Influence of carrier density and energy barrier scattering on a high Seebeck coefficient and power factor in transparent thermoelectric copper iodide

Transparent thermoelectric materials offer a synergetic performance for energy harvesting as smart windows. Among them, p-type copper iodide (CuI) is preferred due to its low synthesis temperature, moderate conductivity and mobility, and high optical transparency. X-ray absorption spectroscopy results showed a pre-edge feature in the Cu 2p_3/2 spectrum, which suggested the presence of Cu⁰-like defect states in γ-CuI films. Interface and grain boundaries of CuI and Cu⁰ act as a potential energy barrier for energy filtering of charge carriers, which along with the decrease in charge carrier density enhanced the Seebeck coefficient, α. The α value increased by 298% upon annealing at 100 °C, α = 789.5 μVK⁻¹, which resulted in a 480% increase in the power factor (α²σ = 740.9 μWm⁻1 K⁻²). Our results showed that a high Seebeck coefficient resulted from a decrease in charge carrier density and energy filtering of charge carriers at the interface and grain boundaries in optically transparent (T_visible ∼ 60−85%) γ-CuI films for energy harvesting as smart windows.