Exceptional Thermoelectric Performance of Cu2(Zn,Fe,Cd)SnS4 Thin Films

Abstract

High-quality Cu₂(Zn,Fe,Cd)SnS₄ (CZFCTS) thin films based on the parent CZTS were prepared by aerosol-assisted chemical vapor deposition (AACVD). Substitution of Zn by Fe and Cd significantly improved the electrical transport properties, and monophasic CZFCTS thin films exhibited a maximum power factor (PF) of ∼0.22 μW cm^-1 K^-2 at 575 K. The quality and performance of the CZFCTS thin films were further improved by postdeposition annealing. CZFCTS thin films annealed for 24 h showed a significantly enhanced maximum PF of ∼2.4 μW cm^-1 K^-2 at 575 K. This is higher than all reported values for single-phase quaternary sulfide (Cu₂BSnS₄, B = Mn, Fe, Co, Ni) thin films and even exceeds the PF for most polycrystalline bulk materials of these sulfides. Density functional theory (DFT) calculations were performed to understand the impact of Cd and Fe substitution on the electronic properties of CZTS. It was predicted that CZFCTS would have a smaller band gap than CZTS and a higher density of states (DoS) near the Fermi level. The thermal conductivity and thermoelectric figure of merit (zT) of the CZFCTS thin films have been evaluated, yielding an estimated maximum zT range of 0.18-0.69 at 550 K. The simple processing route and improved thermoelectric performance make CZFCTS thin films extremely promising for thermoelectric energy generation.

Keywords: Cu2ZnSnS4 (CZTS); Fe and Cd doping; aerosol-assisted chemical vapor deposition (AACVD); density functional theory; thermoelectrics; thin films.

Figure 1

(a) Grazing-incidence X-ray diffraction (GIXRD) patterns for Cu₂ZnSnS₄ (CZTS) and Cu₂(Zn,Fe,Cd)SnS₄ (CZFCTS) thin films prepared using different molecular precursor ratios. (b–e) Scanning electron microscopy (SEM) images and cross-sectional SEM images of the thin films of (b) CZTS, (c) CZFCTS-1, (d) CZFCTS-2, and (e) CZFCTS-3. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns of the CZFCTS-1 thin film: (f) Low-magnification bright-field TEM image, (g) SAED pattern taken from the entire area shown in (f) but cropped to concentrate on the core details, (h) HRTEM image, and (i) HRTEM image taken from the white region in (h) and FFT image (inset).

Figure 2

High-resolution X-ray photoelectron spectroscopy (XPS) measurements of the Cu 2p (a), Zn 2p (b), Sn 3d (c), and S 2p (d) binding energies in CZTS and CZFCTS-1 thin films, together with the Fe 2p (e) and Cd 3d (f) binding energies in CZFCTS-1 thin films.

Figure 3

Temperature-dependent Seebeck coefficient S (a), electrical conductivity σ (b), and power factor PF (c) of the CZTS and CZFCTS thin films. The uncertainty bars indicate uncertainties of 5, 3, and 10% in the measured S, σ, and PF, respectively.

Figure 4

Calculated total electronic density of states (TDoS); (a) and atom-projected (partial) density of states (PDoS; (b)) for CZTS and a model of CZFCTS.

Figure 5

(a) XRD patterns for as-prepared CZCFTS-1 thin films (0 h) and films annealed for up to 36 h. Impurity peaks assigned to Cu₂S and FeS are marked by circles and diamonds, respectively. (b) SEM and cross-sectional images of a CZFCTS-1 thin film annealed at 390 °C for 24 h.

Figure 6

Temperature dependence of (a) the Seebeck coefficients (S), (b) electrical conductivity (σ), and (c) power factor (PF) for as-prepared (0 h) and annealed CZFCTS-1 thin films. (d) Dependence of S, σ, and PF at 575 K on postdeposition annealing time. (e) Comparison of the maximum PF value obtained in this work and published data for single-phase polycrystalline quaternary sulfide (Cu₂BSnS₄ (CBTS); B = Mn, Fe, Co, Ni) bulk and thin film materials.^,,,,,,− The uncertainty bars show uncertainties of 5, 3, and 10% in the S, σ, and PF values, respectively.

Figure 7

(a) Temperature dependence of the total thermal conductivity κ_tot and the electronic and lattice components κ_ele/κ_lat for our 24 h annealed CZFCTS-1 thin film. As described in the text, the κ_{tot_TFA} and κ_{lat_TFA} are based on TFA measurements, while the κ_{tot_est}/κ_{lat_est} are calculated based on the thermal conductivity of bulk CZTS/Ag taken from Sharma et al. (b) The estimated temperature-dependent zT values for the 24 h annealed CZFCTS-1 thin film. The uncertainties in zT are estimated to be ±20% based on the uncertainties in the other parameters in eq 1 and are comparable with values reported in earlier investigations.^,