Thermoelectric properties of Ag-doped CuI: a temperature dependent optical phonon study

Due to the natural abundance and non-toxicity of copper (Cu) and iodine (i), gamma-CuI has been widely explored as a potential transparent thermoelectric material for near room temperature applications. Here, we report the effect of doping of an heavy atom such as silver (Ag) on the evolution of temperature-dependent optical phonon modes and thermoelectric properties of chemically synthesized single-phase nanocrystalline gamma-CuI. We found that Ag doping reduces the lattice parameters of CuI and thereby confirms the occupancy of Ag atoms at the vacancy sites of CuI. The decrease in phonon lifetime with the increase in temperature, which strongly influences the lattice thermal conductivity, is established from temperature-dependent optical phonon vibrations study. The four-phonon/Umklapp scattering is found to be more prominent in undoped CuI, whereas three-phonon scattering is prominent in Ag-doped CuI. At low temperatures, an almost 90% increase in the Seebeck coefficient is observed for Ag-doped CuI compared to undoped CuI, which can be understood by taking into account a net decrease in the hole carrier concentration in doped CuI.