Defect and dopant complex mediated high power factor in transparent selenium-doped copper iodide thin films

Copper iodide (CuI) is a promising p-type transparent thermoelectric material for near-room temperature energy harvesting. We report a high-power factor for selenium (Se)-doped CuI films. Ion beam-sputtered CuI films were doped using 30 keV ⁸⁰Se⁺ implantation with Se concentration varying between 0.50% and 6.50%. Hall effect measurements showed a ∼34% increase in electrical conductivity (σ ≈ 36.1 Ω⁻¹cm⁻¹) due to a ∼54% increase in carrier density (p_H ≈ 5.4 × 10¹⁹ cm⁻³) in the p-type γ-CuI film implanted with 5.0 × 10¹⁴ Se.cm⁻². A high Seebeck coefficient, α ≈ 388.9 μVK⁻¹⁻¹, and moderate electrical conductivity, σ ≈ 29.1 Ω⁻¹cm⁻¹, yield a nearly 85% increase in the power factor, α²σ ≈ 439.7 μWm⁻¹K⁻², for a 1.0 × 10¹⁵ Se.cm⁻² implanted film compared to the unimplanted film (α²σ ≈ 236.4 μWm⁻¹K⁻²). Monte Carlo simulation and ab initio density functional theory calculations revealed that the increased displacement per atom values and the {Se_I−V_Cu} defect complex-induced shallow acceptor could be attributed to the observed increase in hole density. Our results highlight that native defects and defect complexes are beneficial for enhancing the power factor in transparent CuI for thermoelectric applications.