Assessing the thermoelectric properties of CuxO (x = 1 to 2) thin films as a function of composition

Abstract

Series of CuxO thin-films in the entire range of compositions 1≤x≤2 were obtained by varying the oxygen flux in an rf-sputter deposition process. Growth windows for three crystalline phases, i.e., the thermodynamically stable cuprous oxide Cu2O and cupric oxide CuO as well as the metastable paramelaconite Cu4O3, were observed. The crystalline phases persist non-stoichiometrically over a wide range of compositions. These flux-range windows are separated by ranges where highly disordered, almost amorphous material is obtained. All samples were analysed with respect to their thermoelectric properties, i.e., Seebeck coefficient, electrical, and thermal conductivity. Clear trends of these transport parameters were found and used to determine the thermoelectric figure of merit ZT. The ZT-values at room temperature are highest for the two thermodynamically stable crystalline phases CuO and Cu2O.

Lattice dynamical properties of antiferromagnetic MnO, CoO, and NiO, and the lattice thermal conductivity of NiO

“…Another study, this time on thin films of CuO by Hartung et al has similar values of S = 550 μV/K and σ = 0.3 (Ω m) −1 , but with significantly smaller carrier concentation ρ = 2 × 10 17 cm −3 . 31 Comparing that to our calculations, our σ drops down to 4.1 (Ω m) −1 with the 1 fs relaxation time, much closer to experiments. Even though the match on electrical conductivity improves with smaller carrier concentrations, our calculations overshoot the Seebeck coefficient, rising up to 860 μV/K.…”

“…Specifically, earth-abundant transition metals such as zinc and copper have received a lot of attention. [27][28][29][30][31][32][33] In addition to being comprised of abundant and non-toxic elements, transition metal oxides are generally very stable at higher temperatures in air, a property that partly mitigates the lower zT by allowing the Carnot prefactor in Equation 1.1 to be larger. By exploiting the strategies described in earlier sections, the thermoelectric efficiency of transition metal oxide materials has improved significantly during the last few decades.…”

“…Comparisons with experiments are further complicated by the carrier concentrations in the experimental studies possibly not being entirely accurate, as Hartung et al pointed out in their study, and Jeong and Choi have estimated ρ only on the atomic density of copper in the samples. 31,140 Electrical properties of nickel(II) oxide have been studied immensely, owing much to its characteristic strongly correlated d-metal oxide behaviour. The experimental measurements have massive variation with different levels of sample purity, as can be seen in the 6 orders of magnitude difference in σ at the temperature of 600 K in the comprehensive review of Keem and Honig.…”

Thermoelectric Properties of p-Type Cu2O, CuO, and NiO from Hybrid Density Functional Theory

“…Our calculations with T = 300 K at a hole concentration of 3 × 10 15 cm –3 predict a thermopower of 1200 μV K –1 , whereas their measurements reach around 900 μV K –1 . 71 They do mention in the paper, however, that their values on the hole concentration should be taken with a grain of salt as the assumption of diffusive transport might not hold. Additionally, the phase purity of the thin film samples will surely not match the perfect Cu 2 O crystal we have in our calculations.…”

“…71 This would bring the conductivities better in line with our RT approximation results as we obtain for this hole concentration σ = 4.1 (Ω m) −1 using τ i , k = 1 fs, although our Seebeck coefficient would then be rather heavily overestimated with S = 860 μV K –1 at a temperature of 300 K. With the same ρ as in the Jeong and Choi’s single crystal study, Hartung et al. have a Seebeck coefficient of 300 μV K –1 when we have 330 μV K –1 , and higher concentrations up to 1 × 10 21 cm –3 bring S down to around 200 μV K –1 for the thin film study and 135 μV K –1 for the theoretical results, both at a temperature of 300 K.…”