Insights into stability, transport, and thermoelectric properties of transparent p-type copper iodide thin films

Abstract

Despite the potential applications of γ-CuI thin films that involve significant temperature variations, few studies focus on the thermal stability of this material. We carried out post deposition annealing at different temperatures under argon and air atmospheres on γ-CuI thin films to stabilize and optimize the optoelectronic and thermoelectric performances. Electrical temperature measurements reveal an irreversible evolution during the first annealing treatment, with a systematic decrease of the conductivity irrespective of the atmosphere used. The annealing process under argon at 300 °C followed by air at 150 °C allowed stabilizing and optimizing the electrical conductivity at 152 S m⁻¹. The measurement of the Seebeck coefficient (S) of the stabilized film at various temperatures (S = 287 μV K⁻¹ at 44 °C and S = 711 μV K⁻¹ at 139 °C) confirmed the p-type degenerated semiconductor behavior of the γ-CuI thin films. Light hole (3.37 × 10¹⁸ cm⁻³) and heavy hole (7.26 × 10¹⁹ cm⁻³) concentrations are calculated and attributed to the presence of and defects. Hall effect measurements confirm the light hole carrier density and mobility. The total transmittance of the stabilized γ-CuI thin films is 65%, giving a transparent conducting Haacke's Factor of Merit FOM = 7 × 10⁻⁷ Ω⁻¹. A thermoelectric power factor of PF = 12 μW m⁻¹ K² is obtained at close to room temperature, and reaches 66 μW m⁻¹ K⁻¹ at 139 °C. In view of its interesting combined conduction, transparency and thermoelectric properties, a new coefficient of performance for transparent thermoelectric materials (COP_TTE) is proposed. COP_TTE = 8.3 × 10⁻¹² A² m⁻¹ K⁻² is calculated at room temperature for the stabilized γ-CuI thin film. This result is discussed with respect to the performances noted in the literature on other p-type materials.