a promising semiconductor with many potential applications in the fields of opto-electronics, solar cells, photodetectors and thermoelectric for energy harvesting. 30 keV 32S+ ions were implanted with fluences between 5.0 × 1014 and 1.1 × 1016 ions cm−2 which results in 0.3–5.5 at% peak sulfur concentration at an average depth of ~ 30 nm. Sulfur implantation increased electrical conductivity by up to 100 % (42.3 Ω−1 cm−1) for film implanted with the highest fluence (1.1 × 1016 ions cm−2), which was due to an increase in hole density of one order of magnitude. The thermoelectric power factor increased by over 300 % for low-fluence (≤1.0 × 1015 ions cm−2) implanted CuI films which is attributed to a simultaneous 22 % increase in electrical conductivity (25.9 Ω−1 cm−1) and a 95 % increase in the Seebeck coefficient (498.6 µVK−1), which overcomes the typical trade-off. XPS results showed that doped sulfur ions are in S2- states for low-concentration doped films, whereas for high-fluence (≥ 2.5 × 1015 ions cm−2) implanted films, a fraction of the sulfur ions are also present in an elemental state. Sulfur solubility impacts its valency which alters the defect chemistry and thermoelectric properties of the sulfur implanted CuI films. Sulfur doping by ion implantation is a promising strategy to improve the electrical conductivity and power factor in wide bandgap thermoelectric materials.